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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jacob B (guest: search)
Date: Tue, Jun 14th, 2005 @ 23:58 ( . )

Rather than hi-jack old threads...

Does anyone have experience with/thoughts on the use of the 6g6/6ak6 tube family and their use in a push pull amp?

From what I have read the 6g6g octal will run comfortably at 250 volts in SE, but cant find any schematics or info for push-pull. The tubes are rated at 2.75 watts maximum dissipation, which should give the 2-3 watts of audio power that I want and could actually use (As in the 6n1p thread...planning on a jtm45/plexi build with low power).

The spec sheet I found also states that typical operation uses a 10k output impedence in SE. What does this indicate for output impedence? would 12500k be a good starting point?

I know these tubes are no longer in production, but I want to get the overdrive/distortion that is only possible with pentodes....at low volume without an attenuator for a number of reasons...I dont need the high power, the iron is a heck of a lot cheaper, the amp with be easier to transport, and I wont need to mess around with an attenuator...


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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jacob B (guest: search)
Date: Wed, Jun 15th, 2005 @ 00:36 ( . )

correction...Typical operating voltage of 180V for the 6g6...

dang...thats pretty low...Some old threads confirm that these must be held around 200 for long term reliablity...

Any other power pentodes available for something in the same power range with higher voltage handling?

If there isn't I guess I'll just rework the plexi preamp for the lower voltage values...I doubt that will give anything close to the plexi sound...but hey, it doesnt matter too much...Guess I really just want a small PP amp that I can play with...

Just "found" the 6k6...which at 10watts audio power in push pull is a little much, but a more useful voltage levels...

So my question is...Will the pentodes sound drastically different than the 6n1p? or should I go for the simplicity of the 6n1p?

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jon Fleig (guest: search)
Date: Wed, Jun 15th, 2005 @ 07:55 ( . )

I'm probably the person who posted stuff about the 6AK6, 6G6, and 6K6. I also love the cranked pentode sound with no attenuator. This guy built a P-P 6AK6 amp with a 190-0-190 power transformer:

[link]

(just scroll down the page).

He didn't have a schematic, and doesn't seem to be too eager to answer questions.

10K or 12.5K should be a fine primary OT impedance. It's easy to adapt the Hammond 125 OT to whatever impedance you need.

Have you considered trying a SE 6K6GT? About 3W out, and can live at 315V.

My moonlight test rig currently has push-pull 6K6-GT's in it, and I like it.

At some point, I'll try 6G6/6AK6 at 250V with a lower screen voltage. The max voltage rating is 300V, so it should be possible. I suspect the reason some people have had problems is that they exceeded the plate and/or screen dissipation rating.

Another power pentode that may be useful is the 6CL6/6197. Others have experimented with EL95, but no one has made a detailed report yet. Power output is going to be similar to (or even more than) 6K6.

-Jon


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'6G6-G, 6AK6...small pentode Push Pull'
Author:Pete Allen (guest: search)
Date: Wed, Jun 15th, 2005 @ 08:54 ( . )

I tried El95's. For me I hit grid cuttoff before distortion. I might as well have used transistors and diode's.

I briefly tried solid state cathode followers, but screwed up the bias so it didn't work out (lots and lots of crossover distortion). Experience with a 12au7 and cathode followers is that it sounds great in mild distortion but then you _want_ it to hit grid cuttoff (or certinally in an 18watt) to get that marshall grind.

That's my guessing anyway.

Pete

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jamie Anderson (guest: search)
Date: Wed, Jun 15th, 2005 @ 10:14 ( . )

Adam Alpern once built an amp with a 6AK6 and to get preamp voltages he was looking for, he put the 6AK6 later in the B+ chain than the preamp. Not sure how well this would work with a push-pull power amp. Dmitry created a 6AK6 PP power amp that they worked pretty hard on. Search for "Dmitry 6AK6", not sure if there is a link to a schematic anymore, but there might be.

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'6G6-G, 6AK6...small pentode Push Pull'
Author:CraigB (guest: search)
Date: Wed, Jun 15th, 2005 @ 10:24 ( . )

Hey guys, did you forget about me :)

[link]

this amp could be built just as easy with 2x6AK6, instead of 4, all you need to change is the cathode resistor and play around with what pri impedance you like.

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jamie Anderson (guest: search)
Date: Wed, Jun 15th, 2005 @ 11:26 ( . )

Somehow I missed that one. Cool design!

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Luap (guest: search)
Date: Wed, Jun 15th, 2005 @ 14:14 ( . )

Real nice design Craig!
-I missed that one too (you've designed too many amps to digest)

I built a 5e3 type thing that uses 6ak6 at around 130V, nice sound but more fender/clean/rounded breakup than you're looking for.

