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RH807 with a pentode driver?

dnewma04

dnewma04

The Healer
So, I have been eyeing my small collection of 5B/255Ms that I have put together and decided that it was about time I make an amp out of them.

For those not familiar, the 5B/255M is a loctal 807 without a top cap, or, it could be considered a loctal 6L6. They aren't inexpensive, unfortunately, but they are pretty valves.

5b255m.jpg


I'm thinking about using some output iron that I purchased from Heyboer about a year and a half or 2 years ago now or possibly utilize some similar Edcor iron. My original intention was to use some 6AB4's in place of the 12AT7, but the more I think about it, the more I think I might want to use some pentodes as drivers. I've heard the circuit lends itself well to the use of pentodes. The heyboer iron has a 40% UL tap to go with the 6k:4/8 primary/secondary.

This is the RH807 circuit.
se807_sch_2.gif


Unfortunately, my knowledge of utilizing pentodes is limited in this application. I'd be tempted to strap them as triodes but that would seem like it would defeat the purpose to some degree.

Kegger recently built an amp using the 6EJ7, but I can't recall if it's strapped as a triode in his application.

Lastly, I have some of Micah's CCS boards that I wouldn't mind utilizing.
 
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I did use the 6EJ7's triode strapped, they work extremely well.
(I could draw the circuit for yu)

No reason they could not be used as an pentode though either.
 
I love that 6EJ7s are great tubes that cost next to nothing. But my guess is that there are many dozens of tubes that would qualify.

Keg, I'd love to see the circuit. Or any other ideas you might have as an option to the RH807 circuit.
 
If you wanted to use a CCS, the best place to put one would probably be as a load for the driver stage. Then you'd need to use a triode or triode-strapped pentode (a plain pentode would likely not bias right and have too much gain - similar to CCSs working into each other).

A pentode would do great in the first stage, though. No need to worry about gain then, you'll have plenty of it. And, they work well with plate feedback due to the high plate resistance. You'll probably get the same distortion cancellation that the 12AT7 offers by properly biasing the pentode. You know I'm a fan of the 6CL6, that would work. 12BY7, 6AU6, and a few others would probably work too.
 
Here is how I run the 6EJ7 triode strapped as a driver, (works well for 6L6-KT88, ect.)
(attached is the prototype amp KT88 schem, 6EJ7 triode data, an my driver sheet)
(this amp runs 6L6GC through KT90 with cathode bias, just drop in the tubes there)

This will give you the layout of how I run there driver circuit, if you wire them in such a
way there are several other PINOUTS that will drop right in the driver stage, as seen on
the schematic of my prototype amp I used a pot to adjust bias v/current/plate voltages
which alll attribute to the amount of feedback and distortion(both type an amount) that
the amp ultimately produces. From a fair amount of testing the following post contains a
couple schems of how I run my driver circuits nowadays on single ended ultra linear amps.

If you look at that Excel spreadsheet, the top right hand side gives the wiring pinout, the
sheet contains pinouts that are compatible along with the list of different tubes to try out.
(I've used a fair number of those tubes but not all as of yet)

From what I've been doing the past few years SE,UL needs little additional feedback that a
triode used as a driver in this fashion works extremely well, if running the outputs in pentode
then using a pentode for the driver with additional feedback the Schade style would produce
is certainly something I would look at, probably even Doug's E-Linear hookup I would consider.
(careful saying "pentodes work better" for Schade fbk, it's all in how much you "need/want")

EDIT: Can't upload excel sheets, it's now a GIF file.. :)
 

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Here is an schematic I recently tweaked for a stereo 807 amp using the 6DJ8, 6BK7
family of driver tubes, there are plenty of driver tubes that can drop in here as well.
(single tube to drive both channels, works well, (ECC85 is a "12AT7" in this pin-out)

Also another driver some may find interesting, it is a single triode 9pin tube, with an
good amount of gain and transcon, in that roughly 40-50mu range an 8-10k trans, I
have used these recently an they perform in circuit similar to how the 12AT7's do in
how they react to distortion in an output stage. (6BC4, you can grab um for a buck)
http://www.esrcvacuumtubes.com/dollar_days.html (you'll need 2, for a stereo amp)
(this amp will also run 6L6GC through KT90 tubes)

----------

When running the 807 type tubes in a cathode biased circuit 375-435v B+ is where I
would run, 430-500ohm cathode resistor, higher voltages I'd run the higher resistors.

