Amp gurus , PT questions and others...

[StrummersOfThunder]

Looking for the photos too, on imgur maybe not set for sharing or something?

Hmm it was working

[StrummersOfThunder]

50V equals 50V

ah sorry i meant 53v on some and 50v on others.

[Jay]

50V equals 50V

ah sorry i meant 53v on some and 50v on others.

Check out the specs for the valve re bias voltage range. The range will be wider than 3V so both your values will be okay

[RectifiedAmps]

If it’s cathode biased you won’t be using the 50v tap, so it’s irrelevant.

I’m not sure it’s the PT causing sag in this case. Usually a sagging PT is under a pretty heavy current draw (relative to its rating). Sag/compression is usually a combination of self-regulation from cathode biasing, sometimes power supply caps that are underrated (5-10uf in high power amps), and often series resistance in the power supply (a bit in the PT, a lot in a valve rectifier (if you had one), then any dropping resistors between the PT output and the supply nodes). Low efficiency speakers also cause compression, which might be the case here, since you don’t have the volume up high enough to suspect current-limiting compression.

The replacement PT should be fine. 6vac difference won’t amount to anything significant. Remember the voltage coming out of the wall can often vary by as much as 10VAC, which would mean ANY PT output could vary by >25vac on a day-to-day basis.

[StrummersOfThunder]

If it’s cathode biased you won’t be using the 50v tap, so it’s irrelevant.

I’m not sure it’s the PT causing sag in this case. Usually a sagging PT is under a pretty heavy current draw (relative to its rating). Sag/compression is usually a combination of self-regulation from cathode biasing, sometimes power supply caps that are underrated (5-10uf in high power amps), and often series resistance in the power supply (a bit in the PT, a lot in a valve rectifier (if you had one), then any dropping resistors between the PT output and the supply nodes). Low efficiency speakers also cause compression, which might be the case here, since you don’t have the volume up high enough to suspect current-limiting compression.

The replacement PT should be fine. 6vac difference won’t amount to anything significant. Remember the voltage coming out of the wall can often vary by as much as 10VAC, which would mean ANY PT output could vary by >25vac on a day-to-day basis.

Thanks for that , super helpful
Like is said , I might not be explaining what I’m hearing that well
I think I’m going to grab a Hammond super reverb PT that has 710vac 300ma and go from there
Agree the 50v/53v is irrelevant just wondered about it as an aside

[StrummersOfThunder]

ok ive done some measurements off the PT, B+, output tubes and preamp tube to normal channel
I havent drawn the schematic yet but plan to during the weekend
Like I said, it appears to be one preamp tube short, but i wonder if there is sime sharing of an unused half some where.
Will try and photos again too.
From measurements compared to stock schematic it appears that there is higher voltage as expected by the increased PT VAC (360 vs 330 stock)

trans former 2ndry 360v ac
pins 3 + 4 output tubes 468v 470v
bias voltage pin 5 = 37v
phase inv pins 1 = 300v 6 = 290
preamp tube from notmal (12ax7) pin 1= 260v 3 = 1.9v 6 =260v
first node after 100k resistor off pin 1 of preamp 12ax7 for normal ch = 406v
B+of cap can 460v

[StrummersOfThunder]

[StrummersOfThunder]

[StrummersOfThunder]

ok ive done some measurements off the PT, B+, output tubes and preamp tube to normal channel
I havent drawn the schematic yet but plan to during the weekend
Like I said, it appears to be one preamp tube short, but i wonder if there is sime sharing of an unused half some where.
Will try and photos again too.
From measurements compared to stock schematic it appears that there is higher voltage as expected by the increased PT VAC (360 vs 330 stock)

trans former 2ndry 360v ac
pins 3 + 4 output tubes 468v 470v
bias voltage pin 5 = 37v
phase inv pins 1 = 300v 6 = 290
preamp tube from notmal (12ax7) pin 1= 260v 3 = 1.9v 6 =260v
first node after 100k resistor off pin 1 of preamp 12ax7 for normal ch = 406v
B+of cap can 460v

is that plate voltage gonna be ok?

[MikeC]

Specs say 300V max on 12ax7 plate. 406V too high.

[StrummersOfThunder]

Specs say 300V max on 12ax7 plate. 406V too high.

Not overly surprising given the transformer specs etc

So will this just burn through tubes ?

Voltages on the output tubes ok?

[MikeC]

Specs say 300V max on 12ax7 plate. 406V too high.

Not overly surprising given the transformer specs etc

So will this just burn through tubes ?

Specs says 300V max. Exceed (by miles) at your peril.

[MikeC]

Plate voltages are fine for 6L6 tubes but who knows what they are dissipating at idle until measured. The cathode resistor is mostly controlling this.

[StrummersOfThunder]

Specs say 300V max on 12ax7 plate. 406V too high.

Not overly surprising given the transformer specs etc

So will this just burn through tubes ?

Specs says 300V max. Exceed (by miles) at your peril.

To be clear thats the voltage after the 100k resistor . Fender calls it 390v
The voltage on pin 1 is 260v on the dot

[StrummersOfThunder]

My poor describing sorry