Luap

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jacob B (guest: search)
Date: Wed, Jun 15th, 2005 @ 18:57 ( . )

On 06/15/2005 @ 14:14, Luap wrote :
:Real nice design Craig!
: -I missed that one too (you've designed too many amps to digest)
:
: I built a 5e3 type thing that uses 6ak6 at around 130V, nice sound but more fender/clean/rounded breakup than you're looking for.
:
: Luap
--

If you build a 5e3 type thing and got fenderish tones...SWEET! Hopefully the same will be true for the plexi/jtm45/bassman type thing i want to build (cant decide on the preamp i want to use...)

CraigB: How does that amp of yours sound? Can you get decent overdrive before cutoff? any clips? any problems with 6ak6's at that voltage?

Everybody: How does a 12ax7 sound at 180V or so if biased correctly? Im guessing it will be less linear :)

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jacob B (guest: search)
Date: Fri, Jun 17th, 2005 @ 22:03 ( . )

Just digging this out for craig...

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jon Fleig (guest: search)
Date: Wed, Jun 15th, 2005 @ 18:46 ( . )

Well done. Below 200V B+, pentode preamps and these "radio" type power pentodes rule.

-Jon

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'6G6-G, 6AK6...small pentode Push Pull'
Author:FredH (guest: search)
Date: Sat, Jun 18th, 2005 @ 19:50 ( . )

I built a 6G6 push-pull amp back a few months ago.
Hey, it sounds really good and the tubes are cheap. A definite 6V6 vibe.
What I don't know is if the 6AK6 sounds the same. The bottles and internal structure are hugely different ,even if the electrical characteristics are similar.

For you folks wanting gory details: I used a Fender preamp/tone stack and BF phase inverter with Fender spec global negative feedback. I'm using an old RCA OT and 16 ohm of speaker load to get around the 10K impendence.

The B+ is 260VDC or so with a 1K/5W resistor on each screen. Screens are about 257.

The tubes haven't red-plated yet. I think the ratings of these older tubes are pretty conservative, especially in push pull AB .

hope this helps


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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jon Fleig (guest: search)
Date: Sat, Jun 18th, 2005 @ 21:43 ( . )

I couldn't tell the difference between a single ended 6G6 and 6AK6 in my prototype of the "pipsqueak pentode" build. I run at 160V / 18 mA, with the screens attached to the other side of the OT primary.

[link]

I agree that the bottle looks different, but the stuff inside the plate oval looks about the same. I am keen to try these guys around 250V, with suitable bias and screen stoppers.

-Jon



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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jacob B (guest: search)
Date: Thu, Jun 23rd, 2005 @ 23:19 ( . )

what are you running for ot impedence on those transformers with the 6ak6's?

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jon Fleig (guest: search)
Date: Fri, Jun 24th, 2005 @ 22:21 ( . )

I'm running a single-ended 6AK6 into a 14K primary impedance. It's a 7K:4 ohm OT with an 8 ohm speaker. 10K is the "book" value for 6AK6.

-Jon

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Jacob B (guest: search)
Date: Sat, Jun 25th, 2005 @ 00:28 ( . )

So would 10k be a good start point regardless of P-P or single ended? I'm not very well informed with P-P vs. SE output impedences...

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'6G6-G, 6AK6...small pentode Push Pull'
Author:HTH AMPS (registered user: 51 posts )
Date: Sat, Apr 10th, 2010 @ 08:37 ( . )

ok, I know I'm reviving this one from the dead, but I'd be interested to know if you ever got this amp off the ground using 6G6-G tubes.

I'm wanting to build a SE amp using a 6G6-G for no more than 2w, but since there are no guitar amps out there using this tube, its tough to get any info on it.

Best I can find is that this tube likes to be run a fair bit below max plate and screen spec (300v).

So, would I be best off assuming that 250v on the plates and 240v on the screens into a 10k OT is a good starting point ????

Any help appreciated.

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'6G6-G, 6AK6...small pentode Push Pull'
Author:tkwink (registered user: 76 posts )
Date: Sat, Apr 10th, 2010 @ 13:02 ( . )

I looked at these tubes a little. They have a maximum plate dissipation of 2.75 watts. With a 10k load, you wouldn't want to go much over 180volts. You could use a larger load at a higher plate voltage, but then you'd want to lower the screen voltage quite a bit. The most output I would expect with this tube is about 1-1.25 watts.

