Problems with extra gain stage in 2203

[HTH]

I've added an extra gain stage into the front end of a 2203 and it sounded great until I switched in the Mercury Magnetics OT, now it just oscillates wildly.

I realise this is a lead dress issue since the amp plays fine with the extra gain stage bypassed.

any thoughts on why the OT swap made a stable amp unstable?

also, any thoughts/tips on getting this extra stage working again without oscillation? - I'm already using shielded wire on all the signal runs and 120pF caps across all the plate resistors.

[Jag]

You probably need to reverse the secondaries on the OT. If the OT is installed with the secondaries switched, your negative feedback becomes positive and the amp oscillates like crazy.

That's my guess on your issue.

[HTH]

You probably need to reverse the secondaries on the OT. If the OT is installed with the secondaries switched, your negative feedback becomes positive and the amp oscillates like crazy.

That's my guess on your issue.

thanks for the suggestion, but I tried that and it's not the problem.

[mytco]

Try these:
1. Ensure your lead dress is clean.
2. Check for a microphonic tube.
3. Look for bad solder joints.
4. Try a bigger plate resistor bypass cap on one of the secondary gain stages.

[HTH]

Try these:
1. Ensure your lead dress is clean.
2. Check for a microphonic tube.
3. Look for bad solder joints.
4. Try a bigger plate resistor bypass cap on one of the secondary gain stages.

1. lead dress fine and I've used shielded wire too
2. ECC83s are all fine
3. I'll check the solder joints, but I'm not a beginner by any stretch AND the extra gain stage was functioning fine before the OT swap, so I doubt it's dry joints.
4. would you have bigger bypass caps on gain stages down the line (the 2nd or third) rather than the very first gain stage?

Lastly, thanks for the suggestions - all input welcome and appreciated.

[CarlZ]

I'd put my money on a bad solder joint. Just because somebody has a lot of experience doesn't mean you can't get a bad joint. What sometimes happens is that there's usually a little bit of surface oxidation on the wire strands, and what you wind up with is NOT a cold joint but rather an encapsulated joint for lack of a better way to put it. The solder just kinda lays on top of the wire and never actually bonds to it....and this makes it a real bitch to find when it happens because the joint LOOKS good.

Carl

[HTH]

I'd put my money on a bad solder joint. Just because somebody has a lot of experience doesn't mean you can't get a bad joint. What sometimes happens is that there's usually a little bit of surface oxidation on the wire strands, and what you wind up with is NOT a cold joint but rather an encapsulated joint for lack of a better way to put it. The solder just kinda lays on top of the wire and never actually bonds to it....and this makes it a real bitch to find when it happens because the joint LOOKS good.

Carl

I'll go back and suck the solder out since theres only a handful of joints on the extra stage, but I'm really doubtful it's a bad joint. Why would the extra stage 'mod' work fine with one OT and then squeel with another?

anyway, I'll get the iron heated up and see if I can get this sorted.

[CarlZ]

because the bad joint is probably on the transformer lead and not in the circuit wiring.

Carl

[dai h.]

are the leads coming out from the bottom (through the chassis) the right way (i.e. plate leads from hole near back, secondary leads from hole near front)? I recall reading something (perhaps on the Marshall site or maybe an amp review) that Marshall got it wrong on a few amps (HW series?) and had to correct it. My understanding is that since the plate leads radiate lots of powerful signal, this can cause instability from positive feedback (and maybe too much negative as well?).

[HTH]

because the bad joint is probably on the transformer lead and not in the circuit wiring.

Carl

ahh, cheers Carl - I get you now. I've checked that over, but the amp works fine with the standard 2203 setup, so if it were a bad joint on the OT it would show up with or without the extra gain stage, right?

just to clarify, I'm only getting instability with the extra gain stage switched into circuit - the standard 2203 preamp is fine.

[HTH]

are the leads coming out from the bottom (through the chassis) the right way (i.e. plate leads from hole near back, secondary leads from hole near front)? I recall reading something (perhaps on the Marshall site or maybe an amp review) that Marshall got it wrong on a few amps (HW series?) and had to correct it. My understanding is that since the plate leads radiate lots of powerful signal, this can cause instability from positive feedback (and maybe too much negative as well?).

I've put the transformer in the same way as the stock one and also the same way as my '76 Superbass. That is, the primary side (plate leads) closest to the pots and the secondary side (speaker taps) closest to the output tubes.

[CarlZ]

Have you thrown it on a scope? You may not hear anything in clean mode but with the hotter signal from the extra gain stage it may be making the problem pronounced enough to really get it going. You could easily have a small parasitic at a 100 kHz or so that's not enough to influence the clean channel but when switching over to high gain it's making the amp go nuts.

