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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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22nd Oct 2018, 8:51 am | #1 |
Hexode
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6V6 push pull amp overheating
Hey People
I am refurbishing a very old amplifier, ca 1940s/50s design. I have a pair of 6v6, one 5y3 rectifier + 6SJ7 and 6SL7, stancor power supply, and output transformer. Attached a quick drawn schematic of some of the components and pin outs. This is where I have replaced the two electrolytics. One of the 6V6 gets really really hot and burning/ smoking, also there is a ceramic Ohmite resistor of 300 Ohms connected to (both) 6V6. That resistor is also super hot. The other 6V6 is cold. Not sure what is going on and why the 300 Ohms resistor gets hot. Measures spot on at 305 ohms on my DMM. Huge buzz is coming out of the speaker and distorted sound. What would you advise? Thanks! |
22nd Oct 2018, 8:57 am | #2 |
Moderator
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Re: 6V6 push pull amp overheating
Replace all those 0.05uF capacitors, the black ones with yellow writing on them. 0.047uF is the nearest preferred value.
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22nd Oct 2018, 9:00 am | #3 |
Hexode
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Re: 6V6 push pull amp overheating
Thanks Michael! wooo...i thought these caps are never go bad as these are not electrolytics...but I could be very very wrong indeed...
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22nd Oct 2018, 9:11 am | #4 |
Dekatron
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Re: 6V6 push pull amp overheating
It's the other way around. Electrolytic capacitors can be reformed, most capacitors that use paper treated with oil as a dielectric need to be replaced. Paper capacitors can be in cardboard tubes covered in wax, built into metal tubes or boxes filled with wax or bitumen, or covered in plastic but they all seem to go leaky. The only exception to this is some military grade capacitors that are in a sealed can filled with oil.
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22nd Oct 2018, 9:40 am | #5 |
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Re: 6V6 push pull amp overheating
Swap the 6V6 valves over, does the hot one now become cold? And visa versa.
Measure the voltages on the valves. List them here. |
22nd Oct 2018, 9:45 am | #6 |
Hexode
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Re: 6V6 push pull amp overheating
I had 3 6v6s and all were tested OK...
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22nd Oct 2018, 9:51 am | #7 |
Octode
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Re: 6V6 push pull amp overheating
If one valve is getting excessively hot, it's probably due to a faulty coupling or cathode bias cap.
The smoke is likely coming from a stressed/faulty capacitor or resistor. Personally, I'd replace all of the capacitors, for how little it would cost. Make sure to use the same value and same voltage (or higher). It would also be worth checking any cathode and anode load resistors at the same time incase they've gone faulty. Would also be a good idea to check the 6V6's and other valves if you can to make sure there are no internal shorts in any of them. Also check your power supply is behaving properly and giving stable voltage, proabably of ~270-300V for 6V6's Once all that is done, you've a good chance to have sorted the most likely of faults. Good luck, and work safely! Last edited by whiskas; 22nd Oct 2018 at 10:00 am. |
22nd Oct 2018, 10:02 am | #8 |
Hexode
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Re: 6V6 push pull amp overheating
thanks! all these caps are 600v rated...but doubt that there is 600V there...what would be the lowest voltage I can go with these caps?
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22nd Oct 2018, 10:04 am | #9 |
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Re: 6V6 push pull amp overheating
Some of the soldering looks rough, check that there are no dry joints.
630v and 400v are standard voltage ratings for caps, I would use 630v to be on the safe side, 400v could be too low in some positions. Last edited by Boater Sam; 22nd Oct 2018 at 10:06 am. Reason: Added. |
22nd Oct 2018, 10:18 am | #10 | |
Octode
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Re: 6V6 push pull amp overheating
Quote:
If you have a schematic for the amp, check for the main power supply's output voltage (before smoothing etc). I would measure the voltage at the main rectifier's DC output/first smooth cap. If the voltage there is reasonably under 400V (which for a 6V6 amp is usually around 300V or a bit less) then you should be OK to use 400V caps, but do check this very thoroughly and carefully. Test the DC voltage that each capacitor is loaded with to be doubly sure before replacing with a 400V type. |
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22nd Oct 2018, 10:21 am | #11 |
Dekatron
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Re: 6V6 push pull amp overheating
The cathode bypass capacitor for the output valves looks like it's connected the wrong way round....?
Lawrence. |
22nd Oct 2018, 10:26 am | #12 |
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Re: 6V6 push pull amp overheating
Indeed. The +ve terminal of the cap. should go to the cathode, and the -ve to pin1 (Ground) of the 6V6. If the amp has been run for any length of time with this electrolytic reversed, it may be as well to fit a new capacitor, connected the correct way round.
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22nd Oct 2018, 10:30 am | #13 |
Octode
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Re: 6V6 push pull amp overheating
Absolutely, an electrolytic cap will be distressed greatly if it's been installed backwards. I'd just replace it anyway (observing the polarity carefully) - they cost so little, it's not worth leaving the old cap(s) in place.
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22nd Oct 2018, 10:31 am | #14 |
Hexode
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Location: Budapest, Hungary
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Re: 6V6 push pull amp overheating
Thanks Guys...that cap is connected to the 6SJ7 pin 3 . So that should be the negative or should I not connect it to the 6SJ7 pin 3 at all, but the cathode of the 6v6?
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22nd Oct 2018, 11:09 am | #15 |
Octode
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Re: 6V6 push pull amp overheating
The cathode on a 6SJ7 is pin 5, so to comment accurately about the capacitor placement to the 6SJ7 pin 3, it would be best to see a schematic..
Do you have a model number/maker for the amp? |
22nd Oct 2018, 11:28 am | #16 |
Hexode
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Location: Budapest, Hungary
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Re: 6V6 push pull amp overheating
thanks, that pin 3 on the 6SJ7 is connected to pin 5 and that is connected to ground with a 16kOhms resistor...no schematic tho sadly....and I have tried to follow everything how it was when I got this but I am not aware of who and how did anything on that...
Last edited by Levente; 22nd Oct 2018 at 11:29 am. Reason: missing info |
22nd Oct 2018, 11:30 am | #17 |
Dekatron
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Re: 6V6 push pull amp overheating
You should amend your schematic for the correct polarity.
Lawrence. |
22nd Oct 2018, 11:30 am | #18 |
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Re: 6V6 push pull amp overheating
As this amplifier is of U.S. origin, where the domestic power is usually about 120v, is it being supplied with the correct voltage?
Just a thought. Tony |
22nd Oct 2018, 11:33 am | #19 |
Hexode
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Location: Budapest, Hungary
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Re: 6V6 push pull amp overheating
amended the schematic drawing...
will test it again with another cap and report back... thanks guys |
22nd Oct 2018, 11:34 am | #20 |
Hexode
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Re: 6V6 push pull amp overheating
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