Single Ended Power Amp with 4 pcs EL84/6П14П/6BQ5/6P14P (Welter HE-ES PSE)
A friend of me uses this one-stage SE power amp together with a pair Klipsch modell "Cornwall" and temporarely the model "Heresy".
In the attachement you will find the schematic - all tubes operates in parallel mode (PSE means here "parallel single ended" - mode).
Follow questions rises up:
1) how is to determine the voltage gain factor (I guess between 10-times and 30-times resp. 20-30db) ?
2) What happens in respect of the sonic quality, when I remove the four cathode capacitors (reducing of voltage gain factor is to compensate with higher gain factor of the line stage in the previous connected preamp) ?
3) What happens in respect of the sonic quality, if I enhance the values of cathode resistors (addidtional reducing of voltage gain factor is still to compensate with higher gain factor of the line stage in the previous connected preamp)?
4) what happens in respect of the sonic quality after linearising of the speaker impedance to 8 ohms, if I use the the windings from output transformer for 4 ohms (decrease output voltage and output power) ?
5) Are there advantages, when the voltage for the heaters are present in DC and regulated?
6) Which value is to calculate for the damping factor ?
Thank you very much for advices.
A friend of me uses this one-stage SE power amp together with a pair Klipsch modell "Cornwall" and temporarely the model "Heresy".
In the attachement you will find the schematic - all tubes operates in parallel mode (PSE means here "parallel single ended" - mode).
Follow questions rises up:
1) how is to determine the voltage gain factor (I guess between 10-times and 30-times resp. 20-30db) ?
2) What happens in respect of the sonic quality, when I remove the four cathode capacitors (reducing of voltage gain factor is to compensate with higher gain factor of the line stage in the previous connected preamp) ?
3) What happens in respect of the sonic quality, if I enhance the values of cathode resistors (addidtional reducing of voltage gain factor is still to compensate with higher gain factor of the line stage in the previous connected preamp)?
4) what happens in respect of the sonic quality after linearising of the speaker impedance to 8 ohms, if I use the the windings from output transformer for 4 ohms (decrease output voltage and output power) ?
5) Are there advantages, when the voltage for the heaters are present in DC and regulated?
6) Which value is to calculate for the damping factor ?
Thank you very much for advices.
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The "sonic quality" is not an unambiguous fact as the measured performance can be.
Therefore you will get answers that are mainly opinions that vary.
Therefore you will get answers that are mainly opinions that vary.
Interesting... Something i would deal with... Really fun schematic, i recently designed an PSE amp...
And I am asking myself those questions too...
And I am asking myself those questions too...
The gain is some 7 dB.
Because output tubes are pentode connected and no negative feedback is applied, the output impedance is high and so the damping factor is low, just some 0,22.
Technically that circuit is at the same performance level as the (simplest) tube radios at 1950'ies, but obviously with higher quality output transformer.
However, the maximum output power is reached at some 10% distortion. Good hifi-tube amplifier has less than 0.5%.
Still some (maybe many) will say that it has excellent sonic quality, i.e. sounds good to them.
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EL84 max rating for the grid resistor is 1 Meg.
You have 4 tubes in parallel. R10 should be 250k Ohm or less.
The tubes are at 9.5V/220 Ohm (43mA each), and 280V (290 - 9.5V bias).
They are dissipating 12W, the max tube rating.
Unless you want to reduce the plate dissipation, do not change the cathode resistors.
Raising the resistance to more than 220 Ohms will reduce the tube current.
Removing the cathode bypass capacitors will make the damping factor even worse.
There is no need to use DC filaments on these output tubes (unless the tubes are broken).
If you have too much signal from the preamp (I don't expect that), you can turn the volume down.
Or, you can wire the EL84s in triode mode. Disconnect the 150 Ohm screen resistors from the B+ side of the output transformer, and connect them to the EL34 plates side of the output transformer.
You will get less power, but you will get a better damping factor, and also less gain.
But the distortion will be lower, and the frequency response will be wider.
You have 4 tubes in parallel. R10 should be 250k Ohm or less.
The tubes are at 9.5V/220 Ohm (43mA each), and 280V (290 - 9.5V bias).
They are dissipating 12W, the max tube rating.
Unless you want to reduce the plate dissipation, do not change the cathode resistors.
Raising the resistance to more than 220 Ohms will reduce the tube current.
Removing the cathode bypass capacitors will make the damping factor even worse.
There is no need to use DC filaments on these output tubes (unless the tubes are broken).
If you have too much signal from the preamp (I don't expect that), you can turn the volume down.
Or, you can wire the EL84s in triode mode. Disconnect the 150 Ohm screen resistors from the B+ side of the output transformer, and connect them to the EL34 plates side of the output transformer.
You will get less power, but you will get a better damping factor, and also less gain.
But the distortion will be lower, and the frequency response will be wider.
Just curious about the logic of such product, in what situation would you want to use it? Even by paralleling 4x EL84's, the Rout is still quite high, since no negative feedback is used, the damping factor is all but useless. In terms of the output power, you also get very little for your money, why not make it PPP instead of PSE? Surely, a cheap off-the-shelf PP OPT is cheaper than the SE OPT.
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