Lilliput amp

[Stixx]

Hi all,

as a second project (being an absolute tubez beginner) I am planning on building the lilliput amp (E88CC and 6080 SE) that once was mentioned on this forum. It was fdegrove himself who put my nose on it.....

This should be a lovely project, and for starters I have redrawn the schematic to make it readable. I have also converted the psu to tube rectification and added a RC stage.

The only point that remains unclear to me is the choice of output transformer (...I know this question is being asked all the time...). The original schematic sez something like 1.4K (hardly readable), the Megahertz schematic says nothing at all about that! I was wondering to go a little bit higher (say 2.5K...lower power but also lower distortion?), also trannies with around 2.5K are readily available at a number of places....

So, what would be the right choice?
Any advice/recommendation is greatly appreciated!

Thnxs,
Oliver

[Yvesm]

Hi Oliver.

Not really critical.
I bet any load from 1K5 to 3K will work with this such lo Rp tube.
Anyway, don't expect more than 2 watts !

Oh, I feel the 800R in PSU is really too hi in value (and too lo in wattage).
80R seems more acceptable (typo ?), and why not one per channel ?

Regards, Yves.

[bigbulb]

Hi,
The 800 ohm is correct, it drops the 300V of DC (after the EZ81) to 220V,when both channel drawing 100mA.The wattage is too low, should be 50W i think,.80 V voltage drop across it with 100mA will give 8Watts of heat from that resistor.rules of thumb, give 6-7 times extra for lower temperature.
I used 1K output trans for my own design(similar schematic) give me 2.8W at clipping.

[Yvesm]

I used 1K output trans for my own design(similar schematic) give me 2.8W at clipping.

Ho ! interresting setup

Would also make the OPT very simple and easy to built

Yves.

[Geek]

I was thinking that too Yves

I was pondering the loadlines of a 6080 the other day and see 1.33K is good for 200V Va in class-A

[Stixx]

I was pondering the loadlines of a 6080 the other day and see 1.33K is good for 200V Va in class-A

Merci Yves, thanks Geek and bigbulb for your input.
I know this little amp won't make a lot of power, but since my open baffles are about 96dB efficient (old Telefunkens from '53) it should do.
I think I will talk to one of the local winders in Germany to have opt's made with the spec'd 1.4k. AE Europe also makes very nice opt's with c-core at a reasonable price and is basically just around the corner....

And btw...I will quickly change that psu resistor in my schematic to a higher wattage one....

Oliver

[jlsem]

The 800 ohm is correct, it drops the 300V of DC (after the EZ81) to 220V,when both channel drawing 100mA.The wattage is too low, should be 50W i think,.80 V voltage drop across it with 100mA will give 8Watts of heat from that resistor.

Instead dropping the B++ with the 800 ohm resistor, why not just eliminate the 2uF cap and have the power supply choke-input?

By the way, these little two-stage, two-tube stereo SE amps using pass tube outputs DO sound lovely.

John

P.S. Is the AE transformer a standard item or is it wound to order?

[Stixx]

Instead dropping the B++ with the 800 ohm resistor, why not just eliminate the 2uF cap and have the power supply choke-input?

Modelling this in Duncan's psud results in B++ of 195V, plus you're loosing one cap for smoothing....

The AE transformer hasn't been wound yet and is not a standard item, but as far as I know the owner happily accepts custom orders.

Oliver

[fdegrove]

Hi,

The original schematic sez something like 1.4K (hardly readable), the Megahertz schematic says nothing at all about that! I was wondering to go a little bit higher (say 2.5K...lower power but also lower distortion?), also trannies with around 2.5K are readily available at a number of places....

1K4 it is.But...into a 6 Ohm load.

To answer your question once and for all, the Megahertz iron is extraordinary both in measurement and sonics: it's about the only OPT I ever heard that sounds like no OPT at all.
Coming from someone who's been religiously devoted to OTL amps for well over two decades that's very high praise indeed.

The 6 Ohm secondary is no problem as it will drive both 4 Ohm and 6 Ohm speakers without any problem.
I heard this amp on 8 Ohm Lowthers and was impressed: it was one of the few times I actually heard the music and not the Lowthers' notorious nasality for a change.

Of course you can use other iron with a slightly higher Zprim on 8 Ohm nominal speakers but why would you?
The Megahertz OPT will do better, the amp doesn't show any sign of distortion under normal listening conditions and the OPTs are pretty cheap on all counts...

plus you're loosing one cap for smoothing....

You can't look at it that way: doing away with the input cap changes the PS filtering topology from cap input to choke input.
Choke input being the prefered one.
If you need to have a higher B+ use a hybrid FWB rectified B+ choke input filter type iso a FW tube rectified one.
That's what I'd recommend anyway: no need for fancy diodes as most people would have you believe, the two diodes can be as daft as a pair of 1N4007s, it won't make any difference except in your wallet.

Cheers,

[Stixx]

Hi,

Of course you can use other iron with a slightly higher Zprim on 8 Ohm nominal speakers but why would you? The Megahertz OPT will do better, the amp doesn't show any sign of distortion under normal listening conditions and the OPTs are pretty cheap on all counts...

...which implies that the kit from audiokit.it still comes with those megahertz OPT's.....

1K4 it is.But...into a 6 Ohm load.

My Telefunkens are 5 Ohm-ish, so that'll do.

regards,
Oliver