freestompboxes.org

Hello,

i'd like to show you the amp i'm working on the last few months.
The aim was to build an amp using the least parts count possible, but including some unusual details making it some kind of "special" among others.
It's not as "simple" as a Champ and i had to compromise with the powersupply and the output-transformer aswell as with the chassis as i took them from an old DIY-amp bought on evilbay for about 100€.

Here's the schematic

The first tube-stage is taken from the "bassomat" from kpt.Maritim (very nice guy i owe him a lot) from the tube-town-forum, i'm still testing different anode- and cathode-resistors there but i cant decide what sounds best, same with the cathode-capacitors. Frankly said i prefer the sound without any cathode-capacitors in the first two stages.
Still enough gain for my Sandberg bass.

The phase-inverter stage is inspired by the Ampeg Portaflex amps, a floating paraphase. I included a switchable "symmetry-control" allowing asymmetry up to some kind of "pseudo-single-end", giving a lot of warmth and a slight distortion.

The output-stage is nearly 1:1 the one of a Dynacord Bass King, nothing special in there. There i had to compromise with the power-transformer as it gives me about 500VDC, i'd like more to have a cathode-biased class-A/B power amp.
Screen voltage on the EL34 is 495VDC, pretty high. The Valvos don't care, nothing glows except of the heaters, but i won't rely on that.
Next thing would be therefore a choke-input-power-supply lowering the HT to approx. 320VDC.

That's all so far, further changes will be mentioned here aswell as in german at the tube-town-forum.
Ideas, warnings, tips are welcome as i have all my knowledge on tube-amps from reading in books or the internet, but they don't tell you if you misunderstood something.

regards

Mathias

I like the idea of the "Symmetry" control. How does it affect the sound?

looks like a way to increase power amp distorsion. It should make the amplifier sound like a smaller amp (more dist / less volume).
Am I right?

The "symmetry-control" produces some kind of "smoky"-distortion, as i named it.
It's less brilliant, warmer and not as loud, but i'd say it sounds like another amp not like a smaller one.
Poweramp distortion sounds different in both modes.

There's still a lot of thinking to do, a lot of things happening in there when fiddeling with that knob. (more 2nd and 3rd order harmonics beeing produced; what does the other half of the phase-inverter when its input is bend to ground, whats happening to the EL34 with no signal at its input)

My guess is that half of the circuit is muted and only one-half of the waveform is amplified. This explains the distortion and lowered volume.

I thought that too, but when the grid of V2b in the phaseinverter is at ground potential dosn't it act like a little-long-tailed-pair, because of the common cathode-resistor with V2a?
Just something that came into my mind

Hey Keppe was the old amplifier a Geloso? Was it already with el34?
They are cheap here in italy, but now I know that they're good for rewiring a guitar amp.

You may want to flip the V2b schem symbol upside down from what it is. I kept thinking "man, that's a weird mu-amp".

It looks beautiful. It's not a "true" floater though. But it's close. The negative fixed bias voltages feeding the power side won't allow for it. That's the reason you'll see coupling caps, C6 and C7, rearranged like that (moved to the right hand side of the 1M/1M voltage divider). They pass the split signal to the power grids and block the backward dc bias. In your amp that's the reason for C5 and the 1Meg grid resistor. It couples signal to the grid and works against the 1meg resistor to ground to supply the grid; blocking V2A plate supply voltage. No resistor, no signal voltage. It's got to be there.

With a self biased power stage there wouldn't be dc bias voltage to hassle with so you'd run C6 and C7 to the left side of the divider resistors, blocking V2A plate dc voltage, and you could tap the V2B grid directly in the centerof the 1M/1M divider, letting grid signal develop across V2A's 1meg resistor. The other 1Meg forces V2B plate current back into the 1M/1M node. The currents are close enough in magnitude to cancel which creates a spot which "looks" like zero volt reference point (always some offset though, I guess) and you can just tie or "float" VB2's grid to it. No caps, nor more resistors, nothing. Straight in to the grid. You know all this, I'm sure. I just thought I'd throw it in as a refresher.

What I don't get is the switch to the 470k VR. It's looks like a volume control, just like at the input of V2A. When you switch to that configuration and turn the VR all the way CCW you're grounding the signal. There's no more resistance to build a signal against. Zero volts at the grid. One half of the splitter is shut off leaving no inverted signal left for the other side of the power tube.

Anyway if you say it sounds good then I'm sure it does. I must just be missing something. I'll tell you what though - it looks excellent. Nice, clean work. I just can't seem to get my builds to look good like that. My stuff looks like crap

Thank you,
schematic is corrected. Shame on me because i had drawn that part months ago, and never noticed V2b beeing upside down

You're right with your thoughts on the paraphase and different power-amp-biasings. I love this variant because it's a gain stage too, so you can get real loud with a two-tube preamp including the phase inverter stage.

When turning the symmetry-470K VR CCW, the negative amplitude of the signal gets less amplified than the positive one, the asymmetry caused through this leads to a greater amount of harmonic-distortions (hope i get the right term here).

Theres an audible difference between the 470k VR and the 1M R at the grid of V2b, even when the 470k VR is turned fully CW.
As in my case the sound gets significantly "warmer" i guess there are mainly 2nd order Harmonics.
A lot of mathematics could give me an answer which ones exactly, but as long as my ears say "yeah!" thats not needed.