Mike express build

[Miket]

Hi guys, I put this amp aside for 6 months while I focused on other projects.
Now im back to settle the score with its issues.

I tried what Geetarpicker suggested, and turned the guitar volume off, and there's no oscillations. I can max all the pots and it's stable. (even with no negative feedback loop)

But I'm still trying to work out the swirl issue that I described in my original post. I have removed the VVR, removed the bright caps, removed the negative feedback completely, swapped all the tubes. And the swirl remains.
I can reduce it by putting lower gain v1- 12at7. Also, the volume knob regulates the buzzy swirl as I turn it (more gain more swirl/buzz) the lower frequency notes really buzz out too.

Any ideas on what it could be?
I think I might try bypassing the tone stack next, see if I can isolate the issue....

Regards. Mike

[KellyBass]

You might want to double check your resistor values (with a Meter) around your PI. An out of spec value here can cause some swooshy swirl. BTW, what's your bias set at?

[Miket]

Every component was checked with a meter before being installed. So they should be fine. I'll have a look in that area to.

Bias is 70% plate dissipation at idle for the new jj's I've got in it. I've tried various bias levels, on various tubes and none seem to effect the swirl.

Could this be result of some kind of faulty component?

[LeftyStrat]

Every component was checked with a meter before being installed. So they should be fine. I'll have a look in that area to.

Bias is 70% plate dissipation at idle for the new jj's I've got in it. I've tried various bias levels, on various tubes and none seem to effect the swirl.

Could this be result of some kind of faulty component?

Can you give us a recording of the swirl you're experiencing? Swirl can be a nebulous term. It's sometimes used to describe something good.

[Miket]

Yeah it's a hard sound to describe. I'll make a Recording tonight and put it up.

[Miket]

https://soundcloud.com/mtuck

theres two sound samples there. the sound should be fairly aparent. not the best recording, all i have to use is my phone.

[Noval_novice]

Miket, sounds like you have a synth machine there! I feel your frustration as I had similar (but less intense) sounds coming out of my Express build. One thing you may want to try is reducing the value of R7. I changed mine to 67.9k and it cleaned up the abnormal phasing sound but still left plenty of gain. YMMV.

[Miket]

Yeah I would be happy with that sound if it was coming out of a fuzz, pedal. Lol.
But not an amp I've spent a lot of money building!

Thanks for the tip noval

[Colossal]

Mike,

Do you have or have access to a 'scope? Before you change anything, if you are sure that all part values are correct and wiring is correct, I would find where that distortion is coming from on the scope. An Express built with the correct values and lead dress and voltages should have a smooth, rich grinding distortion. Have you replaced each tube one at a time with known working tubes? Dont go down the rabbit hole of soldering parts in and out without taking a full set of voltages and posting. An erroneous value will stand out.

[Miket]

Colossal, I've run lots of known good tubes through her. The only way that seemed to make any difference was by putting low gain tubes in v1, but hat was only slight difference. I'll take some voltages, post them, then start with the scope.

[Miket]

Voltages

[Miket]

ok here are the scope pics.
all controlls set to 12 oclock.

[Miket]

the last ones.
i think that little dip that appears at v3 pin 1 grid, is the cause of the swirl... the top of the waveform getting chopped at v2 is also an obvious problem...

[Tillydog]

If I'm reading your scope shots correctly, it looks like you're getting about 25V P-P at V1b plate. Since your voltages are OK, it looks to me like you're hitting the amp with way too much signal to learn much from the scope. It only takes ~3mV at the input to drive an Express to full (clean) output, which should give ~~200mV at V1b plate.

FWIW, you won't get the Express response / distortion without NFB, but you should at least be able to get a clean sound.

From your sound clip, I would guess that your amp is suffering from ultrasonic oscillation, which largely boils down to lead dress, lead dress and lead dress. If everything there is *perfect*, then you could try adding grid stoppers to V1a, V1b & V2 - you can always take them out again later.

My 0.02p, anyway!

[Miket]

Yes tilly. I was sending a 1V peak to peak sign wave into it. I did it again with a 0.3v sine wave, and it was a tad different.

The top of the sine wave is still getting clipped at v2 however... (gain at 12 o'clock) does that seem right?

FWIW I have 68k grid stopper on v1b, and 1.5k on v1a. I think I'll try a grid stopper on v2a.
Also, nfb is connected. And bright caps are out of the circuit.

I did just notice tho, at pin 2 phase inverter, signal is 10v peak to peak. At pin 7, it's 48v peak to peak. Is that normal? Also the corresponding power tube v4's screen resistor is running very hot. (while v5 is only warm, the tubes are a matched pair, and at idle v4 is running slightly colder than v5)

I have also been using my audio probe, listening around the phase inverter, the phaser swirl seems to fist appear at the nfb r22, and make its way to the pin 2 grid of the phase inverter. While the pin 7 grid sounds normal.