5C5 Circuit Build (Re-purposed old PA)

[old tube]

I posted that I was going to build this a couple of months back looking for advice on the High B+ voltage, people were very helpful then. Many thanks to all.

This chassis with the right arrangement of octals for a 5C5 came up on eBay. Id no idea of the transformer spec only clue was that the amp had originally been a 25 watt affair, so I reckoned it would not be too far away.
I did a bit of research into the British manufacturer (Romec Audio) and they went out of business in 1951. So that most likely dates this to the late '40's, early 50's. I decided I'd use it to house a 5C5 circuit Fender Pro build.

After reading on here of the "Hum" issues some people had with the “B” circuits on tag boards, I decided to go point to point. Also the arrangement of components on the chassis really didn’t lend itself to a tag board.

For parts I used Jupiter (Astron red and yellow repro's) caps, carbon comp resistors, Jensen electrolytics and period correct valves. 50's Tungsol 5U4G and 40's Sylvania branded RCA 6L6G's. Got unused 50's Tungsol 6SC7's from a UK supplier, bought 4 at a great price, they are genuine NOS and none are microphonic, which is apparently a big problem with these valves.

The unloaded output voltage of the mains transformer was quite a bit higher than I was ideally looking for at 360V out; yielding around 450V after the rectifier. The Fender 5C5 seems to have had a B+ of 360-390V on the anodes of the 6L6's which is already high, they would certainly not be happy up at 450V. I looked at various ways to bring the post rectifier voltage down, doesn't seem to be too many eloquent solutions for that problem, so in the end I decided to build it and see what I got. I could always use later, higher spec 6L6's if I needed to, or change the transformer. Needn’t have worried, with everything wired up and the full circuit load on the Transformer, it's putting out 387V on the first node of B+ .. perfect! The voltages round the amp at key points are also pretty much perfect too. Transformer must be a bit soft / under spec, but it doesn’t get warm at all during operation.

With the circuit built, I fired it up using my variac, watching for runaway current at the same time. No problems at all and at mains voltage there was no smoke, flames or horrible noises ...too good to be true!

Still learning to use it. The single volume and tone control are really interactive and you can change the dynamics of the amp a lot just with those two controls. Interestingly, the tone control almost acts like a master volume while the volume is more like a gain control.

Tone wise, I expected it to be jazz in character, dark with knarly break up. Well it's not! It's bright and chimy clean and has fairly late break-up. It’s very touch sensitive and when pushed, it compresses and sags in a very good way. It gives great controllable harmonic feedback at volume. Through an open back Jensen 15" Alnico it sounds very "big" possibly more "Vox" than Fender cleans, real chime to it, lots of harmonics. When it breaks up and compresses it definitely sounds old Fender tweed, like Deluxe grunge with a fair helping of JTM45 type harmonic feedback. Through my old WEM closed back 1 x 12, with a Celestion Gold, it has the same chime, though is a little more scooped in the mids, but the bottom end is fantastic, it farts out but you can still feel the bass in each note, it doesn’t “mush”.

I've tried Fender single coils, P90's and PAF type humbuckers and I think for me, the best pickup / speaker combination is a humbucker with the open back 15" Jensen and single coils with the Celestion. Shouldn't that be the other way round?

I've still got a little work to do on it, firstly the noise floor is higher than I'd like, most likely down to not having the earth positioning in the circuit optimised, so will do some moving around to see if I can improve it. The 6L6's are also running biased a little cold (~70%) just now, so I'll get some more high wattage cathode resistors and taste test, seems I should be targeting 85-90%.

[M Fowler]

Alway nice to see another PTP early tweed built, congrats.

[JoeCon]

Really nice!
Great to see old stuff re-purposed and not just trashed.
And PTP to boot...wow.

[martin manning]

A pure like it!

[Structo]

Cool!

Great looking job on that re-purpose amp!

Love those big bottle tubes.