Amp help needed - Marshall JCM800 2205

[FELINEGUITARS]

Hi
I have a great Marshall JCM800 - split channel reverb

It has 5 preamp tubes  (12AX7/ECC83) - three in a row like on most Marshalls and then two in a separate line next to the power amp tubes

I want to know which tube is doing what...

the reason I ask is that I started to get some instability in the amp and it started to squeal and oscillate when the gain on channel 2 was turned up.
It gets weirder as when you turn the tone controls or the gain control it seems like a short wave radio trying to tune in or like a signal generator sweeping through its ranges.

What is going on here?

This revealed itself at last years LGS and stopped me using the amp on the last day.
I assumed it was a dodgy valve soI revalved what I thought were the first 3 tubes and the problem seemed to stop for a while, but recently flared up again.

So I took the back off and tapped the sides of each valve with a pencil to see if any seemed microphonic and was surprised that the valve that made a rattle wasn't one of the first 3 but what I would call valve 4, and I would assume it was to do with the reverb or the FX loop.

But have I got it wrong?

Is the valve layout on the 2205/2210 different to what I would expect and was that in fact the first preamp tube.

And what is going on here - it doesn't actually sound like a microphonic valve problem.

I'm hoping one of the amp guys here can advise me on this one

[FELINEGUITARS]

OK - an update
I stuck a new Groove Tubes 12AX7M in V4 position and the problem seems to have gone away (the old valve went straight in the bin once the problem went away).
So it was a simple duff valve problem from the looks of things

Seems like maybe it was a loop/reverb valve problem that would show itself at high gain (but at all volumes low to high)

Still happy to have this confirmed if any of you guys know about the insides of the 2205/2210.

Amp sounds great again - I still want an Egnater but happy with the Marshall sounds again - maybe I'll just sell the Wolftone, Fender Prosonic and Atomic amps to finance the Egnater

[greg]

V4 is actually a reverb drive / reverb recovery circuit.
all other are:
V1 - 1st preamp stage
V2 - an extra gain stage for a drive channel (as well as 2nd half of V1)
V3 - mix stage for both channels and mix with a reverb
V4 - as above - reverb send/return stage
V5 - phase inverter

That's actually 2205. The 2210 has 4x 12ax7

cheers,
g.

[FELINEGUITARS]

V4 is actually a reverb drive / reverb recovery circuit.
all other are:
V1 - 1st preamp stage
V2 - an extra gain stage for a drive channel (as well as 2nd half of V1)
V3 - mix stage for both channels and mix with a reverb
V4 - as above - reverb send/return stage
V5 - phase inverter

That's actually 2205. The 2210 has 4x 12ax7

cheers,
g.

Thanks Greg
Why does the 100w version (2210) have less valves
The amp seems fixed but problem came back momentarily - could it be the socket, maybe loose pins ? Do I need to get that checked over?

[gwEm]

if we are to believe the schematics on drtube, the 2205 and 2210 have somewhat different preamp sections... of course though it must depend on when the amp was made - we all know marshall and more than likely they went through many revisions. (compare difference between 83 and 88 schematic)

http://www.drtube.com/marshall.htm#JCM800

[FELINEGUITARS]

Mine is an '89 so it wshould be the latter but it does have 5 valves in the preamp

[greg]

yes, 100w version head has different pre-amp and a quad of valves in the power-amp section.
if the problem (a squealing tone) come back momentarily it seems like you replaced the most worn valve, but another is dying as well. i doubt it is a socket, but you never know. i would start from replacing another valve in the pre-amp. Try V5 (phase splitter) or V1, V2.
the 'short wave radio' issue is not that easy to trace without an oscilloscope, etc. check if there are no missing shielding cans in the preamp stage. it can be also a ground loop or internal wiring issue.

[jpfamps]

The reverb send return stage can always cause problems as the signal returning from the tank is very small so there is usually quite a lot of gain in the reverb recovery stage, and the tank is wired externally to the amp with a long piece of wire. If the shielding in this wire is poor or has deteriorated then it can pick up EMI.

Grid wires in amps act as antenna for EMI and should ideally be a short as possible and shielded. This is especially true in high gain stages in valve amps which are usually also high impedance. The stability of many Marshalls is questionable. Turn up the presence and treble on a JMP Super Lead and they almost all oscillate.

Regarding the JCM800 100 W, I've seen one amp that had an oscillation problem with the reverb and this was due to a very strange grounding arrangement inside the amp that looked factory wired. Often with these sort of problems if you move the offending wires around with a chopstick with the amp running (carefully!) then you can stop the amp oscillating. If the wire is shielded check it is only grounded at one end as this can cause
a ground loop.

Another approach to avoid RF interference is to add grid stop resistors directly to the grids of the valves (which you can only do in an amp that doesn't a have PCB mounted valve sockets). Remove the grid wire (usually green in Marshalls of that era) and solder a 10 or 15k resistor to the valves socket. Put some heat shrink around the free grid wire and solder the grid wire to the other end of the resistor (I make a loop with the resistor wire). Slide the heat shrink over the resistor and shrink wrap. This usually kills most RF interference and should not affect the should of your amp adversely.

I've posted about this elsewhere on the forum.

Hope that this is useful and good luck.

Frank

[Lucifer]

Hello. I am sorry for my English, I'm from Ukraine. Friends, could not make you for me some qualitative photo PCB of this Amp? I wish to make same. Thankful in advance. My e-mail: sera.luciferchik@gmail.com
 :P