diytube.com

Ughhh.

I purchased an Triode ST-70 Clone with all options,
including matched JJ Tesla EL-34s, 12au7.
The other tubes are russian EF86. Solen caps.

It has the Triode ST70 Cap Board.
The original instructions were for the SDS board.
Triode sent me an email explaining that and gave me the pinout.

My problem is that my bias will not go below 13v on either side.

when I measure pins 6 and 21 on the driver board (s/b -30 to -40 volts) I get -48 on both sides.

When I measure each tube grid resistor, they measure right at -48 volts
with a drop of less than 1v on the other side of the resistor.

The amp actually makes sound from my audio generator to the rca ins to my cheesy 8 ohm speakers. Equal volume and sound on both channels.
The sound is low and slightly distorted, but I thought I would mention this to eliminate some possibilities.

This is my first tube amp. I thought a full kit would be the way to go.

Triode pointed out that I forgot the 12au7 jumper (j1 to j2) but that didn't change a thing. (and I never found that in the instructions anyway?)

They have looked at a large photo of the wiring and told me everything looks good.

That was the last I heard? It has been a couple of weeks.

You folks have any ideas?

Based on what I can derive. The Tubes are all matched. I get similar numbers all around, just way too high?

Here are some large photos of the st70 wiring.

There are also a couple of photos of a grounded grid preamp
I built to drive it. Of all the goofy things, I got that working
just from schematics?

The images are huge, so load them manually in your browser.
Note the wiring used to be super clean, but I keep having to mess
around and test the unknow.

http://www.maximumclix.com/st70/st70.jpg

http://www.maximumclix.com/st70/st70driver.jpg

http://www.maximumclix.com/st70/ggtop.jpg

http://www.maximumclix.com/st70/ggbottom.jpg

I built one of those a few months ago, and it worked fine.

I would suggest double checking the value of every resistor
in the bias circuit with an ohm-meter, to make sure you
have the correct values in the correct places. It's easy
to get them wrong. Make sure the 10 ohm resistors to
ground are really 10 ohms and not 1000 or something.

I have some (lower res) pics on http://tmamps.com
unfortunately I don't have the amp here to get you a
better pic.

Are you measuring 14 volts from the 10 ohm resistor to ground ?
I would think that could only be possible if the value was wrong.
If it was 14 volts across 10 ohms the tubes would be bright
red and hot.

... tom

Tom,

You are my new hero.
The resistors were Dales with no stripes, they were marked 10 alright
"10K".

How embarrasing.

Where do you want me to send the beer?

The output tubes are just a little too hot to touch,
the sound is great.

I set the bias at about .9v.

Having never built a tube amp before, I am not sure if I should
do anything else but listen?

Hah ! Great - I needed a smile, it's always great to help someone !

.9 seems a little high - what do they suggest in the book ?

You probably want to check the bias (with no music playing)
after a few hours and reset it, it will drift a little as things
warm up.

... tom

They suggest under a volt....
I have full range to put it anywhere I want.
I was using dummy plugs when I set it.

Now I am ready to build another one.
Maybe an st35 clone a board and spares.

Just think, an hour ago I was ready to run it over in the driveway.

hahahahahah

coop402 wrote:
Just think, an hour ago I was ready to run it over in the driveway.

hahahahahah

To save yourself grief in the future, just check each resistor with a multi-tester <i>before</i> installing it.

You got that right kheper!

The sad part is that I measured every part as I built the driver and cap boards. I must have missed this one!

BTW, the amp sounds awesome. I am hooked.