Troubleshooting Selmer T'n'B Mk III

[EFK]

Steve I feel like I'm running up against a wall. I did check the PT as soon as I got the amp and it is set to 120. Today I replaced the main 2X50uf can; now it has three brand-new cap cans, and just for the hell of it I replaced the 100uF/100V bias cap. Also rechecked all the ground straps and resoldered a few just to satisfy myself. Can you guess? (Drum roll please.....) - no difference. If I ground out the PI grids or if I ground out the grid side of the .022 cap feeding the PI, sweet silence. Otherwise, I have a beautiful vintage boat motor. I also replaced both volume pots but I've noticed that if I fiddle with them while the amp is on and singing its lovely humming song, there is crackly static here and there. Both channels - so there is DC getting through to the pots? I replaced those tiny 220pf and 500 pf caps, but I guess I'm going to have to say goodbye to those lovely mustard coupling caps too. I can't think of what else to do.

If there was a grounding problem in one of the LTPI resistors, could it cause this? And if so, which resistor would be the likely culprit? One of the 1M?

BTW - anyone know how this power transformer is supposed to 'read?' It's a lay-down, and the terminals are numbered 1 to 10. What would happen if the rectifier diodes were installed incorrectly? As I mentioned somewhere previously I think, someone replaced the BY128 called for on the schematic w/ two hexfreds. The two negative sides of the diodes are joined and soldered to the red wire which runs to one of the + terminals on the 50uF can, and then the two positive ends are respectively soldered to PT terminals #5 and #3. Aside from the hum everything seems to work properly so I assume they are installed correctly, but just thought I'd doublecheck in the event that someone might be familar w/ this transformer.

This amp has the biggest replacement 3-prong power cord I have ever seen in my life. 14 ga wire was used. Trans-atlantic cable anyone? Whoever did it got the colors wrong but it is just a piece of cable with a replacement screw-together plug as well, and I checked the wire routing and things work out to where they are supposed to be so the colors don't really matter. A standby switch was installed also and it all works properly.

[melvin]

I think you should go back to the preamp section. I still haven't read whether or not you have tried pulling only V1 or V3 to see if the hum goes away. And it would help if we knew whether it was hum (60 or 120Hz) or if it was "motor-boating" which usually gets worse as you turn up the amp and is some kind of parasitic oscillation.

If it's a parasitic, and it involves something before the PI, you could try shortening the pre-amp grid wires, but more importantly, grounding the last filter caps before each preamp section near the cathodes of their respective gain stages. You might also try replacing the 27K resistors they are attached to, assuming you haven't already done so...

But tell me how it goes when you pull V1 and V3.

[Steve Conner]

Also, can you measure your main B+ (at that red wire going from the diodes to the cap can) and heater voltage, and get back to us with the results? Should be about 420V and 6.3V. We still haven't determined why the PI grid voltages are half of what they should be.

I'm beginning to suspect a grounding SNAFU. When this replacement power cord was installed, where was the ground wire connected?

And when the 2x50uF cap can was replaced, what about the negative wire of it? You should find that a wire runs from the negative directly to the PT center tap, maybe via the HT fuse holder. And another wire should run from the cap negative to the rest of the amp's ground system. The wire from the PT center tap should not share any other grounding function. This keeps charging current out of the ground system.

If the rectifiers were wired wrongly, it would most likely blow fuses or destroy the PT.

[EFK]

OK guys this is a lot so forgive me for being terse: (but not with the THANKS for hanging in there with me!!!! Thank you!)

Off the main 2X50uF can negative terminal:

(1) Black to HT fuse and jumper from black to a chassis ground. Purple off tail of HT fuse holder to terminal 4 on Power Trans.

(2) Another black wire to terminal 2 on PT, which jumps over to terminal 7 (which is between 6 and 8 which are filament taps), then jumper from terminal 7 to a joined ground with both 32uF can negative terminals.

(3) Ground strap to brass plate (across front behind pots) which has all the preamp ground straps ties to it (the brass plate). Positive end of bias cap is tied to this ground strap.

(4) Bias resistor off the adjustable bias pot is also tied to negative term. on main 50uF can.

