# Is this just too whacky?



## loudtubeamps (Feb 2, 2012)

Because of the potential to really hit the output tubes...
Any thoughts on strapping 2 x ef 86's for a long tail phase inverter application?


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## Paul Running (Apr 12, 2020)

Strapping them as triodes?


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## Paul Running (Apr 12, 2020)

For headroom, at 400VDC plate voltage on the 86s, you are looking near 230VP-P clean input to the grids of the power tube. If you are not operating AB2...few issues.


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> Strapping them as triodes?


Yes, musing how they would drive the output section (to replace a 12 AT7 ) in one of my Class a/b guitar amps.
Wondering if anyone has ever seen this done.....if so,the advantages...if not,why not? 
Other than higher heater draw which is not a problem with my mains xformers.


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## Paul Running (Apr 12, 2020)

I would verify on the test bench. First, set-up with signal generator, fed directly to the input of the PI and scope along as you crank up the level of the generator. When you begin to notice appreciable distortion...the fun begins.


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## Paul Running (Apr 12, 2020)

If you are still having issues with blocking distortion, a 12BH7 will source more current than a 12AT7...lower gain however, you can always recoup in prior stages.


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## nonreverb (Sep 19, 2006)

loudtubeamps said:


> Yes, musing how they would drive the output section (to replace a 12 AT7 ) in one of my Class a/b guitar amps.
> Wondering if anyone has ever seen this done.....if so,the advantages...if not,why not?
> Other than higher heater draw which is not a problem with my mains xformers.


If you're wondering from a production standpoint, I'd say cost/benefit. More money to do something that can essentially be done for less money.


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## Paul Running (Apr 12, 2020)

loudtubeamps said:


> Yes, musing how they would drive the output section (to replace a 12 AT7 ) in one of my Class a/b guitar amps.


What are you looking for in sonic characteristics?


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## Paul Running (Apr 12, 2020)

This part of the circuit is replaced by active components:


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## Paul Running (Apr 12, 2020)

You can even go with 12AX7s for driving the output tubes. Run them as cathode followers and you will have plenty for those AB2 situations. Here's an example of keeping the channels separate...right down the line and using a 12AX7 to drive the grids of a power tube.


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## loudtubeamps (Feb 2, 2012)

nonreverb said:


> If you're wondering from a production standpoint, I'd say cost/benefit. More money to do something that can essentially be done for less money.


Hey Richard....thanks for the input.
I guess I'm not articulate enough. 
I am tinkering with the idea for myself , not necessarily from a commercial production aspect.
I see the potential output gain and wondering how a pair of these would drive an output section with the same input signal... basically comparing a 12AT and a pair of these EF 86's. Wondering if it would be like adding a turbo charger to the exhaust.  
Guess the only way to find out is to hook em' up and see how they perform.
Cheers.


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> What are you looking for in sonic characteristics?


See above reply...Thanks for your input Paul, much appreciated!


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## Paul Running (Apr 12, 2020)

loudtubeamps said:


> basically comparing a 12AT and a pair of these EF 86's. Wondering if it would be like adding a turbo charger to the exhaust.


If you use the EF86s, configured as pentodes, there would be much more gain produced than the 12AT7s, with greater sensitivity however, the 12AT7s will dissipate more power for AB2 operation. I have not tried pentodes as PI/driver but definitely worth experimenting with.


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> If you use the EF86s, configured as pentodes, there would be much more gain produced than the 12AT7s, with greater sensitivity however, the 12AT7s will dissipate more power for AB2 operation. I have not tried pentodes as PI/driver but definitely worth experimenting with.


Thanks Paul.
I'll post some results when I've done the deed, Cheers.


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## Paul Running (Apr 12, 2020)

Here's some design info for the EF86:


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> Here's some design info for the EF86:


Thanks Paul....good info!


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## Paul Running (Apr 12, 2020)

loudtubeamps said:


> Thanks Paul....good info!


Lots of info on the net, sure makes it easy nowadays, just have to be cautious of false information.


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## Paul Running (Apr 12, 2020)

Here's an example that uses 6AU6 pentodes and a grid choke for the driver/PI...note the B+ voltage to the drivers will provide lots of headroom...those 813s will be singing.


