Choke in blackface amps.

[Bill Moore]

I told my amp tech friend, (that moved back to HI last year), that I had ordered a DR kit from Weber, and he sent me a box full of transformers.
I had intended to use the Weber iron, but now I have a Vibrolux PT, a Hammond output, and assorted chokes, and a Fender # reverb x former.
My question is, does it matter, or is there any advantage to use a 50 ma or 90 ma choke on the little 6V6 amp? They are both 4 h.
Thanks.

[cdw2000]

Given that they are both 4uH, I would say that it does not matter except that you need to ensure that the rated current is adequate for the amp.

The 50ma choke would be adequate for a single 6V6 champ-type amp. You would need to use the 90ma choke for any two-6V6 PP output amp like a DR.

[Bill Moore]

Thanks for replying, on the CE site, they list the 50 ma for the DR, and the 90 ma for Twin etc.
I've thought the choke acts somewhat as a storage for current that helps maintain the proper power to the tubes. Does having more, (higher ma rating) allow for a more stabillized voltage during high demand? Would it be desireable, or too "clean"?

[Kap'n]

No, that's inherent in the Henry rating. The current rating is just that how much current it can handle before its integrity is compromised.

[Bill Moore]

Thanks Kap'n. Does the inductor value, (henries), change the tone of the amp, or the characteristics, (sag, breakup)?
Did Leo only use 4 h in the blackface amps?

[Kap'n]

Thanks Kap'n. Does the inductor value, (henries), change the tone of the amp, or the characteristics, (sag, breakup)?
Did Leo only use 4 h in the blackface amps?

They do. The larger the henry value, the better the choke's ability to stabilize voltages downstream from it. Marshalls, for example, use larger chokes - one of the reasons they are punchier (or colder depending on your viewpoint) than the tweeds they are derived from. The more stable the screens, the less likely it is to sqaush.

Leo didn't publish the specs on the chokes, so I don't really know what they are, but I think that's the value he standardized on for all of the BF/SF amps from Deluxe to Twin, with a couple of different sizes, depending on the amp.

[Bill Moore]

Thanks again!

[cdw2000]

Thanks for replying, on the CE site, they list the 50 ma for the DR, and the 90 ma for Twin etc.

I see that now. They must be conservatively rated.

I was basing my recommendation on what I figured the bias current would be plus some margin assuming a 350V plate voltage. The result came out to more than 50mA.

Looking at a BF Deluxe Reverb schematic (AB763), I see that the nominal plate voltage is actually 415V, so bias current for each tube at 70% of max plate dissipation would be about 20ma. Therefore current for the power tubes plus allowing 5ma for all the other tubes would be about 45mA. So, yeah, it looks like the 50ma choke would be adequate.

[Bill Moore]

Thank you again, cdw2000. I used the Hammond 90 ma replacement in my "Twin" build, (because that's what the catalog called for), if there's no advantage, I'll use the 50 ma for the DR build this time.
As an aside, I got an e-mail that my project has shipped!

[fran]

Looking at a BF Deluxe Reverb schematic (AB763), I see that the nominal plate voltage is actually 415V, so bias current for each tube at 70% of max plate dissipation would be about 20ma. Therefore current for the power tubes plus allowing 5ma for all the other tubes would be about 45mA. So, yeah, it looks like the 50ma choke would be adequate.

the choke on DR (and most other amps really) is placed in power supply after the plates, so it only has to withstand screen + preamp current. And screen current is much smaller than plate current (although it rises considerably in full tilt), say 3mA in idle (?) per tube, plus preamp
50mA is I think more than adequate