Nice to read that you solved the problem! That gives directions how to solve it in my case. I checked the OPT and it has 3 secondary windings in each chamber, so I would have to think how to combine these. Probably only trial and error will do...
I found that the best arrangement of parallel secondary windings is:
If the bobbin has dual or more chambers, connect parallel one coil from each chamber.
Each individual secondary coil must be surrounded (in a sandwich fashion) by two primary windings in the same chamber.
If the bobbin has dual or more chambers, connect parallel one coil from each chamber.
Each individual secondary coil must be surrounded (in a sandwich fashion) by two primary windings in the same chamber.
Eventually I applied global negative feedback as baudouin0 recommended in this post. I did it in order to reduce the output impedance and also reduce THD. I wanted to avoid it, but it seems GNFB can have benefits, too. As for the R20-C4 complex, I used a single 510pF capacitor for avoiding self oscillation.
Really? How many dB and at what frequency?
Usually GNFB extends frequency range both at bass and high frequencies.
Or do you mean that your ears told you that bass is weaker with gnfb?
Usually GNFB extends frequency range both at bass and high frequencies.
Or do you mean that your ears told you that bass is weaker with gnfb?
It is subjective. In general the sound is more "civilized", more boring, but probably closer to truth.
The more I listen to it the more I like it. "I can hear new things from my old well known LPs"-effect.
I was going to remark, respectfully, that if the feedback makes the amp sound "boring" then there's something lacking in the design. ;-) It's not an unusual phenomenon. When I switched from no-feedback SE amps to push-pull designs with feedback, it took me a while to appreciate the better control and wider bandwidth of the push-pull amps. I do sometimes miss the midrange "realism" of the SE amps, but on the whole my Williamson amplifiers play more music better. ;-)
I like amps to be blameless that is transparent and 'boring' in the sense they don't add their own colour. Adding GNF will reduce the bass resonance as the output impedance of the amp is much lower.
I agree...although I've heard amps that are "boring," in the sense that they lack dynamics and don't convey the color or nuance of the music. My Williamsons are very lively and colorful, but only in the sense that musical expression is conveyed rather than muted.
The main reason why i would use gNF in this case is because the 807 has a poor distortion profile with lots of higher order harmonics. Without feedback I would expect them to sound quite nasty. If applying feedback makes the amp sound dull then what you are hearing is the abscense of those higher harmonic components which tend to make things sound lively. Still that lively presentation is unnatural and very fatiguing to listen to for long periods.
Zero Negative Feedback designs should be reserved for the more linear power triodes.
Shoog
Zero Negative Feedback designs should be reserved for the more linear power triodes.
Shoog
Last edited:
Zero feedback Push Pull amps can be the best of both world - but there are almost no examples of them around so most people assume that all PP amps have to have loads of feedback to work - which is a very wrong assumption.
Shoog
Maybe they don't get to hear a perfectly balanced push pull output and phase inverter. If done properly it cancels even harmonic distortion while adding odd harmonic distortion produced by the output stage. Overdriving output causes debalancing of the phase inverter which is rare in playback systems. Most tubes are used for guitar amplifiers where tube balancing often isn't good.
I wish more sellers would offer serious matching at a cost as option. This certainly influences sound of FB/no FB amps, preamps, whatever is build with tubes needing matching/balancing.
P-P pentodes need feedback to work well. Ultralinear or triode strapping is a form of feedback. Local instead of global, but feedback nonetheless. There are complete feedback-o-phobes, and global feedback-o-phobes. The first kind balk at triode strapped EL34’s, which can sound quite good even in P-P with no GNFB. Make no mistake about it - there IS feedback.
A low power SE pentode (with zero feedback) can have a pleasing sound, even if it lacks “realism”. An old 50C5 output stage in an old radio or phonograph. They don’t sound “bad” at all. But go to Push pull without any feedback and heaven help you, unless you’re playing an electric guitar thru it. Might be “ok” if you bias it up to class A, but nobody ever did that because the whole idea was to get the improved power/efficiency.