Thoughts on a 5C3 Scratch Build

DocEmmettBrown

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Greetings All,
I'm in the planning stages of building a 5C3 circuit, and have a few questions regarding sourcing and substituting parts, and what (if any) consequences there would be in doing so.

I'm planning on re-building an old homemade PA amp and re-using the chassis, power and output transformers. The old amp used a 5y3, (2) 6v6gt's, and (3) 6sj7's. So I have the Octal tube sockets and tubes. My plan was to order a 5E3 small parts kit, modify the fiberboard to the correct layout by moving a couple of holes, and utilizing as many caps and resistors from the kit as possible. By comparing the 5C3 and 5E3 schematics, it looks like I would still need to source some parts. So here are some of the questions:
(1) The 5C3 schematic shows the coupling caps as .05mfd-600 volt and the 5E3 shows them as .1mfd-400 volt. Can I safely substitute these, or should I stick to the .05mfd caps (or can I use .047mfd-600 as I can't seem to find .05mfd anywhere)? Will it make any difference?
(2) The Load Resistors on the 5C3 are listed as 250k. As I can't seem to find 250k resistors anywhere, can (should) I use 270k instead? And I'm not sure of the wattage rating.

I want to stay as true to the original 5C3 schematic as I can (component wise), and as I mentioned, I would still need to get various other resistors, but those were the most significant differences that I could see.

Any input and suggestions would be greatly appreciated.

Cheers!
 

Lowerleftcoast

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(1) The 5C3 schematic shows the coupling caps as .05mfd-600 volt and the 5E3 shows them as .1mfd-400 volt. Can I safely substitute these, or should I stick to the .05mfd caps (or can I use .047mfd-600 as I can't seem to find .05mfd anywhere)? Will it make any difference?
You can get by with the .1uF. It will provide a little more lows. Many 5E3 owners install .047uF or even .022uF to cut lows.
The .05uF and .047uF are close enough to be indistinguishable. Caps have a tolerance of like 20% so all is good.
(2) The Load Resistors on the 5C3 are listed as 250k. As I can't seem to find 250k resistors anywhere, can (should) I use 270k instead? And I'm not sure of the wattage rating.
Yes either 220k or 270k could be used. Consider two resistors in series or parallel to be more exacting. 1/2W is enough for the 250k.

For the most accurate 5C3 use the .047uF and create 250k.

EDIT: Welcome to TDPRI
 

vampwizzard

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1) use the 0.047's but also buy 0.022's as well. They used low efficiency speakers back then and the bass got lost at the speaker. Also any build ive done using donors has had beefier than advertised output transformers and itll pass all that bass through.

The first coupling caps at the input should remain 0.1.. they were an upgrade that fixed a problem with high gain input throwing the grid bias off and making ugly sounds.

2) I would go 220K unless you want to globally adjust the B+ to be lower. These amps were designed at 100-110VAC and we have higher wall voltages now.

Some good info: RobRob's discussion of Deluxe Models
 

bebopbrain

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(1) The 5C3 schematic shows the coupling caps as .05mfd-600 volt and the 5E3 shows them as .1mfd-400 volt. Can I safely substitute these, or should I stick to the .05mfd caps (or can I use .047mfd-600 as I can't seem to find .05mfd anywhere)? Will it make any difference?

0.047uF would be perfect. Probably you will hear no difference with 0.1uF. Capacitors are often specified with a tolerance of +100%, so your 0.047uF cap could measure about 0.1uF.

(2) The Load Resistors on the 5C3 are listed as 250k. As I can't seem to find 250k resistors anywhere, can (should) I use 270k instead? And I'm not sure of the wattage rating.

270K resistors are nominally within 8% and will work. I am surprised 250K resistors are hard to find. Try Mouser or Digikey. Or put 500K resistors in parallel. Or buy from Marlon P. Jones and use 240K resistors:

https://www.mpja.com/Pack-of-230-2-Watt-Resistor-Assortment-High-Values/productinfo/32763+RS/

OK, the power rating. If a tube shorts the resistor could see B+ or maybe 400 VDC. We don't want the resistor to catch on fire in this scenario.

