Ampeg G-12 Gemini I on the bench - Never been cracked - Trem issue.

[LooseChange]

Just thought I'd share this with all of you.
It's an Ampeg Gemini I from 1966. The chassis has neever been out of the cab.
It has all the issues you would expect from a little played amp that's 44 yeaars old.

[john_t]

Loosechange, I have a 1966 ampeg g-2 (curb find) other than a new power cord, light for the front, speaker recone, and a couple of power tubes. I have yet to change a thing. I keep meaning to recap the amp but it sounds great as is. really nice reverb. The down side, it weighs a ton.

Enjoy.

[LooseChange]

It has a problem with the tremolo... All caps and resistors around the tremolo circuit have been replaced. Still no tremolo.  I think it just may be the "special" tremolo bug Ampeg made back then.
I see a good voltage swing at the plate but hitting the trem bug it's knocked down to around 50v.
Anyone have an idea as to what I should be reading at the bug?
Biggest problem is replacing it... Not available.. Any thoughts?

Link to the schematic:
http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_G12.pdf


Thanks!

[sluckey]

Fliptops has it.

[LooseChange]

Thanks Sluckey... I just want to be sure that the part is bad before I spend almost $40.00 to replace. What is so special about this thing?

[chocopower]

Hi there!

I,m working in the same amp for a friend.

He loves the tone, but sometimes he need a little more volumen. He upgrade the speaker with a Weber Blue Dog (ceramic) and it helps a lot, but not enought.

Looking at the schematics is very similar to the Ampeg G-15. The main diference is that the g-15 is fixed bias.

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_GS15.pdf

My question:

Is posible convert the G-12 to fixed bias without damage the mains or power transformers?

thank in advance and sorry for the hijacked tread!

[sluckey]

Is posible convert the G-12 to fixed bias without damage the mains or power transformers?

Yes, but since the G12 uses a bridge rectifier, you cannot copy the G15 bias circuit. Instead, copy the G20 bias circuit.

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_G20.pdf

There are big differences between the G12 and G15 that are not apparent on the schematic. Things like bigger iron, higher B+, and bigger speaker. I don't think converting to fixed bias will give your friend the extra volume he wants.

[LooseChange]

Anyone know what is so different about the tremolo bug in an Ampeg?

[sluckey]

From an electronic standpoint, I don't think there is anything special about it. It looks like an LDR and a NE-2 neon bulb on the schematic, just like an old Fender roach. Can you open yours up? If it's just encased in a heat shrink tube it will be very easy to confirm if the bulb is indeed a ne-2. If it it a ne-2, just use a Fender roach.

I think the reason Fliptops wants so much for their replacement is because they are the only people offering a replacement. Using a metal hex standoff to house their bug is just a gimmick.

You've already replaced everything else in the circuit and verified that the oscillator is working. The ne-2 bulb (if that's what it is) fires at about 90v. Your meter is probably averaging that voltage waveform down to your measured 50 volts. A scope would show what's actually happening.

[VMS]

Anyone have an idea as to what I should be reading at the bug?

Luckily the tremolo module chapter is visible in the preview of the Ampeg book:

http://books.google.com/books?id=O36Ro9MSkccC&lpg=PA1925&dq=ampeg&pg=RA1-PA282#v=onepage&q&f=false


I think this place might also have those modules:

http://vintage-amp.com/ampegpts.htm

[chocopower]

Is posible convert the G-12 to fixed bias without damage the mains or power transformers?

Yes, but since the G12 uses a bridge rectifier, you cannot copy the G15 bias circuit. Instead, copy the G20 bias circuit.

http://www.el34world.com/charts/Schematics/files/ampeg/Ampeg_G20.pdf

There are big differences between the G12 and G15 that are not apparent on the schematic. Things like bigger iron, higher B+, and bigger speaker. I don't think converting to fixed bias will give your friend the extra volume he wants.

thanks for you repply.

Why the bridge rectifier in the G12 make not usable the G15 way?

just learning... thanks

ahhh! one more question. In my friend´s amp the reverb is working even with the reverb mix control al cero. What could be the problem?

[sluckey]

Why the bridge rectifier in the G12 make not usable the G15 way?

I don't have an easy answer and I'm not sure I totally understand myself. I do know for certain that it won't work though. You must use a cap to pick off the voltage from the PT. Wire it exactly like the G20 and it will work. BTW, there are some big Marshall amps that use the exact circuit, same components as the G20.

In my friend´s amp the reverb is working even with the reverb mix control al cero. What could be the problem?

Bad reverb pot or the pot has a poor or broken ground connection.

[LooseChange]

VMS, Thanks for that link to the Ampeg book.  I tested the module. The light side is good but the resistor side is reading way low around 30k. I guess that's the problem.

[Justa]

VMS, Thanks for that link to the Ampeg book.  I tested the module. The light side is good but the resistor side is reading way low around 30k. I guess that's the problem.

I just checked the data on my Reverberocket rebuild that had a barely functioning trem unit in which the LDR measured 25K.  I got a used Ampeg trem unit that worked great that measure 120K as a reference.  Apparently the LDR's shift in value with age.

[LooseChange]

Justa, Where did you get the used LDR??

[Justa]

I lucked out.  Member Jbrew73 here was kind enough to help me out.   He had an extra from an amp he stripped.

[terminalgs]

I used a fender LDR/bulb in my Ampeg G12.,  if you have one handy, you can test it with that  and then later get the fliptop version.

[PRR]

> resistor side is reading way low around 30k.

LDR is light sensitive. 30K is maybe correct for room light. You MUST contrive to get it DEAD dark to measure the higher resistances.

In the Ampeg plan, you need above 1Meg in DEAD darkness and below 100K in light.

These high resistances (higher than Fender zone) mean that microscopic pinholes in the casing may leak enough light that volume is not wobbled.

[PRR]

>> Why the bridge rectifier in the G12 make not usable the G15 way?
> I don't have an easy answer

The Bridge Rectifier, used that way, NEVER swings negative.

With the 2-diode CT plan, each end swings to +400V and to -400V. You can tap that negative swing, wastefully, and get -40V.

With the 4-diode FWB, any time any end starts to go negative more than 0.6V, a diode grabs it and holds it. This levers the other end up to +400V, our main goal. This does not leave a simple way to get even a little negative voltage.

> You must use a cap to pick off the voltage from the PT. Wire it exactly like the G20 and it will work.

Right. The cap-input plan has limits, and needs a GOOD cap, and the math is a brain-buster, but works fine for bias-supply.

[LooseChange]

UPDATE: Got the TM-1, tremolo module in from Fliptops. Installed and all seems to be good.
The Gemini I is not a bad amp.  It has that classic Ampeg reverb sound too.

[plexi50]

They are specail amps. My Gemini II is labled ECHO for the reverb. Just looks cool labled that way. Nice tone for sure