JCM800 2204 - preamp section not working

Motterpaul

Tone is in the Ears
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13,680
I have an 1980s JCM800 2204 where I seem to be getting output but no audio getting through the 12ax7 preamp stages (especially tubes 1 & 2). I have some decent electronics skills, I build pedals, I installed a Metropolis loop on this amp five years ago and it has been working fine (it still is working, but I just uninstalled it to see if it was the problem. It wasn't, still no sound going through the preamp stage.)

I can use a DMM. I also seem to be getting audio through the output stages, loud sounds from the speaker when I tap the output of the loop (in loop on mode, so I am sure it is not the loop).

The amp was working but seemed weak just before it died. First, the high input quit, then the low quit about 30 minutes later. We reset V1 tube in its socket and it brought back the input for an hour, but then the high input died, and 30 minutes later the Low input died. Now I cannot get any sound out of it. On the bench at home I can still get sound from the Output stage, though.

I do not see any bad components and I have been testing for continuity of ground throughout and also along the audio path wherever I can do it.

If anyone can just give me a clue on where to look next it could be a big help. I am suspecting someplace around the small tube receptacles.

I do not know a lot about OT and tube plate voltages, but if you tell me where to look at what I should be seeing, I could tell you what voltages I see. I do see about 385v on the OT plates and something like 250v on the small tube voltages.


In another forum, the responses all focussed on the loop I installed. I put it inside the chassis in a manner where I could just use the jacks on the actual loop PCB to access it (no holes drilled in the chassis or extra wires or jacks added). I have a couple of short cables coming out of the loop that go into the housing that I used to connect a spring reverb. No holes were drilled and nothing was added except for the reverb unit and some hot glue (which comes right off). The wires to install the loop are soldered to the amp pots (floating old-style vertical input pots) as instructed by Metropolis. I did not disrupt anything on the PCB. There is a bypass switch by the loop that is operational.

Before I put the loop in it was a stock 2204 as far as I know (bought it used). The tubes in the V1 and V2 (and all the tubes) are a brand new full set of JJs - they are all lighting up. I hear a loud sound coming out of the output when I touch the cable attached to the loop return. Also - when it is powered up I can tell that V1 and V2 seem to be alive because I hear a less loud sound output when I tap on some of the preamp tube connections inside the chassis. I just don't get any sound from a cable inserted in either input jack.

I should add that I do hear the amp working (a hum when I take it off standby). Bias controls are also working. Once again, just no sound from the input jacks.

[update] Okay - I did swap out V1 - no help. I checked the spacing on all the tube inserts - all seem good. I have pictures here:

Code:
https://motterpaul.imgur.com/all

(click on "albums" and select 2204 JCM800)

The pictures show the loop as it was installed, but I just took it out and I still have the same problem. I can get sound coming from V1 if I tap on pin 6 (even louder on V2 and + more on V3). But taking the loop out did not solve the problem.

TL/DR - My JCM800 2204 has a working output stage, but I can't get any sound into the preamp section from the front guitar insert jack. All tubes are lighting up and are new.
 

glpg80

Member
Messages
297
I haven’t seen you mention whether you’ve verified all preamp tubes are working? New doesn’t necessarily mean working.

Rule one of any build: verify voltages, grounds, and tubes. The voltages you mention all seem normal for plates and preamp, the loop was verified to work beforehand, power section works, so the issue is something to do with the preamp section only.
 

Motterpaul

Tone is in the Ears
Messages
13,680
I just went and cleaned the sockets in V3 and put in a new tube, still no joy. When I measure voltage for pin 6 I also see that V1 is 300v V2 is 220 and V3 is 200. I also see that V1 has pin 6 tied to ground (through a cap).

I really don't know how to verify whether a tube is working - sorry. That is a hole in my electroknowledge. I tried different tubes in different sockets.

I guess I could try them all in a different amp that I know is working - is that it? I see no burnt components or loose wires anywhere, and I agree it must be in the preamp.
 

Motterpaul

Tone is in the Ears
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13,680
Very good question. I started to read these and V1 seemed pretty stable, but V2 was fluctuating a lot and on pin7 started at 0 and came up slowly and then the amp came alive - but only for a while (like a CAP was getting charged up and then going down).

I need to test these tubes in a different amp and make sure they are all working.

Roughly I got these readings:

V1

1 - 257v
2 - 0
3 - 2.78 v
6 - 216 v
7 - 1.5 mv
8 drained down to just about 1.7 v

V2

1 - 243v
2 - 0
3 - 200 mv
6 - 296 v
7 - 1.5 mv
8 - didn't get there

I tried putting a different tube in V2 to see if it would work, but it would not even charge up as the first one did.

But like I said - it was on pin 7 on the first V2 tube that it started at 0 and kept climbing and then suddenly the amp worked - from the input. But the charge did not hold once I disconnected.

I will test these tubes & report tomorrow. Happy Holidays to All and thank you!
 

glpg80

Member
Messages
297
Yes you’re correct - without a tube tester, it’s not possible to test them for their electrical parameters. I was simply oversimplifying by assuming you had tubes on hand that were known to work and not brand new out of the box with unknown electrical characteristics/verified performance.

