Source Of Squeal On 5C8 Clone?

[rp]

I got some glass 6SC7s to try, and waiting for my 6J5 to 6SC7 adapter parts to arrive. Still haven't had a chance to take it to a studio to try it flat out, but I've got a touch of squeal that I want to troubleshot before I do that. I initially thought it was tube quality related but it's not.

It's on V2 only, not V1, and only with the treble full up and on the final 10 degrees of the volume pot. (Or if you like, on the last 10 degrees of the treble pot with the Vol 2 all the way up.) It's not a very loud squeal and it's all high end - dog whistle range. V2 is the modded cathode biased channel, V1 I kept stock grid-leak as in the schematic. V2 BTW has a 1 meg grid load and one side straight in, the other with a 33K stopper.

First thing I did is lift up the yellow wire (<- image 1) going to the 6J5 grid from the 2 68K resistors from the Vol 1 and Vol 2 wipers, in case it was too close to other wires (the picture is before I lifted it). That didn't change things even a little and I pushed it back down. Since the wiring is short and minimal and mostly ptp I'm sorta stumped - not much left to chopstick. V2 does it with all tubes rolled metal and glass. I'm wondering whether the circuit is inherently unhappy with the mod to V2 - the cathode-bias change, but I really doubt this. Also thinking maybe the silver mica on the treble is bad but that should affect Channel 1 too.

V1 / channel 1 works perfectly so off-hand I don't think it's the tonestack. Amp sounds great otherwise and voltages look good.

No scope, I need to learn to use one.

You guys are way better than me, maybe one of the maestros will see it right away?

This just in: problem seems to be above not inside the chassis: the squeal goes away if i put a cage in the form of a stainless steel cocktail shaker over V2. And yes, V2 pin one is grounded to the chassis and it also has the squeal with metal cased tubes. Now get this, I can play Three Card Monty and cover any preamp tube and the squeal goes away.

Did I just pick the wrong layout? My tubes are further spread out than Leo's ever were - how the hell did he get away with this stuff??? Now I don't think it'll ever handle the added gain with the future planned 6J5 -> 6SC7 swap. Good God, I hope the cocktail shaker isn't the only solution I don't want to Kluge this amp.

[martin manning]

What happens if you put a piece of ferrous metal between the OT and the preamp tubes?

[Milkmansound]

try chopsticking that lead from the mica cap on the treble pot away from the ground bus

Fender circuits are not super high gain like Marshalls - but lead dress plays a big part in keeping oscillations down in the preamp section. I find that if the volume pot squeals or sounds scratchy, its usually got something to do with the lead to the treble pot being to close to something. That wire is hot!

[M Fowler]

I'm wondering if the voltage divider would have to be increased since you modded V2. Stock is 75k/75k going into 6J5.

[Structo]

That's too bad you are having trouble with the amp.

It is a beautiful build.

[martin manning]

rp, you are saying only channel 2 squeals. Does that mean that it doesn't squeal if nothing is plugged into either channel, and it doesn't squeal if you plug into channel 1 (leaving channel 2's input grounded)?

[rp]

I'll get on this tonight, starting with Milkman's suggestion.

One thought, are OT (and PTs too) supposed to have the endbells grounded to the chassis? I re-sprayed these and I think they are insulated, I'll check it later. And the OT (Stancor 3851) has the ultra-linear and oddball tertiary taps trimmed back and tucked into the bells. Could they be acting as antennas?

I'll play around with shielding when I get home.

[M Fowler]

I had the same problem with my EH150 two channel amp one side normal input the other OD input and it would start to squeal past 1 o'clock on OD volume due to the 1m/100k voltage divider. Once that voltage divider was sorted out no more squeal. Also as you already tried lead dress is very important because of the increased gain of V2.

Very nice build and it's a damn shame the amp isn't working to full potential without having to go in an take things apart.

Mark

[rp]

Once that voltage divider was sorted out no more squeal. Also as you already tried lead dress is very important because of the increased gain of V2.

The divider is pretty easy to access. Any suggestion on what values to try?

Very nice build and it's a damn shame the amp isn't working to full potential without having to go in an take things apart.

At least it's a tony Alessi cocktail shaker.

[Cliff Schecht]

I'll get on this tonight, starting with Milkman's suggestion.

One thought, are OT (and PTs too) supposed to have the endbells grounded to the chassis? I re-sprayed these and I think they are insulated, I'll check it later. And the OT (Stancor 3851) has the ultra-linear and oddball tertiary taps trimmed back and tucked into the bells. Could they be acting as antennas?

I'll play around with shielding when I get home.

I've got a 3851 in my 5F6A Bassman build and another one in my Tweed Twin build. I just tape off those goofy windings and tuck 'em away. Neither of those amps squeal or are really noisy in any way. No RF from taping off those leads, that shouldn't be your problem!

