#1
I have a not-so well known Tube Works Provalve 60 that I am having some issues with. So my problem started when I began work on the amp to add in some relays for a more robust switching system. While preparing to test it out, I had accidentally left the output transformer disconnected and when I took the amp off standby, the tubes did not like it. One screen resistor had blown and I believe that at least one of the tubes died. After replacing the resistor and connecting the OT, I did get output to my speakers for about 10 seconds until the signal gets really fuzzy and one of the tubes begins to red-plate pretty bad. I have isolated that issue to the tube as when I switch the tube the red-plating stays with the tube. So I bought a new pair of GT 6l6GE's and here I am now. When I fire up the amp I am getting signal but my concern is that both of the tubes are red-plating and getting very hot within 30 seconds of having the B+. I believe that I need to rebais the amp but, even after having read a lot of info on the subject, I am not sure of how to go about doing it with this amp. I have drawn up a small diagram of what I think is relevant to this issue. Right now I am thinking that the 360 ohm 10W resistor is what needs to be worked with but I have come here to make sure. I am planning to convert it to adjustable bias while I'm in there as well. Let me know if you need more info.


BTW the 3 points toward the bottom left is the stand by switch. B+ goes to the middle point.

Thanks a ton.
Last edited by Shepdog at Dec 9, 2009,
#2
sounds like somethines not converting the current and the tubes are being maxed..
White/Cream Faded 1966 Stratocaster
100 Watt All-Tube JVM410H
1960 Lead Cab-B
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#4
I have also read that the screen resistors could be bumped up to 1K to reduce the power dissipation which is what I'm needing to do, correct?
#5
Obviously, the reason your tube glows red is there's way too much current pumping through it.

The most obvious thing that can cause too much current, is the Grid voltage not being sufficiently negative (vs the cathode) - have you mesured the DC voltage between the cathode & grid to see if it is within range?
#6
So between the grid 1(pin 5) and cathode (pin 8) i am getting - 21.5v. Between cathode and plate I am reading 600v. What could be causing this? Does it matter that I took these readings without the tubes in?
#7
Replace your screen resistors and output tubes. Those 470R screen resistors should really be 5W.

You need to be more specific about the OT. When you say you forgot to hook it up what do you mean? Did you leave the primary or secondary leads unconnected?
Your OT could be shot...
#8
I left the OT disconnected from the amp circuit. Based on that, the OT should be safe right? I can test it by measuring the resistance between windings too right? The output tubes that are currently red-platting are less than a week old. Have only seen about 5 minutes or so of B+ voltage and were not the ones in the amp when it was turned on without the OT hooked up.
Last edited by Shepdog at Dec 9, 2009,
#9
Re-measured the plate voltage with tubes in and got 500v which is still high correct? Did not check the negative voltage on the grid again. Should I?
#10
Without a schematic, this is going to be difficult to troubleshoot via a forum! (I used to work with Valves in the military some 30 years ago...I then dabbled with valve amps as a hobby...but again it's some time ago!)

You're in a tricky situation...you're presumably worried about having the OT connected, but the current through the valve will be dictated somewhat by the output load that the valve sees, so (IMHO) by taking the OT out of circuit you're screwing up the valve's biasing (I'm assuming it has a self biasing resistor between the cathode & ground?)

How I'd tackle this is to carry out some DC resistance checks on the OT...confirm nothing is severely amiss, then connect it back to the valves' output. Additionally, you really are going to need to establish a 'ballpark' cathode->grid DC voltage figure for that particular valve.

Once everyting is normalised, I'd then have the meter in position with croc clips & take DC readings at the cathode, grid & anode ....when the valve strats going red, something should leap out as being amiss (ie a DC voltage will go out of kilter) ...just be sure to hit that off switch pronto!
#12
Well I do have the OT connected back up and i checked the resistances between windings of it and seems to be fine. I mean I do get clean sound through it in the few seconds that it does work. Cathode to anode is 500v with tubes and OT plugged in and cathode to grid is -27.5v .
Last edited by Shepdog at Dec 9, 2009,