Help with strange distortion

[guitarboy58]

I've just finished a Princeton (non reverb) clone build using a long tail pair for the PI and cathode bias.  I'm using Mercury Magnetics transformers and an EV 12 speaker.  It sounds great at volumes up to about 50%, but after that things start to sound strange.  At higher volumes, for lack of better description, it sounds like the speaker is mechanically distorting; a speaker "flub" sound, not that Fender farting sound.  I've been all through the circuit and don't see anything obvious.  There's no way two 6V6's are producing enough wattage to overload a 200W EVM 12L speaker.  The power supply is beefy (pretty much identical to the Trainwreck and other similar amp schematics posted through out the internet) so I don't think I'm straining anything there.  The thing is dead quiet, chimey and rich until it starts to seriously distort around a volume setting of 7 or 8 and you get that "flub" sound.  It really sounds like it is about to blow the speaker.  And yes, it does it on other speakers, all EV's, but I have tried different speakers  Any ideas?  Thanks....

[PRR]

Voltages?

[kagliostro]

This comes from an italian forum and I try to translate for you

(some people, like I, got an oscilloscope, but are not able to use it at his max performance, so I hope this can be of some interest for those, as I, that aren't old skilled wolfs)

Have you checked what kind of distortion you, view the oscilloscope?

you can not fix a distortion on an amp only "measured" with the ears.

Injects a pure sinusoidal input signal amplifier, put the dummy load (or even the speaker if noise is not a problem) look with the oscilloscope to the signal coming out from OT and see how the wave is deformed

If you find swelling waves, search for triggers and self oscillations

If the wave is clipped and/or square, are saturating the tubes, the tubes or OT are working badly

If you see that half-waves are in phase (but broken at the Zero line) push-pull is working bad - primarily responsible for the OT

Does that also make the measurement stage by stage, for each tube see what gains and what compounds

Maybe restricts the fault injection signal to the grid of a tube stage only to see its anode what comes out

Have a Nice Easter


Kagliostro

[guitarboy58]

Sorry for the delay in providing voltages.  Here's the schematic with the measured voltages shown at various points.  Immediately I'm concerned that the second stage of the preamp has an issue with 100k resistor not really being 100k...have not checked this yet.  All the other numbers seem to be in line with what I'd expect with an initial voltage of 450vdc. The volume pot was at the "off" position when the measurements were made.  All others were at 50%.
I think I have it drawn correctly and wired as drawn, but if anyone sees an obvious mistake, by all means, point it out...and feel free to make design suggestions...I'm here to learn.  Thanks.

[sluckey]

that the second stage of the preamp has an issue with 100k resistor not really being 100k

Also check the voltage on pins 7 and 8, and verify that the cathode resistor (pin 8) is really 2700 ohms.

[guitarboy58]

OK, I've verified that the 100k resistor on the second stage is 100k.  The 2.7k was 27k.  Oops.  Fixed that and the amp sounds worse. Chords are now very garbbled (if that makes sense) and the typical "hiss" volume has increased. The voltage on pin 6 is now 244Vdc.  Pin 7 is 0V (volume is off) and pin 8 is 2.2Vdc (just for completeness, the voltage on pin 3 is 1.6Vdc).  The voltage on pins 1 and 6 of the PI is 231 and 229 respectively.  So I've found a problem, but the issue of poor sound at high volume remains and fixing the incorrect resistor value has introduced a higher level of background hiss.  The search continues.  Thanks for the help.

[PRR]

> Pin 7 is 0V (volume is off)

The problem is at high volume? Short the input jack, turn Volume to 7 or 10, turn the Bass pot up and down, and check that grid (pin 7) voltage stays near zero. If not, one of the tone caps is leaking.

After that..... I dunno. Maybe the NFB is phased wrong, but I would expect that to sound odd at low volumes also.

[guitarboy58]

Thanks for the suggestions.  I have another interesting twist to the problem.  Last night I was pushing the amp hard (my ears are suing for spereate maintenance now).  Volume on 10, hitting cords on a Stratocaster and listening for any clues as to why it sounds so bad (and it's not my poor playing).  The amp shut down after a few minutes.  Literally, power light went off, sound stopped.  I'm thinking I've blown the fuse, so it hit the standby switch and the tubes start glowing again....after a minute I turned the standby back to on and the amp fired right up.  I couldn't duplicate the problem last night, but it happened again today. 
I'm going to check every solder joint, switch, and wiper for mechanical continuity tonight. 
This one is really stumping me, but a mechanical issue seems to make sense...higher volume is causing vibration and a breakdown...worth a look.
Any other ideas?  Thanks.

[OldHouseScott]

Your measured voltage drop between nodes 2 and 3 in your PS is 74vdc, but you're only drawing between 4-5 mA from the 4 12AX7 triode sections. I'm guessing the 4.7K resistor in your PS is really an 18K like the next one downstream. I'm not sure what effect that would have and if it would cause the problems you're describing, but it would certainly make for less headroom in the PI.

[dynaman1]

I've had bad coupling caps do that to me a couple of times. Great at low volumes but farted right out when the amp was pushed. I've no clue what was wrong, other than I guess they were leaky. Didn't notice any odd noises or hum. I simply changed caps and all was good.

[Leevi]

It could be a bad contact/component somewhere that causes the problem first on a higher volume.
Turn the volume to level when the distortion starts. Take soup stick and touch the circuit/and wires with it and
try to localize the root cause. Very often a bad pot can be the reason for that kind of phenom.
/Leevi