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Norris Wilson 21st February 2007 12:43 AM

Hammond 270 series power transformer?
Could anyone that has had experience with Hammond 270 series power transformers tell me if they are accurately rated?

If their Classic 272JX transformer has a secondary rated at 300V, with a 115V primary input. Is the rated 300V output what I should expect to get within a few Volts plus or minus?

Or, is there a variance between manufactured batches and could possibly have a high tolerance level?


Norris 21st February 2007 01:40 AM

I have no experience with the 272JX, but every other Hammond (and the Hammond sourced Allieds) that I have used ran on the high side, often 10 to 15% high. My line voltage is typically 122 to 124 volts which only explains 5 to 8% of it. The voltage will vary somewhat with load, but I still measure 10% high with a transformer operating at, or slightly above its rated capacity.

I have used at least 10 each of the Allied 5K56VG and 6K7VG. All of them have been consistently high.

sorenj07 21st February 2007 02:12 AM

just one thing that I noticed only today was the current ratings for the transformers on Angela and, I assume, most other places are for CAPACITOR INPUT. Which means, for choke input, you can multiply the current by (95/65). A 200mA transformer becomes a 290mA transformer, etc. The current bonus is great, and the reduced stress on a vacuum rectifier is great. I'm having problems figuring out why people would ever NOT use choke input, it's barely more expensive to get a bigger choke if you already were planning a pi filter :smash:

Norris Wilson 21st February 2007 05:50 AM

Thanks Tube lab and sorenj07 for your input.

It looks as if I will have to search for another affordable alternative power transformer.

Does anyone have any recomendations?

I plan on using parallel 5V3's with a choke loaded pi filter.

Norris 21st February 2007 02:22 PM

I wouldn't abandon the Hammonds just because they run high. Take this in to account when designing your circuit.

I have been using the Allied version because they are cheap. I routinely run them at or slightly above their ratings. The only failure that I have had so far was a transformer that got wet during a hurricane, and it lived for about 3 months after the storm. Autopsy revealed that the transformer was all rusty inside, and it still worked for 3 months. The amp has played fine for 2 years since I replaced the transformer.

EC8010 21st February 2007 02:35 PM


Originally posted by sorenj07
Which means, for choke input, you can multiply the current by (95/65). A 200mA transformer becomes a 290mA transformer, etc.
No. Transformer manufacturers base their ratings on a resistive load. They certainly don't take account of what you or I might do with nasty capacitor input supplies. For choke input, provided that you have used a big enough choke, you may take the required transformer current rating to be very nearly the same as your DC current requirement. 21st February 2007 04:46 PM

Hammond does seem to rate their transformers in DC output current with a cap input filter. From their "Classic Tube" power transformer specifications page on their web site:


H.V. DC current below, measured with cap. input filter, full wave (two diode) C.T., rectifier circuit.
Many other transformer vendors, and non "tube" transformers from Hammond are rated in RMS AC current into a resistive load.

I would assume that Hammonds figures are for a typical tube rectifier with a normal sized input cap. My experience with Hammond transformers is consistent with this observation. SS diodes follower by a monster sized cap will require some derating (and the transformer may produce a buzzing sound).

A choke input filter puts out less voltage for a given transformer so you can draw somewhat more current than a cap input filter for a given amount of transformer heating. Keep the total DC output power under the original intended specs.


I'm having problems figuring out why people would ever NOT use choke input, it's barely more expensive to get a bigger choke if you already were planning a pi filter
One reason is the lack of (or extreme cost) transformers of much higher than 800 VCT. If you need 450 volts or more from your power supply, you can use a cap input filter on a $42 transformer (Allied 6K7VG) or you can pay $100 plus for a 1000 VCT transformer that will work with a choke input supply. For a 350 volt supply it is an easy choice if your amplifier draws enough idle current.

Another reason is an amplifier design with widely varying current demands. I have a class B sweep tube amp that draws about 20 mA at idle and over 200 mA at full power from the HV supply. You can't easilly feed this with a choke input supply.

Norris Wilson 21st February 2007 05:40 PM

Thanks Tube lab for all of your input.

I have found a Hammond 270 series transformer that will fit my needs within plus or minus 10V. This is based on your 5% to 10% over Voltage Hammond experience.


Poindexter 21st February 2007 06:33 PM

I have had problems with all the Hammond transformers (2XX series, and the enclosed single-secondary trans) humming mechanically; about one in three have done this. I think it possible that both the reason they run high on the heaters and the reason they hum is that they have 115v primaries, do we're running them high to start with.

On the other hand, the 3XX series have proven bulletproof and silent, as well as having dual 100-110-120 primaries so the voltage can be adjusted somewhat. Besides, they don't have that ugly knock-out in the bell.



Norris Wilson 21st February 2007 08:03 PM


What should be the expected tolerance accuracy for the secondary Voltage ratings of the 300 series transformers?

Looking on Hammond's website, I do not see a primary Voltage used to determine their rated secondary Voltage.

Which 300 series model do you use for your Musical machine?

Also, would you please tell me what primary Voltage you use on which tap, and what secondary VAC you obtain before rectification?

That way I can make an accurate choice before I layout for two transformers for monoblock amps?



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