Raz, I think that's an excellent idea......and will likely be doing that to mine!! TSD
Fisher SA-300B Set-up and Bias Adj. Question
- Thread starter Wharfcreek
- Start date
I want to read up on that diode thing first. There was a thread either on one of the Dynaco sites or DIY audio sites on the subject a few years ago. I think it takes a "perfect storm" of events to cause a flash-back, something like your AC mains power has to blip off while the rectifier is at the bottom half of it's conduction cycle. The charge in the capacitor starts to flow back into the rectifier, but if the mains voltage blips back on exactly when the AC is at the top of it's sine wave, the two charge meet and flash, destroying the rectifier. The diode is supposed to act like a back-flow preventor placed on an outdoor hose bib.
I think Don's unit had inrush limiters that he was going to remove. I hope it is simply a cold solder joint and not blown 5AR4's-those Mullard tubes are expensive.
Rich
I think Don's unit had inrush limiters that he was going to remove. I hope it is simply a cold solder joint and not blown 5AR4's-those Mullard tubes are expensive.
Rich
Rich, I had a long discussion w/ Dave Gillespie about that mod one evening on the telephone. I've had several rectifiers 'flash' on me.......and honestly, I recall several more from days past in my old 'band' days...when they'd go 'on stage' in an amp while playing out! Actually..... a more common occurrence there than in home audio....but I believe the cause was more due to the amp's being banged around. However, that begs the question: Do you know the history of YOUR rectifier? For that matter .... does anyone? How many of these things did we actually buy 'new'??? I dare say.....NONE of mine!! So, who knows how much 'mechanical' shock they've been subjected to in the past, and what affect that might have now or in the future?
Anyway..... I've had 'new' rectifiers flash out on me.....those from Russia and Asia. While there may have to be a 'perfect storm' as you call it....... I think those circumstances happen often enough that it's worth the effort to install the diodes. Again, I'm just regurgitating some of my recollections of the conversation....but IFRC, the diodes simply act as 'peak inverse voltage' limiters. And, I do believe that there is some of this inverse-action taking place most all times the amp is functioning..... like every time the dynamic load is on a downward cycle???? I may be wrong about that, and if so.....please educated me!! But, as I remember the conversation, the diode would act to limit the inverse side of the tube rectifiers duty cycle. So, in effect, it was 'protected' from inverse overload...which, as you mentioned, is the common cause of internal pyrotechnics!!
I spent several days adding diodes to a number of my amps.....and then sold a bunch of them that I'd added that mod to. Those amps acquired after that haven't had the effort given to them.....but were I to be using them regularly..... I'd probably take the time. I've found absolutely NO sonic affect from doing this...though I won't argue with someone who claims they have heard a change in the sound. My Platinum ears have passed through the gold stage.....now into 'silver'.....and steadily working their way to 'tin'! Wish there was a 'viagra' for my ear drum..........lol.
Don...... glad to know you got that project done! I just spent 3 weeks 'on the road' and living out of a hotel room....... and I HATE that kind of life!! I think I gained 10 pounds..... and feels like I fell behind on a LOT of stuff here at home! Plus, I like sleeping in my own bed....or sofa.....lol. Hope your 'recovery' happens quickly!!
Tom
Anyway..... I've had 'new' rectifiers flash out on me.....those from Russia and Asia. While there may have to be a 'perfect storm' as you call it....... I think those circumstances happen often enough that it's worth the effort to install the diodes. Again, I'm just regurgitating some of my recollections of the conversation....but IFRC, the diodes simply act as 'peak inverse voltage' limiters. And, I do believe that there is some of this inverse-action taking place most all times the amp is functioning..... like every time the dynamic load is on a downward cycle???? I may be wrong about that, and if so.....please educated me!! But, as I remember the conversation, the diode would act to limit the inverse side of the tube rectifiers duty cycle. So, in effect, it was 'protected' from inverse overload...which, as you mentioned, is the common cause of internal pyrotechnics!!
