Bias Voltage Source with GZ34

[dorrisant]

When using a GZ34 rectifier, is it possible to tap the AC going to pin 4 or 6 for a bias supply? Am I just pissing up a rope trying to do so? Space is limited and the PT didn't have a bias tap. I was going to do the usual Marshall trick but getting no bias DC. I also tried tapping the HT with a class X cap and then onto the bias circuit a la Marshall as well. Neither seems to generate any DC voltage at all.

I think I could use a tiny toroidal PT to get the bias AC, although that will be an exercise in cramming 10 lbs of s#@* in a 5 lb sack. I can do it that way but want to verify that I have painted myself into this corner before I try ordering any more parts.

If there is any reading material available that explains what the rules are surrounding this situation, please let me know. I have Slukey's Amp Scrapbook (p6), but I'm looking for a little more depth into the the reasons why.

Thanks in advance!!

[martin manning]

I don't know why the "Typical Marshall" circuit wouldn't work. You have a FW rectifier set-up with a grounded CT, right?

[pompeiisneaks]

That's sluckey's amp scrapbook, he could likely give you an exact answer if you give an explanation and details of the transformer. You can almost always pull a feed of the secondary ac windings and peel off into a half wave rectifier to get the negative bias.

~Phil

[sluckey]

That typical Marshall circuit is the one to use with a GZ34. It needs no explaining. It's very simple. If it doesn't work for you then you have a wiring error or bad diode or caps.

EDIT... BTW, you need to be looking at page 5. There is nothing on page 6 that applies to your situation.

[dorrisant]

I swapped the tap over to pin 4 of the rectifier socket rather than 6, tried it again. All good now. Had to have been a connection issue.

Thanks for the direction to all.

[M Fowler]

Advice from Dr Z to me last fall was dump the JJ GZ34's and use Sovtek 5AR4's and that was a good tip.

Just burnt up a new JJ GZ34 in a new JTM45 KT66 build while the owner was on a gig. Oh how embarrassing for me, he was a good sport about it.

Mark

[pdf64]

Perhaps the JJ would have had a long operational life if it hadn’t been used in an unfortunate hot switching standby arrangement (for reasons unknown DrZ amps use that too)?
It seems unfair to give JJ a bad rep because they break if thrown at a brick wall.

[M Fowler]

Throw them all at that brick wall

Most of my JJ GZ34 were not hot switched they failed while the amp was being played.
Replace with Sovtek and the amps are still operating at gigs.
I did check all grounds and solder joints as well as power tube bias etc. in these amps. I wasn't just merely blaming JJ tubes without full inspection of the amps.
In the last two weeks there were 3 fails.

Mark

[didit]

As I reported in another thread a while ago, after a few frustrating capricious failures I too have given up on JJ for GZ34. Too fragile compared with available alternatives.

Best .. Ian

[pdf64]

sorry for jumping to the wrong conclusion, the mention of JTM45, DrZ and rectifier tube failure set the ‘hot switching’ bee in my bonnet a buzzing.

[Colossal]

I have had good results with the Sovtek GZ34/5A94. Quite similar sound and feel to a Mullard too at 1/10th the cost. The old Mullards are the standard though.