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Discussion in 'The Workbench' started by scat7s, Apr 26, 2011.
yeah, I'm an 800 guy, and the jtm seemed to work for me.
When I had the JTM 60, I'd crank the clean channel for crunshy rythm (master volume adjusted to taste) and adjusted the dirty channel for overdrive/solo boost. Sounded great, as I remember it. Works well if you don't care for clean sounds.
yep thats exactly how i plan to use it. its got a great clear but fat crunchy tone on that clean channel. im bypassing the diode clipping section on the boost channel as i type, that should clean up whatever fizzyness/harshness is in the boost channel.
so far a great sounding rock amp. i love the serial and parallel loops too, thats a nice feature. not that im a huge effects guy, but ive got a nice old tube tape echo id like to experiment with at some point...and i like being able to have pre-out and pwr amp in as well, not something i would use all the time, but its a nice feature when you need it.
and some good news, i cracked the chassis open, and its pretty clean inside. no burns, not even any significant discoloration on the boards or tube sockets or anything i can see so far. im going to install a muffin fan anyway though, and bias pot and test points externally mounted for easy biasing.
Yeah, i installed a little bias board right on the side of the chassis, with the bias pot and a couple of test points on it.
How's the board look by the bridge rect for the pre-amp tube heaters? That was the only area on mine where there was any evidence of heat damage.
it looks pretty clean, like i said, not much discoloration anywhere i can see, BUT, i do have a problem, the heater cct pops 6.3A SB fuses. and i believe the bridge is the cause.
when i power up it peaks at 12A and then settles in at about 4A+ with all tubes installed. im pulling 2+ amps with just the pre's installed, which seems high.
my power tubes are good, no shorts. those pre's should be pulling only about 500mA total correct?
anyway, my tubes are ok, tested for shorts, no probs, so the only other thing it could be is the bridge. im going to swap it out on monday and mount it to the chassis on the power transformer side with some service leads. that should take care of that for a good long time.
Could be the bridge, but could also be the smoothing caps on the DC side gone bad. Mind you, that's a fair bit of current you're drawing! Would have thought they'd be showing signs of pain if it was the caps...
thanks i'll keep that in mind
ugh....mine just died...again. Last time it was a loose connection at the OT, they look good this time.
whatcha got for symptoms?
No sound at all, power button lights. Wondering about the fuses, would power still light. Also I seemed to remeber a relay clicking when I switched between channels that I don't hear anymore....very frustrating amp, because it really sounds good
plug your guitar into the effect return, do you have sound?
also, are all of your tubes heaters lighting up when power is on?
if your mains fuse is blown, you wont have power lamp, but if your heater cct fuse is blown, it will power up from the mains, but your tubes wont be lit. that fuse is a 6.3A slo-blow, internally mounted.
you can also try jumpering the effects loop with a patch cord, also cleaning the speaker out jack and work the jack in and out a few times.
do one thing at a time so you know what it is if it comes back during one of these tests.
as ive been poking around in mine, i found that if the speaker jack is not plugged in, the power tubes cathode to ground connection is not made. this is a safety feature to prevent blown transformers if you accidentally forget to have a speaker plugged in i would assume. so if you have a meter and know how to use it, you can check for cathode to chassis gnd continuity. you have to have the chassis removed for this check and a speaker plugged in. i noted this when i found it, as the symptom willl be no output, but all other systems will check out fine.
thanks....I really appreciate it.
can somone tell my how to drain caps, I got a pretty good jolt last night.
alligator clip from one of the positive terminals of the mains filter caps, to a resistor in series (100 ohm should be fine), with a second set of clips from the resistor to the bare metal chassis. let it drain for a couple minutes.
you can leave it like that while your working, sometimes they can recharge themselves, but DONT FORGET to remove your setup b4 you power it back up.
for those tests i mentioned, you shouldnt need to be anywhere near the filter caps though...for the cathode to gnd check, you can use the solder pad on the speaker jack as your cathode connection, you dont need to try to wiggle your way into the guts of the amp to connect directly to the tube socket.
but be careful changing that 6.3 fuse, gently use some small plyers, preferably plastic, cuz you could catch a zap if your not careful in that area.
I think 100ohms is a bit too low, initial bleeding current would reach around 4.5 Amps(dissipating around 2000W), it could burn the resistor.
thx gitnote, i usually grab whatever is handy, my bad. what would you recommend? 1k?
Tubes are not lighting up....sounds like the internal fuse?
Thanks for the help.....
i just had that problem with mine, kept blowing em. i changed the bridge rectifier, and although it didnt seem to make any change on my measured current draw, im not popping fuses anymore...i havent really reconciled why or how that is yet, but time will tell if it comes back or not.
I read elsewhere that the fuse should be a 10....marshall spec'd it too low. Sound reasonable or dangerous?
And thanks, I really appreciate the help.
that is beyond my ability to answer. i dont know if that would be safe or not.
you could start a new thread and see what you get from some of the real techs on here. id be very interested to know myself...if you could safely increase that fuse value, no more problems there...i think it would be most dependent on what the bridge rectifier is rated for...got a link where you read that?