7199 - 6u8a

[jaymanaa]

Try to find some Tungsol 6U8 (both blackplate and grey plate are excellent), really nice sounding tubes.

The black glass ones are also very good.

 

[Audiovet]

While the pentodes of the mentioned tubes are roughly the same depending on operating point, the triodes differ markedly and may need circuit redesign. The 7199 triode has µ=17 and gm=2100µS. The 6U8A has µ=40 and gm=7500 µS., quite close to that of a 6GH8, but a little different from the 7199. The 6BL8 (equivalent ECF80) has a µ=20 with gm=5000µS, the 6AN8 similarly, but with yet again different pin-out.

If these happen to do concertina phase-splitter duty the difference may not be noticable, in other amplifier duties there might be a redesign requirement.

The choice is yours; I might very slightly prefer ECF80.

 

[jaymanaa]

While the pentodes of the mentioned tubes are roughly the same depending on operating point, the triodes differ markedly and may need circuit redesign. The 7199 triode has µ=17 and gm=2100µS. The 6U8A has µ=40 and gm=7500 µS., quite close to that of a 6GH8, but a little different from the 7199. The 6BL8 (equivalent ECF80) has a µ=20 with gm=5000µS, the 6AN8 similarly, but with yet again different pin-out.

If these happen to do concertina phase-splitter duty the difference may not be noticable, in other amplifier duties there might be a redesign requirement.

The choice is yours; I might very slightly prefer ECF80.

Good post.

 

[Drugolf]

I went ahead and purchased the adapters from Kevin at Dynakitparts to use the 6GH8's. this way I can always pick up a set of 7199's if I wanted to down the road.

now I need a GZ34 and at least two matched EL34's.

 

[jaymanaa]

FWIW, they make a solid state plug in for the GZ34. Most of the new production are not very reliable, and the NOS ones are going for 100 or even more.

 

[Wharfcreek]

Drugolf,

Get back in touch with Kevin and ask him about the 'mods' that he recommends on the 'stock' ST-70 driver board when using the adapters. I purchased some adapters from Kevin a few years back......and he informed me of 2 resistors and 2 capacitors that should be changed on the board as well in order to 'optimize' the driver board. See post #24. I've since 'modified' several 'stock' boards......changed the pin-out wiring on the board itself, as well as changed out the parts recommended by Kevin. Sold a few on ebay that way too. As to the 'sound' of the amp after the changed tube and passive parts....... in all honesty, I'd have to say the 'difference' (if any) falls into the same category you'd get if comparing different 7199s. A really GREAT 7199 compared to a mediocre 6GH8 or 6U8......and I think you'd probably condemn the change. But, an 'average' 6U8/6GH8 compared to an 'average' 7199.....and you might not hear any difference at all. I didn't! Overall, IMHO the change is well worth it, and certainly not something that if you REALLY weren't happy with.... you could easily enough change back to 'stock'. FWIW. WC

 

[Wharfcreek]

PS: I'm all for a carburetor thread!!.........lol

 

[valentin01]

Hello,
Could somebody help me with the above ? I have a Dynaco ST-70 that I would like to modify in order to get 6BLS ( ECF80 ) instead of the 7199. Could you please indicate, which parts are to be changed and by what ? Knowing that I am completely aware that I will use a socket adapter.
Thank you in advance !

 

[valentin01]

Del

 

[valentin01]

6BL8 isn't a good candidate for the ST70 because the max heater to cathode voltage limits for this tube are too low for the triode side. You would have to make too many modifications to the supply voltages to be under the limits and that would necessitate changes to the feedback components. Good luck.

Sorry for the spam...

Thanks for your reply, which tube would you suggest in that case ?

 

[valentin01]

The ST-70 Series II amp switched to the 6GH8A with no driver circuit modifications. There was only a small change in the feedback circuit do to a change of FB tap points but if you leave that original, then there is nothing to mod for the 6GH8A other than socket rewire or use adapters, too.

To tell you the truth, as I live in Europe, I preferred something that is easily accessible in Europe. For example the 6BL8 ( ECF80 ) has an Russian Equivalent as well, the 6Ф1П, and all those can be found very very easily and on very good prices.

