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More Fun With Magnavox: The 9300 Series
- Thread starter dcgillespie
- Start date
They'll probably be pretty close to right, just may not be absolutely perfect. You'd just want to verify that its stable. Mine was not, but it turned out to mostly be tube problems rather than circuit problems. A reasonably matched set of old RCA tubes works perfect, the grossly mismatched Russian/Soviet tubes did not work well. Maybe one of these days I'll pull the good set of Valvo tubes from my German console amp and give them a run in the Magnavox to see how they do. Those actually match pretty well, and have been living together since the early 60s.
Got a pretty nice bonus this year and decided to splurge on the Z565s. They should be here any day now.
I assume this is still the schematic to follow with the new OPTs?
It is, and I think you'll really enjoy the results! It produces the sound that Maggie owners love, with none of the Maggie OPT limitations. Let us know how it turns out!
Dave
Dave
Harlon
Active Member
Question on application of the buffer circuit or input sensitivity adjustment.
My 9304-20 is basically going back into the console from which it came. I don't intend for it to stay there in perpetuity, but it will initially receive its signal from the original Maggie preamp/tuner assembly, MPX and TT.
From reading Dave's explanation of the buffer, I'm thinking that it will work fine within the console setup. The preamp/tuner is the signal source and the buffer will handle that signal as if it was from any other source. Kind of massage it and present it to the amp.
Forgive my ignorance, but am I correct here, or do I need to use the sensitivy reduction network?
Thanks,
Harlon
My 9304-20 is basically going back into the console from which it came. I don't intend for it to stay there in perpetuity, but it will initially receive its signal from the original Maggie preamp/tuner assembly, MPX and TT.
From reading Dave's explanation of the buffer, I'm thinking that it will work fine within the console setup. The preamp/tuner is the signal source and the buffer will handle that signal as if it was from any other source. Kind of massage it and present it to the amp.
Forgive my ignorance, but am I correct here, or do I need to use the sensitivy reduction network?
Thanks,
Harlon
I actually did a quick test last night with a good set of Valvo tubes and the power level is the same. The bias doesn't even shift by any real amount, so either the Valvos are exactly as tired as the RCA tubes, or both are fine. The good meter barfed on me, so I was using the old Simpson 260 just as a comparison between the two. I know those particular tubes do more power in the German console amp they came from so I suspect the Edcor iron just isn't very efficient. I guess I could figure out what the voltage swing is on the primary side and do the math to figure out the efficiency but that will need to wait until I fix my bench meter.
Harlon -- If you use the design without the buffer stage or the sensitivity reduction network, and install the unit in the console with that configuration, the biggest difference you would notice is that the volume control would tend to react slightly quicker. Theoretically, the console tuner/preamp provides up to 1.0 vac of audio signal, since that is what it took to drive the original stock design of the amplifier to full power. With the modified design as described above, the amplifier only requires .60 vac to develop full power, so you can see that the volume control would tend to react quicker due to the increased sensitivity. Frankly however, it should be fine, as the increased sensitivity won't be that readily apparent unless you're looking for it. I would only install the sensitivity reduction network if you're a stickler for the details of the console's operation.
On the other hand, if you include the buffer stage, it will ensure the very best performance possible, regardless of how the tuner/preamp circuits are designed. The buffer stage allows the amplifier to interact perfectly with any source impedance driving the amplifier -- even if it is a volume control -- which it likely is, almost surely being driven by the output of the volume control on the tuner preamp chassis. If you take this approach, connect the .01 uF input cap directly to the input jack, as well as the 470K resistor shown connected to ground as well. The series 470K resistor between the input jack and .01 uF cap would not be included. In this configuration, the amplifier will have somewhat more gain than the original amplifier did, but again, it just won't be so different as to be of concern.
I hope this helps!
Dave
On the other hand, if you include the buffer stage, it will ensure the very best performance possible, regardless of how the tuner/preamp circuits are designed. The buffer stage allows the amplifier to interact perfectly with any source impedance driving the amplifier -- even if it is a volume control -- which it likely is, almost surely being driven by the output of the volume control on the tuner preamp chassis. If you take this approach, connect the .01 uF input cap directly to the input jack, as well as the 470K resistor shown connected to ground as well. The series 470K resistor between the input jack and .01 uF cap would not be included. In this configuration, the amplifier will have somewhat more gain than the original amplifier did, but again, it just won't be so different as to be of concern.
