Russian TrainWreck - Expresso Lite(Liverpool), Soviet tubes

[TheGimp]

The brown blob on the right is a super secret current source that I won't discuss as it is a super secret FET with a bipolar driver. (See Firefly SET amp at DIY for details).

I was going to use a LED for the cathode bias, but I can't figure out how to bypass one with a cap to get the proper frequency response so I used a 750 Ohm resistor and 22uF cap.

The Pentode front end works nice for holding down the hiss.

[Ian444]

375V B+? or 275V?

Love the first amp, turned out great, well done!

[TheGimp]

375 no load, ie no tubes in place. I'm pushing the 6P1P to see what it will handle.

Screens are now up to 250V with -16V fixed bias on the grids and B+ is down to 346V with 25mA and 27mA on the two output tubes with just them in place. B+ may drop another volt or two when the preamp tubes are installed. I have to finish wiring them still.

[TheGimp]

Amp assembly is complete and I'm tweaking it as I've got a nasty 36KHz oscillation that shows up occasionally. It looks like it is in the first stage and is dependent on volume control setting (loading sensitive?).

I pushed the 6P1Ps into AB1 and have an honest 10W out at the onset of clipping. If I let it clip both peaks, I show about 12W max output.

I'm getting about 25V sag on the B+ under full load compared to no load.

The current no signal bias point is -13.5V on the grids, 35.5mA and 34.6mA cathode current, 328V B+, 250V Screen voltage.

If I drive it to a square wave output I get 12.2Vrms which into 8 ohms translates to 18.6W.

So, how does one rate the power out? I consider it to be a 10W amp since it will deliver 10W at the onset of clipping.

True output power is measured at over 18W if you don't mind massive distortion.

[TheGimp]

Top and bottom without cab.

[TheGimp]

Unfortunately (or maybe Fortunately considering my condition) it's too wet and cold to go outside and work on the speaker cab.

Here is my complete (to be) system.

The speaker is supposed to be a 1970s Rola Celestion G12M, 25W, 8 Ohm.

The markings are T1871 or T1S71, and JJ28 with what appear to be two acceptance stamps. If anyone can confirm the manufacture year I'd be thankful.

In this site it looks like a pre 1970.


http://www.cathedralstone.net/Pages/Cel ... intage.htm

But the date codes in thegearpage say Sept 1976.

http://www.thegearpage.net/board/showthread.php?t=67646

Or I could be miss-interpretating everything.

[M Fowler]

Very nice cabinets

[azatplayer]

6P1P is a pretty cool tube. One could even call it a little sister to 6V6 in a 9 pin noval bottle. An Raa=8k OT and Ua=325V gives ca 10W clean before clipping.
The amp in these clips: http://www.youtube.com/olekelo
runs power amp with 2x6P1P per above Raa & B+. No TW DNA in the circuit.

Love that clip!

[TheGimp]

That is indeed a nice clip.

Finished set with speaker installed (upside down since the leads are up that way).

I decided to do round-overs on all corners. I think it looks better this way.

[Bob S]

That is one sexy looking rig. Stunning.

[azatplayer]

That grille cloth looks a lot like whats on the XITS stuff, very cool!

[TheGimp]

Thanks, the speaker panel and grill cloth were re-purposed from a Magnavox Console Stereo.

I've got one more left with the 12" speaker in it that I might use with my 5F1 inspired amp.