 

[peterh]

ECF82 / 6U8 has some vendors specift Vfk to > 200V :
https://frank.pocnet.net/sheets/049/6/6U8A.pdf ( RCA spec for 6U8A )

However even the +-90V limit on ecf80 might not be a problem, especially if the filements is
raised to 30 - 50V with a resistive divider from B+. ( connector A on the can cap, 300k and 68k )

 

[Wharfcreek]

I suggest sticking with the 6GH8A as a 'sub' in the ST-70 for the 7199. Since having made that post above, I've done some further reading and research and came across an article by VanAlstein where in he did some rather in-depth and extensive testing on the attributes of the 6GH8A vs 7199 before deciding upon it for his rather sophisticated driver board for the ST-70 amp. In that article he pretty much covers all the usual areas of concern; frequency response, noise, drive voltages, current draw, capacitance match, etc. Honestly, I don't recall all the details. But I do recall that in the end he basically concluded that the 6GH8A was a superior tube to the 7199 in virtually every way. So, in reading that, one has to question reports where the old 7199s sound 'better' than the 6GH8As? Over time I've come to the conclusion that, when using 'good' tubes of either type, I don't really hear any difference between the two tubes in the ST-70, even when using 'adapters' and 'not' performing the suggested modifications for using the 6GH8A. I believe those suggested modifications may enhance the life-span of the 6GH8A, but not so sure they really produce an audible sonic difference. In any case, when this question is put to me these days, I just suggest going with the adapters and putting in the 6GH8A and being done with it. The 6GH8A is, to me, a pretty good tube and worthy sub for the 7199 when 're-pinned' or 'adapted' for use. But, like the 7199, you HAVE to make sure you're using GOOD ones!! Of all the 6GH8As that I've purchased, I dare say that a good half of them are sub-par! In testing them, I've found that one section or the other was NOT where is should be. This is particularly true with 'used' tubes..... where the number jumps to perhaps 75%! But, as cheap as they are, I can afford to buy a bunch to cherry pick the good ones for my own use.....and just 'toss' the rest. Even with NOS tubes, I suggest 'testing' them.....or buying from a known reliable source vs just a random ebay purchase. Finally, if you happen to run across the old Sovtek version of the 7199, be careful of that tube. It's been shown to have some issues, particularly in being used in the Dynaco ST-70 amp. Again, I don't recall all the details of this, but I seem to remember that it was 'unstable' in the ST-70 and could lead to some other problems, like a super-sonic HF parasitic oscillation that could do some damage to the output transformers without actually 'hearing' anything while the damage was taking place. WC

 

[valentin01]

I suggest sticking with the 6GH8A as a 'sub' in the ST-70 for the 7199. Since having made that post above, I've done some further reading and research and came across an article by VanAlstein where in he did some rather in-depth and extensive testing on the attributes of the 6GH8A vs 7199 before deciding upon it for his rather sophisticated driver board for the ST-70 amp. In that article he pretty much covers all the usual areas of concern; frequency response, noise, drive voltages, current draw, capacitance match, etc. Honestly, I don't recall all the details. But I do recall that in the end he basically concluded that the 6GH8A was a superior tube to the 7199 in virtually every way. So, in reading that, one has to question reports where the old 7199s sound 'better' than the 6GH8As? Over time I've come to the conclusion that, when using 'good' tubes of either type, I don't really hear any difference between the two tubes in the ST-70, even when using 'adapters' and 'not' performing the suggested modifications for using the 6GH8A. I believe those suggested modifications may enhance the life-span of the 6GH8A, but not so sure they really produce an audible sonic difference. In any case, when this question is put to me these days, I just suggest going with the adapters and putting in the 6GH8A and being done with it. The 6GH8A is, to me, a pretty good tube and worthy sub for the 7199 when 're-pinned' or 'adapted' for use. But, like the 7199, you HAVE to make sure you're using GOOD ones!! Of all the 6GH8As that I've purchased, I dare say that a good half of them are sub-par! In testing them, I've found that one section or the other was NOT where is should be. This is particularly true with 'used' tubes..... where the number jumps to perhaps 75%! But, as cheap as they are, I can afford to buy a bunch to cherry pick the good ones for my own use.....and just 'toss' the rest. Even with NOS tubes, I suggest 'testing' them.....or buying from a known reliable source vs just a random ebay purchase. Finally, if you happen to run across the old Sovtek version of the 7199, be careful of that tube. It's been shown to have some issues, particularly in being used in the Dynaco ST-70 amp. Again, I don't recall all the details of this, but I seem to remember that it was 'unstable' in the ST-70 and could lead to some other problems, like a super-sonic HF parasitic oscillation that could do some damage to the output transformers without actually 'hearing' anything while the damage was taking place. WC

