Marshall Artiste with low P/T secondary and low output

[christarak]

If you can't get more negative than -24VDC at power tube grids, I think you should pull the power tubes and fix that before doing anything else.

Thanks for the tips G1. I put a 680K across the 220K supplying the Bias rectifier. That gave me enough negative volts to have -32v on pin 5 in the middle position of the bias pot. -32 v gave me 40ma at 370v on the plate.

The change in output level with the reverb tank disconnected makes no sense to me. There is no signal path through the reverb then. Makes me think it is just oscillating.

With the reverb tank connected, I can hear the anaemic reverb when I have the pot at around 3 o'clock. Now here's the brain scrambler: when I disconnect the reverb tank the reverb pot acts like another gain stage. I can hear dramatically this with either my signal generator or guitar. I agree with you G1, I can't see a signal path to V3b. The actual amp seems to match the schematic. Its got me stumped!

[christarak]

Very interesting that you have a low voltage Marshall, too. I had to jiggy some values to get the bias voltage (-32v) to the point where the EL34 was drawing 40mA at plate voltage of 370v. I can't get it any higher than 370.

[christarak]

Thanks Nick. Weirdest thing. V1 had a cathode bypass cap tacked on, but I'll be damned if I could find the bypass cap on V2. I attached a 25uF/25v across the cathode resistor and boom ...the gain doubled. I haven't swapped the reverb leads yet but I have bigger issues to solve first. The main one being the Reverb pot acting like an extra gain stage when there is no reverb tank but then acting like a reverb mix pot when the tank is connected.

Channel 1 was oscillating when I removed the 270K between grid and plate that a previous tech had installled....so I reattached it. Channel 1 sounds like crap, as if the treble pot is on 0 when its on 10.

[christarak]

Yes, modding the bias circuit to get a -32v bias certainly helped a lot. The was no red plating, but I don't think I was too far away. I saw 70mA through one of the tubes before I dived for the off switch. Its now at 40mA.

Nick, what would you expect when you disconnect the reverb tank (pulling out RCA connectors at amp end) and you dial up the reverb? I'm with G1 and expect nothing, and yet my ears don't lie. Its like another gain stage has kicked in.

[dstrat]

Hi Chris, from the looks of the wiring, some of it does not look stock to me. Its possible someone has modded the reverb to act as a gain stage. I can not see a 1M resistor anywhere around the 3 470K mixers.
also make sure your bias is feeding from the switch side of the stand by switch, and not diode side as that could cause blown fuses and no bias for a few seconds while it charges to its desired voltage.
maybe its already been taken care of, hard to tell from the photos.

Nick I get a error when clicking your attachment?

[nickb]

Yes, modding the bias circuit to get a -32v bias certainly helped a lot. The was no red plating, but I don't think I was too far away. I saw 70mA through one of the tubes before I dived for the off switch. Its now at 40mA.

Nick, what would you expect when you disconnect the reverb tank (pulling out RCA connectors at amp end) and you dial up the reverb? I'm with G1 and expect nothing, and yet my ears don't lie. Its like another gain stage has kicked in.

Look at the way the dry and wet mix are done. The reverb level pot when set to zero shunts the dry path with 470K to ground so attenuating the signal by half. When the level pot is about halfway you'll get less attenuation. The difference is around 1.5dB if everything is as it should be. If it isn't then all bets are off.

[nickb]

.....hard to tell from the photos.

Nick I get a error when clicking your attachment?

Thx - I've re-uploaded the pic. It seems to be there now http://music-electronics-forum.com/a...2&d=1531577029 . Ignore the "Attachment" text. It's not real and doesn't show up in advanced edit. Weird...

Anyway, so now you have a second image to compare the reverb wiring with.

[Chuck H]

70mA through ONE of the tubes... What about the other?

Obviously you'll replace power tubes if one is drawing substantially more current than the other. I only mention it because I think that (after the signal chain is working and) once you get the bias sorted out with known good tubes you'll probably have around 380Vp. And that's likely normal. I've had two 2204 amps. One from the late 70's and the other from the mid 80's I think. The 70's amp had 380Vp and the 80's amp had 420Vp. This is a pretty well known anomaly for some Marshall 50 watt models. Though I don't have an encyclopedic knowledge of the models, years and details this is probably what I would chalk it up to and I wouldn't sweat it.

