please Peter: VH4, bias probe, confusion and issues...

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yu-man

yu-man

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Hi,

it's the second time I change the powertube tubes on my VH4...I used to follow the old tutorial provided in the tech corner on the Diezel Website...the first time it was ok, but the second time, I had a lot of trouble:

I bought Ruby EL34 BHT matched quartet....I replaced the tubes and carefully foollw the tutorial to makes current measure with my DMM... first weired thing, the current was first very high compared to my old tubes (nearly 120-130 mA), so I turned it down to have about 65mA (-65mA between red and yellow wires)...it was ok...but I wanted to see how it was on the other side of the transformer (positive value between blue and red wires)...so I turned off the amp, made my plugs with the DMM between blue and red wires, turned on the amp, turned standby to run position, and ther,ther was a big "ploc" followed by a permanent loud hum comiing out out from the speakers, and the DMM didn't show any value anymore: the DMM's fuse was blown...to make sure it was nothing but the tubes that were broken in the amp, I decided to put my old tubes again...it worked...i could read current between red and yellow wires and adjust bias...but when i wanted to read positive values (between blue and red) again, same thing: Big hum, and DMM's fuse blowing... I turned off the amp, then turned it on again, and I could read current between red and yellow again and adjust bias, no hum..everything seemed to be fine...So I decided to put my new tubes again, but this time it doesn't work: big hum and DMM's fuse blowing when turning standby to run position, even when having my DMM plugged between red and yellow wires (I've blown 5 DMM fuses making the operation again and again)... So I deduce that my new tubes were broken (at least one of the four)...the only way for me was to put my old tubes again until I get new tubes....

So I purchased a new quartet on Rubys and went back on Diezel Site's tech corner...and saw that there was no biasing tutorial anymore, just the advice to buy a bias probe...so that's what I've done: I'm in France so I purchased a bias probe at tube town like this one:

http://www.tube-town.net/ttstore/produc ... mbled.html

I just received it today and I have to say I'm a bit confused about how to use it: there's something strange with the instructions paper that came with the bias probe: they say to set the DMM to 200 mV, but you at diezel give current values (mA)...so how to know the value I have to set with this bias probe and my DMM measuring voltage (mV)?....on tube town site they say that the probe measures a voltage but with 1ohm resistor so as U=RxI, it's like measure cathode current...is that it?
and am I true if I say that the bias probe gives a measure for just one tube?
so when you say on Diezel Site: 60mA per pair fore EL34, does it mean that I have to set the bias to read 30mV on the DMM, using this bias probe?

Now I'm waiting for my new tubes (they come from USA, couldn't find EL34 BHT at european dealers) and your answer to set bias and rock'n roll....
 
First, the new tubes are rated higher than the old ones. That is why you say 120mA. Be sure that the black lead/common stays on the center
tap or red wire. You may have simply crossed them up and blew the fuse on the DMM. The DMM may have caused the noise due to how you connected
the leads. If the tube were bad or biased too high it would have blown the tube fault fuse or went into cutoff or even red plating.
It is likely that the original set of tubes was fine.

With a bias probe like the one you bought you put the common/black lead in and the red/hot lead into the socket to read 200mV DC.
When you get a reading, it will be for one tube only so set the bias to 30-35mA (=mV) to get 65-70mA (=mV) per pair.
 
You need to read mV on your DMM - inside the adaptor is a 1Ohm resistor, which "converts" (the reading) of currents (mA) into volts (mV).

So set your DMM to read 200mV DC and act as if you would read mA.
And with such an adaptor you are reading the idle current for a single tube !!!
 
first, thank you guys for your quick and instructive answers....

Be sure that the black lead/common stays on the center
tap or red wire. You may have simply crossed them up and blew the fuse on the DMM.
Click to expand...

Yeah, the black lead/comme stayed on the center....that was the rd lead that I hooked up on the blue wire...and I'm partially sure that I didn't make the lead touch to each other (which yould have cause a short-circuit I guess)...the weired thing is that even with my old tubes, each time I tried to makes measures between red and blue wires (to get a positive value), it blown my DMM fuse...the second weired thing is that with my old tube, I had to replace the DMM fuse and everything was fine, but with the new ones, I had the big hum even with no DMM hooked up on the transformer...

If the tube were bad or biased too high it would have blown the tube fault fuse or went into cutoff or even red plating.
It is likely that the original set of tubes was fine.
Click to expand...

Are you saying that my apparently broken new tubes are finally good? Actually, none of the tube fault fuse has blown when I had the issue...Although, it was very strange because, after replacing the DMM fuse to get it working fine, when I had my old tubes in place, it was good, but each time I tried put the new tubes, it blown my DMM fuse and I had the big Hum, but the tube faulty fuses stayed fine...Also, the second time I tried to put the Rubys, when turning standby to run (and so get the "plock" with the big hum), I thought I've seen a spark in one of the four tubes, but I'm not sure...

Anyway I can't try right now, because the amp is in my band's rehearsal room, so I don't have it at hand...but I will try again with this apparently faulty Ruby kit and the bias probe...if it works, it will be all good for me: I'll have a spare kit with the new one I'm waiting for...The important thing for me is that, apparently, all thoses issues didn't broke anything in the amp, because everything seems to be fine with the old tubes (except the fact that the sound is not the best, and that I have some kind of crackle/rattle (I don't know exactly the english word for this) just after switching standby to run position, but it's because of the tubes's oldness, right?)

So again, thank you guys, for thoses precious informations...
 
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