EL84 parallel Single Ended power amp

[Firebottle]

I was given an incomplete Chinese SE amp and it seemed churlish not to do anything with it. So an up-rated 'Dulci SE' is the goal.

It wasn't built very well and was missing the output valves and mains transformer. As the output transformers are supposedly rated at more than one EL84 can provide I thought I would build it as a parallel EL84 SE, particularly as I have a few spare EL84's.

I have sourced a mains transformer that provides the HT and rectifier heater winding so have fitted a second transformer under the chassis for all the other heaters.



So far so good, the mains and heater wiring is finished. The rest of the bits will go in tomorrow.

To be updated

[JimC]

Very nice, I have a soft spot for the EL84, a lovely Valve indeed.

Where do you source your Transformers please?

Jim.

[Firebottle]

This is the mains transformer I bought:
http://www.ebay.co.uk/itm/3817928553...%3AMEBIDX%3AIT

The output transformers were already on the chassis, so they are of Chinese origin.
They are approx 3.6K ohm primary loading so OK for a parallel EL84 drive.

[Bigman80]

Wow !!! I love it and I think I've seen the guts before ?

More Firebottle magic

Sent from my EVA-L09 using Tapatalk

[Radford Revival]

This is the mains transformer I bought:
http://www.ebay.co.uk/itm/3817928553...%3AMEBIDX%3AIT

The output transformers were already on the chassis, so they are of Chinese origin.
They are approx 3.6K ohm primary loading so OK for a parallel EL84 drive.

Sounds like a good load for the low distortion / higher damping factor end of things (with parallel EL84s). Are you going with pentode, UL or triode mode?

[Firebottle]

Pentode operation Will.
However at the moment the driver stages don't appear to be working very well

[JimC]

This is the mains transformer I bought:
http://www.ebay.co.uk/itm/3817928553...%3AMEBIDX%3AIT

Thanks. They look very good and well priced.

.......However at the moment the driver stages don't appear to be working very well

What Valves are being used?

Jim.

[Firebottle]

Chinese 6N1P, as that's what was in the kit build. I have some other contenders

[Radford Revival]

Pentode operation Will.

Just don't forget to add some feedback

[Firebottle]

Well it's been a bit of a voyage of discovery.

Still can't quite make out what the primary impedance of the output transformers are; with 8 ohms on the secondary the measured impedance at 1KHz was reading 3.5K. So presumed it would be ideal for parallel EL84's.

However with one or two EL84's I could only get a maximum of about 3.5W
Turns out to be a bit of a juggle of compromises, I suppose that's typical when throwing together a random bunch of 'junk box' parts.

The HT is close on 300V so I am keeping the current flow (Ia) through each valve on the low side to avoid excessive anode dissipation. Also I was trying to keep a valve rectifier in the psu, but the one I have is only rated to 120mA max. That's now been ditched as I need more than that.

I tried EL86 output valves and with just one I could manage 4.5W, but that was with rather excessive Ia, resulting in an anode dissipation of 19W
Not conducive to long valve life. With two in circuit and the Ia reduced a bit still only gave about 4.5W. Hmm ....

So I tried the only other alternative valve in stock, the 7189. This turned out to be more promising, so with a respectable anode dissipation of about 10W per valve, a pair of 7189's is giving me 5W just before symmetrical clipping. I'm happy enough with this although was hoping for more.

The driver stage utilises an ECC83 per channel in a half mu configuration.

Finished amp:



Lovely clean sound with plenty of punch in the bass.