Carl

[HTH]

Have you thrown it on a scope? You may not hear anything in clean mode but with the hotter signal from the extra gain stage it may be making the problem pronounced enough to really get it going. You could easily have a small parasitic at a 100 kHz or so that's not enough to influence the clean channel but when switching over to high gain it's making the amp go nuts.

Carl

unfortunately, all I have is a software scope which I've never got round to learning how to use.

the normal 2203 mode is quite dirty already since I've lowered to 10k on V1b to 2k7, so theres no real clean channel to speak of.

the extra stage is just blasting the front end with more gain really (much like an OD pedal, except achieved internally with a single ECC83 stage).

I should really buy a scope and a sig. gen. for stuff like this - would make things MUCH easier to track down.

[Bruce / Mission Amps]

Sounds like just way too much gain at the front of the preamp sections to me.
I wouldn't be surprised to see there is an oscillation going too, and maybe the OT is ringing at some high frequency that now has much more gas behind it with the extra gain stage.

Just a passing thought:

Just for fun, try a high voltage capacitance like, .001uF to .0047uF with a 2w-5w 10K to 22K resistor in series with it... all that across the output tranny primary... lug 3 to lug 3 on the tube sockets. Hmmm... maybe lug 3 to ground on each side will be better.
If it works, the resistor will need to be a large one cause the amp can make a ton of power at the frequencies effecting and the cap will probably need to be at least 1000v... I'd use two 630v caps in series for a start... like, two .0022uF in series, two .00047uF, two .01uF caps... ETC., ... that will give you some hi-v protection.
120pF across the PI plate loads just might not be enough.
120pF across a 100K resistor has a -3dB point of around +13KHz... I bet the average guitar player can't even hear much of 15KHz anymore so your squeal is probably much much lower.
How about 360pF to 470pF across the plates but one at a time... that will limit the gain by a few dB starting from around 4500Hz (360pF) down to 3500Hz (470pF).
By comparison, the brown Deluxe uses a big 3000pF across it's 220K plate load resistor for a -3dB point of 2200Hz. And it sounds a little duller too.

Also, do you by chance find that the presence control changes the treble tone of the oscillations in anyway? That can be a sign of an ultrasonic oscillation.

[Steve Conner]

I reckon it's a lead dress issue like Dai H. mentioned. The OT plate leads do indeed have a very high signal voltage on them, and it only takes a few picofarads of stray capacitance from them back to the early stages to make the whole mess oscillate.

Maybe you dressed the new plate leads just a little closer to your new gain stage than the old plate leads were. Try moving the plate leads as far from the preamp section as you can. Prod them around with a chopstick while the oscillation is happening, and see if it changes in volume or tone.

To make really sure, try putting some aluminium foil over the plate leads, grounded to the chassis, so the preamp can't "see" them at all. The coupling is capacitive, not inductive, so foil is all you need to stop it.

Maybe you dressed them the same, but the new O.T. has more leakage inductance, which blocks the high frequencies from the speaker and makes them hang around more on the primary side. Try Bruce's snubber network.

P.S. Also make sure your tube shields are on, especially on the tube for that extra stage you added. (While we're brainstorming, if you've just added one stage, what did you do with the other triode in the bottle?)

[Wakculloch]

Just a thought, but have you decoupled your HT (Vb) supply to the stage. I've had problems in the past with cascaded stages coupling by positive feedback through the HT.

[HTH]

Just a thought, but have you decoupled your HT (Vb) supply to the stage. I've had problems in the past with cascaded stages coupling by positive feedback through the HT.

Not at present because it worked when I tried the extra stage at first, but I did consider that the extra stage was not sufficiently decoupled as I'm running three triodes from the same node.

I'll create a seperate node for the extra stage just to be sure - can't harm.

The unused side of the extra ECC83 is just simply not connected, i.e. the heater to that side isn't wired up.

[dai h.]

I've put the transformer in the same way as the stock one and also the same way as my '76 Superbass. That is, the primary side (plate leads) closest to the pots and the secondary side (speaker taps) closest to the output tubes.

hmm...well I looked at a couple of 100-watter chassis pics and I guess it's backwards to the wiring in the 50Ws(?).

the logic as far as the wiring is that I'm mentally picturing additional gain (from the gain increasing mod) equals more radiation (which can explain why moving wires can make it better or worse, i.e. moving one interacting with another to help induce problem oscillations can mean less capacitive coupling as well as putting flat against chassis = more capacitive coupling to ground plane = less capacitive coupling to another wire nearby), otherwise shielding (capacitive or electrostatic) shielding = blocking out/isolating. Worse as gain/volume control (plus tone controls) are turned up = more/less radiation of signal = more/less capacitive coupling.