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## tomee2 (Feb 27, 2017)

Paul Running said:


> Here's an example that uses 6AU6 pentodes and a grid choke for the driver/PI...note the B+ voltage to the drivers will provide lots of headroom...those 813s will be singing.
> 
> View attachment 347509


I was going to suggest looking at old PA amps.
Many circuits are here...




__





Triode Electronics: On Line Schematics Index






www.triodeel.com





I'm not going to say I really understand them in any way, but I'm guessing that somewhere in the past someone has tried this or something similar.
Harman kardon citation II maybe? Another is VTL, a modern "high end" audio company.


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## Paul Running (Apr 12, 2020)

Not a LTP but maybe of interest...from the Mullard cookbook:


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## Paul Running (Apr 12, 2020)

loudtubeamps said:


> Because of the potential to really hit the output tubes...
> Any thoughts on strapping 2 x ef 86's for a long tail phase inverter application?


If you want to really hit the output tubes, such as 6146 types or others that require 100V+ grid swing...try what Guild does with their J100. The PI uses 2 pairs of 12AX7s to increase the output swing...a bit of overkill for 6L6s however, the switch to overdrive will be smoother than your conventional single pair LTPI.


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## Paul Running (Apr 12, 2020)

Here's another oddity...not a conventional LTPI however, close to it's semi-con brother, the op-amp:


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## jb welder (Sep 14, 2010)

Paul Running said:


> Here's another oddity...not a conventional LTPI however, close to it's semi-con brother, the op-amp:
> View attachment 357739


I see that as the 12AX7 being typical cathodyne PI, with 6SN7 halves acting as "drivers" ?


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## Paul Running (Apr 12, 2020)

Some may recognize it as a concertina or split-load PI going into a differential amp that looks similar to a long-tailed-pair PI, with R8 (15KΩ) being the tail...could be the way the circuit is presented.
Danelectro applied the same configuration for their Challenger model:


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## loudtubeamps (Feb 2, 2012)

@Paul Running ...sorry for not replying....busy with stuff.
Thanks for thinkin'.
So far....the only thing I've played with is the EF86 as the only tube in the audio path to the phase inverter in my guinea pig.
No eq....no nuttin' with the exception of a Pre Phase inverter volume control.
I have an a/b switch to select the 86 or my regular 3 gain stage AX7 and tone stack.
I'm amazed at the wallop from the single 86.....very cool.
I run both Pre and Post Phase Inverter level controls in my amps now.
This offers another simple overdrive option
A pair of combined 86's to act as a long tailed P/I will be next.
Cheers. Doug


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## loudtubeamps (Feb 2, 2012)

Just finished putting my new phase inverter mod through it’s paces,
a pair of EF 86’s strapped together.
Final values that sounded best were 270k plate resistors with a 25 k pot feeding them to balance them out.
A 2K4 cathode resistor to strap the pair and a 24k on the tail with a 1K5 grounding out the circuit.
I have pre and post P/I volume controls and I find the 86’s clip very well...nice and smooth compared to a12AT or AX.
The amp now has a silky lush tone with more presence than ever,highly recommended to those who like to tinker.
This mod is a keeper.


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## Paul Running (Apr 12, 2020)

Good to learn of your success. Did you come across any oddities in this project...any technical surprises?


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## loudtubeamps (Feb 2, 2012)

Paul Running said:


> Good to learn of your success. Did you come across any oddities in this project...any technical surprises?


Thanks @Paul Running ....it’s one of those “I wonder what would happen “ moments that I have, far too often.
No negative surprises, just went with stock values posted online and did some fine tweaking with respect to pairing them for P/I application.
One thing other than the punchy presence and smooth clipping is a noticeable tight low, low end that is not there with an AX.
I really like the sub woof, it’s there but sounds well dampened and breathy without the woof factor.
I’m assuming it’s the .1 (I have an .047 in it’s place)
I’m going to have to play around with this cap and see how much it can be reduced before the flub flub starts up.


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## Paul Running (Apr 12, 2020)

I came across another interesting PI that is used in an Italian model amp, the Davoli 207. Here's a look at the PI:








and an article about the PI is attached.


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