distortion on leading edge of note

[pula58]

I re-wired the grids of the output tubes: less wire, and re-routed them away from any signal bearing wires. I also consolidated the grid stoppers. Previously I had 1.5K at the socket, and 15K on the board (the extra 15K was an experiment from a previous post in this thread). Now I have 15K at the socket.

Made no difference.

I am beginning to think that it is either a grounding issue, or the power tranny.

Does anyone see any problems in the ground scheme that I have used in this amp?

[LeftyStrat]

Have you scoped the B+ rail at various points, to see how much ripple you have? Just wonder if it might be a bad filter cap somewhere.

[pula58]

Have you scoped the B+ rail at various points, to see how much ripple you have? Just wonder if it might be a bad filter cap somewhere.

The ripple on the 1st and 2nd power supply caps ("A" and "B" in Fender nomenclature) look reasonable.

The 120Hz ripple and the "signal" part of the ripple (the part that has the same freq as the output signal) both increase smoothly as the output vol of the amp is increased.

[pula58]

I just got finished trying two other PT's with this amp. One Heyboer, and a Hammond.

The problem is unchanged.


Well, at least I know it's not the PT!

[pula58]

I still have not solved this problem! LOL!

Problem was not solved by any of the following

1- different OT
2- different PT
3- different choke
4- different 1st and 2nd filter caps
5- b-gger cap for screen supply.
6- increasing 6L6 grid stoppers from 1.5K to 15K
7- cleaning-up the grid wiring to the 6L6's.
8- new groudning scheme
9- new PI tube
10 - new main board with different layout
11 (added to post later) new power tubes

The distortion/low freq artifact seems to occur when the output tube grids get fwd-biased.


This happens even if I drive the phase inverter input directly with a signal generator (thus bypassing the preamp completely).

To repeat the problem: When I play it loud, and pick hard, for the first moment or so I hear hum overlaying the note I played.

[JazzGuitarGimp]

Have you tried a different speaker? Maybe you've got some voice-coil-rub goin' on...

Also, does the amp have an effects loop that would allow you to plug out of the efx send into another power amp? That way you could isolate whether the problem is in the preamp or the power amp.

[pula58]

Have you tried a different speaker? Maybe you've got some voice-coil-rub goin' on...

Also, does the amp have an effects loop that would allow you to plug out of the efx send into another power amp? That way you could isolate whether the problem is in the preamp or the power amp.

Hi Lou,
Thanks for your post.
I get this problem even if I drive the input to the phase inverter directly with the signal generator (preamp effectively disabled).
Also, it happens no matter what speaker I use, and it happens whether I use a guitar or signal generator.

But here is something I just figured-out. If I put in, lets say, a 300Hz test signal and crank up the amp I start to get two spikes (in the FFT) That occur to either side of 300Hz. The peaks are at 180Hz and 420Hz....exactly 120Hz above and below the 300Hz test tone. Ah ah...this must mean that the 120Hz power supply ripple is modulating the combined PI and output stage. But, it only happens once the grids of the output tubes are fwd-biased (grid voltage above cathode voltage). I think I am getting closer to figuring this problem-out that has plagued me for quite some time. I'm not there yet, but getting closer......

[pula58]

By the way, I have tried swapping out all of the power supply filter caps to see if one was bad. No luck.

I tried disconnecting the feedback, no luck.


Could the 1.5K screen R's I am using make this circuit more prone to 120Hz sneaking-in through the screens ond modulating the output....Added to this post: Nope, I tried smaller screen R's, same problem still exists.

[Tonegeek]

I still have not solved this problem! LOL!

Problem was not solved by any of the following

1- different OT
2- different PT
3- different choke
4- different 1st and 2nd filter caps
5- b-gger cap for screen supply.
6- increasing 6L6 grid stoppers from 1.5K to 15K
7- cleaning-up the grid wiring to the 6L6's.
8- new groudning scheme
9- new PI tube
10 - new main board with different layout

The distortion/low freq artifact seems to occur when the output tube grids get fwd-biased.


This happens even if I drive the phase inverter input directly with a signal generator (thus bypassing the preamp completely).

To repeat the problem: When I play it loud, and pick hard, for the first moment or so I hear hum overlaying the note I played.

Did you try some other power tubes? They go microphonic on the low end sometimes. If you don't have any tubes to try, then before you go buy some, just turn the amp on and try tapping on each power tube and listen for any noises. If you hear something odd especially a low rumble, then take it to the next level and try other tubes.

[pula58]

Hi Tonegeek,
Yes, I did try another set of power tubes. Made no difference.

I have some intermodulation going on here is whats happeneing. I get two sidebands in the FFT of the output when the output tubes are overdriven. The sidebands are exactly 120Hz above and below the primary tone.

It appears, audibly, as a non-harmonic lower note when I play, say, an "E" on the G string, 9th fret. But also, if I strum a chord really strong I here a nasty kind of distortion.

Man, the power supply ripple is somehow getting into things and modulating the output.

[JazzGuitarGimp]

This is just a stab-in-the-dark, what-if idea, but I wonder if the _type_ of output tube you're using just doesn't play nice in class AB2. I don't know what output tube you're using, but have you tried a different type, like KT66 instead of EL34, etc?

[Firestorm]

It's hard to make out in the photo you posted, but it looks like you've got the heaters on your output tubes cross-wired which will induce a 60Hz carrier frequency in each side of your output. This will modulate the signal and produce sum and difference frequencies, the strongest of which will likely be the first harmonic (120Hz) which may be why you're seeing that frequency on your scope. Check to see. Happened to Fender half the time, because they used all green heater wire, too.

[pula58]

Holy cr#p, you're right, the output tube socket heater wires are crossed! I was real careful to NOT do that, but, seems I did it just the same.

Can't wait to get down in the lab and get heater wires un-crossed and see what happens!

[Reeltarded]

(Crosses fingers) quietly watching this one get worked out. Excellent.

[pula58]

Indeed, the heater wiring to the 2nd output tube was crossed (Thank you Firestorm, good eye!). So, I re-wired it...and...darn, the problem is still there, I am dissapointed!

I did try, earlier this AM, doubling the capacitance of the first power supply filter cap. It is a 47uF 500V F+T. I wired-in an additional 47uF temporarily, to bring the total capacitance there to (nominally) 94uF. The two sum and difference tones got smaller by a factor of 2 or so. But, I can't believe this amp needs ~ 100uF there since all my other builds used ~50uF and were just fine (the other builds used two 100uF spragues in series, this amp uses just one 500V 47uF F+T). The unloaded B+ voltage in this amp is 475V.