I hope this helps!
Dave
trobbins
Super Member
Dave,
Broaching the consequence of a valve failure may be worth a note. A gassy tube, or internal short, or short to heater, could cause a continuous current through the LM337. I would expect the T0-220 package would be preferred by most, and the grounded tab allows for easier heatsinking (if used), and even bolting to chassis if grounding noise is not a concern. I can't think of any failure modes that would short a cathode to ground, and hence warrant a protection diode from adjust to output terminal for peace of mind.
Tim
Broaching the consequence of a valve failure may be worth a note. A gassy tube, or internal short, or short to heater, could cause a continuous current through the LM337. I would expect the T0-220 package would be preferred by most, and the grounded tab allows for easier heatsinking (if used), and even bolting to chassis if grounding noise is not a concern. I can't think of any failure modes that would short a cathode to ground, and hence warrant a protection diode from adjust to output terminal for peace of mind.
Tim
Thanks Tim. Such a diode will in fact add a level of protection, but not the same level it does were the 337 used in a conventional negative regulator circuit.
In a typical conventional scenario, the regulator input is fed from (say) a -30 vdc source, the output is adjusted for (-16) volts, with the appropriate caps connected from all three terminals to common ground to maintain regulator stability. In that case then, the adjust to output terminal protection diode protects the regulator by immediately hauling down the adjust terminal voltage if the output terminal should become shorted or nearly so.
In the case of the 337's application with EFB(tm) however, the configuration of the regulator is somewhat different, and easiest to understand (in the case of the circuit at hand) if the regulator is considered as actually operating from a -370 volt source, with the actual 370 volt source itself representing common ground. Further, because the adjust terminal's voltage divider is sourced from the regulator input terminal (instead of output terminal), the output terminal then seeks to track any change in current flow through the adjust voltage divider, with the divider adjusted (typically) for an output voltage that is (about) 16 volts below that of the input voltage, or in this case, -354 vdc under quiescent conditions.
With this scenario, the biggest enemy of the regulator would almost certainly be a screen to cathode or screen to suppressor short, which would effectively act to place -370 vdc across the regulator. The devices are internally current limited, but not over voltage limited. I have sought to protect the regulator by the presence of Screen Stability resistors, and individual cathode current sampling resistors, which if made small in wattage will add a degree of protection, but in the case of the Screen Stability resistor, primarily protects by preventing any transient arcs to begin with that could take the regulator out. The point being that since the regulator is effectively operating from such a large voltage source, any catastrophic tube failure is likely going to take the regulator out immediately due to over voltage, with a protection diode adding little protection in this case.
I've always felt that the best regulator protection could be had by placing a 25 volt 5 watt Zener across the input and output terminals of the regulator, so that then, such a tube failure would most likely blow out the cathode sampling resistor in conventional manner. In addition, increasing the value of the regulator output cap (which is also effectively across the input and output terminals) and then bypassing it appropriately with a small film cap would really act to protect the regulator against an over voltage scenario. In that instance then, an adjust protection diode would provide the same benefits as it does in a conventional regulator setting.
To date, I know of no regulator failures, other than due to improper installation, that I have been made aware of anyway. The one in the original development model has continued to chug away over the years, and there have now been many hundreds of installations world wide using this basic configuration. Still, your comment is not taken lightly, as peace of mind is huge when intermixing SS and vacuum tube technologies. The suggested protection diode is certainly a good thing, but again, maximum benefit from its installation will only come from adding the additional elements discussed as well. For those wishing to install it, a conventional 1N4007 diode should be installed with its banded end connected to the adjust terminal, and the other end connected to the output terminal.
I appreciate the comment Tim. It has caused me want to re-examine regulator protection for the very peace of mind you mention!
Dave
In a typical conventional scenario, the regulator input is fed from (say) a -30 vdc source, the output is adjusted for (-16) volts, with the appropriate caps connected from all three terminals to common ground to maintain regulator stability. In that case then, the adjust to output terminal protection diode protects the regulator by immediately hauling down the adjust terminal voltage if the output terminal should become shorted or nearly so.
