Leak Point One

Indeed without the schematic is hard to give you any suggestion. First of all, what you mean with "blew"? Does the filament on the GZ34 burn open? In this case I'd first determine if the heater voltage on the GZ34 socket is 5V or not. Then check the AC voltages on the two anodes pin on the socket, it should be no more than 450V (rms) from each anode pin to common ground, probably less. If you don't feel confident with high voltages, bring the amp to someone who knows how to deal with it, it could be very dangerous.

Francesco

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A schematic of the Leak Point One can be found here

It may or may not be the same as your amp since there were a few iterations.

Some references

I hope I am not understanding you correctly... "I bridged and replaced some capacitors)... One should _NEVER_ bridge a cap if the goal is to replace a cap that has failed. The old cap *MUST* be removed from the circuit and the new cap put in its place.

If the goal is to add additional capacitance, it is much better practice to remove the 'small' cap and replace it with a new 'larger' cap. First, after 20 years the existing electrolytics may be tired, and second, modern caps are typically much smaller than their older cousins such that space is not an issue. Parallel caps _of the same type and value_ *are* a legitimate and often used practice, however.

If you are replacing non-electrolytics, the same general logic obtains.

In some circuits, excessive capacitance can cause excessive B+, so be careful there.

It is poor and risky practice to run a system with the rectifier in place and not the load... that is, remove all the tubes but the rectifier.

Lastly (and long): The 5AR4/GZ34 is a very rugged tube unless you are using some of the Chinese versions. Those tend to fail, often spectacularly and often also taking much else with them. You have not supplied and apparently do not have a schematic. Therefore you probably do not know the correct voltages that should be coming off the transformer and also what the correct B+ should be. Verify that there is no more than 5V at the filament pins on the 5AR4. After that, check all your work _VERY_ carefully. Polarity of the caps, solder- blobs, poor connections, and so forth. Apply power through an ammeter, preferably through a variac. Typically a tube rectifier will not pass DC until it reaches ~70%+ of filament voltage, and the 5AR4 is more like 75-80%. But when the needle on the ammeter starts to rise, you will be able to learn much.

With the absolute final caveat that you are working in the dark and without a schematic. So everything you are learning is more-or-less guesswork.

Peter Wieck Wyncote, PA

There is usually a maximum allowable reservoir capacitance stated in rectifier tube data sheets, exceeding this value can strip the cathode as heavy charging current is drawn before a full space-charge has developed at switch on from cold.

Thanks a lot already for your reply but I forgot to say it's a Stereo

20 with the 2 EL84 and ECC83 and a varislope stereo pre-amp. I did bridge some capacitors on the lf side to make it sound better (although the original cap's sound already really good!) and I had a hum in of the channels so I replaced one of the rectifier cap's and then it dissapeared (worked for 20 years). Btb I'm a television/audio repairman but not familiar with tubes. Oh and the first the GZ34 went white and the second time I saw a spark when starting up and then nothing worked anymore. Doesn't seem to have done more damage than that. I'll check the voltages and will be back. Are there also schematics for the stereo 20? Would be great, thanks. Derk-Jan

Going white suggests that the getter deposit has been exposed to air (cracked glass!) other than mechanical damage before or during fitting (bent pins etc) - this would suggest a very severe thermal shock, such as might be caused by a dead short on the HT rail.

Certainly the tube lost its vacuum. I hate to harp, but Chinese 5AR4/ GZ34s tend to fail either that way, or through internal melt-down.

Again, unless the OP has a schematic or finds one, much of the troubleshooting at this point will depend on some basic instruments and some experience in what the readings mean. And here goes, climbing swiftly out on a limb:

4 x EL84/6BQ5 = ~ 3.5A comes to ~22 watts 2 x 12AX7/ECC83 (run at 6.3V) = ~0.6A comes to ~38 watts 1 x 5AR4/GZ34 = ~2A comes to 10 watts

Altogether, comes to ~ 70 watts, then losses, figure your quiescent power use will be something around 75 - 90 watts. I would expect about

80 watts, but that is a blind SWAG guess.

With a good current meter and a variac you should be able to run the current up to the point where the rectifier *just* begins to pass DC, and see what happens on the current meter. If it starts to climb anything above 90 watts, or so, then you have a problem that requires further attention before you go any further. And at 90 watts, be exceedingly careful and look for hot-spots, transformer leakage and so forth.

Note: when a strange piece of equipment crosses my bench for the first time untested, usually I make the calculation above and then see the actual current draw (vs. expected draw) as my very first pass at diagnosis. It can be very helpful.

And, looking at the schematic that Ross offered, I see a couple of places where a shorted small-value cap could create the precise dead- short situation that Ian mentions. I know it is not the same as yours, but it does appear pretty typical of the species. Check them all.

Note also that _VERY_ high voltages exist inside tube equipment. Do not become a part of them. Be careful and treat the amp with great respect at all times.

Peter Wieck Wyncote, PA