Subminiature P_P amp -- Talk me out of building it.

linuxslate

Well-Known Member
Again (Still?) I am threatening to do my first actual build.

My goal is the same -- I am trying to build a small PC speaker amp.

Then I got the idea of using subminiature tubes to keep size and PS requirements down. I'd like to use small boost power supplies, as I mentioned in this thread:
http://audiokarma.org/forums/index.php?threads/modern-power-for-tube-projects.843647/

With a little research, I found this, from a 2011 copy of AudioXpress:

Subminiature_P_P.png
(Note error in caption)

Here is what I like about it:

-- "Proper" Push-Pull with a "proper" phase inverter.
-- Affordable, available OPT's ($17.95 each from AES)
-- Tubes seem to be available on eBay.
-- Can build directly on prototyping PC board(s).

Thoughts/suggestions?
 
Don't do it.

Sorry. Couldn't help myself with the title. If it was in AudioXpress it should work fine.
John
 
it would be different, as long as you're OK with only a couple watts of output I don't see a problem.
 
Here's one of my designs that you can build for very small money: http://audiokarma.org/forums/index....-pentode-power-amp.464003/page-2#post-9046145

Very nice, and very nicely built underneath.

Those OPT's look pretty small. Can you provide any detail on them?

I know there are some "tricks" for avoiding large, costly OPT's -- using 70V line transformers, for example. I also saw a design that used a power transformer that had 2 equal primary windings, connected in series, and that connection used as the P-P center tap.

I wouldn't want to use anything that wasn't rated for the voltage -- i.e. small signal transformers. I obviously also don't care too much about bass response in a design like this, (or any thing that is going to be connected to <= 5 inch speakers.)

it would be different, as long as you're OK with only a couple watts of output I don't see a problem.

The spec's for a 5902 says 3.7W plate dissipation @165V plate voltage. I plan to run ~130V, and figure about 5W P-P. (no math, of course, just estimating), and yeah, that's fine for a PC speaker.

Setting the bar at "better than the lousy, tiny (and tinny), rear-facing speakers in the monitor" should make for a pretty achievable goal.
 
The 555-7120 output transformer used in Parsimon is available from Newark now, at the same price. It's a "10W" speaker line matching transformer specified for 70V systems. The primary voltage that it's exposed to in Parsimon isn't very stressful. I've checked a number of cheap line matching transformers from different sources, and they all passed a 1KV insulation test. That's as high as my insulation tester will go. One of them has been serving as a power transformer with 120VAC on the primary side for several months now.
 
Make sure to connect the suppressor grid of the output tubes (pin 2) to the cathode, it's shown as floating in the schematic.

I have no experience with subminiature tubes, but I think you should make good on your threats and build it! Building is way more fun than discussing, and the product will at the very least teach you something. I see nothing goofy or wrong with this design that stands out.

Only real down side design wise is no. Negative feedback with pentodes means high output impedance. Not a showstopper but something to consider.
 
There were an amazing variety of sub-miniature tube types designed back in the day,and thanks to much use in military equipment,there are still lots on offer for small money today. I like the little buggers,so I say go for it! Sockets may be tricky.....



Here's one of my designs that you can build for very small money: http://audiokarma.org/forums/index....-pentode-power-amp.464003/page-2#post-9046145

That's really neat Mike! I have become a bit of an ''endorser'' of designs using turd-tubes.Before (in pre internet days;)) I would always stick with and counsel other builders to stick with readily available common types,just because the others were hard to find.
 
I don't see anything obviously wrong with the topology of the circuit in your opening post. Sure, why not build it. The Parsimon from Binary Mike will probably sound a little better though...it uses a little feedback whereas this one does not.
 
-- Suppressor grid: Thanks for pointing that out. If I can't /verify/ that it is internally connected, I'll connect it. I can't easily get to the data sheet at the moment.

-- Feedback: Another good point. I really want it to have feedback. I get the basic idea: Inject a little output audio into the cathode of the driver without messing it up from a DC (bias) point of view. I'll scribble on the schematic, and post once I have some time to think about it.

Some of the tubes I plan on buying are used, so I figured I'll essentially need to make a breadboard tube tester. I think it just turned into a whole breadboard amp.

-- Transformers: Thanks for the info about them passing a Hi-pot test. That expands what I am willing to use.

-- Turd-tubes: Heh. I like that. Military-grade turds, if possible!
 
