• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Hammond 272 FX plate voltage

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.
 
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.
 
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...
 
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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.
 
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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.
 
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?
 
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?
 
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.
 
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I have the exact story as your it was my first build after a few post on here I finally went with Edcor problem fix ever since I have not purchase Hammond PTX.

+1

The 115v primary (fixed with the new ones though), as well as the buzzing finally drove me to Edcor, better selection, cheaper, but slower delivery. I never sprang for the 300 series though.

Athos
 
I never sprang for the 300 series though.

I've got a Hammond 374BX. It cost more than the cheaper alternatives, but I've never regretted the money I spent on it. The primary is wound for 120V (or 240V) so the secondaries come out at the correct values. Everything in it seems varnished tight, and it is the quietest transformer I own (virtually no mechanical hum whatsoever). I'd buy another without hesitation.

I've used an Edcor power transformer, too. It's also a good unit, but I think the Hammond 300 is better.
 

Mr_Zenith

Member
Paid Member
2009-08-20 3:16 pm
KC Metro
Hey fellas...

I feel your pain, having just completed my own RH84 build just a few minutes ago.

For the PS I used a Hammond 270HX with a HV secondary rated at 275-0-275 VAC, as opposed to the 300-0-300 shown on the schematic - and my B+ is still high (about 341 VDC on the plates). I haven't yet checked the bias voltage, so I can't calculate the actual plate dissipation. The plates aren't glowing, so we'll see. I'll probably rig up a bucking transformer to set things straight.

That said, I had just about given up on single-ended designs until I fired up this little jewel about an hour ago. I've never heard a single-ended amp sound this good - and I've built several (including the 6550 job in the September 2001 AudioXpress, which sounded horrible to me, BTW). It has excellent volume, great imaging and the best bass response of any SE amp I've built to date. It's dead quiet, too; there's almost zero noise on my 98 dB RF3's with the volume turned down. I now see why folks rave over the EL84/6BQ5! :D I'll try to get some pics up soon.

I feel an urge to order a Simple SE board coming on...

Ty: it sounds GREAT through the buschorns, too!
 
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B+ : 373 V
EL84 plate: 361 V
12AT7 plate: 93 V
5U4G 5-Volt winding: 5V
heaters: 6.5 V


Hi FullrangeSR,

Completed my RH84 yesterday using old console iron. My B+ is spot on 300v

However measuring pin 1 and pin 6 of the 12AT7 to ground I'm getting 155v and 160v, I have 1.2v at the cathodes, is this how you measured your plate voltage?

Hi Boywonder,

I too have 6.5v on the heaters, itchin' to try diodes:D
 
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?

When it comes to lowering the value of the C1 - I'll try it, just not now...

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

I have no idea... Maybe the effect of less voltage=more current somehow stressing the choke? Just guessing.

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

No, not bothered.

However measuring pin 1 and pin 6 of the 12AT7 to ground I'm getting 155v and 160v, I have 1.2v at the cathodes, is this how you measured your plate voltage?

This is what I have:

B+: 305V

EL84 plates: 214V/219V
EL84 cathodes: 7.68V/7.56V

12AT7 plates: 76V/82V
12AT7 cathodes: 0.51V/0.53V

That said, I had just about given up on single-ended designs until I fired up this little jewel about an hour ago. I've never heard a single-ended amp sound this good - and I've built several (including the 6550 job in the September 2001 AudioXpress, which sounded horrible to me, BTW). It has excellent volume, great imaging and the best bass response of any SE amp I've built to date. It's dead quiet, too; there's almost zero noise on my 98 dB RF3's with the volume turned down. I now see why folks rave over the EL84/6BQ5!

Yeah, it does sound mighty fine. I whole heartedly recommend it...

Parts selection is part of the learning curves for me.

Right on! Hopefully, I'll smarten up...
 
This is what I have:

B+: 305V

EL84 plates: 214V/219V
EL84 cathodes: 7.68V/7.56V

12AT7 plates: 76V/82V
12AT7 cathodes: 0.51V/0.53V


Thanks for that.

My output transformers were designed for an ECL86 whose pentode is a derated EL84 so not ideal. I have another set of output transformers in an EL84 based se amp that I think I'll try. Maybe it'll bring our readings closer together.

Brgds Bill

p.s. my EL84 cathode voltages both measure 10.25v
 
B+: 305V

EL84 plates: 214V/219V
EL84 cathodes: 7.68V/7.56V

12AT7 plates: 76V/82V
12AT7 cathodes: 0.51V/0.53V

This doesn't make a whole lot of sense. How can B+ = 305V and the El84 plates be running around 216V ? Surely you can't be losing 90V through the opt primary. :eek:

The current operating voltages are way too low - you should be getting around 155V on the 12AT7 plates and around 1.4V on the cathodes.
 
This doesn't make a whole lot of sense. How can B+ = 305V and the El84 plates be running around 216V ? Surely you can't be losing 90V through the opt primary. :eek:

The current operating voltages are way too low - you should be getting around 155V on the 12AT7 plates and around 1.4V on the cathodes.

My bad... I don't know how it happened, but instead of measuring EL84 plates I managed to measure grids #2... The actual EL84 plate voltages are 294V/294V. My apologies to Soonerorlater...

I also double-checked the 12AT7 plates and cathodes - the voltages are as previously stated.