What kills a valve rectifier?

I've had a number of GZ34s fail short circuit, or suffer from serious flashover, and knock out fuses over the years, but I've always put this down to saggy filaments or whatever, where the valves are mounted horizontally, or upside down.

Arfa

"Nutcase Kook "

..... Phil

failed

end

make/break

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down

What is the state of the fuse with a flashover? vapourised and blackened fuse barrel presumably.

Yes, vapourised

Arfa

If I pop a new valve back in, take some measurements and all seem respectable, I don't try to dive further in and create scenarios in my head as to the failure mode. UNLESS it comes back a few hours of usage later with the same failure. However, after it is placed in a cabinet I stress test all amps into a non-reactive 300 wrms 8 ohm resistor, one of two I have mounted on a large heat sink from a CNC power supply. I use a 400 hz tone and monitor the output adjusting the output until I see just a taste of crossover distortion. I leave it this way until I can smell and feel the amplifier/ output transformer, choke, mains transformer etc get more hot than under normal use. Then it's tested with a guitar and cabinet if it's just a head. I'm a musician so I have those things laying around. I usually use my 4x12 Peavey cabinet for amps that aren't combos. And a Fender 4x10 for Bass. I know what the instruments should sound like though both cabinets with good amps.

What do you think about subbing the 5AR4 with a 5U4?

Never tried it, but looking at the specs, they look rather different from one another, see -

and

The 5U4 appears to be able to handle more current, but at a lower max plate voltage. Also, the heater requires substantially more current. Might be an issue for the transformer supplying it ?

Arfa

"Arfa Daily"

The big difference between the 5AR4 and the 5U4 is in amount of voltage drop across each diode when conducting.

The 5AR4 is uses an indirectly heated cathode, close coupled to each plate and the voltage drop is very low, about 35 volts at 700mA peak.

The 5U4 uses a directly heated cathode with lotsa space between the plates and the cathodes - so there is a much bigger drop in voltage, more like

So, if you drop a 5U4 in place of a 5AR4 - expect to lose 60 to 70 volts DC straight off the B+.

Also, the 5AR4 is rated to work reliably with a large filter cap on the cathodes ( 60 uF) compared to the 5U4 ( 40uF).

So the proposed sub is bad news all around.

..... Phil

om

rop

like

DC

Considering that the vacuum tube rectifier was the first tube to be eliminated (in receiving sets at least), why not replace it with a semiconductor diode circuit? What effect would it have on the amplifier's sound?

"spamtrap1888"

Considering that the vacuum tube rectifier was the first tube to be eliminated (in receiving sets at least), why not replace it with a semiconductor diode circuit? What effect would it have on the amplifier's sound?

But that is what these loonies believe.

.... Phil

from

drop

DC

Considering that the vacuum tube rectifier was the first tube to be eliminated (in receiving sets at least), why not replace it with a semiconductor diode circuit? What effect would it have on the amplifier's sound?

I totally agree a couple of 1N4007 and sag Rs would be fine by me but we are talking musos here and the "valve sound" would magically dissappear. Of course if I told them nothing about swapping to SS, the valve sound would not disappear - strange old world isn't it.

"Nutcase Kook"

he dud GZ34 can stay for ornamental value.

Tell the owner you fixed it.

BTW:

I fix lots of octal a valves, for bad soldering on the pins and flash burns between plate and heater pins - even if it means removing the octal base to do so.

Well worth it with newish 6550s, KT88s etc.

.... Phil

I'd go for 2x 4007's in series to be on the safe side - not forgetting equal value x-class capacitors in parallel with each diode to damp & equalise any spikes.

As well as an added sag resistor, you need to make sure each rectifier path is protected by a fuse, when silicon rectifiers fail short circuit - they do it for keeps.

"Ian Field"

Class X caps are more likely to fail than 1N4007 diodes are.

The AC supply fuse will blow immediately.

You are obviously not very familiar with valve guitar amps.

.... Phil

volts

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do

As for sag R I tried 50R and 100R and a couple of GZ34 on Avo CT160 tester set for 120mA per anode. each GZ34 showed the usual 75 percent "goodness" .

"Arfa Daily"

Sooo, the cathode is a solid tube of metal, with a heater wire inside it.

Ergo - it simply cannot sag.

Only directly heated cathode types do that

- like 5Y3s, 5U4s, 5AS4s etc.

.... Phil

Agreed, but I did say "or whatever". I've not taken that much notice of the actual failure mechanism. I think you can spend too much time in a commercial world, agonising over exactly what went wrong. I have a roof to keep over my head, and I don't have time to analyze how every fault occurred, and why and how every component expired in the way it did, interesting as that may be.

Suffice to say that in 'modern' amps that use a GZ34, I have seen more than a few with a vapourised fuse and no obvious short causing it. Often, a replacement fuse holds up, until you clout the valve with the butt end of a screwdriver, whereupon a miniature fireworks display is launched inside, and the fuse goes out again. Sometimes, it's spontaneous, and only needs the voltage to be present. More often than not, it seems to occur when the valves are not upright. This would imply that gravity acting on the structure has something to do with it. But what is touching what or why - I really don't care. Popped fuse - new fuse - clouted valve - fireworks - new valve - new fuse - soak test - write bill - phone customer. Job done. Cash in bank.

Arfa

"Arfa Daily"

Cos failure mechanisms that are not dealt with by the service tech are bound to be repeated.

Owners of same can operate the amp any damn way up they like - and do.

Works a treat in Quad Mk2s, long as you get rid of those dreadful oil filled filter caps.

... Phil

So no thoughts on generator use , followed by rectifier failure.

It seems the mains fuse I have here was not necessarily the original one. Someone looked at the amp and replaced the fuse, probably with this probably previously mechanically broken one (knocked about in a toolbopx). The original may not have failed but assumed to have fused and so just rectifier failure. Could generator brownout or spikes or something weaken a valve rectifier so it is ok while warm but gives up the ghost on the next powerup from cold?