• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Leak Stereo 20 hum and buzz problem

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi all,

After hearing a mates stereo valve amp i thought gotta get me one of those,
enter 1 leak stereo 20. Over the last few months all caps have been replaced along with all out of spec resistors(as a newbie this was an easy intro into servicing and understanding the basics of how valve amps work) anyway all was working well untill the other evening. The amp was playing for a few hours, i then decided to swap the interconnects for a pair that a friend had given me, when i turned the amp on a hum had developed on the left channel
swapped the interconnects back but still the hum??
Now the hum(and a slight buzz) is there with nothing connected to the inputs also with them connected.
When power is turned off the hum stops imediatley(it does not decay)
It also is effected by moving my hand near to inputs, also if i move near to V1.
I have inspected for bad solder joints by probing with a plastic probe all seems ok but when i probe the the positive wire and asociated resistors( 1meg/22k) at the left channel rca a thud can be heard through the speakers almost like microphonics??
I have tried different valves to no avail.
I have replaced the offending resistors,
I cannot understand why a change of connects would cause this and i suppose i could take it to a repair man but i would learn nothing from this, so have come here for a few pointers into tracking this problem down.
many thanks
have now replaced rca sockets
all the previous symptoms are still there, except now when probing to find dry joints etc the wire that goes to pin 3 on the preamp socket (B+) now becomes microphonic. ie. heard through speakers when tapped?? no noise at the resistors now, hum and buzz increase when i move hand near said socket.
The circuit is original so no extra earths have been added/removed,

I have a Stereo 20 which had similar refurbishments and upgrades over a period of time and there's no hum at all. Have you replaced the rca sockets at the left rear as these are devils to replace due to limited workspace. Are they correctly isolated and are the fasteners tightened firmly?

Also, are you running earthed ac to the amp?

You could try wiggling the valves around with the unit on- your input valve might have a bad connection. You could also try swapping a couple of the 12AX7s around in case it's a bad valve.

Otherwise sounds like you have a dodgy solder joint- I'd try quickly reflowing the connections at the input socket and first valve socket.
Hi M Gregg

I think mine is the later model (grey chassis) the same schematic with R12/13 470k
C5 0.1 also R5 has a 300pf cap in paralell.
I am researching the causes of this problem and can confidently take measurements, solder etc so any suggestions /advise would be welcomed.

bulgin hi,

both phono jacks have been replaced, the speaker terminals also they have teflon spacers to isolate them and tight.
Yes the mains plug is earthed.
Mine was sounding good before i changed the interconnects, i do not think this caused the hum, i think something is breaking down (have reflowed all solder joints)
I think its time to get the dmm out, but having a plan of attack is taking some time.

regards DM
Disabled Account
Joined 2010
If your amp is close to the schematic I posted.

I cannot stress enough make sure the HT is dead!

We will compare left channel with the right!

Power on Be careful! Speakers must be connected any time power is on!

So on the first valve V1 check the voltage on the cathode each side top of R3 to ground. Tell us the value.

Power OFF and Discharge HT.

Disconnect speakers.

You will notice that on the bottom of R3 is a link called "R" this goes to the link on the output transformer 16 Ohm tap. This is the feedback. If this connection is broken your amp will have increased gain!

With the power off and discharged HT measure the resistance from 16 ohm tap to R3 on each side.Tell us the value!

M. Gregg
Disabled Account
Joined 2010
Power on speakers connected or load matching the speakers.
Voltage test!

If you can use a load resistor for this test then please use one! You may get a loud buzz!

Voltage pin 1 & pin 6 anode to ground.
Tell us the value!
Last edited:
Hello M Greg

thankyou for taking the time to post very grateful for any help on this.

my circuit has some component values that are different from any leak 20 ive come across, the amp was working fine for months.
In the feedback circuit the values are R3 1K5, R18 18K also C9 is 68pf. R2 is 5K6.
have taken some measurements with no shocks, smoke etc....

R3L to 16ohm tap= 1k5/ 100ohms (both sides of resistor measured)

R3R to 16ohm tap= 1k6/ 100ohms

Pin 2 to ground 1M

Pin 7 to ground 1M

Pin 1 V1 = 149v

Pin 6 V1 = 145v

cathode voltages = 1.7v & 1.8v

have checked all earths less than 0.5ohms everywhere.

thanks in advance
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.