I'm betting that your squealing is from the two long yellow wire runs from the input to under the pot labeled Vol1 or the black wires running from the input to V1. I think chopsticking might not reveal the problem because your leads are pretty much set in place, an oscope probably will reveal where the squealing starts at least if you have access to one. I'd start by tap testing stuff but I bet that only reveals how microphonic 6SC7's usually are .

[M Fowler]

At least 220k/75k to start with.

220k for the modded circuit.

It could be many things causing this and Cliff's notes are good as well.

Cliff Notes I remember those in High School.

Mark

[rp]

I'm betting that your squealing is from the two long yellow wire runs from the input to under the pot labeled Vol1 or the black wires running from the input to V1. I think chopsticking might not reveal the problem because your leads are pretty much set in place, an oscope probably will reveal where the squealing starts at least if you have access to one.

Those two yellows could be changed to shielded after I've exhausted everything else. It's actually a short run, I've done worse and gotten away with it. All the blacks are grounds. The 4 at the inputs are home runs from the jacks to the bus bar. Also every socket has pin one to ground using a nearby terminal to the chassis.

I'd start by tap testing stuff but I bet that only reveals how microphonic 6SC7's usually are .

Exactly!

What happens if you put a piece of ferrous metal between the OT and the preamp tubes?

Hahaha see the picts, like my cheese-grater Faraday cage?! That and the 2qt All Clad do have an effect on the squeal and lessen it. Covering any pre tube with the cocktail shaker seems to control the squeal, but best of all is with it over V2 - kills it dead, everything works perfect with V2 covered.

rp, you are saying only channel 2 squeals. Does that mean that it doesn't squeal if nothing is plugged into either channel, and it doesn't squeal if you plug into channel 1 (leaving channel 2's input grounded)?

Squeal happened on channel two with any input jack open or closed. And no difference in the squeal either way. I discovered I can make it a bit worse by bringing up V1 while V2 and the treb are maxed.

At least 220k/75k to start with. 220k for the modded circuit. Mark

This sure would be a sweet fix. I will try it soon as it's easy. What's the logic behind it? Does the cathode-biased side have more gain than the grid-leak side? I noticed the 5D8 has 270K/270K in the same place. Is this where you're getting the 220K?

try chopsticking that lead from the mica cap on the treble pot away from the ground bus. Fender circuits are not super high gain like Marshalls - but lead dress plays a big part in keeping oscillations down in the preamp section. I find that if the volume pot squeals or sounds scratchy, its usually got something to do with the lead to the treble pot being to close to something. That wire is hot!

Moving it as much as I could, which isn't much, made no difference. I may have to run shielded here too. Normally I hate having to use shielded wire, amps seem to have a little more sparkle w/o it, but it's just a few inches total, should have no effect on the highs.

So for now:
1) try Mark's idea on the voltage divider.
2) shielded wire for the treble pot run
3) "" for the run to the 6J5
4) "" for the input jacks to V2

Questions - if the cause is internal why can I control it externally with the cocktail shaker over the V2?

Update as I run back and forth to the amp: I rolled so many combinations I forgot if I was coming or going. With metals in V2 no issues now. Just with the glass tubes - all the glass tubes. I was wondering why the metal case wasn't helping with shielding - it was. But they are so f'ing microphonic and limiting it only to metal tubes seems like a lame solution.

Worse case scenario: I could always make V2 grid-leak and leave it at that Just so lame to have 4 identical channels. The cathode-biased side is real nice sounding too.

[Structo]

I remember one of Gerald Weber's rants was how Fender put the tone caps on the board then ran wires to the controls.
He said that if you mount the tone caps to the pots and eliminate some of that wire that it greatly improved them.
Whether or not this is the case with your amp, not sure.

I would probably try the shield cable.

BTW, on my 100w Dumble clone, I can get some squeal if I crank the presence and treble and volume if I recall.
It would never normally be set like that so I don't worry about it.

[Firestorm]

I remember one of Gerald Weber's rants was how Fender put the tone caps on the board then ran wires to the controls.
He said that if you mount the tone caps to the pots and eliminate some of that wire that it greatly improved them.

That's one source of the "Fender fizz." I could always get rid of it on affected amps by replacing the tone leads with shielded quad wire, so I never tried moving the caps.

[martin manning]

Squeal happened on channel two with any input jack open or closed. And no difference in the squeal either way. I discovered I can make it a bit worse by bringing up V1 while V2 and the treb are maxed.

That's 'cause you're decreasing the mixer's attenuation of signal out of V2 by bringinging up the V1 channal volume control. That's about the same as you'll get miss-matching the mix resistors. I guess I would try increasing both 75k's to say 220k. The larger they are the more HF roll-off you will get from the 6J5's Miller capacitance, which might actually help.

Questions - if the cause is internal why can I control it externally with the cocktail shaker over the V2?

That's what I'm wondering. I was thinking that the feedback might be coming from the OT, but I see now that V2 is not very near to it.