Marshall clone, what to do?

[Randall]

I have a Blockhead 100 watt Marshall plexi clone on the bench. Guy says "it just doesn't sound as good as it used to when cranked up with my LP and 4x12 cab". It is a very nicely made amp, all nice components and wire, etc. It sounds fine to me, but the guy insists it has "lost something". Visual inspection finds nothing. I measure the four output tubes and they are all running at 34 -38mA @ 484Vp. This amp has no bias adjustment, but pin 5 measures -36v on all four. It does have a screen adjustment pot, which I have never seen before. The screens are sitting at 441v. Could this be a factor? I don't know what else to do.

[xtian]

If the customer is to be trusted in his opinion that amp has "lost something", then it's tubes or filter caps, right? Install a fresh set of power tubes for him and see if it makes him happy.

How old are the filter caps?

Maybe he broke in his speakers, and is missing the harsh high end? ...

[Randall]

Does anyone have a comment on the screen adjustment?

[g1]

It's a little unconventional, but I'm pretty sure the screen adjust will function as a bias adjust.
You should see the idle current of the tubes change as you adjust the screen voltage.

[alexradium]

Does anyone have a comment on the screen adjustment?

how is it implemented,like a mosfet voltage regulator?is there an external pot for the user to fiddle around?

[Randall]

My bad, since I have no schematic and the wires run under the board I believe the multi turn pot is actually a bias pot after all. It is located on the board, and is quite difficult to turn. It does effect the bias and the screen voltage. But I'm happy where the idle currents are, and they are pretty close together, so I doubt that's it. I did measure power at 60 watts, does that seem low?

[alexradium]

if all 4 tubes are active,and the drive signal from the Pi is right,its very low,try a new set.

[kanks]

Altering the bias or the screen voltage will change the bias. Just try setting it really hot or really cold and hope or do as I do and tell then there is nothing you can find wrong then send them on there way.

My bad, since I have no schematic and the wires run under the board I believe the multi turn pot is actually a bias pot after all. It is located on the board, and is quite difficult to turn. It does effect the bias and the screen voltage. But I'm happy where the idle currents are, and they are pretty close together, so I doubt that's it. I did measure power at 60 watts, does that seem low?

[Pedro Vecino]

485V with 38mA is a high work regime in EL34. It´s probable that originally it was even greater. It is not surprising that the sound degrades in a not very long time. It also depends on the model of EL34 but the natural tendency is that the punch decreases and the sound becomes softer, less solid and more dispersed.
I do not understand a circuit that manages bias and screen grid voltage at the same time. With what purpose?
If so, you should be able to identify a mosfet somewhere...

[g1]

60W for a quad of EL34 in a Marshall style amp does seem low to me.
And a difference of 45V between plate and screen seems a bit more than usual. But without a schematic that is pure speculation.
Agree with alexradium, try a new set of power tubes out on the owner's ears and see what he thinks.
"Doesn't sound quite as good cranked" is often 'code' for aged power tubes.

[Randall]

"I do not understand a circuit that manages bias and screen grid voltage at the same time. With what purpose?
If so, you should be able to identify a mosfet somewhere..."

There is no mosfet. I suspect that screen voltage is not what is being managed by the multi-turn pot, but a side effect of bias adjustment, just as plate voltage goes down as bias adjustment causes current draw to go up. I am also leaning towards tubes. I actually got contact with Ossie from 3 Monkeys Amps, the company Blockhead turned into, and he seems game. So I may get some real info soon.

[Enzo]

I think you are right about the screens.

Have we had the guy bring over his speaker cab?

Alternatively, he thinks it lost something, you think it sounds fine on YOUR speakers. Does he think it sounds fine on your speakers?

[Randall]

Haven't gotten to the point of him bringing his 4x12 cab. He says his cab sounds good with other heads. He also says it don't sound too bad thru my cab, which is a bass cab with the horn turned off. I really don't like these type of, "it just sounds different" complaints.

[Chuck H]

Just an observation, if he thinks it sounds "not too bad" through a bass cab it would imply a final EQ favoring more bottom end is leaning toward putting the mojo back in. Poor LF is a typical complaint of old power tubes. How old ARE the tubes? If they're not very old then it would be a real burden having to keep really new tubes in the amp for that extra few percent of goodness. If the tubes aren't very old maybe try cooling the bias a little. Tubes that draw more current with age can sound mushy in the bottom end.

[Mick Bailey]

Most 100W Marshall heads will give 80-90W before showing any signs of clipping. Usually at idle there's only a small difference between the screen and plate voltages, the difference being marginally increased with increase in idle current. If the tubes have been in a while, then a swap would be my starting point. I have a few 'bench sets' that I sub in in these cases, just to eliminate tube issues.