EL84 Amp - Baby Huey

[Brouilly]

Thanks Bill,

I remember that post about the resistor in the bias block. There seems to be light at the end of the tunnel to finish this tonight, but the amount of money and time ive spent on it already, I think I need to take my time.

But at least 42mA is in the right ball park, instead of being in a completely different game as it was before.

Charlie

 

[Soonerorlater]

http://www.diyaudio.com/forums/tubes-valves/72536-el84-amp-baby-huey-10.html

Post 95

 

[malcolmfraser]

Charlie,

This is ok for a test outside the amp. You wont know the true current through the bias until it is fitted to the amp as other things may have an effect such as your B+ value. IIRC Gingertube explains how to lower current by increasing the 16r resistor to 18r should this become necessary.

HTH Bill

Cool, thanks Bill. Will get the PSU built and then throw a load onto it and see what needs to be fine-tuned.

Basically, the first part of the build will be the PSU and the ‘box’ to house it, the OPTs and to make sure all the earthing is spot on.

The output stage/tubes I will be using Morgan Jones suggestion of a ‘plate’ to carry the tubes that then floats on top of the box. That way if I want to swap out the tubes, or go PPP, it not a major rework ;-)

 

[Soonerorlater]

An ultralinear ppp baby huey sounds interesting - not to mention more work for your winder.

 

[Brouilly]

Quick question about the heaters,

When wiring the ECC803s heaters, in my build they are both reasonably close to the heater winding. Can I just wire the heaters with short leads, or do I still need to make an effort to twist the wires together?

At the moment they are wired in series, but if I did them individually I could twist the wires. Should I do this, or will it not make much of a difference?

Thanks

Charlie

 

[adamus]

twist, its a good habit to always twist the ac heater wires.

use a cordless drill, super quick and effective.

 

[Brouilly]

thanks adamus,

yeah drill is in hand ready to go

 

[malcolmfraser]

Heater Wiring

Hi Charlie, to quote Morgan Jones "Building Valve Amplifiers" P118:

To minimise the external electromagnetic field, the heater wire should be tightly twisted. This means that although any given twist induces a current of one polarity, the twists either side of it induce opposie polarity, and so the fields tend to cancel. This twist should be maintained as close up to the pins of the value as possible.

 

[Brouilly]

Just fire up up my BH for the first time, reading a B+ of 372v. The PT secondary is 0-250-350, and am reading 264v AC before the rectifier. I'm confident that my wiring is correct, but that's not saying a great deal.

What would the cause be for such a high B+?

Charlie

 

[hidnplayr]

264 * 1.41 is 372 so that makes sense.
You should wait a couple of seconds for the tubes to warm up and start conducting, this should lower the voltage to the normal level ?

If not, something is wrong..

 

[Brouilly]

I'm confident that my wiring is correct, but that's not saying a great deal.

Charlie

My wiring was wrong haha

Its now a stable 329v, still a little high no?

Im dropping it with 47R like, should I try a little higher? I know it would increase the output impedance of the power supply, but at the moment buying different value caps isnt an option, speaking from my wallet.

 

[Soonerorlater]

If I had 329vdc stable I would be thinking of my record collection at this moment!

 

[Brouilly]

Everything is up and running, but there is a fault in the signal line, im getting nothing at all. So Close!!

Wish I thought more about the layout now, going to be a tricky job finding the problem

 

[Brouilly]

Is 320vdc too high for the ecc803s plates? I would of thought it would be lower. maybe there is an issue with my ccs? When I touch that part of the path with a probe it comes through the speakers, so the fault is definitely before that.

 

[Soonerorlater]

Yes, IIRC it should be around 170v dc on pin1/6 of ecc803S.

 

[Brouilly]

MUSIC!!!!! oh wow...

... its not quite singing yet, but... that's amazing

 

[Soonerorlater]

Cigars all round - it's another one

 

[Brouilly]

just trying with and without feedback, cant really tell the difference...

... maybe ive done something wrong, or maybe my ears arnt trained enough yet.. or maybe its the excitement. Should there be a big difference?

Sounds absolutely amazing, granted, its the only the 4th valve amp ive heard, but big thanks to Gingertube

 

[Brouilly]

A few results...

I took my recently finished BH into the uni lab for some test.

I used the Prism Sound Dscope, which is really impressive, although Im still not fully up to speed with its features.

Didnt manage to grab any screen shots but from what I remember...

THD+N at 1W - 0.45%
THD+N at full power 5%

At full power the amp is clipping, in fact, it has to go down to 3/4 power before the signal doesnt clip. This would explain the high harmonic distortion, but it does not explain the very high 3rd harmonic distortion. So high in fact that it is more prominent than the second harmonic distortion.

I also tested the NFB of the amp, as posted before, I heard no difference with and without feedback.

When connecting the the 12K feeback resistor to the circuit I looked up at the screen amazing, no harmonic distortion visible on the FFT. But, this was due to the noise floor being raised well above the harmonic distortion.

So...

My wiring may be wrong? Or is there other answers...?

The third harmonic issue may be sorted by Allen Wrights suggestion of the resistor in series after cathode caps...? But surely this is still too high...?

The amp is clipping at full power (input was 1.5Vrms), so am I running the valves too hot? Or maybe its a biasing issues?

All of the above are guesses really...

What do you guys think?

Charlie

 

[Brouilly]

Oh also the frequency response was really good, using the Hammond 1608 and the response was accurate to the 1608 specs, although it did rise slightly towards the bottom end.

And a 10KHz square looks a lot like a broken umbrella