Found the source of the buzzing noise finally !!

[stevesuk]

I recently built an 18w lite II and although it has a nice 18w sound, it had a loud buzz which increased with the volume control. It made it unuseable.
I tried all the usual suspects, rerouting heater and AC wiring, ripping out all the earths and putting in a bus bar, changed the valves, changed the OT connections around, changed the heater connections to the valves.
AND THEN after several weeks and three hours spent today I found I had put the two 100K balancing resistors on the EZ81 heater supply instead of the other valves, swopped them around and instant cure. Now i have to redo all the temporary lash ups and try outs. That one error has taken me longer to find than it took to build !! that is one more mistake I won't make again.

[loverocker]

Steve, I love it when there's a happy ending I got your email and was going to undo my WEM conversion to check the OT wires to see if there was a clue there.

Thing is... I don't think the WEM had 100r (not 100K, shirley?) balancing resistors on *either* heater supply... I'll have to check. I deliberately didn't touch anything other than the signal path of the WEM amp, so I didn't put any 100r in, and its hum/buzz is no louder than anything else I've got. Weird...

Wonder - if I put a pair in, will the amp get even quieter Or should I just count my blessings and leave it alone.

[stevesuk]

Barry, you are right, I used 100 ohm not 100K balancing resistors to make a dummy centre tap as I used an old oscilloscope PT which didn't have a centre tap for the 6.3V heater supplies.
The WEM OT works just fine, very loud with a G12H. I am going to make a mini marshall type box for it tomorrow, 12" deep X 7" wide and 7" high. With all the ' remedies ' I fitted like shielded input cable, star earthing, bus bar etc. it is now very, very, quiet. So perhaps it was worth the effort.

[Benboom]

Barry, you are right, I used 100 ohm not 100K balancing resistors to make a dummy centre tap as I used an old oscilloscope PT which didn't have a centre tap for the 6.3V heater supplies.

I don't know anythng about oscilloscopes; did you mean you used an oscilloscope PT for the AMP's power supply tranny or the HEATER power supply? Just curious.

[stevesuk]

Both, most valve equipment has the same requirements, Now I think about it, it actually came from a signal generator. Normally a valve oscilloscope has a very high voltage transformer for the cathode ray tube, like a TV around 1000 + volts. I have a scope PT with 1250 volts and every other tap you could ever want, bit risky in a guitar amp.
The sig gen PT I used has 300v 0 300V and two 6.3V heater taps, unfortunately not centre tapped. This is why I made the dummy centre tap with two 100 ohm resistors, unfortunately putting them on the EZ81 tap by mistake. From what I have learnt making 18 watters, JTMs, 100 watters, Fenders etc. Most valve gear, old test equipment, hi fi, radios etc. yield some quality Mullards, PTs, valve bases, nuts, bolts, brackets, even chassis' some times.