sub bass odd harmonics?

[lowell]

Ok so I scoped it and I'm not seeing the sawtooth, at least I think. There is definitely clipping and some angled peaks though. I changed the B+ supply to series 100uf and 220uf w/ 220k balancing resistors. Seems that problem persists. Should I up the screen supply cap values? How much current do screens draw?

[loudthud]

The screen supply could do it but the choke would have to be bad. Is the choke something new or from a working amp or something unknown that's been laying around for 20 years?

You still haven't talked about the two switches for the power tubes and the rectifier. Does the amp have to be in one particular mode to make the sub-harmonics? Will it do it when either the 6V6s or 6L6s are removed from the amp?

Have you tried a different speaker? Can you rule out a bad connection in the amp or a bad tube that is affected by vibration? Can you extend the speaker connection so the speaker is across the room and still create the sub harmonics?

[lowell]

I beefed up both supplies, choke is brand new Mercury Magnetics. The tube rectifier switch hasn't been installed yet so just solid state rectifier as of now. The output switch just puts the 20uf cap across the 6L6 grids thus canceling the signal. Could be a bad tube I guess, I've had these tubes in here for the 6 months I've been tweaking this, and I've cycled the power on/off many many times. maybe I've stressed the tubes over time. I'll try new ones. Oh and I also reduced the 6v6 coupling caps to .022uf and their grid leaks to 220k.

[pdf64]

I'd measure the Vac riding on the B+ at the OT and Screen grid supply nodes, at no signal, max sine wave, and max square wave. Best to use a true rms meter, but any should give a reasonable indication.
Also think about the ground return paths for each power supply cap, the power tubes, phase splitter and pre-amp. Best to keep the path for the power tubes and reservoir cap (which are going to carry large ripple currents) seperate from the others. Peter.

[guitician]

A fresh matched set of tubes my changes things indeed.

[lowell]

I'd measure the Vac riding on the B+ at the OT and Screen grid supply nodes, at no signal, max sine wave, and max square wave. Best to use a true rms meter, but any should give a reasonable indication.
Also think about the ground return paths for each power supply cap, the power tubes, phase splitter and pre-amp. Best to keep the path for the power tubes and reservoir cap (which are going to carry large ripple currents) seperate from the others. Peter.

Ok, what am I looking for in the results though? Increased AC on B+ at higher levels?

I originally laid this amp out quite well concerning grounds so pretty sure that's not the issue.

[loudthud]

There has got to be something in your amp causing the problem. Other than the extra gain stage, the amp is pretty close to a 59 Bassman. Everything looks fine except there is that output stage with the 6L6s and the 6V6s. The switch is a little unusual. We are kind of on a treasure hunt here. None of us can see or hear what's going on. Could the output stage be oscillating during parts of the input signal?

Does the amp create sub-harmonics with just the 6V6s alone or with both 6V6s and 6L6s enabled?

Does it do it with either set of tubes removed?

Does it do it with any of the preamp tubes removed? Some if the V numbers in the preamp don't make sense. Are they correct on the schematic? In a preamp with lots of gain you really don't want two tubes in the same bottle if one is too far down the signal chain.

What is the B+ voltage? Is it too much for the 6V6s to handle?

[lowell]

let me know post a correct schem w/ correct tube numbers. i'll look into those other things.

[RudeBoy]

Ok so I scoped it and I'm not seeing the sawtooth, at least I think. There is definitely clipping and some angled peaks though. I changed the B+ supply to series 100uf and 220uf w/ 220k balancing resistors. Seems that problem persists.

Do you hear the problem when playing only one note or does it require two or more?

A spectrum analyzer plot of the problem would help. For a two-tone input, if the sub-bass harmonics are related the intervals by f_lo = 2*f1 - f2 and f_hi = 2*f2 - f1 then they are normal distortion products. And if they are prominent and depending on the frequencies of the two fundamentals they can be non harmonically related to the two fundamentals (would sound bad). What you are hearing may not be related to the power supply ripple but might be normal products of a badly distorting amplifier.

Just thought I'd float the idea...

[lowell]

it's only when minor 3rds and 6ths are played. i don't have a spectrum analyser. are they expensive?

[loudthud]

What are the voltages on V6? One side is marked 12AX7 and the other 12AU7. What type tube is in that socket?

[RudeBoy]

it's only when minor 3rds and 6ths are played. i don't have a spectrum analyser. are they expensive?

There are a number of FFT type analyzers available that use a PC sound card. A google search will produce several.

[Steve Conner]

My personal favourite is Spectrum Lab. You could easily see 60 or 120Hz sidebands with it. The only drawback is that it has so many features, you could spend a day figuring out how to make it work.

Minor 3rds and 6ths are kind of marginal with distortion, anyway. Church music never changes key because of the long reverberation times of cathedrals, and metal ended up all 5ths and octaves because they had to be intelligible through all that distortion.

[lowell]

very good reminder of some things I forgot Steve.