The cathode voltage for V2B is basically zero when it should be larger. If it charged up then there’s likely an electrical capacitor in parallel with the cathode resistor, correct? Could it simply be a bad electrolytic capacitor needing replaced at that location? If no capacitor, and it worked when a meter was placed on it, either the socket for pin 7 is not tight enough to make electrical connection so the tube pin is floating/open circuit, or the cathode resistor itself is not tied to ground and is open circuit and the stage began to bias on through the meter.
 
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Dan40

Silver Supporting Member
Messages
1,888
A quirk in the design of the 2204 is that the high input signal travels through the low input jack on it's way to the next stage. If the contacts in the jack become dirty or oxidized you can get a weak or no signal condition occurring.
 

Motterpaul

Tone is in the Ears
Messages
13,680
Yes you’re correct - without a tube tester, it’s not possible to test them for their electrical parameters. I was simply oversimplifying by assuming you had tubes on hand that were known to work and not brand new out of the box with unknown electrical characteristics/verified performance.

The cathode voltage for V2B is basically zero when it should be larger. If it charged up then there’s likely an electrical capacitor in parallel with the cathode resistor, correct? Could it simply be a bad electrolytic capacitor needing replaced at that location? If no capacitor, and it worked when a meter was placed on it, either the socket for pin 7 is not tight enough to make electrical connection so the tube pin is floating/open circuit, or the cathode resistor itself is not tied to ground and is open circuit and the stage began to bias on through the meter.

Thanks - I need to contemplate what this means. (I will get it). What exactly is V2B ? Basically yes, it did seem to charge up slowly after I had the DMM testing for voltage for about 30 seconds. Then when I took it off it discharged. Seems like a cap.

I also noticed the cable that brings the audio to V1 also has a ground wire (inside a shield) and that ground was wired to (I think) pin 6 which was the voltage for that V1. I guess that is just some kind of RF filter but I am not sure.

I was thinking of buying a tube tester on Ebay. But I don't want to spend $800. I don't do this for a living yet, although this area could use a good amp tech. Any recommendations on what to get? One YouTube said the Orange is good.

I guess I should build a basic push/pull amp and get an idea of how these things really work. But the more research I do the more I see, it is getting more clear already.
 
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rockon1

Platinum Supporting Member
Messages
13,681
Very few pre amp tubes just die. Ive got hundreds of them. Some test weak (but still work) some go too microphonic. Couple known good spares is all you need -unless your buying or selling old tubes imo.
 

Motterpaul

Tone is in the Ears
Messages
13,680
FYI - I also just thought to check the filter caps as well. I thought they were done about 8 years ago, but they are reading:

1. 390 / 390
2. 390 /340
3. 240 /240

So I guess I need to replace those as well, and also the 10 uF 100v bias caps. I found another post on which resistors to replace if any pin currents on the OTs fluctuate. I am just wondering if anyone knows a good resource where all this info is listed?
 

Motterpaul

Tone is in the Ears
Messages
13,680
Very few pre amp tubes just die. Ive got hundreds of them. Some test weak (but still work) some go too microphonic. Couple known good spares is all you need -unless your buying or selling old tubes imo.

Yeah, I probably have 30 or so myself, including some Mullards, Phillips & more. They all seem to be about the same really, I can't remember one that ever fully died on me although I am sure it happens.
 

rockon1

Platinum Supporting Member
Messages
13,681
Yeah, I probably have 30 or so myself, including some Mullards, Phillips & more. They all seem to be about the same really, I can't remember one that ever fully died on me although I am sure it happens.

Yeah just an observation over the years of collecting them. A couple oldies have lost their vacuum (cracked)during that time but thats about it. I enjoy reading about troubleshooting though.
 

paulg

Member
Messages
3,128
Check the plate voltage (done), pop the grids. If theres no sound work back from that point. I’d suspect an open coupling cap.
 

glpg80

Member
Messages
297
I think there may be a bad solder joint on V2 socket, eg the link between 3 & 7.
V2 is the direct coupled cathode follower DCCF.
For voltages, see https://el34world.com/charts/Schematics/files/Marshall/Marshall_ampchart.gif

Yeah I’m thinking along the same lines - something may have a cold solder joint related to V2.

@Motterpaul V2B simply states which side of V2 is potentially problematic. Each preamp tube is more formally called an envelope of two triodes, an A side and a B side. The B side is related to the triode with higher numerical pin numbers.
 

Motterpaul

Tone is in the Ears
Messages
13,680
Thanks for the definition. I know in schematics they usually show tubes that way, I just never saw the term in conversation.
 

Blue Strat

Member
Messages
30,687
I skipped a few posts. Have you resolved why the DC voltages are changing on some of the tube pins? If not, you need to focus on that. If they're changing while you're probing, resolder all connections (after discharging the power supply capacitors) in that voltage path (tube pin, wire, resistor, other end of the resistor, etc. You could have a broken connection on a PC board pad underneath the board.

Testing your tubes in another amp is adequate for what you're trying to do here.
 




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