redpalting...need suggestions please

[guitarsnguns04]

I recently built a express with 4 output tubes. After playing the amp for about 15 hours I notice the 2 power tubes on one side redpated. I shut the amp down and took voltage reading throught the entire amp. I found v1 voltages were quite high and changed the 2 100k resistors to 150k to get the voltages within 10 volts of the design voltages for the express on v1. Everything else looked good voltage wise. I have -33vdv on the output tubes bias. I had the power supply board mounted a little to close to the chassis and thought it may be grounding out one side of the bias supply so I made the standoffs longer to ensure of no grounding. I also put in a new pi tube with matched triodes and after about 10 minutes of hard playing it still replates on the same side. I have looked all the solder joints over and am a bit stumped. Any suggestions would be greatly appreciated.The amp seems quite stable for an express so Im not thinking parasitic oscillation but I guess its possible maybe. I also had my allesandro bias meter on the amp and when it redpltes one side stays ay 40ma and the other side that redplates takes off and pegs the meter. I have red the voltages across the screen and grid resistors on the power tubes and they seem to be fine. Im stuped? Output transformer problem?
thanks in advance.

[Bob Reinhardt]

i would first swap those tubes with the ones on the other side, to see if the redplating follows the tubes, or stays in the same location. you may just have a bad tube. If it stays in that location, then you can start to blame oscillation or something.
Now by the "100k resistors to v1", do you mean the plate resistors? If so, put them back to 100k, and up the dropping resistor for that node. By changing the plate resistors to v1 that much, you have changed how v1 will react, a lot. I guess unless you like it there, you arent going to hurt anything, it just may make it a bit grainy.

[jjman]

Ditto on the preamp plate resistors. The voltage "across" the grid resistors should be zero at idle. The (negative) voltage on the grid pins of the output tubes is the target area. Probably a bad connection or resistor between the bias tap and the bias feed resistor on that side. But do the tube-switch thing 1st.

[guitarsnguns04]

I was just going throught the amp again..double..triple check..lol. I found the nfb to be positive. I will flip the ot leads and see what happens from here I guess. Its weird though...no squeling like the usual out of phase ots? I hope this fixes it. I have tried the tube swap thing too. It stays at the same sockets with different tubes. I got my fingers crossed. If my cap on the presence pot has the polarity reversed would this cause the nfb to be positive ?

[jjman]

Reversed cap on the presence should not change anything. The cap would not be polarized normally anyway. Such as an orange drop. One side is connected to the shield but that's close to meaningless in that spot.

Be careful switching the OPT leads. Be prepared for screaming in case it's currently correct.

What is the (negative) dc voltage on the grid-pins while the tubes are red-plating?

[guitarsnguns04]

the reason I was wondering about the polarity of the cap there is because when I flipped my test leads on my meter..red to ground on the cap and black to the lug on the presence pot it appeared as negative voltage so I didnt catch it right away. When I put the black meter lead to ground and the red to the pot lug like I should read it.. it read positive. Its a good dmm and I am efficient with it etc. But it made me wonder. I will try to get a reading on the grids when its redplating before I switch the ot leads thanks for the help thus far.

[geetarpicker]

Red plating is typically rare in an Express circuit due to the low B+. However your amp having 4 output tubes is obviously a different animal. What is your B+ at the output tubes? Bias readings for current would help too.

[guitarsnguns04]

I have 409v at the plates pin3 396 st the screens pin4..-33v bias pin5 on all putput tubes 42ma bias with my allesandro bias meter..thanks

[Cliff Schecht]

Red plating is typically rare in an Express circuit due to the low B+. However your amp having 4 output tubes is obviously a different animal. What is your B+ at the output tubes? Bias readings for current would help too.

My Express was red-plating nicely until I got the bias set to the proper -30V.

Also, I'm not so sure you can just throw the two extra tubes in the output without modifying your transformers output impedance and altering the bias of your amplifier a bit. Try setting up your amplifier to get closer to -40 to -60V on the grids. I'm not sure about the transformer impedance though, I think that the usual trick involves removing two output tubes and setting the output impedance selector to 4 instead of 8 Ohms (for an 8 Ohm cab). This gives you half the transformer impedance on the primaries so perhaps you need to double your transformers output impedance. Not sure what the "standard" trick here is though as I've never implemented a four tube amplifier yet alone actually designed one.

[M Fowler]

Cliff Allynmey came up with an 80watt Express and others have also built some four power valve Express amps.

300-0-300 450mA
3.3k OT

[Cliff Schecht]

Very cool! That must be an incredibly loud amp though ..

[gbruzzi]

I recently built a express with 4 output tubes. After playing the amp for about 15 hours I notice the 2 power tubes on one side redpated.

It sounds like a bad coupling capacitor

gio

[guitarsnguns04]

thats the layout I used and I am using sozo caps for this build. I will check the coupling caps. Orientation of these is a bit confusing if it matters. I put the banded end of the caps to the grounded side throughout the preamp section. The tranny are designed for this build and are the same specs as listed above. The amp performed great for about 15 hours and then the redplating started. No issues prior. I will uadjust the bias to -50 and see what happens

[gbruzzi]

thats the layout I used and I am using sozo caps for this build. I will check the coupling caps. Orientation of these is a bit confusing if it matters. I put the banded end of the caps to the grounded side throughout the preamp section. The tranny are designed for this build and are the same specs as listed above. The amp performed great for about 15 hours and then the redplating started. No issues prior. I will uadjust the bias to -50 and see what happens

I was talking about these 2 caps. If number 1 is leaky (I don't think it's shorted) it conducts a little of the DC from the plate of the preceding stage into the grid of the two EL34 connected to the yellow cable and the plate's current raises dangerously. It's the same for number 2 and blue cable.

gio

[guitarsnguns04]

thanks for the suggestion. I will try to read them to see if this is the problem.