Stereomour Troubleshooting

[tristan]

Hi all! I just finished building my Stereomour (not without my share of muck ups and frustrations, but now all seems *almost* all well!)

After finishing the build, I realised that the orientation of one of the 2A3 sockets was different to the other, checked the manual, saw no mention of orientation so I checked the forum and found my answer here - (http://www.bottlehead.com/smf/index.php?topic=1485.0) - thanks to all on that one! So I reversed the one 2A3 socket that was round the wrong way (the smaller sockets were closest to the hum pots instead of the larger ones) and resoldered all the bits. Unfortunately in this process I managed to destroy the 220 ohm resistor that connects to A3. So I replaced it with a 200 ohm carbon comp resistor I had lying around that was measuring at 210 ohms. How critical is this value? I became very confused about this because when I measured the other 220 ohm resistor that was still nicely connected to the C socket (original one supplied with the kit), it read close to 600 ohms, quickly rising in value from about 350 ohms (when first connecting the multimeter). I thought perhaps this was due to me trying to measure it whilst it was soldered in with other components, however after soldering in the 200 ohm resistor on the A side checking it still read 210 ohms. Does this indicate a problem with the 220 ohm resistor on the C socket or is it perhaps some bad solder joints on my behalf?

Stranger still, after putting it all together, all my resistance checks are OK and all my voltage checks are OK. All were pretty much spot on. The LEDs on the board glow when I turn the amp on (though not super brightly). However the tubes barely glow. I mean barely. I can just see a glow when I switch all the lights off. One of the two 2A3 tubes has a blue ring of glow towards the top. There's almost nothing in the preamp tube. However like I said all my resistance and voltage checks were OK? And it plays music. Quite loud it seems (it's almost midnight here so I can't really push it too far just to test right now unfortunately) but I can't say it's right because I'm not 100% sure that it's working correctly. I've had the amp on and playing for about 30mins and all seems well. No smoke, no other apparent issues, just no tube glow. Is this normal? It would seem like I should be getting low output or nothing at all with no tube glow?

I seem to be almost there (after a bad start - I somehow managed to snap a binding post in half! (funny as it's not solid metal as I thought it to be) and after I used the power switch about 3 times it broke - replaced it with a spare I had for a guitar amp - it's a bit more solid now) - I'm awfully confused as to how to troubleshoot this issue with the tube glow so any help would be greatly appreciated!

Thank you kindly in advance,
Tristan.

[Jim R.]

Tristan,

Don't worry about the lack of glow from the tubes -- some tubes can barely be seen when they're on and working properly.

Since it is playing music, and there's no smoke or burning smells, and if the channels are balanced, then I'd say you're done.  Now just give it some time to break in and enjoy.

BTW, the value of those grid stopper resistors is not critical, though your description of the wandering readings is a bit odd, but may just have been a test lead connection issue.

HTH,

Jim

[tristan]

Hi Jim,

Thanks so much for the quick reply. I thought this may be the case but wasn't sure. I thought that at least the preamp tube might glow a bit brighter as the tubes in my Seduction glow a fair bit brighter. Also similar tubes in my valve guitar amps tend to glow brighter (however I have no doubt they'll be being driven harder!?).

Thanks for the info on the grid stoppers. A question if you feel you have the time - why are they carbon comp resistors? Is there any reason to use carbon comp resistors for this purpose instead of, say, metal film? Just curious - would like to understand more about such choices!

thanks again,
Tristan.

[Jim R.]

As for the tubes, some glow more than others and definitely, the tubes in guitar amps are being driven, typically, beyond their limits.

Grid stoppers are usually carbon comp as they have very low inductance and they are there to prevent hf oscillation, but yes, metal films will work, and also Rikens can be a good choice.

A friend was recently working on a pair of huge 211 monoblocks (now there's some glowing tubes!) and the driver tubes were oscillating badly, and when he inspected the circuit board, he found that there was a carbon comp resistor alright, but spaced more than an inch from the tube socket, so, two soler joints instead of one, an inch of PCB trace and the result was a completely ineffective grid stopper.  He soldered a new carbon comp resistor to the tube socket pin directly and that stopped the oscillating.  Not sure what the driver tube in questin was, but sure it has a fairly high mu like the 12at7.

On the other hand, with the 12au7 in the crack, the mu is low enough that a grid stopper is not required.

Not the best or most complete explanation, but it makes the point.

-- Jim