I spent several days adding diodes to a number of my amps.....and then sold a bunch of them that I'd added that mod to. Those amps acquired after that haven't had the effort given to them.....but were I to be using them regularly..... I'd probably take the time. I've found absolutely NO sonic affect from doing this...though I won't argue with someone who claims they have heard a change in the sound. My Platinum ears have passed through the gold stage.....now into 'silver'.....and steadily working their way to 'tin'! Wish there was a 'viagra' for my ear drum..........lol.
Don...... glad to know you got that project done! I just spent 3 weeks 'on the road' and living out of a hotel room....... and I HATE that kind of life!! I think I gained 10 pounds..... and feels like I fell behind on a LOT of stuff here at home! Plus, I like sleeping in my own bed....or sofa.....lol. Hope your 'recovery' happens quickly!!
Tom
gullaz
Active Member
Tom, do you have the "B" version? If so your B+ is correct I think. The plain 300
is supposed to have a higer B+ than the "B" version.
The SS diode modification is primarily applicable to the use of modern rectifier tubes, that don't always meet the Peak Inverse Voltage (PIV) rating of the bogey device. In those cases, the SS diode's PIV rating adds to the that of the tube's, to ensure enough PIV rating for proper operation.
If a HV winding puts out 400 vac on each side of the winding's CT, then multiplying that value by 1.414 produces the peak value at the crest of the AC waveform being supplied to the rectifier, or 566 peak volts in this case.
Under a worst case scenario then (cap input filter with no load on the power supply), the rectifier has to endure +566 peak volts at it's output because the filter cap will charge to this value, while a fraction of a second later, the waveform at the plate has swung to -566 peak volts in the opposite direction. The rectifier now has 1132 peak reverse volts across it. But this doesn't allow for any error at all in terms of high line voltage or transients. So, we multiply this figure by a 50% safety margin to arrive at a 1698 volt PIV requirement for safe operation. Original GZ34s were made to withstand a 1700 volt PIV condition, so it will work very well in this scenario -- and even more so because it is slow heating, ensure that the load is there when the tube warms. But modern tubes don't always have the integrity of meeting this rating, and can flash over as a result. The diode modification helps to resolve this issue.
However, the other issue is plate dissipation. Rectifier tubes like the GZ34 are constructed with little more than a human hair's distance between the plate and cathode. That's how they reduce the voltage drop through the tube. But if the plate or cathode overheats, then these elements can warp and touch, causing the flash produced under conditions of distress. Those conditions can be created by simply overloading a good NOS tube -- as I showed in my thread on the stock Dynaco ST-70 --, subjecting a good tube to "hot switching" events with excessive capacitance attached to the cathode, or simply using an inferior tube.
The maximum capacitance rating for the first filter cap connected to a rectifier tube has nothing to do with regular use under normal conditions. It has to do with hot switching events, where in the power is interrupted for just a second or two, and then quickly reapplied. In this scenario, the circuit discharges the great majority of the normal B+ operating voltage, but the cathode is still at operating temp. If the power is then reapplied with the conducting AC cycle high enough in its cresting to cause excessive current to flow in trying to restore normal operating voltages, the tube -- still very much at operating temp -- overheats, warps and flashes. If you include an automatic shutdown on your equipment as I do on much of mine (any loss of power requires a manual restart -- it cannot restart on its own), then hot switching events cannot occur, and maximum capacitance can easily be doubled over the maximum value published. When two rectifier tubes are employed, the value can be doubled anyway, because the current capability of the effective rectifier tube has been doubled as well.
I hope this helps!
Dave
If a HV winding puts out 400 vac on each side of the winding's CT, then multiplying that value by 1.414 produces the peak value at the crest of the AC waveform being supplied to the rectifier, or 566 peak volts in this case.
