1968 Pro Reverb rectifier flash

[tdlunsfo]

I got a wierd one. Here sits a 68 Pro Reverb AA165 that has been completely redone. All electrolytics changed, new output tubes and three new preamp tubes. New power cord with the obligatory death cap removal and fuse wired to the hot side. The amp came in with a hard short on the standby switch that I traced to the reverb transformer being shorted primary to secondary.. which is rare. The rectifier was a new JJ GZ34 provided by the owner that kept arcing and poping fuses. I assumed it was damaged when the owner attempted to power up the amp with the short. I just threw in another new JJ GZ34 and another flash and fuse pop. Here is what is wierd.. I can throw in one of my vintage 5U4GB or my 5R4GYB and the amp will run and play all day long. Another odd thing is I get a decent pop when taking out of standby (more so than normal but could be a red herring). Note the standby switch ohms out fine. Thoughts or similar experiences?

[Helmholtz]

What is the value of the reservoir cap?

What is B+ (measured before the standby switch) with standby in and out?

What is DC current measured between the lugs of the open standby switch?

[Mick Bailey]

Does the tube arc when the amp is initially powered up, or when only when the amp is taken off standby?

[tdlunsfo]

Only when taken out of standby. Reservoir caps matches the schematic two 70u/350v in series. Give me a few mins on the other readings.

[Leo_Gnardo]

Thoughts or similar experiences?

Sounds like 2 bad rectifiers in a row. FWIW JJ did issue a run of baddies about 10-12 years ago, I thought they had cleared that up by now. In any case the lightning inside the glass isn't lying. Try another brand & better luck.

[tdlunsfo]

Ok GZ34 back in and ramping up slow on the variac. Let me know if you were expecting min /max here.
(measuring on the standby switch)
*511v standby off
*462v standby on
*0mA DC between open standby switch terminals.

[Helmholtz]

*0mA DC between open standby switch terminals.

Not possible with a powered amp.
Make sure the switch is not closed. The switch needs to be in standby position.

[tdlunsfo]

Apparently the mA fuse in my meter was blown. Duh.. I should've caught that. 98mA is the reading.

[Helmholtz]

Apparently the mA fuse in my meter was blown. Duh.. I should've caught that. 98mA is the reading.

All measurements look good - except somewhat high B+ with GZ34. What is the designated rectifier for this model?
A 5U4GB will lower B+ by around 30V.

I assume a GZ34 quality problem.

[tdlunsfo]

Schematic says 440v plate with a GZ34 and I'm sitting at about 456v on the plates with today's line voltage. I may just throw a 5U4 in this thing and call it good.

[garytoosweet]

Are you able to power up the amp without using standby? Just make sure the unloaded voltage doesn't exceed your filter cap rating. (i.e. Off/COLD then flip both switches at once).

This is my experience with JJ GZ34's as well, purchased in the past couple years. They can not handle the inrush current in almost any amp when switching from standby to play mode.

They seem to hold up if the amp is started from cold (i.e. tubes have fully cooled down), without standby, and everything warms up together.

If standby is used and all tubes are hot, they will arc, and blow the fuse and potentially the tube.

They experience failure when someone turns off the amp then decides to turn it back on again before it has fully cooled.

I like JJ tubes, but I no longer buy the GZ34.

You may consider adding diodes before the tube a la RG Keen. Also I have found using Inrush Current Limiters to be helpful.

[garytoosweet]

Schematic says 440v plate with a GZ34 and I'm sitting at about 456v on the plates with today's line voltage. I may just throw a 5U4 in this thing and call it good.

That's an option. Just make sure to re-bias the tubes. Will change the sound/feel of the amp significantly. Could be a good thing if it's what the owner wants.

[tdlunsfo]

Good info. Any GZ34's you recommend?

[Mick Bailey]

Occasionally I've wired a 47K resistor across the standby switch to reduce the surge. 2W or higher is fine. It charges the cap after the choke to around 70v by the time the amp is taken off standby and prolongs the life of the rectifier. I've had better luck with Sovtek than JJ GZ34s.

[Zouto]

Take Sovtek GZ34, they are solid and reliable, I use them for years in different amps without any complaints. Using JJs is just a waste of time, money and resources.