Got these parts, what to do now?Design help needed!

[Labrador]

Hi folks, i just found this old italian farfisa amplifier in the basement, dual EL34 with a rather complex preamp section, ECC83´s, ECL86, EF86..

I ripped everything out of the chassis and made sure the PT and OT are working properly.The PT is giving 500V-CT-500V, 29V with CT and 6.3V at no load!

The amp had a EZ81 tube rectifier, on the same heater winding as the other tubes but i think i just want to go fr a simple diode bridge rectifier on this one.

If i didnt want to use the full 500V-CT-500V power, can i just use one half of those windings and rectify 500V-CT...not sure if i really need that much (close to 1000V with 1N4007 bridge rectifier, no load, of course...) of power?
Opinions more than welcome!!!

I would love to make a simple dual EL34 amplifier on that Farfisa chassis and where possible with the Farfisa parts, lots of mustard caps & all tubes are Telefunken except for the ECL´s wich im not going to use.

I would like to make it having a EF86 and a ECC83 input channel,and simple ECC83 PI.
Maybe a Cut control ala Vox and rotary selectable bass-cut pre-PI.

Problem is that i build quite some amps to date but have really no big experiences in designing circuits from the ground.

[Jazz P Bass]

Sounds to me like you have your homework cut out for you.

[Steve Conner]

Probably best to just mix the Vox AC50 and AC15 schematics to taste. (The AC15 shows one way of using the EF86.) Both available at drtube.com.

An EZ81 is a bit too small for two EL34s. The best thing to do is replace it with two silicon diodes. Using a bridge rectifier on one half of the winding isn't recommended: that half will overheat, while the other half does nothing.

Use two 1N4007s in series for each "diode" to get a 2kV rating.

[jfutral]

WHat a beautifully odd looking amp! Did it work before your deconstructed it? How did it sound?

I like Steve's idea of the AC30/15 hybrid. But then I would also add a Fender-ish or a Marshall pre-amp channel if the parts were there. Just my gig, I guess. I like variety.

Joe

[Labrador]

thanks for the comments, i had the AC50 in mind as well but i do have a good number of way too big amps and would probably want to go for something using the EL34´s in the most inefficient way possible.
maybe something cathode biased like an tw express power amp with the EF86 in front!?

my power transformer puts out massive AC...i could get b+ lower using the EZ81 instead of silicon diodes, right?
How about using only one half of the secondary winding with the rectifier and starting out with 450ish Volts AC?
anybody knows a smart way of converting the /about/ 29V bias winding (that i wouldnt need in a cathode biased design) to a 6.3V heater winding for an EZ81?
Dont want to hang it on the same winding as the audio tubes but then the original design had 7 tubes on that single winding, including the EZ81.
Or should i just go that way?

The amp was kind of cool looking before i took it apart, the speakers where blown & the cabinet was totally wrecked.
It was working, somehow.
Not very well made but a quality selection of parts...by todays standarts.

[Albert Kreuzer]

my power transformer puts out massive AC...i could get b+ lower using the EZ81 instead of silicon diodes, right?

Yes. As the EZ81 is underrated for 2 x EL34, it will drop more voltage than usual.

How about using only one half of the secondary winding with the rectifier and starting out with 450ish Volts AC?

See Steve's post #3.
When you use the center tap of the HV winding correctly, you will get 450 something Volts, not 1000. Read up on full wave rectifiers (NOT full wave bridge).

anybody knows a smart way of converting the /about/ 29V bias winding (that i wouldnt need in a cathode biased design) to a 6.3V heater winding for an EZ81?
Dont want to hang it on the same winding as the audio tubes but then the original design had 7 tubes on that single winding, including the EZ81.

The 29V winding probably will not deliver enough current when you divide it down to 6.3V.
The EZ81 was designed to run off the same winding as the other tubes. It has been done many thousands of times.

That said, I think it's a good idea to keep the EZ81 and going cathode bias on the EL34 if you want an "inefficient" amp.

Cheers,
Albert

[Ronsonic]

Opinions more than welcome!!!

My opinion is you should have just fixed it. Maybe it was brilliant. Maybe it was so-so but would have benefitted from some mods. Now we won't know. We will certainly never have that funky cool old amp again.