Marshall 8100 head preamp

[jessadr]

I have a Marshall Valvestate 8100 head. The power amp is out. It blew fuses when I got it. I removed all the poweramp transistors. The fuses hold now. The tube glows. My question is can I just just it as a preamp? Can I come off the wiper of the VR13 pot direct into anouther amp or should I use FX send ? I am sure this is a simple question for you guys

http://www.drtube.com/schematics/marshall/8080-pre.gif

http://www.drtube.com/schematics/marshall/8080-pre.gif

[olddawg]

Yes, use the FX send or Pre-out if it has one. But ....why not fix it?

[jessadr]

That amp was ran in to a dead short. The speaker cable was crushed by a heavy object while the amp was being played. A piece of lighting truss. The speaker cab was fine.
That is how I got it It blew fuses and few times and I thought it would be too bad too fix. It had a strong smell of smoke. After pulling the outputs out of it it didn't blow fuses and all the footswitch lights worked. I just was going to mount it in a wide rack I have and plug it into a spare rack amp I have. It looks like new and I hate to just put it away. I wasn't sure if the FX send would be the correct way to use the preamp and reverb or come off the circuit board. I know the Line Outect out would not work. It runs though the outputs.

[mozwell]

I agree, why not fix it. Shoudl be easy. Even if you fix it, you can still mount it in a rack & just use the preamp as you are already planning to do.

The speaker box is OK, you just have to fix the power amp & fix the speaker lead.....
You can test the head without connecting a speaker.
Running the amp into a short, blow up power transistors, solution replace power transistors & driver transistors (if any), rebias (if necessary) & you're done.

First step, as you have already removed (all ?) the power amp transistors - hopefully just the two big ones....
With power off, measure all the big ceramic power resistors, in the power supply & the power amplifier, replace any that are faulty.
Then, turn the power on & measure the power supplies, 0V on cathode of ZD1, +15V on cathode of ZD2, -15V on cathode of ZD1, approx +40V on one end of R85, approx -40V on one end of R75.
Also measure the HT supply (careful high voltage) on each end of R110.
All voltage measurements with repsect to pcb 0V.

If all these are ok, then we can replace the two power transistors, & power the unit up. Measure the DC voltage on the speaker output, it should be a few mV from 0V. If so, conenct a speaker, check with a guitar & you are done.

Make sure any transistor insulation is retained & you use suitable heatsink grease to mount the transistors.

Once you fix it, you will have a spare amp......

[jessadr]

I just went upstairs and pulled it out a box. I did only remove the 2 big transistors. Looks like the center pin on each is melted at the board. I had forgotten that. I will give it a checking with your info . Thanks It would be nice to have it working to use as a loaner.

[Enzo]

Me too. Of course you can just use the preamp. But repair to the blow power amp should be simple, and it won;t get in the way of just using the preamp.

Those BDV65,64 power transistors? Just replace them with the common TIP142/TIP147 pair.

If the damage went farther back, then TR4,TR7, and even TR9 are small common and cheap. Check for open resistors and voila.

[jessadr]

I will check every thing and the traces on the bottom of the board. I have it out of the head. I think I have a pair of those tip/142/Tip147. Are there any subs on the drivers in case they are toast ? The smell could just be those outputs, I hope so.

[Enzo]

MPSA06,MPSA56 are extremely common types, so is that TIP29C. The C is important by the way.

[jessadr]

I think I can handle those jobs. But may need a lead on how to rebias. I know how to check and adjust the bias on my tube amps when I put in new power tubes, but have never did that on a SS.

[Enzo]

TR9 is the bias transistor. Note that there is no bias adjustment.