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Hi Again All,
Seems I have managed to get myself into another head scratcher (my Ampeg VT-120 that some of you assisted with is still with a second amp builder trying to figure out where the hum is coming from!).
I bought an early 60s Vadis 408R a couple of months ago. These were point to point amplifiers hand built in Sydney in the 60’s and very much based on Fender designs of the day. This one has two channels (“Normal” and “effects”) with dual inputs, tube driven tremelo and tube driven reverb and sounds great.
It has been serviced / rebuilt at some point recently (not by me) and all electrolytics and a lot of the original mustard caps have been replaced throughout. The work looks decent enough and voltages all seem in the ball park across the board.
Now to the issue - even with volumes down there is still sound (input signal) coming through the speakers. It is quite loud (ie approximately equivalent to how loud it would be with volume on half). It does this on all inputs on both channels. The grounding scheme looks stock and whilst not perfect I have read elsewhere that these amps are usually surprisingly quiet. Ive tested the volume pots with my multimeter and both go to zero ohms when turned down and make good connection to ground.
In an attempt to fault find I have disconnected the tone stacks, the reverb and the tremelo, however the issue persists. I tracked down the attached schematic of the preamp (note Vadis and Electrovox were One and the same, just different retailers and distributors at the time) and the red lines indicate where I have disconnected. If I disconnect the dry signal from the preamp to the phase inverter the sound stops so guessing the issue is emanating from Preamp (V1 or V2) or maybe phase inverter.
In comparing the preamp to the schematic everything seems to be wired ok, although I obviously might be missing something as gradually going cross eyed after staring at this for some
time.. I can’t see how with the disconnections I have made, that input signal audio is still getting through the second half of each triode in V1 and V2 and ultimately getting to the phase inverter and power amp.
I’m aware of instances where audio has been transferred through chassis / ground but what is the best way to test for this and would it likely be this loud?
Also aware that lead dress can cause induction and I have spent a bit of time poking around inside moving various signal and power connections to ensure things are paralleling or crossing unnecessarily to no avail.
So have pretty much exhausted the extent of my tube amp / electronics knowledge and need some assistance. If I can’t resolve she will be heading off to the amp tech.
As always, appreciate any thoughts / advice you might have.
Cheers
Paul
Seems I have managed to get myself into another head scratcher (my Ampeg VT-120 that some of you assisted with is still with a second amp builder trying to figure out where the hum is coming from!).
I bought an early 60s Vadis 408R a couple of months ago. These were point to point amplifiers hand built in Sydney in the 60’s and very much based on Fender designs of the day. This one has two channels (“Normal” and “effects”) with dual inputs, tube driven tremelo and tube driven reverb and sounds great.
It has been serviced / rebuilt at some point recently (not by me) and all electrolytics and a lot of the original mustard caps have been replaced throughout. The work looks decent enough and voltages all seem in the ball park across the board.
Now to the issue - even with volumes down there is still sound (input signal) coming through the speakers. It is quite loud (ie approximately equivalent to how loud it would be with volume on half). It does this on all inputs on both channels. The grounding scheme looks stock and whilst not perfect I have read elsewhere that these amps are usually surprisingly quiet. Ive tested the volume pots with my multimeter and both go to zero ohms when turned down and make good connection to ground.
In an attempt to fault find I have disconnected the tone stacks, the reverb and the tremelo, however the issue persists. I tracked down the attached schematic of the preamp (note Vadis and Electrovox were One and the same, just different retailers and distributors at the time) and the red lines indicate where I have disconnected. If I disconnect the dry signal from the preamp to the phase inverter the sound stops so guessing the issue is emanating from Preamp (V1 or V2) or maybe phase inverter.
In comparing the preamp to the schematic everything seems to be wired ok, although I obviously might be missing something as gradually going cross eyed after staring at this for some
time.. I can’t see how with the disconnections I have made, that input signal audio is still getting through the second half of each triode in V1 and V2 and ultimately getting to the phase inverter and power amp.
I’m aware of instances where audio has been transferred through chassis / ground but what is the best way to test for this and would it likely be this loud?
Also aware that lead dress can cause induction and I have spent a bit of time poking around inside moving various signal and power connections to ensure things are paralleling or crossing unnecessarily to no avail.
So have pretty much exhausted the extent of my tube amp / electronics knowledge and need some assistance. If I can’t resolve she will be heading off to the amp tech.
As always, appreciate any thoughts / advice you might have.
Cheers
Paul
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