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Post new topic Output Impedance After 6L6 Conversion
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Author Topic:  Output Impedance After 6L6 Conversion
Randy Owens


From:
West Central Indiana, USA
Post Posted 20 Mar 2017 4:49 am     Reply with quote

I finally got the Ampeg Gemini II amp that I wrote about back in October. It's a 1965 model and according to the tube diagram, came with 7591 output tubes. Someone along the time crossed out the two 7591 notations on the chart and wrote in 6L6. The output transformer looks to be much newer than the power transformer. There is a sticker on the chassis that states that the amp was serviced in 1976 in Bloomington, IN. The single 15" speaker is an 8 ohm Jensen Concert Series which sounds very good. I tried one of my 4 ohm D130 equipped Walker cabs and the tone went from very good to excellent for a 52 year-old amp. The volume was much louder as you'd expect.

What would be the chances that this amp now has a 4 ohm output impedance and how can I tell? It probably doesn't even matter on a tube rig that but I like things to be as right as possible!

Thanks,
Randy
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Bill Sinclair


From:
Hagerstown, Maryland, USA
Post Posted 20 Mar 2017 6:14 am     Reply with quote

I'm not a tube amp specialist but like tinkering with them. I believe that you could take it to an amp tech and have them them crank it to clipping (on a scope) into different load resistors. (4, 8, 16) The one that produces the most power (not just voltage) before clipping is the most efficient and "correct". P=V(squared) divided by R. There are some real amp techs and builders on the forum who can correct me if I'm wrong.
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Bill A. Moore


From:
Silver City, New Mexico, USA
Post Posted 20 Mar 2017 8:09 am     Reply with quote

Randy, can you get numbers from the OT?
The 6L6 and 7591 tubes both use a 6.6K ohm primary transformer impedance in AB configuration. I believe changing tubes shouldn't make any difference to the output impedance.
I suspect the difference you are hearing is the greater efficiency of the JBL. I put a D 120 in a 5E3 clone, and it is LOUD!
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Randy Owens


From:
West Central Indiana, USA
Post Posted 20 Mar 2017 9:41 am     Reply with quote

I will attempt to get that chassis out again. It can be a real bugger to remove. The angled front panel provides a springy, friction fit with the heat shield inside the top of the cabinet and it really is held in there well! I had it out just last week and I did see a number on the newer output xformer but it was only 4 or 5 digits. The chassis itself is pretty cool to see. It's either chrome or plated with some other substance but it's quite shiny both inside and out.
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William Rasch


From:
Vermont, USA
Post Posted 20 Mar 2017 1:17 pm     Reply with quote

Is this Gemini the black covered model or blue check? I've seen a few black models that came stock with 6l6's and seen fenders and Ampegs with crossed off numbers on the tube chart. I've seen Gemini's with 7591a, 7868 and 6l6 power tubes. All were stock. As someone else stated try to get the numbers off the output tranny it could help with originality and the ohm rating
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Randy Owens


From:
West Central Indiana, USA
Post Posted 20 Mar 2017 1:26 pm     Reply with quote

Here she is: The cabinet has the blue checkered tolex.

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Randy Owens


From:
West Central Indiana, USA
Post Posted 20 Mar 2017 1:31 pm     Reply with quote


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David Gertschen


From:
Phoenix, Arizona
Post Posted 20 Mar 2017 2:03 pm     Reply with quote

Randy, instead of pulling the amp apart, you might just try this: (Power off, of course) Plug a guitar cable into the jack where the speaker goes. Measure the other end of the cable with a meter set to ohms.

Example: my pro reverb reads 3.1 ohms, so you can easily determine it wants a 4 ohm speaker load. My deluxe reverb measures 6.8 ohms, it requires an 8 ohm speaker load.

Might not be exact science, but it has worked for me on every amp I've worked on...
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Randy Owens


From:
West Central Indiana, USA
Post Posted 20 Mar 2017 2:15 pm     Reply with quote

David Gertschen wrote:
Randy, instead of pulling the amp apart, you might just try this: (Power off, of course) Plug a guitar cable into the jack where the speaker goes. Measure the other end of the cable with a meter set to ohms.

Example: my pro reverb reads 3.1 ohms, so you can easily determine it wants a 4 ohm speaker load. My deluxe reverb measures 6.8 ohms, it requires an 8 ohm speaker load.

Might not be exact science, but it has worked for me on every amp I've worked on...


I'll try that, David and I'll get another data point for your survey. The Jensen speaker in there now is clearly marked 8 ohms so it should be interesting to see what I find.

***UPDATE*** It reads 1.6 ohms Whoa!
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James Phillips


From:
Michigan, USA
Post Posted 20 Mar 2017 5:00 pm     Reply with quote

The info I found on the OT-213 is :6.5k, 8/16 ohm taps.
8 ohms for the speakers , 16 for a little negative feedback. No 4 ohm tap, so I would not run a 4 ohm load on it for too long, as your amp will work harder, and run hotter.
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