EL84 mystery

[blue]

hello peoples,

something has confused me  :?

i have an amp, it's supposed to have been built by Matamp, although there's no badge on it.  the little plate on the back gives the model name as AB15 and rates it at 15watts.  i kind of assumed there'd be a pair of EL84's inside.  now, i know a lot of you guys get an amp and immediately pull the chassis out and take explicit gut shots! but i've only now gotten around to having a look inside.  and i'm a little bewildered.  it says it's 15 watts; it sounds like it's 15 watts;  but it's got 4 EL84's.  shouldn't this mean 30watts?  there's also a little screwdriver adjustable micropot between the two pre-amp valves, i have no idea what that might be for.  i took a couple of pictures which i'll get posted up in a little while.

just wondered if any of you amp gurus might be able to shed a little light on this?  ultimately, the amp sounds great, and that's the main thing.  i just find it a little odd that a supposedly 15 watt amp has a quartet of EL84's.  and i'd like to know what that little pot is for.

[martinw]

Matamp sold off a few amps they'd assembled from leftover prototype parts a few years ago. Those are the only 4 x EL84 amps I've ever seen that they did.

As to why it's labelled as '15', I think you can take that with a pinch of salt. There was no model number as such on those amps. Some Matamps run on low anode voltages, and employ other factors to bring the power down, but even so I'd be surporised to see less than 20W (clean) from 4 x EL84 in cathode bias, normally its nearer 25-30W.

[HTH AMPS]

I remember Dave and Jeff demoing such an amp to me YEARS ago, probably as much as 8 years back and I seem to remember the demo entailing each EL84 being pulled while the amp was playing to demonstrate that it would still run if one valve went south.  I seem to recall the volume didn't change a whole lot either between one EL84 and all four of them.

If each cathode has separate cathode resistors with no bypass cap, the volume could be kept quite low - pictures!

[MisterMuncher]

Is that the one you bought in Lisburn?

[blue]

thanks for the info guys.  from what little else i've found out about the amp, that all sounds right.  i did see somewhere else that they built up some amps from leftover parts.

Mister Muncher, yes, it's the amp i got in Lisburn.  i have this forum to thank for alerting me to it! :)

here are some pictures.  i suspect, HTH, that what you want to see is inside the chassis.  i'll try to sort that out this evening.

oh, one of the pictures shows the little pot in between the preamp valves.  anny idea what that might be for?

[martinw]

Yes, that's the one. I had a look round one with a view to re-voicing it when I was working at Matamp, but we were too busy and never got round to it. Jeff told me the story behind it, but I can't remember much. Something about Hiwatt prototypes for Music Ground. I think Carlos in the States bought most of the parts.
No idea what the pot does, would need to see innards.