1965 Ampeg Gemini G-12 Unboxing Fun - with IDIPA, cigar, and bald spot

Tron Pesto

Silver Supporting Member
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920
North New Jersey is definitely a hotbed of old Ampegs - I guess that makes sense. This is the SEVENTH Ampeg I've stumbled upon in the last year (and actually worked on a couple of others), and the third Gemini from 1965!

I picked it up today and thought it would be fun to make a "unboxing" video as every single one of these that I've opened have been different - let me reiterate that I'm talking about 3 different amps from the same year and not one was the same!

In addition to a curious trem bug, the hair is still standing up on the back of my neck thinking about the pre-amp tubes that I found in the amp.

The videos include a glorious IPA, a cigar, and dermatologist-worthy shots of my bald spot(s). Yeah, I'm no videographer, but hopefully these are somewhat entertaining and mebbe a little eductional.



But wait, there's more!

 
Messages
1,657
North New Jersey is definitely a hotbed of old Ampegs - I guess that makes sense. This is the SEVENTH Ampeg I've stumbled upon in the last year (and actually worked on a couple of others), and the third Gemini from 1965!

I picked it up today and thought it would be fun to make a "unboxing" video as every single one of these that I've opened have been different - let me reiterate that I'm talking about 3 different amps from the same year and not one was the same!

In addition to a curious trem bug, the hair is still standing up on the back of my neck thinking about the pre-amp tubes that I found in the amp.

The videos include a glorious IPA, a cigar, and dermatologist-worthy shots of my bald spot(s). Yeah, I'm no videographer, but hopefully these are somewhat entertaining and mebbe a little eductional.



But wait, there's more!



The Bugle Boy ECC83 tubes are original. They are not replacements. Amperex ECC83 tubes were standard issue from the mid thru late 1960's Ampegs, and maybe earlier.

For having scored 7 Ampegs this year, I would be surprised if you haven't seen others of this era without Amperexes.

I've owned 3 Ampegs from 1967 thru 1969. In each case, all ECC83 tubes were Amperex. The 1967 had Bugle Boy labels the later 60's ones had Orange Globe labels. The 6CG7, 7199 and 7591 tubes were all Ampeg-labeled Sylvanias.

I've mostly found Mullard ECC83's labeled with "Magnavox," when Magnavox maintained operations in the 1970's.

That said, I'd be very confident that the Telefunken ECC83 is a replacement. These are not particularly rare, but your over-the-top enthusiasm was entertaining. I find the smooth plates to be more desirable for home Hi-Fi than guitar amps.
 

Tron Pesto

Silver Supporting Member
Messages
920
The Bugle Boy ECC83 tubes are original. They are not replacements. Amperex ECC83 tubes were standard issue from the mid thru late 1960's Ampegs, and maybe earlier.

For having scored 7 Ampegs this year, I would be surprised if you haven't seen others of this era without Amperexes.

Thinking/looking back, I may have come across one or two Bugle Boys in the Geminis - but they were not as original, there were RCAs, Sylvanias and GE - my guess, a lot of replacements.

The other amps were an Echo Twin (all octal preamp tubes), an SB-12 from 1965 with Raytheons (replacements maybe), a 1979 VT-22 (different animal/vintage) and an SS-70 (soldistate).


I've owned 3 Ampegs from 1967 thru 1969. In each case, all ECC83 tubes were Amperex. The 1967 had Bugle Boy labels the later 60's ones had Orange Globe labels. The 6CG7, 7199 and 7591 tubes were all Ampeg-labeled Sylvanias.

I've mostly found Mullard ECC83's labeled with "Magnavox," when Magnavox maintained operations in the 1970's.

The VT-22 has several Magnavoxes - the 12DW7, 6FQ7 (both labeled made in Japan), 12AX7 hard to read the "made in tag" maybe USA, and 6K11 made in USA. The other 12AX7 is a GE and the 12AU7 has lost it's labeling (the 6189 is basically the only thing showing). So this one is a mis-mash.


That said, I'd be very confident that the Telefunken ECC83 is a replacement. These are not particularly rare, but your over-the-top enthusiasm was entertaining. I find the smooth plates to be more desirable for home Hi-Fi than guitar amps.

Yeah - I still get carried away - love this stuff. Love the look (and sound) of Telefunkens - love that "Made in Western Germany" label. It's just so cold-war! Just look at my avatar (it's the inside of a Mganatone M-8 I had - such a cool topology).
 
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1,657
The VT-22 has several Magnavoxes - the 12DW7, 6FQ7 (both labeled made in Japan), 12AX7 hard to read the "made in tag" maybe USA, and 6K11 made in USA. The other 12AX7 is a GE and the 12AU7 has lost it's labeling (the 6189 is basically the only thing showing). So this one is a mis-mash.

Yeah, I had a 1975 SVT, and the 12DW7 tubes had the Magnavox label. If I recall, I believe they were Japanese, as you had stated.

The 12AX7 tubes were GE, which I figured were replacements, as the 6550 tubes were also GE but with a 1986 date code, so I assumed a tech with GE products swapped out everything he had available (as they were prone to do) back in the 80's.
 

