plexi clone issues

[phsyconoodler]

I just built a plexi clone with 6v6 power tubes and it seems to have an oscillation when jumpering channels.
  If you plug the guitar into the bright channels and ramp up both volumes it goes into cutoff when the bright volume is above 7.Doesn't do it when you plug the guitar into the normal channel and crank the volumes.
  I have a cap across the plates of the PI and that doesn't stop it.
The differences between this amp and a regular plexi are these:
-it has a rectifier tube and the voltages are about 360-380v on the plates.
-im using 6v6 power tubes
-I'm using a smaller ot suited to 6V6's an Allen amps unit.
-it's cathode biased

 Is it just a case of too much gain at those settings or can I expect something else? I don't typically play at those extreme settings but the owner does and he insists it's happening no matter what input he plugs into.I can't get it to happen when I plug my hot-as-hell epiphone humbucker into the normal channel and crank both volumes.
Any ideas would be welcome.

[silverfox]

I used to provide computer support for corporate clients. From time to time a call would come in and the client would insist nothing had been changed. Only to find out they had installed some home software or changed a setting the system.

I would ask your customer to bring his setup in and have them send the amp into oscillations for you.

Fox.

[John]

  If you plug the guitar into the bright channels and ramp up both volumes it goes into cutoff when the bright volume is above 7.Doesn't do it when you plug the guitar into the normal channel and crank the volumes.

Maybe it's simply dumping too much signal into the next stage with them jumpered? Especially since you can't make it happen with only one channel being fed.... regardless of what the customer says. And I agree with SF, tell him to bring in and make it happen so you can "better troubleshoot the problem". ;)

[plexi50]

I have run accross this a few times in some of my plexi builds. Nothing i did solved the problem. Then i started to tinker and spead the OT wires out and away from the tubes and chassis area they were lying on. Even at the entrance of the OT wires comming in from the top of the chassis. The OT wires were twisted tight but i wound up untwisting them giving me more slack. This worked on the last amp i built. Still dont know why. Also the input grid wires going to the 270k VD would tended to couple both channels if too close to one another. I know you know all of this stuff. It's a PITA when this happens. Microphonic power tubes are least thought of too

[TIMBO]

Hi Phsyco, The build i'm doing was at one stage M Huss's 6V6 Plexi but it suffered just about everything that could happen to an amp. The brite channel was way too brite and the normal was very dead sounding.

The main problem was it had a loud hiss and would go into oscillation at high volume. I found that a 820p cap across each plate resistor of V1 helped with the HISS and also the oscillation. 

[tubenit]

I'd try a variety of 12AX7 tubes in V1 and even some 5751 tubes to see if that resolved the issue?  I like the 5751 in the Marshall topology.

You could also try a 12AT7 in the LTPI  and then move the 12AT7 towards V1 & see where it stops the oscillation. That might pinpoint which tube is the culprit or which gain stage is the culprit.

You can try a smoothing cap (220p-390p) on the bright channel from plate to cathode.

With respect, Tubenit

[Leevi]

it seems to have an oscillation when jumpering channels

I have had similar problem with plexi.
If I remember correctly I got rid of the oscillation when I used a longer  (3m) jumper cable.
But the root cause for the oscillation is inside the amp. Check the cabling there.

/Leevi

[phsyconoodler]

The oscillation did stop with a longer jumper cable so I assumed adding capacitance from the input to ground would do the job and it helped but didn't kill it.
   I moved the grid resistors right on the tube sockets and used shielded cable,but not much difference there.
I moved wires around near the tone stack and that helped too.I got rid of 90% of the high pitched noise(which my old ears could not hear anyway) and adding the longer cable killed it 100%.
  It picks up some RF signal on the tone stack cause if you put your hand into the chassis it stops too.
Never had this issue with 2204 builds I've done.They are super stable.

[plexi50]

Im wondering  
Has anyone ever documented a resistor that could cause this problem? Grid,screen,plate,vd r. That would be a wild hunt.
Maybe dougs listening device and a scope could locate the oscillation point.

It seems like some preamp grid inputs are overly sensative and semi microphonic even when the preamp tubes themselves are not microphonic

[Leevi]

I have normally succeeded to solve the oscillation problems by
changing the wiring or replacing wires by shielded cables.
Critical areas are often around PI. I always solder the grid stoppers to the power tube pins.
Extra metal sheets inside the chassis can help as well.
/Leevi

[jojokeo]

Plexi's advice is good and works (not that others' isn't) but with high gain circuitry lead dress is super important which also includes the OT wiring and grounding. Timbo, the Huss Plexi is also a beeyotch if not careful. If it were me and experiencing cutoff issues just drop in a temp voltage divider or split load divider to lower the voltage gain just a tad and you're golden. Very likely w/out a bunch of tone-sucking caps where they don't belong. These are the fun challenges that test you in a number of ways but help you learn, grow, and truely appreciate the skill involved to triumph on another build. You got this Psycho

[macula56]

i was thinking of building one of those Huss Plexi's. i did not know there were any problems with them. what should i look out for?

[jojokeo]

It's a solid build but be careful of the usual problematic culprits - iron/parts placement, lead dress, and grounding.

[macula56]

thanks jojokeo. you mentioned in your previous post that it was a beeyotch so i was wondering if there were any real problems other than the usual suspects.

[plexi50]

i was thinking of building one of those Huss Plexi's. i did not know there were any problems with them. what should i look out for?

There are no problems in most plexi builds. Once in a while an oscillation problem will happen. But not something you need to worry about while building one.