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    Building, troubleshooting and testing of these amplifiers should only be
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Faulty rectifier valve possibly damaged my amplifier - help

Looking for help with my 300B SE amplifier. I recently purchased a NOS GZ34 valve. I put it in my amplifier and on switch on it immediately sparked and I immediately switched the amplifier off.
I replaced the original new GZ34 and checked the amp again and it's working but the transformer seems to buzz more than normal. Other than that I have only had it running for 5 mins and nothing seems to be running hot and no smoke or sparks. I did look order the cover before I plugged the amplifier back in and nothing in the power supply side looks burnt, shorted or melted.
When I run the amplifier normally in the past the transformer gave off a very quiet buzz and I may be paranoid.
I have access to a good multimeter and have worked on kit valve amps before so I can work carefully and safely with high voltages.
Any suggestions on testing to see where the fault may lie please?
 
Which transformer is buzzing? Power transformer or out put transformer? Can you post a schematic for the amp? I am looking at a 300B Single ended schematic but not sure it is the same. Also, was the GZ34 a Mullard tube? Did the new tube blow the fuse?

If pin 4 and pin 6 shorted it could have caused issues with the power transformer but perhaps it did not, and if it did not blow the fuse most likely you have no issue to wory about.

If you have access to a good tube tester that will tell you what shorted that would help to understand what could have happened.

This is a pretty simple direct coupled amp with not too much stuff that can make it buzz. I am assuming two 6SN7 pre amp tubes and one 300B.

Need a bit more info.

I live in the United States in Florida. [email protected] is a good email contact. Be glad to help.

Cheers,

Billy
 
In answer to my replies:
The main transformer not the output transformer(s) is now excessively buzzing. I haven't checked the Limiting Resistors which I installed between the 450-0-450 HT PRIMARY and the GZ34, they were 100 ohm each, I remember I was working to about 125 ohms per plate. The toroidal transformer was rated at .5 amp on the HT and 3A on the heater secondary which was 5 - 0 - 5 v. If memory serves me correctly: The supply is a 40uF polypropylene capacitor rated at 900 v dc (it doesn't look damaged) then a 10H choke then a 80 uF polypropylene capacitor rated at 900v dc.

It's a 12 sn7 input valve and 2 12sn7 driver valves. All the heaters bar the rectifier valve are run off separate transformers.
I didn't install a fuse on the power transformer which I should have done as it is only protected by the plug fuse.
The amp plays fine with no hum at the speakers but the buzz is louder than before.

I have to put the amp upside down to look inside it and this does mean that it balances on the top of the toroidal encapsulated mains transformer. I haven't looked to see if I have disturbed its seating and it may be that it's partially vibrating on the top enclosure plate and thus making more noise than normal....
When I first connected the transformer, last year, I measured the HT on load it was 413v and the heater voltage when on load was 5.3v.
 
Possibly psychological, you observed something untoward when you installed that rectifier tube, and now as a result you are looking for/noticing other things. (Well this is what happens to me.. lol)

I would check the input capacitor just to be sure. Disturbed transformer mounting while investigating? Got scope and meter? Check voltage and ripple.
Could a sign of a less than ideal rectifier tube as well. (ripple amplitude consistent from one peak to the next?)

Good luck!
 

teemumm

Member
2014-04-22 3:18 pm
One thing that can make power transformer buzz is that for some reason only half of the sinewave is rectified. I once had lot of buzzing from the amp. There was two fuses between transformer and rectifying tube anodes. One of the fuses had blown and only one anode was rectifying.

So, measure the voltages of both GZ34 anodes and ground with AC-voltmeter and see if they have equal voltages.
 
Been a while and I have been sidetracked working on my other amp.
I am now back on this one. I have tried two different GZ34 rectifiers and each time I still get transformer buzz that with no music volume I can hear 10 feet away. When I switch on it is completely quiet for about 42 to 45 seconds then the hum ramps up over a second then stabilises.
I haven't checked the limiting resistors to see if they are damaged yet.
The sound quality is great with very very low hum you can just hear with your head to the bass units. After checking the resistors I can only start replacing the power supply capacitors and the choke which is quite depressing...
I can't ignore the hum it's driving me nuts.
 
I suggest you replace the other one as well as it is likely compromised.

As a further thought I now wire a UF4007 in series with each GZ34/5AR4 plate as this greatly increases the PIV rating of the rectifier tube and is effective in preventing warm up arcing in most cases - baring an absolute defect this is what usually kills the GZ34. You can place them right in series with those existing resistors. Still technically functions as a tube rectifier would. (Same warm up, and dynamic characteristics you would expect from a tube rectifier)