495V on pin3/4 of output tubes, new Triode ST-70 build

[gregk]

Hi all,

Triode has confirmed that their version of the st-70 runs fine with a 2.5A slo-blo fuse. I have put about 10 bucks worth of fuses through mine trying to find out why the amp is drawing to much power but I don't really know what to look for and they have not given me a complete version of Triode schematic. As soon as i get the chance I am going to test all the resistances on different parts of the circuit as well as the voltages when the amp is way below the normal bias level so the fuse doesn't blow.

I am waiting to hear back from Triode on what they think might still be wrong. I would appreciate if anyone on here might have further clues on this one. It's been two months now that I've had this amp and it is so far useless, well it makes a great dead fuse holder. Wish I understood more about this stuff.

Thanks all,
Greg

[TomMcNally]

I built one of the Triode kits without any problems, although I may
have had to figure out that jumper in on the driver board too.

Have you tried setting the bias to about 400 mv (.4 volts),
and running the tubes cooler to start/ It sounds to me
like you are pushing those tubes too hard and popping fuses.
I have build dozens of amps, and very rarely even measure
any voltages other that when setting bias. If you have the
right components in the right places, it should work.

If you have JJ tubes, they are known to be crappy, swap them
around, bring up one channel at a time and see how things
work. It's ok to run the amp with tubes in just one channel,
yeah the voltages will be a little high, but it won't hurt for
testing.

[gregk]

Tom,

Thanks for the response, I do have the bias set at about .4V and the amp will run continuously for a long time (with music playing through it). I had it running for something like 5 minutes before I slowly tried to bring the bias up to 1V, switching between both sides after a I increased each side by about .1V. When I get both sides much over .5V the fuse blows. Maybe I am doing something wrong?

According to Triode I should see about 420VDC on pins 3 and 4 of the power tubes, I measured mine and found I was getting 440-442VDC which is a bit high. I also measured the rectifier tube and found pin 2 - 439VDC, pin 8 - 441VDC and pins 4 and 6 were 366VAC, which is normal according to Triode. Could the high voltage at pins 3/4 of the output tubes be related to the low bias I have set?

thanks all!
Greg

[TomMcNally]

Don't worry about the B+ ... it's not important here.

I don't have the documentation handy for that amp, but
tell me this. What is the value of the cathode resistor ?
Is there a resistor for each EL-34, or one per channel ?
Have you measured them to make sure they are correct ?

What does the manual say to set the bias to ?

Are you absolutely certain you are measuring correctly,
proper scale on your meter, negative lead grounded OK ?

It really seems like you are setting the bias so that the tubes
are drawing way too much current and popping the fuse.

Let's concentrate on that. Pull the tubes and tell me what
the resistance from Pin 8 of the EL-34's to ground is.

[gregk]

The cathode resistor is 10ohms and mine measures about 10.7ohms.

The manual says that the bias should be about 1V or slightly below. When I measured the DC voltages yesterday I was in 1000VDC mode, when I measure to bias the amp I put the meter in 2000mVDC mode

I grounded to the chassis using an alligator clip, I have also tried putting the VMM ground lead through one of the screw holes and then setting a pair of pliers on it so it cant move around. Either way seems to work ok for ground as I can measure 10ohms roughly to ground from the bias point which is correct according to the manual.

Thanks,
Greg

[gregk]

Ok, try not to laugh to hard, but I might I have just realized what I have been missing - RCA shorting plugs. I have just had my cd player hooked up but no shorting plug, I totally forgot about that. I have just been jumping down to the bias section of the manual which doesnt mention them until the end. Could it be that simple? So then, how do I make shorting plugs, do I just need a certain value resistor? I searched online and it seems theres a wide range of resistor values used, from 50ohm to 5Kohm. I will keep looking...I feel pretty dumb that it could be that simple, but I would be so glad to have this thing completely working.

