Variety of Noises to come from an AB763

[deuce42]

Hi guys

I've posted before bout my project 15 watt Ab763 (single channel without trememlo) based upon an existing Princeton reverb re-issue PCB. I thought I'd start a new thread if ok to ask for some help with my confounded noise issues. I also apologise in advance if I ask some really silly questions but I'm trying to learn

The first thing I note is I've had to commit a bit of surgery to the Princeton's PCB to make the phase inverter a Long Tail version rather than the Princeton version that goes through the tremolo.

For the life of me this thing is producing hum and buzz. It's frustrating given I'm purely rehousing an existing pcb with the same original wiring used from the original Princeton amp other than a few changes to the PI circuit. Accordingly, issues such as ground points should theoretically be less of an issue as the PCB only has one ground lug to attach to the chassis in a different position to the power plug's ground lug.

Now as for the noise, when I inject a signal from the 0.001uf Phase inverter input onwards the amp sounds fine without any noise at all. I've deduced the problems must therefore be in the preamp and/or reverb sections. Noise goes crazy turning up the volume pot and hum becomes huge when I turn up the reverb pot.

As for the first thing I did, using the oscilloscope I seem to get a strange shape at the treble tone cap. Kind of jagged at the top and bottoms of the wave. The sine waveform is flowing and even along my signal trace prior to this capacitor. I've attached a amateur photo of the scope's screen and wondering if this is normal? The shape changes a bit when turning the treble control however the uneven waveform looks like noise or something. Could you guys confirm if its normal for the signal to change shape like this at the treble cap?

Thank you

[pdf64]

Please provide a schematic. Just draw it up manually if (like me!) you haven’t mastered schematic drawing software.
From your text, it seems you’ve swapped a cathodyne for a long tailed pair (LTP). The former has a gain of 1, the latter a gain of about 30.
That’s a significant increase, the amp may not be stable and I suspect that if you plugged a guitar in, the amp would overdrive even at the very lowest volume settings.
The scope pic is blurry; adjust controls to have only a couple of wave cycles on screen. Have brighter lighting in the room so that the camera’s exposure time is reduced.

[deuce42]

Ok I've tried to attach my hand edited version of a Schematic that was floating around the web.

I hope its readable. Basically it has a bias circuit that comes off the 325VAC HT secondary of the power supply and a solid state rectifier.

I take your point PDF64 about the amp having high gain. Im actually trying to make it a higher gain amp with the ability to play cleaner by rolling off guitar volume. Ive done this circuit to a super reverb and its worked out well. This one stumps me though when it comes to hum and other noises all going on.

[pdf64]

What’s the rationale for the zener diodes in the bias supply, especially for use as a high voltage rectifier?
Regarding the input stage; for a given degree of heater cathode leakage, a partial bypass will introduce more heater hum into the signal path than a full bypass.
Regarding the reverb tank, do the input and output 0V returns really have a series 220pF cap; or is it in parallel with a direct connection?
What’s the idle anode / cathode current of the power valves?
With and without the LTP valve fitted.
Why change to silicon rectification?
How’ve you got an additional 6.3V PT secondary?

[deuce42]

Thanks PDF64 - I've responded below

What’s the rationale for the zener diodes in the bias supply, especially for use as a high voltage rectifier?

Just using the existing bias circuit that was on the Princeton PCB I pillaged. I removed the tremolo intensity pot that the Princeton's bias supply usually goes through and replaced this with a wire link.


Regarding the input stage; for a given degree of heater cathode leakage, a partial bypass will introduce more heater hum into the signal path than a full bypass.

Ok I changed it to original fender specs 22uf/1.5k. No real change in hum and buzz it seems

Regarding the reverb tank, do the input and output 0V returns really have a series 220pF cap; or is it in parallel with a direct connection?

Yes sorry the caps are parallel with their input and output sockets - ie one leg of the cap attaches to the signal lug and the other to ground on each socket.

What’s the idle anode / cathode current of the power valves?

90uA? Im not sure why I get this reading and suspect I'm doing something wrong here.

With and without the LTP valve fitted.

Didn't go this far as I suspected the meter reading above is incorrect

Why change to silicon rectification?

Prefer the stiffer feel when playing power chords. Amp sag isn't a sound that I particularly like, although I respect a lot go guitarists love it.

How’ve you got an additional 6.3V PT secondary?

Yes the power xformer actually has 3 different low voltage supplies. It has 6.3vac and 0 @2A, 3.15vac-0-3.15vac@2A, and 6.3vac-5vac-0@1A. As I believed I needed about 2.1A for heaters, I used the first 2A coil for heaters (understanding that I was about 100ma over its rating). Accordingly for the pilot light and a little 8v fan too keep it cool I used the last coil. As I suspected noise here could be a problem I disconnected the fan and pilot light from that last coil altogether but the hum and buzz remained.

Thanks for all your help:)

[pdf64]

It is very beneficial to fit 1ohm current sensing cathode resistors to the 0V return of the power valves.
They should be on the amp already, R20 and R32, and their related test points TP16 and 17?
You seem unconcerned about not knowing whether the power valves are conducting heavily at idle or not?
https://www.thetubestore.com/lib/thetub ... ematic.pdf
The point being that if one or both power valves were conducting heavily, hum would probably increase.
Note that the bias supply rectifier is a 1400V BYD33V, not the 1N537 zener you noted.

[deuce42]

I
You seem unconcerned about not knowing whether the power valves are conducting heavily or not?
https://www.thetubestore.com/lib/thetub ... ematic.pdf

Thanks PDF64. I'm not unconcerned, was just unclear how to do so. I'll now test from these two resistors.

Thank you for all your help. I'm sorry I'm still trying to absorb amp building info

[pdf64]

Haha, it’s a refreshing attitude, threads where people obsess over the exact idle anode dissipation, and its % of the published anode limit, are all too common.
Don’t be reluctant to ask about things you’re unsure of; we’re all on a learning curve, no one is born knowing this stuff.