How does this EF86 amp work?

[Alex R]

These little British amps were made in quite large numbers in the early 60s. I don't understand the circuit too well. Can anyone explain its oddities to me? What I don't get can be summed up thus:

1. 1 meg plate resistor?
2. No coupling cap?
3. No ground reference for the EL84 grid?
4. B+ is about 350v. What sort of plate voltage ought to be on the EF86 do you think? (supplement to point 1!)

Thanks in advance.

Schematic to follow...

[Alex R]

Here's the schem.

[loudthud]

Talisman: (2) Something producing apparently magical or miraculous effects.

What a wacky circuit. Just a guess: the EF86 is operating in grid leak mode where it produces a negative voltage on it's grid. The screen voltage feeds back from the cathode of the EL84 such that the EF86 is nearly saturated. I would expect the screen voltage to be around 30V and the plate voltage to be half of that. It might not work so well with current production EF86's.

[Alex R]

Talisman: (2) Something producing apparently magical or miraculous effects.

Nice quote!

Yep, plate volts hover under 30v somewhere, impressive prediction.

I have two of these in at the moment, and have replaced all bad components (there aren't many). Even with NOS EF86s they still present problems. One of them works kind of OK with lots of crunchy gain, but occasionally does a blocking distortion kind of drop-out, and the pots are very scratchy, just due to their odd position in the circuit and all the gain I guess. The other one has very little gain - you can only just hear the buzz if you touch the input - it just hums.

I haven't wanted to blame the circuit for these woes but am wondering whether there's a simple way to convert the EF86 circuit to something more conventional. They are however PCB amps. Oh boy. Suggestions welcome for taming the current circuit or for simple conversions.

[Steve Conner]

That is indeed a strange circuit.

As far as I can see, there are two negative feedback loops around the output stage. The first one is from the speaker output of the OT, back to the cathode of the EF86. This is fairly standard.

The second is from the cathode of the EL84 to the screen of the EF86. This loop is bypassed by the two 50uF capacitors and 22k resistor, so it's only really functional at DC and sub-audio frequencies, and its main purpose is to stabilise the DC bias point of both valves.

I guess the main purpose of this loop was to lower the component count, because it does away with the DC block capacitor to the EL84's grid. But I bet it also makes the amp sound horrible when overdriven.

I suggest rewiring it in the style of a Fender Champ, or if you can't be bothered doing that, at least break both feedback loops, add a DC block cap and grid leak resistor to the EL84, and change the EF86 stage to something closer to the recommended circuits on an EF86 datasheet.

[Alex R]

Thanks Steve, I think I get it now. Is this it?...

The loop through the 22K from the EL84 cathode, smoothed into DC by the big bypass caps on it, holds the voltage on the EF86 screen at a certain level.

This controls the current through the EF86, a relatively small amount of which causes a big voltage drop across the 1 meg plate resistor.

This holds down the plate volts on the EF86, and thus the grid volts on the EL84, to a level that balances out with the biasing effect of the big cathode resistor on the EL84 to set that valve's bias too... I s'pose...

Well (throws up hands in despair) I can either try to bring the thing into some kind of balance by adjusting that 22K finely, or give up and wire it like an AC15 or something.

They're saving the cost of two caps it would seem... I guess that 'normally' wired the EF86 would draw a bit more current? - and give a lot more gain - but the mains transformer looks quite meaty. Given the nature of the PCB I think the AC15-type preamp approach is the simplest, with a big cathode resistor on the EF86 to tame things a bit...? I have one or two more pressing jobs now, but will attempt this rewire in a few days and report back, just in case anyone else has the rare luck to have two of these on their hands at once!

[drewl]

Oh come on now....this thing is just aching to be a Vox circuit!

[Alex R]

Yes it is isn't it? Pleease make me a Vox, it mutely pleads. But with no tone stack after it the gain would have to be reined in a bit.

I've told both the customers that their amps are never going to work that well unless they give me more money to play around with them in, you know, an open-ended kind of way, and for some reason they are still circling the bait warily. Guess I should have gone ahead and done it anyway.

[drewl]

I had some weird no-name amp like that which a customer wanted me to turn into a Marshall 18w clone.
Turned out real nice, sounded great and the customer was very pleased.
Perhaps if you tell them what the amp COULD be with a little work, they could have a great amp for not alot of money.

[Alex R]

Well I was out a jam with one of the owners last night, and he says he's going to get a new Super Champ instead - didn't know there was such a thing. I took the amp back though, just to fiddle with - so maybe I'll try voxifying it anyway & see what happens. then of course I won't be able to charge him properly. Oh boy.

I'm such a lousy salesman. I keep stocks of tubes which I buy wholesale, but if someone comes in and asks for a full retube some demon of honesty makes me say, well I'll test what's in there, you might not need the whole lot renewed. I need a special prayer - please god, make me more of a ripoff bastard. Then I could get the money to get the car fixed.

[Steve Conner]

Sounds like you have the old problem with mixing business and pleasure. :P Next thing you know, you'll be wanting to keep the amp once you've modded it.

I've always had the same problem, and my way of resolving it is that I don't work on tube equipment for money, and I refuse to touch solid-state equipment for free.

[Alex R]

I don't work on tube equipment for money, and I refuse to touch solid-state equipment for free.

Sanest thing I've heard all year...

[Robert M. Martinelli]

pleasure and pain

Hi folks,
Just for this time I won' t do tech talk....beg your pardon!

Passion is at the same time an horrible and beautiful thing....
Steve is absolutely right, the bad news is none of us is gonna heal and recover from this disease.....

I do something else for a living, but I' m both playing and tinkering with circuits since the age of 8, so a lot of fellow musicians know me and ask for help when their beloved amps/guitars fail.....

The other night I finished restoring a 1969 Super Lead Plexi, I came to the point of carefully washing by hand the original cover to be sure not to ruin it, still I am disturbed by the thought I will at least have to ask this friend the money I spent for the new tubes, caps, resistors, pots and so on......my wife is beginning to look at me in a strange way, still I' m unable to raise my eyes from the bench until the job is done......the eyes get weary, your back aches like someone just stabbed a knife there, but, hey.... you just won your daily battle, and only the thought that your wife and children are sleeping keeps you from cry out your triumph scream!

We' ll all be luckier in our next life, or maybe it' s just we don' t realize how lucky we already are because we' re allowed to do things we love.....how many men in the whole world can say this?

Regards

Bob