GuitarIV
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So, where do I start?
I got myself a 1982 Marshall JCM 800 2203 about 3 years ago, fulfilling myself my teenage dream of owning one.
The amp was totally mint and serviced, had all the original tubes and filter caps and I got to open it up once with the preamp on 8 and the master on 6. Unbelievable experience. Sexual.
About half a year ago I was playing it at home and after half an hour the sound faded and a low frequency hum started fading in. I turned it off and phoned up my amp tech.
He came over, we fired it up and noticed that one of the EL34s was red plating.
Recently I finally had the cash to order a quartett of Tung Sol EL34Bs and a Bias-Master from TAD.
I am still a newbie in regards to tube amp electronics but I have my amp tech who helps me, I ordered some books he recommended and after I talked to him today on the phone and made sure I understood the biasing procedure I cleaned the table in my kitchen, took the amp out of the chassis and went for it.
Some pictures:
I was a bit nervous as I have never done this before, but my tech told me to take my time, not rush things, follow the "one hand in your pocket" rule and go for it.
After spending the better half of the day on this I can tell you it's not that hard if you're not an idiot. Respect electricity and be patient, then you're golden.
Now to get to the techy part of the thread where I need your input guys:
When I fired the amp up with the new tubes in the bias pot was turned up about 2 thirds of the adjustment range. Going with the TAD biasing guide the EL34s go from 25mA cold to 40mAs hot. That's the range.
Now I let the amp sit for 10 minutes, all the pots were turned down, I noticed one of the tubes in the middle was running quite hot. Somewhere around 46mAs. I turned the bias pot down a bit and got it to 40, after a minute or two it started rising up again untill I eventually ended up turning the bias pot all the way down to the end of its adjustment range. When that specific socket reached 47mAs I turned the amp off. The socket in question was one of the middle ones, I think V5 (when counting starting with the first preamp tube being V1 and the last poweramp tube being V7).
I turn the amp on again after a couple of minutes and suddenly the middle tube next to V5, so V6, starts rising up the most. The Bias pot was still all the way down.
After I let the amp cool down and turned it around so the electronics were facing the table downwards again (bias pot still turned all the way down) I fired it up and everything stabilized.
I had readings of 18, 21, 22 and 19mAs. It stayed stable.
I plugged the amp in, played it, it sounded good although a bit sterile which is to be expected with such a cold bias and my amp tech wrote me he'll call me tomorrow after I messaged him with my results.
Now I didn't want to call it a day and fired the amp up again. Bias was still stable with the same readings of 18, 21, 22 and 19. I started to adjust the bias pot up again and suddenly I had the middle tubes run away again!
Turned it off, let it sit, fired it up and this time I managed to slowly turn up the pot till I had something like 28, 32, 33, 29.
This was steady, after half an hour it stabilized somewhere around 32, 35, 36 and 34. (I'm giving numbers here from memory)
I plugged the amp into my Two Notes Captor and played it that way into my interface and my DAW and it sounded glorious! I did notice that the values would rise up to around 40mAs after having played the amp, but they would come back to the previous 32-36ish values after 5 minutes or so when I let the amp sit.
Now again I am no expert, but from my observations the following things come to mind:
- When the tubes run too hot the heat causes a resistor or cap to leak which then in turn increases the mAs and that could have caused the redplating of the original tube in the first place.
- it might be a mechanical issue (cold solder joint?) as the problem seems to be less severe or go away when I turn the amp around the "right way" as opposed to upside down
- it seems to be centered around the inner pair of the tube sockets as I have higher readings in the middle than the outside tubes. I might also just have a not so well matched quartett of tubes.
Now as of tonight the amp was stable with the readings of 32, 35, 36 and 34. I am tempted to fire it up tomorrow again, adjust the bias pot a wee bit down till the lowest reading tube is smack at 30 and call it a day.
However my instinct tells me there is an internal issue and if I don't fix that my bias will eventually run away again and it'll fry some of the new tubes. I don't want that.
I will be talking to my amp tech tomorrow anyway, but I figured some of you knowledgeable folks here might have a clue what might be causing this. The amp sounds great, I wanna have it running smoothly.
Sorry for the long thread, but I wanna be as accurate as possible. If you have any ideas, your input is very appreciated!
Cheers
p.s.: my amp tech developed a tremor in his hands, he can't work on amps anymore. I t would be a no brainer to have him fix the amp otherwise. This way he's helping me out and I might become the apprentice.
