Supro inspired amp to build soon just finished 1st schematic

[The Dude]

No cred lost. I do believe you may have prematurely "played the troll card". I liked Jazz P's post for his "call to order" and staying focused. Just didn't want to see this get any further out of hand. I guess the old hippy came out in me. I just want everyone to be happy.

[swingarm]

And I didn't say we were being trolled. I wondered if we were. I thought we might be. That is all. I didn't even make a big deal out of it, call anyone out, use colorful expletives or make any implications about anyone's intellect. In fact I even offered some constructive observation in that same post.

Everything else surrounding this situation is of swingarm's construction.

[swingarm]

So, on the off chance that anyone is interested I'm going to use a Davis 106 as a donor amp. I see Paul Ruby did a PDF on this little PA head. Hopefully I don't have this problem. I had one before that I built an 18 watter out of and sold it to a friend. That one had no buzz issue.

18-watter_buzz_info_311.pdf

I will continue using the EL84's and the 6eu7's (I have several good ones) I will need to add another 9 pin socket of course.

[EFK]

EL84 for a Supro?

You might be better off looking into some of the old Watkins, Selmer and WEM small amp designs.

[Chuck H]

I think you'll find the Paul Ruby zener trick eminently useful if you build amps with EL84's that are intended to be driven hard. Crossover distortion doesn't present as a problem every time, but often enough that you'll be glad to have this trick in your arsenal.

Incidentally, I only skimmed your long rants so I'm only vaguely aware of any defamation of my character on your part. Since you're still addressing amp issues I'll offer that I probably jumped the gun. And I haven't failed to apologize because I think I'm above it. I'm just going to see how this goes first. Try not to take it too personal in the meantime

If you REALLY want to switch between cathode and fixed bias a neat trick is to simply parallel a zener valued at the cathode voltage across the cathode bias resistor. No more self adjusting bias. The relationship between the grid and cathode remains "fixed". Easy peezy and fewer switching functions to potentially fail and leave the power tube grids compromised.

Onward and upward

[Dave H]

Onward and upward

OK

Back to the A/Both/B switching, one way to do it would be to connect both channels to the input jack and to use a double pole centre off switch on the wipers of the tone pots. One pole of the switch connects channel A tone pot wiper to ground in one direction and the other pole connects channel B tone pot wiper to ground in the other direction. Centre position is 'Both' with neither wiper grounded.

EDIT
I've just sketched it out. It only needs a single pole centre off switch. Connect switch wiper to ground, connect one side of the switch to channel A tone pot wiper and the other side of the switch to channel B tone pot wiper.

[swingarm]

Hey that's great, thanks Dave! When / If you get a chance would you post the figure? I thought about it last night and decided 2 inputs might be better but i could put the switch in too.
Here is a look at the donor amp named after my new friend. Thank you for the info Chuck.



Edit: Dave I'm actually using the the 1624t schem EFK posted. It only has one tone pot

1624T.pdf

[Chuck H]

I like the two input config better. It would allow you to use an A/B/Y switch for pseudo channel switching that still gives you both channels simultaneously.

I'll sketch some figures and post back. Probably after work today.

[swingarm]

Damn work, it always gets in the way of life!!

Thanks!

[swingarm]

EL84 for a Supro?

You might be better off looking into some of the old Watkins, Selmer and WEM small amp designs.

Hey EFK I'm also building the Super Scaler from Tut 5 right now it uses a Hammond 10w 1609 as the input transformer wired in reverse. The output transformer is a 1650p and the power is a 278x. I want to put power scaling in this amp too. I'm also currently building the standard from tut5 as well. I should be able to use the same power scaling circuit for the Super Scaler project. So I can feed Chucky into the super scaler that uses 6L6, EL34 6550 etc. So diming Chucky into the super scaler with power scaling might be a good combo no? I'm not sure what the upper limits of of the power scaling circuit are but if not that circuit there are others rated for higher voltage. What i like about this idea is that future project can start using much smaller tube types and iron which saves $.

I know el84's are not the traditional power tube but as long as i get close I'm happy. I need to keep the Mrs from getting irritated about needing 6973's for an amp that came with strong el84's I could put octals in that amp but its kinda small its possible though I think.

[Dave H]

I'm actually using the the 1624t schem EFK posted. It only has one tone pot

I that case the A/Both/B switch could be connected to either the vol pot's wipers or cw terminals but as the donor amp already has two inputs I'd do as Chuck said and use a A/B/Y switch.

[g1]

Thanks for sorting it out fellers.

[Chuck H]

Just using generic drawings to demonstrate the principals...

This two input arrangement should allow for independent channel operation with the switch in one position and change one input into "both channels" with the switch in the other position. This would allow two channel operation with or without an A/B/Y pedal.

The bias arrangement would be set up with the zener voltage the same as the measured cathode voltage. The zener actually doesn't need to be high wattage because the resistor will still be doing most of the work. I use three watt DO package zeners I buy from Mouser.

[ron vogel]

If you haven't bought trannies for it yet, Magnetic components was the OEM for Valco/Supro/National...and they are still in business. They may not have the transformers listed on their site, but they have the drawings and tooling for it still.

I work across the street from them, and they did a non-catalog item for me before. You would have to call and talk to the engineer to find what you need.

[swingarm]

Cool bias trick Chuck. Thanks for posting these. I have the input switch installed. I should be ready to fire up the iron this weekend. Have a good weekend!