6688 = E180F.
I used these once as triodes - just as voltage amplifiers; mu is about 50 and they give pretty good gm: about 18mA/V from memory.
They should make nice cathode followers with one important condition: they have a low Vh:k rating so you'll have to float the heater supply - preferably a bit positive relevant to the cathode.
For the g2 stopper I would use 100 ohms. Grid stopper depends on so many things; layout, wiring, operating conditions etc. that it's best to start lowish (e.g. 1k) and work your way up as necessary but for this you need a good 'scope.
[Edit] One clue is that when things sound "glassy" it usually means that there is oscillation; sometimes microphony is a clue too.
I once built a pre-amp with 6SN7 diff pairs DC coupled to cathode followers (can't remember which valve) sitting on EF91 pentodes. SOunded OK to me, but when an expert had a look at it for evaluation he found it was going off at about 2.5MHz!
7N7
I used these once as triodes - just as voltage amplifiers; mu is about 50 and they give pretty good gm: about 18mA/V from memory.
They should make nice cathode followers with one important condition: they have a low Vh:k rating so you'll have to float the heater supply - preferably a bit positive relevant to the cathode.
For the g2 stopper I would use 100 ohms. Grid stopper depends on so many things; layout, wiring, operating conditions etc. that it's best to start lowish (e.g. 1k) and work your way up as necessary but for this you need a good 'scope.
[Edit] One clue is that when things sound "glassy" it usually means that there is oscillation; sometimes microphony is a clue too.
I once built a pre-amp with 6SN7 diff pairs DC coupled to cathode followers (can't remember which valve) sitting on EF91 pentodes. SOunded OK to me, but when an expert had a look at it for evaluation he found it was going off at about 2.5MHz!
7N7
I put together a linestage for my 6688 last night. Cathode resistor is 160R, plate is about 130V. I used a pair of 120H chokes on the plate. I do not have any spare chokes, so using CRCRC with a SS regulator for the B+. Getting some low level humming from my sub. Floated heaters about 30V above cathode, which is under 1V (!) Heaters are DC regulated (because they were already in the chassis.
What should I do to get rid of the hum from the sub? AC heaters? B+ choke? Float heaters more? Cathode seems a bit low on voltage?
thanks a bunch!
What should I do to get rid of the hum from the sub? AC heaters? B+ choke? Float heaters more? Cathode seems a bit low on voltage?
thanks a bunch!
Thanks Kevin and Jh, I got rid of the hum all right. But now I'm having this buzzing sound on the right channel, left is much less, barely audioable. I am gussing some of my hasty soldering efforts were not good enough. I'm going to try to resolder every and see how it goes. It sounds pretty good BTW, though gain is a little high! 
Yes, likely the heater elevation. You need a low impedance common mode connection to ground. Once lifted, couple the center tap to ground with a big cap.
If you don't have a center tap, then make one. Try something like two 220 ohm resistors in series on the winding. Add the big cap to their newly created middle point.
jh
If you don't have a center tap, then make one. Try something like two 220 ohm resistors in series on the winding. Add the big cap to their newly created middle point.
jh
Brian Beck said:
Thanks Brian,
My Russian is effectively non-existent. However this looks to be an impressive valve: only 330mA for the heater, yet 7.5W Pa max. I note with interest that there are the usual rather wide tolerances for emission and gm, with which we are familiar from our experiences with 6C45P! Best to bet on them generally making the lower end...
7N7
7N7 said:
Probably true. A cursory look suggests that the 6J52P is similar to the 7788 in the key specs. The 6J52P is specified to have a gm value of 55mA/V at 42mA, while the 7788 is spec'd at 50mA/V at 35mA. These values are within the tolerance range of each other, especially if both tubes were run at the same plate current. Given the occasionally optimistic nature of Russian tube data sheets, it's hard to make the conclusion that on is "better" than the other, whatever "better" means.
I've never touched a 6J52P, but I think it would be worthwhile looking into it.
Another thing
I just remembered something about 6C45Ps.
After they arrived I used to put them on the valve tester of course. The ones that read low on emission or gm, I would leave on the valve tester overnight, running more or less flat out.
Quite often by the next day, things had improved considerably.
7N7
I just remembered something about 6C45Ps.
After they arrived I used to put them on the valve tester of course. The ones that read low on emission or gm, I would leave on the valve tester overnight, running more or less flat out.
Quite often by the next day, things had improved considerably.
7N7
Seeing the Russian tubes going cheap on ePay I bought and tested some ‘standard’ ones as the 6P15P-ev and the 6N6P in existing circuits. As experience was positive I went chasing the high GM ones. It was not hard to find the E180F, the E280F, 437, 417 ‘clones’, all selling for very modest prices: the Russian sellers are kind enough to state the western equivalents in their listings, once in a while they rather mention E180F in the title instead of 6J9P. Anyway, I never found any offering a E810F or a D3A, so I went looking for their ‘clones’ on google, and found references towards the already mentioned 6J52P (also called 6sh52P or 6zh52p – in Cyrillic it is a kind of X) and the 6P38P, this one reaching (on datasheet) values above the 60ma/V. Still I have not found any ePay seller with these tubes.
There seems to be a rather extensive Russian tubes list at Klausmobile (apologies if everyone already knows about this). Datasheets aren't available for all of them, but quite a few, are.
D3A Datasheet question
The D3a datasheets that I can find
http://www.mif.pg.gda.pl/homepages/frank/sheets/009/d/D3a.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/128/d/D3a.pdf
both suggest (when triode strapped) a plate voltage of 160V, Rk of 470R and an anode current of 24mA. Now, 24mA through Rk of 470R is going to bias the grid up over 11V, which is not possible according to the curves ... so what am I missing?
The D3a datasheets that I can find
http://www.mif.pg.gda.pl/homepages/frank/sheets/009/d/D3a.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/128/d/D3a.pdf
both suggest (when triode strapped) a plate voltage of 160V, Rk of 470R and an anode current of 24mA. Now, 24mA through Rk of 470R is going to bias the grid up over 11V, which is not possible according to the curves ... so what am I missing?
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