[Helmholtz]

With the reverb tank connected, I can hear the anaemic reverb when I have the pot at around 3 o'clock.

Did you verify the oscillation without tank on your scope? Oscillation can produce very high voltages across the reverb transformer primary and may eventually damage the transformer, cause arcing in the tube and give false meter readings
I recommend to short the reverb transformer secondary while the tank is disconnected.

[christarak]

Did you verify the oscillation without tank on your scope? Oscillation can produce very high voltages across the reverb transformer primary and may eventually damage the transformer, cause arcing in the tube and give false meter readings
I recommend to short the reverb transformer secondary while the tank is disconnected.

Thank you Helmholtz. I checked for oscillation with my scope, but I couldn't see or hear any. With the tank connected, I can hear reverb. Both the In and the OUT of the tank measure 166R. Is it OK to short the reverb secondary terminals or is it kinder to the transformer to connect a 100R resistor between them? I get this (nice sounding) gain boost when increasing the Reverb pot with the tank removed and the reverb transformer secondary left open (which I only do very briefly).

I have just emailed the customer to see if he is aware of a mod in his amp to convert the reverb to provide more gain. I doubt it as then I would not expect the reverb that I can hear. I just fail to see an electrical connection to pin 7 of V3 (the recovery half) other than the reverb tank.

[christarak]

Thx - I've re-uploaded the pic. It seems to be there now http://music-electronics-forum.com/a...2&d=1531577029 . Ignore the "Attachment" text. It's not real and doesn't show up in advanced edit. Weird...

Anyway, so now you have a second image to compare the reverb wiring with.

Thank you Nick. I will be studying your photos when I get home on the bigger monitor to pick up any discrepancies. Thank you for taking that trouble.

[g1]

See nicks post #21 about the gain boost. Or you might think of it as a 'cut' when reverb is turned down.

I think a 100ohm resistor to load the reverb TX should be fine.

[christarak]

Look at the way the dry and wet mix are done. The reverb level pot when set to zero shunts the dry path with 470K to ground so attenuating the signal by half. When the level pot is about halfway you'll get less attenuation. The difference is around 1.5dB if everything is as it should be. If it isn't then all bets are off.

You are bloody brilliant Nick!! Does my little sketch match what you're telling me? If so, then there is no mystical voodoo happening at all.

[christarak]

70mA through ONE of the tubes... What about the other?

Obviously you'll replace power tubes if one is drawing substantially more current than the other. I only mention it because I think that (after the signal chain is working and) once you get the bias sorted out with known good tubes you'll probably have around 380Vp. And that's likely normal. I've had two 2204 amps. One from the late 70's and the other from the mid 80's I think. The 70's amp had 380Vp and the 80's amp had 420Vp. This is a pretty well known anomaly for some Marshall 50 watt models. Though I don't have an encyclopedic knowledge of the models, years and details this is probably what I would chalk it up to and I wouldn't sweat it.

Sorry Chuck, I should have been more specific. Both tubes were drawing large currents, but I only put one value down, The lettering on the tubes have browned from the overheating. I have now corrected the bias to give me good levels of current. My B+ is still low at 370, but I feel much better about it from what you have said. Thank you so much for sharing your experience and knowledge with me.

[Helmholtz]

Thank you Helmholtz. I checked for oscillation with my scope, but I couldn't see or hear any. With the tank connected, I can hear reverb. Both the In and the OUT of the tank measure 166R. Is it OK to short the reverb secondary terminals or is it kinder to the transformer to connect a 100R resistor between them? I get this (nice sounding) gain boost when increasing the Reverb pot with the tank removed and the reverb transformer secondary left open (which I only do very briefly).

I have just emailed the customer to see if he is aware of a mod in his amp to convert the reverb to provide more gain. I doubt it as then I would not expect the reverb that I can hear. I just fail to see an electrical connection to pin 7 of V3 (the recovery half) other than the reverb tank.

Shorting the secondary or terminating with 100R shouldn't be a problem. Perfect would be a load resistor corresponding to the tank's input impedance (might be found from the model number). Does this take care of the electrifying/shock when touching?

What is the bandwidth of your scope? Oscillation may be in the MHz range.