Marshall 9100 Voltages?

[Gregg]

Hi,

I have a Marshall 9100 for repair.
Can you suggest what voltages should be expected at the following junctions:

1/ R39/R40
2/ R39/R37
3/ ECC81 pins 3-8

Right now I'm having:

1/ 438V
2/ 230V
3/ 2.87V

Below is the schematic:

http://www.schematicheaven.com/marsh...9100_2x50w.pdf

Thanks in advance.

[AdmiralB]

I can't help with the voltages (don't have a 9x or EL34 version anymore), but there was a factory service bulletin issued on the early 9x00s. The transistor in the cathode circuit of the ECC81 would fail - the net result was that the amp would just not put out any sound, there was no damage.

I don't recall the original part, but the MPSA43 shown on the linked schematic is the replacement. It also installs backwards from the screened 'footprint' on the PCB.

The 9x00 amps also have a potentially catastrophic design flaw in the bias circuit (corrected in the EL34 versions).

[Gregg]

Thanks for this information. Now after you pointed it out I noticed that the transistor has already been changed because I see a MPSA42 there which has somewhat higher vollage ratings than MPSA43. Still these voltages on ECC81 don.t seem right to me.
Would Marshall give any info on that?

[Mars Amp Repair]

Gregg,
I am only looking at the schematic link you provided. First of all it would be helpful to know what the symptom is...also some history if you have it on how/when it failed...did power tubes fail...etc.

Not certain which ecc83 your referring to..v2 or v1. I'll assume by the resistor junctions you've referenced, you're talking about V2.
this is basically just a triode preamp stage as far as I can tell.

1. I'd say 438 at the junction of R38/R40 sounds about right for plate voltage...
2. I'd say that with the tube conducting, 238v at the junction of R39/37 sounds about right, too. Pretty much a voltage divider 3/4.
3. At the cathodes of the driver tube (pins 3&8), 2.87V controlled by what looks like a constant current source (or simply for muting) is probably ok, too. Anyway you look at it, the base would have to be about 1.4v by virtue of the series diodes to ground & the emitter would have to be about 0.7v.

I need more info about what the amp is doing wrong...thanx, g

[Gregg]

Hi,

The amp is not doing anything wrong. It has been heavily modified by somebody and now the new owner wants it restored back to original. This PI is not something you see everyday in a tube amp - I just wanted to make sure it's properly biased.
The voltage at the base of my transistor is 1.0V, the emitter 0.44V. The second tube /PI/ is actually an ECC81.

[AdmiralB]

Just out of curiosity, what are the mods?

I've done more than a few of these where I chop up the bias supply to get around the fusing issue the 9X00 series amps have.

[Gregg]

Amongst other things the PI has been transformed into the usual PI you can see in so many amps. There's a separate secondary for the bias - the PT doesn't look like the original one. Bias trimpots for each tube have been added - I'll definately leave those.

[AdmiralB]

They changed both PTs? Wow.

The stock PCB layout is flawed (the PS daughterboards) - the bias winding also feeds the rail that powers the voicing relays and the fan, and the schem shows the tap for the relay feed being fused downstream of the bias circuit.

But the board was created with the fuse *upstream* - so if the relay fuse blows, that side of the amp loses bias.

The EL34 versions of these amps fixed that, but all the 9100/9200 have the problem.