oldman55
Well-Known Member
With the low current draw, why the 25 watt? Don't have a bin of those.
Why would the mfg leave it out? A 25 watt stands out.
old
Edit: a 1w resistor burns quite nicely in that circuit.
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Increase of 299C power
- Thread starter oldman55
- Start date
Why would the mfg leave it out? A 25 watt stands out.
old
Edit: a 1w resistor burns quite nicely in that circuit.
That actually handles a lot of power. Its a voltage divider. The 15K should be dissipating something like 12 watts if its got 430 volts across it.
It may have been left out by a prior repair person, not so much the mfg. Does this one have those standee resistors that poke up through the top or no? I'd half expect it to be topside for heat reasons. If not, look for where the wiring is baked and thats where it would have lived originally.
It may have been left out by a prior repair person, not so much the mfg. Does this one have those standee resistors that poke up through the top or no? I'd half expect it to be topside for heat reasons. If not, look for where the wiring is baked and thats where it would have lived originally.
I know that at one point, Scott stopped using the screen grid bleeder resistor, due to the heat it gave off, and its propensity to fail as a result. It may be that Old's unit is one that did not use the bleeder -- one of (possibly) a few differences between the power amp circuit in his unit, and the 208, which definitely used the bleeder.
Dave
Dave
oldman55
Well-Known Member
Every solder joint, as well as unused tabs/pins had a green marker on it. As if QC checked off each connection.
It does have standee resistors but they are accounted for in the HV rail. The only change when I got it was the first dropping resistor was changed from 80ohm to 160ohm.
I put in a gang of 2 and 3 watt resistors. Before they burned I got a reading on the screens. 420V. But didnt have time to see how it affected the other voltages.
I got it down to 423v earlier by adding another 5k ohm in series with the 1.2k without affecting any other circuits (but it lowered current on the output tubes). What would be the disadvantage of going that route other than that as it is liable to be the case when adding this resistor also? That will be alot of heat when/if added.
old
Edit: all wiring is clean and pliable. It was in great shape when it came to me. Wondering now if I shouldnt rewire the HV chain back through that 1.2k ohm screen standee. I thought it was a mistake but now think it is possibly correct as this was a different circuit.
Why else a 10w resistor on that screen circuit?
There are no schematics in the Scott data base/radiomuseum/etc that match this unit.
It does have standee resistors but they are accounted for in the HV rail. The only change when I got it was the first dropping resistor was changed from 80ohm to 160ohm.
I put in a gang of 2 and 3 watt resistors. Before they burned I got a reading on the screens. 420V. But didnt have time to see how it affected the other voltages.
I got it down to 423v earlier by adding another 5k ohm in series with the 1.2k without affecting any other circuits (but it lowered current on the output tubes). What would be the disadvantage of going that route other than that as it is liable to be the case when adding this resistor also? That will be alot of heat when/if added.
old
Edit: all wiring is clean and pliable. It was in great shape when it came to me. Wondering now if I shouldnt rewire the HV chain back through that 1.2k ohm screen standee. I thought it was a mistake but now think it is possibly correct as this was a different circuit.
Why else a 10w resistor on that screen circuit?
There are no schematics in the Scott data base/radiomuseum/etc that match this unit.
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with a 1.2K resistor between the rectifier and the first filter cap the voltage is going to sag horribly with any kind of power output. Same with really big value resistors in the screen circuit, the voltage will sag badly under load and the performance of the amp will suffer considerably.
oldman55
Well-Known Member
Bottom line as I am buttoning this one up:
I have two B+ voltages out of line, others are fine;
1) 470v to the OPT which is in line for a 208 but 50v high for a 299C
2) 450v to the screen which is 50v high for a 299C and 40v high for a 208
I can drop the screen down to 423v by adding a 5k in series to the screen (or put it somewhere in between) or just leave as-is.
Any last advice on on whether to leave alone and run the screen high or drop the screen voltage? Putting tube life aside, which would be the best sounding and still be safe?
