trinityamps.com

Hello everyone,

I've hit the wall in trying to get my new Triwatt kit to work. It's almost a "no sound" situation, but if I plug a cable into the input and touch the end, I can *just* hear the tiniest amount of sound through the speaker. Behaviour is the same on both channels and is affected by both the channel and master volume pots---they have to be way up for anything to be audible at all, this is a *very* faint signal.

My voltages are generally in the "slightly low" range, except for pin 3 of the power tubes which is negative! I assume this is likely the core of my problem, but since these pins go straight to the OT, I'm at a loss to understand how they can be seeing a negative voltage.

Here's the full list of voltages, with tubes in (12AT7 in V4, 6V6 in V5/V6).

V1
1: 105
3: 0.4
6: 75
8: 0.7

V2
1: 135
3: 1.1
6: 65
8: 0.6

V3
1: 82
2: 120
3: 120
6: 120
8: 1.2

V4
1: 150
2: 120
3: 120
6: 180
7: 90
8: 120

V5 and V6 identical readings
3: -6
4: >300
5: -31

Any ideas are more than welcome.

Thanks and Happy New/Old Year, depending on when you're reading this :-)

-NT

Looks like there is something wrong in your power supply. Check all your wiring very carefully. Make sure all the resistors in the power supply chain are in the right place and are the right values.

What are the voltages at the + terminals of C20, C21, C18 and C19?
What is the voltage at the center tap of the primary on the output transformer?
Also what is the incoming AC line voltage?

So you know how sometimes just asking the question fixes the problem?

I went in to answer the questions from Mitch M above, and immediately discovered that the center tap of the OT primary had come loose from its join. Hooked it back up and I have a working amp! I need to get in and do the biasing now, and some chopsticking to get the overdrive squeal under control. And I'll go back through and check out the voltages, but considering the audible results I expect to see them in reasonable ranges now. [EDIT: nope, see below]

This thing sounds nice! I haven't really wrung it out yet, but I'm getting lovely cleans while testing, and it's dead quiet.

Thanks

-NT

Edit after biasing and testing: Good sound, but still low voltages in a lot of places.

V1
1: 105
3: 1
6: 80
8: 0.75

V2
1: 135
3: 1.2
6: 70
8: 0.7

V3
1: 220
2: 150
3: 140
6: 150
8: 1.6

V4
1: 160
2: 140
3: 140
6: 185
7: 90
8: 140

V5
3: 400
4: 380
5: -30

V6
3: 430
4: 360
5: -30

Wall: 120
OT center tap: 440
C18: 220
C19: 260
C20: 440
C21: 400

The HT1 voltages look OK but the voltages associated with HT2 and HT3 are low. Check R40, the 2K5 5-watt resistor.
C21 has two positive terminals with a 1K resistor across them. What are those voltages?

It looks happy to me; it tests at the right resistance, the joins at the ends have continuity. 33 V across it.



400 and 380.

I guess my next step is to trace through HT2 and HT3 and see where the voltage drops along the way?

Thanks for spending your time on this, by the way. I appreciate it.

-NT

Glad to help if I can.

Yes, trace the HT2 and HT3 paths and see if you get any abnormal voltage drops. Start with HT3 since HT2 isn't that bad. Also recheck your wiring very carefully. Look for bad solder joints on the board and the wiring out to the tube sockets. Make sure all the resistors are the right values. Sometimes it's hard to tell the difference between red and orange on the color codes.

You can try pulling V1, V2, V3 and V4 one by one and see how that affects the voltages. One of them might be bad.

Voltages in tube circuits vary a lot from one build to another. Don't be too concerned if they are about 10% off and the amp sounds good. Your voltages at V4 are way off, though. Start by looking for problems there.

Got something: It looks like the 22 nF cap across V3 is dead, so pins 2 and 6 were shorted to each other, bypassing the 1M resistor. This surely explains some of the out of whack voltages on V3; I pulled the cap and the short it gone, but I'll need to come up with a replacement and see how things look then. It's possible I cooked it while working in the crowded space around that socket.

It isn't obvious to me what that cap does. What should I expect if I run without it?

-NT

Good work finding that bad capacitor.

It looks like it's there to help pass the higher frequencies. Are the voltages more reasonable around V4 and V3 now?

That did it! All voltage readings are now within 10% or so and the amp sounds good. I'll drop the replacement cap in when it gets here, but for now I'm going to turn my attention to enjoying this thing. Thanks a million for all the assistance.

-NT

Glad you got it working!

It will be interesting to see what affect that capacitor has on the tone when you get it replaced.

Thanks mitchm for your help.

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