I got myself a 1982 Marshall JCM 800 2203 about 3 years ago, fulfilling myself my teenage dream of owning one.
The amp was totally mint and serviced, had all the original tubes and filter caps and I got to open it up once with the preamp on 8 and the master on 6. Unbelievable experience. Sexual.
About half a year ago I was playing it at home and after half an hour the sound faded and a low frequency hum started fading in. I turned it off and phoned up my amp tech.
He came over, we fired it up and noticed that one of the EL34s was red plating.
Recently I finally had the cash to order a quartett of Tung Sol EL34Bs and a Bias-Master from TAD.
I am still a newbie in regards to tube amp electronics but I have my amp tech who helps me, I ordered some books he recommended and after I talked to him today on the phone and made sure I understood the biasing procedure I cleaned the table in my kitchen, took the amp out of the chassis and went for it.
Some pictures:
I was a bit nervous as I have never done this before, but my tech told me to take my time, not rush things, follow the "one hand in your pocket" rule and go for it.
After spending the better half of the day on this I can tell you it's not that hard if you're not an idiot. Respect electricity and be patient, then you're golden.
Now to get to the techy part of the thread where I need your input guys:
When I fired the amp up with the new tubes in the bias pot was turned up about 2 thirds of the adjustment range. Going with the TAD biasing guide the EL34s go from 25mA cold to 40mAs hot. That's the range.
Now I let the amp sit for 10 minutes, all the pots were turned down, I noticed one of the tubes in the middle was running quite hot. Somewhere around 46mAs. I turned the bias pot down a bit and got it to 40, after a minute or two it started rising up again untill I eventually ended up turning the bias pot all the way down to the end of its adjustment range. When that specific socket reached 47mAs I turned the amp off. The socket in question was one of the middle ones, I think V5 (when counting starting with the first preamp tube being V1 and the last poweramp tube being V7).
I turn the amp on again after a couple of minutes and suddenly the middle tube next to V5, so V6, starts rising up the most. The Bias pot was still all the way down.
After I let the amp cool down and turned it around so the electronics were facing the table downwards again (bias pot still turned all the way down) I fired it up and everything stabilized.
I had readings of 18, 21, 22 and 19mAs. It stayed stable.
I plugged the amp in, played it, it sounded good although a bit sterile which is to be expected with such a cold bias and my amp tech wrote me he'll call me tomorrow after I messaged him with my results.
Now I didn't want to call it a day and fired the amp up again. Bias was still stable with the same readings of 18, 21, 22 and 19. I started to adjust the bias pot up again and suddenly I had the middle tubes run away again!
Turned it off, let it sit, fired it up and this time I managed to slowly turn up the pot till I had something like 28, 32, 33, 29.
This was steady, after half an hour it stabilized somewhere around 32, 35, 36 and 34. (I'm giving numbers here from memory)
I plugged the amp into my Two Notes Captor and played it that way into my interface and my DAW and it sounded glorious! I did notice that the values would rise up to around 40mAs after having played the amp, but they would come back to the previous 32-36ish values after 5 minutes or so when I let the amp sit.
Now again I am no expert, but from my observations the following things come to mind:
- When the tubes run too hot the heat causes a resistor or cap to leak which then in turn increases the mAs and that could have caused the redplating of the original tube in the first place.
- it might be a mechanical issue (cold solder joint?) as the problem seems to be less severe or go away when I turn the amp around the "right way" as opposed to upside down
- it seems to be centered around the inner pair of the tube sockets as I have higher readings in the middle than the outside tubes. I might also just have a not so well matched quartett of tubes.
Now as of tonight the amp was stable with the readings of 32, 35, 36 and 34. I am tempted to fire it up tomorrow again, adjust the bias pot a wee bit down till the lowest reading tube is smack at 30 and call it a day.
However my instinct tells me there is an internal issue and if I don't fix that my bias will eventually run away again and it'll fry some of the new tubes. I don't want that.
I will be talking to my amp tech tomorrow anyway, but I figured some of you knowledgeable folks here might have a clue what might be causing this. The amp sounds great, I wanna have it running smoothly.
Sorry for the long thread, but I wanna be as accurate as possible. If you have any ideas, your input is very appreciated!
Cheers
p.s.: my amp tech developed a tremor in his hands, he can't work on amps anymore. I t would be a no brainer to have him fix the amp otherwise. This way he's helping me out and I might become the apprentice.