Troubleshooting Selmer T'n'B Mk III

[MWJB]

Find the resistors that feed pins 1 & 6 of V1, stick one probe of your ohmmeter on the juction of these resistor, where they join the B+/HT supply rail. Now, with the other probe find the filter cap section that reads "0" ohms between these points. When you have found this filter cap, remove its ground wire from where it is currently situated and run it to your V1 ground (or as near as possible on the buss wire).

Now repeat the process with v3, try the ground wire at V1 ground, or also at LTPI ground, whichever gives least hum.

Ground the remaining filter caps & bias supply to a PT bolt, along with any PT centre taps.

It would be helpful if we knew where the hot leads from the filter caps go, you seem to have a lot of filter sections there..

[Steve Conner]

Also grounded to this can is what I think is the CT of the power trans, and from there a wire runs over to ground both 2X32 uF filter cans. A second wire then runs from this shared ground (all 4 32uF filters) back across the chassis to the bus on the back of the pots.

Where is "there"? if it were the CT of the power tranny, then you have exactly the problem I mentioned in a previous post. To repeat, the connection between CT and filter cap negative must be totally isolated from the rest of the ground system. The only connection with the rest of the ground system should be at the filter cap negative terminal itself.

This is the biggest mistake you can possibly make in terms of inducing hum. And your sketch suggests you have something like that going on, with the chassis ground being at the HT fuse holder.

Chirps and whistles imply some other problem, though. Could be some sort of noise pickup from bad grounding, could be a bad connection or bad pot somewhere. The 1M volume pots on my SV were completely shot, first thing I had to replace. (apart from the broken heater wiring which was why I got it cheap in the first place )

After running my SV for a couple of years, the PI plate load resistors were beginning to go crackly, and when I fixed them, soon more mysterious crackles and pops came from somewhere else. That was my main motivation in tearing it down and rebuilding from the ground up.

Voltages: You said 485 at the main cap can, then 425 at the power tube plates? That's quite a drop. Was the amp in standby when you measured 485, and running when you measured 425?

I've got a real busy week at work coming up, may have to drop out of this thread. Thanks for popping in MWJB and Alex.

[EFK]

Steve, thanks for hanging in as long as you have. I truly appreciate the help. Hope you make at lot at work to make up for the money you surely must be losing taking time to help me out. Thanks! (BTW, yes "there" is CT; wire runs from CT to ground both 2X32uF cans, then a second wire runs from that dual can ground all the way back across the chassis and soldered to bus along back of pots, for some reason. This was an add-on by someone.)

It'll take me a few hours to digest the past few posts but thanks MWJB and Alex - really, thanks!

Here's the wiring of the cans:

[MWJB]

The 2x50uf cap handles the main B+ & the screen supply. These get grounded to a PT bolt with PT B+ centre tap.

The 32uf caps are the filters for V1-V4, ground these to the front panel buss wire, near input jacks.

[EFK]

MWJB - I'm going to try to implement some of the advice both you and Steve have offered and will report back with the results. BTW, do you see any problems with the manner in which the output jacks are grounded? They are isolated Cliff jacks, but as they are grounded right now, the bus bar for the jacks is attached to both the OT black (ground or common? I guess ground) as well as the cathode pin 1&8 ground for one of the EL34 tubes, which is grounded at the chassis. Would it be better to leave the power tube ground right next to the tube, where it is currently located, and ground the OT and output bus to a different chassis ground closer to the OT?

[MWJB]

Fine for the OT common to be grounded where it is, close to the speaker jacks.

[EFK]

Alrighty! Grounded as per many of the suggestions here and what I was able to understand about star grounding/bus grounding as per Aiken's site, with both volumes down and me not touching anything a lot of the loud hum is much reduced. Not gone completely, but much reduced. I think going with some shielded grid wires might help with what's left too.

NOW: if I turn up the volumes (new volume pots), with nothing plugged in, some of the hum comes back, and if I turn the volume up to, say, 10 o'clock, and touch (not turning it, just touching it!) the knob, there is still all manner of whistling and screeching noises. If I plug in a cable (not guitar, just cable), loud hum of course which goes away almost completely - *if I touch a volume knob!* What the heck is going on here?


BTW: what is the purpose of the 1M resistor following the two 470 mix resistors from each channel? IS it attenuating signal prior to final gain stage (V2)? Serving some other purpose?

[Alex R]

Any noise that goes away when you touch a ground is going to be hum induced from the atmosphere via your body - ground your body and it goes away.

The screeching on the volume knob still sounds like oscillation to me - internal feedback caused by induction into a signal line somewhere. Is it just touching the volume knob that makes the difference, or does it happen the same if you touch the chassis anywhere?


.

[MWJB]

Hook up your common probe of your ohmmeter to the main B+ filter cap ground, now probe the grounded points of components in the preamp (pot grounds, jack grounds, cathode grounds) do you read any significant resistance between these points & ground. Is there any rust/oxidisation between the chassis & the control panel grounding plate? If, so remove the pots & brass plate & remove rust.

