SORTED: Dr Z Amp Blowing Fuses - Preamp To Blame. Safe to test??

ICBM

That's fine on the solid-state-rectified amps. The big problem on the valve-rectified ones is that because the standby switch is in the wrong place in the circuit, every time you switch from standby to play, you stress the rectifier valve heavily which can cause it to fail.

To avoid this the switch needs to be after the first filter cap, not before it, so the valve does not have to supply the full charging current for the caps, which will exceed its current rating. This mistake is common in modern valve-rectified amps, Dr. Z isn't the only one by a long way.

If you don't want to modify the amp, the best thing to do is never use the standby switch, just leave it in the 'play' position permanently.

ecc83

"This mistake is common in modern valve-rectified amps, Dr. Z isn't the only one by a long way."

Yes IC and, as is often the case, many mnfctrs pay scant attention to valve ratings and fit far to high a value res' cap'. For the GZ34 that is about 60mfds IIRC . The transformer's 'impedance' should also be taken into account, never is. ICBM do YOU know of any valve rectifier amps with anode stopper resistors?

Dave.

ICBM

ecc83 said:

ICBM do YOU know of any valve rectifier amps with anode stopper resistors?

No, none that I can think of.

RiftAmps

ICBM said:

ecc83 said:

ICBM do YOU know of any valve rectifier amps with anode stopper resistors?

No, none that I can think of.

I add Rlim on a few of my larger amps, but it’s very uncommon to find them these days in gtr amps

jellybelly

So with the help of @SunDevil (thanks mate!!) I think it’s cracked - now just a little idling noise on a par with any valve amp up loud but higher ‘post’ settings are very quiet right up toward the end. Reverb is a bit noisy at high levels but I’ll go back and forth til I find the quietest 12at7 I own. The new rectifier helped but now I’m out about £70 in new valves!

Sounds bloody wonderful but I’m really wondering if it’s the amp for me - I’ve always quite like blackface amps but never really gelled with one. They’re ‘hard’ sounding amps and I can really appreciate why blues players like them. This one holds together better than any I’ve tried and takes pedals brilliantly, but I might have got too use to the slightly spongy softness of my MJW....

hywelg

You can try a different rectifier.  According to DrZ a 5V4 works well in there, a bit more sag in the power supply. When the EZG was in prototype he did run it with a 5U4GB for a very spongy attack. 

Of course you'll have to rebias if you swap to a different rectifier type. 

ICBM

hywelg said:

You can try a different rectifier.  According to DrZ a 5V4 works well in there, a bit more sag in the power supply. When the EZG was in prototype he did run it with a 5U4GB for a very spongy attack.

I would not try a 5V4, unless you're definitely not going to use the standby switch. A 5V4 has a lower current rating than a GZ34, so will be even more likely to fail.

The 5U4 should be OK, or at least no more at risk than the GZ34.

jellybelly

That all seems like a lot of effort! Been enjoying it as a pedal platform both at home and up loud with a drummer etc - just so firm and clear, but I think I’m trying to make it sound like my JTM-a-like...

hywelg

jellybelly said:

That all seems like a lot of effort! Been enjoying it as a pedal platform both at home and up loud with a drummer etc - just so firm and clear, but I think I’m trying to make it sound like my JTM-a-like...

I did too. A Menatone Workingmans Blue does exactly that including the slightly farty bass response!!

jpfamps

ecc83 said:

"This mistake is common in modern valve-rectified amps, Dr. Z isn't the only one by a long way."

Yes IC and, as is often the case, many mnfctrs pay scant attention to valve ratings and fit far to high a value res' cap'. For the GZ34 that is about 60mfds IIRC . The transformer's 'impedance' should also be taken into account, never is. ICBM do YOU know of any valve rectifier amps with anode stopper resistors?

Dave.

As in the Blackstar Artisan!

jpfamps

ICBM said:

ecc83 said:

ICBM do YOU know of any valve rectifier amps with anode stopper resistors?

No, none that I can think of.

I think the early valve rectifier AC50 had one (I assume you mean current limiting resistor; it's not really a "stopper" in accepted sense).

DJH83004

jpfamps said:

ICBM said:

ecc83 said:

ICBM do YOU know of any valve rectifier amps with anode stopper resistors?

No, none that I can think of.

I think the early valve rectifier AC50 had one (I assume you mean current limiting resistor; it's not really a "stopper" in accepted sense).

Quite a lot of early valve rectifier designs did have a dropper resistor straight off the cathode B+ leg, as per the early AC50, but can't say I have seen designs with resistors in-between the anodes and the tx HT secondary, (which I think must be the anode stopper?) 

jpfamps

DJH83004 said:

jpfamps said:

ICBM said:

ecc83 said:

ICBM do YOU know of any valve rectifier amps with anode stopper resistors?

No, none that I can think of.

I think the early valve rectifier AC50 had one (I assume you mean current limiting resistor; it's not really a "stopper" in accepted sense).

Quite a lot of early valve rectifier designs did have a dropper resistor straight off the cathode B+ leg, as per the early AC50, but can't say I have seen designs with resistors in-between the anodes and the tx HT secondary, (which I think must be the anode stopper?) 

They are both current limiting resistors to prevent the ripple current exceeding the maximum allowed.

Doesn't matter whether you put them before or after the rectifier.

Technically a stopper is to prevent oscillation.

DJH83004

Appreciate a grid 'stopper' is used to reduce Miller effect oscillations, but never heard it referenced to rectifiers.