Tron Pesto

Silver Supporting Member
Messages
920
But more importantly, what stogie were you smoking?
LOL - I pretty much smoke a cigar everyday, so I mainly smoke JR branded cigars for the economy. That one was a JR Nicaraguan Torpedo. Decent enough. I'll step up my cigar game for occasions - in this case, maybe once I get the Gemini running.

Which by the way, I'm not firing it up until I put in a grounded chord. While I am confident enough after checking the amp I could fire it up with the original two-prong, I'm have no compelling reason not to wait until I have a nice safe set up in place.
 

Jeff Gehring

Gold Supporting Member
Messages
7,209
Should be a sweet amp once you've got the TLC taken care of! I'm not permitted to stogify in the amp bunker here, but I will be setting fire to an Undercrown Shade later on the deck. It's a nice day for once!
 

Tron Pesto

Silver Supporting Member
Messages
920
Should be a sweet amp once you've got the TLC taken care of! I'm not permitted to stogify in the amp bunker here, but I will be setting fire to an Undercrown Shade later on the deck. It's a nice day for once!

At risking of jinxing myself, this is going to be a real easy 100K maintenance and it will come out well. This particular G-12 has about the simplest board/wiring. Although it would be cheaper to go with discrete parts for the filter caps, I'm going to spend a little more and get the same-spec can and a dual F&T 33/33-450 to replace the dual 20/20. Easy swap-outs. And besides the three-prong (which updating in these amps is a breeze), I've got only a handful of 'lytics and at most three power resistors to replace. (The trem is always a crap shoot, maybe I'll be lucky. Everything checks out on the reverb, so I'm confident about that.)

Another reason I'm going with the cap can is because it will be simpler to stay wired within the parameters of the single point contact ground bus topology. I know you know about this design in a lot of Ampegs, but I've seen people not realize this and get themselves into deep water. I made a "PSA" vidoe about it - you actually get a cryptic shout-out in the video. (I didn't want to name names without permission.):

 

Tron Pesto

Silver Supporting Member
Messages
920
Thought I would up my video game - three cameras (Macbook, iPad, and iPhone), proper mic'ing, intending to actually try out editing, etc....

But apparently God does not want me to make amp videos...

 

Tron Pesto

Silver Supporting Member
Messages
920
Although I did make some videos (I have to put them together), I put in the tubes and fired the amp up. I got some very interesting results (at least to me) – one thing that came out of it was the potential rabbit hole one could fall into when setting up a cathode biased amp. (Spoiler alert – did my tube checker actually provide a valuable warning if I had been listening???? OMG! Imagine that!)

I put in the tubes and fired up the amp using a variac and light-bulb-limiter. All checked out well and I went to house power.

Filaments are running at 6.4V – awesome. That was probably my biggest concern. I was afraid they would be running too hot on house voltage.

I did a “down and dirty” bias check – the amp is cathode biased and it’s easy enough to see in general if things are ok. The plates were running around 375V or so – nominal is 360V, so all was looking good. The 143.8Ω cathode resistor was dropping 10.84V, giving a “shared” power of about 27.4 Watts. That put it just over 70% dissipation (19W max plate dis), which if you deduct a watt or so for the screen dissipation, that works well.

But I knew the tubes were old, and there was a few volt difference in the plates of each tube to make me wonder if one tube was drawing significantly more current compared to the other.

I took reading of the voltage difference between the center taps and the plates and one of the tubes had more than double the voltage drop compared to the other – uh oh. After shutting down the amp and getting the OT primary resistance on each side, I calculated that one tube was running at 8.47W and the other at 17.41W of plate dissipation*. No good.

I switched the tubes to see if maybe it was a circuit issue (leaky PI coupling cap perhaps) and the issue followed the tube.

I thought I had some 7591s in my stash, but alas no. Order has been placed. When I get the new tubes (JJ 7591S), I’ll report on how they are biasing. Even if no one is interested LOL- I need to keep my typing chops up.

Side note: even with the significant mismatch and wonky tube, the amp sounded pretty damned good!

Side Side note: Tremolo works!!! I was dreading a crapped-out trem nearly as much as the spectre of high filament voltage. Replacing tubes and adjusting bias is a walk in the park compared to dealing with high filament voltages or shot trems in Ampegs.

* Yeah, yeah, yeah, I know the pros and cons of tube testers, BUT, there's a video of me testing the tubes for this amp and lo and behold, the 7591s measured significantly different in transconductance from each other. One was pretty damned weak according to the tester. That result didn't (and wouldn't) stop me from trying the tubes, but the tester did provide some correlating info - or at least an inkling I should take a close look at what was going on.
 

Tron Pesto

Silver Supporting Member
Messages
920
Here's a video showing the bias checking. It's long and boring - particularly for most people here as it discusses things that you already know. But it does discuss why just checking the bias across a cathode resistor leave some things to be desired - if not potentially misleading you if you don't have a good understanding of what is actually being measured.

More importantly (ha), I try to make "fetch" happen. If you do nothing else, watch the 24:50 through 26:55 minute section to see what I mean - and maybe the last 50 seconds for a laugh.

Hold on - gotta edit something...
 
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