AB763 PI mods?

[Jake1524]

Hey there!

I have an old, bastardized 68 AB763 Bandmaster that has been a great learning experience for me over the past 5 years. I knew very little about tube amp operation before getting this amp(and still dont hah).. but it has been very enjoyable to modify, tinker and play this amp over the years.

That being said, I'm still educating myself on the different parts of the amp. Perhaps this is a naïve question, but are there any mods that can be done in the PI or output section of this amp to further increase headroom? I would love to get this amp to the point where it breaks up much later.. Ideally no break up all the way up the volume dial.

Is this something that can be done? I would love to use this as an opportunity to learn in the process.

Thanks!

[pdf64]

What’s your definition of ‘headroom’?
From my reading of your query, it seems to be the inverse of gain.
Whereas really it’s determined by an amp’s maximum clean power output. And it’s unrelated to the number on the dial at which it overdrives.
Yes, many guitarists seem to use the term incorrectly.

A faster recovery from overdrive induced bias excursion can seem to make an amp cleaner. The 70s silver panel variants are an example of how that can be implemented, eg see the circuit around V6 of https://el34world.com/charts/Schematics ... tr_rev.pdf

FYI attempting to mod an amp that isn’t working right isn’t a good idea.

[bepone]

Hey there!

I have an old, bastardized 68 AB763 Bandmaster that has been a great learning experience for me over the past 5 years. I knew very little about tube amp operation before getting this amp(and still dont hah).. but it has been very enjoyable to modify, tinker and play this amp over the years.

no need to touch anything on bandmaster, this is mine on 9/10 vol, still light crunch, you cannot have better "headroom"
https://www.youtube.com/watch?v=jLGXxq5HZB4

[Jake1524]

What’s your definition of ‘headroom’?
From my reading of your query, it seems to be the inverse of gain.
Whereas really it’s determined by an amp’s maximum clean power output. And it’s unrelated to the number on the dial at which it overdrives.
Yes, many guitarists seem to use the term incorrectly.

A faster recovery from overdrive induced bias excursion can seem to make an amp cleaner. The 70s silver panel variants are an example of how that can be implemented, eg see the circuit around V6 of https://el34world.com/charts/Schematics ... tr_rev.pdf

FYI attempting to mod an amp that isn’t working right isn’t a good idea.

I understand what you are saying.

Also agree with your last comment about not modding an amp that is not 100%. This post was more of a curiosity.. maybe something I would mess around with after getting the amp up to speed.

[Jake1524]

Hey there!

I have an old, bastardized 68 AB763 Bandmaster that has been a great learning experience for me over the past 5 years. I knew very little about tube amp operation before getting this amp(and still dont hah).. but it has been very enjoyable to modify, tinker and play this amp over the years.

no need to touch anything on bandmaster, this is mine on 9/10 vol, still light crunch, you cannot have better "headroom"
https://www.youtube.com/watch?v=jLGXxq5HZB4

Fair enough! This further drives home the fact that my amp is not functioning properly. I wish i could get mine to sound like that when its cranked....

[Stevem]

My 67 bandmaster which is exactly the same as yours, even with its stock output transformer, but with strong output tubes will do 42 watts of clean power.
This level of wattage thru two efficient 12” drivers is loud. So we need to figure what’s going on with yours there!

Just to put down some basics here about how SPL works let’s say you have a 10 watt amp and you wanted to go twice as loud by means of just wattage, well for the Human ear to precive that you would need to step up to a 100 watt amp.

Now if you wanted get that result another way you can just double up on the speakers.

[pdf64]

…
Just to put down some basics here about how SPL works let’s say you have a 10 watt amp and you wanted to go twice as loud by means of just wattage, well for the Human ear to precive that you would need to step up to a 100 watt amp.

Now if you wanted get that result another way you can just double up on the speakers.

For a given electrical / sound power, I think doubling the number of like, close coupled / aligned speakers provides about a 3dB increase in forward SPL.

Whereas 10x electrical / acoustic power should be a 10dB increase (speaker compression will tend to reduce that though).

[dorrisant]

A very invasive mod for headroom would be to add a cathode driver tube between the PI and the power tubes. It will still break up but it will be later and with a wonderful transition as well as increased bandwidth.

[Reeltarded]

Run a balanced PI and lower the output grid leaks to 150k. It is just in the curve of not pushing the 6l6 into blat. It sounds louder cleaner, but it isn't. It's bolder.

[Reeltarded]

https://ampgarage.com/forum/viewtopic.php?f=5&t=35962

First page disassembled and a proper repair job shown in several steps here

[JD0x0]

Get B+ as high as you reasonably can.
51k/47k plate load resistors with a 12AT7 tube. Maybe add a plate balance pot ala Dumble/Ampeg. Also try a 610ohm bias resistor as opposed to 470 ohm.

Reduce the grid leaks out of the PI to 100k or the silverface value of 68k and reduce the coupling cap size out of the PI a bit.

If you want to get really crazy, you could try an Ultralinear output transformer from a HiFi amp or later Silverface amp.

[Jake1524]

A very invasive mod for headroom would be to add a cathode driver tube between the PI and the power tubes. It will still break up but it will be later and with a wonderful transition as well as increased bandwidth.

Do you have a little more info on how to go about this.. After doing some research, I really think this is what im after!

[WhopperPlate]

A very invasive mod for headroom would be to add a cathode driver tube between the PI and the power tubes. It will still break up but it will be later and with a wonderful transition as well as increased bandwidth.

Do you have a little more info on how to go about this.. After doing some research, I really think this is what im after!

Listen to clips of an SSS type circuit . If you like this sound then it’s a viable solution . Not everyone’s cup of tea . I prefer higher voltage and a standard long tail PI. Fwiw

[Jake1524]

A very invasive mod for headroom would be to add a cathode driver tube between the PI and the power tubes. It will still break up but it will be later and with a wonderful transition as well as increased bandwidth.

Do you have a little more info on how to go about this.. After doing some research, I really think this is what im after!

Listen to clips of an SSS type circuit . If you like this sound then it’s a viable solution . Not everyone’s cup of tea . I prefer higher voltage and a standard long tail PI. Fwiw

Hi Charlie, thanks for the input. Would you mind providing a little more information and your reasoning here? I'm still learning and a little goes a long way!

[WhopperPlate]

As stated by Dorrisant, The cathode driver will increase headroom and bandwidth . The amp behaves more like a bass amplifier . Less non linear mojo from typical LTPI. Might be exactly what you want , maybe not . Listen to steel string singer clips and you will get an idea :