As Taj says, he need to trace the circuit on paper, from the OPT backwards, to discover what tubes are doing what. Not an easy task, but not impossible..
Could be a 3rd leg shunt? DoNothing waste o' current when not pushing nor pulling...
I guess that makes it a regulator of sorts??? Try to keep the cathode resistor at a
fixed voltage... I'm just guessing in the dark.
Yvesm sais RCA engineers were never silly. In light of that, my suggested function
for the mystery 3rd 6V6 does seem unlikely.
I guess that makes it a regulator of sorts??? Try to keep the cathode resistor at a
fixed voltage... I'm just guessing in the dark.
Yvesm sais RCA engineers were never silly. In light of that, my suggested function
for the mystery 3rd 6V6 does seem unlikely.
Sometimes the RCA Engineers designed some pretty strange circuits so as to avoid paying royalties to the owners of patents. This was especially true with the FM Ratio Detector. RCA Engineers designed the circuit to avoid paying Royalties to Armstrong.
I am the type of person that will pull ANYTHING apart... I buy an iphone and rip it apart to see whats different and try to forsee what parts will be the most common areas that people will be breaking/repairing etc...
Having said that I was insanely scared pulling this amp apart even as much as you see. I don't want even the slightest change to how it sounds. 🙂
I think I have foind a solution though.... I am planning to rebuild the whole entire amplifier exactly as it is even with matching old style resistors etc... I have boxes and boxes of old tube sockets, old style wire wound resistors and old old old caps and I should have a match for every component. I am going to test my final product and see how the tones compare.... then I will have a test subject to rip apart as well.
Thank goodness for all the craigslist finds collecting old style resistors and ham radio stuff...
Can't find anything on the chassis... I did take a chance and pull tubes and "see what happens" I pulled each of the 6v6's (1 at a time and never more than 1 missing from a slot) with each 6v6 missing the tone was not changed whatsoever, however with position 2 and position 3 missing a 6v6 there was a hum that the amp produced.
That tube was probably used to power the exciter lamp. You can safely leave it out if it's removal doesn't cause the B+ to go too high.
here are the measurements with / without
0 / -.01
24.3 / 24.5
379 / 385.4
363.5 / 384.1
2.75 / 3.41
22.82 / 24.5
22.82 / 24.49
I looked up everything i could on 6v6's and those numbers seemed odd but i guess not near as high voltage as a lot of fender amps run them. *shrug*
0 / -.01
24.3 / 24.5
379 / 385.4
363.5 / 384.1
2.75 / 3.41
22.82 / 24.5
22.82 / 24.49
I looked up everything i could on 6v6's and those numbers seemed odd but i guess not near as high voltage as a lot of fender amps run them. *shrug*
To be absolutely confident that it is push pull you have to trace the plate circuit pin 3 tube socket wires for position 2 and 3 6v6. If they connect to different wires at the output transformer primary then it is push-pull. If they end up being connect at the same point on the OPT then it is paralleled SE. If that was the case you'd want to double check that the 3rd 6v6 plate isn't also in parallel. Also you might want to find out how they wired the screens, g2 which are on pin 4. That could have a lot to say about the sound you are getting. Triode and pentode mode will change the sound a lot. I'm sure you know that.
I'm assuming the 24 volt measurements were taken at the cathodes of the 6v6. If that is the case it makes sense because a 6v6 at 375 is running "hot" and you want to decrease the idle plate current to keep it in the correct plate dissipation range. With a lower plate voltage around 18 or 19 volts is normal. You should never change one without the other if you want to keep a constant plate dissipation on the tube.
I would also note that the 24 volt measurement, if it IS the cathode bias, is in line with class AB push pull. I would still trace those wires to be double sure.
Hope that helps.
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So if it is push/pull you feel that the 385volts it jumped to without the 1st 6v6 would be safe to leave out and is in line with 6v6 class AB (fenders push up to 400volts I guess.)
Alvarez,
Please don't get ahead of me. Have you checked where the pin 3 wires of the first 2 6v6 go yet? I would certainly do that first, with power off of course.😱 Also double check that the approximately 22-24 volt difference exists between pins 5 and pin 8 of each 6v6. That will tell you that you ARE measuring the cathode/grid bias with that voltage.
As far a what that third 6v6 does I really don't think we know yet. At least I don't know. I wouldn't assume anything just yet.
best
Actually, I'm starting to feel uncomfortable with long distance troubleshooting. For instance, pin 5, which is the grid connection - I don't think you want to do a direct measurement on the pin but rather on the other end of grid stop resistor connected to it.
I don't want to be in position where troubleshootng wanders into an unsafe area because it is being done remotely. Alvarez, if your skills are up to this good, but if they aren't it will be too easy for me to get you into trouble. I don't want to be responsible for that. In person I could stop you from doing something unsafe but remotely I can't. And if I misspeak about anything I wouldn't actually do you will suffer the consequences and not me. Sooo,
for now I'm going to sign off.
I advise you to do any education you may need for you to feel more comfortable with this. I know it might make me more comfortable.
The best to you
exeric
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