Under a worst case scenario then (cap input filter with no load on the power supply), the rectifier has to endure +566 peak volts at it's output because the filter cap will charge to this value, while a fraction of a second later, the waveform at the plate has swung to -566 peak volts in the opposite direction. The rectifier now has 1132 peak reverse volts across it. But this doesn't allow for any error at all in terms of high line voltage or transients. So, we multiply this figure by a 50% safety margin to arrive at a 1698 volt PIV requirement for safe operation. Original GZ34s were made to withstand a 1700 volt PIV condition, so it will work very well in this scenario -- and even more so because it is slow heating, ensure that the load is there when the tube warms. But modern tubes don't always have the integrity of meeting this rating, and can flash over as a result. The diode modification helps to resolve this issue.
However, the other issue is plate dissipation. Rectifier tubes like the GZ34 are constructed with little more than a human hair's distance between the plate and cathode. That's how they reduce the voltage drop through the tube. But if the plate or cathode overheats, then these elements can warp and touch, causing the flash produced under conditions of distress. Those conditions can be created by simply overloading a good NOS tube -- as I showed in my thread on the stock Dynaco ST-70 --, subjecting a good tube to "hot switching" events with excessive capacitance attached to the cathode, or simply using an inferior tube.
The maximum capacitance rating for the first filter cap connected to a rectifier tube has nothing to do with regular use under normal conditions. It has to do with hot switching events, where in the power is interrupted for just a second or two, and then quickly reapplied. In this scenario, the circuit discharges the great majority of the normal B+ operating voltage, but the cathode is still at operating temp. If the power is then reapplied with the conducting AC cycle high enough in its cresting to cause excessive current to flow in trying to restore normal operating voltages, the tube -- still very much at operating temp -- overheats, warps and flashes. If you include an automatic shutdown on your equipment as I do on much of mine (any loss of power requires a manual restart -- it cannot restart on its own), then hot switching events cannot occur, and maximum capacitance can easily be doubled over the maximum value published. When two rectifier tubes are employed, the value can be doubled anyway, because the current capability of the effective rectifier tube has been doubled as well.
I hope this helps!
Dave
audiodon
Addicted Member
Swapped the two rectifiers and the amp came right on. I retensioned the sockets with a dental pick with the rectifiers out of the sockets and things are looking good again.
WooHoo. I had cleaned the sockets, but sometimes I guess you have to shake things up.
Next up is to clean the electronics room so I can get test instrumentation close enough to the workbench to play with.
WooHoo. I had cleaned the sockets, but sometimes I guess you have to shake things up.
Next up is to clean the electronics room so I can get test instrumentation close enough to the workbench to play with.
gullaz
Active Member
Glad you solved, Don. I had to work a lot inside my sockets in order to get them acceptable (had to use metal polish).
I have a question about bias settings for this amp:
The manual says to adjust bias to have 7.8 volts at pin 8 which, if I am correct, across the 50 ohm cathode resistor makes 156 mA current for a couple of el34 so almost 80 mA each tube...With 370v on the plates this seems way too hot...
But maybe I am not calculating correctly?
I have a question about bias settings for this amp:
The manual says to adjust bias to have 7.8 volts at pin 8 which, if I am correct, across the 50 ohm cathode resistor makes 156 mA current for a couple of el34 so almost 80 mA each tube...With 370v on the plates this seems way too hot...
But maybe I am not calculating correctly?
Gullaz -- You're very close! When figuring out plate dissipation, you always use the voltage that appears between the plate and the cathode, and also, if figuring from current draw as seen at the cathode, subtract an appropriate amount of current to account for the screen current that flows through the cathode as well. For EL34 tubes, an appropriate amount of screen current to subtract out would be 5 ma per tube.
Therefore, 362 (effective plate volts) X .073 (effective plate current) = 26.4 watts real plate dissipation.
The EL34 is rated for a plate dissipation of 25 watts in tube data publications. But this was under the old Design Center rating system, which equates to 30 watts under the newer Design Maximum rating system, that was in during the last days of the vacuum tube era. Therefore, the operating point that Fisher specifies is acceptable for the tube, but does run the tube fairly close to its maximum rating, as was so common back in the day. This was due to an effort to get maximum power at minimum distortion from the tubes. But the EL34 tubes of the day were very tough cookies, and could easily take it, and still return a very good life expectancy. There are even a number of EL34 amps from the day whose instructions specifically state that a dull red glow in the plates of the output tubes was no cause for concern!
