Vibrochamp - choke?

[angelodp]

On a donor Vibrochamp build. Real close to firing it up. The Chassis came with a large choke transformer. I don't see any VC's with a choke. Anything to gain with throwing it in the build or live without?

[matt h]

(deleted)

[angelodp]

Here is the original schematic. Cap input filter I believe. Make sense to continue using the choke or is it optional. I have ripped everything out but the transformers and I am going with a stock VC build layout.

[IMG790]http://i260.photobucket.com/albums/ii9/angelodp1/ScreenShot2014-08-29at61709PM_zpse02264ca.png[/img]

[angelodp]

Just waiting for some pots to complete. I reckon i'll throw in the choke.

[IMG768]http://i260.photobucket.com/albums/ii9/angelodp1/IMG_0379_zps7010ba2b.jpg[/img]

doner amp w/ choke

[IMG768]http://i260.photobucket.com/albums/ii9/angelodp1/IMG_0364_zps0d9a2400.jpg[/img]

[matt h]

(deleted)

[angelodp]

ok matt, good deal. After stock I may take this to + gain territory with another 1/2 12ax7.

best Ange

[Leo_Gnardo]

I'd definitely keep using the choke with the CLC before taking the plate tap. Get a quieter build that way.

True & I've done it. A couple of my customers use their Champ/VibroChamps in studio & to minimize hum for them, I installed a CL pre-filter. 22 uF/500V + "Twin" choke rated for 90 mA. It's about 4 Henry if I'm not mistook. It's like spraying the amp with a big can of Hum-B-Gone. Also just worked on a tweed Super last week, it had a pre-filter since the mid 50's. where do you think I stole the idea? Great design, thank you Saint Leonidas of Fullerton.

[Stevem]

The biggest hum reduction change that can be made to the champ line of models is to wire the heater string so that one leg is not grounded to the chassis.
A choke does help somewhat, but the voltage stabilizing good effect that a choke has on a power supply is wasted a good amount on a amp with a class A output stage and its near constant current draw.

[Leo_Gnardo]

The biggest hum reduction change that can be made to the champ line of models is to wire the heater string so that one leg is not grounded to the chassis.
A choke does help somewhat, but the voltage stabilizing good effect that a choke has on a power supply is wasted a good amount on a amp with a class A output stage and its near constant current draw.

I've done the filament-balance change on Champs/VC's and surprise, not much change in hum. The CL pre filter did make a very noticeable improvement. Good practice would have us doing both, no question of that. I'm sure on angelodp's project amp he's got the filament balanced, right? Could even "float" it on the output tube cathode for further hum reduction. Doll it up with a humdinger pot & you have the best of all worlds.

Just last night I did a filament-balance operation on a Gibson GA-6 Lancer and it DID reduce hum to tiny fraction of what it was with one side of the filament line wired to chassis ground. With volume controls turned way up, hum went from obnoxious down to a level where it's competing with hiss. I'd call that a success. Not that difficult to do so it's worth a try on any amp that's lazy-wired for filaments. Could help, can't hurt.

- - - - - - - - - -

edit:

Newsflash: just for a lark I tried an old 60's Ampeg trick. Took the Gibson GA-6 filament virtual center tap, and inserted a 0.1 uF film cap between that and chassis ground. Hum at full volume dropped to half what it was, now reduced to 7 or 8 % what it was originally. I never understood how the Ampeg trick works, but it does. And if there's ever an internal short hi voltage to filament, there's a 600V cap between the filament line and ground, that might save a transformer in a worst case power tube meltdown.

[matt h]

(deleted)

[Leo_Gnardo]

Your options for a quiet SE build are either mammoth filter cap values, or through inclusion of a choke.

Pick the choke, I'm choking up, I'm verklempt! Pass the coffee please.

Leo--I'm intrigued--does this mean that the heater tap no longer has a dc ground reference? (since the connection is broken with the .01uF?)

Thats 0.1 uF and yes, there's no DC ground reference for the filament. 60's blue "shower curtain" Ampegs had this, but not all of 'em. Typically Ampeg used a 200V rated part. Since I keep in stock mostly 600 & 630V film caps, that's what I use. When I'm flailing around for a hum solution I try it. Sometimes it works better than anything else.

I have tried different values but 0.1 uF seems to be the ideal.

[matt h]

(deleted)

[Leo_Gnardo]

thanks for the update! and apologies for getting the value wrong in my question. (what's .099uf between friends anyway?)

I can see how the cap is shunting some noise to ground, but should we be worried this means the heaters get a floating DC reference? I've gotta think that's... less than ideal.

If you tried it with 0.01 probably wouldn't work so well.

As long as the cathode is at or near ground everything's OK, that's where the electrons are after all. Also as long as the heater-cathode voltage isn't exceeded. Lots of old Jets, Geminis, Reverberockets & such still rockin', and they don't seem to wear out tubes at an excessive rate.

FWIW I did try floating the filaments on the output tubes' cathode R voltage, no joy, same hum as at ground. With the 0.1 uF cap, hum cut in half, a noticeable change in a good direction.

I s'pose a resistor could be placed in parallel with the 0.1 uF cap to provide a nominal ground reference. Will have to experiment with this, try values & find the smallest resistance R that doesn't add hum. Hmmm? Mmmmm.

I wonder if our Garage experts can explain why it works, and any drawbacks. Martin, Gingertube, anybody?

[martin manning]

Leo, do you mean this? That would seem to offset the the center of the heater VAC, but it would be swinging above and below ground. I have not noticed this technique before. Maybe the trick is that the peak voltage with respect to ground is reduced?

[Leo_Gnardo]

Leo, do you mean this? That would seem to offset the the center of the heater VAC, but it would be swinging above and below ground. I have not noticed this technique before. Maybe the trick is that the peak voltage with respect to ground is reduced?

Without that 0.22 uF on one side of the filament line, yes. Can't say I've seen one with the 0.22 and the only guess I have for that, is they're trying to panel-beat the hum down even more. Now a double cap mystery.