I have a Apollo 500 and the transformer is bad it is a 1300v can I replace it with 2 transformers out of a D&A Phantom they are around 850v i believe. Here is one from Apollo.
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Apollo Amplifier transformer replacement.
- Thread starter Jimbo165
- Start date
It appears that transformer has been operated above its maximum altitude of 12,000 feet... I'm assuming this amplifier runs sweep tubes and 900 to 1,000 volts AC before rectification and filtering, seems very high for sweep tubes. As I recall the D&A transformers are 660 volts AC. They work great with a full wave bridge rectifier on sweep tubes.
Running 125 VAC on 170 VAC primary windings would drop B+ down to something closer to "Normal" :-0 (still 1000+VAC) ...I am thinking this is some surplus Xformer they grabbed for price and made it work. D&A did the same...never knew until the covers were off to really know what you had. (and sometimes not even then!) 
To answer your question: The change of a different transformer, will most likely require additional work, unless it is a direct replacement. (which the D&A's will not be) rectifier config/Cap configuration...possibly many more components and circuitry will need to be examined. IMHO
This transformer was probably not the only thing that burned up. Don't quite see how to get less than 1200 Volts from it judging by the markings.
D&A used the same transformer in all their sweep-tube base amplifiers. The HV secondary should be around 650 Volts depending on your local line voltage. Gets you just over 900 Volts DC from a bridge rectifier. D&A used three 350-Volt rated filter caps in series with those transformers. We use a pair of 500-Volt caps when replacing original HV filters.
If this amplifier really was running with 1200 Volts DC on the tubes you might take a closer look at the condition the tubes are in. Running sweep tubes that hot tends to shorten the service life.
Wasn't this model originally made with two fairly-small transformers with one on each rear corner? Those were famous for being too small and breaking down before the first five or ten years of regular use. Seems to me the problem with that model was to find a single transformer that wasn't too tall for the top cover to fit.
Best of luck.
73
D&A used the same transformer in all their sweep-tube base amplifiers. The HV secondary should be around 650 Volts depending on your local line voltage. Gets you just over 900 Volts DC from a bridge rectifier. D&A used three 350-Volt rated filter caps in series with those transformers. We use a pair of 500-Volt caps when replacing original HV filters.
If this amplifier really was running with 1200 Volts DC on the tubes you might take a closer look at the condition the tubes are in. Running sweep tubes that hot tends to shorten the service life.
Wasn't this model originally made with two fairly-small transformers with one on each rear corner? Those were famous for being too small and breaking down before the first five or ten years of regular use. Seems to me the problem with that model was to find a single transformer that wasn't too tall for the top cover to fit.
Best of luck.
73
I think it's a good idea to replace that original transformer with a pair of the D&A transformers. That will certainly give you headroom and I really don't see where there would be a requirement for much of any modifications. That original transformer is certainly not being run through a voltage doubler and the original capacitors would more than handle the voltage of the replacement transformers. That assumes the capacitors, rectifiers and tubes are still in good condition. Which would be an unusual assumption for their age.
The only other thing I can think of, is in order to get these tubes to run at the ridiculous voltage the original transformer ran at, I question if they're not applying a lot of negative bias in order to keep the tubes from melting down? If they are, switching to the lower voltage transformers is likely to push this amplifier down into class C operation. Getting it back into the linear mode, may require a slight bias modification.
The only other thing I can think of, is in order to get these tubes to run at the ridiculous voltage the original transformer ran at, I question if they're not applying a lot of negative bias in order to keep the tubes from melting down? If they are, switching to the lower voltage transformers is likely to push this amplifier down into class C operation. Getting it back into the linear mode, may require a slight bias modification.
The 170 volts is the maximum that the full primary can handle. The rated voltages are printed on the top. With a primary of 115 volts, that has an additional 4.3 volt winding, that could be added in series to make less than a 5% drop in the secondary voltage. I don't see how to get below 900 volts AC out of this transformer, when it was working. That works out to be a no load voltage of over 1270 volts DC. The most I've ever seen on sweep tubes being used in a linear amplifier.Running 125 VAC on 170 VAC primary windings would drop B+ down to something closer to "Normal" :-0 (still 1000+VAC) ...I am thinking this is some surplus Xformer they grabbed for price and made it work. D&A did the same...never knew until the covers were off to really know what you had. (and sometimes not even then!)
I was always told to be careful doing that, using a center tap transformer for a full wave rectifier in a full wave bridge and not using the center tap. I was told that sometimes the installation is not rated high enough for that across the transformer
That is true however, the transformer in question, from the D&A amplifier was designed to be used with a full wave Bridge rectifier. Since the original transformer doesn't have a center tap, this should be a good replacement, with plate voltage that's appropriate for a sweep tubes.
Older transformers would have a separate breakdown-voltage rating for the center tap. Blew up a few old military transformers that were meant to be used in a full-wave circuit with the center tap grounded, or close to ground voltage. I wired them to a full-wave bridge and the secondary winding would arc over to the frame after a while. Any transformer that tells you the max voltage to ground on the center tap, believe it.
The D&A transformers have no such limitation built into them. They were meant to be compatible with a full-wave bridge rectifier circuit.
73
The D&A transformers have no such limitation built into them. They were meant to be compatible with a full-wave bridge rectifier circuit.
73
From not so fun experience, I can tell you that the breakdown voltage on the D&A transformer, is three secondaries. Put four secondaries in series and a humid day will find the breakdown voltage.
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