Hammond 272 FX plate voltage

[fullrangeSR]

Some time ago I bought Hammond 272 FX power transformer meaning to use it in RH84 SE build. Which I did. The amp sounded awesome. Being a tube newbie at the time (still am), I never bothered with checking voltages – I just enjoyed the music. Output transformers were Hammond 125 BSE and the choke was Hammond 193 C…

I did notice that the 272 FX would get hot to the touch and that both pre and power tubes would somehow get spent faster than usual, but I never paid any real attention to all that…

In spite of the diminutive OPTs the sound was not lacking, but I still wanted to go for the bigger OPTs – I bought a pair of 5K Transcendars. I decided to retain the 272 FX and the 193 C… Now, the varnish on Hammonds started peeling off and the cloth which covered laminations on the Transcendars plain bugged me, so I decided to sand down and refinish all the transformers.

The build progressed to the point where almost all the components were in place, some of them soldered in, when I remembered the ‘bad’ reputation of the Hammond 272 line, the reputation so many times talked about, and checked the voltages…

Red/red: 680 V
Yellow/yellow: 5.2 V
Green/green: 6.6 V

Mains: 120 V

I kept on checking the voltages again and again: different times of day, different outlets, and still the results (with slight variations) were the same…

Will the voltages go down once they’re under load? Is the 272 FX defective, or is it ‘normal’ for this line? Was I not knowingly ‘cooking’ the tubes in the previous build? Am I panicking without reason? If 680 V indeed is too high for a no-load, what can be done then?

Comments? Suggestions? Solutions?

Help is muuuch appreciated.

[fullrangeSR]

I forgot to mention that the Hammond 272 FX plate voltage is advertised as 300-0-300...

[Ty_Bower]

Originally Posted by fullrangeSR

Will the voltages go down once they’re under load?

Absolutely. You need to measure your voltage with a load on the supply. Another option might be to measure the DC resistance of the primary and secondary windings, and we can try to plug the results into PSUDII and estimate the voltage under load.

[boywonder]

Get everything soldered back together and check the voltages in circuit with the amp warmed up. Clip leads are your friend here or use one hand with a probe and keep the other in your pocket. Since it sounds like you are using a CLC supply with the hammond 193C, you can adjust your B+ voltage a bit by adjusting the value of the first cap.

[fullrangeSR]

Well, I finished the amp, turned it on, the tubes lit up... music, sweet music out of speakers...Then I did a set of measurements:

horror:

B+ : 373 V
EL84 plate: 361 V
12AT7 plate: 93 V
5U4G 5-Volt winding: 5V
heaters: 6.5 V

I didn't feel like doing any other measurements... It seems obvious now that either digikey.ca supplied me with a defective 272 FX or somebody at the Hammond factory put a 272 FX sticker on a transformer other than 272 FX...

[Ty_Bower]

Originally Posted by fullrangeSR

horror:

It doesn't look so bad. Measure the voltage drop across the EL84 cathode resistor and calculate the total tube current. Make some assumption about combined plate & screen dissipation. Compare vs. the data sheets, and adjust your cathode resistance as needed to ensure that things stay within limits. Check the plates in a totally darkened room after an hour of operation, and confirm there isn't the slightest hint of red glow coming from the plates.

Yes, the Hammond 200 series do have a reputation of running hot. The specs do say they are designed for 115VAC on the primary. Who's got that low of a line voltage these days? Not me.

For what little it's worth, I've built an EL84 amp which runs the finals ultra-linear. My B+ is over 400 VDC. I keep the idle current around 30 mA or less. The tubes (Russian 6p14p) don't seem to mind the high voltage at all.

[TheGimp]

I've got a Hammond 270FX that has a boiler plate stating 138VA, 550VCT at 150mA.

If you go to the Hammond web site, the 270FX is specified as 550CT @173mA.

I called them and never got a straight answer about the descrepency.

My experience is that Hammond has a bit more DC resistance than the older Stancor transformers I'm used to. So the IR drop can result in much higher output voltage if lightly loaded, or lower output if max load.

YMMV.

[boywonder]

Originally Posted by fullrangeSR

Well, I finished the amp, turned it on, the tubes lit up... music, sweet music out of speakers...Then I did a set of measurements:

horror:

B+ : 373 V
EL84 plate: 361 V
12AT7 plate: 93 V
5U4G 5-Volt winding: 5V
heaters: 6.5 V

I didn't feel like doing any other measurements... It seems obvious now that either digikey.ca supplied me with a defective 272 FX or somebody at the Hammond factory put a 272 FX sticker on a transformer other than 272 FX...

The voltages are a little hot, but not too bad.

What are your target voltages for B+, 12AT7 & the EL84? Assuming that you have a CLC power supply topology you can reduce the value of the first cap to reduce the B+ voltage a bit. What is the value of the first cap after the rectifier?

[fullrangeSR]

Here's the link to the schematic I followed:

RH 84 - Tube Audio ...... RH DESIGN

On top of the existing PI filter (47 uF/450 V, 20 H/100 mA, 220 uF/350 V, 0.47 uF/400 V) I added 1.2 K/12 W, 0.47 uF/600 V, 120 Ohm/ 12 W, 0.027 uF/600 V (that's what I had on hand at the time). The addition did bring the B+ from 373 V down to 305 V... The 1.2 K resistor was getting hot a lot so I had to mount it on a fairly large heatsink... The heaters are at 6.5 V... I let the amp run for more than an hour. Everything was fine except for one thing: the choke is now humming/buzzing (it didn't do that before the addition)... What's causing it to do so?

[boywonder]

Instead of adding the RC sections, you can also lower the value of C1 from 47u to around 1uf-4uf or so, that will drop the voltage down to around 300V without burning up a bunch of power as heat in your large wattage resistors.

Of course, the added R's and C's will provide additional filtering. Do you know what the amp current draw is?

Hmm......I can't see how adding additional RC sections would make the choke hum.

You can also put fractional ohm value resistors on the filament windings if the 6.5V bothers you.