Ampeg Gemini OT primary impedance

Snapcase · 06-21-2007, 11:49 AM

Hi all, I'm wondering what's the primary impedance of Ampeg Gemini I & II trannies. (OT-151 & OT-225) They both use 7591A tubes @ 365 & 430 volts respectively giving 20 and 30 watts.

Looking at the tube 7591A data, in most operation conditions they need 6K6 aa load resistance in AB1 class. But with these voltages and biasings (15V Cathode biased & -22V fixed) power should be higher. Am I missing anything? Are trannies impedances higher than 6K6 in these amps?

I have a bunch of matched NOS Sylvania 6L6-GA tube pairs. I'm wondering how good would they work on these amps. That voltages and a 6K6 OT would be perfect for them (of course rewiring the sockets). Is this a good idea?

Thanks in advance.

Alex/Tubewonder · 06-21-2007, 02:48 PM

My calculations say both these Ampeg OTs are 8k Raa. Based on 7591 data, B+ 360 re 430 and given rated output power of the amps 20/30W. The 30W OT should be marginally bigger than 20W.

You can rewire for 6L6GA without problems, in the 20W amp expect slightly more output power around 25W because 8k Raa is close to optimum 9k for this tube at 360V, provided B+ doesn't sag more than to around 350V.

The 30W amp with 430V B+: a 6L6GA might not take it too well.

Snapcase · 06-21-2007, 03:58 PM

Thanks a lot for your reply. I'll try the 6L6GAs. The JJ 7591s sound good setting the amp around halfway volume, but pushing it further they compress too baddly and doesn't give enough volume for gigging. 25 real watts and more headroom look a sweet improvement for the Gemini I.

Enzo · 06-22-2007, 12:41 AM

My factory service info calls for the OT215 in the Gemini 2 (G-15).

It lists 6.2k and 8/16 ohms.

Snapcase · 06-22-2007, 07:13 PM

Quote:

Originally Posted by Enzo

My factory service info calls for the OT215 in the Gemini 2 (G-15).

It lists 6.2k and 8/16 ohms.

Yes, this is the OT for the 6L6 version of the Gemini II. The earlier G-15 version used 7591A tubes, an OT225 tranny and lower plate voltage.

7591 G-15

http://www.kbapps.com/audio/schemati...ampeg/g15.html

6L6 G-15

http://www.kbapps.com/audio/schemati...mpeg/g15b.html

Anyway, it's quite interesting knowing the OT215 data. Thanks a lot, Enzo.

Enzo · 06-23-2007, 02:36 AM

Actually that tranny is listed with both 6L6 and 7591s.

I have found the OT151, and it shows as 6.6k and 8 ohms.

I am running through the book colecting this data and will post in a couple hours what I find.

Enzo · 06-23-2007, 06:36 AM

I found these, impedances and the tubes they were used with in the examples found:

OT-149 10k, 8 ohms 7591
OT-150 10k, 8 7591
OT-151 6.6k, 8 7591
OT-208 6.5k, 8/16 6L6
OT-213 6.5k, 8/16 6L6
OT-215 6.5k, 8/16 6L6/7591

and for comparison:

8950004 1850 ohms, 4/2 ohms 6146B/6550 (SVT)

Snapcase · 06-23-2007, 09:02 AM

Hey, this info is great, Enzo. Big thanks!

06-21-2007, 11:49 AM	#1
Snapcase Member Join Date: Jun 2007 Posts: 25	Ampeg Gemini OT primary impedance Hi all, I'm wondering what's the primary impedance of Ampeg Gemini I & II trannies. (OT-151 & OT-225) They both use 7591A tubes @ 365 & 430 volts respectively giving 20 and 30 watts. Looking at the tube 7591A data, in most operation conditions they need 6K6 aa load resistance in AB1 class. But with these voltages and biasings (15V Cathode biased & -22V fixed) power should be higher. Am I missing anything? Are trannies impedances higher than 6K6 in these amps? I have a bunch of matched NOS Sylvania 6L6-GA tube pairs. I'm wondering how good would they work on these amps. That voltages and a 6K6 OT would be perfect for them (of course rewiring the sockets). Is this a good idea? Thanks in advance.

06-21-2007, 02:48 PM	#2
Alex/Tubewonder Senior Member Join Date: Jul 2006 Location: Goteborg, Sweden Posts: 144	My calculations say both these Ampeg OTs are 8k Raa. Based on 7591 data, B+ 360 re 430 and given rated output power of the amps 20/30W. The 30W OT should be marginally bigger than 20W. You can rewire for 6L6GA without problems, in the 20W amp expect slightly more output power around 25W because 8k Raa is close to optimum 9k for this tube at 360V, provided B+ doesn't sag more than to around 350V. The 30W amp with 430V B+: a 6L6GA might not take it too well. __________________ Aleksander Niemand Tubewonder amps Life's a party but you get invited only once

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06-21-2007, 03:58 PM	#3
Snapcase Member Join Date: Jun 2007 Posts: 25	Thanks a lot for your reply. I'll try the 6L6GAs. The JJ 7591s sound good setting the amp around halfway volume, but pushing it further they compress too baddly and doesn't give enough volume for gigging. 25 real watts and more headroom look a sweet improvement for the Gemini I.

06-22-2007, 12:41 AM	#4
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,303	My factory service info calls for the OT215 in the Gemini 2 (G-15). It lists 6.2k and 8/16 ohms.

06-23-2007, 02:36 AM	#6
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,303	Actually that tranny is listed with both 6L6 and 7591s. I have found the OT151, and it shows as 6.6k and 8 ohms. I am running through the book colecting this data and will post in a couple hours what I find.

06-23-2007, 06:36 AM	#7
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 10,303	I found these, impedances and the tubes they were used with in the examples found: OT-149 10k, 8 ohms 7591 OT-150 10k, 8 7591 OT-151 6.6k, 8 7591 OT-208 6.5k, 8/16 6L6 OT-213 6.5k, 8/16 6L6 OT-215 6.5k, 8/16 6L6/7591 and for comparison: 8950004 1850 ohms, 4/2 ohms 6146B/6550 (SVT)

06-23-2007, 09:02 AM	#8
Snapcase Member Join Date: Jun 2007 Posts: 25	Hey, this info is great, Enzo. Big thanks!

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