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My amp is up and running but since I've got just enough knowledge to make myself dangerous.. I figured I would ask some questions.

First let me say that even though the Triwatt is a more involved build than some, it fired right up for me first shot. And even though I've got a few things to iron out, it still sounds huge. Powerful complex tones, that really cover some wide wide ground.

In any event.... Here what I've been trying to chase down....

I found that my bias voltage readings were WAY off when I had a 12ax7 in the phase inverter position... as in 47 to 62mv's set to 6v6's. Changing the tube to a 12at7 the voltages dropped way down, but I was barely able to get past 15 mv's with the pot maxed out.. with something like 6 or 7mv's being the cold side of the spectrum. Does this sound off? To be fair, the 6v6's are old and used but within a mv of being matched.

Also, with the 12ax7 in the phase inverter slot, the Presence knob had an audible thhhhwwwwickkk... and a sharp increase in background noise as I move it from 2 o'clock to full bore. The normal operation of the amp is very quiet, otherwise. This problem has been 90% alleviated by switching back to the 12at7... Maybe I've got something screwed up in that area?

Also... Turning the amp off with let's say a chord ringing out as the amp powers down..... should I expect the sound to signal to get nastily distorted as it drifts into silence?

Thanks in advance.

Zack

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Well, I can tell you that a couple of us has similar issues with low bias current/voltage on initial startup, and the cases I recall (mine included) were due to a bad solder join in the bias supply (mine was the ground side). Once I re-flowed everything was fine. I'm sure at least one other person has the same (or a similar) issue. I don't recall that the PI tube made a difference, but it was a year ago.

Not sure about the presence thing. I haven't experienced anything like that. I did have a 12AX7 in the PI at first, but I prefer the 12AT7. I didn't have any strange results with either tube.

As for the ringing chord getting nastily distorted on power down, that wouldn't surprise me as I've heard similar things. Don't know if it would be normal for this amp. I defer to our more knowledgeable friends on that one!

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SG + Brit tube head = Nirvana

The PI tube should make absolutely no difference to bias voltage. The two 47n (0.047uF) caps, C15 & C16, on the power tube grids block all DC voltages from the PI. So what you're saying suggests that either you've got a leaky cap or two there, or else something is slightly shorting across one or both of them.

Something' not quite right there either. I don't know if it's related to the biasing problem, since negative feedback/presence also involves the PI. So I suggest you try fixing the first problem first. I suspect you may have some parasitic oscillation problem with the higher gain of the 12AX7, in which case you may need to play around with the amp's lead dress - try moving some wires around with chopsticks.


You should get a heavily distorted chord fading out. Not sure about "nasty" - that's a very subjective term. In any case there's at least one fairly serious problem in your amp that needs fixing.

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Great tones come in small glass jars!

Thanks Zaphod.... I'll dig into this tonight.

Just plain distorted...

I will get out the white gloves and make sure the connections are good and tight, that the caps are right, no shorts.... and that the wires are routed nicely.

Am I correct in assuming the standard Pin 1 of any preamp tube to ground will drain the caps?

Thanks

Zack

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dig it

Didn't the power tubes you initially used made a big difference to the way things behaved? Presence control excepted.

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

Connecting pin 1 or pin 6 of 12A_7 tubes to ground will discharge the preamp power caps. However, they also generally discharge as the main power caps discharge. Just to be clear the caps that might affect the bias voltage seen at the power tube grids are C15 and C16 (0.047uF).

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Great tones come in small glass jars!

Stephen...
The power tubes were pretty far out of match, to be sure... and the presence thwiiick could've been worse because of them.

But the big change happened with a 12at instead of the 12ax. Perhaps the tube was bad? It's completely possible. My tube tester is pretty rudimentary. And because I got such a late start tonight.. the problem solving will have to wait until tomorrow. I'll definitely post back with what I find.

Zaphod... I know the caps you're speaking of.... they're the smaller electrolytic's near the bias circuit correct?

The funny thing is that even with this anomaly and tubes of dubious quality, the amp still sounds pretty damn good. Can't wait to get it all the way up to snuff.

Thanks guys!

zack

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Wrong. The caps I'm talking about are a pair of 47n signal caps,normally either Mallory 150s or Sozos, which take the signal to the power tube grids. The symptoms you're describing would suggest that DC is getting across them somehow.

Also BTW you don't need matched power tubes in this amp (or indeed most guitar amps). None of the Triwatts that we custom build use matched tubes. The tubes just shouldn't be way out from each other, like more than 5mA say, when you measure the bias currents. Some imbalance in the power stage helps give a sweeter tone, with more even order harmonics.

