Dear Forum,

I finished my PX18 set a few months ago. Since a few weeks I have had strange symptoms:
- sometimes unexpectdly low gains and volumes on the bright channel, either at start-up or coming suddenly after half an hour. This would disappear if I strum heavily my guitar (or hand-touch the input jack, see below).
- then came some microphonic sounds (very very high frequency hiss, that would stop by switching on/off the stand by)
- now, if I do any change on the input jacks (e.g. changing the jumping, or jack from normal to bright), the gain/volume will drop a lot (like 30/40%).
- then recently I discovered that if such drop occurs, I can restore the standard volume/gain of the bright channel by touching the input jack (guitar side) with my hands. THis will produce a microphonic sound (very high pitch) and when I replug the guitar the gain/volume on the bright channel are ok.
- the normal channel sounds muddy and low-gainy no matter what, but it has always been and I guess that is the 'crux' of Plexi - anyway I use normal channel only to blend the trebbles of the bright channel.
I have checked (again) the bias and voltages.
All voltages are Ok (like 10% deviation at the most on all valves, which I believe is acceptable according to Dirk's post on measuring).
Re: the bias, I must underline that the 1R resistor used for bias setting has an actual value of 1.3R, so when measuring 60 mV (value recommended), that would mean much less than 60 mA current. But if I try to have 60 mA, then increasing bias voltage will increase all B1/B2/B3 voltages at values well above standard deviations, so I don't dare doing it - or should I until I get really 60 mA current through the bias resistance?

Before I investigate more deeply each component/soldering, I was wondering if that could come from the input area or if the microphonic sounds would indicate a flawed valve? For that latter point, reading some forums I have tried touching with a pen the pre-amp and power valves when the microphony starts, but it does not stop anything - and therefore I wonder if the super high-pitched noise is valve microphony or a defect item that triggers high trebble.

Any hint would be more than welcome!

Thank you and kind regards
David

Hello David,

it sounds like there is more than one problem.

While reading your list, the first thing that came in my mind was: grounding. I'd guess that you have a bad ground connection somewhere.

- I'd unplug the guitar and check if the amp still makes sound. Just to make sure that it's really the amp.
- Check for any connection issues around the input jacks
- check anode resistors in the preamp and screen resistors in the poweramp since they can produce these sudden gain drops

Regarding your bias: Are you sure that the resistor value is 1.3 Ohm? I would assume that you get an extra 0.3 Ohm from your connection wires.

Cheers, René

Thank you very much René for your answer!
What I did:
- check that the horribl noise stays if guitar unplugged -> yes, tried tonight.
- I checked absolutely all resistors, both preaamp and power stage...all Ok.
- check all ground connections
- I found only a possible cold soldering for the bright cap in the treble volume but re-soldering did not improve...still got the high-pitched hiss.
Where I stand:
- Noise only in the Treble Channel, normal channel usable.
- I also found that the Presence and Treble know can lead the noise to come when turned beyond 3 o'clock when using the Treble channel (normal channel alone is ok even with full presence and treble).
- I also observed something beyond natural  : when unplugging and then approaching the guitar jack a few centimeters from the jack inputs, it would trigger the noise if presence and/or treble are fully engaged.
Suspect an electromagnetic interference somewhere (so yes, probably grounding issue?) but can't find further.
Any idea whereby the Presence and Treble influence could help narrow down the cultprit, or is it just that Plexis always make presence and treble noisy anyway by design?

Any suggestion welcome !

Kind regards
David

Hello,

is the noise still present when you pull the first tube out? Since the bass channel seems to work fine I'd look for the error around V1. Changing the tube woulod be the first thing to do.

Can you post a photo of your build? Maybe your lead dress isn't optimal.

Have you tried the amp in another room? Sometimes lights or other electrical devices can cause interference. A few years ago I also had problems with interferences while working on an amp. In the end it was caused by the soldering iron on the desk...  :facepalm:

René

Thank you René !

In bewteen, what I did:
- remove the shielding wiring from Treble Pot to main board that I had installed trying to reduce Plexi hiss. I suspected there could a cold lead or ground issue but it did not improve anything, still squealing noise
- re-re-set bias at 60 mV, and this improved a lot the accuracy of voltages..+/- 1 V versus the spec for most voltages, so this is quite cool indeed. But squeal persisted
- then I removed V1...amp totaly muted, no sound whatsoever. I don't know if this is normal, as I thought that removing any preamp valvle would produce a stop in the signal chain. Is it supposed to produced a sound with preamp valve removed?
- out of despair, I exchanged V1 and V2 from their place, and it improved a lot: squeal comes only at Bright volumes above 7 when pushing Presence or Treble to high values.
So I have a workable area, where I can use OD pedals to push gain.
Generally, I find the amp not gainy at all, nothing happens when pushing volume from 7 to 10...Maybe normal for vintage amps with no gain setting?
Anyway, comparing the gain with Helix plexi under same conditions (including cab sim) shows a big difference in gain. Maybe a sign of weak preamp valve (the ex-V1 now V2 one?).

Regarding possible interferences at home, that possible, as for example the amp is worse in the cellar as in the play room, but I could not nail a possible root cause, and other valve amps seem immune to that.
Finally, I don't understand what is 'main lead' so I attach a pic of the wiring - disclaimer: this is my 2nd amp build, so any suggestion to improve more than welcome.

Many many thanks again
David