feedback compensation capacitor on the Tubelab SPP

[Alco]

I've been enjoying listening to my recently completed Tubelab SPP and comparing pentode with ultra linear modes, with and without GNF.

I'm using the Edcor 25 watt, 8 ohm, 7.6k output transformers, with the 40% ul tap.

The amp sounds really good - and best to me when wired in UL mode, with GNF. The bass sounds a bit boomy to me in pentode mode, both with and without GNF. Better with GNF. The bass seems downright flabby at higher volumes in pentode mode.

I have not installed C101 / C201, but would like to know what the effect would be, and what values to consider ?

The tone of the amp in UL with GNF and no C101 / C201 is a bit on the shrill side for the horns in my klipsch kg4's. It's much better in a speaker with silk domes. Will these caps help this problem ?

thanks,

 

[Tubelab_com]

Will these caps help this problem ?

Yes. The capacitor is used to tweak the feedback compensation for the high frequencies. The usual way to do this is to apply a 10 KHz square wave to the amp with a resistive dummy load attached and tweak the cap value for the best square wave with a scope.

Most builders do not have a scope, or a good square wave generator. So the next best thing is to use your ears and tune for the best sounding highs with your speakers. After all you don't run the amp with square waves and dummy loads. I have seen values from 270 pF to 880 pF work the best using the scope and square wave method. The value depends on the OPT being used. It you must guess, start with 470 pF. That is probably better than no cap.

There is much speculation among audiophiles as to the type of capacitor used here. Some prefer polystyrene and some prefer silver mica. Most agree that ceramic disks suck. I got a box full of assorted silver micas on Ebay several years ago, so thats what I use. It is usually possible to stick the cap leads in the PC board holes without soldering for testing until you find the value that you like. Just make sure that they aren't touching anything on the other side before powering up.

 

[Alco]

thanks so much for the informative reply.

I went to all electronics saturday and fortified my selection of mica caps to sample in the SPP. I started with a 560pf, which sounded good. I don't have a scope just yet. To have something to compare with I installed 270 pf caps and they seemed just as good, so I left the smaller value in.

No polystyrene caps here to compare with, but micas have sounded good to me everywhere I've used them, particularly as tone caps in guitars.

This amp sounds great to me now. I would have liked to have changed the screen resistors to 860r as some have suggested when I auditioned pentode mode. From what I understand, it would have been a more fair comparison as far as distortion goes. But it sounds wonderful now in UL with the 270pf micas, sylvania 12at7wa, raytheon 6bq5, and edcors as above. I like the clean, no distortion sound. And I can't hear any of the compression that gnf is supposed to introduce. Its such a dynamic amp.

My power xformer is the 008 and is probably right at it's top end for current at 6.3v; it's warm but seems fine.

I would very much like to get a scope. I've been looking on craigslist and ebay but I haven't found any bargains. It looks like you've gotta spend around $100 to get a reasonable scope with some probes.

There's a guy nearby selling a tektronix 465 with probes for $125.
should I get it ?

thank you, again for the informative answer.