Friday, October 9, 2015

Ampeg Gemini G15

  Here's a great 'bargain' amp, an Ampeg Gemini G15.

Ampeg amps are not my favorite to work on. They are a bit of a pain in the amp. A lot more screws to unscrew, and you need to navigate the layout which is awkward. I usually put it up on it's side and do some involuntary yoga moves to remove parts and replace them.

One side of the boards and the other:

Also if you consider buying one, the tubes are uncommon and often expensive. This one originally had 7591 tubes which were lovingly converted to 6L6 tubes. I think Main Drag did the work and they did a great job. Makes my life easier when a good tech is there before me!

These also have the very hard to find 7199 as the phase inverter/driver tube:

This one is good for now but it is starting to show its age. I burnished the pins which helped reduce noise but soon enough it's going to go. It does have a bit of hum but not enough to cause alarm in this psych rock machine.

The 7199 was developed by RCA in the 50's from what I understand. An old tech once told me: "The 7199 was never a great tube, it was made by RCA so RCA could build circuits around them so people would have to buy them!" You find them in the legendary Dynaco ST70. There are plenty of other friendlier pentode/triode tubes like the 6AN8 or the 6U8. I've converted a few before with very pleasing results. The good news is you can buy 7199 to 6U8 adapters for cheap, and 6U8 tubes can be had for a song!

  So this amp came in cause it was noisy. The volume pot on channel 2 was so scratchy you could no longer use it. I noticed both volume pots were changed so I looked for culprit #2: leaky coupling caps. Bingo! Replaced the 2 feeding both volume pots and their old 270k 1/2 watt resistors with new ones. Also noticed a burnt power supply resistor. It was working but charred really badly. a 470 ohm 2 watt resistor. I replaced that with s 5 watter. It'll never die again.

  Here's what a burnt resistor can look like:

  Last bit was to fix the reverb. Simple fix, the tank wires broke. I see this all the time. I'll keep splicing them until the tank really dies. I hate to replace these old tanks cause they have their own special sound to them. So I did my job for Mother Earth keeping one less object out of our landfills today!

  Now the good stuff. These amps sound incredible. Really fat and warm. They are the answer to the Fender Deluxe Reverb, about 25 watts into a single 12" speaker. Plus you get the futuristic name. Ampeg really embraced the 50's space race hipness. While Fender had names like "Princeton" and "Harvard" Ampeg had the Rocket, Reverb Rocket, Gemini....... I think Gibson had the Atlas but that's another story.

Update 9:33pm 10/9/15...

Wasn't convinced the 7199 was the hum culprit. I was right. It was a filter cap. It was a replaced filter cap, an 80@450V. I've seen techs use these before. Military sealed Spragues that are gold and New old stock. Trouble is, even if it is NOS it's still just an old electrolytic cap. Don't use them. Go for fresh stock. Amp is quiet as a dead fish running idle now!


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