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Hey Stephen... would your VVR kit work OK for a Fender Super 5D4 build or would I need something a little beefier? (6L6G push-pull power amp)

Thinking of building one for the harp player in my band!

Looking at the schematic (interesting design) the stock VRM should be able to handle the job.

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http://pdfelectronics.com

Thanks Kurt. I'll order one from Stephen when I get ready to build.

I was thinking it would work but I know you can get more than 50 watts out of a pair of 6L6's but probably not that high with cathode bias.

Supposedly, the 5C4/5D4 is the basis for the Sonny Jr. harp amps so I thought I'd give it a try.

Now I just need to figure out a power/output transformer set for it. I'm thinking anything for 6L6 with a 6k primary up to about 30W to 40W should do the job for an OT, but I can't find anything on the plate and screen voltages that an old cathode-bias tweed Super ran at.

Still can't wrap my head around the pi in that amp

But as for the voltage i've seen a few 5d4 layouts with a b+ of 370 how your gonna get there from a 5u4 is an exercise in math
http://elektrotanya.com/PREVIEWS/634632 ... .pdf_1.png

After taking a second look i think its 320v on the layout

lol yeah i'm an idiot... the voltages are on the layout not the schematic. I can figure it out from there.

As far as the PI goes, it's a paraphrase inverter. Not as high output as your typical long-tailed-pair but definitely contributes to the "vibe" of these early tweeds.

The top half of the 12AX7 is actually a gain stage. It directly feeds the upper power valve with its signal. The lower half is the paraphrase inverter. Its grid is being fed by the voltage divider created by the 220k "grid leak" resistor and the 50k resistor below and to the right of it. This divider is reducing the input voltage by about the same amount the paraphrase triode will be increasing it, thus achieving approximately unity gain. The paraphrase triode's inverted signal then feeds the lower power valve.

See http://www.valvewizard.co.uk/paraphase.html for a more detailed write-up and variants.

Of course, I would add in the required grid and screen resistors to the 6L6's.

on another layout think it is 370v
https://www.gbase.com/gear/fender-super ... -twee-1954

triad transformers but hav no found out any more

Yeah it looks like 370V on the plates and 280V on the screens. With the 250R cathode resistor, it is almost verbatim from the RH30 manual for the 6L6G, which lists 360V and 270V with a 248R cathode resistor for about 26.5W into a 6k6R transformer. (p.552-553) So, yeah, even further confirmation that the stock VVR will work just fine. I really like how Stephen includes the switched pot in his kit to use as a power switch so I will order his up soon.

I'm sure I can find a power transformer that will get me within +/- 10 volts of that 370V figure without too much trouble.

This should be a really simple build. I even have an output transformer on the shelf (two of them actually) that will work right.

Here's the schematic and layout I've been working off of:
http://schems.com/schematicheaven.net/manu/fender/super_5d4_schem.pdf

I think I will simplify it with a single input and possibly a switch to go between single-channel input and parallel (jumpered-channels) input or just hard-wired jumpered inputs. Also 5k6 grid and 1k5 screen resistors protecting the power tubes.

Keep us posted. Interesting project. Thought about a harp amp from time to time!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Will-do.

This is all your fault you know... if that TC-15 kit wasn't so fun to build and play I would not be hooked on building amps!!!

Yeah, the TC-15 was the first tube amp I built, too, and I was hooked!

Thanks. Glad you had fun building the amps. That's the best we could ask for

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

What do you guys think... fixed vs cathode bias for this application?

Do both!
Easy mod and fairly practical .

_________________
http://pdfelectronics.com

Yeah I thought about that.

Problem is, this amp needs to be as simple as possible. The guy who will be using it will get overly confused about the volume and tone knobs let alone a bias switch. His current approach is to plug in and play and if it doesn't sound right, he calls me or the other guitar player over to fix it for him.

I can always put it in there and "hide" it I guess. Or put it in and take it out once he figures out which one he likes best.

I got the green-light from the rest of the band to go ahead and use the band's bankroll to order all the parts for the build so hopefully we can surprise him for Christmas. I don't know if I can get the cabinet built by then though.

OK I have parts coming in today and tomorrow so the build will be started very shortly. Need to finalize the layout and I also need to finish a bookcase for my wife's classroom first though. You know the old saying... if mama ain't happy then ain't nobody happy!

I'm going to hold-off on the VVR and see if I need it first. I suspect I probably will but it's easy enough to add after the fact.

I have the board built, the chassis drilled and all the parts installed. Will be wiring up the amp today and doing the initial "smoke test" tonight.

Good luck!

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

It works great. Sounds really nice with my guitar even through a Vintage 30, which is totally not the right speaker for this. In fact, it sounds so nice, I actually wouldn't mind another one for guitar. I used no NFB loop at all so it breaks-up pretty quickly on the volume knob, which is exactly what I was hoping for with harp in mind.

Other than that, nice and quiet, lots of punch and a nice warm overdrive. It is pretty loud, so we'll see if I need the VVR once I get the cabinet with the 2x10's going and him playing it with the rest of the band.

Good job!!

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks. Hoping I can get the cabinet built and a YouTube video of him playing it posted sometime within the next 30 days.