Small Tube Amplifier Conversion

[tedmich]

just need ~30mA per el84, plus 7mA per 12ax7 section. Also if you don't dime it all the time you can go with a small PT. Whats wrong with the OTs? I didn't catch their spec...

EL84s can use 6-10k primaries PP or SE and output 4-8ohm.

Never buy new if you have usable iron!

[UniPlank84's]

Maybe this would work???
As said above why waste money and time converting to PP? If both the SE OT's are good I'd be making a Mini-z, Vjr, or Smokin'Joe into 2x outputs. 1x6-10" and 1x8-12" speaker or whatever would be a cool possibility. You may have great SE OT's right there-would just need passive parts and 2 speakers. I'd do a Mini-z circuit myself if I had that. Good find by the way looks in great condition.

[JKowalski]

just need ~30mA per el84, plus 7mA per 12ax7 section. Also if you don't dime it all the time you can go with a small PT. Whats wrong with the OTs? I didn't catch their spec...

EL84s can use 6-10k primaries PP or SE and output 4-8ohm.

Never buy new if you have usable iron!

Impedance ratio is 1600:1, 12.8k at 8 ohm, 6.4k at 4 ohm. Could I use two SE output transformers in a push-pull arrangement, by connecting the windings to each other for center taps?

[JKowalski]

Alright, well I managed to find a 15-20 watt 8k primary PP on eBay for around $10. It's new in box, and looks perfect for this.

So it's the 18 watt for me.

[unparalleled]

Alright, well I managed to find a 15-20 watt 8k primary PP on eBay for around $10. It's new in box, and looks perfect for this.

So it's the 18 watt for me.

The parts and chassis you have would be perfect for a Vox AC4.
In you case you would substitute silicon diodes for th tube rectifier.
To guitarists this amp's sound is very desirable and with an efficient speaker quite loud!
I have built them.
If you don't want Tremolo just leave out the ECC83. You could also use the input stage from the "minimalist" if you don't want to use EF86,
and you could also add an extra gain stage later with a full tone stack.
As far as power transformer voltages go, connect a rectifier to the darn thing with a 400v or higher rated capacitor connected across +/- and let your meter do the maths!
Just be sure to parallel a bleed resistor across the capacitor so it will discharge when you switch off.
Try 100Kohm for this and leave meter connected, you can watch the voltage fall when switched off.
A handy device to make before you start is a test lead with a 240v lamp socket in series with the phase wire and a power socket to plug in any amp you are testing.
If there is anything drawing excessive current the lamp will light up and limit the flow of current to a level which will save any smoke from escaping!
The wattage of the lamp used will set the limit and you can change bulbs to suit the amp under initial test.

[JKowalski]

I'm still going with a PP 18 watt. I just got the OT today. Picked up a step drill to enlarge the holes on the tube sockets and place some ceramic ones in.


I'm actually going to try a hybrid, with the 18 watt power amp and a preamp with ideas borrowed from other amplifiers. I am also thinking about adding reverb.... The chassis has two RCA jacks on top already installed, and the single ended OT's that were on the amp originally seem good for driving a tank (though a little overkill).

Tomorrow I am going to gut the filter cap can from the original and put new ones inside, as well as some 0.1uF parallels. Right now I am thinking about three 56uF 420V (they fit perfectly, according to my calcs) I may even put in a bleeder resistor (thoughts?). The main reason I didn't want to put them inside the chassis is obvious - there isn't alot of space. Just enough to fit everything in decently. I really like the size of the amp, my only concern is noise... We'll see how that turns out...

[olddawg]

I'm actually going to try a hybrid, with the 18 watt power amp and a preamp with ideas borrowed from other amplifiers. I am also thinking about adding reverb.... The chassis has two RCA jacks on top already installed, and the single ended OT's that were on the amp originally seem good for driving a tank (though a little overkill).

Although adding an onboard spring reverb has been done for 18watt EL84 type amps, the general consensus is that it doesn't work well. The magic in these amps is the way they break up. You end up running a distorted signal to the reverb driver with some pretty ugly results. Most people would tell you just to use a pedal and don't waste your time. The Bugera V22 uses a digital reverb in it's otherwise tube build, but I haven't been able to locate a schematic for one yet.

[JKowalski]

Although adding an onboard spring reverb has been done for 18watt EL84 type amps, the general consensus is that it doesn't work well. The magic in these amps is the way they break up. You end up running a distorted signal to the reverb driver with some pretty ugly results. Most people would tell you just to use a pedal and don't waste your time. The Bugera V22 uses a digital reverb in it's otherwise tube build, but I haven't been able to locate a schematic for one yet.

Yeah, I've read that, but I figured that I'd be redesigning the preamp anyways and I would do so to consider the reverb as well. Regardless, It's a no on the reverb. I have a pioneer tube reverb that would mesh fine with this, and I completely ran out of space after I decided to get a seperate heater transformer.

It's coming together, I just gutted that cap (that was a pain...) from the bottom, and installed all the new sockets and the transformers.

[JKowalski]

Here's what I came up with. It's the basic lite preamp, with an added gain/CF stage.



I read this article:

The Valve Wizard

And It looked interesting (I recently made a SS pedal design that let you get that kind of half-wave smooth clipping behavior, and liked the sound) so I incorporated it. I suppose I would have to see how the variable aspect works IRL to decide whether to keep it or not.

Thoughts on the design? Anywhere I completely screwed up?

Thanks in advance