6K6GT PP AB/1 info?

[Backwoods Joe]

Looking info for running 6K6 tubes class "AB" for a push pull output stage. All the info I can find is for class "A" push-pull. Even a old schematic of such would be appreciated. Opinions welcomed as well. THX, joe

[eleventeen]

"Class "A" push-pull" is something of a contradiction in terms as far my understanding goes. "Class A" usually refers to a single ended "always on" pentode or triode amplifier. Tube data sheets typically list the specs of any "power handling" tube as a "class A" amplifier because it is a sort of baseline way to compare one tube to another. 


That said, 6K6's are kind of a de-rated 6V6. They are something like 8.5 versus 14 watts (6V6) and they can't take as high a plate voltage. They are called "power pentodes" versus "beam power tubes" = 6V6.


Most likely any 6V6-fired amp circuit would work, but you would want to dial back the B+ from 6V6 levels.

[octal]

Class A push pull is real! All that means is that when the "top" tube is fully on, the "bottom" tube is not in cutoff, as it would be in a class AB amp.
Having said that, there really aren't many (any?) true class A amps in the guitar world.


What is your goal with wanting AB operation? You really can't get much more power out of 6K6s (compared to the class A P-P datasheet condition) due to the plate dissipation limits. You can bias them a little colder if you're trying to get the bias shift effects of a larger amp as it's overdriven.

[jazbo8]

You can get about 12W output in class AB1 with the following conditions: Ep=315V, Eg2=285V, Eg1=-30V and Ra-a=10k. If you want more power, the tubes can be run in class AB2 as well.

[tubenit]

Most likely any 6V6-fired amp circuit would work, but you would want to dial back the B+ from 6V6 levels.

I agree with that.  I've done two D'Mars ODS amps using 275-0-275 PT with 150ma.  Using an old stock 5Y3GT, I'd just put in 6K6 tubes and it worked fine.  You could build a 5E3 Deluxe or a Princeton Reverb with a 250-0-250 or a 275-0-275 PT.

And with a 275-0-275 PT with 150ma,  I can also use 6V6's with a 5Y3GT or a GZ34 or a 5V4.  And I can use a 6L6/5881 with a 5V4 or GZ34 or a solid state plug in like Doug sells.  Nothing needed to be done using a 330ohm/10w cathode resistor for the power tubes.

I built one for a church that needed a "zero volume" stage and used a 100w 8ohm dummy load resistor and a H&K Red Box going into the PA system.

This sound clip was used with a 6K6 D'Mars.   http://www.soundclick.com/player/single_player.cfm?songid=11527276&q=hi&newref=1

With respect, Tubenit

[jazbo8]

agree with that.  I've done two D'Mars ODS amps using 275-0-275 PT with 150ma.

What are the voltages on the 6K6 D'Mars? Did you use a VVR on the PS?

[tubenit]

About 305v on the 6K6's plates with the D'Mars.  That amp has been used for over 2 years with those same tubes with NO issues at all.  No VVR.

IIRC,  HotBluePlates has talked about people using 6K6's in their Deluxe 5E3 tweeds with no mods and having that work out OK.  I think he did that himself at one point?  And that would be quite a bit higher voltage than 305v.

With respect, Tubenit

[jazbo8]

About 305v on the 6K6's plates with the D'Mars.  That amp has been used for over 2 years with those same tubes with NO issues at all.

Good to know that these tubes are really rugged, it would suggest that the rating was pretty conservative, since with 305V on the plate (and assuming the same for the screen), the estimated dissipation is ~12W or 41% higher than the 8.5W maximum Pda rating. They are only $4 at the Tube Center, I'll order some. Thanks.

[Backwoods Joe]

Thanks for all the replys. The "plan" is to build something with a mid-gain preamp that will clean up nicely with the guitar vol. The power section will have 4 6K6's pp, fixed bias using a 6K OT that I already have. Probably have to build a quad bias supply as of now I can't find a matched quad of 6K6's. I've ordered some iron that should give about 300V using a GZ34. Hoping the amp will put out no more than about 15W.

[eleventeen]

"They [6K6] are only $4 at the Tube Center, I'll order some. Thanks.


It would not surprise me to find out that 6K6s haven't been made by Sovtek, JJ, etc etc and thus you could reasonably expect to get US made NOS (or at least strong used tested) tubes. 

Let us know what you get, will you?


Another approach is to buy 12V6 tubes, which I have been able to find in NOS Sylvania and GE for $10 a pair. Great deal if you have the double heater winding.

[HotBluePlates]

... I've ordered some iron that should give about 300V using a GZ34.

Could you post the specs of the power transformer? Just the a.c. output voltage and current of the high voltage winding should do.

The power section will have 4 6K6's pp, fixed bias using a 6K OT that I already have. ...

How big a lump of iron is that 6KΩ OT? That impedance sounds like a lower-power/lower-voltage 6L6 OT (maybe in the 30-35w range?).

