Plate voltage w/ tubes in

[lowell]

working on vibrolux reverb and the plate voltage on plates are 440VDC without tubes and a low 365VDC w/ tubes in. this does not seem right to me. The 6L6 in there are brand new JJ.... I know just cause they're new doesn't mean they're good. The amp is also suffering from low headroom and a fair amount of distortion. I have set bias at 35ma, replaced ALL plate resistors throughout the amp, all bypass caps, bias supply diode and filter cap, and all coupling caps. I have not touched filter caps because they test fine for leakage and the VDC on pin3 with no tube seems fine. I have decided to try a different set of tubes. I place one tube in one socket w/ other socket empty, fired up the amp and the voltage was 440VDC for about 3 seconds then blue sparks exploded inside the tube... I shut it down quickly. I put the same tube into the other socket and fired it up. The VDC on the plate fluctuates all over b/t 440VDC and 380VDC. so then assuming that this tube is now bad I put a new one in the second socket and it was fine... 440VDC. I then added another new tube to socket 1 and the amp makes a HORRIBLE farting noise. I went to pull out socket 1's tube and the socket came out of the chassis with it!!!! Could this be the problem? I know this IS a problem, but would a loose/bad tube socket cause fluctuating voltages and farting noises? Does this mean the socket is arcing inside cause I don't see anything as far as burn marks go.

[Mars Amp Repair]

Well,
let's say the grid pin (5) decided to let loose...then the tube is drawing max current...not good. If for some reason the grid goes positive, it starts drawing current & that's not good for a grid.
similarly, if the screen plate pin gives out, then the screen grid tries to act like the plate & draw all the current. It too, will glow & eventually crap-out.
Just running the amp with one tube shouldn't cause that tube to blow at low volume, but still not a great idea.
Once a tube shoots sparks inside, you might as well condsider it shorted & dangerous to put back in.
Sounds to me like your lack of headroom could very well have been a result of bad socket connections. Replace those puppies.

p.s. it would be wise to go back & check for certain that you have all the correct values ie; you have the color code correct. It's easy to replace a 100K (Yel/purp/yel) with a 10K (yel/purp/or) & so on, esp if it's your first time.
When you do something like replace all the plate resistors or all of anything in a device, it's always good to do them one at a time. It's very easy to goof up if you cut them all out first & then attempt to replace them.
Most techs will agree that the most difficult problems to solve are the ones you caused yourself, cause you don't always think to go back & check your own work sometimes. Us older guys dont' do that quite as much as we did in the early days...I'm thinkin' ;-]
I don't know how you did it, but
glen

[Mars Amp Repair]

Oh man...It's too late in the day for me to be thinking. I got those color codes wrong!! See how easy it is!!
Sorry....Yel/Purp/yel = 470K yel/purp/or = 47K I'm going home now....ZZZzzzzzz glen

[lowell]

Thanks Glen,
Yeah makes total sense w/ the grid and screen not being connected and the tube operating at maximum... definitely not good. I'll replace those tube sockets... thanks! I will also double check those plate resistors and whatnot, never a bad idea to double check the work I've done... as I'm constantly being reminded.