Cool tubes for SE with a 10k load and 250v would be the EL85, or EL95. These tubes both have a maximum plate dissipation of 6 watts. You could also do a 10k load on an EL91, but I think a plate voltage of about 225v would be better due to it's 4 watt plate dissipation.

Wink

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Marc Davies (registered user: 25 posts )
Date: Sat, Apr 10th, 2010 @ 13:14 ( . )

This should get you there . . .
[link]

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Marc Davies (registered user: 25 posts )
Date: Sat, Apr 10th, 2010 @ 13:26 ( . )

Ooops, Jon Fleig already linked to it earlier in the thread. I've been kicking around the idea of doing a push/pull based on them with a Fender reverb tranny.

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'6G6-G, 6AK6...small pentode Push Pull'
Author:tkwink (registered user: 76 posts )
Date: Sat, Apr 10th, 2010 @ 13:29 ( . )

Isn't a Fender reverb OT single ended?

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'6G6-G, 6AK6...small pentode Push Pull'
Author:HTH AMPS (registered user: 51 posts )
Date: Sat, Apr 10th, 2010 @ 15:29 ( . )

Thanks for the help so far guys, I really didn't expect such a response.

Reading about people's experiences with these 6G6-G tubes, some people are having to run them around 180v otherwise they'd burnt out, but others are happily running them at 250v. It made me wonder if the various versions by the US manufacturers were spec'd different - the data sheets from Sylvania and Tung Sol list max plate and screen voltages of 180v, while the RCA data sheet lists max plate and screen voltages of 300v. It was fierce competition back then - maybe RCA made their version with higher voltage specs than the competition ????

Its not too big of a deal I suppose - I'll just have a separate node on the HT rail (Vox style) with a big honkin' resistor to reduce the voltage down to around 180v and have the preamp rail up at around 250v. I only want it to do 1w anyway, so its a bonus.

The 6K6 looks to be a better all round choice, but at 8.5w max dissipation I think it'd be louder than I want (data sheet lists 3.4w single ended at 250v).

The reason for going to these lengths is that although this is going to be a home-use/recording amp, I want that pentode tonality in the output stage.

I've got some 6G6-Gs coming my way, so I'll lash up an amp and see how they fare - will post results.


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'6G6-G, 6AK6...small pentode Push Pull'
Author:tkwink (registered user: 76 posts )
Date: Sat, Apr 10th, 2010 @ 16:32 ( . )

You know... You could use on of those 6g6 or 6ak6's at 250v, and use a Fender reverb transformer for an output transformer, but you would want to have the screen voltage at probably about 150v which would require a large screen resistor.

Wink

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'6G6-G, 6AK6...small pentode Push Pull'
Author:HTH AMPS (registered user: 51 posts )
Date: Sat, Apr 10th, 2010 @ 16:53 ( . )

On 04/10/2010 @ 16:32, tkwink wrote :
You know... You could use on of those 6g6 or 6ak6's at 250v, and use a Fender reverb transformer for an output transformer, but you would want to have the screen voltage at probably about 150v which would require a large screen resistor.
:
: Wink
--


and there is the rub - the amp would surely sag with such a high drop between the plate and screen.

what I'm warming to is running the preamp at the usual voltages found in a JCM800 and just having the output stage running at around 180v, then I'll be able to use the 6G6-G or a 6K6. the data sheets for both of those valves seem to indicate that a primary of 10k would be good with 180v on the plates.

use a Hammond 124BSE OT at 10k, and away I go.


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'6G6-G, 6AK6...small pentode Push Pull'
Author:Celeste hall (registered user: 2249 posts )
Date: Sat, Apr 10th, 2010 @ 19:07 ( . )

If you are worried about PS sag dropping all that voltage with a resistor, use an emitter follower

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Celeste hall (registered user: 2249 posts )
Date: Sat, Apr 10th, 2010 @ 19:04 ( . )

Try looking at a 6F6, it is a beam tetrode, so it sounds a bit more agressive

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Marc Davies (registered user: 25 posts )
Date: Sun, Apr 11th, 2010 @ 01:26 ( . )

Yep. About 3.5w, but it's got a pretty high primary impedence--I think 25k. There are some submini builds that have used them quite effectively

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'6G6-G, 6AK6...small pentode Push Pull'
Author:HTH AMPS (registered user: 51 posts )
Date: Sun, Apr 11th, 2010 @ 06:30 ( . )

On 04/11/2010 @ 01:26, Marc Davies wrote :
Yep. About 3.5w, but it's got a pretty high primary impedence--I think 25k. There are some submini builds that have used them quite effectively
--



25k for a SE 6K6 at 250v??? - the data sheet lists 7k6 at 250v and 12k at 100v, so I figured approx 10k at 180v.

