Eico HF85 clone?

[ferninando]

dont mind in this case.
I am learning from it to help make decisions.

Altho my question of how to reduce the stock hf85 gain
has NOT been answered yet.
I really would like to know how. If its not too complicated.
I have put in lower gain tubes to no avail. sounds and operates same. Excellent.

 

[kward]

Lower gain could be achieved in one of two ways that I can think of that would preserve fidelity while retaining the basic circuit topology of this preamp (including retaining absolute phase relationship between input and output). The first idea is more invasive, the second idea I like better.

1. Rewire V3 and V4 for a 12AU7 or 6FQ7/6CG7. This will also probably require changes in the power supply and definitely will require a resistor change in the local feedback loop around the two triodes in V3 and V4. With the lower gain, you may not need this feedback loop at all, or you may want to add less feedback.

2. Rewire the second triode in V3 and V4 for a unity gain non-inverting buffer. This basically looks like a split load inverter in a push pull amp, but where you only use the plate output. This will also necessitate changes in the local feedback loop to accommodate, as described above. I think this approach could be done without changing the power supply. In other words, this change would probably require swapping out only 3 resistors per channel. The output impedance of the stage would probably go up as compared to the stock circuit, but I don't think it will go up enough to fundamentally change the frequency response or other performance characteristics, as compared to the stock unit.

The one method I would not consider for reducing gain of this preamp is to add voltage dividers between the triode sections. The problem with this approach, while simple, is it can mess with the high frequency response of the preamp (due to Miller effect).

 

[Kegger]

My old PC died with my notes on it to when I was working with my HF-85 here,
(so I went back through my backups to find this)

To lower gain in half with my thinking of simple mods this is what I had proposed.
2 resistor changes, and the line-stage tube goes to an lower gain 12AY7 in there.

R31, 32 220k original, make that half the value, easy mod would be to just parallel another 220k..

R37, 38 33k original, make that half the value, easy mod would be again just parallel another 33k..

(this would make it simple and reversible, shouldn't have to make any supply changes)

Attached is the area of the proposed mod, 1 channel, resistor numbers follow on the other channel..

 

[Kegger]

The problem with the HF-85 and just using a lower gain tube in the line-stage is the actual output doesn't change because the feedback lowers with it here.
I simmed my proposed mod an found that if I cut the feedback resistor in half with using a 12AY7 I get the same feedback "voltage" but now gain is reduced.
(then reducing the plate resistor on V3A to get the operating parameters in a decent place as well, so 2 resistor changes per channel)

 

[kward]

Yeah, I wasn't sure how much feedback is applied, but it definitely needs to be dealt with, in order to correctly address the issue of lowering overall gain.

 

[Kegger]

And Yah, I was writing up my response at the same time you were apparently..

 

[ferninando]

kegger, do you mean a 12ay7 for each ch and 1 for the output for a total of 3?
or leave the output a 12ax7?
thanks

 

[BrokenHill]

Kegger,
I checked those caps over the weekend and they are 200v just like the schem shows, I looked through some extra
components I have collected over the years and found most the parts I will need, I will order the other parts I need
this week and build this simple preamp.

Thanks for your help.

 

[Kegger]

You are welcome, I'll be curious how you get along..