Marshall Artiste with low P/T secondary and low output

[nickb]

You are bloody brilliant Nick!! Does my little sketch match what you're telling me? If so, then there is no mystical voodoo happening at all. [ATTACH=CONFIG]49723[/ATTACH]

You are on the right lines. When reverb is at max it will be loaded by the reverb recovery tube so you'll see about a 510K load rather than 1.47meg.

So this is the big question in my mind. If the reverb works normally then the gain variation should not be too bad. If on the other hand it's been modded or the recovery stage is not working as it should be (so that it's output impedance is higher) then the gain variation will be more.

[christarak]

Thanks for the suggestion Helmholtz. My scope is 25MHz. I haven't checked yet, but I have no idea of what amplitude of oscillation is significant. I guess the answer is ANY amplitude is bad.

[christarak]

Loading the secondary of the reverb transformer with 100 ohm (The resistance on the reverb tank was 166 ohm) seems to have sorted the massive gain and the increase in gain is now quite small. I am now getting 38 watts across the speaker on Channel 2 which I should accept as OK when you consider my P/T was down. Channel 1 is putting out a little less but sounds very muddy. Then I need to lift that circuit board without causing more damage to properly solder in my repair work.

[pdf64]

FYI another report of Marshall with lowish HT http://music-electronics-forum.com/s...ad.php?t=21173
I remember plenty others over the years.
And the Marshall voltage chart http://www.freeinfosociety.com/elect...iew.php?id=495
As the HT will sag substantially under load, I'd only expect 30-40 watts from these amps.

[Helmholtz]

Loading the secondary of the reverb transformer with 100 ohm (The resistance on the reverb tank was 166 ohm) seems to have sorted the massive gain and the increase in gain is now quite small. I am now getting 38 watts across the speaker on Channel 2 which I should accept as OK when you consider my P/T was down. Channel 1 is putting out a little less but sounds very muddy. Then I need to lift that circuit board without causing more damage to properly solder in my repair work.

If the reverb input DCR is 166 Ohm, its impedance will be 1k to 2K. So you could increase the (dummy) load resistor up to 2K without a problem, if you like.
Somewhere above you mentioned that the reverb wiring carries "supply voltage". What made you think so?

In your post#10 you mentioned 47W output (before clipping?). This means that the power stage voltages are sufficient for this kind of output power and it must be possible to get the same power again.

[nickb]

If the reverb input DCR is 166 Ohm, its impedance will be 1k to 2K. So you could increase the (dummy) load resistor up to 2K without a problem, if you like.
Somewhere above you mentioned that the reverb wiring carries "supply voltage". What made you think so?

In your post#10 you mentioned 47W output (before clipping?). This means that the power stage voltages are sufficient for this kind of output power and it must be possible to get the same power again.

He was referring to the to the B supply to the reverb transformer primary, I believe.

Chris -sounds like you had the reverb primary signal crosstalking into the reverb recovery circuit.

We still need to look into the low gain reported above in post 10. You had a gain of 11 where is should be 50-ish...

[christarak]

FYI another report of Marshall with lowish HT http://music-electronics-forum.com/s...ad.php?t=21173
I remember plenty others over the years.
And the Marshall voltage chart http://www.freeinfosociety.com/elect...iew.php?id=495
As the HT will sag substantially under load, I'd only expect 30-40 watts from these amps.

Thank you pdf64. I missed that series of posts from 8 years ago. I'll have a read through that this morning. I'll print out and keep that Marshall voltage chart, too. Many thanks.