Keep grid wires (12AX7 pins 2 & 7) away from plate wires (pins 1 & 6) or heater wires (pins 4&5, 9).

[EFK]

I have triple checked every ground in this amp. The amp is well-grounded - everything reads no resistance at all between any given ground and filter cap negs, chassis, brass plate etc. Everything is grounded as per all of the recommendations I have received here. Today I said goodbye to the last of the mustard caps - I have replaced every cap in this amp as well as the two 1M resistors in the LTPI. I have also now replaced ALL the pots. All long grid wires are shielded as is the wire b/t the two 470K mix resistors and into last gain stage.

Hum is barely tolerable with volumes off, but still there. Loud playing would drown it out. With volumes rolled up, there is a good deal of background hiss - I can live with that too. Hum gets a little louder but not much. But, it is actually vibrating the amp a little right down into the table it's sitting on. I can feel the vibration through the table. Worse, as previously, if I roll up the volume and then just grab the volume knob - not turning it, just touching it - there is horrendous crackling and static like a bad radio. Volume pots ONLY (which are brand new) - it does not do this when messing with the bass or treble pots, nor when touching the chassis. Only the volume knobs!

If if I touch the side of the .022 cap (off V2 feeding the LTPI) with a grounded meter, the amp goes dead silent but there is 7VDC at that point. That 7V is coming back off the PI grids, which both have 7VDC on them. The PI plate voltages are about where they should be (as per a slightly later TNB SV schematic) at 202 V, the cathode voltages are about where they should be at 22VDC.

If I bias via OT shunt, I get 37ma and the plate voltage is 438; but, when I measure this by pin 3 to ground, there is a terribly painful high pitched squeal which remains steady, as long as the meter is hooked up.

I feel like I have some kind of weird problem revolving around this PI DC voltage on the grids, and also some kind of weird oscillation revolving around all this squealing.

I am losing my sanity! I'm about ready to toss this amp into the trash, seriously! I have repeatedly heard from a number of people how great these TNB are supposed to sound, kind of a Vox meets Marshall hybrid. Well not this one!

[MWJB]

Is the vibrating a serious juddering/rattling...or just a faint "tingling"? If the former, it might suggest an excessive load on the PT. Pull all tubes, still there? Look at layout of the PT secondaries. Re-install tubes, does any particular tube make it worse.

What's your heater voltage?

If the latter...it may have always done this.

7vdc on the PI grids seems low. Have you cahanged the cap feeding PI pin 2?

Are you positive that the volume pots are tight in the chassis, have you changed the coupling caps that feed the tone stacks (from V1 & V2 pin 1)?

[MWJB]

What happens if you disconnect the NFB loop?

[EFK]

It's a pretty serious vibration timed with the loud humming. Heater voltage is 6.1vdc. The heater wires are twisted very tightly and run up high above everything. I've tried jiggling and moving them around to no avail.

PT terminals are numbered like so (looking at inside of chassis, pots facing forward away from me):

5 4 3 2 1
6 7 8 9 10

6 and 8 are filament taps, 2 is grounded to main filter can negative and has a jumper wire over to 7 which I believe is the ct for the filaments. Nothing else is grounded to this. The rectifier diode outer legs are attached to 5 and 3. 4 runs off the tail of the HT fuse holder. 1 and 9 are wired to the indicator light and 1 is also attached to the power cord. 10 is wired to the tail of the Mains fuse holder.

Every cap in this amp is new. Every single one!

If I pull either the PI or V2, the amp goes almost silent. I still hear a faint humming from the PT if I put my head down close to the chassis (not TOO close... ) but I've heard this in every amp I've ever owned. It sounds like the same hum, though - it's as if somehow the preamp is picking up this hum through a microphone. I believe the problem is somewhere in the preamp but by this point cannot figure out where it is. All caps are new, I've rolled tubes, jiggled wires, checked every resistor on the meter etc. BTW, if I pull the PI or V2, my squealing and crackling problem with the volume knobs vanishes. And believe me they are tight. The brass plate is basically redundant as a pot ground now, because someone ran a bus from the input jacks all along the back of each pot and I have that grounded to chassis with a lug right near the bass inputs (other side of the amp from the PT).

Lifting the NFB made no difference at all.

Thank you VERY MUCH for maintaining any interest at all in this apparently insane problem!

[MWJB]

"If I pull either the PI or V2, the amp goes almost silent." Weird, let's just make sure we ar on the same page, your PI is V4. You can leave this in and pull V2 (input tube & tone stack recovery for one channel) & amp functions normally?

Please post dc voltages for all tubes.

Please post a few pics of the chassis, tube socket & pot wiring.

[MWJB]

6.1VAC is on the low side for heaters.