Review? MOSFET CF & "Mix" control

[TheTinMan]

The attached schematic is my first attempt at combining several different circuits and ideas. I bought the transformers for an AX84 "SEL" over a year ago and then got involved in another project. Now I don't feel like building that amp, so this is an attempt at a single-ended "November" (another AX84 project) using one EL-34 The basic concept is a Plexi-type preamp with the two channels internally jumpered. November documentation.

Having a tone stack & master volume right before a single-ended power tube seems like it leaves out the PI stage of a Plexi (and other classic push-pull amps). That's the reason for the MOSFET cathode follower - to use two 12AX7s and still have a gain stage after the tone stack. However, I have no idea whether or not the last preamp stage is biased properly to simulate a PI. Several circuits use a really big, unbypassed cathode resistor on one gain stage to add something to the tone. Also, I may have too much signal hitting the EL-34 and a voltage divider (or tweak to the MV) might be needed.

The thing I'm most uncertain about is the MOSFET cathode follower. The part # came from R. G. Keen's "MOSFET" follies. I'm guessing that the B+ will be about 350 at that power supply node. Is that part OK here? Also, Merlin (aka Valve Wizard) recommends a resistor/diode between the grid & cathode of a tube/valve CF. Any point to carrying that concept over to a MOSFET cathode follower? (not shown here) Last but not least, is the 100K load resistor appropriate?

The 220 ohm resistor between the plate of V2a and the gate is supposed to be a "gate stopper". Is it wired right, is that the right value, and do I need it? I saw several references to a "gate stopper" here and elsewhere, but never saw an actual schematic which included one.

The "Mix" control and single "Gain" replace two separate volumes, one for "Bright" and the other for "Dark". The Mix & Gain are going to be interdependent - higher Gain reduces the effect of the Mix control. Also, there's a 20% loss when the Mix is in the middle and the Gain is maxed out vs. separate volumes (or having Mix at one end or the other). This idea came from Doug Hoffman's "Mix" control in his setup for adding reverb to the "Western Electric Circuit". What do you all think? The good news is that I can always revert to two separate volumes if this approach doesn't work out well.

This build is going to require a LOT of tweaking along the way. I'm just trying to avoid basic bozo blunders at this point.

I'd really appreciate any comments, suggestions +/or improvements.

Thanks in advance for your thoughts,

Chip

[loudthud]

4K seems high for an OT primary. It would be handy to use one of those Hammond Universal OTs to allow you more flexability with the primary Z to squeeze more power out. The 470 5W in the B+ line might smoke, leave room for a 10W. The MOSFET looks ok but the gate stopper should be as close as possible to the transistor. If you have trouble blowing the FETs, add back to back zeners from gate to source. Don't forget to tweek the cathode resistors in the preamp. The Mix control is an interesting idea. I don't think the loss at mid position will be a problem.

[pdf64]

Some great ideas, keep us updated.
I would get a beefier choke and put in place of the 470 ohm resistor, to make a proper pi filter feed to the whole amp, in order to minimise that SE hum. Then take the feed for the screen from the same node as the plate, as it should be pretty smoothed.
Have you got the bright switch shown as you intended - usually connect to the vol control wiper?

[TheTinMan]

Thanks guys! Some good input.

Tube data sheets seem to indicate 3.5K to 5K as good for OT primary with 300-375 B+.

Thanks very much for the info about locating the gate stopper.

I was wondering about that 470 ohm resistor in the first pi filter. Going as low as 100 ohms should filter out 60 Hz but I'd like to use 220 or so.

A choke before the plates would be nice for single-ended operation. Hammond makes a 159P rated at 10 Henries, 125ma, 155 ohms & 500 volts. Calculated total current as follows:

2 x 12AX7 = 6ma
MOSFET = ?
EL-34:
Screen = 8 watts / 350 volts = 23ma
Plates = 25 watts / 350 volts = 71ma

Total = 100 ma + MOSFET

Would that be adequate? The problem is that it costs $31 and is one honkin' BIG choke.

