1966 Epi Pacemaker EA-50 alternate schematic...more gain?

[chopstuck]

I have a very minty Pacemaker. Looks just like a grey Princeton.

I gave it PS caps and a tune-up. Sounds fine.. Working well. Nothing to write home about.

Problem is with it's transformer driven PI it's not real gainy. Other versions of the EA-50 had conventional tube driven PI circuits.

Is there any way I can get some more gain out of this without changing every last detail ? I was considering a dissimilar dual triode for the preamp maybe ?
I tried an 12ax7 - sounds terrible in this circuit. A 12AT7 sounds better but not great.

Here is the schematic

[10thTx]

You can try a 12AY7 instead of 12AU7 and lower the cathode resistor value on the V1-a to 15.k or 1.8k.

with respect, 10thtx

[Phil_S]

I've got a Minuteman GA20-RVT. The normal channel is nearly identical, though yours is considerably simplified. I know this circuit quite well.

I suggest there are reasons not to pull the PP tx. If you amp is minty, I think it is worth north of $500 on eBay. Once you bugger it by pulling the tx and adding a tube, it will be worth 1/3 of that, if that much. The tx itself is a rare find. Mercury Magnetics makes a replacement for $65. I'll bet an original could sell for $90-$100 for a desperate eBay bidder. But I urge you not to do it! Of course, it's your amp, so you do as you like.

Agreed, the circuit is nothing to write home about. Consider it in the context of the era. This amp dates to c. 1965-66. That makes it all about clean tone for jazz players. It wasn't made for those of us who want some grind/grunt/drive or whatever you want to call it.

I found that tube rolling produced unsatisfactory results. You may do better. At least that's not at all invasive. I second 10tx's suggestion. I might even try it myself.

The circuit itself is problematic. The notch filer at C5/R6 contributes to ice-pick treble. There are other considerations, too. If you had to rework the whole circuit, refer to my comment about eBay value. If it were mine, I wouldn't do anything unless it is fully benign and totally reversible. Otherwise, you are, in my view, doing the wrong thing.

If you can bring yourself to sell it, you'll do better to put the money into an amp more to your liking. Meanwhile, it will go to a good home.

Just my 2¢. Good luck.

[chopstuck]

Thanks Phil for the response and 10thx.
Problem is we think that these amps should be worth $500 - they're not, especially in this economy. I have had it on CL now for 3 weeks at half that price and not much response.
The two folks who came to try it noted that it can't produce enough volume or gain. The Gibsons from the early 60's and 50's are increasing in value approaching Fenders but these later Gibsons / Epiphones and Maestros just don't have the same appeal.

So here I am with an amp I don't play and that I can't sell because it's too cute and clean for the $175 I might get for it.

I will try the tube substitution. I do hate to change a vintage amp.

I will investigate how to change something useless to useful.

( I have a bunch old old Gibsons collected over the last 25 years)

[Phil_S]

Well, OK, I did a quick survey of eBait. Recent Crestline GA5 Skylarks were $200 to mid $300's. The Pacemaker, the Epi equivalent, recent sales around $200. Man, this is sad. IMO, the iron alone is worth at least $100 just because people are looking for it and it's scarce.

I have an alternative suggestion that is more expensive. Pull the chassis and either mothball it or sell it on eBay. Get a new chassis and put whatever amp you like in the cab.

On the less radical front, you might bypass C5. Either use a jumper or lift one end. See if it gets you anything.

C2 and C7 are 5u and 2u respectively. Try increasing them to 10u or 20u. This will bring up the bottom end considerably. I don't think you have anything to lose at all by doing either or both.

The "problem" I see with a 12AY7 is that you really should rebias as 10thTx says. You might get by without doing that using a 12AT7. Both strategies are worth trying.

Probably the best thing you can do, and I'd try this first, is to change C1 from .0047 to .015 (or maybe .022). This seems to be a tonal bottleneck in the circuit.

I'd find it hard to hack the amp. Maybe you can bring yourself to do it. Like I said, it's your amp....

[VacuumVoodoo]

As for tube driving the transformer PI, it's not voltage gain that is important but plate current. 12AY7 in that duty will not perform satisfactorily.
My suggestion is a 12DW7/ECC832, one triode is 12AX7 and the other is 12AU7. Seems to me it could be just the steroid you're looking for.
You'll have to swap connections to the socket so that triode 2 connects to where triode 1 is connected now and vice-versa.
http://www.mif.pg.gda.pl/homepages/fran ... /12DW7.pdf
http://www.mif.pg.gda.pl/homepages/fran ... ECC832.pdf

[Phil_S]

12DW7, a great suggestion. This tube is also known as 7247.

[chopstuck]

Great, I have a few of those. I will check the socket wiring first and switch triodes if need be.

Should I rebias V1a then and do a standard 1K5 and a 20mfd on the cathode ?

[martin manning]

Inspired idea Aleksander! Chopstuck, I think if you have to flip the section 1/section 2 triodes you would put 100k/1k5 on the 12AX7 section with a 20 uF bypass cap for full-range frequency response, but you might want to play with the Ra and Rk as well as the cap to see what that can do for the tone. Consider increasing the value of the Ck on the 12AU7 section to fatten the tone up a bit... the 2 uF shown on the schematic will roll the bass off. You might need to tweak a power supply dropping resistor somewhere to recover the voltage you had due to the reduced draw. Thw 12AX7 will be ~1 mA lower than the 12AU7 section it replaced. On the other hand, higher voltage may not be a bad thing!