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Great tones come in small glass jars!

Got em. Their the pair with the 100k resistors between them..
the caps feed pin 6 of the power tubes. I'll check that out asap.

Thanks again king Z.

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Problem Solved... I was missing a jumped connection between the 7k5 2watt resistor and the 2k2 resistor to the ground buss.

The amp jumped up with a lot more power... much cleaner attack on notes etc. Before, I could tell it was cool... but there was something wrong under the notes... Now that's gone.

Question... I'm still only able to get about 17 mv's with the bias cranked... Does that bely something else futzy with my construction?

thanks again.

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dig it

On the 6V6 setting, I would say yes. 17 mv is about 5 mv too low. Check the bias resistors are correct values and in the correct places; grounding is correct; that you're measuring one tube only etc.

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

Everything in that area looked right, but I'll check again.

On another note, what's the difference between using the 620k 2w resistor over the 910k 2watt ? My layout had the 6720k pencilled in, so that's the one I used. Could that be why my volts are a little low?

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dig it

Reducing the bias resistor increases the bias current.

The two bias resistors in parallel (i.e. less & approx 140K) provide another setting for the KT66 in conjunction with the increased voltage (i.e. 18V zener out of circuit).

So, maybe check to make sure in the 6V6 position that the B+ voltage is low, and the resistance is high (180K).

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

+1

As a little background, in the Triwatt prototype we originally used the 910k, which gets switched in parallel with the 180k for KT66s. After trying more tubes we found 620k was a better value to use for most KT66s, but since tube characteristics can vary quite widely, we also put the 910k alternative value in the kit. For 6V6s this resistor is disconnected by the KT66/6V6 toggle switch.

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Great tones come in small glass jars!

Sweet.

I ran through the layout... Made sure all connections were nice and tight, sent some fresh solder through there and now the head will run for 3 minutes or so and then pop a fuse. Even on standby.

The only other "clue" i'm getting is that when turning on standby I get a flash in v2 then it settles down into warm up mode.

I should mention that while it's working all seems good. Amp performs fine, sounds clear, and is quiet on the noise front. Then pops a fuse.

arrrgh!

What could need to "warm up" before shorting out? Seems like that happens only with tubes. I'm going to start by cleaning up all the wiring to/from the tube sockets, take some readings and see what happens next.

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I rewired a 12ax7 tube socket... rewired the power switch like Zaphod's pictures and what I've got is an amp that works great for 45 seconds, then starts to hum... Switched into standby, Now, it will idle fine. But Once switched into power it'll work fine....45 seconds or so start to hum. The humming will get louder then blow the fuse.


Ive got fuses aplenty... let's blow some more.

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dig it

You will need to take voltage readings on all the tubes right before the hum starts, and then after. This means you will need to work real fast, and take a few readings at a time.

For the fuse-popping on standby issue, check what happens if you disconnect the PT secondaries from the rectifiers (tape the wire ends up carefully to avoid dangerous accidents).

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Great tones come in small glass jars!

The fuse isn't popping on standby anymore.

Can you walk me through what I'm looking for and which pins to check against which pins?...to check pin 3 and 7 etc...

Thanks

zack

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There's a voltage chart in the build guide, which shows which tube pin numbers need to be measured. Also check for a large imbalance in the power tubes' bias after the hum starts.

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Great tones come in small glass jars!

Ok... after pulling one of the legs off the 6vv6/kt66 switch, checking out the bottom of the board, and some re-wiring I seem to have a stable amplifier.

It works as it should ... all controls are functioning as I'd expect them to. Overdrive swith works.. Tonally, the amp seems to be firing on all cylinders... the Bias still seems low.. 8.5 mv to 17mv is the swing I'm getting.

However, the voltages I was getting seem to be off a bit from the chart. Here's what I'm reading as taken from each pin and the leg of the rectifier closest to the bias pot. I wouldn't be surprised if my readings were wrong due to meter setting? I'm no expert when it comes to the nuances of volt meter readings.

V1 (1): 155v (2) 0.8v (3) 2.3v (6) 136v (8) 2.0v

V2 (1) 207v (2) .08 (3) 2.2 (6) 128v (8) 1.7v

V3 (1) 290v (3) 76v (6) 179v (8) 2v

V4 (1) 284v (3) 80v (6) 283v (7) 69v (8) 80v (9) 1v

V5 (3) 473v (4) 44v (5) 44v (6) 44v (8) 1.1v

V6 (3) 474v (4) 444v (5) 44v (6) 44v

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