I hope this helps!
Dave
Therefore, 362 (effective plate volts) X .073 (effective plate current) = 26.4 watts real plate dissipation.
The EL34 is rated for a plate dissipation of 25 watts in tube data publications. But this was under the old Design Center rating system, which equates to 30 watts under the newer Design Maximum rating system, that was in during the last days of the vacuum tube era. Therefore, the operating point that Fisher specifies is acceptable for the tube, but does run the tube fairly close to its maximum rating, as was so common back in the day. This was due to an effort to get maximum power at minimum distortion from the tubes. But the EL34 tubes of the day were very tough cookies, and could easily take it, and still return a very good life expectancy. There are even a number of EL34 amps from the day whose instructions specifically state that a dull red glow in the plates of the output tubes was no cause for concern!
I hope this helps!
Dave
gullaz
Active Member
Thank you Dave, I didn't really know about the old vs new el34 plate dissipation rating (I sticked to the 25W rating) .
Actual voltages on my 300-B are 376 on the plates, 370 on the screens and I didn't dare going higher than 6 volts on the cathodes ( that makes 370 between plate and cathode), resulting in -20/-21 on the grids on the respective couples, if I'm not wrong this makes around 23W plate dissipation per tube. I run RFT el34 on the amp...
So you think I could dare going a bit higher?
Actual voltages on my 300-B are 376 on the plates, 370 on the screens and I didn't dare going higher than 6 volts on the cathodes ( that makes 370 between plate and cathode), resulting in -20/-21 on the grids on the respective couples, if I'm not wrong this makes around 23W plate dissipation per tube. I run RFT el34 on the amp...
So you think I could dare going a bit higher?
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Gull -- At 6 volts at the cathode across a 50 ohm cathode resistor equates to 120 ma total draw for each channel, or 60 ma per tube. Subtracting out 5 ma for the screen grid means that 55 ma is flowing between the cathode and plate. With the voltage potential between these two elements being 370 vdc, then the plate dissipation would be 20.35 watts per tube. At 81.4% of the conservative Design Center rating, THD will be higher than with the recommended Fisher setting, but there is likely little danger of any crossover distortion, and any version of the tube should be able to take this setting in stride.
Dave
Dave
Dave,
Very good explanation with regard to the Diode mod you performed on your Dynaco ST70 thread. Thank you.
I will be replacing the first smoothing capacitor (60uf @500VDC) during this restoration. That's a difficult value to find in a FP capacitor, but two options are available, 20uf-20uf-20uf @525vdc and 30uf-30uf-30uf @525vdc. I purchased the later capacitor (30-30-30) originally intending to use only two of the sections in parallel for a total of 60uf.
The HV winding on the SA300B puts out 300vac on each side. Using your math that results in 424 vac and a total of 848 peak reverse volts across two rectifiers. Fifty percent more for error is 1273 v and still below the 1700 piv you quoted for a single GZ34 rectifier tube.
I think I understand how the maximum capacitance and hot switching event applies to rectifier flashover but am unsure when two rectifier tubes are applied. From what I am reading, the only way to prevent such a flashover is with a shutdown option. Therefore, it seems to me that it would be fine to use all three 30uf sections for a total of 90uf on the first cap, given the SA300B has two rectifier tubes, even if one of the tubes failed. Am I correct and is there any benefit, or will the GZ34's and Mains Transformer have to work too hard making the GZ34's more prone to flashover?
What would you do, 20+20+20, 30+30, or 30+30+30? And if you were to use the diode mod, would you use two for the combined ouput of each rectifier or four, two per rectifier? I should think each GZ34 should have it's own extra PIV protection. Am I correct? WWDD?
Thanks. Rich
Very good explanation with regard to the Diode mod you performed on your Dynaco ST70 thread. Thank you.
