Help me to work on the damping factor of my 845 SET

[rdk845]

Hello,

I would like to ask an help to all the experts in tube amp, particularly in SETs.

I have a Dared VP-845, which I frequently use with my AKG K1000 ear speaker. Since K1000 have impedance of 120hm, it is often said that high damping factor or at least adjustable negative feed back amp works best with them. I would like to know what parts in the schematic I can play with to achieve damping factor for this 845SET best suited for K1000. ( except the output transformer)

Specifically, the schematic shows 91K connected from 8 ohm tab of output transformer to the cathode of 12AX7. Is this some sort of negative feed back?

Thanks!

-John

[Robert McLean]

John :
General answer
The 91K resistor is indeed the feedback resistor. Lowering the value will increase feedback, decrease gain, decrease output resistance and thus increase damping factor. But if you lower the resistor too much you may get instability. Try some different values and see (hear) what happens.

Specific to your situation
But, having said all that, you say you want to lower damping factor in order to best suit some 120 ohm headphones. If so, I must assume you have some sort of attenuating/matching resistors between the amp and the headphones. 1) so the amp sees 8 ohms, not 120, and 2) to reduce the power to the headphones so you dont go deaf. If that is the case then, the attenuator network predominates and sets the source impedance that the 120 ohm headphones sees, and the output resistance of the amp will make very little difference to the damping factor.

Of course you may still want to play with the 91K resistor to change feedback for other reasons, such as distortion reduction.

[nafanja]

May be You need to change the output transformer.
And understand what kind of demping factor do you need.
Dempoing is the Alfa (Ra/R out)
Both amps maker calculated Alfa 3-5,
Some-times it's 6-7...
I am prefer mopre than 10...fery offten 26-30 Alfa
If recalculate its more than 3 or 15 parts.
Feedback have a time delay and bring dirt in a sound and compression is signal.
Than impedanse low in secondary than demping better like the force for push the speaker and stop it. And sound more concret.

[rdk845]

Thanks for the replies.

The AKG K1000 is designed to be driven directly bfrom speaker amp. No resistor network. so the name earspeaker. They have very low, 74dB efficiency, but one of the best dynamic headphone around. My amp supposedly output 22W, and my volume is about 30-40% using it.

Regarding the negative feedback, I'm not sure the following thinking is correct, bieng a real noovie... Since K1000 impedance is 120ohm, I don't need as much of damping factor (or as low amp output impedance) as speakers. So I can have lower negative feedback (for clearer sound) and stable operation.

If this is right, based on your experience, how high resistance would you expect I could replace 91K with before I will be running into stability issue?

Also do I need to change the bias current to 845, when I change the negative feedback?

[JLH]

You forgot to post the part of the schematic that has the largest impact on an amp's damping factor --- The Power Supply ---

You can always try to play games with feedback, but if the power supply can't supply the demanded current the tube is asking for all the calculations in the world will not help. More than likely your amp has a high impedance power supply. The result is flabby uncontrolled bass and poor dynamics.

[rdk845]

Right. Here it is.

[wfmali]

John,
you also might get stability issues with "global" feedback in such a design, means around two transformers. It can work, but I would definetly check the phase shift before trying to implement such a feedback loop. An alternative might be local feedback around the 845.
Marcus

[wfmali]

Hoops, sorry, just saw that the "instability issue" was stated before already... never mind!
Marcus

[JLH]

Originally posted by rdk845
Right. Here it is.

What is the operating point for the 845? Vp, Ip, Vg? Also, if possible could you tell me what the DCR of the 1H choke is, and the 700V winding for the power transformer. Lastly, is this a mono block, or does this one power supply feed both channels?I'll see what I can come up for you.

[rdk845]

O.K. I may not have answer for some of these parameters.

845 Plate voltage seems to be ~ 850V.
The biass current is currently set to be ~ 70mA. The factory set would be more like 100ma, based on 22W per channel rating. I reduced them, since I'm currently using 845W which has probably lower dissipation current rating than Shuguang, and also I don't need high power for K1000.

I think the 845 grid voltage is ~120V.

I don't have DCR of the chokes and tranny winding, but I guess I could measure them, if they are important. Do I just desolder connection and measure the DC resistance?

Thanks.