the "cheapo"

[the_manta]

Hi !
Just wanted to show you my little amp which didn't cost me more than 20 bucks.
It's a strange design and IMHO one of the only designs where u can really use a mains-transformer at the output.

The original circuit comes from an old Philips TV. See
here.

This circuit can cause some headscratching. This amp was built to directly drive the 800Ohms Speakers in the TV-set. (Yeah..we know it..Philips) There are two different tubes working in the Load. So it could be regarded as an unsymmetrical serial-push-pull amp or from a single ended view as a SE Parafeed Amp with an active / bootstrapped load.

To fit usual speakers, I had to use an output transformer. Because of the low ratio (resistance ratio is 800Ohms : 8Ohms = 100. So voltage ratio is 10) and due to the fact that there is no DC current present, I could use a usual mains transformer without to fear getting bad results. I'm currently using a 230V - 15V transfo.

So I started with the first breadboard constructions And finally built it in a nice Case.

The final Amp has some additional negative Feedbacks. ANd sounds really wonderful. But a Zobel-network is necessery and should be well adjusted. (depends on your speakers)

I expect Output Power is about 6W

completed: 1 2 3

Drilling a steel chassis really sucks......thats why I love aluminium

Chassis was painted in what you call "hammered blue". I love this finish. Just like the Altec green or the Telefunken/Klangfilm grey. Industrial style rawks

Best regards, Simon

[kosmin227]

wouldn`t been cheaper if you have built just a simple Single-Ended with an ECC82/83 and 2 EL84 ?
the same output power...6W

[the_manta]

Usual single ended fashion needs an appropriate output transformer. I would need 5,2K Ra which results in much higher ratio..the higher ratio, the worse the transformer, so I need multiple windings, expensive laminations [grain oriented steel] and of course an airgap. Due to that airgap the transfo needs much more windings to provide the same inductance etc.....

And of course, EL84 & ECCXX are much more expensive. PL84 and PCL86 are bulk cargo here.

Believe me, u can't go cheaper. I know, there are many sceptics and doubters out there, but if someone wants something not dull and mainstream then this is a nice inspiration for some experiments.

Regards, Simon

[Svein_B]

There is a very comprehensive thread on Geek's site for a similar (SRPP) ECL82 amplifier with mains toroid outputs.
ECL82 SRPP Flea Power Amp

The link is to the end of the thread (finished result). Reading the whole thread is highly recommended!

SveinB.

[kosmin227]

Originally posted by Svein_B
There is a very comprehensive thread on Geek's site for a similar (SRPP) ECL82 amplifier with mains toroid outputs.
ECL82 SRPP Flea Power Amp

SveinB.

yes it`s interesting and why not a cheap one because toroids are cheaper than E+I transformers

[Shoog]

Toroidals definately sound better in this type of capacity. I experimented with mains EI's and the difference was huge when I swapped them out for toroidals. Other than that a great idea.

Shoog

[the_manta]

Thanks for the info with the toroids ! Maybe when I built another SEPP stage I will experiment with that.

And yes, I know Tom's amp. Looks very good. He was also at the "Röhrenstammtisch" (Tube meeting) here. But without his amp.

Regards, Simon

[Merlinb]

Nice work! I tried using a mains transformer for a PP amp, and it "worked", but it didn't sound so great I'll be honest! Maybe parafeed is the way to go. I definately plan on using a 100V line matching transformer in a 6V6 prafeed design.