Thanks!
Greg

[TomMcNally]

Don't worry about the shorting plugs, the CD player can be
connected, just make sure it's off or not playing. The goal
is to measure the bias with no signal. A dead short is unimportant.

It sounds like you are doing everything right. If you are blowing
3 amp slo-blo fuses, something in the power supply can't take
the load. Have you tried another 5AR4 just out of curiosity ?

For a line fuse to blow, something has to be drawing way too
much current, or it's on the edge to begin with, which doesn't
seem probable. Does anything seem to be running too hot
while the amp is playing ?

[gregk]

I am using 2.5A slo blo, I know folks on here have recommended 3A, but the Triode manual, and I confirmed this when emailing Triode that a 2.5A should be used. I dont have another recto tube, the one I am using is a JJ GZ-34. I was thinking about picking up another recto tube for testing and backup though.

Nothing seems to be running hot, the transformers dont heat up significantly in the time that I am running the amp, the tubes seem normal from my experience with other tube amps.

Thanks again,
Greg

[Writer Frog]

I am using 2.5A slo blo, I know folks on here have recommended 3A, but the Triode manual, and I confirmed this when emailing Triode that a 2.5A should be used.
....
Nothing seems to be running hot, the transformers dont heat up significantly in the time that I am running the amp, the tubes seem normal from my experience with other tube amps.

Greg,

Your faith in what Tride recommends is commendable.
For your reference, this is what Dynaco recommends (excerpt from the original Dynaco ST-70 manual, page 13):
http://www.curcioaudio.com/st7_mnl.pdf


Good luck!

[gregk]

I understand where you are coming from, but I don't see how triode is able to run their version of the amp on a 2.5A fuse if a 3A is required. I am still working with them to troubleshoot the amp. If that doesn't get me anywhere I will try a 3A fuse. I just don't see why triode would be inclined to lie or be wrong about this since they claim to run their amps this way.

Thanks again,
Greg

[TomMcNally]

My opinion of JJ tubes isn't very high, try a Sovtek if things
don't calm down after you change to a 3 amp slo-blo fuse.

[TomMcNally]

Greg ....

It's just a fuse, experience shows that a 3 amp slo-blo is
the way to go. Are you using slo-blo or fast acting fuses ?

If you are happy to spend another $ 10 on fuses, go
right ahead.

[Ty_Bower]

I understand where you are coming from, but I don't see how triode is able to run their version of the amp on a 2.5A fuse if a 3A is required.

If you add up the (idle) power consumption of all the tubes, including the heaters, I think the amp only "needs" about 150VA at idle. I'm assuming each EL34 takes 50mA at ~410VDC and 1.5A at 6.3VAC. Maybe there's three driver tubes, each needing 300mA at 6.3VAC for the heaters. Total combined B+ for the drivers shouldn't be more than 20mA or so. The rectifier tube takes 2A at 5VAC. By this logic, a 2.5A fuse should be way more than enough, at least to properly bias the idle amp.

Is there some way you monitor the current draw with the amp running? Do you have an extra AC ammeter you can temporarily wire in series with the fuse, just to monitor the draw? I'd be interested to know if the draw is right at the limit with the amp slightly underbiased. I'm fearing it probably isn't.

[Writer Frog]

I understand where you are coming from, but I don't see how triode is able to run their version of the amp on a 2.5A fuse if a 3A is required.

Hi Greg,

Certainly, Triode is free to specify a 2.5A fuse for their kit. This is their product.

But having done that, it is incumbent upon them to make sure that their customer's line voltage falls within a certain range, that they supply the kit with parts with closer tolerances, supply an accurate and complete assembly manual and troubleshooting guide, the schematic with voltage measurements, the correct version of the driver board, etc.

Otherwise it is exceedingly difficult to diagnose which part of your build is "out of tolerance" with Triode's intended design.

[gregk]

That makes sense, I hadn't thought about it that way. I will see if I can swing by Radio Shaft tonight and get some 3A fuses to give it a whirl.

Greg