The Bright switch is as intended for the initial experiments, but who knows where it might end up? A 470pf across the 470K grid stopper for V2a might work better, going to the Gain wiper, or none at all. I'll just have to listen to it.

Cheers,

Chip

[Steve Conner]

Pdf64 has a point. SE amps tend to hum somewhat. I had a little Champ-style SE EL84 amp that hummed a little even after having all its filter caps replaced. I tried replacing the resistor in the Pi filter with a choke, and the hum was completely gone!

So the question is just whether the hum bothers you bad enough that you'd pay $31 to get rid of it. I already had a bunch of chokes lying about from other projects.

[tubeswell]

The .1uF (that is in parallel with the 16uF) in the power supply wouldn't be doing anything (except making it 16.1uF). 2CW

[TheTinMan]

PDF and Steve - I had the same experience in my first build - a modified 5F2-A. The incremental cost actually is only $13 ($31 - $18 for the 156G).

Is 125ma a high enough rating for this entire amp?

The .1uF (that is in parallel with the 16uF) in the power supply wouldn't be doing anything (except making it 16.1uF). 2CW

I keep reading posts from people who believe otherwise so I figured this would be a good build to try it on. My guess is that you are absolutely right but there are plenty of 0.1uf capacitors in the world .

Chip

[mooreamps]

The .1uF (that is in parallel with the 16uF) in the power supply wouldn't be doing anything (except making it 16.1uF). 2CW

No. You really need to start understanding how this stuff works, before making a knee jerk comment like that. The 0.1 uF cap improves the high frequency performance of that first gain stage.

-g

[Steve Conner]

I don't believe that film capacitors in parallel with electrolytics do anything either.

At least, not in tube circuits in the audio band: they have their place in RF circuits, switchmode power supplies and the like, where the frequencies are high enough that the ESL of the electrolytic and its wiring become significant.

They also have their place in solid-state audio power amps, where they help to short out parasitic oscillations in the 1s to 10s of MHz (or 100s if you're working with MOSFETs and having a bad hair day) that could otherwise be provoked by the stray inductances I mentioned above.

But in a tube amp, the frequencies are low enough and the impedances high enough that the electrolytic does all the work. The ESR of even a cheap and nasty electrolytic is negligible as far as tubes are concerned, and even if it wasn't, it's still less than the reactance of the film cap anywhere in the audio band.

I've argued this point on the forum before and posted calculations to support it, showing that a typical film bypass cap only makes 0.1% of a difference at 20kHz.

[GibsonLover]

The MOSFET could use a 12V zener with cathode on the gate. I don't see where back to back is necessary. I see Hi-Fi designs implemented that way often. It's just to protect the gate when B+ is turned on, and the triode before it is not conducting yet. It's a protection circuit that is often overlooked. The same is true of a triode cathode follower in the same place, yet you seldom see protection there (on its grid). The 100K load is fine.

The 0.1uF on the power supply - leave that for the Hi-Fi camp. For guitar amps, where high frequency response is low past 8-10KHz - no, they are not needed. If you need test equipment to quantify such a small improvement, it is insignificant.

[TheTinMan]

Guys - thanks so much for the thoughtful replies.

GL - Does the zener go between the gate and the source? I understand the basic issue - that's why the plate resistor has a 2 watt rating on the triode right before the MOSFET. The Valve Wizard suggests something similar with a tube cathode follower HERE, but it's a diode and resistor combination. Would the 12 volt zener replace both the regular diode and the resistor or would I still have a 10K resistor in series with the zener? Also, what current rating do I need for the zener? I'm assuming that the current would be very small, but hate to assume anything...

Is 1n4742A an appropriate part? (12v/1W/5%)

Can I use a regular 1/2 watt plate resistor if I add the diode/resistor or zener?

Thanks again - you all are a big help!

Chip