By any chance, would you happen to have the needed mods for the 6GH8A ? I have one extra PC 3, that I could modify. I shall listen to you and to 6DZ7 and use the 6GH8A.
So let me repeat, there won't be a significant change in the sound, meaning it will not become a "solid state" one. As I always say, the reason for buying a classic tube amp is to have a classic tube sound
Thanks a lot to everybody.

 

[Destructor]

I have rotated my ST70 Series 2 back. I figured I would listen to it while on the final leg of building a new Latino ST70. I have used various 6GH8’s in my series 2 but haven’t noticed an appreciable difference. It originally came with national labelled Russian 6GH8’s.

 

[Bob]

there are 3 pins to change to go from a 7199 to 6GH8/6u8/etc, or buy adapters to avoid
spending time/effort and worry about whether it loses value with the change.

if you're changing the pins, you add short wires to each of the 3 affected pins
on the bottom of the tube socket, and flatten them against the bottom of the
socket then solder them into the new positions. then push down and solder
the other non-affected of the socket into the remaining holes of the PCB.

google the change, get the reasoning, etc. and get the rest of the mods.

BTW, having had 6 of these or so, with various driver boards, the BIGGER
sound difference comes from the output tubes and the respective biasing,
so Mullards XF2s sound different than Winged-C, than RCAs, than EIs,
than any other tubes I tried (zillions). that's just EL34/6CA7s not the
KT66/77/etc.

I gave up due to gremlins and parts failure. so my advice is to blue-print
that ST70 if you're going into it - I've replaced fuse holders, power cords,
can caps, sockets, boards for reliability after failure.

let's not talk about 5ar4 vs 5U4s. starts religious war.

but while and after you do all this,

enjoy the music.

 

[valentin01]

there are 3 pins to change to go from a 7199 to 6GH8/6u8/etc, or buy adapters to avoid
spending time/effort and worry about whether it loses value with the change.

if you're changing the pins, you add short wires to each of the 3 affected pins
on the bottom of the tube socket, and flatten them against the bottom of the
socket then solder them into the new positions. then push down and solder
the other non-affected of the socket into the remaining holes of the PCB.

google the change, get the reasoning, etc. and get the rest of the mods.

BTW, having had 6 of these or so, with various driver boards, the BIGGER
sound difference comes from the output tubes and the respective biasing,
so Mullards XF2s sound different than Winged-C, than RCAs, than EIs,
than any other tubes I tried (zillions). that's just EL34/6CA7s not the
KT66/77/etc.

I gave up due to gremlins and parts failure. so my advice is to blue-print
that ST70 if you're going into it - I've replaced fuse holders, power cords,
can caps, sockets, boards for reliability after failure.

let's not talk about 5ar4 vs 5U4s. starts religious war.

but while and after you do all this,

enjoy the music.

And are there any resistors or capacitors to be changed for such with other values ?

 

[Bob]

no. however, if you have a replacement board do not use the 60YO stuff off
another, use modern day caps (in some order of price/performance/street-cred:
Teflon, polypropylene, mylar, electrolytic, tantalum,...) and metal films.

 

[valentin01]

no. however, if you have a replacement board do not use the 60YO stuff off
another, use modern day caps (in some order of price/performance/street-cred:
Teflon, polypropylene, mylar, electrolytic, tantalum,...) and metal films.

I was thinking the same, I wanted to try with Russian PIO capacitors, and change the resistors as well
Thanks !

 

[wa2ise]

If you rebuild the audio amp boards (like in the SCA35) with a metal sheet as a chassis, you could use separate triodes and pentodes in place of the 7199, like a 12AU7 and a pair of pentodes like 6DK6s.