Normally, you would design an output stage by picking a desired power output, and looking at available OT primary impedances and PT output voltage & current. You'd pick a good PT/OT pairing to deliver the desired power output, then look at tubes which could support that PT/OT pairing. So while I understand how you got where you are, it doesn't strike me as an intuitive plan to use 4x output tubes when you could likely run 2x bigger output tubes in the same amp.

However, I suppose if you had a stash of 6K6's (I do), or noticed they're a lot cheaper than 6L6's or 6V6's (I did), then maybe it makes sense.

The 6K6GT is an ~8w tube. So push-pull class A can certainly give 8w for a pair, 16w for a quad. The data sheet shows conditions for ~10w per pair with a 12kΩ OT and ~300v B+, suggesting you'd net 20w with a 6kΩ OT and 4x 6K6's. Which lands you right in Deluxe Reverb territory, using only 2x 6V6's (and maybe higher supply voltage). Are you sure you want to install and wire 2 extra sockets?

IIRC,  HotBluePlates has talked about people using 6K6's in their Deluxe 5E3 tweeds with no mods and having that work out OK.  I think he did that himself at one point?  And that would be quite a bit higher voltage than 305v.

Yep, that was me. I once bought a bunch of 6K6's on ebay because I had a Fender standalone reverb and wanted to use the correct tube. Then I got curious and popped them into a 5E3 build I had (don't recall the supply voltage, but probably a bit of 400vdc). I was thinking at the time I drop the power output of the 5E3 for home use, but it didn't quiet it down that much. And the tubes survived just fine in the amp.

I can tell you the 6K6 is a true pentode (it has a suppressor grid and not beam forming plates, as in the "beam power tube" 6V6), and therefore has a different distortion sound that's a tiny bit more raucous than a 6V6.

[Backwoods Joe]

HotBluePlates, The PT I've ordered is a Edcor XPWR093. It's 500V (250-0-250) @ 300ma, 6.3V CT @ 7A & 5V @ 5A. It's way more power than required but that's good so if I want to use bigger bottles later on. The OT is actually one I've had for years that was supposed to be for a trainwwreck express somewhere around 6K @ 30-35 watts. The whole idea is to have a low watt amp that runs the 4 output tubes with as you say the "more raucous distortion". The plate characteristic curves for the 6K6 look more like a EL34 than anything else. 20 watts output would be fine. Thanks for your input.

[printer2]

Running a pair of 6K6's at 350V and they do not seem to mind. 310V on the screen, cathode resistor 24V (484R, 49.5mA), 7W plate dissipation.

[HotBluePlates]

HotBluePlates, The PT I've ordered is a Edcor XPWR093. It's 500V (250-0-250) @ 300ma, 6.3V CT @ 7A & 5V @ 5A. It's way more power than required but that's good so if I want to use bigger bottles later on.

Yowsa! Yep, that's a ~50w PT, and a 35w OT. You might want to expect 350vdc (250v*1.414 = 353vdc) due to the severe under-loading of the PT high voltage and filament windings. You can usually adjust easily if you have less voltage than expected, but not often as much the other way around.

The OT will limit output power to ~20w regardless of the tubes (350v²/6kΩ =  20.4w); you'd need a higher supply voltage and/or a lower OT primary impedance to enable more power delivery from the PT and supply. For example, 450vdc would enable closer to 35w (~34w calculated, less real power & 100% clean, more measured power with distortion).

[Backwoods Joe]

Hey printer2, do you remember which OT you used with your amp & what was the value of the feedback resistor?

[DummyLoad]

The plate characteristic curves for the 6K6 look more like a EL34 than anything else.

respectfully disagree.

--pete

[printer2]

Hey printer2, do you remember which OT you used with your amp & what was the value of the feedback resistor?

24k, Hammond 125E.

[tubeswell]

You can run any PP output stage in Class AB1 if the output tubes are biased on the 'cold' side of centre-bias, and the input signal (at each output tube's signal grid) is big enough to put each output tube into cutoff* for part of (less than half of) the signal cycle (during the part of the cycle when the input signal voltage is at the negative peak for that tube).


* The 'Class B' operating condition occurs when one side of the output stage is in cutoff, causing the OT primary on the cutoff side to stop conducting, and thereby halving the reflected load resistance seen by the tube(s) on other side of the OT primary, which causes the steeper 'B' load line for the tubes that are still conducting. This causes more current to be pulled through those tubes (and therefore more induction through the OT to the speaker). Tubes can be run this way without harm, because the time that each tube spends in cutoff compensates for the time that they are running hotter than they otherwise should be.

[HotBluePlates]

The plate characteristic curves for the 6K6 look more like a EL34 than anything else.

respectfully disagree.

--pete

I think the intended statement was that the 6K6's plate curves look like a true pentode (EL34, EL84), rather than like a beam power tube (6L6, 6V6). Meaning there is a long, sloping knee to the curves rather than a beam tube's sharply defined knee.

Being a much smaller tube, there are no other similarities aside from general-pentode-nature.

[Backwoods Joe]

Thanks HotBluePlates, that is exactly what I meant. I will try be more specific next time.