I will be replacing the first smoothing capacitor (60uf @500VDC) during this restoration. That's a difficult value to find in a FP capacitor, but two options are available, 20uf-20uf-20uf @525vdc and 30uf-30uf-30uf @525vdc. I purchased the later capacitor (30-30-30) originally intending to use only two of the sections in parallel for a total of 60uf.
The HV winding on the SA300B puts out 300vac on each side. Using your math that results in 424 vac and a total of 848 peak reverse volts across two rectifiers. Fifty percent more for error is 1273 v and still below the 1700 piv you quoted for a single GZ34 rectifier tube.
I think I understand how the maximum capacitance and hot switching event applies to rectifier flashover but am unsure when two rectifier tubes are applied. From what I am reading, the only way to prevent such a flashover is with a shutdown option. Therefore, it seems to me that it would be fine to use all three 30uf sections for a total of 90uf on the first cap, given the SA300B has two rectifier tubes, even if one of the tubes failed. Am I correct and is there any benefit, or will the GZ34's and Mains Transformer have to work too hard making the GZ34's more prone to flashover?
What would you do, 20+20+20, 30+30, or 30+30+30? And if you were to use the diode mod, would you use two for the combined ouput of each rectifier or four, two per rectifier? I should think each GZ34 should have it's own extra PIV protection. Am I correct? WWDD?
Thanks. Rich
WWDD.......pretty clever Raz!! Now....back to my silent 'Watch, Read, & Learn' mode! WC
Hey -- When two rectifier tubes are paralleled -- either with the plates of each tube directly paralleled, or the plates of separate tubes paralleled, the following occurs:
1. Heater current is (of course) doubled from the use of two tubes.
2. PIV rating is unchanged from that of a single tube.
3. DC current capacity is doubled.
As a result, the maximum capacitance specified to prevent damage from hot switching events is doubled as well.
While I believe there is something to the sonic improvements received from increasing power supply capacitance, I believe that there is a definite point of diminishing returns -- and importantly -- that it occurs at a much lower value than many today would have you believe.
Improvement from increasing the value of power supply capacitance that directly supplies the output stage is very much power output capability related. Therefore, adding 200 uF of capacitance to a 10 watt amplifier is way, way overkill. Such a design may have originally used 30 uF to supply the stage, with increasing beyond 60 uF producing no further significant return.
When power output capacity starts reaching the 100 watt level and greater (total power), returns diminish rather quickly after 1 uF / watt is reached supplying the stage. With the SA-300 rated at (I believe) 90 watts RMS total, then using greater than 90 uF will return little further improvement in my experience.
The real concern with the power supplies of higher powered units, is the capability of the power transformer/rectifier combination. Fisher addressed this wonderfully with the use of dual rectifier tubes, as did others of the day, notably, H.H. Scott, Pilot, and even Eico in some of their units. The bigger the caps get, the bigger the reservoir they represent -- but also the more charging current they pull when recharging, which the transformer/rectifier combo has to supply, along with the needs of the amplifier itself. You never get something for nothing. In this case, if you want more reserve, then you need more ability to produce that reserve. That's why a point of diminishing returns is reached quickly enough with increasing just power supply capacitance alone. At some point, if you really want more reserve, then more than just the caps are going to have to be upgraded.
The power transformer/rectifier combo on the SA-300 is really excellent for the needs of the amplifiers it supports. I'd use the 90 uF combination of 3x30 uF -- only because that might leave room for an additional cap section in the can to help somewhere else if needed.
Use of the diode mod is really based on the tubes you use. Real, good NOS tubes won't need the modification. But if you're using modern tubes, I'd install it. If the plates of each tube are directly paralleled, then one diode per tube will suffice. If the plates of one tube are paralleled with the plates of the other tube, then one diode will be required for each plate connection (a total of 4).
While keeping the capacitance within ratings will normally protect a tube during hot switching events, it still stresses the tube(s) significantly. I include a manual reset power strip for all my equipment with vacuum tube rectifiers. It prevents these unfortunate events from damaging the tubes at all.
