Vadis / Electrovox 408R Volume / Channel Bleed

[q9522678]

That existing 22uf is the decoupling/filter cap for the C+ node. You don't need another one.

Oh right - if I remove the 10UF I’ve soldered in the volume bleed issue comes back. With it in place there is no volume bleed. Does this mean the 22uf is not high enough and I should just increase that

At present I have 22UF -> dropping resistor -> 10UF ->plate load resistors (x4)

[The Dude]

If I understand you correctly, channel 2 sounds fine with v5 out, but when it's installed sound is thin/distorted. If that's correct, I'd start with this:

What is the cathode voltage of V2b with V5 in?
What is the cathode voltage of V2b with V5 out?

[The Dude]

Oh right - if I remove the 10UF I’ve soldered in the volume bleed issue comes back. With it in place there is no volume bleed. Does this mean the 22uf is not high enough and I should just increase that

At present I have 22UF -> dropping resistor -> 10UF ->plate load resistors (x4)

No. You need a separate filter/decoupling cap on each node. In other words, you do need to add one to the 186V node (since there wasn't one), but there is already one for the C+ node. I believe you have that part sorted. You can leave it alone.

[q9522678]

If I understand you correctly, channel 2 sounds fine with v5 out, but when it's installed sound is thin/distorted. If that's correct, I'd start with this:

What is the cathode voltage of V2b with V5 in?
What is the cathode voltage of V2b with V5 out?

Yep, that’s exactly what’s happening,

Cathode voltage of V2B with V5 in is 2.03v with tremolo controls at zero and oscillates between 1.6v and 3.5v with intensity at Max and speed at min.

Cathode voltage of V2B with V5 out 0.4v

[q9522678]

Yep, that’s exactly what’s happening,

Cathode voltage of V2B with V5 in is 2.03v with tremolo controls at zero and oscillates between 1.6v and 3.5v with intensity at Max and speed at min.

Cathode voltage of V2B with V5 out 0.4v

So I think I’ve solved this one. The voltage on V2A seemed a little low. Not by much.. about 10v. But played with some resistor values to bring voltage on pin 1 of V2A up 110v and problem gone away. Channel 1 and 2 now same volume and channel 2 sounds loud and proud even with tremolo turned up. Let the amp run for a couple of hours with audio playing through and seemed happy enough. No magic smoke, no pops and crackles and nice clean sounding output.

So I think it’s pretty much done and ready to be stitched up.