Making sense of the AC10 circuit

[LeftyStrat]

I've been fascinated with the tone of the AC10 for some time. I plan on doing a clone, but looking at the schematic, I'm wondering if there is a mistake. The plates have a 100R resistor, and the screens are connected directly to B+. If this is correct, why would they design it that way?

[Structo]

Isn't fairly common for EL84's to use 100R screens?

Vintage AC30 also uses 100R screens but feeds them after a choke.

Later ones did use a dropper or another node to screens.

Probably has to do with the Class of the amplifier which I think is Class A although I have read different opinions on that.

[LeftyStrat]

That's what I thought. But in the schematic, the resistors are going to the plates, and the screens have no resistors.

[Structo]

Oh geez, I guess I didn't look close enough.

You are right, resistors on plates.

That is a bit strange.

Schematic shows 305v on both plates and screens.

[LeftyStrat]

Did some more research. According to this thread (about tenth post down), they are really wired this way:

https://tubeamparchive.com/viewtopic.ph ... light=ac10

I wonder if this was just a design mistake that made it in to production.

[shane]

It's not unknown for 100 ohm resistors to be used on EL84's Anodes.
I've seen data sheets that recommend it. it helps prevent unwanted oscillations. One of the project amps on AX84 uses them.

Probably not a bad idea on modern EL84's especially if you're gonna push them hard.

I would also definitely use screen resistors on them with 1K as a minimum. I prefer to use 2k2 to 2k7.

Cheers
Shane

[LeftyStrat]

Thanks, I also found a quote from Merlin in an ax84 thread:

"They reduce the Q of the filter formed by the OT inductance and interelectrode capacitances, to reduce any chance of HF oscillation. They're not essential, just cheap insurance."

It's somewhat confusing that Vox would go to the trouble to put these on the plates and then turn around and not use screen resistors.

[shane]

Yeah it does seem a bit quirky doesn't it! Lots of old amps missed out screen resistors, Fenders, Marshalls et al..But hey just cause they're classic amps doesn't mean they were well designed!
Especially for clones using today's current production tubes.

Cheers
Shane

[gingertube]

The big clue here is that the screens are at only 305 Volts. No screen resistor is required. The power amp is only pushing 10 Watts.

Things degenerated late 60's early 70's. This was when you could be as crap a player as you liked as long as you played at ear bleeding sound levels. Your "standing" as a guitarist was not how good you were but how loud you were (and if you were loud enough no one could tell if you were any good or not). Amp manufacturers started a power war and started to run really high voltages in an effort to screw the last watt out of an amp (Our 60W Amp has to be better/louder than their 55W Amp - RIGHT?) and players started to run their amps in to heavy overdrive. That required that screen resistors became necessary to keep the tubes from melting.

Now I've been there and done that!!!

I'm only just on the south side of 60 now and in recent times my favorite amps are:

A Trainwreck Clone I built running cathode biased EL34s at 360 Volts and a common 470 Ohm screen resistor for 35 Watts out (instead of the 55 Watts most try to screw out of a pair of EL34).

A 6V6 push pull amp (also my build) running at 300V for 10 to 12 Watts Out (individual 270 Ohm screen resistors).

Both sound smooth and effortless and I'm convinced its due to the conservative voltage levels and hence output powers. Note that both have very low value screen resistors compared to most designs.

Cheers,
Ian

[LeftyStrat]

That's a very good point. In those years there was sort of an arms race wrt wattage. Heck, I can even remember a time when companies would advertise peak wattage instead of RMS.

[rp]

I owned one, a 1963 fawn, it's correct. Had it 20 years ago, to this day I wish I'd documented the voltages throughout, oh well, I blew it for the both of us. I do recall it had like 345V plates, 130 Rk.

Mine was one of the best amps I ever played or heard, took off like a missile. HUGE swirl of deep rich distortion. A great big essential part of the sound were the original Elacs, maybe someone here could direct you to a speaker clone. The cab / baffle might have played a role as Silvertones also have the same big swell, maybe the AC10 had a thin baffle?

I cloned one but it didn't sound right, and my screens came out a couple volts higher than the plates, don't recall of the orig did that too. I also used Hammond iron (guessing at the values), metal films, MIT caps and all that bad stuff. I never got the vibrato sounding right with the pots I had so it's a good idea to drop it. It was dark and ran wicked hot too so maybe I had an ultra sonic oscillation. It got rebuilt into a most excellent Lightning, an other amp with a big swell, though less organic.

[rp]

Hey Lefty, FYI found this looking for an OT for my Rocket:

http://livinginthepast-audioweb.co.uk/i ... rmrclassic

-> scroll down to AC10 xformers.

Those PT primary Vs are wacked, 0 - 105 - 145 - 160 - 205 - 245V! no wonder I had such a high Pv. What the hell is 145, 160, 205, Timbuktu??? Soviet submarine??? If you want to be accurate in the USA you'll have to build with a bucking tranny in mind or just go with a Hammond PT.

My orig sounded so good I'd push towards the higher end, 345V isn't so out of place on old Vox or Matchless, and maybe it's the secret?

[LeftyStrat]

Those PT primary Vs are wacked, 0 - 105 - 145 - 160 - 205 - 245V! no wonder I had such a high Pv. What the hell is 145, 160, 205, Timbuktu??? Soviet submarine??? If you want to be accurate in the USA you'll have to build with a bucking tranny in mind or just go with a Hammond PT.

Thanks! Those primaries are crazy. I have a Hammond 270Dx 275-0-275, but it is only 90ma, which is cutting things close, even if I drop the ECF82.

I wonder if the Weber Axis 10, which is a clone of the Fane AX10 would work.

I've heard the Celestion Blue sounds great with this circuit, but I haven't gotten over the sticker shock on that one.