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Any tips on testing for dummies?
ie: where do I place the lead to check the B+

I've been through my wiring 3 times and can find no errors (not to mean that there aren't any ). Here are my measurements.

Pin 1 2 3 4 5 6 7 8 9

V1 53.4 0 .5 - - 85.3 0 1.08 -
V2 81.2 0 .32 - - 53.8 0 .221 -
V3 102.7 24.2 25.7 - - 69.1 0 .64 -
V4 369 25.7 29.0 - - 370 23.5 29.2 -

V5 0 - 449 429 -43.5 -43.6 - .021
V6 0 - 452 430 -43.5 -43.6 - .021

Pin 1 on V1 through V4 are way off. Definitely a problem. Pull the preamp tubes and check the pins, maybe you bent a pin on installation and it's shorting across to another pin.

Tube pins ok. Changed out for new set. Same result.

You need to measure the preamp DC rail voltages for V1 to V4. I suspect the problem may lie somewhere along there. Possibly there's dropper resistor in the DC rail that's the wrong value.

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Great tones come in small glass jars!

Appreciate the advice, just dumb it down as much as possible. Are you talking about the cap and resistor on V3?

I measured the resistors on V3 & V4. they are supposed to be 1M. The one on V4 measures 1017 but V3 is only 487. Same colour codes. Could this be an issue or is it because the V3 is piggybacked on a cap?

In order to accurately measure resistors and caps you need to take them out of the circuit. You can also just lift one end, this is basically enough as taking the element out of the circuit.

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Then I'm doubting that there is a problem with the resistor on V3. I am interested in checking Zaphod's suggestion about DC rail voltages but i need to be explained exactly where to measure. Also where do I measure B+?

B+ is marked HT1 on the diagram. Ht2 feeds the screens on the o/p tubes. HT3 feeds the preamp tubes. Ht3 gets to the preamp via that dotted blue line on the diagram. Those resistors near the bottom of the 33uf 450V cap are of concern.
Also check that 22K at the right end of the dotted blue wire.
Stew

Follow the schematic and the positive voltages from the first filter caps along the path to the resistors that feed the plates of the 12 AX7s / AT7. There has to be a wrong resistor value in the power rail chain.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Specifically: you need to cross reference the schematic to the turret board and trace the voltages down the line to the plate resistors. These go to pins 1, 6 of the AX7, AT7. Also measure the B+ - voltage at point labeled, HT1. Measure HT1, HT2, HT3 and all the voltages at the 2W Power resistors 22K, 10K and then pins 1 & 6.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I think I've located the problem. For some reason I installed a 220K2W instead of a 22K2W. I'll pick one up today and get it installed. Thanks for the advice. I've learned some new things.

That would do it alright!

That did the trick. Amp is working well with no noises or hum except................................
when I engage the OD all is sweet and amazing until I hit the 3 o'clock position. At that point I get a static whistle. Stephen thinks it may be an OD lead dress issue. I am open to suggestions.

Well not sure what happened but I loaded it up with all the serious tubes (1950's Sylvania 6V6's, RFT-V1, Sylvania-V2, Bugle Boy-V3, RCA12AT7-V4) and everything is perfect.

Sounds like you may have had a slightly flaky preamp tube, possibly in the V2 position.



No, I've never tried to test for dummies myself. Maybe someone's invented a dummy detector by now...?

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Great tones come in small glass jars!

Thanks for your help. The photos of your build where very helpful to me.
I figure after this (my 3rd) build that I actually understand about 50% of what i'm doing. Lots of fun on this one. Really took my time, probably about 45 hours including troubleshooting.

Congratulations, it's a great amp that you'll surely love. I'm surprised (shocked actually) that you can crank the OD to 3 O'clock before you get hum. That's a ton of gain!

Edit - I re-read you comment, maybe you mean you get oscillation at that point. I hear background hum above 10 O'clock. I don't have uncontrollable feedback at higher settings, but I have to take care that I'm not letting strings on the guitar ring or it can get out of control quickly if I'm set above 1 O'clock. I rarely use that much boost.

I have no reference point to any other Triwatt amp but with my "vintage" tubes in place, and using the 12AT7 in V4, I'm getting virtually no noise or hum at any setting, even with the OD at full. It must have been a tube issue previously (I was using some Tungsols that I thought were good) that was causing the OD problem past 3 o'clock. I'm going to experiment with some other 12AX7's in V4 to see how much more gain I will get. Perhaps I might get some "noise" then but who knows. Perhaps I got lucky with my wiring layout. I used to think that Tungsol 12AX7's were good but I'm reading more and more negative postings about them. Not as reliable as I thought I guess.

Head cab is on order (white with black piping, black handle, gold hardware). Can't wait to gig this baby.