Hi all, my first post (I think, been a stalker for quite a while), usually find what I need without making myself intrusive and annoying 
.
Got one on the bench doing something I've never come across before and has got me a little stumped (which in some ways is great, I'll have learnt something new by the time I get to the bottom of it, as frustrating as the journey may be).
A brief history of me... Muso and been a repair tech on stage equipment for 20+ yrs, I'm not an EE but I've got a pretty good grasp on fundamentals of tube amps and have done upwards of 50 ground up builds (with varying degrees of success of course.. hehe), some of my builds have seen 10 years or more of touring and are still going strong so I figure I'm not totally crap at it...
The one on bench.....
Loosely a "Matchless" DC30 build in a dead Laney VC30 1x12 combo chassis I had kicking around.
stock 4xEL84, hanging around 300V main reservior/plates, 68ohm cathode/220uF bypass, looks good ball parking 10V on cathode, push to edge of clip, looks good, push into medium clip, start to see a touch of x-over notch, push it a little more and x-over notch goes horrid, reservior/plate Climbs as much as 30V, cathode drops as low as 3V, back it out of clip, x-over notch still ugly, over biased to the hilt, rail still up 30V cathode at 3V, slowly rail comes down and cathode comes back up (takes a minute or two with signal a little below clip) all looks good again, good strong clean sine into clip, push it and away we go again....
signal is currently injected at the PI (LTP), have subbed out the output tranny, rebuilt half the supply, swapped cathode resistor/cap, different set of tubes, I'm running out of things to replace, so I'm now leaning toward design issue being the culprit. Supply is close to the matchless in spec. with the differences being... main res cap 330uF, the other side of choke 56uF, and 33k instead of 22k droppers to pre supplies (these are the bits I had on hand, you know how it goes).
Has anyone ever had this kind of thing happen before?
I'm sure I'll slap myself when I work it out, but I feel like it's defying ohms law with the rising plate, dropping cathode, yet super ugly X-over nastiness.....
Cheers all....
.Got one on the bench doing something I've never come across before and has got me a little stumped (which in some ways is great, I'll have learnt something new by the time I get to the bottom of it, as frustrating as the journey may be).
A brief history of me... Muso and been a repair tech on stage equipment for 20+ yrs, I'm not an EE but I've got a pretty good grasp on fundamentals of tube amps and have done upwards of 50 ground up builds (with varying degrees of success of course.. hehe), some of my builds have seen 10 years or more of touring and are still going strong so I figure I'm not totally crap at it...
The one on bench.....
Loosely a "Matchless" DC30 build in a dead Laney VC30 1x12 combo chassis I had kicking around.
stock 4xEL84, hanging around 300V main reservior/plates, 68ohm cathode/220uF bypass, looks good ball parking 10V on cathode, push to edge of clip, looks good, push into medium clip, start to see a touch of x-over notch, push it a little more and x-over notch goes horrid, reservior/plate Climbs as much as 30V, cathode drops as low as 3V, back it out of clip, x-over notch still ugly, over biased to the hilt, rail still up 30V cathode at 3V, slowly rail comes down and cathode comes back up (takes a minute or two with signal a little below clip) all looks good again, good strong clean sine into clip, push it and away we go again....
signal is currently injected at the PI (LTP), have subbed out the output tranny, rebuilt half the supply, swapped cathode resistor/cap, different set of tubes, I'm running out of things to replace, so I'm now leaning toward design issue being the culprit. Supply is close to the matchless in spec. with the differences being... main res cap 330uF, the other side of choke 56uF, and 33k instead of 22k droppers to pre supplies (these are the bits I had on hand, you know how it goes).
Has anyone ever had this kind of thing happen before?
I'm sure I'll slap myself when I work it out, but I feel like it's defying ohms law with the rising plate, dropping cathode, yet super ugly X-over nastiness.....
Cheers all....
Relocated to Instruments & Amps per forum policy.
Seems like everything is normal up to a point and then the EL84s are conducting much less current then they should be, for some reason. This will pull down the cathode voltage and push up the plate and screen voltages. But why? Bad connection somewhere in the supply? EL84 heater voltages look OK during the occurrence?
All is well with the world!!!! I should go to the dunce corner now!!!
post PI MV build matchless style, no reference for the grids through the MV, forgot some tie down resistors needed..... DUH..... couple of 220k to ground and it's crankin!!!
All that work chasing this gremlin... hehehe... I'm a silly old buggar....
post PI MV build matchless style, no reference for the grids through the MV, forgot some tie down resistors needed..... DUH..... couple of 220k to ground and it's crankin!!!
All that work chasing this gremlin... hehehe... I'm a silly old buggar....
I built a 80 watt quad el34 valve amplifier.
I powered it up and got a red plating el34.
Took me a while to figure it out but I had completely missed putting in a grid stopper resistor. Of course the valve was just passing loads of current and getting red hot.
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