Ampeg Gemini 1 G12

[terryrocks]

I have half of the original schematic for my 1965 Gemini 1.
The layout of the schem matches some i've found online, but the voltages are sometimes way different. What to do?

[img:800:598]https://farm8.staticflickr.com/7406/15810405503_17734f75c4_c.jpg[/img]image

[xtian]

What do to about what? Are you trying to solve a problem?

[terryrocks]

I want to check values and restore to spec.

Replacing electrolytics later today or tomorrow. I already havethecaps and know where they go, but i also want to do voltage measurements on all tubes.

[Leo_Gnardo]

I have half of the original schematic for my 1965 Gemini 1.
The layout of the schem matches some i've found online, but the voltages are sometimes way different. What to do?

Great amps those ol' Ampegs. Since the time of their release, our power companies have been inching up the line voltage. 110-117 VAC was standard back then, now it tends to be 120-125V and sometimes more. That can account for a 10% jump in voltages you're checking.

Also failing filter caps can draw current and drop the hi voltage readings.

What readings are alarming you, then we can concentrate on what's the problem, if any.

[xtian]

IMO, you shouldn't worry about matching 60 year old voltage readings--yours will be higher. Just confirm that the power tube's plates are within reason, and that they're biased properly. Preamp tubes have a WIDE range of functional voltages, so your concern should just be, does it sound good?

It's usually smart to replace the dropping resistors in the power string in these old amps, along with the filter caps.

I don't want to make any assumptions about your level of knowledge, so let me know if you have questions.

[terryrocks]

Thanks guys. I just bought this amp a couple weeks ago. I've built some mic preamps, lots of cables, couple of paint by number stomp boxes.
This is my first time working with high voltage, but I've spent the last 5-6 years reading about it quite a bit and I'm now confident enough to dig in.

The amp works, but doesn't sound great to me. And the verb isn't working.
The guy that sold it to me said the verb worked last time he used it, so I'm hoping i can sort it out myself.

My plans are to recap the electrolytics, replace the power cord with grounded one, clean all pots/knobs and cabinet, and get the verb back online.

[Stevem]

The problem with the verb may just be that the pan on one end has gone open due to one of the small wires at each rca Jack busting off as they due a lot!
If you do a ohm check you should read about 178 ohms on the input side of the pan and 150 on the output end.
If you find one end is open and the wires to each rca Jack are hooked up than the pan is toast!
If your luck and find just a busted wire solder it back up and place a nice dab of silicone glue or hot glue on each wire so that the spring tray bounching around does not snap them off again!
I would also plan on replacing the output tubes cathode resistor with a 15 watt one and try to move it away from other circuit parts that it can heat up.
Many of these Ampeg amps use the same small OT and they many times go open or short on the primary side so keep that in mind if the amp still sounds crappy through another speaker.
If you need a new OT filptops has them!

[Leo_Gnardo]

My plans are to recap the electrolytics, replace the power cord with grounded one, clean all pots/knobs and cabinet, and get the verb back online.

Good to have a plan. Also good reverb advice from stevem. Here's some more. Sometimes the failure is at the drive end. If you can shake the amp and hear reverb bashing noises thru the speaker, then the problem's at the drive end.
A: transducer open or broken wire to it, inside the tank itself.
B. corroded connector to tank
C. footswitch stuck in mute position
D: bad capacitor or resistor in drive circuit - they use big ones - 0.47 uF film cap and 10K 10W resistor - either one can quit. Occasionally the drive tube goes bad too.

[terryrocks]

Ok, so I pulled the tank, measured resistance at each end and both match Stevem's numbers.

The output works as I can hear it through the speaker.

Then I opened up the footswitch to check it out as the previous owner told me he'd had a problem with the termelo circuit until he replaced the switch. As it turns out, both switches were replaced at the same time so I would assume they work. It is still soldered to the amp so I'm not certain how I can test the switches and cable up to where it's connected.

Next, I checked the plate voltage of the reverb driver tube and it is exactly 300v. Also heard a pop thru speaker when probed.

So, it's likely one of the transducers, or the cap/resistor on the input side?

How can I test these components without removing them from the circuit?

Thanks again everyone. This place is awesome!

[pdf64]

Regarding the increased line voltage thing, what voltage are the heaters running at, eg 6.3V?

[terryrocks]

Ok, so further exploration of the 6cg7 driver tube yields some interesting results.

http://www.drtube.com/datasheets/6cg7-ge1956.pdf

Pin 6 plate - acccording to schem should be 205v. I'm reading 290v

Pin 8 cathode - should be 4.5v. I just read 10v.

Pin 1 plate - should be 100v and is measuring exactly that.

Pin 3 cathode should be 3.5v and is measuring right on the money.

Does that mean it's likely a component between tube a reveb input?

[Stevem]

If you tap hard on the pan with the reverb up can you hear the pan picking up the taping?
If so unplug the input side of the pan and with a AC meter hooked across it while you play do you read any signal voltage output from the driver?
Also that foot switch shorts the pans input signal to ground so I would for now lift the ground point for that switch action !

[terryrocks]

Just measured the heater voltage on the 6cg7. Both heater pins read 2.6vac.

Tried measuring voltage on input cable with signal. Should I see a varying voltage?

[sluckey]

Measure AC filament voltage directly across tube pins 4 and 5, not from one pin to chassis.

[terryrocks]

So one probe tip on each pin?