5C1 amp project

[armillary]

I'd like to share some early pictures of a 5C1 circuit amp project I'm building. It's a repurposed Webcor 210 tape recorder. I stripped everything from the chassis but the PT and tube sockets. The 210 power supply supplied 400VDC to the plates of a 6SN7 in PP mode, as well as powering 3 other misc octal tubes and a magic eye tube, so I was looking forward to see what it could do in a Champ 5C1 circuit.

I tried to stay as close as possible to the Champ 5C1 schematic but had a few issues to address. The PT has only a 6.2 VDC filament secondary, so I couldn't use a standard 5Y3GT rectifier, and had to stick with the original 6X5 used by the tape recorder. Other differences between my build and the 5C1 schematic include using 10 uf power filter caps instead of 8uf, a 1000 ohm dropping resistor instead of a 500 ohm, to bring the B+ down a bit. I used new carbon comp resistors where possible and vintage 6SJ7 and 6V6GT tubes that I test myself. It needed a SE OT so I used a Hammond 1750C.

Here are some voltage readings, along with the target 5C1 layout spec values (in parens.)

6X5 output: 393 (340)
at the second filter cap: 355 (320)
6V6 plate: 344 (300)
6V6 screen:289 (280)
6V6 cathode: 18.3 (16)
6SJ7 plate: 151 (130)
heater voltage: 6.7 (6.3)

The PT runs surprisingly cool. Overall I think it sounds good, but more clean and bright than I was expecting. Right now it's still on the bench while I make a baffle board, and it will all go back in the 40's era Webcor cabinet it came out of. I have several 8 inch speakers I'm trying out.

Comments, suggestions and/or errors I've made are appreciated. I'll try to remember some questions I wanted to ask. Thanks!

[Structo]

Looks like a fun amp!

[Phil_S]

I butchered one of those Webcor chassis a few years ago. Today, I would do much better with it. I wonder what I did with that amp? Mine came complete with a small speaker which I put in a box so I wouldn't lose it. Where's that box now? Anyhow, it's a decent project chassis. I love the copper. Nice.

[armillary]

Thanks! A few days ago I had a Hammond 155H (5H, 50mA, 270 ohm) choke in place of the 1k ohm power resistor, like in the 5E1 schematic. It was noticeably quieter, but the 6V6 plate voltage was 369, which is above what the RCA manual says is the design max. So I replaced the choke with the 1k resistor. But could I use both the choke and the resistor? In what sequence? And how strict are the design maximum values in the RCA tube manual?

[JazzGuitarGimp]

Thanks! A few days ago I had a Hammond 155H (5H, 50mA, 270 ohm) choke in place of the 1k ohm power resistor, like in the 5E1 schematic. It was noticeably quieter, but the 6V6 plate voltage was 369, which is above what the RCA manual says is the design max. So I replaced the choke with the 1k resistor. But could I use both the choke and the resistor? In what sequence? And how strict are the design maximum values in the RCA tube manual?

Place the choke first, then a 750R resistor. To answer your second question, just look at a bunco of old Fender schematics! Leo didn't follow max place voltage recommendations very well, and it seems he did alright. This is a cool build. I'll be curious to know which 8" speaker makes the cut.

Welcome aboard!

[armillary]

Thanks! I see you're right. I was just looking at the octal Champs, but now I see the Vibro-Champ is way past spec on the 6V6s. I'll give your suggestion a try and post back when I get the thing back together.

[Structo]

Tubes were a bit more "robust" in Leo's day.

Just sayin'