Especially for Tubelab: 845 amp 2.0

[mario2005]

Giaime do you see in your life 845 tube.


Do you desing only PC Simulation.

regards
mario

 

[Miles Prower]

I am engineer too! And I use gobs of feedback to get the accuracy required for the ATE instruments I design - in audio I generally eschew the use of global feedback and these days I even use local current feedback pretty sparingly. Global feedback really seems to "kill" the sound, it measures better, it just doesn't seem to serve the music...

That can be a problem if you misuse and/or over do it. In all too many cases it seems as though they add way too much just to make for impressive numbers. For one design I did, I used some 6.95db(v) of local NFB + 4.0db(v) of gNFB. This being a pentode design, the gNFB was necessary to kill overly bright mids and highs. Without any gNFB, some program material (Ozzy especially) just had a fingernails-on-blackboard effect. A small amount fixed that, and too much made it sound as bad as any SS amp.

 

[kevinkr]

Good points Miles, I think this is particularly necessary with pentodes because of the high source impedance and the usual preponderance of odd harmonic spectra in the output.

I used cathode feedback in the output stages of most of my pp 6550/KT88 pushpull amplifier designs, seemed to work very well in some instances and not so well in others.. I was never quite sure why - I suspect it had a lot to do with certain characteristics of the output transformers I was using.

 

[Tubelab_com]

I used cathode feedback in the output stages of most of my pp 6550/KT88 pushpull amplifier designs, seemed to work very well in some instances and not so well in others.. I was never quite sure why - I suspect it had a lot to do with certain characteristics of the output transformers I was using.

I tried cathode feedback on a P-P KT-88 amp. (ground the 4 ohm output and use the 0 amd 16 ohm taps for feedback). Then I swapped several OPT's through that amp. I found only two where the CFB made an improvement. A UTC LS-57 and a toroidal power transformer. None of the transformers that I tried had dedicated CFB windings. Some transformers were so bad that the amp distorted worse (more higher order harmonics) than without CFB. I came to the conclusion that the OPT had to be symmetrically wound for it to work with CFB in push pull. This is not usually the case with budget OPT's.

In SE amps I have only met one transformer that didn't show some improvement with CFB. It was the little $14 "Fender Champ replacement" transformer from Triode Electronics. It was neaver meant for HiFi and has far too much phase shift across the audio band to work with CFB. CFB works wonders on the $18 Edcor and the Hammond 125CSE. It also makes a big improvement in the big Edcor SE OPT's. A kick drum at full volume actually sounds like a drum instead of something going "splat".

I have been looking for a way to test for this phenomonon, and I think that I have found it. I measure the distortion VS power for several frequencies. Then I connect up the CFB and test again. I look for transformers that show improvement at the frequency extremes, 44Hz (100Hz for small transformers) and 10KHz. The data correlates well with sound improvement. I am putting the data on the web site as I finish it.

 

[Bandersnatch]

Hey-Hey!!!,
I have played with CFB with a few outputs. Dynaco's 441, and a BOH-6. No other NFB was needed. Drive requirements were increased. These are both ~15% or 1/6 of the anode winding. Using the speaker secondary is more like 4%...

Also when using CFB, one is also runnign that fraction of g2 FB as well. I have not found an OPTx with taps on the anode winding to run g2 from to allow full pentode operation( cross connected g2 like the 1:1 McIntosh ). Not that U-L is bad, but it's always nice to have something to play with... guess I'll have to wind them myself.
cheers,
Douglas