Re: Calling all jtm45 kit experts..cuz I ain’t one...
Posted: Mon Mar 11, 2019 12:45 am
Progress note:
I measured all the voltages, everything seemed to come out in reasonable ranges, so I think I’m on the right track. In general, I think I’m measuring a little higher than expected in some areas, but it think I’m ok.
My problem right now is with the bias adjustment. I am measuring 54mV and 46mV on V4 and V5 respectively. Too high (shooting for 40-45 in the documentation for KT66. I even waited for about an hour to see if things would settle down. They didn’t. Unchanged.
As was pointed out previously the 1 ohm resistances can screw up bias current measurements. I was never able to verify the true resistance (1 ohm is at the fringe of most meters because it’s on order of other resistances in the measurement circuit), but now I think I have to, before I consider changing a bias adjustment resistor or something. The solution is a 4 wire resistance measurement. Fun.
For anyone interested, here are my recorded measurements thru the testing phase. V1-V3 have 2 numbers, the top number is before KT66 tubes were added. These valves are expected to be high without output tubes in. As I indicated earlier, this is with the Mercury Magnetics power transformer. The line voltage was tested at 120vac. Sorry, I wish I kept track of which tube I put where.
https://ibb.co/FBQQ4Sy
https://ibb.co/KFXtkSF
Annnnnnnd,for fun, I mounted it all up to see what it would look like. (Still not ready to rock until I get the bias worked out and test the output transformer wiring). I like the look of my Lunchbox JTM. Head cab is from Mojotone. Bottom is a reissue of the circuit with tremolo from Marshall (head version of the famous combo). Gut shots in a previous post in this thread. Looks like my amp had a baby.
https://ibb.co/zX1TZ3Y
Cheers!
I measured all the voltages, everything seemed to come out in reasonable ranges, so I think I’m on the right track. In general, I think I’m measuring a little higher than expected in some areas, but it think I’m ok.
My problem right now is with the bias adjustment. I am measuring 54mV and 46mV on V4 and V5 respectively. Too high (shooting for 40-45 in the documentation for KT66. I even waited for about an hour to see if things would settle down. They didn’t. Unchanged.
As was pointed out previously the 1 ohm resistances can screw up bias current measurements. I was never able to verify the true resistance (1 ohm is at the fringe of most meters because it’s on order of other resistances in the measurement circuit), but now I think I have to, before I consider changing a bias adjustment resistor or something. The solution is a 4 wire resistance measurement. Fun.
For anyone interested, here are my recorded measurements thru the testing phase. V1-V3 have 2 numbers, the top number is before KT66 tubes were added. These valves are expected to be high without output tubes in. As I indicated earlier, this is with the Mercury Magnetics power transformer. The line voltage was tested at 120vac. Sorry, I wish I kept track of which tube I put where.
https://ibb.co/FBQQ4Sy
https://ibb.co/KFXtkSF
Annnnnnnd,for fun, I mounted it all up to see what it would look like. (Still not ready to rock until I get the bias worked out and test the output transformer wiring). I like the look of my Lunchbox JTM. Head cab is from Mojotone. Bottom is a reissue of the circuit with tremolo from Marshall (head version of the famous combo). Gut shots in a previous post in this thread. Looks like my amp had a baby.
https://ibb.co/zX1TZ3Y
Cheers!