The Edcor meets the 6AV5

[Fuling]

I´m getting second thoughts about using E280F as input tubes.
They are great tubes for sure, but the plate current they require will probably heat my power transformer a bit too much.
In triode mode I´m worried about the Miller capacitance causing trouble when used together with my passive preamp and wiring them (correctly) as pentodes means extra work and components*.

The chassis are already cut for two noval sockets and there´s no room for upgrading to octal input tubes so I´m thinking about triode wired 6BX6/EF80 with cascoded bipolar CCS´s as plate loads.
According to these curves they should do well with a CCS load and Ia>8mA:
http://www.tubes.mynetcologne.de/roe..._as_triode.pdf

*Go achieve proper pentode operation the G2 voltage must me constant WRT the cathode. With a non-decoupled cathode resistor the G2 voltage can no longer be referred to ground but to the individual cathode, hence the need for extra circuitry.

[the_manta]

I've got about 100 EF80. It's just junk here in germany. They sound pretty good with higher currents, but nearly all have a major problem: Ultra strong microphony. Wheter it's Telefunken, Siemens or some Eastern-Germany type.
So be sure to pick good ones.

Regards, Simon

[Fuling]

the_manta:
Thanks for the reply! They´re considered junk here too, I threw away ~50 of them a while ago (before I saw Tom´s triode curves... )
Since they come almost for free it might be worth the effort to gather up a bunch and test them for microphony, I´m planning to use EF80 as input tubes in a pair of OTL monoblocks too
(For some very specific reasons that doesn´t belong in this thread).
Thanks for the warning about the microphony!

[Fuling]

Changing my mind for the N:th time:

Upon further inspection it seems to be room for a pair of octal sockets instead of the novals. This allows me to use a tube that I´ve read good very things about: 7193 / 2C22.
Somewhat similar to 6J5 but with top caps for both plate and grid
and deadly linear with a CCS plate load.

[Fuling]

I forgot to drill holes for the plate and grid connectors to the 7193 input tubes before I painted the chassis, so guess what: I changed my mind one more time!
Instead of two 7193s I used a single 6SN7 and left the other octal socket unused.

The amp is almost finished by now and I listened to it a couple of minutes ago, except for a (quiet) hissing sound there is nothing to complain about. No audiable hum.
For the moment I run the output tubes fairly cool, about 270V plate to cathode and 50mA. The original plan was to push them a bit harder (300V 60mA) but I got nervous and installed cathode bias resistors (390R + 1000uF) to drop some voltage and get a mixed bias arrangment. I will increase the plate current later on.

The next step is to finish the chassis to make it cat-safe, then I will take a closer look at that hissing sound and start playing around with different bias points.

[jaymanaa]

Wow, this thread turned out to be a great way to spend a Sunday morning. I'm just now in the process of gathering parts for a sweep amp and this is the best reading I've found so far. Tubelab has done an excellent job of documenting his experiments, and for that I thank him. The outputs I plan on using have a primary Z of ~6K and came from an old Fisher that claimed 35 watts per channel using 7591s (no UL taps). I went to my tube shed yesterday and gathered up a few 6AV5s, 6DQ6s and 6BG6Gs for experimenting. I'm working on this with a friend (who is way past me as far as tube circuit design) and we haven't actually settled on a driver yet. I was thinking something in the 6BK4/6CG7 family. I mostly just wanted to thank the original posters in this thread and see about any drivers that have been tried with good results. Thanks, Jay

[Miles Prower]

Originally posted by jaymanaa
Wow, this thread turned out to be a great way to spend a Sunday morning. I'm just now in the process of gathering parts for a sweep amp and this is the best reading I've found so far. Tubelab has done an excellent job of documenting his experiments, and for that I thank him. The outputs I plan on using have a primary Z of ~6K and came from an old Fisher that claimed 35 watts per channel using 7591s (no UL taps). I went to my tube shed yesterday and gathered up a few 6AV5s, 6DQ6s and 6BG6Gs for experimenting.

Given your OPT, your best bet here is to go with the 6BG6. This type is the octal version of the 807, and will definitely work with a 6K P-2-P load. It will also like a bit of local NFB to tame the open loop pentode harshness. Most of these horizontal deflection PAs like a much lower P-2-P load. I considered the 6DQ6 for a project, but that type gave a bit more power than I was looking for, and would definitely stress the power xfmr I had in ye olde junkbox, so I opted for a 6BQ6GTB design instead. Still, nearly 40W out, and it pushes that power xfmr to the limit. As for loads, the 6DQ6 came in at under 4K P-2-P, and the 6BQ6GTB likes 4K4 P-2-P.

I'm working on this with a friend (who is way past me as far as tube circuit design) and we haven't actually settled on a driver yet. I was thinking something in the 6BK4/6CG7 family. I mostly just wanted to thank the original posters in this thread and see about any drivers that have been tried with good results. Thanks, Jay

Here, you lost me. The 6BK4 is a high voltage regulator that's designed to handle up to 60KV, and the characteristic doesn't begin until 5KV. It is definitely not a driver type, and is completely different from a 6CG7. As for grid drivers, I've used cathode followers of the 6SN7 and 6FQ7. The 6CG7 is comparable to the 6SN7, and the main difference between the 6FQ7 and the 6CG7 is that the latter type includes a shield between the triode sections that the 6SN7 and 6FQ7 lack. These will make excellent cathode followers for grid drive for any Class AB1 pentode.

[jaymanaa]

Sorry about that, what I meant to say was 6BK7B. I hadn't had my second cup of coffee.

[Miles Prower]

^^^^

Follower duty isn't all that demanding, and the 6BK7 ought to do well there. Like a 6BQ7, it looks like it would also make a fine cascoded LTP as well.

The Pd= 2700mW means that getting enough current to drive the Cgk + Cmiller + Cstray at high frequencies should not be a problem either.

[kegger]

Interesting meeting you over here, I'll get with yu soon!

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Anyway I had been contemplating building a 6AV5 amp for a while now, finally did it!

These tubes are very nice, (thanks for all your hard work Tubelab) mine sounds great!

Got a few cosmetic things to finish, like routing/painting the base and output trannie board.


Here's a couple pic's if anyone is interested.