• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

small shrouded PP OPT choices ?

I am considering to build a small PP desktop amplifier and the priority is size and cost over absolute performance. For this I need small OPTs. And I'd prefer if the OPTs went on top of the chasis, so shrouded is best. I've seen some small cute looking amps on eBay but I'm not sure where they get their OPTs. Low power should be fine (e.g. 5W).

p.s. didn't see any recent threads on this so thought it best to open a new thread.
nicoch₅₈;4619774 said:

[1] Because its Bigun posing the problem. He's posted hundreds of problems; they're click bait for new discussion.

[2] Because Bigun loves tubes, so the amorous tube love drives the choice of amplifier topology and parts mix.

[3] Because nominally a push-pull amplifier will offer more attainable power using cheap if but “odd” TV type tubes in a small package. Bigun has always seemed to be attracted to quirky amp designs. Generates more discussion.

These three … and the idea that this is DIYaudio, so therefore it has to be a creative project 'done on your own' so to speak. Me? I'd probably cobble together a slick little PCB amp and be done with it. My driver isn't the pleasure of endlessly building more amps, but rather listening to music coming out of their attached speakers or headphones.

Just noting…
I think you got me pegged there GoatGuy, addicted to the quirky I am.....

Having 'done' a SE spud amp but never a PP, seems like time to try my hand at it.

This one is for my dad, where a SS design is just 'another desktop box' some bottles would be a bit more interesting :D

besides, my SS amps are too big and too powerful for a small desktop computer amp.
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[4] Because you can make a lighter and usually smaller P-P amp for a given power level that a SE amp due to the gapless OPT requiring less iron.

I gave been trying to come up with a tiny tube amp that requires minimum power.....how tiny. Small enough to fit INSIDE a solid body electric guitar. The best tiny OPT I have found is this one.....no it isn't designed to be a vacuum tube OPT so it isn't priced like one. For $5 try one. Yes its kind of ugly, but maybe some covers from another transformer will fit.

70V 10W Speaker Line Matching Transformer

hook the tube plates to the purple and black wires, B+ on brown. Speaker connected as normal. Be careful there are two black wires. Check continuity with an ohmmeter first. I can get 5 watts through these with good results down to about 50 Hz on 170 volts of B+ (rectified output of an isolation transformer).
My Parsimon design might be useful. It uses cheap unshielded speaker line matching transformers from MCM, but it has enough global feedback to essentially eliminate the bad consequences. With the layout in this photo, I got noise down to about 1mV at the speaker terminals. Schematic is in the second attachment.


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That's a pretty novel heart-shaped chassis you got there Mike - just in time for Valentine's Day!

The line transformer are the right size and cost ! It looks like Edcor also have some low cost open frame OPTs although not as cheap as the line transformers, e.g. the XPP10-8-10k but the issue is making them pretty. I could mount them face down (horizontal) on the chases and put some kind of end bell over the top - Edcor make something called the HC-100 but I'm not sure if it fits.
Let me know if you decide to build Parsimon. I've got a commented bill of materials and some decent under-chassis photos free for asking -- or I'll attempt to post 'em here if that's appropriate. They were previously posted on another forum. I believe all of the tubes are still on the dollar menu at ESRC.
...the issue is making them pretty. I could mount them face down (horizontal) on the chases and put some kind of end bell over the top - Edcor make something called the HC-100 but I'm not sure if it fits.

Imported die-cast aluminum enclosures might be a good choice. Or even plastic enclosures that come pre-threaded for securing the cover plate with screws in the corners.
output transformer_MABLE AUDIO
I've used the 86PP fr guitar amps but not for HiFi.
It should be fine for "medium" quality HiFi.
10K to 4 and 8 Ohms, with 23% UL taps to give a bit more power than the usual 43%, suits EL84, 6V6, ECL86.

Wow, these Mable people have a lot of handy stuff.
I'm getting very interested in this project now (will have to change the thread title...).

So I got good choice for OPTs and I'll be using an ECC99 as the power tube.

I'm thinking about how I want to drive the output tubes. I'd like some help to evaluate some hybrid options - using some SS parts. For inspiration I decided to look at the ARC Classic 30. I've read good reports from this design. It has a lot of things going on in this design and although I won't use all these ideas all I'm interested to direct the design a little to see what I can learn before I decide how to proceed with my own project.

The ARC 30 uses an inverter right at the input of the amp, all discrete, a common source JFET does the inverting and a source follower MOSFET buffers it. There's a feedback loop around the inverter too. See attached. Why did they use this approach, what are the benefits ?


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