Troubleshooting Selmer T'n'B Mk III

[EFK]

I've got this old Selmer head that I've been trying to kick start. When I got it, all of the main filter cans were replaced and an adjustable bias pot was added. One power tube socket is a replacement along with filament wiring to power tubes only. Standby switch (stdby-off-on) added. New rectifier diodes (16A 1200 V hexfreds). All of the old mustard caps (save one) are still in place and much of the wiring etc. is original. I had some problems biasing, initially, but have managed to get it biased. With NO input cable and both volume pots all the way down, there is still a fairly annoying hum which comes through the speakers. When I bias via OT shunt, with black test lead on center tap and hot lead on the respective plate, reading direct mA, the hum completely vanishes. Amp works - bass channel (2 inputs) seem fine, but when I plug into treble channel inputs the hum gets louder and sound is kind of weak. Also, when plugged into treble inputs, if I work the volume knob on any guitar it is very scratchy (oddly enough the amp knobs do not seem scratchy). Some of the hum may be AC filaments, but what's up with the treble input? Anyone have any ideas where I should start? Input jacks/resistors are all original but look ok, pots are almost all original but also seem to work fine. I enjoy working on these (carefully!) but am not particularly knowledgeable when it comes to troubleshooting so I'm a bit at a loss as to where to start. Anyone have any ideas? Thanks!

[melvin]

Do you have a schematic?

First of all, there should be no sound coming out of the amp when using the shunt method to bias the amp. So that's normal.

As far as the hum... Are the new heater wires connected each to the same heater pins on the output tubes? (pin2 to pin2) Is the bias supply cap new as well? Did the power supply resistors get changed with the cans? Are any preamp tubes noisy?

Most likely though, it sounds like the input jack and connections in the treble channel needs to be resoldered. And if that is so, there may be other solder joints that are adding to your woes....(ground connections?)

[Enzo]

My first thought is the amp is unstable and oscillating at high freq, above audio. The result is usually hum along with a weak signal. A scope to the output would detect that.

You may have replaced all the B+ filters, but did you replace the bias supply filters? If the old B+ caps were in a can, did you ground hte new caps to the same place? As opposed to grounding them here and there?

[EFK]

Thank you both very much for the initial thoughts! I will check the filament wiring and ensure whoever rewired it kept 2 on 2 and 7 on 7. Should be simple as they're two different colored wires. The filter caps and bias cap all look pretty new. I didn't replace them so I don't know how long ago they were replaced but they sure look new. The power resistors in this amp are all 27K and they look like they are the original resistors with some new solder on them. I will take a harder look at the inputs and the treble channel soldering. All the input jacks are original and I have to admit that when plugging in they feel kind of loose - however, it didn't seem to make any difference if I wiggled the cable input around. The scratchy guitar volume pot was interesting, though - is that DC leaking back into the guitar wiring?

Enzo I didn't do the cap job so I don't know if the grounding scheme is different than when new. I'll attach a schematic I found online which does seem to match up as best as I can determine with what is in the amp. I think what I would like to do first and foremost is to check all the grounds I can find - some look recently resoldered but some are definitely original soldered grounds. Anything that involves a scope is going to not involve me because I don't have one! If I can't figure out the problem myself with a meter and soldering iron, I'm going to have to send it out to someone or drive an hour + to hit civilization.

Is there a safe and/or effective way to check all the coupling caps for leakage while they are in-circuit? I'd like to do that too but I don't like to just helter skelter start poking around with a meter in there.

[Steve Conner]

Have you tried swapping the preamp tubes around? A scratchy guitar volume often means a bad first preamp tube. Try swapping the tubes between the normal and bass channels, and see if the fault follows the tube.

The OT shunt biasing method shorts out the OT, so it's hardly surprising that the hum disappears. Are the tubes matched? (ie, similar bias reading for both sides?) If they're not, the hum will be worse. These amps have quite a well filtered power supply, so they should only hum a little.

I had a T'n'B 50 SV, they're great amps. I ended up rebuilding it completely and turning it into a combo.

Where are you? These were British made amps, and it would be interesting to see where they ended up.

[EFK]

Steve it was tubeless when I acquired it so I added all brand spanking new tubes. The (2) EL34s are biased within about 1 mA of each other so they seem pretty well-matched. I'm in northcentral PA in the middle nowhere, meaning it has definitely wandered quite a distance from home!

