Quad II to Federal Am864/u

[Tubetec]

Just getting back to work on a project I started a good few years ago. I have an old Quad II power amp chassis ,the output transformer on this unit is fried ,so I decided to re-purpose it in to a Federal 864. Ive read a few other threads on various Federal builds. Firstly the ht of the quad is alot higher that whats required for the fed, I guess there no problem adding extra rc smoothing stages to drop the ht voltage a bit , and reduce hum at the same time. I see alot of people used the edcor tranformers in their build .
Space on the quad chassis is a bit limited . Ideally id like to keep the input transformer seperate,and use a potted output transformer . The original input transformer is centre tapped secondary ,Im wondering if it will work ok with a non centre tapped ,the grid bias on the pair of 6sk7's should make it through the dual pot arrangement at any rate .
The gain on the original is very high, I think a 10k:10k mu metal bridging transformer would be better for mostly line level sources that Ill use with it.

I'm considering making the input gain control dual concentric that way I can adjust drive balance  ,as well as adding a balance pot on the anodes to balance the dc conditions of the vari mu stage,this might help reduce any odd ball effects due to  differences in the pair of 6sk7's .

The compactness of the quad II chassis means alot  less cableing than the original ,but of course carefull layout will be needed .

Any suggestions for output transformer would be great ,
Ill post a pic later on .

 

[gridcurrent]

Just getting back to work on a project I started a good few years ago. I have an old Quad II power amp chassis ,the output transformer on this unit is fried ,so I decided to re-purpose it in to a Federal 864.

the Quad II is such a classic amplifier.
sincerely hope you don't cobble up that beautiful unit for a "one trick pony" compressor.
there are folks that can re-wind the output transformer even if not exactly like the original;
CJ could figure it out.

 

[Tubetec]

I already own a working pair of the quads, nice bit of kit for sure.
I have all the quad parts still ,so the unit can be put back to original condition with the addition of 9ba adapter plates.,if I ever get the output transformer rewound .

The original federal compressor comes with two different transformer sets  ,narrow and higher bandwidth ,I'm gonna use a carnhill VTB-2291 for the output ,does anyone know what kind of bandwidth I can expect from this transformer driven by a 6sn7 ?
It seems good value at 20 pounds sterling .

I agree that in its in its original form the federal is probably one of those things that either works on a sound source or doesnt ,Attack/release are preset,but making the these adjustable will make the unit a lot more usable

I might try adding an extra choke to the ht rail and use a foil cap something like a 2 or 4 uf after the rectifier to keep the voltage down a bit.

I might add a seperate ht decoupleing  for each of the sections ,but Im not sure if this would affect the operation of the circuit.

I'll post a few pics later

 

[My3gger]

I already own a working pair of the quads, nice bit of kit for sure.
I have all the quad parts still ,so the unit can be put back to original condition with the addition of 9ba adapter plates.,if I ever get the output transformer rewound .

The original federal compressor comes with two different transformer sets  ,narrow and higher bandwidth ,I'm gonna use a carnhill VTB-2291 for the output ,does anyone know what kind of bandwidth I can expect from this transformer driven by a 6sn7 ?
It seems good value at 20 pounds sterling .

I agree that in its in its original form the federal is probably one of those things that either works on a sound source or doesnt ,Attack/release are preset,but making the these adjustable will make the unit a lot more usable

I might try adding an extra choke to the ht rail and use a foil cap something like a 2 or 4 uf after the rectifier to keep the voltage down a bit.

I might add a seperate ht decoupleing  for each of the sections ,but Im not sure if this would affect the operation of the circuit.

I'll post a few pics later

I feel very sorry for classic Quad, i'm quite sure windings are known, if not CJ would probably want to rewind two for stereo pair. You will have to drill chasis most likely, why not use cheap chasis for Federal.

Anyway, haven't found use of VTB-2291 as push pull. Sowter, Cinemag and even Edcor seem to make those a bit different, you can find Sowter's comments in vintage audio section. Ian might know if 2291 work ok. Btw, i like at least 50%Ni in cores, probably got spoiled by Cinemag and their custom work.
Why not regulate or cap multiply at least vari-gm tubes, it would be cheaper because output shouldn't need so low ripple. Later could be done with lytic.

Btw, i did my best to "simulate" some of Panasonic lytics to replace foils. If used properly it seems Al part would corrode before cap failure. Those are numbers and guesses from people i asked what they think. Any thoughts from you, or others since you are thinking of using choke along expensive cap?

 

[Tubetec]

Around 50uf is about the maximum you would want to use with a tube rectifier ,
The quad chassis usually has a plate supply of around 340v and a 16uf cap,this is quite a bit higher than the federal,so by intentionally using a small value cap ,the ht will be reduced ,the rest I can scrub off with Rc and LC stages. I most likely will use a  motor run cap as reservoir , only costs a few euros .
So far Ive only needed two extra mounting holes drilled under the output transformer housing ,of course Ill need to mount the pots as well ,
If it turns out a usable 764u I'll be very happy ,
heres how things are looking so far
All the parts except the output transformer is is from reused components  or new old stock .

 

[Tubetec]

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