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'6G6-G, 6AK6...small pentode Push Pull'
Author:Marc Davies (registered user: 25 posts )
Date: Sun, Apr 11th, 2010 @ 08:12 ( . )

On 04/11/2010 @ 06:30, HTH AMPS wrote :
On 04/11/2010 @ 01:26, Marc Davies wrote :
: Yep. About 3.5w, but it's got a pretty high primary impedence--I think 25k. There are some submini builds that have used them quite effectively
: --
:
:

:
: 25k for a SE 6K6 at 250v??? - the data sheet lists 7k6 at 250v and 12k at 100v, so I figured approx 10k at 180v.
--


This was actually a reply to twink's question about whether a fender reverb tranny is single-ended. My computer apparently froze after I hit "post message" and walked away, there were several posts uploaded before mine finally processed, and it ended up looking like a reply about the 6K6. Got lost in context. Second time it's happened, so I'm going to have to make it a habit of doing quote-replies until I get a new computer.

Anyway, the reverb tranny is not the most ideal here. Also, at such low voltage and power requirements, have you given any thought to maybe using submini tubes in the preamp like 6021, 6011, etc? They'll work at a comparable voltage range to your output section and will make your power rail a little easier and more traditional to design. They may not have picture-perfect mu specs for what you have in mind, but there are plenty of great low-wattage designs involving them.

If you go the submini pre route, you can probably save your Vox iron for a bigger build in favor of a cheap switchmode Nixie supply like this [link]

If you need a little more current, do Google Nixie power supplies, and you'll find a number of guys who have been able to get 50ma or more. Just something to consider . . .

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'6G6-G, 6AK6...small pentode Push Pull'
Author:tkwink (registered user: 76 posts )
Date: Sun, Apr 11th, 2010 @ 11:00 ( . )

I've actually had pretty good luck using the P1 preamp with a 12ax7 at 142v (154v B+). It clips harder with the gain cranked, but it cleans up real nice as you turn it down. I did this on a build using a 12au6 as a power tube, and a reverb transformer (22.8k) for an OT. Here is a video of my son playing on it:

[link]

Wink

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'6G6-G, 6AK6...small pentode Push Pull'
Author:slackjaw (registered user: 132 posts )
Date: Sun, Apr 11th, 2010 @ 18:57 ( . )

Here's a 6k6 se at about 180v

[link]

dirtmonkey amp

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'6G6-G, 6AK6...small pentode Push Pull'
Author:tkwink (registered user: 76 posts )
Date: Sun, Apr 11th, 2010 @ 21:26 ( . )

That's pretty sweet. What kinda pre-amp?

Wink

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'6G6-G, 6AK6...small pentode Push Pull'
Author:slackjaw (registered user: 132 posts )
Date: Sun, Apr 11th, 2010 @ 22:04 ( . )

like a P1 but with a single tone control

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'6G6-G, 6AK6...small pentode Push Pull'
Author:slackjaw (registered user: 132 posts )
Date: Sun, Apr 11th, 2010 @ 23:18 ( . )

[link]

dirtmonkey schematic

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'6G6-G, 6AK6...small pentode Push Pull'
Author:slackjaw (registered user: 132 posts )
Date: Mon, Apr 12th, 2010 @ 15:54 ( . )

Actually more like a champ preamp with a MV

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'6G6-G, 6AK6...small pentode Push Pull'
Author:HTH AMPS (registered user: 51 posts )
Date: Mon, Apr 12th, 2010 @ 05:10 ( . )

On 04/11/2010 @ 18:57, slackjaw wrote :
Here's a 6k6 se at about 180v
:
: [link]
:
: dirtmonkey amp
--



thanks for that - what primary Z are you running?

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'6G6-G, 6AK6...small pentode Push Pull'
Author:slackjaw (registered user: 132 posts )
Date: Mon, Apr 12th, 2010 @ 15:30 ( . )

I believe it's a 5k primary for 8 ohms. So I use a 16 ohm spesker. i plugged it once in to a 4x12 Randall cab and it sounded pretty good. cab was 8 ohm though.

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'6G6-G, 6AK6...small pentode Push Pull'
Author:HTH AMPS (registered user: 51 posts )
Date: Tue, Apr 13th, 2010 @ 06:22 ( . )

On 04/12/2010 @ 15:30, slackjaw wrote :
I believe it's a 5k primary for 8 ohms. So I use a 16 ohm spesker. i plugged it once in to a 4x12 Randall cab and it sounded pretty good. cab was 8 ohm though.
--



so without mismatching, we're looking at a 10k primary - thanks for that.


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