Line stage preamp filament voltage question

[Lavane]

The only difference between using the 6J5 and 6SN7 is you would need two 6J5's instead of one 6SN7.

 

[FlaCharlie]

The 6J5 is a strong possibility. That tube is very similar to the 6SN7 but less expensive, and would make the build a lot more straightforward and simple.

Straightforward and simple is exactly what I've been advocating.

If your system will actually benefit from using a cathode follower, then use one. If not, what's the point?

And I would still suggest that it's not necessary to use DC to heat an indirectly heated tube unless you're building a phono stage. Easy enough to try AC first since your PT likely has a 6.3v heater winding that can supply all the tubes. Why not see if there actually is a problem before you decide to "solve" it?

You might also want to consider the various loctal base versions of the 6SN7 (7N7),12SN7 (14N7), 6J5 (7A4), 12J5 (14A4). I think all the loctals are made by Sylvania, regardless of branding, and many of the most desirable ($$$) versions of the 6SN7 are Sylvanias. The loctals just use a different base which is less common and, as a result, they're cheap.

My local dealer (https://vacuumtubes.net/) lists 7N7s for $10 and Sylvania 6SN7s listed in their "audio" section range in price from $35 to $150.

 

[nerdorama]

There is some opinion that a high gm tube, i.e. a pentode, makes a better cathode follower. Plenty of options there in single tube packages. A mosfet makes a great follower and no filament issues.

 

[Lavane]

There is some opinion that a high gm tube, i.e. a pentode, makes a better cathode follower. Plenty of options there in single tube packages. A mosfet makes a great follower and no filament issues.

Are there any pentodes that were used in the driver stage currently being made besides the 12DW7?

 

[nerdorama]

Can't say about current manufacture. I think the EF86 is a current pentode but not sure if it is a good candidate for a CF. There are a number of older pentodes that would work fine and are inexpensive. For that matter an EL84 would work great.

 

[THD+N]

Thanks for all of the input. I've got a lot of possibilities to consider. The thing I find the most interesting is that nobody recommended using half of a *SN7 - in fact the recommendations are opposite and to use both halves of the triode. That's good news - I'd rather only use one tube when one tube is needed. I guess all of photos using half of two 6SN7's was for aesthetic reasons. The 6J5 is a strong possibility. That tube is very similar to the 6SN7 but less expensive, and would make the build a lot more straightforward and simple. The transformer is a lot easier to find, too.

This is typical example of the use of 1/2 of the 6SN7 that I've seen so many times in my web searches. I get that glowing tubes are cool looking but this just doesn't make too much sense to me.
View attachment 2730269 View attachment 2730275

If only half of a dual tube is used, the half not being used should be properly terminated, and not left floating. Open tube connections are “undefined”.

 

[THD+N]

And I would still suggest that it's not necessary to use DC to heat an indirectly heated tube unless you're building a phono stage. Easy enough to try AC first since your PT likely has a 6.3v heater winding that can supply all the tubes. Why not see if there actually is a problem before you decide to "solve" it? .

For a normal line stage, AC indirectly heaters (DHT’s are another topic) should be fine. A phono or microphone preamp would benefit from DC heaters for the lowest SNR.

 

[TriodeLuvr]

\I think the EF86 is a current pentode but not sure if it is a good candidate for a CF.

That's correct. The EF86 doesn't flow sufficient current to drive a low-Z load. The 6SN7 or 12AU7 are good candidates, and often used for this service. There are many examples online.

Jack

 

[gadget73]

Maybe a 6au6? Not sure if that works any better as a follower though. Honestly not sure why or if a pentode would make a better follower than a triode would. Seems to me it would end up effectively triode wired anyway.

I suppose if you want real current supply you could use something that will serve as an output tube. A 6v6 or something in that general power range. Seems overkill for any normal preamp job though. That's the kind of thing you do to drive big output tubes into grid current operation.

