Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: CascoSieg on April 07, 2022, 02:20:54 pm

Title: Reverb Transformer Question
Post by: CascoSieg on April 07, 2022, 02:20:54 pm
I've searched here for a while but just can't quite find what I'm looking for. Apologies if I missed it or just can't quite interpret the info...
I have a Fender Princeton Chorus SS donor amp and would like to reuse the reverb tank in a new EL84 SE build.
The tank is the 8EB2C1B and lists input Z of 800 Ohms, and output Z of 2575.
Assuming I'd like to do a one-tube (12A_7) reverb circuit, how do I figure out the best transformer to get?
Thanks for any clues or experience anyone might offer!

Title: Re: Reverb Transformer Question
Post by: CascoSieg on April 07, 2022, 06:43:11 pm
Ha! Found a clue in the post 3 lines down on reverb tank selection!

If the driving impedance of a tube of a given mu, is calculated as:
Rout = Rp / (mu +1) || Rk
then for a 5751 with a Rp of 58000, a mu of 70 and a 10k cathode R, we'd have:
58000 / 71 ||  10000 = 754 Ohms
Correct?
Close enough it seems to he 800 Ohm input impedance of the tank, but I'm still not clear how to select a transformer for this scenario? Or am I way off? ... will keep looking tho. Any help most appreciated! :)

Title: Re: Reverb Transformer Question
Post by: tubeswell on April 07, 2022, 10:21:15 pm
Quote from: CascoSieg on April 07, 2022, 02:20:54 pm
The tank is the 8EB2C1B and lists input Z of 800 Ohms, and output Z of 2575.


That Pan input impedance is wrong for a standard Fender Blackface type amp reverb transformer. You need a pan with an 8 to 10 Ohm input impedance.


You could use that pan with a transformerless reverb circuit of some sort
Title: Re: Reverb Transformer Question
Post by: CascoSieg on April 08, 2022, 08:37:17 am
I see - that makes perfect sense, since the amp it's from doesn't use a transformer. Would you suggest trying to tell from the schematic what an equivalent tube driver might look like, or just measuring the voltage at the tank under operation, and going from there?

Title: Re: Reverb Transformer Question
Post by: PRR on April 08, 2022, 11:47:43 am
It's just not real suited to tubes. A 2K tank "kinda works OK" with a fat tube. Anything lower, you want a transformer. Or a low-volt supply and a chip, like that SS amp. Saves heat also. But costs about as much as just getting a suitable tank.
Title: Re: Reverb Transformer Question
Post by: CascoSieg on April 08, 2022, 12:23:56 pm
Ok, Darn.  :sad:
I maybe could pull the parts from the donor amp, but the +/- 16v business doesn't seem worth the trouble.
Thanks very much for the guidance!