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Nope, that's normal for a cap. You cant check resistance of a cap.

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

I know. That's why I surprised that it beeped at all.

The meter battery charges it up. Initially, it's a dead short until then so you'll get a beep.

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

I reflowed everything again.

Voltages all check out except for
pin 1 of the 12AX7 which is high at around 234V.
pin 3 of the 12AX7 which slowly, slowly creeps up to just over 2V.
pin 8 of the 12AX7 which is low at around 1.5V.
These have been about that far off all along. I chalked it up to component tolerances at first.
The VRM is maxed when these measurement are taken.

I experience the same issues with a 12AT7, although I haven't measured the voltages with it in yet.
So, I don't suspect the tube... unless I've got 2 bad ones.

One thing I noticed playing with the VRM, the circuit takes a while to catch up when you change it.

What I mean is, I can dial it up and things slowly build up - either in voltages at pin 3 or pin 8; or in sound level when I have guitar plugged in. It does the same when I dial it down. It takes a while to settle where ever I leave it. That doesn't seem right because I'd have to turn the knob and wait for things to settle to find the level I want and then tweak from there, wait, tweak, wait.. etc...

and I'm still working with settings over 8 in tweed mode and over 6 in tude mode.

That's normal.
It's just the large filter capacitors charging up/discharge. Charging up as you move the VRM control higher. Releasing their HV charge as you dial the VRM control down.

So the amp passes sound.

If pin 1 and 3 are both high, I'd just attribute that to a slightly off spec triode in your 12AX7. Try a different 12AX7. If it sounds good I wouldn't worry about it though. The voltages are +/- 10% or so. They also depend on your main's voltage, assumed 120V AC.

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Cool. Thanks.

When I say the voltage creeps up slowly on pin 3, I'm talking minutes.

But other than that, the only thing that bothers me is that I can't get any sound unless I crank the VRM up beyond 8... Volume and gain settings need to be high as well - they vary depending on the tweed/tude switch, but anything below around 6 is useless.

At 8, what is the voltage? Could be a bad resitor in the VRM ( 100K to ground possibly) . Can you measure the two 100K resistors in that circuit?

Or a bad triode in the 12AX7 as suggested as well.

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

At 8 the voltage at pin 3 is around .660V

R21 is 99.3Kohms
R22 is 92.1Kohms

I'm going to try a different cable to the speaker - just because...

What is the B+ [.66 is pretty low]


These are OK.

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

I just got a new 12AX7 in and tried it.

Same issue AND after it warms up, sound really starts to break up.
I'll have to play with it a bit more to better characterize what it's doing.

If VRM, volume, and gain are maxed, though, it stays clean.
More twiddling will bring better info. I bet...

I'm sort of having this issue as well but I'm not sure if that's the way this circuit works. If the volume isn't fairly high, there is little or no sound when the power level knob is under about 8. The sound sputters out and breaks up below that. Although if I have the volume high I can reduce the power level below 8

Guys, start by shorting out the VRM. We need to eliminate that from the equation and first get the main part of the amp working. Solder a shorting wire from the VRM end of 100 ohm resistor R23 to the + end of the second filter cap C12.

With the VRM out of the way, you should have a nice loud amp. We can deal with the VRM later.

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I'm not that good at reading the schematic and co-relating it to the layout. Is it possible to show me how to short the VRM on the layout?

The instructions I gave were based on the layout diagram. I don't know how to spell it out any simpler than I did.

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This is what Zaphod is suggesting but I have not done this.



Assuming it works, it should put 400+ volts onto the plate of the 6V6. Make sure the bias switch is in the correct position for 6V6.

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

Thanks for the image. I actually was pretty sure I figured out where to short it but I used the Capacitor next to the one you indicate and it didn't work. I'll try this later today.

Hmm...I was also confused by that. I shorted to what I thought was the "2nd Cap", meaning from left to right of the pair. Perhaps you could put that image into the docs for troubleshooting?

If you are getting variable B+, your VRM is probably OK.

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

I'll try this later on tonight.

The image makes sense to me.
If you shorted to the top of that cap beside C12, you'd just be bypassing the 100 ohm resistor - because the top leg connects to the top of that other cap with the jumper wire under the board.

How about CAREFULLY putting in a nice beefy jumper wire instead of soldering?

Solder a beefy jumper wire if you want. But the connection has to be nice and solid, so it has to be soldered in until the rest of the amp has been debugged.

After that, retest your volume levels and *carefully* re-measure voltages at all the tube pins.

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