Preamp section of my JCM800 1987

[JamesHealey]

I've checked all the parts of this amp are stock following the schematic and checking the component values, Im almost 100% sure it's stock after removing a master volume mod.

Im sure the phase inverter is wired up stock but V1 and V2 look a little suspect anyone with a bit more knowledge than myself wanna chime in and tell me if im right or wrong?[/img]

[Mr Ed]

I see the problem, all them wires are in the way.

[Twinfan]

Looks OK to me, but I'm no fully qualified tech.  I'd wait for HTH or MartinW to chip in  ;)

[martinw]

Shouldn't there be a 100k resistor between pins 1 and 6 of V2?

[JamesHealey]

I don't know you tell me haha.. thats the idea.

[HTH AMPS]

Shouldn't there be a 100k resistor between pins 1 and 6 of V2?

Martin, they've put that resistor on the PCB for reasons best known to themselves.

It looks fine from what I can see.

Did the person you bought it from tell you there were any other mods appart from the master volume?

[JamesHealey]

nope it's just got a problem where if im plugged into the amp and i turn up the volume it starts to squeel.. it's like i dunno a microphonic tube.

But it can't be since i've changed all the pre-amp tubes.. and just changed the power tubes.. i've also replaced the Bias and Power Filter Electrolytic caps.

Im just a bit miffed as to why it's doing this, because by all other accounts it's absolutely stunning.

[JamesHealey]

as i turn up the volume it sweep across a frequency range.. like it's a radio tuning in.. and it's full of RF signal.

I think i've solved the background noise problem by replacing the power tubes and caps, it's just stopping it picking up RF which seems to be the biggest issue now, and it seems the master volume control is like a tuning wheel for the RF lol.

Im just suspecting the pre-amp section of this, earlier it was making strange crunching noises and sounded microphonic but i think i solved that with a dry solder joint..

[Twinfan]

Would shielding the head box (like you would a strat scratchplate) help with the RF interference?

[JamesHealey]

see RF could be coming through the AC.. if thats the case an AC conditioner would solve it. Im not 100% sure wtf is going on though.

I don't think it's a faulty component anymore, I think it's something to do with it picking up RF whether that be because someones changed the Pre-amp or because of my house's electricity supply?

[HTH AMPS]

I thought you'd removed the master volume?  :?

Post some info and pix of what master volume you've put in there.  Depending on where you've put the master volume pot, it could be introducing crosstalk into the signal path somewhere.  A bulletproof way of guarding against this is to use shielded wire.

[JamesHealey]

sorry i didnt mean to say master volume.. lol

Ok the amp was bought, and it had a master volume installed, old noisy tubes and failing electrolytics.

It was noisy beyond half way on the master volume to the point where it was unusable and it picked up RF signals but with the master volume below half it was fine. so...

I removed the master volume, It was a typical Marshall Pre-PI Master Volume, simple enough to remove.
I replaced the bias caps with 10uf 100v caps (as spec'd)
I then replaced the LCR caps with JJ's 50+50's (as spec'd)
I then installed new power and pre amp tubes and biased to 32mA which is well within specification for this amp about 60% dissapation.

and it's still picking up RF, it's not as noisy it's a lot quieter, i can turn it up full without the guitar plugged in and it's only minimal noise, but you plug a guitar or even just a cable into the front end and turn the volume up and it starts to pickup RF in the background and it has an almost 'tuning' like sound as you turn up the volume. tis rather strange.

im running all the propper fuses so if something we're majorly wrong it should blow out a fuse and turn off but it hasn't it's stayed on just fine..

[jpfamps]

Tracking down oscillation in an amp can be a tiresome process, however here are some pointers that might help:

I would double check that the amp is wired correctly. You can get a Schematic from the web. When working on an amp that someone has modified don't assume anything!

If turning up the volume control increases oscillation then the oscillation must be being injected into the amp before the volume control so you should focus your efforts here.

Firstly ALL the wire in the amp acts as an aerial for oscillation/RF, so it is likely that it is being induced into some component of the wiring.

Check the integrity of ALL the grounds in the amp, especially of any shielding. Check all solder joints on the board especially those that go to the pots.

The wire from the board to the grid of the first valve stage is unshielded. You could try shielding this wire.

For RF it is best to attach both ends of the shield to ground; however this will potentially make an earth loop. To avoid this attach the non-grounded  end of the shielded wire to ground with say a 0.01 µF cap. The less noisy end of the shield should be connect to earth. Also you can try Randall Aikens recommendation of tying the ground side of the input jack to earth via a 0.01 µF cap.

RF spikes from HT rectification are induced onto the filament wiring in the power transformer. To subdue these put 100 nF 1500 V cermaic disc caps across the rectifier diodes. Also tie the filament wiring to ground with 0.1 F
 film caps. Do this at the VI valve. Fix a solder tag to the chassis and solder a 0.1 µF cap from each side of the filament wiring to the tag. This is worth doing anyway as it reduces noise on the filament supply.

Try putting a grid stop resistor into the first valve stage. Start with 10k. The grid stop should we wired directly onto the socket. I solder the resistor onto the socket, then loop the grid wire around the other end of the resistor and solder this. Finally I cover the resistor wire junction with shrink wrap (remembering of course to slide the shrink wrap onto the wire first!)

Is yours a reverb model by the way, as I remember sorting out a similar sounding problem on a reverb model by rewiring the reverb circuit to improve shielding and grounding, although I can for the life of me remember exactly what I did!

Anyhow good luck and let us know the results.

[jpfamps]

Oh and also you could try shortening the grid wires and moving them away from each other. The longer the wire the bigger the aerial!

[Henk]

Came along these, both original 70's 2204's, note the cap on V2.