Ampeg Gemini OT primary impedance

[Snapcase]

When this amp was designed 3 prong plugs were not used. Safety regulations have changed since then and it's a clever idea replacing these old two connector cords altogether for brand new three connector power cords and rearranging the power grounding in any old amp. For a good reason that cap is commonly called the "death cap". Better avoid this old and outdated "hum removal" arrangement.

About supply voltage, I cannot remember it and I'm not at the computer where I have all the schems and amps data. I would check it later. Sorry... But if I remember well, there are some Ampeg voltage charts somewhere in the web. Do some searches.

As Tom said, did you blew the PT by directly plugin it to the 230VAC supply in Spain? If so, Check all your power rail filter caps too. They might have blew or become damaged due to excess of heat and voltage before the PT finally died. Burning the PT takes some time. Plenty of time to send all your power supply caps south. Specially aged ones. If the caps were still alive and fine before (I really doubt it in an amp that old) this stress surely drastically reduced their life if any of them survived. If this is the case I wouldn't waste time checking old overshot caps. Just replace them all for fresh ones. Something that should be done in any old amp like this anyway.

PS, By the way. Check all the power rail resistors too, even the plate resistors in every stage. I've repaired a few amps burned like this. If the PT takes a long time to die it's not strange finding some resistors fried or out of specs due to overheating.

[torrevieja]

QUOTE=Tom Phillips;230523]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." [/QUOTE]

Ok, so that makes sense ... I guess this "death cap" is the same in Fender Blackfaces hum switch. I'm a beginner in electronics so I still have fundamental lacks.

About supply voltage, I cannot remember it and I'm not at the computer where I have all the schems and amps data. I would check it later. Sorry... But if I remember well, there are some Ampeg voltage charts somewhere in the web. Do some searches.

As Tom said, did you blew the PT by directly plugin it to the 230VAC supply in Spain? If so, Check all your power rail filter caps too. They might have blew or become damaged due to excess of heat and voltage before the PT finally died. Burning the PT takes some time. Plenty of time to send all your power supply caps south. Specially aged ones. If the caps were still alive and fine before (I really doubt it in an amp that old) this stress surely drastically reduced their life if any of them survived. If this is the case I wouldn't waste time checking old overshot caps. Just replace them all for fresh ones. Something that should be done in any old amp like this anyway.

Thanks very much, Snapcase. I haven't found any Ampeg voltage charts, but I guess that this schem could be enough
http://www.el34world.com/charts/Sche.../Ampeg_G12.pdf

The amp is already revised. Caps, tubes and out of value resistors have been changed. Even power transformer was replaced by one with European voltages from Fliptops. The problem is that this pt is designed to work with 240AC (British voltage) whereas in Spain we have 230. This cause drop voltages and decrease headroom. My first idea was using an step down but voltages where very low. I don't know the scientific reason, but it has to do with cyclage. The step up and down doesn't transform 50Hz to 60Hz and that cause a voltage drop.

A couple of days ago I was checking the amp because there was some hum (the cause was a burned resistor) but (please don't lough) I checked with a metal screwdriver. I caused a short in the rectifier bridge, there was a spark, blown fuse, pt bled wax and no is in short. Now I've found a guy who rewire transformers (there are very few now) and that's why I want to know the specs of this PT.

So, are these right?
Two red wires: B+, 280 VAC each (or 140VAC each?)
Two green wires: heaters, 6,3 VAC each (or 3,15 VAC each?)

Thanks very much again and regards
PS: sorry for my English

[Enzo]

Hi

Transformer windings create an AC voltage. The wires have a voltage then with respect to other wires. They do not have a voltage all their own. In other words, consider a 9 volt battery. One terminal has a voltage of 9 volts, compared to the other terminal. But you cannot sit a battery on the table and say it has 9v on one end, or 4.5v on each end. Your volt meter has two probes, and it measures the boltage between them.

If you say the 280v winding has 140v each wire, it implies you were reading the voltage with one probe on ground. But the transformer is not connected to ground. If the transformer had a center tap, AND that center tap was grounded, then and only then would ther be 140v on each wire. 140 with respect to ground. As this one is used, ther is no center tap, there is only 280v AC between the two red wires.

Once the transformer is wired into the circuit, then ultimately it has some sort of ground connection, so you would get readings at each red wire, but the readings to ground would not tell a transformer maker what he needs to know.


RUnning a 240v transformer on 230v won;t cause much trouble. 230v is only 4% low from 240v. That means your 280v would drop only to 268, and the B+ it creates would also only be 4% low. The 6.3v for heaters would drop to 6.05v. This might be enough to notice, but not enough to cause any problems.

[torrevieja]

Thanks, Enzo, very clarifying to me. As I've said, I'm a beginner in electronics and as you can see I have important lacks.

However, I've been said that voltages differences between primary and secondary are exponecial. It means that a 4% difference in AC primary may result in a bigger difference in secundary. It usually means less headroom, cold bias, looser bass and harsher highs, though you can live with it. Anyway, low powered amps such Ampeg Gemini I can't afford any headroom loose.

Anyway, I'll give the schem to the transformer maker and I hope he knows what to do. When I have the "new" pt installed I'll post if I notice any relevant headroom and volume increase (I hope it would be enough to rehears with the band...).
Thanks again!

[Tom Phillips]

...However, I've been said that voltages differences between primary and secondary are exponecial. It means that a 4% difference in AC primary may result in a bigger difference in secundary. It usually means less headroom, cold bias, looser bass and harsher highs, though you can live with it...

A 4% change on the primary still only translates to a 4% change at the secondary. The absolute voltage change on the secondary may be more but it is still only 4%. Note that the line voltage itself will change more than that from location to location and from hour to hour during the day.

Also you will note on old service information that the reference voltages listed sometimes have a tolerance of 20%. This makes your 4% change insignificant. I know it's hard to change onece you get the idea of rewinding in your head but I'd recommend using a stock off the self transformer. The 230V vs. 240V difference is no big deal. You will see on the schematic that you linked that the specified line voltage for the USA model was 117V. Today we see 120V or more. Over one day at my shop I have logged a range of 113minimum to 128V maximum.

If you still decide to go with the rewind then be aware that a good transformer designer knows how to size the wire and the number of turns to produce the proper voltage when the transformer is loaded down by the current draw of the circuit. The open circuit voltage (OCV) that you measure before the transformer is connected to the circuit will be higher. There are additional factors too such as how much sag is desired. The basic principles of a transformer are simple but there are lots of factors that affect the final performance of a given design.

By the way, if it's not already clear, no transformer will change 50Hz into 60Hz. As Enzo said it only transforms voltage.

Cheers,
Tom