# Deciding which circuit to start building -- PP UL EL86 w/ shunt NFB

#### rongon

A few years ago, and after more than 20 years of service, my tube power amp broke down. I just never got around to rebuilding it, and I've been thinking I should try something new. I'm interested in the Baby Huey PP EL84, but I was thinking of adapting it to PP UL EL86 (actually 6P43P-E). The problem is that the max plate and screen grid voltage for EL86 in UL is likely to be about 275V at the very most. Since I'd like to go with self bias for simplicity's sake, a 300V B+ minus about 25V of Vk should get me there. I have a Triad N-77A 115V:230V 150VA isolation transformer that should work with plenty of headroom. BUT...

Here's the Baby Huey EL86 circuit I was thinking of building:

The phase splitter/driver stage (12AX7 LTP) has to work from a measly 300V B+. That looks like a limiting factor. So I got to thinking...

I have a couple of 700VCT 40mA power transformers. I could use one to get about 430V for the B+ to the phase splitter/driver stages, but still use the beefy isolation xfmr for the output stage 300V B+. But to do that, I'd have to abandon the Baby Huey's clever feedback loop arrangement and go with the traditional single stage inverter/'anode follower'/plate-to-grid feedback arrangement. With 300V to the output stage and 430V to the driver stage, I came up with this:

This would be my first build with a shunt feedback output stage. I'm hoping to get a more 'push-pull triode' kind of sound but with a nice 15W max from the pair of EL86s. I'd be very appreciative if anyone cares to give that a look and let me know what they think.

Thanks!

#### 6A3sUMMER

That is an interesting circuit.

The parallel impedance of a 12AX7 plate, rp; that is in parallel with 330k RL . . .
Is that low enough to drive the MOSFET gate capacitance all the way up to 20kHz?
(I did not look at the MOSFET capacitance curves versus voltage).

Because of the LTP cathodes, the plate impedance of the 12AX7 is pretty high.

#### tikiroo

That mosfet has a fairly flat Crss of only 3 pF for Vds>40V.

#### Tubelab_com

The mosfet depicted in the second schematic has a Crss in the less than 3 pF range for any Vds over 40 volts. My previous experience with 6CW5's and UL84's (a 45 volt 6CW5) tells me that they work great with 340 volts or more on the plate and 170 volts on the screen. That rules out UL though. I have a guitar amp that runs everything from an isolation transformer. Heaters are series wired from bridge rectified and filtered 120 volts as are the UL84 screens. The output plates get 340+ volts from a voltage doubler on the same transformer. Power output is about 20 Watts.

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#### huggygood

A few years ago I bought an amp that looked like this

#### rongon

Will the 6N43P-E (EL86) burn up running UL with 270V from its plate and screen grid to cathode?
Will adding 1k screen stopper resistors in series with the OPT primary screen taps keep the 6P43P-E screens from melting down at that voltage, if the plates are also at about 270V plate to cathode?

For MOSFETs, I have ZVN0545A or FQP2N60C rattling around in my parts bin. Both have Crss around 4pF at Vds of 25V or higher.
For FQP2N60C:

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#### rongon

Right you are Francois G!

It seems I've been pestering people about this for a long time, and still haven't gotten to work on the actual amp. Well... I've picked out a chassis with holes punched for stereo PP EL84 with one 12AX7 per channel, and I have an Edcor 250V-0-250V @ 250mA power transformer that will fit. I also have a pair of 30W-rated 5k p-p primary OPTs I can use after drilling some holes. I will have to wire this up point-to-point, as the holes are in the wrong places for the Baby Huey PCBs I bought years ago. But that's OK.

It will make a good experiment. If I melt down some 6P43P-E tubes then it will be easy enough to switch to EL84s (they have the same pinout).

I have to get to work on this.

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#### Francois G

Paid Member
Dear @rongon,

Sounds like you have all the goodies in place, and the stars are aligned just right for this experimental build. 😁🤞

I am most interested in your findings.

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#### rongon

Francois, did you end up building your PP EL86 Baby Huey?

I forgot that if I the 6P43P-Es can't survive 275V plate+screen to cathode, and I decide to switch to EL84s. I'll also have to switch out the OPTs. I guess that's not too difficult.

#### Francois G

Paid Member
I skipped the EL86 Baby Huey build because I had the same concerns you expressed about the “measly” supply voltage to the phase splitter/driver stage. In stead I am slowly working on two Tubelabs SPP boards/amplifiers for 6P43P-E and 6P15P-EB respectively, that I customized for Dave Gillespie’s EFB scheme that modulates the grid bias and the screen voltage according to B+ sag. For the 6P43P-E amp I‘m planning on using Ua=300V, Ug2=170 V, and supply voltage of 330V for the front end. The 6P15P amp will light-up first though.

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#### v4lve lover

PL84 is the 300mA series heater variant (16V) of the EL86