Is it normal for the preamp voltages to be quite low on these? The plates on V1 were pulling only @ 114/116 V, V2 133V, V3 132/119V and V4 209/199V. Maybe I don't know what I'm talking about but those seem low for a 50W amp that supposedly does not dirty-up very fast.

[EFK]

Well so far I've found a bad 1M resistor on the treble inputs. Also, the power resistors which are supposed to be 27K have drifted (apparently) upwards to about 32-33K by now.

Take a look at the attachment below: what is the purpose of the 1M resistor in-line with the signal *after* the two 470K resistors?

[Gtr_tech]

Check for DC on any grid in the weak/noisy channel. If you have a leaky coupling cap it'll cause exactly what you describe.

[melvin]

What is in the circuit at that spot?

[EFK]

Despite some mods to the bias (added pot) this circuit is essentially intact; it matches the schematic. Where I indicated the 1M resistor above, there is a 1M resistor. Of the few other amps I've worked on, Gibson and Marshalls, there is nothing following the mix resistors so I'm wondering why this 1M resistor was put in the signal path b/t the two channels and the next gain stage (via the echo loop). ???

[EFK]

I have the amp biased and the inputs sorted-out, everything meters out fine there, and all of the grounds have good continuity. But, there is still a godawful loud hum as soon as I take it from standby to power. I would REALLY like to check caps for leakage but I'm not sure how - do I need to desolder all of these? Can I check for DC somehow on the grid side of any cap without sparks flying? Frankly I'd just replace them - there's really not that many - but part of me hates to mess with the old hallowed "mustard" caps and I'd like to keep them there if possible.

A stupid question: if the circuit specifies a 1M pot, and I take a resistance reading across the outer legs while the pot is in-circuit tied to tone pots, should it still read about 1M? The pots all seem to work but I get some REALLY funky readings off them.

Any HELP!!!!!! very much appreciated!

[melvin]

The tone pots should give you strange readings in circuit. Is the hum just momentary or is the amp always humming? If not the latter, it could be the standby switch or any number of things.

From the schematic, the 1M resistor should be safe to shunt. It may have been necessary for the echo in/out but I don't see why. I would think the amp will sound a lot better without it.

[Wakculloch]

As the bias circuit is the area where mods have been done, i would be tempted to check it over and make sure the smoothing cap is OK and there is no ac there.

[Steve Conner]

To answer the earlier question: The 1M forms a potential divider with the 47k, to lower the signal level to something more suitable for an outboard echo unit. Or you could say, to lower it to a level that won't overdrive the loop recovery stage.

The designer could have achieved the same by changing the two 470k's upstream of it to 2.5M, or changing the 47k to 10k. I'm not sure why he added this extra resistor, unless he didn't want to buy 2.5M just for that one position. IIRC, my T'n'B had the same circuit and I thought it was weird too. One of the first mods I did to mine was to change it to a parallel FX loop, with higher level to drive a digital reverb, and a cathode follower (there was a spare triode in there) driving the send.

How does the amp perform if you go straight into the "echo in" jack, disabling the preamp? Is the hum and weak signal gone then?

Does it still hum if you pull the PI tube? If so, we're probably looking at badly mismatched power tubes, bad filter caps, shorted choke, and/or a problem with the bias circuit.

[EFK]

I like a fairly straightforward circuit so I cut out the echo loop and did away w/ the 1M resistor. The less in the signal path, the better. Chased the hum around today, actually I'm not sure if I should describe it as a hum or maybe motorboating. Pulling all the preamp tubes, the amp is dead silent. I mean, REALLY quiet. Putting them back in one at a time, the very loud noise kicks in when I plug in V2. If I pull V2 but leave all the other pre tubes in place, the amp is dead silent. If V2 is left in place, as soon as I kick it from standby to full power, the noise starts low and quickly - about 3 to 4 seconds - gets VERY loud. Like really loud TV loud, through the speaker. It actually vibrates the amp chassis and speaker. V2 uses only half the triode, and the solder joints all look good. I cleaned the socket and tried different tubes but it makes no difference. It kind of sounds like a hummy motorboat. Not a slow 'put put put' though, a pretty fast one. This is with nothing plugged into input. The cathode cap on V2 is new and oddly enough the only coupling cap in the entire amp which has been replaced is this one - the .022 uF (see above schematic) off the V2 plate is some pink russian looking thing - K40Y or something like that. Looks very new. So where do I go from here? The preamp voltages seem slightly low to me, but I don;t know what they are supposed to be so maybe they are normal. Voltage on V2 pin 1 is 133V and it stays stable. So where do I go from here? I've never had this problem before!