DIY EL34 amp and "jerky" notes decay

[gitarzysta]

Hello

I have a 2xEL34 tube combo loosely based on 18W TMB but with 2xEL34 power section.
It seems to have awfully sounding notes decay - the "grain" volume seems to oscillate up and down - have a listen (zip with mp3 attached). Notice how chords decay - grain suddenly disappears to go up with volume and down again. Almost like tremolo.
Do you have any suggestions where I should look/what to tweak?

Regards
Gitarzysta

[dude]

You need to post the schematic


al

[shooter]

Almost like tremolo.

typically the "tremolo" effect has been some sort of parasitic osc.
could be from many places, a scope is real handy if that's your issue.

test caps can help, bump preamp bypass caps up, like double what you have, help?
add 470pF to .001uF across plate R's in the preamp, help?
***both WILL effect tone*** but we're troubleshooting now, playing comes later

DO only 1 cap at a time, then evaluate.

[dude]

Maybe check lead dress too, like the OT sec's to close to the PI.


al

[tubeswell]

Are you using a LTP inverter? If so, try biasing it with 820R or 1k.

[pompeiisneaks]

Also, if you have a negative feedback, which the original 18W TMB does, you may want to try disconnecting it, and if the issue goes away, swap the OT leads on the power tubes, this is a possible phase issue with NFB, sometimes it's loud and horrid, other times subtle.

~Phil

[gitarzysta]

Are you using a LTP inverter? If so, try biasing it with 820R or 1k.

Thanks, I will try and see if it helps.

Also worth mentioning is that the issue started recently.
Maybe indeed some caps went bad. Food for thought 

[tubeswell]

Also worth mentioning is that the issue started recently.
Maybe indeed some caps went bad. Food for thought 

In that case, it is possible that it could be tubes or caps.


You swapped the pre-amp tubes for known good tubes?


How old are the output tubes? What are the bias (and other) voltages looking like?

[hesamadman]

Unfortunately I wasn't able to listen to the audio clips. But just to mention. One time I had a tremolo effect in an amp. I posted I thread called "tremolo in a non tremolo amp".


It turned out to be that I had the Mid pot wired wrong. I had the wrong lug grounded. This was due to a layout error. I didn't find it until I threw away the layout and followed an actual schematic.

[gitarzysta]

Thank you all for your suggestions.
I gave up and had the amp serviced for me, the "tremolo" effect is gone, circuit is more like JCM800 (2xel34 though) now, power tubes were also changed, however I noticed that:

- as it is a cathode biased amp, it has a common 270ohm cathode resistor and NO BYPASS CAP - is this OK?
- I also noticed that both heater pins (2 and 7) are connected to ground with 150ohm 5W resistors - why is that? Any idea?

Thanks

Marek

[pompeiisneaks]

Yes some amps just don't use a cathode bypass cap on the power tubes (I'm redoing my Gibson EH-185 right now that has no bypass cap on the power tubes).

As for the heater pins, using anywhere from 50 to 200 ohm resistors to ground is common to create a fake center tap if the transformer didn't have a center tap for the heater lines.  Even better is to use a 200 ohm variable resistor (Potentiometer) so that it can be balanced.  This is often called a humdinger or hum balance pot.

~Phil

[shooter]

however I noticed that:

Phil covered the filaments,
the single Rk and no cap, works, it also works to double Rk and use 1 for ea. tube, use a bypass cap starting at 10uF to 47uF on ea. Rk to get a gain bump and probably tighten up bass.
but if you like it now, play it til you're bored, then tweak n see

[labb]

One of the 18 Watters I built was set up to run 6V6's. I had to change one of the resistors at the PI from the original that was set up for EL84's. Don't remember just what or which. I can look it up if you need the info.

[tubeswell]

- as it is a cathode biased amp, it has a common 270ohm cathode resistor and NO BYPASS CAP - is this OK?...

Fine. If you want you can add a 25-100uF bypass cap in parallel without any harm. (+ve pole pointing to the cathodes.). Make sure the cap voltage is rated to withstand the cathode voltage plus a safety margin (of say an extra 30%).

[PRR]

In a push-pull amp, common cathode resistor, each tube "bypasses" the other.

As a general statement, a cap is not needed or wanted.

In extreme cases, "it depends". In Hi-Fi you get different THD numbers. Sometimes better sometimes worse. And people hear different things. In overdriven guitar we do drive tube to cut-off, "bypass" doesn't happen on the distorted peaks, so there is a difference with/without a cap. My impression is that no-cap overdrives more smoothly and predictably. With-cap may give a more ratty sound good for some players. The difference is only when OVER-driven and still may not be much.

If you are addicted to mods, and skilled at doing them right, try the cap. I don't think it would be worth paying a tech to do it.