Can I use this PT for something?

[Phil_S]

Phil, I'm unfamiliar with any blackface fenders (or ac30s) using a choke input filter. Are you thinking about using a choke for the screen supply?

It appears I am guilty of misinformation. I was thinking the 6G8 Twin, 6G7 Bandmaster, 6G6 Bassman, 6G14 Showman and the AC30. All of these place the choke between the plates and the screens. Memory isn't to be trusted. Next time I'll look before I post.

[Leo_Gnardo]

I have not read the whole thread, but personally I think you are in for more work than expected. Don't most choke input filters use specially designed chokes to handle the swing voltage / current, hence the term swing chokes? Most are big $$, big core, air gapped etc. Use a regular choke = let the smoke out. IMO for the money you would spend on choke, you could buy the PT you want.

A couple months back I had such a suggestion from an electronics expert, a very smart feller with lots of college degrees, even a double doctorate. So . . . I did a bit of research on swinging chokes. Now it's no big secret any ordinary power choke DOES change its value (filtering ability in Henries) depending on how much current passes through, with the Henry rating given at the choke's rated current, and somewhat less for lower currents. But swinging chokes are designed to make the most of this feature, however they are mostly used in gear employing very high voltages and very large current swings. The closest thing to amps we're all familiar with would be a modulation amplifier for hi power radio transmission. After all that I reasoned it's best to stick with standard chokes for MI and hifi designs.

meat&beer, you may want to keep a bit of chassis "real estate" around your choke mounting site just in case the cheapo version doesn't work out. I've read reports of the input choke humming & buzzing physically in choke-input power supplies. If your $12 choke does that, you may need to replace it with a potted one to keep the buzz racket down to a livable level.

[David Root]

Just make the choke larger than it needs to be, ie heavier, (not higher inductance), than if it were in a C-L-C arrangement.

For example if it's a Twin type circuit, the standard 022699 choke is 3 to 4H at 90mA. This is designed to handle only the screens and pre tubes and rvb/trem, NOT including the plate current. If you use a 300mA 3H choke it will weigh about triple what the 022699 weighs. A potted version, as Leo recommended, would be even less likely to buzz/rattle.

I recently changed over a Gibson EH-150 build from 400V CLC to 270V LC power supply. The originals were 270V plate and as it happened I was able to simply reroute the wiring to change from CLC to LC, using the existing choke. (I also changed the rectifier tube from a 5R4G to a 5AR4 to get the B+ back up some). This is because this PS was like a tweed PS, the choke takes the plate current as well as the screen current, so it was the right size for LC use too. Hammond 158Q. If the original choke had been designed like BF amp, ie for screens/pres only, it would have buzzed out for sure!

[matt h]

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[David Root]

matth, in this particular case I think getting the plate voltage down where it was originally designed to be is a big part of it.

The tubes are NOS 6L6GAs which is pretty close to the metal 6L6s used in the originals, and 400V on them is about it. I'm sure they are more comfortable at 270V. No, not as loud nor as much headroom, but they sound good to me. Hell, anything sounds good thru a '61 P12N!

As to choke feed vs cap feed, the amp is noticeably quieter now, as is no surprise. Not quite dead quiet though. 6SQ7s don't have humbucking heaters.

Only a 5H choke, with a 40uF plate cap, but a PSUD2 simulation gives ripple at the plate cap of 1.3V RMS, which is pretty good vs the CLC figure which is 5.7V RMS.

I may have been kidding myself, but I think the amp sounded a bit smoother, more "at ease", for want of a better description.

[matt h]

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[Meat&Beer]

I can't believe what a wealth of knowledge just this little thread has become for me. Sincerely, thank you everyone.

As for the choke input deal, I'll order an appropriate one as well as some other needed goodies, get this thing together from PT to the beginnings of a power supply, and bring her up slowly.

I'll report to you all when this happens (and before) of course!

This guy would be lying if he told you he wasn't pumped about this thing in the works here. I even told my Mom.

[matt h]

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[Leo_Gnardo]

rarer than rocking horse turds

that made me laugh very heartily, thanks ya goof!

Before I forget, credit that one to Andy Moore, who was Tony Levin's tech in the 1980's. He had lots of UK expressions, some he borrowed from Robert Fripp.

Looking forward to your burger recipe.

[David Root]

matth, no I didn't try a humdinger. It never occurred to me since it's not an SE amp. I just assumed it was the 6SQ7s and I'd live with it. I generally balance the heaters with very closely matched 100R resistors.

[matt h]

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