Does a 'bright cap' really boost highs?

[maxkracht]

Others have mentioned this, but a low value cathode bypass cap will boost only highs. Say, .22-.68 somewhere around there. You can also cut some volume early by changing the volume pot to 500k or even 250k instead of 1M. A shortcut for that is to add another 1M resistor in parallel with the 1M pot to give you something like a 500k. You might also experiment with just removing the power tube cathode bypass cap altogether for a bit less bass and volume.

[martin manning]

Lastly, you can increase perceived highs by cutting highs in the NFB loop. This is exactly how a 'Presence' control works. Cut highs in the NFB loop, which in turn reduces less gain in the high frequencies, when the NFB is applied.

I like this idea! Note that it is not just perceived highs, in this case you are increasing the gain at high frequencies. Try a 100n or 220n across the 1k5 cathode resistor on the second triode. Put it in series with a 10k linear pot wired as a variable resistor for a presence control.

[Gregg Hermetech]

I finally got a load of caps to try in my 5F1 based amp.

Quick question: Do I put the bright cap between lugs 1 and 2 of the Volume pot, or between lugs 2 and 3? I've seen it mentioned both ways. Or doesn't it matter?

[martin manning]

I finally got a load of caps to try in my 5F1 based amp.
Quick question: Do I put the bright cap between lugs 1 and 2 of the Volume pot, or between lugs 2 and 3? I've seen it mentioned both ways. Or doesn't it matter?

Might depend on how you number the lugs. If lug 3 is the input, lug 2 is the wiper, and lug 1 is ground, then the bright cap goes between 3 and 2.

[Gregg Hermetech]

Thanks Martin. This is how the designer labels the pots, 3 2 1 left to right looking from the back of the pot. From the schematic it seems 1 is input, 2 is wiper, and 3 is ground, so I should put the bright cap between lugs 1 and 2, right?

[martin manning]

Correct. The cap is a bypass around the upper part of the resistive divider.

[Gregg Hermetech]

Brilliant, thanks again, will have a go tomorrow and report back!

[TUBEDUDE]

Ha, thats great Max.

I have from time to time imagined electron flow in strange analogous ways to give some visual to the actions.

"But, to truly know the electron, you must "Be" the electron" - Bruce B+

Semi-sincerely yours,

Phil D.

To truly know the electron you must know quarks, quantum chromodynamics, and probability theory..

[Gregg Hermetech]

OK I've been tinkering today, ran wires out from the Volume pot so I could easily swap out caps while playing. I tried 100 pF, 150, 220, 250 and 470. It seems 150 or 220 are the charm in this amp. It's amazing the difference (and improvement!) with the cap in or out. Sounds like it has a blanket over it with no cap. I'll live with swapping between 150 or 220 for a couple of days before soldering one in. Can highly recommend for anyone in an apartment having to keep a Tweed Champ style amp at low levels, but wondering why it sounds so dark...

Thanks for all the help on here, brilliant as always!