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?
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. planobillyfl@gmail.com is a good email contact. Be glad to help.
Cheers,
Billy
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. planobillyfl@gmail.com is a good email contact. Be glad to help.
Cheers,
Billy
I think you will find the transformer is quite robust. I wonder why the NOS GZ34 sparked ... mystery without testing it. I doubt any damage has been done as the GZ34 is the weakest link.
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.
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.
Try to take off first GZ34, put back it and disconnect first or second el. caps. Some times buzzzing is shorting in PSU.
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!
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!
And to amplify that, *do* check your limiting resistors. It's easy to imagine a toroidal power transformer complaining about unbalanced current flows.
All good fortune,
Chris
Thanks for all the helpful replies. I will come back with an update as soon as I have done the investigations I can with the test equipment I have available.
I have recently moved home so I still haven't unpacked all the man-cave gear yet...
I have recently moved home so I still haven't unpacked all the man-cave gear yet...
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.
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 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.
are you using a steel chassis? a picture of your amp can help.....
if aluminum, then the transformer can buzz, check that the screws are as tight as they can be..
if aluminum, then the transformer can buzz, check that the screws are as tight as they can be..
Thanks to everyone for all your input. I finally got round to checking the limiting resistors and one was an open circuit so I duly replaced it and now no buzz from the transformer. So I'm now pretty sure the faulty GZ34 had blown the resistor.
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)
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)
Thanks for the further advice and I did replace both resistors as I too thought the other one may be damaged. Once again thanks to everyone.
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