You'll notice in all these circuits the LED and small unbypassed cathode resistor used
on the driver tube, that is the combination along with the Schade style feedback res
and output tube UL connection that make these particular circuits work well together.
(pentode drvr gives you "more feedback when doing Schade" as with Bricktops post)

Also: Just for example, couple Pic's of an amp using those 6BC4's.
 

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Like you don't have enough going on at the moment Dave? I am wondering if we won't see a loctal forum in the next year or so. :scratch2:
 
Kegger said:
Here is an schematic I recently tweaked for a stereo 807 amp using the 6DJ8, 6BK7
family of driver tubes, there are plenty of driver tubes that can drop in here as well.
(single tube to drive both channels, works well, (ECC85 is a "12AT7" in this pin-out)

Also another driver some may find interesting, it is a single triode 9pin tube, with an
good amount of gain and transcon, in that roughly 40-50mu range an 8-10k trans, I
have used these recently an they perform in circuit similar to how the 12AT7's do in
how they react to distortion in an output stage. (6BC4, you can grab um for a buck)
http://www.esrcvacuumtubes.com/dollar_days.html (you'll need 2, for a stereo amp)
(this amp will also run 6L6GC through KT90 tubes)

----------

When running the 807 type tubes in a cathode biased circuit 350-400v B+ is where I
would run, 430-500ohm cathode resistor, higher voltages I'd run the higher resistors.

You'll notice in all these circuits the LED and small unbypassed cathode resistor used
on the driver tube, that is the combination along with the Schade style feedback res
and output tube UL connection that make these particular circuits work well together.
(pentode drvr gives you "more feedback when doing Schade" as with Bricktops post)

Also: Just for example, Pic of an amp using those 6BC4's.
Click to expand...




Hey Kegger, you know how guys do those "junkbox" builds? Let's you and me do a "crap laying on my bench" build some day. Nothing that isn't already on the bench/s. Nice amp!
 
Keg, that's pretty sweet. I wonder if that tube might even be a suitable replacement for the 5842 in the SimpleSE amp. Interesting.

The 6EJ7 just looks really cool and I know it sounds good.

Micah, I quite like the 6CL6, as well. The Bogen makes me a believer.

Looks like I need to make some decisions. For some reason I like theme amps and I had originally envisioned all loctals with a triode strapped 7C7 as the driver. Unfortunately, I've been unable to find gain, transconductance or plate resistance numbers for it, triode strapped.

The curves look pretty decent, however.

http://www.tubes.mynetcologne.de/roehren/daten/7c7pentode_as_triode.pdf
 
7C5 is pretty much equivalent to a 6V6, which should be close enough to a 6BQ5/6AQ5.

I think strapped as a triode, either of those options is only going to have a gain of 10-20. I'm not sure that's enough in the RH circuit. Someone can correct me if I'm wrong there. I've got a bunch of 6AQ5s that I've held on to for triode strapped driver applications, but never pursued anything yet.
 
hey-Hey!!!,
The pentode input stage does not absorb the NFB signal through a signal-variable plate resistance like a triode would. This is not a small benefit IME. There are lots ov good pentodes; 6AC7, 12BY7, 6CL6, EL84, EF184/6EJ7, 12HL7, EF86 to name a few.
cheers,
Douglas
 
Kegger said:
Here is how I run the 6EJ7 triode strapped as a driver
Click to expand...

Those curves look really good, perhaps I'll have to try one with a 10ma ccs load. It appears they're reasonably priced so I'll pick up a handful the next time I order some stuff.
 
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Scuzzer said:
Those curves look really good, perhaps I'll have to try one with a 10ma ccs load. It appears they're reasonably priced so I'll pick up a handful the next time I order some stuff.
Click to expand...

Did you get any of the Micah CCS boards? I may have one pair I could "loan" you for awhile.:yes:
 
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