Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Topic started by: fvale on March 20, 2010, 08:13:52 AM

Title: Why should I replace BH-7 plate choke?
Post by: fvale on March 20, 2010, 08:13:52 AM: I have a pair of SR45 Paramour amplifiers. Plate choke, especially, and output transformers are not optimized for this amp as they are the MQ BH-7 and BH-5 upgrade created for the Paramount. This plate choke should be rated for 70mA current while the 45 needs about half of that. I'm really satisfied with this wonderful amplifier, the best I've ever had, but I'm wondering which improvements I would get with a "lower" current (and higher inductance) choke. Unfortunately I don't have others to test myself.
I expect to be told that I'd gain better, deeper, basses, but it's only a thought. If there's something more than this I will replace them, otherwise amp will remain untouched because it'll be soon used as a tweeter amp (>1500Hz/6db oct.). Thank you for suggestions/comments.
Title: Re: Why should I replace BH-7 plate choke?
Post by: Paul Birkeland on March 20, 2010, 08:29:27 AM: As a tweeter amp, I wouldn't bother replacing the plate choke. In fact, I would recommend considering the TFA-2004 to be the sensible upgrade for a tweeter amp, and while you are in there, you can replace the parallel feed caps with something around .022uf to augment your crossover and knock any low frequency hum down from the 45's.

I currently run the SR-45 circuit on my tweeters with stock Paramount iron, and I am quite happy with it.

-Paul
Title: Re: Why should I replace BH-7 plate choke?
Post by: fvale on March 20, 2010, 09:20:58 AM: This is interesting!
What are the differences between the TFA-2004 and the BH-5? I thought these to be quite similar.

Yes, the parallel feed cap will be reduced heavily (now 6.8uF) but main frequency cut will take place before input tube in order to relieve it from low frequencies (thank you PJ !)
Title: Re: Why should I replace BH-7 plate choke?
Post by: Paul Joppa on March 20, 2010, 06:06:52 PM: Yes, the BH-5 is derived from the TFA-2004Jr, with a few internal improvements which Mike at Magnequest is not divulging.
Title: Re: Why should I replace BH-7 plate choke?
Post by: Paul Birkeland on March 20, 2010, 07:13:59 PM: Is the BH-5 a 5k like the TFA-2004 Jr, or still around 3k? A 5k OT for a #45 tweeter amp would be quite nice.
Title: Re: Why should I replace BH-7 plate choke?
Post by: Paul Joppa on March 20, 2010, 07:28:40 PM: BH-5 specs are I believe identical to the TFA-2004Jr - certainly it's a 3K unit, like the stock Paramount part. It is, in my experience, excellent with the 45 if you choose the operating point for that impedance - and especially if you spring for the nickel core.

I should note that not everyone agrees on what core material sounds best to them. :^) I like nickel.
Title: Re: Why should I replace BH-7 plate choke?
Post by: Paul Birkeland on March 20, 2010, 07:32:57 PM: Weird, the TFA-2004's I have here are 5k units, but I look on the MQ forum and I see 3k. They must have been custom ordered. One of these days we will have to try the "coil only" approach that Dan keeps talking about...
Title: Re: Why should I replace BH-7 plate choke?
Post by: Doc B. on March 21, 2010, 06:17:17 AM: If you are talking about the brass bell ones you got from my old amp chassis those are not TFA-2004s. They are EXO-050s and are quite a bit bigger. For a tweeter only 45 amp one might look for a pair or all nickel core EXO-45s.
Title: Re: Why should I replace BH-7 plate choke?
Post by: Paul Birkeland on March 21, 2010, 06:52:28 AM: No, these were custom Ni/Cobalt Jr's. It looks like 3-4 pairs were made a long, long time ago.