In the case of the 337's application with EFB(tm) however, the configuration of the regulator is somewhat different, and easiest to understand (in the case of the circuit at hand) if the regulator is considered as actually operating from a -370 volt source, with the actual 370 volt source itself representing common ground. Further, because the adjust terminal's voltage divider is sourced from the regulator input terminal (instead of output terminal), the output terminal then seeks to track any change in current flow through the adjust voltage divider, with the divider adjusted (typically) for an output voltage that is (about) 16 volts below that of the input voltage, or in this case, -354 vdc under quiescent conditions.
With this scenario, the biggest enemy of the regulator would almost certainly be a screen to cathode or screen to suppressor short, which would effectively act to place -370 vdc across the regulator. The devices are internally current limited, but not over voltage limited. I have sought to protect the regulator by the presence of Screen Stability resistors, and individual cathode current sampling resistors, which if made small in wattage will add a degree of protection, but in the case of the Screen Stability resistor, primarily protects by preventing any transient arcs to begin with that could take the regulator out. The point being that since the regulator is effectively operating from such a large voltage source, any catastrophic tube failure is likely going to take the regulator out immediately due to over voltage, with a protection diode adding little protection in this case.
I've always felt that the best regulator protection could be had by placing a 25 volt 5 watt Zener across the input and output terminals of the regulator, so that then, such a tube failure would most likely blow out the cathode sampling resistor in conventional manner. In addition, increasing the value of the regulator output cap (which is also effectively across the input and output terminals) and then bypassing it appropriately with a small film cap would really act to protect the regulator against an over voltage scenario. In that instance then, an adjust protection diode would provide the same benefits as it does in a conventional regulator setting.
To date, I know of no regulator failures, other than due to improper installation, that I have been made aware of anyway. The one in the original development model has continued to chug away over the years, and there have now been many hundreds of installations world wide using this basic configuration. Still, your comment is not taken lightly, as peace of mind is huge when intermixing SS and vacuum tube technologies. The suggested protection diode is certainly a good thing, but again, maximum benefit from its installation will only come from adding the additional elements discussed as well. For those wishing to install it, a conventional 1N4007 diode should be installed with its banded end connected to the adjust terminal, and the other end connected to the output terminal.
I appreciate the comment Tim. It has caused me want to re-examine regulator protection for the very peace of mind you mention!
Dave
trobbins
Super Member
Hi Dave, my initial concern was having a cap on the adjust terminal, which could discharge through the adjust to the output terminal if the cathode somehow shorted to ground. I couldn't envisage such a fault scenario, but for completeness I'll keep going (although the rest of the post is a bit dry, and iffy/butty).
The datasheet section on protection diodes indicates " The bypass capacitor on the adjustment terminal can discharge through a low current junction. Discharge occurs when either the input, or the output, is shorted.", although I just noted that the datasheet figure showing the placement of protection diodes also notes "When the adjust terminal bypass capacitor is larger than 10 μF and −VOUT is larger than −25V, the protection diode protects the LM137 in case the output is shorted." - which relates to the voltage on the adjust pin. Any such discharge through the adjust terminal sort of depends on how quick the cathode is shorted to ground, but as the EFB is only using 4.7uF to keep a low 100ms time constant, and the bias is only 17V, then no issue.
Your comment on belts-and-braces protection with a zener is somewhat similar to protecting a 317 when used for cathode CCR. In either case, if the regulator fails open-circuit (for whatever reason) then the cathodes can rise and possibly stress the cathode heater insulation - for that scenario, especially if deploying a cathode regulator or fusing expensive big bottles, a 150V 5W zener is a simple means to avoid collateral damage.
The only other weird operation that I can think of is if both sides of a PP went in to cut-off, such that 337 current went below a few mA, but that would also need to occur on both channels. The datasheet also doesn't clarify what loss of voltage regulation then occurs.
Tim
The datasheet section on protection diodes indicates " The bypass capacitor on the adjustment terminal can discharge through a low current junction. Discharge occurs when either the input, or the output, is shorted.", although I just noted that the datasheet figure showing the placement of protection diodes also notes "When the adjust terminal bypass capacitor is larger than 10 μF and −VOUT is larger than −25V, the protection diode protects the LM137 in case the output is shorted." - which relates to the voltage on the adjust pin. Any such discharge through the adjust terminal sort of depends on how quick the cathode is shorted to ground, but as the EFB is only using 4.7uF to keep a low 100ms time constant, and the bias is only 17V, then no issue.