Mine is included with the buck transformer strip that I power my vintage equipment from. Therefore, not only is the AC voltage level corrected, but it blocks hot switching events as well. A simple 120 volt relay wired in latch mode, with a momentary "push-to-start" button to initiate the latch is all it takes to eliminate such events.
I really enjoy the work you do. It is always a first class effort, and it always shows!
Dave
1. Heater current is (of course) doubled from the use of two tubes.
2. PIV rating is unchanged from that of a single tube.
3. DC current capacity is doubled.
As a result, the maximum capacitance specified to prevent damage from hot switching events is doubled as well.
While I believe there is something to the sonic improvements received from increasing power supply capacitance, I believe that there is a definite point of diminishing returns -- and importantly -- that it occurs at a much lower value than many today would have you believe.
Improvement from increasing the value of power supply capacitance that directly supplies the output stage is very much power output capability related. Therefore, adding 200 uF of capacitance to a 10 watt amplifier is way, way overkill. Such a design may have originally used 30 uF to supply the stage, with increasing beyond 60 uF producing no further significant return.
When power output capacity starts reaching the 100 watt level and greater (total power), returns diminish rather quickly after 1 uF / watt is reached supplying the stage. With the SA-300 rated at (I believe) 90 watts RMS total, then using greater than 90 uF will return little further improvement in my experience.
The real concern with the power supplies of higher powered units, is the capability of the power transformer/rectifier combination. Fisher addressed this wonderfully with the use of dual rectifier tubes, as did others of the day, notably, H.H. Scott, Pilot, and even Eico in some of their units. The bigger the caps get, the bigger the reservoir they represent -- but also the more charging current they pull when recharging, which the transformer/rectifier combo has to supply, along with the needs of the amplifier itself. You never get something for nothing. In this case, if you want more reserve, then you need more ability to produce that reserve. That's why a point of diminishing returns is reached quickly enough with increasing just power supply capacitance alone. At some point, if you really want more reserve, then more than just the caps are going to have to be upgraded.
The power transformer/rectifier combo on the SA-300 is really excellent for the needs of the amplifiers it supports. I'd use the 90 uF combination of 3x30 uF -- only because that might leave room for an additional cap section in the can to help somewhere else if needed.
Use of the diode mod is really based on the tubes you use. Real, good NOS tubes won't need the modification. But if you're using modern tubes, I'd install it. If the plates of each tube are directly paralleled, then one diode per tube will suffice. If the plates of one tube are paralleled with the plates of the other tube, then one diode will be required for each plate connection (a total of 4).
While keeping the capacitance within ratings will normally protect a tube during hot switching events, it still stresses the tube(s) significantly. I include a manual reset power strip for all my equipment with vacuum tube rectifiers. It prevents these unfortunate events from damaging the tubes at all.
Mine is included with the buck transformer strip that I power my vintage equipment from. Therefore, not only is the AC voltage level corrected, but it blocks hot switching events as well. A simple 120 volt relay wired in latch mode, with a momentary "push-to-start" button to initiate the latch is all it takes to eliminate such events.
I really enjoy the work you do. It is always a first class effort, and it always shows!
Dave
Dave,
Thank you so much for the explanation, I think I'm understanding more and more as I tackle each stage.
After doing a little research, a Buck/Boost Transformer sounds like a good idea, especially one with a built in manual reset. If not, a manual reset sounds like a fairly easy project.
And thank you for the complement. I have to admit I was smiling all day after I read that! Your knowledge and willingness to take the time to share is very rare and special. I just put the final touches on my phono stage and will be devoting my attention to the SA300B this week.
Thanks again.
Rich
Thank you so much for the explanation, I think I'm understanding more and more as I tackle each stage.
After doing a little research, a Buck/Boost Transformer sounds like a good idea, especially one with a built in manual reset. If not, a manual reset sounds like a fairly easy project.
And thank you for the complement. I have to admit I was smiling all day after I read that! Your knowledge and willingness to take the time to share is very rare and special. I just put the final touches on my phono stage and will be devoting my attention to the SA300B this week.
Thanks again.
