Some Dynaco ST-70/VTA Low Gain voltage questions

[Kegger]

Kegger,
Thanks for giving us rookies a really good explaination of "adjustable bias." As one who finds himself "addicted" to tubes, any bits of knowledge I can absorb is a huge help.

Chuck

You are welcome, I like to try an give back where I can beacuse I was a rookie
not to long ago and had to learn from others. (I still try an learn every day). :yes:

 

[Jeffery]

Almost any new coupling cap is as reliable as any other here.

Just every now and then you get 1 bad one, I've gone through
probably a couple hundred of those Russian oil caps on ebay and
I had 1 fail on me in the same manor you seem to be experiencing.

But I still do like those Russian caps quite a bit an trust them, from
my experience you get a bad part here an there, I also actually like
the Yellow IC film caps for vintage equipment

Since caps are not so likely to fail, I decided to give this thing another go. The caps aren't Russian, they're just Parts Express "Jantzen" brand. I bought a replacement and enough spares to give it another reload, but it's the whole taking out a tube with the cap that's getting me down. Maybe this batch will last, I just don't know.

So I got another quad of JJ EL34s to see if they blow up, and a quad of KT77s if I can make it past a week on the EL34s. I'll slap a Mullard rectifier in it instead of the Ruby, that's about all I'm going to put into it.

I'm close to done with this thing, my MKIIIs just keep on jamming. I should have saved my money and just bought a new set of tubes for them.

 

[bktheking]

That's the spirit! Post how it goes.

 

[Jeffery]

I replaced the coupling cap on that tube, voltages all seemed good so I put a pair of the old tubes in that side - right channel - and just couldn't get the bias voltage under about 50mv on either tube. The tubes have a brown scum in them, so I said heck with it and slapped a new pair of the JJ EL34s in there. And now I could get some adjustment on them. So I put a pair on the other side and everything started getting more reasonable.

So the amp is playing, I'm checking the bias about every minute it seems like.

No flames now for about 30 minutes. I wish I had some confidence in it, I can't even go to the bathroom without checking the bias before I go and peaking out the door looking for flames.

I also got a quad of the Gold Lion KT77s that I can try if this quad lasts a few hundred hours.

The gain seems a little low, and the tone a little thin/bass shy. But it's not flaming.

 

[Kegger]

That sounds good, keep an eye on it as I'm sure you are.
Hopefully everything settles down to a bit more "normal".

 

[Jeffery]

When I was trying to get it up and running, I put the old Ruby 5AR4 in it in case the conflagration returned. This evening, before powering it up, I put the 7 notch Mullard in there and checked the bias before powering up the source.

And it sounds back like it should. I'm only at about 38ma on the outputs instead of 40, but it's back in the range it was before as to output per preamp setting and the bass is back. I don't know if the rectifier had that much influence, or just getting a little time on the new outputs, but it sounds better than it did before it conflagrated. So I got that going for me. Which is nice.