Current Production GZ34

[martin manning]

What if the centre tap fuse blew?

Same result. The reverse biased diode(s) are seeing the end-to-end p-p voltage.

[pdf64]

From http://www.valvewizard.co.uk/bridge.html Merlin's take (perfect storm scenario!) is -
The diodes must also have a Reverse Repetative Maximum (Vrrm) rating that exceeds the peak-to-peak AC voltage (measured from one end to the cente tap), twice the value needed for a bridge rectifier. This is equal to 2.8 × Vrms. A 1N4007 is rated for 1000V. This corresponds to an AC voltage of 1000V/2.8 = 357Vrms. Knock off 10% to allow for variation in mains voltage, plus another 10% for transformer regulation and we are left with about 290Vrms. In other words, we shoudln't use the 1N4007 with anything more than a 290-0-290V transformer.

[martin manning]

0.9 x 0.9 is a very conservative margin for continuous use.

It would be unfortunate if both tube rectifier and the silicon protection diode were to fail short!

But no worse than if there were no silicon back-up.

[tubeswell]

...

I could use higher rated diodes so I only need 1 on each phase and mount them on the valve socket by using the unused pins, but I've heard this shouldn't be done as those unused pins are needed for voltage isolation.

No. The gap between the rectifier socket pins is fine. Lots of guys mount the diodes using the 'unused' socket pins for mounting.

[M Fowler]

I just added diodes to the unused pins on a Dr Z EZG50 for flyback protection from rectifier tube arc plus changed to Sovtek 5AR4 tube since Doc's tech reports problems with JJ GZ34 tubes lately.

Told the owner quit using the standby switch.

6 day straight gig plus numerous other gigs in the past 2 weeks and all is still well with the EZG50, next fix is his reverb pan replacement.

Mark

[pdf64]

I just added diodes to the unused pins on a Dr Z EZG50 for flyback protection from rectifier tube arc plus changed to Sovtek 5AR4 tube since Doc's tech reports problems with JJ GZ34 tubes lately

Good move.
Perhaps if the standby switch didn't have that unfortunate hot switching of the reservoir cap, the rectifiers wouldn't have shorted?
It seems to me that they might look closer to home before blaming JJ.

Told the owner quit using the standby switch

Good advice.
I wired the standby on my friend's amp to disconnect the screen grids, as he insisted he needed the feature.

[Littlewyan]

BYD33V diodes look like they'll do the trick.

I'm thinking of adding a 47K 2W resistor across the standby switch.

[pdf64]

0.9 x 0.9 is a very conservative margin for continuous use.

It would be unfortunate if both tube rectifier and the silicon protection diode were to fail short!

But no worse than if there were no silicon back-up.

I don't think the derating is necessarily applicable to normal continuous use; rather such situations where the amp is flipped on to standby for a while and coincidentally, the mains happens to rise towards its upper limit.
Bear in mind that worldwide PTs are intended for 115 / 230 Vac, so are already running >4% over those at the regular USA / UK mains.

From bitter experience, it is particularly galling when eg a tube short results in a transformer being damaged, whilst the fuse etc intended to protect it resolutely carries on regardless.
Hence if going to the trouble of fitting protective components beyond the bare minimum, it's good to take some care so that they do their job.

BYD33V diodes look like they'll do the trick

Yes, if they are your regular stock type, then fine, but for this application they may be OTT; you'll only get the benefit of their fancy switching characteristics in the event of the tube rectifier shorting.
In normal operation, as they are in series with the tube diodes, the tube's switching characteristics will control the current flow (the tube will be cut off anywhere near the zero crossing point).
So regular slow (& cheap!) diodes can be used without any potential for detriment.
Where were you looking to buy the BYD33V?

[martin manning]

I don't think the derating is necessarily applicable to normal continuous use; rather such situations where the amp is flipped on to standby for a while and coincidentally, the mains happens to rise towards its upper limit.
Bear in mind that worldwide PTs are intended for 115 / 230 Vac, so are already running >4% over those at the regular USA / UK mains.

By continuous use I mean where they are not being used as a safety back-up. I see transformers with 240 as well as 230V mains specs.

[Littlewyan]

My PT is rated for 240VAC I believe. My mains does go up to 242VAC some days.

I only chose those diodes as I saw Fender used them for protection in one of their valve rectified amps and they had the right voltage rating. I was going to look on Ebay. I only have the 1N400* series in my stock.

[pdf64]

By continuous use I mean where they are not being used as a safety back-up. I see transformers with 240 as well as 230V mains specs.

It probably wouldn't be apparent to the user when the tube rectifier shorts and the silicon diodes take over.
It may not even be noticed by a tech, unless they are specifically checking for it.
So the silicon diodes may be handling the rectification function for a prolonged period, possibly years.

Sorry, I should have phrased it as 'MANY worldwide PTs'.

I only chose those diodes as I saw Fender used them for protection in one of their valve rectified amps and they had the right voltage rating. I was going to look on Ebay. I only have the 1N400* series in my stock.

If you've got 4 x 1N4007 then there's no reason not to use them (in series pairs).

[Littlewyan]

The only issue is space. I wanted to keep the diodes fairly close to the pins so it looks neat. If I put 2 x 1N4007s in series they might be a bit too long. I'll look at the weekend. If they are too long then I will probably buy some BY255GP diodes or something similar. I didn't realise the BYD33V diodes were hard to get!

[martin manning]

What is your unloaded HT voltage?

[Littlewyan]

I don't know if I'm honest. Haven't measured it since I built the amp. I'm hoping to get a chance to tinker with it this weekend so I'll measure it then.

[Littlewyan]

I finally measured the amp.

361VAC on both windings when there is no load and 360VAC on both windings when there is load.

I modified it today so there is a 100K resistor across the standby switch. It works great as the HT is 130VDC when the standby switch is off, not enough to make the EL34s conduct so the amp is silent but I think it should be enough to charge the filter caps. I still won't use the standby switch but I wanted to make it a bit fool proof in case anyone borrows the amp. I would put the main filter cap on the Rectifier side of the standby switch but its not an easy mod to make in this amp.

Also I checked and there isn't quite enough room for 2 x 1N4007 diodes in series on the valve rectifier socket, so I'm going to order some BY255GP diodes.