for the SSE. Very happy with mine!There is also Stephe's design, which has the advantage of being documented in a set of videos ...
6sq7-el34-se-amp-design
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Hi. Recentely I built an EL34 SE with a similar OPT of yours (identical power and primary impedance, but different manifacturer). I was happy by running it with 6CG7 pre tubes in SRPP, I tried also cascode. I provided it with a toggle switch to go from triode to U.L. for little more power.
In practice I think you may have many options with primary tubes, anyway if you have a 2.5K OPT you should consider a certain amount of feedback, and work with a not too high supply voltage let's say 290-330V is ok.
In my case I used a small amount of NFB (some percentage units) and a bypass capacitor of 0.1uF from g3 to k.
In practice I think you may have many options with primary tubes, anyway if you have a 2.5K OPT you should consider a certain amount of feedback, and work with a not too high supply voltage let's say 290-330V is ok.
In my case I used a small amount of NFB (some percentage units) and a bypass capacitor of 0.1uF from g3 to k.
I agree with indaco's post.
Look at the triode-wired curves for the EL34:
The EL34 Tube
(first graph, top graph is triode-wired).
With a 2.5k primary, you need to use enough cathode current (plate/screen combined current) to stay in the straight slope portions of the triode wired combo plate/screen curves.
If the current is too low, the damping factor will be low, and the 2nd harmonic distortion will be too high.
Get it set for good performance without negative feedback, before you apply any negative feedback (the negative feedback will not have to work as hard when the stage is fairly linear to begin with).
You might find that the amplifier works OK on your loudspeakers without using any negative feedback (you have both options, with, and without).
Or, you might want to use the Edcor output transformer UL tap to the screen instead of triode-wiring the EL34, but 2.5k primary is a little too low for that, unless you use negative feedback.
And, with low quiescent current the output power will be low, as dictated by the 2.5k load.
Do use a moderate B+ voltage as suggested by indaco, 290-330V (but be sure to increase the B+ voltage to compensate for the plate/screen current x primary DCR of the output transformer, and also increase the B+ voltage by the amount of the self bias voltage.
(I suggest that you use self bias resistor and a bypass capacitor across it).
You do not set the plate current too high though ;
1. Start with (cathode current) x (plate/screen volts) at 25 Watts.
2. Use a less cathode current than the output transformer is rated for, or it will saturate on any bass frequencies (this is a guitar output transformer right? It is for bass down to 40Hz).
Example:
330V plate/screen to cathode, 70mA cathode current, and about 24V self bias
(330V + 24V + 0.70A x primary DCR = required B+ volts).
The plate + screen total dissipation will be 23 Watts, the tube should last a long time.
Happy designing, building, and listening!
Look at the triode-wired curves for the EL34:
The EL34 Tube
(first graph, top graph is triode-wired).
With a 2.5k primary, you need to use enough cathode current (plate/screen combined current) to stay in the straight slope portions of the triode wired combo plate/screen curves.
If the current is too low, the damping factor will be low, and the 2nd harmonic distortion will be too high.
Get it set for good performance without negative feedback, before you apply any negative feedback (the negative feedback will not have to work as hard when the stage is fairly linear to begin with).
You might find that the amplifier works OK on your loudspeakers without using any negative feedback (you have both options, with, and without).
Or, you might want to use the Edcor output transformer UL tap to the screen instead of triode-wiring the EL34, but 2.5k primary is a little too low for that, unless you use negative feedback.
And, with low quiescent current the output power will be low, as dictated by the 2.5k load.
Do use a moderate B+ voltage as suggested by indaco, 290-330V (but be sure to increase the B+ voltage to compensate for the plate/screen current x primary DCR of the output transformer, and also increase the B+ voltage by the amount of the self bias voltage.
(I suggest that you use self bias resistor and a bypass capacitor across it).
You do not set the plate current too high though ;
1. Start with (cathode current) x (plate/screen volts) at 25 Watts.
2. Use a less cathode current than the output transformer is rated for, or it will saturate on any bass frequencies (this is a guitar output transformer right? It is for bass down to 40Hz).
Example:
330V plate/screen to cathode, 70mA cathode current, and about 24V self bias
(330V + 24V + 0.70A x primary DCR = required B+ volts).
The plate + screen total dissipation will be 23 Watts, the tube should last a long time.
Happy designing, building, and listening!
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I have EL34 valves that I need to use so I am reluctant to purchase any more and if new valves are bought then there is a danger of heading into 300b territory….
Yes, it's just like a did. The value reported are quite identical to those I used: for 315V at plate/72mA per tube + about 24V bias + drop from OPT I needed 350V on B+. I made some variations for these values based on my personal better listening (besides calculations and diagram I think you have to see things also empirically).
E.g. I appreciated a certain difference on sound even with 10V of decreasing voltage (300-305V), so I set to a higher value. As for me I think the higher the better, compatiblely with the OPT impedance and a proper D.F.
Also, as you say feedback is not mandatory but can help in some cases if used in the right way.