• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 Amp - Baby Huey

Post a good picture, maybe someone can spot something, never hurts to have a fresh set of eyes look at it.

I had to leave it at work, so hopefully my eyes will be fresh in the morning, and hopefully you kind DIYers will be about for guidence.

I had swapped the EL84s around on the problem channel, and this is when the power supply resistor blew. This didnt happen for the first hour of testing so unless the valve faulted within that time I cant see it being an internal problem...?
 
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Sorry to change the subject, but I was wondering... If one uses a giant wad of LED's for biasing the output stage, is it mandatory to use the MOSFET follower to drive the output tube grids? I know that makes the circuit mo' bettah, but will the plain RC-coupled-12AX7- to-EL84-grid version work reasonably well with the outputs biased by LED's?

I do see that v.2 of the BH amp has fixed bias and the MOSFET followers, just not sure if going to fixed bias makes the MOSFET *mandatory*.

Thanks...
 
I fired it up again after raising the dropping resistor in the power supploy to lower the HT, I also swapped the valves around.

The first 10 sconds was fine, and then the same channel's LED bias lit up very fast and cause it's power supply resistor to burn out.

I checked and double checked the wiring of this channel and it is OK. Might I be looking at a problem with the OPT?

Thanks
 
I think this thing may have quite a few gremlins. I decided to just work on the other channel for now.

Apart from a HT of 350V, I guess due to half the load, I am reading 73V on one anode of the ecc803, and 315V on the other. This doesnt change when swapping valves about. So I guess they are still not drawing current? Or one half is?
 
It seems very strange.

I measured the voltage of the ecc83 anodes at turn on

Anode 1 - Starts at about 500V and ends up at 314V when the el84s start drawing current.

Anode 2 - mirrors that but ends up at 73V when the el84s start to draw current.

If I tap any rail with DC on the '314V' half the LED bias blinks, but when I tap any rail on the other side the LED array doesnt change. It still is drawing about 90ma which is obviously too high.

This is the first time ive delt with a bias techneque like this, can someone suggest some actions to lower the current through the array and maybe suggest some fault finding steps?

Many thanks
 
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I really can't understand why im getting such different readings.

The 220K plate resistor on the lower half of the driver is dropping nearly 150V, where the other plate resistor is dropping 5V.

I have swapped everything over that I can, check my wiring again. Checked the ccs again. Anyone got any bright ideas, I am all out.

Cheers
 
If this was my amp I would like to know that all is well apart from the ECC803s ccs and the RLD bias scheme for the EL84's. I had similar problems with the ccs's in the basic BH version. I simplified the amp by replacing the preamp ccs with a 675R resistor ( IIRC ) and biased the EL84's separately with 270R resistors. Happily this gave me a working amp with all the proper voltage values. I then reintroduced the ccs's one by one solving any local faults as I went along.

HTH Bill
 
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