Hello
It seems like all the odd amps are ending on my desk I currently have this 1978 Marshall 2204 that was damaged during transport. This is the combo version and the back plate holding the chassis got loose and banged the filter caps pretty bad. It seems to be 100% original so I have replaced all filter caps, the bias caps and the mustard cathode bypass cap on V1 (which was hissing like crazy when poking it). I thought it might be a good idea to put in 1R resistors on the power tubes as I only have a single probe for the BiasKing meter, making it easier to bias the amp with my multimeter. I added the caps, pin 1+8 tied together --> 1R --> Ground. the resistors measure 1 ohm on my meter. However when the amp gets warmed up it starts to create this insane noise, its insanely loud. Shorting pin 1+8 to ground on either tube make the noise go away. Any suggestions other than running the tubes directly to ground?
Another fact: I did not have 10uF/160V for the bias caps so I used 22uF/100V instead.. could that be the case? I don't think so (as the bias voltage is about half of that voltage at the first cap, and the bias voltage is stable at the tubes), but you may have another opinion?
those 1 ohm resistors need to be connected to grid g2 electrolytic capacitor "ground" and nowhere else, so you might have problem if you connect them "somewhere random gnd"
50W Marshalls from the 70s have their silver award for bad standby design. In that standby switches off the bias supply, as well as the HT supply.
When the standby switch is closed, the HT comes up virtually instantly, the bias takes a bit longer, subjecting the output valves to a nasty kicking every time.
And with the time constants doubled, the bias supply will take even longer to charge up.
I suggest to look into that, maybe sorting things out will resolve your issue. https://el34world.com/charts/Schematics ... 50watt.gif
bepone wrote: ↑Sun Apr 17, 2022 9:51 am
those 1 ohm resistors need to be connected to grid g2 electrolytic capacitor "ground" and nowhere else, so you might have problem if you connect them "somewhere random gnd"
That could be it, the grounding in here is all over the place, usually to the nearest screw connected to the chassis. So if I understand what you’re saying correctly the ground point for the 1R resistors should be at the filter cap ground..?
I tried to ground the 1R resistors to the GND point of the filter caps, no luck. I've attached a clip I recorded with my phone. Amp on, turning the volume knob. Could it be as silly as one or two bad 1R resistors..?
2204 noise.m4a
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bepone wrote: ↑Sun Apr 17, 2022 8:30 pm
pls give te pictures what you did.. never had anything similar. you made HF receiver with your wiring
I ripped it out and reattached the wire straight to ground. Now there seems to be one tube red plating which I have to address. However, there is no resistors at pin 4 (6-4) or pin 5, I know I had these on my DIY-JMP. I guess that could be a source of oscillation. I also recall that I added a couple of caps to prevent parasitic oscillation in that one. I’ll post when I have poked around some more.
Edit: and then the HT fuse blew. And I have no spares until tomorrow
understand. blowed output section, tubes / grids "floating" in the air, currents increased to abnormal levels.. pictures from internals, output sockets if you want help
bepone wrote: ↑Mon Apr 18, 2022 12:07 pm
understand. blowed output section, tubes / grids "floating" in the air, currents increased to abnormal levels.. pictures from internals, output sockets if you want help
Here you go. There’s a 20mm fuse holder piggy backed to the 32mm holder at the moment because I did not have any 32mm fuses left.
I measured the voltage at the rectifier diodes, they forward a couple of AC volts (ca1.. Is that a clue?
98C80BE3-9DDC-47D6-8BF0-227494A0638A.jpeg
FB52CBDA-0DAF-412A-B8C0-1BBF8F586BA1.jpeg
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now start from 0, like nothing happen before, need to be systemtic. you gave too much informations which are scrambled and not giving any clear point other than output section problem. so
remove output tubes and measure voltage on pin 5, pin 4, pin 3 to ground on each socket and pass it here . also all cathode on preamp section recheck if there is some strange voltage. voltage chart find on the internet or from the head it is simple
sluckey wrote: ↑Mon Apr 18, 2022 8:52 pm
I would put the 1Ω resistors back in but do it this way...
Leave pin 1 connected to ground.
Remove the jumper between pin 1 and pin 8.
Connect a 1Ω between pin 1 and pin 8.
Done, same issue.
bepone wrote: ↑Mon Apr 18, 2022 8:45 pm
where are those 1R rwsistors? removed?
now start from 0, like nothing happen before, need to be systemtic. you gave too much informations which are scrambled and not giving any clear point other than output section problem. so
remove output tubes and measure voltage on pin 5, pin 4, pin 3 to ground on each socket and pass it here . also all cathode on preamp section recheck if there is some strange voltage. voltage chart find on the internet or from the head it is simple
Yes they were removed.
I'm in the progres, I'll post when I have some readings.
sluckey wrote: ↑Mon Apr 18, 2022 8:52 pmI would put the 1Ω resistors back in but do it this way...
Leave pin 1 connected to ground.
Remove the jumper between pin 1 and pin 8.
Connect a 1Ω between pin 1 and pin 8.
Done, same issue.
Meaning it still makes a loud noise unless you short pins 1 and 8 to ground? One power tube, either, or both?
That suggests that the ground connection at the tube socket is poor. I can't see, but it there a solder lug there? Can you take those bolts out and clean the mating surfaces?
Was it working fine before you first installed the 1R's?
I can't seem to find anything very suspect.. the B+ is rather on the lower side, but as my wall voltage is 238V I have the amp set to 240/250V. I have also tried the 220/230V setting, but same results regarding the noise.
The noise seems to be some kind of "motorboating", but the only way to make it go away is to ground one of the output tube cathodes (it does not matter which tube). I have tried with different tubes, both EL34 and 6CA7.
Skjermbilde 2022-04-19 kl. 12.33.18.png
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Last edited by goldenGeek on Tue Apr 19, 2022 10:46 am, edited 1 time in total.
I currently have this 1978 Marshall 2204 that was damaged during transport. This is the combo version and the back plate holding the chassis got loose and banged the filter caps pretty bad. It seems to be 100% original so I have replaced all filter caps, the bias caps and the mustard cathode bypass cap on V1 (which was hissing like crazy when poking it). I thought it might be a good idea to put in 1R resistors on the power tubes as I only have a single probe for the BiasKing meter, making it easier to bias the amp with my multimeter. I added the caps, pin 1+8 tied together --> 1R --> Ground. the resistors measure 1 ohm on my meter. However when the amp gets warmed up it starts to create this insane noise, its insanely loud. Shorting pin 1+8 to ground on either tube make the noise go away. Any suggestions other than running the tubes directly to ground?
. Is that a clue?
