Help me diagnose JCM800 with low almost no output.

[walkman]

JCM800 4210. No sound from speaker except when plugging lead into the guitar. No hiss no hum.

Lineout ( driven from OT secondaries) into a preamp gives a singnal when all volumes are maxed, tone controls respond as expected, sounds guitar like.Preamp into fx return, again no sound from the speaker.

Voltages look normal, 12ax7 anodes 290v, EL34 anode 450v screen 448v bias -40v.

El34s have been tested in another amp and I have tried different speakers.

The preamp has had some mods, but were working. The supressor grid has been tied to bias voltage rather than ground as described at the tone-lizard site Marshall Myths ? Tone Lizard. PI biased a little colder with 1.2k rather than 470 ohms.

Where to look for the issue? Any help would be much appreciated!

[Chuck H]

First, stop running the thing and listening for sound. If there is voltage on the line out then there is voltage on the speaker jack. Not hearing ANY sound from the speakers might mean that there is a problem with the speaker cable or the cabinet and you may be cranking the amp full blast into an open load!!! So, yeah, stop doing that now.

[g1]

Is this the correct schematic? (transistor switching rather than IC)

[Chuck H]

No sound from speaker except when plugging lead into the guitar. No hiss no hum.

I'm sorry, I don't know how to interpret this.?.

Lineout ( driven from OT secondaries) into a preamp gives a singnal when all volumes are maxed, tone controls respond as expected, sounds guitar like.Preamp into fx return, again no sound from the speaker.

See my first post.

Voltages look normal, 12ax7 anodes 290v, EL34 anode 450v screen 448v bias -40v.

290V would be very high for the preamp plates except for, perhaps, the cathode follower. If any of your other preamp stages have 290V there is probably something wrong that is keeping the stage from conducting current.

El34s have been tested in another amp and I have tried different speakers.

Did you try another speaker cable?

Where to look for the issue?

Start by trying another speaker cable (at low volume). If there's no joy I would suggest pulling the power tubes for the moment. There is no reason to have any voltage on the line out with absolute silence from the speaker. That needs to be figured out before you blow your output transformer cranking it up to ten and blanging away into a potentially infinite load. After that the issue is the preamp voltage. If you have 290V anywhere but the plate of the cathode follower (EDIT: or the plate of the reverb driver) there's a good chance there's a problem with that stages cathode circuit or it's ground.

[walkman]

That’s the schematic. Speakers and cables are all good.

290v is probably the b+ not anode of the 12ax7 will check again.

I don’t want to damage the OT, how can I test it is ok.... will I need to disconnect all the leads?

How can I test the power section without the powe tubes in. Or for now just focus on the preamp.

Cheers
Walkman

[Chuck H]

If you have a scope you can trace the signal through the circuit. Power tubes out at this time because we don't now why there can be voltage at the line out but NONE at the speaker jack? If this is happening as stated and represented accurately, cables are good and speakers are good, there MUST be a fault of the + speaker connection at the jack on the amp. If this is not the case then something has been misrepresented. But this is the first order of business IMO.

[walkman]

I have a scope and probes but have never used it before ....

[Enzo]

What better time to learn then?

[m1989jmp]

If you have a scope you can trace the signal through the circuit. Power tubes out at this time because we don't now why there can be voltage at the line out but NONE at the speaker jack? If this is happening as stated and represented accurately, cables are good and speakers are good, there MUST be a fault of the + speaker connection at the jack on the amp. If this is not the case then something has been misrepresented. But this is the first order of business IMO.

Precisely; that leaves output speaker jack and speaker cable as the only suspects.

He did state he gets sound from line out when maxing out the volumes, which can't be good.

[m1989jmp]

That’s the schematic. Speakers and cables are all good.

290v is probably the b+ not anode of the 12ax7 will check again.

I don’t want to damage the OT, how can I test it is ok.... will I need to disconnect all the leads?

How can I test the power section without the powe tubes in. Or for now just focus on the preamp.

Cheers
Walkman

You can do a basic OT test with multimeter only. Measure DC resistance from the OT's primary center tap to both power tube anodes (pin3). I think CT is brown and anodes are white and red.

For a 50 watter it should be around 40 ohms. This should confirm the OT is not partially shorted or completely open.

[walkman]

You can do a basic OT test with multimeter only. Measure DC resistance from the OT's primary center tap to both power tube anodes (pin3). I think CT is brown and anodes are white and red.

For a 50 watter it should be around 40 ohms. This should confirm the OT is not partially shorted or completely open.

OK from CT ( connected at HT 500ma fuse) to each EL34 anode (pin 3) 44 ohms on each side.

[Mick Bailey]

A multimeter will only tell you if the primary is open or not. It will not tell you if it's partially shorted, as a shorted turn has such low resistance that it makes practically no difference to the DC resistance. Only a severe short that goes through several layers of windings will give a meaningful reading. That is, if the reading can be distinguished between the normal resistance asymmetry of the two halves that often occurs. So a faulty OT can have perfectly good resistance readings. For a reliable check you need to use the 'neon trick' or a ring tester.

[walkman]

When the problem first occurred it was after I had modified the phase inverter to be a bit colder and tied the supressor grids to the bias voltage.

As soon as i realised that there was no sound I turned it off and have only very briefly turned it back on with the el34s in it, only momentarily with the volume and gain controls up, so I hope there is no damage.

If it were partially shorted should there still be a signal at the line out?

I have tested the voltages on the preamp tubes and they appear ok at first look.

V1 anodes 213v and 230v, 1.9v and 1.4v cathodes
V2a anode 211v cathode 1.4v,
V2b anode 311v and 203v (cathode follower)
V3 anodes 196v and 199v, cathodes 1.6v

With a guitar plugged into the input there is a good signal from the fx send into an external preamp / headphone amp. I didn’t test the send before. So the issue must be somewhere after that.

(Though I did also modify the reverb input at the same time as the phase inverter but have removed the tube for this last test, as I have the reverb pan disconnected.)

Will have a chance to look at the phase inverter voltages tomorrow.

[g1]

If it did not run after the PI and supressor grid mods, please post pics of that work.

[walkman]

Checked the PI and voltages look ok to me.

V5a anode 305v, v5b 296v, cathode 35.8, top of tail resistor 33.7v
(mod was to increase cathode resistor to 1.2k from 470 ohm, and add a grid stopper.)

Will post a photo as soon as I can.

I was checking the voltage on the anode of the EL34 with the tubes pulled. A sound can be heard from the speaker when the multimeter probe touches the anode.

Also checked the supressor grid voltage ... same as bias voltage -42v.

Also had change the grid leaks from 220k to 100k And added 100k grid stoppers on EL34s but can see that causing the issue.