Question about transparency in ab763 style amps

[espinete]

Don't attempt to put a back biasing circuit in this amp!

I've already had a back biasing circuit in my Two other amps to be able to run these tubes. Anyway i wouldnt do it. For these kind of things , a professional technician. The problem in my area is most of technicians don't have enough knowledges eather, so i would tell your suggestions to the tech

[espinete]

It's a 470Ω resistor and a standard rectifier diode, not a zener. That is your bias circuit
Thanks!!

Can you see in the pics anything like a back-biasing circuit ?

[espinete]

I know of one Super Reverb amp that has a power transformer that is 354-0-354.

A useful general guide I use is this:

solid state rectifiction 1.4 x 354 = 495 volts

GZ34 1.3 x 354 = 460 volts

5V4 (or 5U4GB) 1.2 x 354 = 424 volts

So, IF you are wanting to lower voltage, then you can consider a 5V4 rectifier if you are not drawing more current then the 5V4 can handle. OR use a 5U4GB for more current handling. See attached "tube data cheat sheet".

Having said all that, I don't think changing the voltage will change anything about the tone stack issue needing treble on zero. I would be suspicious of a wiring mistake for the tone stack or some value that is off in the tone stack.

With respect, Tubenit

Thanks!!
I'll have a look

[sluckey]

Can you see in the pics anything like a back-biasing circuit ?

No. Do you know what back biasing is?

[espinete]

Can you see in the pics anything like a back-biasing circuit ?

No. Do you know what back biasing is?

Thanks !!
Back biasing :
http://www.aikenamps.com/index.php/what-is-back-biasing
I hope the technician knows to do it right. Suggestions for making it properly are welcome

[sluckey]

Show that web page to your tech.

[10thTx]

That link to back biasing says:

It is particularly useful when it is also desirable to lower the plate voltage supply by fifteen to twenty volts or more

Changing rectifier tube may lower even more voltage for you and you don't change anything but the rectifier tube. Something worth considering, IMO.

With respect, 10thtx

[espinete]

Show that web page to your tech.

Once installed, i'd like to check it out. What should we measure before and after ? And..How much should it measure to be correctly ? Currently plate voltage exceeds the maximum in 85v because the power tubes inside only can handle 400v . 485v b+ and 39,40 bias

[espinete]

That link to back biasing says:

It is particularly useful when it is also desirable to lower the plate voltage supply by fifteen to twenty volts or more

Changing rectifier tube may lower even more voltage for you and you don't change anything but the rectifier tube. Something worth considering, IMO.

With respect, 10thtx

Thanks, i think this is the first thing i'm gonna try

[espinete]

I've been playing with new 5u4gb rectifier and same bias and i haven't noticed any significant change

[10thTx]

I've been playing with new 5u4gb rectifier and same bias and i haven't noticed any significant change

That seems VERY odd to me? I have always experienced voltages changes between solid state, GZ34, 5V4 or 5U4GB and 5Y3GT.

With respect, 10thtx

[espinete]

Me too. I do hear differences between ss,gz34 and 5u4. I main the ugly crunchy sound when i add any od pedal is still there

[espinete]

I put some Mallory in Tone Stack. The middle cap was in the brown wire instead of along the blue one. One issue is resolved. Fuller sounding now. I can adjust treble pot at 5 now.

What do you think about these two resistors below the caps in the v6 area ?
If i'm not wrong the resistor on the right it should be a 100k one.it measured 72-73k. I put a 100k instead.

I don't know if it's got any sense or gives some clue but i turned de bias pot down at minimum with 5u4 rectifier tube and nothing changed about the issue. All my od pedals sound a little bit like a fuzz pedal

[sluckey]

What do you think about these two resistors below the caps in the v6 area ?

The value and wattage is correct, but they should be 5% tolerance. The ones in the pic are 10% tolerance.

[espinete]

Yes, the right resistor should be 82k. I saw now the right layout