I have 485 B+ directly off red wire from 50uF can to PT diodes. 6.1 filament voltage on all socket pins.

Ground wire on 3-wire cord is attached to chassis w/ it's own ground bolt and soldered. Tested good for continuity.

Negative feedback wire runs from 7.5 ohm jack to tag on board, then 100K resistor goes on from there. Should there be a ground continuity between the chassis and the junction of this wire and the resistor? There is.

Filament taps and chassis have ground continuity.

Merlin will let you know about the tube pulling shortly - I already did this but unfortunately have forgotten what happened. I have swapped in about 10 diff. tubes and no difference.

[MWJB]

If wired stock ALL your filter caps garound to the samp point. Separate the PI & preamp filter caps and run ground wires to the buss wire at the input jacks. Only the main & screen caps should be grounded to the chassis where the caps are physically situated.

If you are familiar with Fenders the filter caps may be grounded like them - e.g. PI cap is grounded to buss wire about halfway down the chassis, preamp cap goes to bus @ input jacks...but those Selmer buss wires are quite busy, so both caps could go to the jack end, try both ways...

Alternatively, keep both volumes turned up at all times.

This hum is a normal "feature" of Selmers that I have seen.

[EFK]

Also here's another funky thing - if I try to measure voltage at power tubes pin 3, there is a horrific squeal even with volumes all the way down. This is whether I measure it pin 3 to ground or pins 3 to 8. Voltage is low too - should be about 430 but it's not, more like 400. Voltage on pin 4 is 425. This is really terrible - a little hum I can easily live with but what spectacularlybad noises this old amp is making!

MWJB - I'm trying to digest your grounding scheme! What is the "PI cap?" The two .047 which feed the output tubes or the 100uF/100V which is in the bias circuit? Or the two .1 off the LTPI?

[Alex R]

Alex R: If you're reading this, the channel I kept was the "Bass" one with the full size coupling cap. Is that a bad thing, would the "normal" one be considered better sounding?

Steve: Dunno - and if I'd heard which was considered best, I wouldn't have listened . As far as I recall there wasn't a whole lot of difference anyhow, am I right?

The amps the guys I mentioned use are the SV types, like yours, with the full aluminium front. They used to be way cheaper than the earlier ones. Beautifully built, though there is that irritating Brit habit of hiding half of the wiring under a board you can't lift without desoldering everything...

EFK: As to this 'ere motorboat problem, I take it you've tried moving signal wires about? Like Merlin said, if it isd motorboating of the classic type it will likely get worse with the volume control up, but not in the same way as a signal would - it will suddenly kick in as you turn up.

[EFK]

Well I spent a while whacking wires around and prodding things with a wooden stick. Sure, there's some microphonic stuff in there, but nothing made any difference through the speaker.

I replaced the output jacks and temorarily lifted the NFB - no difference. If I pull V1 or V3, noise is still there - just quieter. So, I replaced ALL of the preamp caps in the first two channels. No difference.

I can't tell if I should be calling it motorboating or humming but it is loud and it is annoying. I'm going to look for a youtube or sound clip of an amp motorboating as I've never heard it before so I don't know exactly what I'm hearing. I don't think it's 60 cycle - sounds higher pitched. And, my brand new volume pots on both channels are still a bit crackly and every now and then when turning them I get a high-pitched squeak or chirp. Kind of like whistling radio static. Now if this is DC, how am I getting DC on the pots if I replaced all the preamp caps? Is there something feeding back through the ground system somehow? I think I almost have to assume this is in fact traceable to a serious ground issue as I can't see what else it can be at this point. Maybe some of you can but my knowledge is very limited.

I'm losing it!

[EFK]

It sounds just like the annoying hum in this video, but very, very loud and with crackling noises or tapping noises when I tap the volume knobs (even when they are all the way off). Intermittent whistling/static if I turn them (brand new volume pots). Tube swaps make no difference at all. Jiggling and moving wires around do not change the hum. I don't know if this is motorboating or hum but it's what I've got going on. Dead silent on standby.

[ame="http://www.youtube.com/watch?v=NmFPTSXWOg0&feature=related"]YouTube- Vacuum Tube Hum and Buzz in a Guitar Amp[/ame]

[EFK]

Here's another one, complete with some of the whistling I'm getting at 3:00 when I turn up the volume. So it looks like it's 120 hz hum.

Have I mentioned how loud it is?

[ame="http://www.youtube.com/watch?v=SUBk_ZQPLDI"]YouTube- The Hunt for Buzz & Hum - Part 1 of 10[/ame]

[Alex R]

Is there DC on the pots, or not?

You might well have lost a ground under that circuit board or somewhere, some of the wiring goes up into the eyelets from underneath. Check that you have continuity to the chassis from all the points that should be grounded.

[MWJB]

Why do you say your voltage should be 430v? What plate current do you have per tube? Ground your meter to the filter cap ground for the first filter cap after the rectifier. Check that your power tube cathodes are properly grounded with an ohmeter. Does the amp make unwanted noises under idle/normal use?

The PI filter cap is the big axial electrolytic cap that connects to the junction of the phase inverter (12A#7 tube next to the power tubes) plate resistors, the preamp filter cap is the one that connects to the junctions of the plate resistors for V1 & V2.

[EFK]

There are no grounds under the board, nor any wires up through eyelets. In a lot of ways it's even simpler than a plexi. Everything on top of the board, wires from tubes directly up to board. Same with lines from filter caps etc.

I don't get any DC readings to ground on the pots but what then is causing new pots to whistle, chirp and crackle? Or make noise if touched?

I'm sorry I am having a hard time with this - I trying to understand what is going on.

MWJB - got ya, The PI filter cap is in a shared can with V3 so it shares a common ground on the can.

I'm going to play with grounds today. Currently, the way everything is grounded, all of the grounds have continuity where they should and all with the chassis. I've double and triple checked them. But I wonder if some grounds are shared that shouldn't be. My real issue is finding a good, clear, detailed pictures of the inside of a MKIII. I'd just copy the stock ground layout. This has been worked on previously w/ some additions and some of the grounding does not quite look like the blurry little photos I've seen thus far.

The common ground for the outputs (7.5 and 15 ohm) runs to a tag on a chassis bolt and thence on to pins 1&8 on one of the power tubes.

The brass plate behind the volume pots is there to serve as ground for the pots and preamp. The dual input ground bus for each channel runs to the back of the volume pot for that channel. There is an added bus that runs along the back of all six pots although I would assume they'd already be grounded as they run through the brass plate and the chassis. There is a ground strap from this pot bus to the brass plate. All of the preamp grounds are soldered to this brass plate as they run down the line. At the end of the line (PI end) the brass plate is grounded to the negative on the main 2X50uF can. Also grounded to this can is the HT fuse, which then runs to separate chassis grnd. Also grounded to this can is the bias cap as well as the bias resistor. 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.

That is a lot of shared grounding, don't know if some of this is causing the issue. I will try to draw a simplistic sketch of where all the ground wires are running - maybe someone can suggest some changes.

[MWJB]

"My real issue is finding a good, clear, detailed pictures of the inside of a MKIII. I'd just copy the stock ground layout." No it's not. These amps were not well grounded, you would just end up with an anp that hummed as much as every other Selmer.

"That is a lot of shared grounding, don't know if some of this is causing the issue." A good rule of thumb is to have the phase inverter & preamp sections grounded to the control panel buss/input jack and have the main filter (after the rectifier) and the screen supply filter (after the choke) grounded at the other end of the amp, say to a PT bolt.

You might not read significant dc on the noisy pots, you really need to lift the pot side ends of the tone & coupling caps & insert you meter between the cap & ground, replace any caps that leak more tan 0.25vdc...given they wway Selmers are usually wired, I'd just go straight in & change any caps that I was suspicious of. Check any bolted ground points are tight on the chassis.

[EFK]

Here's a roadmap of the grounding. There have GOT to be some hum-inducing loops here. Sorry for the scribbling but this is exactly what is in the amp right now - suggestions VERY MUCH APPRECIATED!