SE 6V6 cathode bypass cap experiment report

[Baguette]

Hello,

I've had a fun and informative experiment so I thought I would share.

I've finished up a simple SE small tube amp in the vein of an early 50' Supro / Valco Spectator.
Tubes are 6SL7, 6V6 a,d 6X5. The topology is very 5F1 Champ style, with some minor tweaks. The 6V6 cathode resistor is left unbypassed.

When I fired it up, the amp sounded great (better than a 5F1 due to the 6SL7 that provides a smoother breakup that is much needed on those small SE amp IMO). But it had a little buzz that was annoying, the kind of buzz I usually associate with a preamp wire too close to an AC wire. Also, when cranked, the amp produced a massive squeal I thought was the notoriously microphonic 6SL7 going out of control. Apart from those two issues, the amp was excellent and virtually hum free.

I pulled out the preamp tube and the buzz was still there, leading me to think it was power section related. So I triple checked my wiring, wiggled a chopstick all around the power section with no luck.
I finally decided to stick a cathode bypass cap around the power tube cathode bias resistor. To my surprise, it solved the two problems all at once! The little buzz is now totally gone, and there's no more squeal at high volume, eventhough adding the cap boosted the gain considerably.

The tone with the bypass cap engaged is indeed different and honestly I do not quite like it as much as without. The cap seems to add gain, volume and harshness. I somewhat compensated it by adding a NFB loop from speaker to second preamp stage cathode and it's better.
(as a little rule of thumb for myself, when designing an amp, I try as much as I can not to use bypass caps - only on the first gain stage)

Do you technical inclined folks have an explanation for this phenomenon (how adding a bypass cap on the power tube cathode res killed a buzz and an oscillation)?

Hopefully you enjoyed the little story and it will be useful for others down the line.

V

[PRR]

Buzz from lack of cathode bypass suggests a leaky heater. Can you try several 6V6es?

[HotBluePlates]

I believe Fender used a 250uF cap for the input stage cathode bypass cap on the tweed Bassman for the same reason. The 12AY7 was supposed to be a low-noise, hum-controlled tube for low-level input stages; however, the extra-big bypass cap assures that the amp won't have hum added if you get a 12AY7 with heater-cathode leakage.

I've talked with a manufacturer of vintage-style amps who had hum problems when reducing the size of that cap, but only in occasional builds. We determined the supplier of 12AY7's used by that manufacturer hadn't culled out tubes with heater-cathode leakage. The oversize cap eliminated the leakage hum.

[Baguette]

Thanks for the great explanations guys
No I did no swap the 6V6. I had this one reserved fpr this project. It's an old Hytron with black glass / base (no idea of the maker). It's my smoothest sounding 6V6 by a noticeable margin. Better than my 50' brownbase RCA. I wanted this bugger to stay in.
HBP I'll keep what you've just said somwhere in my mind and will use that trick if needs be. Thanks.

[DummyLoad]

does your 6v6 look something like these?


http://www.ebay.com/itm/Sylvania-JAN-CHS-6V6GT-G-VT-107-A-Black-Bottle-Tubes-NOS-NIB-Mint-PAIR-1944-/221870969572


--pete

[Baguette]

Yep Pete those are the ones. Only labeled Hytron.