-- Turd-tubes: Heh. I like that. Military-grade turds, if possible!
Some folks used to call 'em plinkers. The ones in Parsimon are still listed for $1 each at ESRC. The finals were actually designed for low-voltage audio power amp service --- in table radios, usually single-ended class A. The driver is just a series-heater version of the 6GH8, a tube that saw quite a lot of use in PP audio power amps back in the day and is frequently chosen to stand in for type 7199 even now.
 
If you want a really fool proof low power amp, a single ended 6V6 or EL84 is really hard to beat, and you can get the transformers nearly for free, for the price of demolishing and disposing of a beat up old console stereo. I'd think with all the old folks in Florida they should be fairly easy to find. On the other hand, this is certainly more unique. You're on the right track about how to add negative feedback, but also will have to be sure the gain is adequate, and that it remains stable.
 
I looked into the sub mini's awhile back. From what I found the only real problem was microphonics with the driver tubes. A couple rubber grommets per signal tube seems to help. Build it!
 
The ones in Parsimon are still listed for $1 each at ESRC

Wow! ESRC is not far from me at all. Thanks, I'll check them out. I ordered a bunch of 5902's on eBay, but if anything goes wrong, ESRC lists them for $6.

BTW, the data sheet shows pins 2 (sup. grid), 4, and 8 (cathode) all connected together internally, so the schematic should be fine as shown.

I'd think with all the old folks in Florida they should be fairly easy to find.

If you want a really fool proof low power amp, a single ended 6V6 or EL84 is really hard to beat, and you can get the transformers nearly for free, for the price of demolishing and disposing of a beat up old console stereo.

I figured the same thing. (not about the old people, but about getting transformers from old receivers.)

That said, BinaryMike convinced me to use 70V line transformers, so I ordered a couple I found for $7 each on eBay.

I was planning a "normal" 12AX7/6BQ5 build (could use same transformers), but the little turd-tubes let me build on a regular prototyping board:

IMG_20181205_104025_sm.jpg

No sockets needed, no custom chassis.

Each channel will be built on one of these boards. Another, small, narrow one will be used on the back for inputs, power, speaker connections.

Here is a basic, planned layout.

submini_amp_Layout.png

The base will probably be aluminum. If it works, I'll build the lexan cover to protect humans and amps from one another. I'll make a cover for the HV supply out of the metal grid from the letter holders they sell at Target.
 
That said, BinaryMike convinced me to use 70V line transformers, so I ordered a couple I found for $7 each on eBay.
I've also experimented a bit with line matching transformers. The details of my testing are on this post (model number of transformers I used, measured primary impedance, output power capable, and frequency response): http://audiokarma.org/forums/index....nsole-comes-for-a-visit.776287/#post-10646530

They performed surprisingly well for their small size. I wouldn't use them for a full range Hi-Fi amp, but for this project, I think they would be perfect!

Neat project! Good luck.
 
Those small block screw terminals might be an idea for securing the tubes leads. That way if one happens to fail you won't have to unsolder them.
 
little turd-tubes.

Old timers used to call them peanut tubes - I find that mental image a bit more pleasant than imagining turds in my amplifiers :)

One comment on the layout. May want to put some distance between the input jacks and the AC supply to the filament transformer. I'd consider putting the inputs on the other end of the chassis, near the front of the boards.
 
Those small block screw terminals might be an idea for securing the tubes leads. That way if one happens to fail you won't have to unsolder them.

I've even thought of using an 8 pin DIP IC socket with these little things. Perhaps a 16 pin DIP to allow more room between pins.

I know I had some of these in my shoe box of loose tubes when I used to play with this stuff years ago. The only gadget I have now with anything close to a subminiature tube is a Russian Geiger counter with an miniature thyratron tube in it.

I'll have to see the actual size when I get the tubes I ordered.

May want to put some distance between the input jacks and the AC supply to the filament transformer.

The filament supply is DC-DC, and linear. The PWR connection is a 12VDC coaxial jack. I am keeping the inputs away from the HV supply, which is a switcher, but the switcher is in it's own aluminum heatsink/shield.

I am also using the OPT's to block the KHz stuff from the switcher from getting into the input of the amp (distance and shielding). Yes, the input wires will have to run the full length of the amp, but they will be shielded (probably a USB cable will give it's life for this, as that is most readily available.)

If it whines because of the switcher, so will I.
 
Old timers used to call them peanut tubes - I find that mental image a bit more pleasant than imagining turds in my amplifiers :)

Actually,my turd-tube comment was not in relation to sub-miniature tubes. It was in reference to tubes that had once been considered worthless or useless.
At one time,you couldn't give away a horizontal or vertical output,damper diode or anything with an oddball filament.And unless you wore an antique hearing aid or worked on military equipment,most people weren't even aware of the sub-miniature types.
 
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