UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Closed Thread
 
Thread Tools
Old 22nd Oct 2018, 8:51 am   #1
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default 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!
Attached Thumbnails
Click image for larger version

Name:	IMG_3262.JPG
Views:	271
Size:	107.4 KB
ID:	171327   Click image for larger version

Name:	IMG_3267.JPG
Views:	278
Size:	158.5 KB
ID:	171328   Click image for larger version

Name:	InkedIMG_3267_LI.jpg
Views:	262
Size:	78.0 KB
ID:	171329  
Attached Files
File Type: pdf 6v6.pdf (14.2 KB, 151 views)
Levente is offline  
Old 22nd Oct 2018, 8:57 am   #2
Michael Maurice
Moderator
 
Michael Maurice's Avatar
 
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,219
Default 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.
__________________
Forum Moderator

http://www.michaelmauricerepairs.co.uk/
Michael Maurice is offline  
Old 22nd Oct 2018, 9:00 am   #3
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default 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...
Levente is offline  
Old 22nd Oct 2018, 9:11 am   #4
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default 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.
PJL is offline  
Old 22nd Oct 2018, 9:40 am   #5
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default 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.
Boater Sam is offline  
Old 22nd Oct 2018, 9:45 am   #6
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default Re: 6V6 push pull amp overheating

I had 3 6v6s and all were tested OK...
Levente is offline  
Old 22nd Oct 2018, 9:51 am   #7
whiskas
Octode
 
whiskas's Avatar
 
Join Date: Mar 2016
Location: Knutsford, Cheshire, UK.
Posts: 1,488
Default 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.
whiskas is offline  
Old 22nd Oct 2018, 10:02 am   #8
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default 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?
Levente is offline  
Old 22nd Oct 2018, 10:04 am   #9
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default 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.
Boater Sam is offline  
Old 22nd Oct 2018, 10:18 am   #10
whiskas
Octode
 
whiskas's Avatar
 
Join Date: Mar 2016
Location: Knutsford, Cheshire, UK.
Posts: 1,488
Default Re: 6V6 push pull amp overheating

Quote:
Originally Posted by Levente View Post
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?
The amp maker may have used 600V caps for inventory convenience when 400V would have done. However we must be very careful and not make any rushed assumptions.
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.
whiskas is offline  
Old 22nd Oct 2018, 10:21 am   #11
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: 6V6 push pull amp overheating

The cathode bypass capacitor for the output valves looks like it's connected the wrong way round....?

Lawrence.
ms660 is offline  
Old 22nd Oct 2018, 10:26 am   #12
'LIVEWIRE?'
Rest in Peace
 
'LIVEWIRE?''s Avatar
 
Join Date: Mar 2006
Location: N.W. Oxfordshire(Chipping Norton)
Posts: 7,306
Default 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.
'LIVEWIRE?' is offline  
Old 22nd Oct 2018, 10:30 am   #13
whiskas
Octode
 
whiskas's Avatar
 
Join Date: Mar 2016
Location: Knutsford, Cheshire, UK.
Posts: 1,488
Default 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.
whiskas is offline  
Old 22nd Oct 2018, 10:31 am   #14
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default 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?
Levente is offline  
Old 22nd Oct 2018, 11:09 am   #15
whiskas
Octode
 
whiskas's Avatar
 
Join Date: Mar 2016
Location: Knutsford, Cheshire, UK.
Posts: 1,488
Default 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?
whiskas is offline  
Old 22nd Oct 2018, 11:28 am   #16
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default 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
Levente is offline  
Old 22nd Oct 2018, 11:30 am   #17
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: 6V6 push pull amp overheating

You should amend your schematic for the correct polarity.

Lawrence.
ms660 is offline  
Old 22nd Oct 2018, 11:30 am   #18
boxdoctor
Rest in Peace
 
Join Date: Sep 2010
Location: Ripley, Derbyshire, UK.
Posts: 785
Default 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
boxdoctor is offline  
Old 22nd Oct 2018, 11:33 am   #19
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default Re: 6V6 push pull amp overheating

amended the schematic drawing...

will test it again with another cap and report back...

thanks guys
Attached Files
File Type: pdf amended 6v6.pdf (14.7 KB, 108 views)
Levente is offline  
Old 22nd Oct 2018, 11:34 am   #20
Levente
Hexode
 
Join Date: Nov 2016
Location: Budapest, Hungary
Posts: 386
Default Re: 6V6 push pull amp overheating

Quote:
Originally Posted by boxdoctor View Post
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
yes, using this with correct voltage, step down to 110 VAC/ 45Va
Levente is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 12:20 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.