Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Geezer on July 13, 2010, 10:11:45 am

Title: What's this filter??
Post by: Geezer on July 13, 2010, 10:11:45 am

What type filter is the attached drawing? Conjunctive? Other?
(the 1200pf/150k)
It's the output couplers to the power section of a well know "physician's" Rx......

Title: Re: What's this filter??
Post by: FYL on July 13, 2010, 12:18:23 pm

It's basically a tone-shaping crossline damped low pass filter, connected between the PI output coupling caps and the EL84's grid R's. A conjunctive or corrective (as per RCA) filter is more or less the same, but is connected across the OT primary (or, sometimes, secondary).

Title: Re: What's this filter??
Post by: Geezer on July 13, 2010, 12:22:12 pm

Thanks FYL!

Title: Re: What's this filter??
Post by: PRR on July 14, 2010, 03:32:37 pm

The 150K is not any different from changing the 220Ks to 56K (except it may not bleed grid-leakage as well). With 40K source and 0.01u coupling caps, bass is cut near 166Hz, an octave up from your bottom.

The 1,200pFd across stuff right-away induces some broadband loss: two 0.01u or 10,000pFd caps feeding 1,000pFd  induces 1.8dB loss. The main effect though is a high-cut near 2,840Hz. (Without such a cap, this network is flat to like 20KHz.)

It is a bandpass, 166Hz-2,840Hz, Q about 0.25 which is not ringy, center near 670Hz which is musically good.

The Rc on the left is, I assume, a common cathode bias. Perhaps for four EL84s. 250uFd is hefty by guitar custom, nothing wrong with that. If you mostly play LOUD, you might try other values down to 25uFd.

> A conjunctive

The conjunct of a cap is an inductor. There's no significant inductance here, so that line of thought is not useful.

Title: Re: What's this filter??
Post by: phsyconoodler on July 14, 2010, 04:06:21 pm

Where does the 22k resistor go?

Title: Re: What's this filter??
Post by: FYL on July 14, 2010, 04:17:24 pm

Quote

Where does the 22k resistor go?

B+ dropping resistor/split plate load.

Title: Re: What's this filter??
Post by: Fresh_Start on July 14, 2010, 04:32:23 pm

The physician's coupling caps look just like Doug's polypropelene Xicon caps! :grin:

PRR - thanks for that really clear explanation.  25th fret on high E string is about 1,400 Hz, so the high end of the bandpass is about 2x that.  If you changed the coupling caps to 0.1mfd instead of 0.01mfd, is there a proportional change you could make to the 1,200pf cap?  IOW would you need a 120pf cap to get the same high-cut point?  (probably over-simplifying the relationships... )

Chip

Title: Re: What's this filter??
Post by: FYL on July 14, 2010, 04:48:16 pm

Quote

The physician's coupling caps look just like Doug's polypropelene Xicon caps! :grin:

If I remember correctly Dr. Z uses ETR polypro caps. Really nice, compact, very affordable (just like Xicon).

http://www.etr.com.tw/