Tube Amps / Music Electronics
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|1/27/2001 1:06 PM|
|sixtystrat||tubes for JCM 800|
I have a (1984 I think) JCM 800 2x12 combo, 100 watt (4211). I wanted to replace the 6550s with EL34s and, after looking through some of the literature, it looks like maybe it should have EL34s in the first place. Does anyone know about my year/model? Is it just a matter of rebiasing? Any info on the strengths/weaknesses of these amps and my year model would be appreciated. Also, I replaced the rectifier tube on my '69 Deluxe Reverb with a lower power tube as suggested by the group and I love the difference! Thanks.
|1/27/2001 2:12 PM|
|Lew Collins||Flying leads?|
Or are the pots attached directly to the circuit board? IMO, the version with flying leads is a better sounding amp and a better designed circuit. You'll have to replace the bias resistor on the bias adjustment pot to get the bias within range. I forget what I changed mine to. Hopefully Bruce or someone will step in here now and take over with some real information!
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|1/27/2001 5:30 PM|
Well, the pots look very different than on my Fender, but it is mounted vertically and is on the circuit board. Thanks.
|1/27/2001 3:58 PM|
||Re: tubes for JCM 800|
If you haven't been inside a tube amp before or are aware that there are things that can kill you even after the amp has been turned off, then take it to a local-tech.
6550's to EL34's
1.) Locate the 47K resistor that is above the bias pot and solder another 47K in parallel.
2.) Set the bias pot to max (in and around -38V)
3.) Move the feedback wire (purple wire) from the
4-ohm tap the 8-ohm tap.
4.) Install the EL34's and adjust the bias (somewhere around 32-35ma depending on the plate voltage and all that--ha, ha)
5.) Plug your guitar in and rock out!
What is it that you want to know?
Just follow the directions above and you should be ready to rock.
Joe, I'm not sure I follow you regarding the strength/weakness thing(?)
Hope this helps
|1/27/2001 5:38 PM|
That's just it, I can't find a 47k resistor anywhere near the pot. That was why I was wondering if maybe it should have had EL34s all along...
As far as the strengths/weaknesses thing, I suppose you make a good point. If I like the way it sounds, who cares what everyone else thinks about the quality, collectability, etc.? I like the way it sounds, it's reliable, but how I hate carrying it up and down the back steps of some of these nightclubs. Anyway, thanks for the help and I'll be careful. I'm clearly a novice.
|1/28/2001 7:20 PM|
What's in there now for a resistor value??
Tone is very personal and highly subjective. If you like it then cool! if you want to improve it then go for it! Make sure you're happy with your tone, after it's not very inspiring to play on a "lifeless" sounding amp. You could trade it in for another amp you like...there's a lot of possibilities! (ha, ha)
It seems all good sounding tube amps are HEAVY (ha,ha) I guess that's the requirement for good tone
|1/29/2001 6:40 PM|
Tip: the first two digits from the serial # are the year of production.
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