#1
Ok, so I had this weird metallic sound coming out of my amp, which I'm pretty sure is a preamp tube going bad. However, while troubleshooting I was Biasing the amp. Egnater suggests that the EL34 Section of the amp should be biased between 65 and 75 mV. The first time I tested it, I was able to set it to that setting. However, after leaving it on for 10 minutes per the instructions, the multimeter started reading into the 100's. The 6L6 side had no issues, and I could not turn lower the bias any farther than I already had it on the EL34s.

I've googled this and can't find anything, so I would like to know what could cause these tubes to bias so hot.
#2
What's the plate voltage?
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#4
Measure the voltage on pin 3 of the EL34 with respect to ground. Be careful, it will be up around 400V. Keep one hand in your pocket.

Edit. You don't need to open up the amp, just remove the tubes and measure it on the socket. That'll be way close enough to do the calculations.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
Last edited by Cathbard at Feb 4, 2015,
#5
First of all, if you don't know what you are doing this could all be very dangerous.

Unless you are having volume drops or some kind of 'warbliness' I wouldn't worry about your power tubes right now anyway.

Let's get to the metallic gallonk sound first.

PS: Get a biasing probe and check out my biasing blog in my profile.
#6
Alrighty then. The metallic gone sound is kind of strange. It's a 2x12 combo, and it only comes on sometimes. I noticed it when I jammed with a few friends the other day. There's two parts to this noise. The first is that when playing, it creates almost a static unplugged sound. The signs, seems fine but when I play a lick and stop it sounds like a bad cable. However, I've switched cables etc and it still continues.

The sound even happens when there is no cable plugged into the amp. I also noticed that if I lightly tap on the outside of the combo, that it makes that metallic gonking noise. It sounds as if something inside the amp can't make a connection. All the power tubes seem to be in working order, but when the amp makes the noise, I also hear some noise coming from the tubes like how they sound when the amp first turns on. I've searched the interwebs about what a failing preamp tube can sound like and I've seen some threads about it sounding like a metallic gonk
#7
Try tapping each tube with a pencil. If any sound comes out of the speakers, replace that tube.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#8
Ok did the pencil thing. Turned out it was one of the el34 power tubes. I noticed that it was flickering combined with the sound from the amp. Removed the tube and turned it on. No more racket.
#9
Thought so, replace both of them.
Gilchrist custom
Yamaha SBG500
Telecasters
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100


Cathbard Amplification
My band
#10
Yup, as soon as I saw the tube flickering I was like oh its a power tube I'm an idiot haha. checked the bias on the el34 side and everything was fine. Now i have a nice new set of JJ's