More headroom in a Ampeg Gemini II

[chocopower]

Hi,

a friend of mine is really happy with his Gemini II amp, but sometimes needs a little more clean sound.

He ask to me for some mod to get it. Before you start to say me... "tell your friend to buy another amp!!" , i already did it....

schematic:


ok... those are the options i found:

.-change it to fixed bias as used in the g-15 model (http://www.drtube.com/schematics/ampeg/g15-jp.gif)
.-Play with the power supply dropping resistors.
.-Decrease the value in the NFB loop resistor.

Any other?

thanks in advance!

[bigdaddy]

What power tubes are you using? Is this an original 1960s Gemini II? Was it changed in any way like to 6V6s at some time? Maybe all it needs is to be put to spec in terms of voltages and the power tubes. A fresh set of JJ or Sovtek 7591a tubes would be a start if not already done. I would check that out before I went into the amp itself.

As far as I know all those old Ampeg schematics were redrawn by Joe Piazza. It says 120v on the bottom then the tranny says 110v, yet it shows a non grounded AC line, did anybody change that? If not that is something to do. Those amps were designed for 110v so all the voltages are high. Maybe too high in the preamp, so once you have the amp all speced out you might want to use another resistor to knock some voltage down. Has he cleaned the pots with Deoxit?

Those amps are what they are and by wanting just a little more of something you will lose a lot of something else. Before an amp is modded it should be brought back to spec and serviced. A friend asked me to fix his beat up gotten from the garbage B-15. I told him him before I even start I have to find out if the caps are good and a lot of other parts, I will not spend 20+ hours trying to fix one thing only to find it was really another and so on. I said you might end up rebuilding it completely except for the trannys and even one of those might need replacing, total cost could be $600, he said forget it. He was like can you make it stop buzzing, I said yes but, here we go again. Unless people have done this work they do not understand.

[eleventeen]

Great advice, Bigdaddy. I usually paraphrase it as: All you have to do is to touch it and you become responsible for every aspect of it and everything that might go wrong with it, even stuff that was wrong BEFORE you foolishly decided to touch it.

[chocopower]

The amp is the 7591 one.
Cap job ans the usual safety work are done.
I installed a european transformer for 240v. That could be aproblem, because in Spain there are 225-230vAC...
Not other mod is donne to the amp.
New JJ tubes installed.. but when i measured them there are more the 15mA betwen them!!! The suplier is gonna send a new "real" matched pair....

When it arrives, i try to measure some voltages.
by!

[Shrapnel]

The amp is the 7591 one.
Cap job ans the usual safety work are done.
I installed a european transformer for 240v. That could be aproblem, because in Spain there are 225-230vAC...
Not other mod is donne to the amp.
New JJ tubes installed.. but when i measured them there are more the 15mA betwen them!!! The suplier is gonna send a new "real" matched pair....

When it arrives, i try to measure some voltages.
by!

the under-voltage probably won't be a problem (Your given voltage is 93.75-95.83% of rated transformer voltage and at 220v, a 91.67% difference.)

[JDarby]

A Gemini II (G-15) is a fixed biased amp.
You posted a Gemini I (G-12) schematic which is cathode biased.
Get a copy of the factory schematic and get the amp back to factory spec.
or as close as you can and then go from there.
The properly working Gemini II is a 30w amp.
If your friend needs much more than that he may have to mic it.

JDarby

[sluckey]

A Gemini II (G-15) is a fixed biased amp.
You posted a Gemini I (G-12) schematic which is cathode biased.

He has a G12, but was wondering if it would increase the clean headroom if he modified it to use fixed bias like the G15.

I don't know if that will do it, but I owned a G15 Gemini II in the '60s. It had plenty of clean headroom for a 30watt amp. 'course it had a 15" Jensen Concert Series speaker that probably contributed to that clean sound as well. I've never heard a G12.

[john_t]

I have a Ampeg g-15 believe it or not was a curb fine. Needed a set of power tubes a speaker recone and a new grounded plug. I love the amp stock. Great reverb. I would not even think about tweaking mine.

john

[eleventeen]

I know that Ampegs were kinda ugly looking, but I swear, I have heard more "curb find" Ampeg stories than any other make! Seriously, at least three, not counting one a pal of mine rescued from imminent toss-out just because he was at the right place at the right time.

[bluesbear]

Curb finds can be sweet! I have a '62 Gibson EB-3 my son found in a garbage can. He was driving by and saw a Gibson head sticking out! Other than a case, all that was missing was a pickup cover and 2 screws. It's not everyone's favorite bass, but if you have short fat fingers, it's way easier to play than a Fender bass.... and people are paying stupid money for a '62 "Jack Bruce" bass.
Dave

[JDarby]

My G-15 was a "set out for trash collection" find.
After going threw it and a speaker replacement it has turned
out to be a killer amp with a big sound and a lot of MoJo.
Put a 15" AlNiCo in it..pure heaven to my ears.

[eleventeen]

My all time best find was a 1960 Fender Precision bass neck at the Alameda Flea market for $1.00. Probably circa 1972. Maybe that disqualifies it since I did pay actual money for it instead of finding it for free. It had a little crack off one of the mounting screws for one of the tuners, otherwise perfectly functional. Later I turned it into a homemade bass of no great quality. That neck was W-I-D-E, geez, it felt like it was 3" wide at the nut. I can remember carrying it around the rest of the day not being able to believe I bought such a cool neck for a dollah.

Wow, Bluesbear, that's an unusual find! Gibson headstock sticking out of a garbage can? Wow!