Eico HF-22 Circuit

[PAK3TA]

I'm currently thinking of building an Eico HF-22 style Amp. Found the circuit @ :Eico,Grommes,HK,and Heath.

Question is: The comment states, "This is another variation on the Mullard 5-20 amplifier, but with low enough output impedence in the driver circuit to accomodate fixed bias should you wish to implement it." How would one go about implementing this modification and would it be different for 6l6's?

Also the picture ( http://www.triodeel.com/eicohf22.gif ) is a bit distorted and hard to read, anyone out there who has built this circuit who has a better version or any advice?

[pchw]

This should complement the other schematic.

[PAK3TA]

Thanks, that's going to help alot!

[Eli Duttman]

"This is another variation on the Mullard 5-20 amplifier, but with low enough output impedence in the driver circuit to accomodate fixed bias should you wish to implement it." How would one go about implementing this modification and would it be different for 6l6's?

Mullard style circuitry is an excellent choice. Unlike Williamson style, which is instability prone, Mullard style can be safely combined with "budget" O/P trafos and still yield satisfactory results.

For Mullard style, with "fixed" bias 6L6GC "finals", I suggest you use the Harman Kardon Citation V, as your model. The schematic in that "Sams" folder is flawed. Look here, for the correction.

[PAK3TA]

Thanks Eli,

It looks like using this "fixed" bais wouldn't be too hard to implement on the existing circuit. My problem is figuring out the value changes for the applicable components, as I wouldn't even know where to start.

I've been looking at some power transformers and unfortunately living in South Africa means that these have to be imported or locally custom winded. I'm unsure of the locally wounded option as I'm unsure how accurately these can be manufactured with ALL the requirements and they would probably carry a hefty price tag(I mean fighting of lions while having to wind some iron). Furthermore shipment to ZAR isn't that cheap either. This limits me to using a budget power transformer. The cheapest one I could find fulfilling to al requiremenst is the Hammond 374BX.

Specs: 375-0-375@210mA, with a 50V bias tap.

Thus having <50V DC balance voltage, with an AC balance voltage of 385V looking at the circuit.

Any help altering the circuit to fixed bias(with http://www.quadesl.com/pdf/citv_photofacts.pdf as model)with the following information?

[Eli Duttman]

Before you make any decisions about "iron", contact Johan Potgieter, a DIY Audio member who also lives in South Africa.

That Hammond power trafo is not up to the task of providing for 4X 6L6 family O/P tubes plus the small signal stuff.

[PAK3TA]

Forgot to mention this is for a mono single channel amp. The Transformer should be up to spec then. Or else I'm missing something?