Your comment on belts-and-braces protection with a zener is somewhat similar to protecting a 317 when used for cathode CCR. In either case, if the regulator fails open-circuit (for whatever reason) then the cathodes can rise and possibly stress the cathode heater insulation - for that scenario, especially if deploying a cathode regulator or fusing expensive big bottles, a 150V 5W zener is a simple means to avoid collateral damage.
The only other weird operation that I can think of is if both sides of a PP went in to cut-off, such that 337 current went below a few mA, but that would also need to occur on both channels. The datasheet also doesn't clarify what loss of voltage regulation then occurs.
Tim
Hi Tim --
The last scenario you describe actually occurs with regularity at start up when SS rectifiers are used for the B+ supply. In that instance, there is no current draw through the regulator until the tubes warm, yet the adjust voltage is present virtually instantly.
One of the things I always endorse as well is to use the EFB bias voltage to bias the heater system as well, just as the voltage from conventional resistive cathode bias is often used for. That covers if the regulator fails open -- but only the the output tubes. Any small signal tubes powered from the same heater winding could then get stressed if the heater bias connection is used.
I keep coming back to the belts and braces approach, but have frankly been slow to really go after that approach due to the almost non-existent failure rate. Still, it's bound to happen some time if it hasn't already (but not reported), with a fail short condition certainly being of greatest concern. Time for some more failure mode experiments.
I thank you for the time you've given to the analysis of the circuit!
Dave
The last scenario you describe actually occurs with regularity at start up when SS rectifiers are used for the B+ supply. In that instance, there is no current draw through the regulator until the tubes warm, yet the adjust voltage is present virtually instantly.
One of the things I always endorse as well is to use the EFB bias voltage to bias the heater system as well, just as the voltage from conventional resistive cathode bias is often used for. That covers if the regulator fails open -- but only the the output tubes. Any small signal tubes powered from the same heater winding could then get stressed if the heater bias connection is used.
I keep coming back to the belts and braces approach, but have frankly been slow to really go after that approach due to the almost non-existent failure rate. Still, it's bound to happen some time if it hasn't already (but not reported), with a fail short condition certainly being of greatest concern. Time for some more failure mode experiments.
I thank you for the time you've given to the analysis of the circuit!
Dave
Harlon
Active Member
Couple of questions if I may.
Dave, on your test mule, I think you're using the black, screw terminal strip on the top of the amp for your test points, am I correct on that? I think on the undercarriage pick I'm seeing wires running from pin 3 on each output tube to the hole beside the terminal.
The other question is in regards to the bias pot. 5k, linear and 1/2watt?
Sorry if either of these are redundant and thanks for the help.
Dave, on your test mule, I think you're using the black, screw terminal strip on the top of the amp for your test points, am I correct on that? I think on the undercarriage pick I'm seeing wires running from pin 3 on each output tube to the hole beside the terminal.
The other question is in regards to the bias pot. 5k, linear and 1/2watt?
Sorry if either of these are redundant and thanks for the help.
Harlon -- That is correct. The barrier strip's four individual sections each represent one output tube, in the same order as the tubes appear on the chassis. Then, on the left side of the chassis, the front pilot lamp connection terminal represents the cathode buss, which is the point that the negative lead of the dvm connects to when checking the current draw of each individual tube.
Right again on the bias pot: 5K, .5W, linear taper.
Dave
Right again on the bias pot: 5K, .5W, linear taper.
Dave
dallen9494
Member
I just completed rebuilding my 9300, it was previously built to the Magnavox mods listed on an earlier thread. I really enjoyed it, but after building a DG-SE1, I had to see what a difference Dave's mods would do to my 9300. All I can say is Wow! This thing is so impressive, I've only listened to it for about an hour, but I'm sold! Maybe I'll finally get to build the base for my DG-SE1 while I enjoy this amp.
Thanks Dave for taking the time to take these remarkable little amps truly to the next level!
Dennis
Thanks Dave for taking the time to take these remarkable little amps truly to the next level!
Dennis
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Harlon
Active Member
dallen9494
Member
Harlon, they are banana jacks. I found them at a local electronics supply store. Thanks for the nice words Gadget. I just look at what everyone else does and try to copy them!
Regards,
Dennis
Regards,
Dennis
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