Ampeg Gemini OT primary impedance

[Snapcase]

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]

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]

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]

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

It lists 6.2k and 8/16 ohms.

[Snapcase]

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]

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]

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]

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

[torrevieja]

PT voltages data

Hello, I take the oportunity to ask about the power transformer specs of the Ampeg Gemini I. I've burnt the PT and now I'll send it to rewire.
http://www.el34world.com/charts/Sche.../Ampeg_G12.pdf

According to this schematic, I assume that black chord and red/black take current from the AC chord. (red/black chord take it through the fuse).
Red wires must give 280 VAC each one.
Green (some times green and green/red) wires go to heaters and each one must give 6,3 VAC.
There's a last yellow wire that is connected to a black wire of the OT througn a 0.1 MFD non electrolytic cap. I'd like to know how many volts would it give, as I can't see it in the schem.

Here's a picture that show the chord I'm referring to


Another question: I can't see where the ground of the amp is connected. Could it go to the on/off switch?

I would appreciate so much this data.

[Tom Phillips]

...Another question: I can't see where the ground of the amp is connected. Could it go to the on/off switch?...

By "ground of the amp" I assume that you mean the main power supply ground. In that case see the "Start Here" note that I added to you photo. That is pointing to a ground potential point in the main high voltage power supply. It is an isolated lug of the filter cap can and is not directly connected to the chassis at that point. You can start there and trace the black wires as they daisy chain around the circuit. I believe that the connection to the cahssis is at one of the input jacks in the Gemini I. It does not go to the ON/OFF switch. The ground layout is designed to minimize ground loops and hum. It's not perfect in most old amps but was good enough.

Regards,
Tom

[torrevieja]

Thanks for the reply.
You're right. The connection to the chasis is at the accordion input jack of the first channel. However the 3 pong chord is grounded to the chasis. Could it couse any ground loop? Would it be better to connect both at the same point?
Thanks again

[Tom Phillips]

The line input earth ground should be connected to the chassis near where it enters through the strain relief. It should be soldered to the chassis or connected via its own dedicated locking bolt / electrical ring. Good practice is to make the ground wire longer than the others such that if the cord is ripped out then the ground will be the last wire to break off.

For best noise performance it is best to have the power line earth widely separated from the power supply DC ground at the input jack. So...NO, don't put them the two grounds in the same location.

Regards,
Tom

[torrevieja]

OK, thanks again Tom.

Now I'm waiting for more news abot pt voltages.
Regards

[torrevieja]

Please excuse me if it's a silly question. Could it be the ground of the power transformer? This chord is connected at the same point of the black wire of the output transformer and this point is connected to ground. This would match with the schematic, but now I don't understand why is this cap between the chord of the PT and the ground.

[Tom Phillips]

...This chord is connected at the same point of the black wire of the output transformer and this point is connected to ground. This would match with the schematic, but now I don't understand why is this cap between the chord of the PT and the ground.

That makes sense. The common side of the OT secondary is grounded and so is the earth ground. This is another case of "good enough." Very few old designs have perfect grounding designs. The cap you mentioned was in the original design to hold one side of the line input at AC ground. In the original design it would minimize hum & buzz. When you upgrade to a grounded power cord it is no longer needed and can be removed from the circuit. It is the part commonly referred to as the "death cap."

Since you are in Spain you have a 230V / 50Hz mains service. Correct? If so is your Gemini I an export model? The Schematic you linked shows the USA version which was originally designed to run from of 117VAC. If you are running from 230VAC then you need a different power transformer or a step down line conversion transformer or you will burn up the PT. Is that perhaps what happened to your PT? For 230VAC mains you would also use a DPST power switch to switch both hot leads of the main service.