Help with 6F5/6L6 rebuild...

[jt1]

New guy here, I am rebuilding an old Capehart amp that was originally a push-pull 6F5/6L6 mono. I would like to convert it to a SE stereo using the original componets as much as possible. Can anyone post a schematic for a SE 6F5 (with grid caps) and 6L6 outputs...new to tubes but not electronics, I have read the tube manuals and have a fair idea of what's going on (kinda-sorta) but any imput would be welcomed. The rectifier is a 5Z3, I have refurbished the mains xformer and the choke, just need output xformers, schematic and some guidance....

thanks
jt

[Eli Duttman]

A look at the 6F5 data sheet indicates that it is electrically similar to the triodes in the 12AX7. It's a very good voltage amplifier, but can't fight its way out of a wet paper bag.

Other than its UX4 base, the 5Z3 is the same as a 5U4G.

Those phenolic wafer sockets are crap and should be replaced.

You have 19 W. plate dissipation 6L6Gs. A look at the data sheet should provide some ideas about operating conditions.

The Edcor CXSE25-8-3.2K could be suitable. Definitely take advantage of the ultralinear tap. Put a 330 Ω current limiting resistor in the line between g2 and the UL tap.

I'm thinking constant current loading for the voltage amplifier and DC coupled ZVN0545A MOSFET buffering too. Remember that the 6F5 is a wimp.

[jt1]

Thanks for the reply and the links. I was thinking the sockets left something to be desired but as they were from the "good ole days" I figured they were OK...I'll replace them with ceramic.

As for the 6F5's, I just scaned the spec sheet's, not really knowing all the in's and out's they looked OK to me.

Perhaps I need to reasses this project and look to a more modern, well documented circuit for my first tube build. I would like to keep this project all tube and not have to use SS parts to compensate for a poor preforming desgin...maybe I don't want it as original as I thought.

Looks like I have much to learn...

[aardvarkash10]

Eli knows his stuff. I'd follow his instructions as they will lead at worst to a reasonable performance.

The only issues you have left to deal with at this early stage are operating points and the ability of your power transformer to reach them.

Sure, there is now a bunch of math to do, but oddly enough thats a part of the fun.

Ask away, I'm sure plenty are watching and just waiting their turn to jump in with advice...

[Eli Duttman]

I would like to keep this project all tube and not have to use SS parts to compensate for a poor preforming desgin...maybe I don't want it as original as I thought.

If you are willing to put the 6F5s away for a future project, I think we can come up with a 100% SS free design.

Ultralinear mode "finals" require some loop NFB, in order to obtain a satisfactory damping factor. IMO, you would be hard pressed to find a better package of high gain, high gm, and low R_P than the 6GK5, whose data sheet is here. The 6GK5 has "stones" and needs no buffering help. To use the 6GK5, cut rhombic shaped pieces of sheet Aluminum of about the same size as the Octal phenolic wafers. "Break" the corners, for safety's sake. Mount 7 pin mini sockets in the pieces and mount the pieces where the 6F5 sockets now are. The technique is known as hole shrinking.

Circuit topology will be quite straight forward. Both the driver and the "final" will be self biased. Simple RC interstage coupling will be used. GNFB will be taken from the speaker tap and applied to the driver's cathode.

Do you have an o'scope, which can be used to optimize NFB phase compensation? If not, a brute force method of phase compensation is available, which delivers pretty good results. So, if push comes to shove, the only instrument absolutely necessary to finish the project is a decent multimeter.

[mashaffer]

Eli, I am having a little trouble visualizing your hole shrinking approach. Do you have any pics/drawings?

[Eli Duttman]

Originally Posted by mashaffer

Eli, I am having a little trouble visualizing your hole shrinking approach. Do you have any pics/drawings?

Bottlehead offers premade hole shrinkers for 9 pin mini tubes. Look here.

[jt1]

Eli - The 6F5's can wait for another application, I really did like the looks of the grid cap wires though...

As far as "hole shrinking" it's not an issue, I'm much better at metal work than tube amp design.

Circuit topology will be quite straight forward

- This is a good thing. I can do good fab/assembly work, but I'll need a good schematic to work from.

I pretty much understand the Ultralinear and GNFB approach and really don't have any issues with it, but I keep hearing some folks talking about the virtues of pure SET sound. Can I have a switchable setting for this, or am I misinformed...the end goal is to make some really good noise.

Do you have an o'scope

- Yes, I have a vintage Dumont single channel that works well and access to modern equipment if necessary.

I really appreciate your time and sharing of your knowledge. I guess to get started I will measure the xformer voltages and test the 5Z3...I'll post the results.

thanks
jt

[jt1]

OK, I got around to measuring the Xformer...I read 880-CT-880/7-CT-7/and 5.6 with no center tap. The 5Z3 output 433VDC.

FWIW here is the original schematic:


Thanks again,
jt

[Eli Duttman]

JT,


IMO, 2 things about that circuitry stand out. Notice the use of a speaker field coil as a PSU filter element. High flux AlNiCo permanent magnets are a post WW2 development. Pre-war, electromagnets had to be used to obtain high flux. Capehart used a differential, AKA LTP, phase splitter. LTPs are quite in vogue, currently.

You will retain the CLC filter topology Capehart used. A 20 μF./500 WVDC Sprague "atom" in the 1st filter position will give you better performance than OEM, without over stressing either the 5Z3 or the power trafo. As the filter choke shields the rectifier and power trafo from the remainder of the PSU filter, it is safe to pile up the energy storage in the 2nd cap. position.