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Leak Stereo 20

Is the hum from one channel or both?
If from one channel swap the EL84s side to side. Does that change anything? If not mark / number the ECC83s and move them round as well, one step at a time. Does that make any difference?
If the hum is equal both sides, chances are the main smoothing caps are in need of replacement.
If you have a multi-meter you could do voltage checks.
 
snowman_al said:
Is the hum from one channel or both?
If from one channel swap the EL84s side to side. Does that change anything? If not mark / number the ECC83s and move them round as well, one step at a time. Does that make any difference?
If the hum is equal both sides, chances are the main smoothing caps are in need of replacement.
If you have a multi-meter you could do voltage checks.
Click to expand...
The hum is equal from both channels, will try swapping the valves around.
The input ECC83 is a vintage Mullard, the others are vintage Mazdas, I do have an EH ECC83 I could swap with the Mullard for now to see if it makes a difference, will report back.
TS
 
Just swapped out the Mullard for the EH & moved the Mazdas L to R, also swapped the EL84Ss L to R & reseated the GZ34. Also tried it with sources disconnected. Made no difference.
TS
 
These F&T 32/32uF from HiFiCollective are great. Fit perfectly and are very reliable, they are what most folk end up with and are favoured by most guitar amp folk too. Bet they fix the hum!

PS You’ll be sent nicer looking black ones with gold text, so not as jarring sticking through the top, but they can be peeled and painted if preferred (I did with mine).
 
As the hum is equal, the prime suspect(s) are the smoothing caps in the power supply which is common to both channels.
If you have a meter measure the HT voltages.
 
Thanks for the replies. I do have a DMM but will need a bit of guidance on where to point the probes to check the voltages.
Now that I have expunged any thoughts about selling the amp I would like to replace the smoothing caps anyway & possibly even swap out the PP caps for some of those Russian military types.
TS
 
Here you go, negative lead on the chassis, the bolt thread is ideal.
Meter on 1000 volts DC and measure 5, 6, 7 and 8. Tell us what you get.
Also tell us which mains transformer you have. There should be a white screen print or label number on it. (The picture has an 8162 for example.)

Remember Do Not run the amplifier without speakers or a dummy load.



 
FWIW John’s Stereo 20 is actually lying. That’s a 8162R 280V transformer, not the 300V 8162. To my knowledge absolutely all grey S20s have the R version, plus you can identify it by a change in the post locations where the mains connects.

Here’s my 1961 bronze one for comparison:

20463132979_707618b4a6_b.jpg


Note the change of blue and orange wire positions.
 
I think you might find it is 300- 0-300 volts.
Easy to check. Meter on 750 or 1000 volts AC . Black lead on the chassis / cap bolt thread. Then red lead / probe to pins 4 and 6 (the White wires).
Let us know. Alan

 
I would like to fit a spark suppressor to my amp as well while I've got the soldering iron out. I am planning on removing the on/off switch on the rear panel (I have a switched mains block) so advice on the best cap to buy (preferably from the hifi collective site so I can order it at the same time as the F&T caps) & the best place to fit it would be appreciated.

Thanks again

TS
 
Thanks Alan, would I be correct in assuming this value cap would also be OK to put across the mains switch in my upcoming Lenco PTP build? The info on the LencoHeaven site recommends 3900pF 400v.

Cheers

TS
 
snowman_al said:
I think you might find it is 300- 0-300 volts.
Easy to check. Meter on 750 or 1000 volts AC . Black lead on the chassis / cap bolt thread. Then red lead / probe to pins 4 and 6 (the White wires).
Let us know. Alan

Click to expand...
I've taken the readings
1st off the AC voltages, pin 4 - 275.5v, pin 6 - 276v. Interestingly the hum increased slightly when I put the probe on pin 6 (I assume were going clockwise).

The DC voltages you asked me to check are;
Pos 1 - 11.06v
Pos 2 - 10.95v
Pos 3 - 11.18v
Pos 4 - 11.14v
Pos 5 - 314.6v
Pos 6 - 297v
Pos 7 - 277.3v
Pos 8 - 215.6v
 
They are pretty close. 1 to 4 are fine.
Interesting you get just a little lower than 280 volts AC to the rectifier, perhaps your mains is a little lower too.
Your DC voltages (5, 6, 7, 8) are all 10 or 15 volts low compared to the service manual. That and the hum would suggest the caps have lost their capacity and are a little leaky too. Not surprising at their age.
You are going to replace them, that should get the voltages back to spec. (5 = 325, 6 = 310, 7 = 295 and 8 = 220 volts.)
If they are still low after cap replacement, then the GZ34 becomes suspect.
The previous work, from the picture, is spot on. It looks like the only things not replaced were the smoothing caps (C10, 11, 12 and 13). Get them in place before considering going to the Russian coupling caps.

Re the Lenco switch; really best to ask at Lenco Heaven. Without knowing what sort of load it is switching etc., it would be guess work to advise. Alan
 
Thanks for that Alan, I have ordered the F&T caps off hifi collective, really hope it's not the GZ34, NOS Mullards are silly prices these days!
Hopefully will be able to fit the caps next weekend, will report back.
Once again, thanks for the help.
TS
 
Given the crazy price of NOS Mullard GZ34s, what are peoples thoughts about the currently available brands, Sovtec, JJ etc?
Would just like to get some opinions just in case the fault with my amp turns out to be the rectifier.
Thanks
TS
 
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