Rich
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audiodon
Addicted Member
Referencing posts #129-132 about how hot to bias the output tubes, I'm wondering if mine is still too hot or just right.
http://www.audiokarma.org/forums/showpost.php?p=7779761&postcount=130
I had my cathode to ground voltage at about 7.3 volts.
I cooled it down to 6.1 volts.
So, across the 10 ohm resistors on the cathode, on average, for all four output tubes, I am reading an average of 60ma, or .60 volts/10 ohms on each tube where each tube has a 10 ohm resistor on the cathode, with the other side of the resistor joined for the pair of tubes and that's the other side of the test point..
From cathode to ground, I'm reading 6.6 volts on channel A and 6.6 volts on channel B.
That's 6.6 volts across the 10 ohm resistor off each cathode in series with the 50 ohm cathode resistor in each channel or 6.6V across 60 ohms.
With this time exposure photo (about 5 seconds), there's still a very slight dull red glow on the plates of the EL-34s.
I have a channel bias pot and a DC balance control for each pair of output tubes.
Mine is a SA-300. B+ reads 400VDC.
AC balance is 0.02 volts on each channel.
I forgot to measure the negative input grid voltage on pin 5 of each EL-34 when the photo was taken, but my test notes from before, when the bias was hotter, had it at about -21Vdc for each EL-34.
Dave had mentioned that if you cool it off too much, the THD will go up. Where's a good balance point for this?
The first photo is the unit with the light on.
The second photo is the same unit with the light off and approximately a 4-5 second time exposure.
http://www.audiokarma.org/forums/showpost.php?p=7779761&postcount=130
I had my cathode to ground voltage at about 7.3 volts.
I cooled it down to 6.1 volts.
So, across the 10 ohm resistors on the cathode, on average, for all four output tubes, I am reading an average of 60ma, or .60 volts/10 ohms on each tube where each tube has a 10 ohm resistor on the cathode, with the other side of the resistor joined for the pair of tubes and that's the other side of the test point..
From cathode to ground, I'm reading 6.6 volts on channel A and 6.6 volts on channel B.
That's 6.6 volts across the 10 ohm resistor off each cathode in series with the 50 ohm cathode resistor in each channel or 6.6V across 60 ohms.
With this time exposure photo (about 5 seconds), there's still a very slight dull red glow on the plates of the EL-34s.
I have a channel bias pot and a DC balance control for each pair of output tubes.
Mine is a SA-300. B+ reads 400VDC.
AC balance is 0.02 volts on each channel.
I forgot to measure the negative input grid voltage on pin 5 of each EL-34 when the photo was taken, but my test notes from before, when the bias was hotter, had it at about -21Vdc for each EL-34.
Dave had mentioned that if you cool it off too much, the THD will go up. Where's a good balance point for this?
The first photo is the unit with the light on.
The second photo is the same unit with the light off and approximately a 4-5 second time exposure.
Attachments
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Does anybody who has a Sams on this unit know what the primary impedance of the OPTs are?
Dave
Dave
gullaz
Active Member
Again about bias setting.
After reading Dave's advice I have tried to set bias closer to manual specs, at 7,5 volts on the cathode... I have the "B" version with 370v at the anodes...
Left it on for forty minutes and although I did not observe any redplating, got a negative voltage of -17 and -18 at the respective couples' grids while schematics calls for -20v.
Don't have the technical knowledge to understand if this may still be ok so I have cooled down again to 6,6 volts (resulting in -19/ -20 volts on the grids).
I wouldn't want to risk my RFT tubes.
After reading Dave's advice I have tried to set bias closer to manual specs, at 7,5 volts on the cathode... I have the "B" version with 370v at the anodes...
Left it on for forty minutes and although I did not observe any redplating, got a negative voltage of -17 and -18 at the respective couples' grids while schematics calls for -20v.
Don't have the technical knowledge to understand if this may still be ok so I have cooled down again to 6,6 volts (resulting in -19/ -20 volts on the grids).
I wouldn't want to risk my RFT tubes.