I have two B+ voltages out of line, others are fine;
1) 470v to the OPT which is in line for a 208 but 50v high for a 299C
2) 450v to the screen which is 50v high for a 299C and 40v high for a 208
I can drop the screen down to 423v by adding a 5k in series to the screen (or put it somewhere in between) or just leave as-is.
Any last advice on on whether to leave alone and run the screen high or drop the screen voltage? Putting tube life aside, which would be the best sounding and still be safe?
Old -- So I take it that R218 is not present in your unit?
Dave
Dave
oldman55
Well-Known Member
That is correct. And I really dont want to burn up that kind of power if it was not designed for it. So I thought that maybe lowering it thru a low power resistor might be the answer if the ramifications were not objectionable. Dont know what is worse, high screens or higher resistance.
old
Edit: BTW, there are some 3.3 ohm resistors in series to the OPT which knock the voltage down to 465v. And I could not find these circuits on either schemactic.
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oldman55
Well-Known Member
New tubes:
With a new "matched" quad of TungSol tubes, all my B+ voltages dropped about 12vdc (about 12v lower than design now). The screen voltage dropped 20v to 430vdc. Plates are 437v.
Heaters on 7591s are 6.14vac and off the bias supply, -44vdc.
But current was too high and after adjusting pots to full, I was able to get the current down to 40ma but the bias is -22.5vdc on average. About 16.5 watts.
Is this an issue or is all good if current is good?
Thanks
Old
BTW, if I adj pots to full in other direction, I can get close to design of -18.5v bias but current is in excess of 55ma. Difficult for me to figure out the optimum value at idle from the tube charts.
http://www.tubezone.net/pdf/7591.pdf
Edit: After about an hour of play, noticed the .01 cap across the wall 117v was sweating liquid wax. The chassis was hot as were the OPTs. Couldnt keep my hand on one of them for over a second or two.
With a new "matched" quad of TungSol tubes, all my B+ voltages dropped about 12vdc (about 12v lower than design now). The screen voltage dropped 20v to 430vdc. Plates are 437v.
Heaters on 7591s are 6.14vac and off the bias supply, -44vdc.
But current was too high and after adjusting pots to full, I was able to get the current down to 40ma but the bias is -22.5vdc on average. About 16.5 watts.
Is this an issue or is all good if current is good?
Thanks
Old
BTW, if I adj pots to full in other direction, I can get close to design of -18.5v bias but current is in excess of 55ma. Difficult for me to figure out the optimum value at idle from the tube charts.
http://www.tubezone.net/pdf/7591.pdf
Edit: After about an hour of play, noticed the .01 cap across the wall 117v was sweating liquid wax. The chassis was hot as were the OPTs. Couldnt keep my hand on one of them for over a second or two.
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Hot output transformers after an hour of play = muy no bueno.
oldman55
Well-Known Member
Yah, kinda figured that. Was looking for some reasons why they do so I could do something about it.
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oldman55
Well-Known Member
Both xfrms are flanked by two tube almost touching them. Only one is really hot.
I put in the original value for the first dropping resistor on B+ (80ohm vs 160ohm). Screen is 444v but other voltages in line (or lower).
However, now the I am getting .48v across the cathode resistor and the bias pots are full turned at -22.5v. Do I need to change the resistor to the pots to get adjustment where I can get the tubes to .35v? Or is running them that high OK?
To get them there, bias voltage will be considerably higher than design.
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Thats probably over about 120F. If it takes that long, it could just be heat from the nearby tube. I don't own one, so I can't say if thats just how hot they get.
Happen to have any aluminum foil around? Maybe try making a temporary reflective shield over the end of the transformer to see if that helps. If it does you can use foil tape on the transformer to make a more permanent solution.
Happen to have any aluminum foil around? Maybe try making a temporary reflective shield over the end of the transformer to see if that helps. If it does you can use foil tape on the transformer to make a more permanent solution.
oldman55
Well-Known Member
Both Transformers or flanked by two Output tubes in an identical configuration. Only one gets hot.
