Trouble Shooting Guidance/Advice?

[The Ballzz]

Hey All,
I built an amp with a preamp section that is a nearly identical clone of a JCM800/2203 with a dual 6AQ5/EL90, 10 watt (nominally) power amp section. Every thing works and sounds quite nice and it is certainly a hellaciously LOUD 10 watts!

There are only two issues that I am trying to track down:

A) The .022 coupling caps between V1A to V1B and from V1B to V2A are microphonic when tapped with my chopstick. This of course is when both the master and gain are turned up. I have swapped both caps for other brands (Mallory 150, etc) to no satisfaction. Is this simply a condition to be expected at higher gain and level settings? There is also a fair amount of hiss, but no humming or buzzing issues.

B) Audible feedback/squealing somewhere in the 1.3khz to 1.6khz range. I'm going to guess that it is also happening in higher frequencies above the audible spectrum. It only happens when the guitar and amp is cranked and is the same frequency with all different guitars and pickups.

I can change the feedback frequency up & down a little through tube rolling, but it is still there in that frequency range.

I can eliminate the unwanted feedback by either reducing the treble or mid pot, but I then lose the desired "controllable" harmonic feedback/sustain that I'm looking for.

It appears to simply be a very narrow frequency peak that I would like to tame a tiny bit without killing the frequencies surrounding it. How do I narrow down the culprit here?

Yet another reason/excuse for wanting an oscilloscope, as mentioned in another recent thread!

Any Help, Comments & Thoughts Are Welcome,
Gene

[John_P_WI]

The three places and ease of attack that I would suggest are:

1) Disconnect the nfb at PI side, this prevents the wire from acting like an antenna if disconnected at the speaker tap (see if this improves anything).

2) Add grid stops to the pre amp tubes.

3) Add shielded cable to all grids.

Of course, proper lead dress - especially grid runs etc is essential - watch parallel runs with other wires. You may have to pad a pot or something too, take a little gain out. Snubber caps maybe... but sounds like it is in the audible frequency,

One of these should quiet things down - hopefully.

[statorvane]

The microphonic coupling caps, I'm betting there is a loose solder joint in there somewhere. These aren't the paper-in-oil variety are they?

[JerryFJA]

Caps make noise if you tap them. Stop tapping and start playing the amp!
Probably need to add shielding on the front end or correct the wiring layout. You can find all this stuff without a scope.

[John_P_WI]

Caps make noise if you tap them.

I'll get flamed for saying this - maybe not so much anymore since there has been a change of members here - my feeling is that the thin chassis of the trainwrecks added to the tone and instability in part via the vibrations coupling with the caps, similarly to the springs on a strat.

Flame on, this concludes the off topic discussion of the day, back to Ballzz's regular programming.

[martin manning]

Yeah, it's that resonance thing ;^)

[The Ballzz]

Well Folks,
I've pretty much determined that the microphonic caps in the early stages are simply a typical thing with higher gain settings. I've had many suggestions concerning the squeal/feedback issue:

A) 7p/1000v cap across plate and grid of first gain stage triode.
B) 100p to 250p across plate and cathode of the first gain stage triode.
C) Energized shield by tying the shield of the cable from the input jack to the plate of the first triode!

Several other suggestions also! All of these procedures seem to be aimed at treating the symptom as opposed to finding and CURING the "CAUSE" of the problem! I personally would rather cure the problem!

Great tips from all here about tracking down my difficulty!

Thanx Again,
Gene