Turning a 60W solid state into a 6V6 bassman

[smuc]

First and most obvious target is C13 and C14, the 0.1uF post-PI coupling caps. Replace those with 0.02uF and see how it sounds. Next would be to replace the 820R shared cathode resistor R7 with individual 1K5 resistors, bypassing the bright side (C1) with 0.68uF, and the normal side with 25uF.

Thanks xtian! I will try that out and let you know about the outcome.

[smuc]

Guys, I am in a need for help... I have been tracking down an issue in this amp and cannot trace it down...

When I play bass notes past 5-6 on the preamp volume, I get a buzzing sound that seems somehow related to the note that I play. I believe the issue is present in the higher notes, but the buzz somehow blends with the original note. I am playing through a reactive load, so it is nothing that resonates in the room. The problem is present on all levels of the master volume, so my money is somewhere around the tone stack and the PI.

This is how it sounds: https://soundcloud.com/user-531861155/6v6-buzz

I have tried the following without any success whatsoever:

• Moving the lead dress around and avoiding parallel wires

• changing the treble capacitor - I suspected that it could be the issue since the bass notes looked very funky on scope on the junction between the treble pot and the treble cap

• different preamp and power tubes

• Tried 22K and 47K tails, as well as a few different PI bias resistors

• power tubes grid to ground caps

The only thing I have not done is to change the filter caps and then I'm officially out of options.

Any kind of recommendation or advice is very much welcome!

Thank you all!

[brewdude]

Does it make the noise plugged directly into an appropriate speaker cab?

[xtian]

+1, @brewdude's suggestion.

Your audio clip does not sound like ghost notes caused by poor filter capacitance, IMO.

Another test: Disconnect the NFB (R26 to OT secondary) to see how that affects the buzzing noise.

More tests: different guitar? different guitar cable (I hope you're plugging guitar directly into amp to reduce variables)?

[smuc]

Does it make the noise plugged directly into an appropriate speaker cab?

+1, @brewdude's suggestion.

Your audio clip does not sound like ghost notes caused by poor filter capacitance, IMO.

Another test: Disconnect the NFB (R26 to OT secondary) to see how that affects the buzzing noise.

More tests: different guitar? different guitar cable (I hope you're plugging guitar directly into amp to reduce variables)?

Tried different guitars, cables, speakers and the issue persists. I have a switchable NfB and the position of the switch had only impact to overall volume as expected, but not to the ratio of the buzz

Now, given that I took a bassman circuit and essentially replaced 6L6 tubes with a 6V6 (and a Deluxe Rev transformers), I might have been too ambitious with the little knowledge of electronics I have. My guess after poking around with the scope and changing bits and pieces was that the issue is somewhere around the phase inverter as the sine looked fine all the way up to the input of the PI. I tried tweaking the PI values a bit and got actually very good results with reducing the PI input cap from 0,022uF to 0,0047uF, as well as reducing the second grid cap from 0,1uF to 0,022uF. Set the tail to 47K and put in the 12AY7 in the PI. This has the least of this nasty buzz and is the closest I got to the sound I expected from this amp. Once I fine-tune it I will post the final schematic and a few clips.

@brewdude & xtian, tbig thanks for your support and ideas, really appreciate it!

[smuc]

I did a whole bunch of fiddling around the amp and I finally got it to where I want it to be. Changed the Master Volume type and settled on one Feedback resistor (removed the toggle), bumped up the power amp grid stoppers to 10K. The amp plugged into a proper cabinet now sounds stellar, no buzzing whatsoever. I also think that I might have had too big of an expectation of how the amp should sound like through IRs, and there was definitely a bit of a learning curve.

Here it is now, played through the reactive load and two different IRs: