Troubleshooting Selmer T'n'B Mk III

[Alex R]

One of the things I'd try (in my reckless way) is grounding the grids of V2 - not on the pins though as there's a big danger of grounding the plate voltage and flash bang! Go back up the grid wires and ground the other end. Use a coupling cap as a probe and a croc lead to ground to be safe from accidentally grounding HT. This sounds like an oscillation problem and if you can stop it by grounding the first input grid then we have isolated the problem to one small area.

I am wondering whether you have lost a ground reference somewhere in the input area, allowing a grid voltage to drift upwards. Those 1 meg resistors on the inputs? I seem to remember some of these amps don't ground out the input contacts when the jacks are out - so if you lost the ground ref resistor too that might cause such a problem.

Agree with Steve that these are great amps. I have a couple of pro customers who use them in preference to anything they can get via endorsements etc, on sound quality and reliability.

[EFK]

I grounded out the input grid of V2 with an electrolytic cap and the amp went absolutely silent. I mean dead silent. What does this mean? Only half of V2 is used and that half is the final gain stage for the signal from both V1 (bass channel) and V3 (treble channel). If I leave V2 in place and listen to the loud noise, and then ground out pin 2 (from inputs) on V1 or V3, there is no change. If I ground out pin 7 on either V1 or V3 at the volume pots, the sound veeeeeerrrry slightly gets quieter, but barely. Hardly noticeable. But ground input to V2 and...sweet silence. I replaced the grid wire to V2 with shielded wire and it made no difference at all. Now what? Unfortunately I can conduct these tests easily and safely, but I do not know what these results mean!!!! Anyone? What next? Thanks VERY MUCH for all the help so far.

[Gtr_tech]

Look for DC on that grid.

[EFK]

So using a meter, do I just hook black lead to chassis ground and start checking the grid wiring and grid side of coupling caps with the red lead while the amp is running? It should all be negative voltage, correct? Any positive DC = bad? Is it absolutely necessary to desolder coupling caps to do this?

[Gtr_tech]

So using a meter, do I just hook black lead to chassis ground and start checking the grid wiring and grid side of coupling caps with the red lead while the amp is running? It should all be negative voltage, correct? Any positive DC = bad? Is it absolutely necessary to desolder coupling caps to do this?

Gnd lead of the meter goes to chassis gnd. Amp on and off standby (play). You should see no dc on gain stage grids....they should be at or *very* near ground potential. Even a couple volts is enough to throw off the operating point of a small triode. Don't disconnect anything when doing this check. Beware the meter probe will cause a fairly loud pop in the speaker when you contact a grid connection. Expect it and it won't startle you as much.

[Alex R]

You've now isolated the problem to the general area of the echo send/return circuit. Since you made some changes there, particularly that 1M resistor, it makes sense to check that area out, and try undoing the changes.

That is, unless the noise stops if you leave V2 in and pull either V1 or V3...

[EFK]

I will test as described above. Thanks!

Alex, I cut out the echo loop in an attempt to find the hum/motorboat - it was already making the loud noise. Didn't change anything. I cut out the loop just before the 1M resistor that follows the 470K mix resistors and tied it back in just before the .022uF cap that leads into the input of V2. I do want to test that .022 cap in-circuit although I tested it for capacitance with an LCR meter when I had it out and it was almost dead-on .022uF, and I also tested it with a battery to see if it would pass DC which it did not.

(BTW, isn't that .022 a little odd or redundant, especially considering that you have caps just before the 470K mix resistors anyway?)

[EFK]

Well now I think it's not really V2 at all. When I pull V2 the hum is largely gone but still faintly there. When I pull PI it is silent. So I tested all the grids on V1, V2 and V3 and have no DC voltage on any of them. But, I hit the grids on the PI and I have 7.58 VDC on pin 2 and 6.97 VDC on pin 7. When grounding pin 2 it is almost quiet (still faint) but when I ground pin 7 is is absolutely dead silent. So which caps are suspect here? Should I suspect the two .047 caps to the output grids, or should I suspect one or both of the .1 caps down on PI pin 7 (can't see from the schematic how they would tie into pin 2???). Getting closer I think! Have my fingers crossed.....

[EFK]

BTW, does anyone have a photo of the board in one of these? Specifically, I'd like to see a good detailed photo of the PI section, caps and long tail pair etc. Thanks!

[Steve Conner]

Here are a couple of pictures of my SV, back when it was relatively unmodded. I may have some clearer ones, but they show a lot of mods that may confuse things.

They're full resolution as they came from the camera.

[EFK]

Steve thank you for taking the time to post those. Those SV amps are considerably different, apparently. Chassis layout is definitely changed-up quite a bit. I found some internal pics on the vintagehofner site (which has a lot of very good information regarding Selmers) but they're not the most detailed quality. Still, I can see that it appears mine is wired correctly. I'm a bit at a loss to explain how 6 to 7 volts DC is getting on my PI grids but whatever is causing it would seemingly be the problem. I replaced one of the .1 caps - the top cap in the schematic into pin 2 - and it made no difference whatsoever. The .022 into pin 7 looks brand new (for a mustard) and tested good (out of the amp w/ a 9 volt battery). So where is this DC coming from? I guess I'll try replacing the other .1 to ground and see if that makes any difference. Anyone have any more ideas or systematic approach to tracing back this DC I'm all ears!!

[EFK]

Take a look at this poorly-drawn sketch (attached). Which diagram matches up with the schematic, and does it make any difference either way? I found two blurry pictures of the board on a Mk II and Mk III, and they appear to be soldered up as per diagram #1. However, when I acquired this amp, mine is soldered up as per #2 (which seems easier given the size of the .01 caps).

Opinions?

[Steve Conner]

The two diagrams are electrically identical.

Am I the only one who thinks that 6-7V DC is nowhere near enough for a LTPI? The grids should have something like 20V on them. The earlier remark about checking for DC on grids doesn't apply to the LTPI, the DC is a vital part of its operation.

The SV and MkIII are almost identical, but the schematic for the SV has voltages marked, and the LTPI cathodes say 20V, so the grids should be around 18V.

So now we have two symptoms, lack of voltage and hum. Both could be explained if the electrolytic caps happened to be bad, and since they're known to go bad with age, this is a possibility you need to check out.

Selmer TnB 50w SV Amplifier Schematic - SV
Selmer Tn'B 50 MkIII Schematic - MkIII

So, I'd replace all of the 32uF caps as a matter of course. The ones labelled "6x 32uF" on the Vintage Hofner schematic. Can you post some pics so we can see if they've already been done? 32uF is hard to get, but 47 will do.

Did we check whether the amp plays properly with no hum, if the "treble" channel preamp tube is pulled?

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?

[EFK]

Steve and everyone - I can;t thank you enough for bearing with me. I'm sure it's tough to try to diagnose from a distance, especially when I don't really know what I'm talking about.

This has three cap cans, 2 which are 2X32uF and the mains which are 2X50uf (someone must have upped the values). I replaced both 32uF cans today, although they were F&T and did not look very old, as well as installed new monster 27K 5W resistors off the cans, and it made NO difference at all. I did not replace the main 2X50uf "Ruby" can as I swear it looks like it was installed yesterday, it's that new. For giggles, I also used a continuity tester on the preamp pins, one end on the respective grid pins then touching all the others in turn, just to reassure myself that none of the pins were shorting out with each other. Still VERY loud hum, 6 to 7 volts Dc on PI grids. There is absolutely no DC on any of the earlier preamp grids, V1 through V3. But, I'm getting that 6 to 7 VDC coming into the PI off the .022 cap which feeds the PI from V2 (last gain stage). I replaced that cap - no difference at all. (How is the DC getting through that cap?) I think the hum might be coming from either V2 or somewhere earlier than V2 (V1 or V3) and the added gain on V2 is amplifying it. IF I touch any of the grids on V1 or V3 with the meter, no voltage and nothing changes. If I touch the V2 grid (one half is used only), the hum gets even louder, almost a double tone hum, and I can feel the vibration right up the probe. But, there is no DC voltage there. I checked for DC coming through any of the coupling caps on V1 or V3 while the amp was running and got absolutely nothing. To reiterate, all of my grounds in the amp check perfect.

This is driving me crazy!!!!! Help!!!

Is there any way something in the pots for either channel could be causing this? I've got replacements lined up. Honestly, I've got enough replacement parts to rebuild the entire preamp but I would REALLY like to figure this out rather than just blindly replace things.

[Steve Conner]

Oh, well if they were F&T, someone has already replaced them. F&T haven't been in business long enough for any of their caps to need replacing yet, AFAIK. How about the bias filter capacitor? has that been done?

Another few things to check: This was a British amp, so it would have been set up for 240V line voltage and a three-prong power cord. (We've always had three prongs over this side of the pond.)

Now, if the voltage selector were set for 240V, that would explain the low voltages on the PI grids, and if an American 2-prong cord had been unthinkingly grafted onto it, that would explain the hum, since the amp would be completely ungrounded.

So check the voltage selector is set to 120V. (should be a little plastic dial on top of the PT if it's like mine?) And check that it has a three-prong power cord with the green wire connected solidly to the chassis. Also check that all of your outlets on your workbench have functioning grounds.

Back on the component front, check all of the components and connections around V2, and try swapping V2 with one of the other tubes, in case it's bad.