 

[THD+N]

You might also want to consider the various loctal base versions of the 6SN7 (7N7),12SN7 (14N7), 6J5 (7A4), 12J5 (14A4). I think all the loctals are made by Sylvania, regardless of branding, and many of the most desirable ($$$) versions of the 6SN7 are Sylvanias. The loctals just use a different base which is less common and, as a result, they're cheap.

My local dealer (https://vacuumtubes.net/) lists 7N7s for $10 and Sylvania 6SN7s listed in their "audio" section range in price from $35 to $150.

I stocked up on 7F7's and 7N7's years ago. A few years ago I went to buy Loktal/Loctals sockets and bought some "ceramic ones" not thinking about buying a socket. Well, they were the ubiquitous Chinese sockets and they are JUNK. Buy NOS ones when you can find them. However, the price has increased significantly, so what started out as a bargain is now so-so. The 12SL7 & 12SN7 are still decently priced though in my opinion.

 

[gadget73]

6cg7 or 6fq7 are options too. Basically a 6sn7 in a 9 pin bottle. One has a shield between the sections but I can never remember which.

Not going to look right behind an octal or pre octal type though.

 

[kentuckycat]

I'm back. I appreciate everyone's input above. I've put some thought into this and I've drawn up a schematic that will hopefully work for me but I'd appreciate if I can get a few experts to weigh in on this because I'm out of my element here. To reframe the goal, I'm interested in the 76/6SN7 line stage preamps that I've seen from JE Labs and Berman. I'm cheap, so I'd like to economize on the tubes and would like to use 12v filaments for the 6SN7's (12SN7's) and I've found NOS 12J5 tubes for less than $6 at tubesandmore. From my understanding the 6J5/12J5 tubes are single triode equivalent of the 6SN7/12SN7, so that's the route I'd like to go. I scabbed the voltage doubler heater supply from the Vacuum Tube Valley 6SN7 line stage power supply;

I kept everything else the same as the JE Labs topography except for the 12J5's.

Here's what I came up with;

Questions;
- Does this look okay to build as shown? I left the elevating the heater part the same as the JE Lab diagram but left the rest of the filament design the same as VTV's. Does that look okay?
- I'm not sure how much voltage I would loose from the diodes in the VTV voltage doubler, but again, I just copied it in and am hoping for the best. I'm guessing the final voltage will be in the neighborhood of 12V.
- Seems like the 4a provided in the Hammond filament secondary should be more than enough. Is that correct? The amp draw for the filaments are .3a, .3a, .15a, .15a.
- I don't know how to calculate the fuse on this amp. Can anyone help me with that?
- The Berman design calls for a 100K volume pot and JE Labs calls for a 250K pot. Which one seems like a better design?
With the price of NOS 6SN7's, I'm wondering why more people haven't done this (if this is a viable design).
Thanks! Jeff

 

[Lavane]

6J5's are great sounding tubes. Very linear also. I used two per channel in a 300B build about 12 years ago. And one per Channel in a preamp build using UTC mic transformers at the input and some 600 ohm Edcores for the output.

 

[kentuckycat]

I'm trying to incorporate the heater voltage doubler from the VTV design into the JE Labs design. Here's a specific question; The VTV has a 22uF electrolytic cap from the neg side to ground and the JE Labs does not have a 'lytic running from the negative side to ground. Should I delete this capacitor?

 

[gadget73]

the 22uf 100v cap ? There should be something to relate the heater supply to ground but it doesn't need to be that large of a cap. Dynaco did it with an 0.01uf cap. Other designs will use a voltage divider from B+ to fix the heater voltage relative to ground in order to control heater-cathode voltages.

 

[THD+N]

I'm trying to incorporate the heater voltage doubler from the VTV design into the JE Labs design. Here's a specific question; The VTV has a 22uF electrolytic cap from the neg side to ground and the JE Labs does not have a 'lytic running from the negative side to ground. Should I delete this capacitor?

In my opinion, the 22uF/100V cap is redundant and not needed. The 100uF/100V cap across the 47k/2W resistor does the same thing... shunts noise to ground.

 

[kentuckycat]

Thanks THD, I'll delete that cap. I'm just about confident enough with the rest of the schematic to start ordering parts.