4 Tube Amp From Scratch

[TubeNoob]

Correction to schematic, the 6v6 cathode resistor R9 is 1W. Not 1/2W.
For the tremolo cathode resistor R19 i have 2W or 1/2W available.

[loudthud]

R9 needs to be at least 2W and it will run very hot. Better off with a 5W. Same kind of thing with R17.

R19, half Watt will be find.

Cathode circuit of V2a needs work.

Grid of V1 needs a 1 Meg to ground.

[TubeNoob]

Hi loudthud, thanks and really good to hear from you.

Will put in the order for those resistors.

Regarding the cathode of v2a, i was wondering about that. What is throwing up a flag for you?

My curiosity was regarding that tremolo intensity pot, when fully cranked, would parallel the cathodes of both sides of the 12ax7, would it not?

Would it work to put the pot straight off the cathode of v2a so the pot would sweep between v2a's cap and resisitor and v2b's?

[loudthud]

I see what you are trying to do, but I don't think the circuit as drawn will get you there. First thing, you don't want to short the cathode to ground at one extreme of a pot's rotation (R18 or R20). Next, R20 is too high a value. It needs to be maybe 10K. Otherwise most of the pot's rotation won't do much. When R18 is at max intensity, R20 won't do anything because C13 will be connected to the cathode of V2a.

The VibroChamp circuit needs an extra triode to buffer the oscillator. One option that might work is the Marshall 18W circuit. Link: http://el34world.com/charts/Schemati...watt_schem.pdf The oscillator modulates the first triode and then V2a (in your schematic) can have the treble boost circuit in the cathode.

[Tooboob]

Tooboob to TubeNoob -

Check my work, but I think you've got the rectifier bridge and PT secondary connected in a less than optimal manner. Certainly, if you plug it in as is, it will be very depressing. You really don't want two diodes in series across the PT secondary, at least not pointing the same way. Two pairs doesn't help. And you should probably be using the center tap on the secondary. Ground the CT, and run each end of the secondary ends to the HV output on the way to the filter cap through one diode or a pair of diodes (two or four diodes total). Don't build a bridge. I live near the San Francisco Bay Bridge. It cost zillions of dollars to build a bridge, even with Chinese parts. Build a high-speed rail system from nowhere to nowhere. That's what we do out here.

[loudthud]

Good eye, the bridge is connected wrong.

[TubeNoob]

TubeNoob to Tooboob: Thank you for the info and the humor; had a good laugh!!! So, i rearranged the PS and attached a screenshot of it. Is that looking a bit less Bay Area now?! :-)

@loudthud: Thanks a lot for looking that schematic up and posting it along with that issue. I am trying to avoid putting another tube in for the buffering. Is there another way to work this? For example, although it's weird:

What if i put a pot on the cathode of V2a that selects between either the cap and resistor on v2a and the trem circuits cap and resistor? I figure there would be a difference in presence, but maybe it'd be interesting? If that'd work at all, of course.

Regarding the pots going straight to ground, would a low value resistor in series before ground do the trick?

Updated all those resistors to 5Watts. Changed R20 to 10k.

[TubeNoob]

Couple quick points:

The Kalamazoo (from which I am ripping the trem circuit) has no cathode cap or resistor on the 2nd pre-amp stage. It also has no 'intensity' control (R18 for me). So, problems abound, i suppose.
Kalamazoo Amp Field Guide: Model 2 Schematic


I can ditch the presence control if that helps / if i have to. The trem is the main thing i'm after.

[Tooboob]

Power looks better.

[TubeNoob]

Kool, thanks Tooboob. So, this half-wave rectification right? why is it i don't need full-wave? Thanks for pointing out the flaw in the PS connection, can't believe i missed that. Curse of the late night poster! :-)

[g1]

If you have 2 diodes, and a CT, then you should have full wave. Half wave would be 1 diode, no CT.

[TubeNoob]

hey you guys, thanks - i went and did some reading up on rectification. So, from what i read it says that you would use the bridge-rectifier when you don't have a centertap -is that about right?


Got a couple diodes (8A/1000V - https://www.fairchildsemi.com/datash...U/RURP8100.pdf) for this.

Also received the 5W resistors.


As Loudthud pointed out that the cathode shouldn't get shorted straight to ground by R20 i'm thinking maybe i'll just try to run the presence control off of NFB with a .1uf on the wiper like on the 5F6a.


That leaves the issue of the intensity control for the trem. I studied several other tube amp trem circuits and didn't find a single amp similar to it that had an 'intensity' pot.

The only thing i can think of is having the v2a cathode go to the wiper of a pot and on one side is a cap and resistor to ground and on the other is the trem circuit with it's own cap and resistor.

Any red flags on this idea?

[überfuzz]

A tremolo is basically an oscillating signal, voltage, that is imposed on the Signal. Question is, where to jam it into the signal path. You don't happen to have a favorite amp or even better a favourite amp with a favourite tremolo? Lets take a look at that shall we, and see what we can come up with.

[TubeNoob]

hey überfuzz!

thanks for chiming in!

coming from an analog synth background the LFO part i have down, but as you say, it is indeed a question of where to stick the modulation!

On push pulls i've seen it done 'tween the output tubes. The majority i've seen use a whole 12ax7 for it (buffer/trem?).
And i've seen it done via resistor between the 2nd preamp stage and the output stage.

But where i'm pulling this trem circuit is, indeed, from a fave amp, the Kalamazoo Model 2. And it does NOT have an 'intensity' control. Therein lies the rub. It's an odd circuit as those smarter than I have mentioned in this thread, saying even that it might barely work (which i totally believe; this stuff is magic that way sometimes). But the trem on it is really very cool. Seems to mod the distortion more than volume. I guess via the varying bias?

Anyway, my knowledge grows via posts and sparks. Thank you guys.

[überfuzz]

Where is the oscillation introduced in this Kalamazoo? If it is, lets call it, a 'varying bias oscillation' you might actually induce it directly on the negative bias, via a coupling capacitor. Lets assume this is where you want to introduce the oscillation, on the bias. Then you have to attenuate the amplitude of the oscillating signal, to get an intensity control. Hmm lets see... A potentiometer! Hey presto! :-)