Power = V * V / R = 400V * 400V / 250,000 ohms = 0.64W.

I would use 1W resistors, although 2W would be just fine.
 

monkeybanana

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Lowerleftcoast

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Would you be using these tubes on the 5C3?

I am curious how these were wired on the homemade PA and how you would wire them for this 5C3. The 6SJ7 is a pentode. Are any of them wired as a triode?

The preamp octals on a 5C3 are 6SC7 which is a dual triode.
 

DocEmmettBrown

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Thank you all for the great information - very much appreciated!

I'll plan on the .047 caps and get some 250K resistors (thanks for the link). I have an additional parts list of various other value caps and resistors that are in addition to what will come in the parts kit.

I haven't started dismantling the old amp yet (other projects to finish), and I need to test the transformers and get their values. Some old components in there might be able to be re-used also.
Would you be using these tubes on the 5C3?
I plan on sourcing a pair of 6SC7s for this build and maybe building a 5F1 at a later date, using the 6SJ7 for the preamp tube.

BONUS QUESTION:
What are your general thoughts on keeping the 5C3 "pure" vs. incorporating some 5E3 differences - ie. keeping the 75K input resistors of the 5C3 vs. the 68K (grid stopper) of the 5E3, using 1.5K grid stopper resistors on the power tubes ala 5E3 or not - will it make any difference...

I'm leaning toward keeping the 5C3 values as close to original as possible (as I already have a 5E3), but am open to more suggestions.

Thanks again for all of the replies!
 

Lowerleftcoast

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general thoughts on keeping the 5C3 "pure" vs. incorporating some 5E3 differences - ie. keeping the 75K input resistors of the 5C3 vs. the 68K
The 75k make a 50/50 voltage divider. The 68k make a 50/50 voltage divider when plugged in to the low inputs. Same same for what they do. Both take away half the signal, just different values of the divider.
1.5K grid stopper resistors on the power tubes ala 5E3 or not - will it make any difference.
The 1.5k work with the miller capacitance of the 6V6. It scrubs off ultra high frequencies which helps to prevent ultrasonic oscillation. The oscillation will make the amp work too hard, causing all manner of trouble. An ear should not be able to hear a difference when using 1.5k grid stoppers.
I'm leaning toward keeping the 5C3 values as close to original as possible (as I already have a 5E3), but am open to more suggestions.
I have not built a 5C3 but I am confident they share some fundamentals that must be adhered to for authentic performance. Imo, the 5E3 needs Goldielocks B+, OT primary resistance, vol and tone pots, 6V6 bias, and speaker. I bet the 5C3 needs the same.
B+ between 350 and 370ish.
8k+ OT primary.
Vol and tone pots that measure 1M or more.
6V6 bias at 100%+ MPD.
A vintage style speaker (poor at producing lows, inefficient, etc = Weber 12a125a, Speed shop, and Jensen equivalents). (Imo, to even things out, the old amps used circuitry to keep lows because the speaker choice did not reproduce the lows well. OMMV.)
 

Jerry garrcia

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5F1 at a later date, using the 6SJ7 for the preamp tube.
That will be a great amp. The 5c1/GA-5 Les Paul sounds amazing. Made a few different 5 W amps with one channel 6SJ7 and the other 6SC7 or 12ax7. The 6SJ7 is my favourite. I’ve been using @printer2 bias switch so it will handle a boost pedal better when it’s cathode biased.
 

JRapp

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A 5C3 is my favorite Deluxe variety. I have a '54 I've been using for 30 yrs. You might wanna have somebody screen the 6SC7 for noise and microphonics esp. if you use the metal variety. About 1/3 of the NOS/ANOS ones I've bought have some sort of problem.
 




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