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Ideas for a low power amp that is bigger than a flea amp. 5-10W out?

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Sacrilege!

Since heat is the major issue here, why not opt for a class-D amp in the summer and revert to a "heating" amp in the winter. Class-D has come a long way in the last few years.
Not sure if you like some second harmonic, as in SE, but if you do, there's Pete Millett's Nutube class D design:
NuClassD 50W Nutube hybrid amp

My class A amp also outputs horrendous watts. I have Pete's amp on the bench now but the reality is that I won't have it in service till next summer (northern hemisphere).
 
Photo.

I'll post an inside shot when I need to fix it lmao.
 

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On second thought, it's closer to 10WPC... Just like it should be...

Can't really go pentode with 6P43P. B+ is 320V like it or not (ok, I could if I used a low screen voltage I guess, but what about linearity?). Still, better triode output for less power than using 6*6 tubes...
 
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Parts in the "junk box" that I paid for once upon a time, and a scrap chassis that I built a 12AV5GA amp in once. The only thing I needed to buy for this was a 6.3V 3VA transformer to power the bias module. $6.95 at Home Hardware :p
And the 6P43P is nothing like a EL84 in triode connection. Even in pentode, you can get about 25WPC from 6P43P vs 15WPC or so with EL84?

Actually, the curves are very similar to 6P36S in triode connection, just way lower Pd.

I feel that I need to waste another 10 - 15W or so to add a driver between PI and output though. I usually do this with 6SN7. I'll probably use 6CG7 instead, but I'd prefer to use 6N3P because of the lower heater current requirements... Still, this would only save a couple of Watts.
 
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The RH84 is an incredible sounding amp! I built one a few years ago and it was one of the better sounding tube amps I had ever built.




I happened to check out your facebook page list....
That one particular post regarding a Zenith radio....
"Scored this today ... America’s first portable transistor radio! Cost $189.95 back in 1960!"


Is utterly inaccurate.
The date, cost, and "first portable" is funny, who dreamed that up?
After some good research, hopefully you'll correct the mistakes.
 
I happened to check out your facebook page list....
That one particular post regarding a Zenith radio....
"Scored this today ... America’s first portable transistor radio! Cost $189.95 back in 1960!"


Is utterly inaccurate.
The date, cost, and "first portable" is funny, who dreamed that up?
After some good research, hopefully you'll correct the mistakes.


I got my info here: 1959 Zenith Royal 2000 - 1958 Thunderbird advertisement

Pretentious much? :eek:
 
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The Zenith Royal 2000 is one of the first portable transistor radios with FM; could be the first one that was built in the United States. Previous portable transistor radios, with the exception of at least one Sony model and a European product (B&O), were AM only. Nevertheless, it is a milestone and deserves a spotlight on a radio collection. Useful informations for Zenith radio collectors are on the book "Zenith Transistor Radios - Evolution of a Classic" ISBN 0-7643-0015-6
 
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Of course you got your info from an internet source, I already knew that, and know about several internet "advertisements", dated 1961-1965.
And dates and details can be easily "photoshopped' to look authentic, I've done it myself for laughs.



However, I also know that Zenith stole the idea and some of the design from a 1958 Sony Am/Fm portable.
Yes, perhaps, the Royal 2000 was "made" in the USA, but with no mention of Sony, of course.
Aggressive marketing, leaving out certain details is the norm.



I got "my" information straight from a Zenith executive back in the 1970's when our repair shop was invited to participate in a Zenith dealer symposium related to their new Chromacolor TV's.
 
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Ok.

So I added that gain stage. I would have used a 9 pin tube, but I didn't have any sockets so octal it is.

Also added an STR75/60 VR tube to drop -110V to -78V. This makes the amp make sound sooner. It was taking 2m30s to bias up but now it's only 45 seconds.

Of course, this meant changing a 390V module for a 280V module into a voltage doubler for 560V. Changing the 12V supply to deal with the extra inrush, and adding a 150R resistance in series with the AC supply from the 560V doubler as well.

Still, after ironing out a few issues (I used 27k instead of 2k7 for the cathode of the driver, oops!) it's pretty amazing if I say so myself.

I've also increased the bias in an attempt to chase out some distortion the sounded like blocking/clipping, but that wasn't the problem. It was a bad tube, but I was too lazy to lower the bias point.

So now the OP is 320V plate, 0.4V cathode, 40mA current for a Pd of 12.8W per 6P43P.

The 6N8S is running 310V plate @ 4.5mA per section.

Of course, I'm using DC boards for the VAPIDRIVE and bias... I even annotated a picture :)

Oh, and the heaters are untwisted AC but since it's a power amp there is still no hum. The PT for the heaters is 12.6VCT. I grounded the CT and use one half of the transformer for each channel. Chassis ground return for everything, but I ground the power supplies to the same point. I wanted to see just how sloppy one could get without it being detrimental. As long as I can put my ear to the cone of the speaker and not hear hum, I'm good.

My Canadian friends might recognize the "lucky coin" from the Mandarin (Canadian Chinese buffet restaurant).

Also, the relay only brings in B+ for the output tubes if the supply for the bias supply is working. When I get a 120V relay, it'll be connected to the actual bias instead of the supply.
Thoughts?
 

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An update: This is the final schematic and a snap of the amp. All recycled parts, and the performance is excellent. I will measure the power output properly soon, but I can tell you it's at least 20W... All from triode connected tv tubes! The secret lies in the fact they support a high peak current, allowing a 3k plate to plate load. The 1Vin == 11VOut is taken with a 6R load == 20W.

The middle tube is an SR75 used for the -bias supply. As usual, I'm using AB-4 from Audioamp.eu because I had it from a previous project, and it works well. The 530V supply is derived from a DC-AC boost converter (delon doubler from 280V) running from a 12V SMPS, so it the -110V bias from another DC-DC converter which is filtered and dropped by the SR75. The heater supply is AC and as an experiment, I ran it untwisted, single feed with chassis return. No hum. The left channel runs of one half, the right off of the other, transformer is center tapped to chassis. I figured if there was any hum, I could easily convert it to DC. The relay is controlled by the 12V SMPS and connects B+. I figure this was if there is a bias failure, the B+ won't energize either.

Thanks to this community. Without your help over the years, this wouldn't have been possible.
 

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