Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on July 25, 2016, 11:15:30 am
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I've got a '73 SF DR here. Came to me with two blown 470 ohm screen resistors. I replaced those and checked the 6V6s that were in it and one really glowed brightly and the other had fluctuation in the plate voltage. So I have a new set of EH 6v6 installed. I set the bias for -35v and turned it on. Came up with 419vdc and .043ma! Shut it right off! So i adjusted the bias up to about -44v and now the 6V6 are running 8w and 11w dissipation respectively. I think the neg bias value of -44v is not right and the matched tubes should be a little closer in current for the given voltage. What should I be looking for here? It should be biasing up around -35v +/- a few volts. So having not seen this before I am wondering if there is a problem.
The amp has some peculiarities. First the PI coupling caps are 224K, .22uf, which is over double the spec ,1uf.
Someone has also removed the cathode bypass capacitor from the reverb driver. To decrease gain, increase output impedance and filters out some low frequencies. I haven't played through the amp yet so I don't know what the reverb sounds like. (also don't have the pan hooked up on the bench, but can do)
The amp also had a 5AR4 rectifier in it, but now its got a vintage USA made 5U4GA.
I'm getting the voltages on the 6V6 pins shortly and post them.
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Try another set of 6V6s.
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For consideration: I just finished building an AB 763 lite. I have 439 VDC to the plates of the 6V6's. I had to set the bias voltage at -44 volts. This gave me a measured cathode current of .021. which should give me a PD of about 65%. I would say that your numbers are correct. I would think that the matched tubes should be a lot closer.
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The amp has some peculiarities.
1) First the PI coupling caps are 224K, .22uf, which is over double the spec ,1uf.
2) Someone has also removed the cathode bypass capacitor from the reverb driver. To decrease gain, increase output impedance and filters out some low frequencies. I haven't played through the amp yet so I don't know what the reverb sounds like. (also don't have the pan hooked up on the bench, but can do)
3)The amp also had a 5AR4 rectifier in it, but now its got a vintage USA made 5U4GA.
1) No sweat, unless one or both are leaking
2) That's a good sounding move and just results in a more useable range of reverb on the pot min-max...I like it
3) More sag,,,plus more filament draw...check your filament voltages to see how far it's draggin them down. That's a complete additional amp+.1 - probably not wise, especially on todays far-east windings
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voltages
V7-1 43.4
2 3.25
3 432.9
4 433.2
5 43.4
6 435.0
7 3.22
8 0 (of course)
V8-1 43.7
2 3.22
3 437
4 436
5 43.7
6 437
7 3.26
8 0
I don't see anything amiss I will try a set of Tungsol supposedly new and matched I played through the amp and it sounds OK ..
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Thanks SG - I checked the specs for this PT and it should have a 3 amp recto filament rating ... The tube chart on this '73 has a 5U4GB so I think it is OK to run one. But you're right it is close to the rating. Thanks for the info on the effect of no cathode bypass cap on the reverb driver.
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The original PTs in the SF DR's with 5U4GB rectos had to be under-rated. Fender using a PT winding that was running 100% doesn't seem logical nor prudent. The classic tone replacement for bf and sf also have a recto winding rated at 3 amp and they know people are going to use 5U4s with them.
Or did I mean over-rated. Oh well you know what I mean.. can handle higher current than rated.
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The original PTs in the SF DR's with 5U4GB rectos had to be under-rated. Fender using a PT winding that was running 100% doesn't seem logical nor prudent. The classic tone replacement for bf and sf also have a recto winding rated at 3 amp and they know people are going to use 5U4s with them.
Or did I mean over-rated. Oh well you know what I mean.. can handle higher current than rated.
Yup...that's all fine...
I was initially just going off of what you said about the 5U4GA being subbed in for a stock 5AR4...dicey
And yes, for whatever reason (cost), they run them at 100% rating..the windings themselves can handle more, it's the insulation that breaks down from the heat.
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Installed a new set of Tuns Sols and it biases at -40 ........ 434v x .025ma = 10.85 w. That's appx 77%. I'd say all is good for this amp.
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Oh and also thank you Labb for your input. Much appreciated..