4 Tube Amp From Scratch

[Chuck H]

Very elegant solution!..

I have some dual-gangs - will they have to be 5k or do i have some room on that?

That's a nice compliment. I really like it when I manage designing circuits that exhibit some eloquence.

I figured the dual pot value as 5k because it's a common value and the original Kalamazoo circuit uses a 4.7k cathode resistor. Close enough for that, BUT... The original design used a shared resistor. That means the current from both triodes is creating the voltage drop across the resistor and therefor the bias point. The resistance for a shared, self bias cathode resistor is typically about half what you would normally see on a single triode stage for the same bias point. However, being a trem, so it's going to be wiggling, I wasn't sure how to manage it. I don't think the hotter bias on the unaffected signal should be a problem. Probably better in fact. And I don't think it will stop the oscillator either. When at full intensity the 5k pot value is pretty much stock Kalamazoo. If you experience any problems we may need to experiment with 10k values and perhaps a parallel resistor on the signal triode. Don't trouble interpreting everything I just wrote, I actually did it so I'll remember my train of thought if it comes up later on this build

[TubeNoob]

Thanks Chuck, I do pretty much follow what you wrote :-)

I have a 17k dual-gang pot on-hand. Reckon i'll need to order up some 5k and 10k's.

If anyone has some ideas for how to add a presence control, i'd love to hear them. Been looking around for non-NFB alternatives, as well. No huge luck on that yet.

[Chuck H]

I have a 17k dual-gang pot on-hand.

You could use that and parallel fixed resistors across the circuit. But you strike me as a "less is more" kind of guy that might be happier with a circuit that has the fewest peripheral components

[TubeNoob]

ha, yeah i suppose i do subscribe to that! but strapping some resistors on is no biggy, especially if i don't have to wait on another order to arrive!

so, a 12k resistor from wiper to cathode, correct?

and Chuck, just to clear this up in my head - with this solution of yours in place, will i not be able to run NFB to the preamp cathode since at no time is it fully bypassed?

[TubeNoob]

Happy Holidays everyone!

So, i self-Santa'd this year and got myself a scope. Tek T935a; dual trace; 35mhz; delayed sweep. Yay!

And what better way to spend today than running my analog synths through it and seeing what Korg and Roland call a square wave and what it actually is.

I also managed to finish up the layout for the amp and double-checked each connection and component twice, just like Santa does.

It is indeed ugly, you are forewarned!

[Jazz P Bass]

Have fun with your new scope.

Dual trace capabilities is a real nice feature.

[TubeNoob]

Thanks a lot, Mr. Bass!

I am watching lots of vids and trying to go slow so i learn it and know it's limitations. One thing is the 250v peak max on the probes, so I'm a little nervous about using it on tube stuff, but I have much to learn.

Very cool that with dual-trace you can check out phase issues.

Sounds like you have one, any input for me on using it with tube gear?

By the way, what bass amps have you made/bought? After this project (and my tube eq) i'm building my brother a B-15n, already have all the parts and the iron from Mercury (like $450 worth!!!!!). I've also seen a killer project that allows you to switch between 3 different B-15 models (like switching between SS and Tube rectification, etc).


Hope you slept in, did what you wanted and ate like a fiend today! I sure did. :-)


Re: the layout. I have to look into the heater wiring a bit more, this isn't right.

Re: next holiday season. Going to find a way to take all my dead tubes and stick a bunch of LEDs in them and string them up as the ultimate tube-lover xmas lights!!! Anyone interested in a set?! I have like 200 TV tubes to spare!

[TubeNoob]

a wee update and a new question...

I figured I'd just dive in on soldering it up and see where that lands me. I'm about 3/4 of the way done on it; probably only one more session to go and it should be ready for the smoke and poke testing.

Hoping I find my variac in storage tomorrow. If not, i still have my lightbulb-test setup here on hand.
(Will be double checking all the wiring before then)



Question: Since it seems i can't get a presence control happening on this design, (and since i already drilled the hole for its pot; what a gun-jumper!), I'm wondering if i can make it a pre-amp output jack, instead. Since the signals rolling thru the 6AT6 and the 12AX7 it could be a fun pre-amp sound. Any input on that output anyone?! :-)

[Chuck H]

You might use a preamp out, and you might not. The real tone generator in this design is the power tube and speaker, it's such a simple amp. How about a power reduction switch? The switch could insert something like a 5R/5W series resistor and another 5R/5W parallel to the speaker making the amp about 1/3 power. Since even a small, single ended amp can be too loud at times for home practice when cranked it would probably get more use than a preamp out... You asked

[TubeNoob]

Hi Chuck!
I think that's a grand idea.
Though I am curious about the preamp, so if that's doable, i might give it a go. I'll take a look at some schemos and see if i can figure something out.
So, for the power reduction, that's one resistor in series between the OT and the speaker jack, and another going across the two output jack leads?
Also, couldn't I just do an extra jack on the back without a switch?