Hammond PT's: Is the 115V primary a problem?

[rockybottom16]

If you use one of the Hammond PT's, does the 115V primary cause problems with the filament secondary being way too high? My service runs around 122V at the wall. If so, how do you compensate? I need a 300-0-300 PT for a SS rectified 18 watter with 6v6's, rated at least 120mA, and the Hammond 272DX looks promising.

[eschertron]

Check the Hammond documentation. IIRC all the 200 series PTs (and probably many more) now come with 115vac and 125vac primary taps. I know the 270CX that I just picked up did...

[g1]

That is correct. Just received a 272DX last week and it has both 115 and 125V primary taps.

[Chuck H]

The last 270EX I bought had the 115 primary. Filament volts end up over spec a tad. A recent discussion here shows this to be a bigger issue than I think most previously thought. There are easy fixes though. HV will be higher by ratio. If, for example, if 115V mains equal 600V CT then just divide 600/115 = 5.217. If you have 125V mains then it ends up being 125x5.217 = 652, or 326-0-326. So there you go. With a diode rectifier and no sag resistor before the plates that's going to roast EL84's and stress 6V6's. But... If you go with a 275-0-275 @ 115V primary but put in 125V you get 597, or 299-0-299. Bingo. Now you just need to deal with the filament voltage.

This was for educational purposes only since Hammond does offer a 125V mains primary now.

[rockybottom16]

Thanks guys for the advice (and the education!). I'm thinking it may be better to go with a 270EX and use the 115 primary. My outlet is at about 122-123V, so that should put me pretty close to my goal. And I'd rather have to bring my filaments down a little than come up lacking. I'm thinking they would end up at about 6.7V. If I use the 272DX and use the 125 primary, my rectified HV would be about the same; but the filaments would only be around 6.15V. So, if I use the 270EX, what would be the best way to knock the filament voltage down half a volt or so?

[g1]

Absolutely nothing wrong with 6.15V heaters, if anything it may improve tube life. You need to go a lot lower than that before you will have any problems. And consider that line voltage keeps creeping up, it will never go down.
I don't have the 272DX installed yet, so I don't have loaded voltages (maybe in a couple days). However, unloaded with 124V on the 125V tap, I am measuring 647V HV, 5.6V and 6.9V heaters.
For heaters to be "lacking" you would need to be down maybe even lower than 5.5V.

[Chuck H]

+++3

Not mentioned with their info, but commonly known among users, Hammonds generally volt out a little high on the filaments. My filaments in the amp I haven't corrected yet measure 6.9V to 7V loaded! The 272DX will likely be spot or still a tad high with 122V to 123V in.

[rockybottom16]

Makes sense to me, so I just ordererd a 272dx. We'll see how it goes!

[rockybottom16]

I got the 272dx wired up and the voltages are just fine. HV is exactly where I wanted it (using the 125v primary), and the filaments are a tad above 6.1VAC, which seems close enough! And it runs cool as a cucumber, which equals peace of mind... ;-)

[g1]

Sorry I forgot to post the loaded data I got for the 272DX. Rating is 300-0-300V @ 125mA, 5V @ 3A, 6.3V @ 4A.
Tube complement: 5U4, 2x6L6, 12AX7, 12AU7, 6SK7 (with 2x250 ohm virtual ground heater resistors and pilot lamp), for heater filament draw of approx. 2.8A.
Power amp cathode biased with 250 ohm, B+ approx. 350V, cathode voltage approx. 25V.
Yellow = 5V winding, green = 6.3V winding, red = 600V winding (end to end)

115V tap wired, 115VAC applied:
Yellow = 5.3V
Green = 6.6V
Red =620V
B+ = 355V, Vk = 24.8V

115V tap wired, 123V applied:
Yellow = 5.65V
Green =7V
Red =660V
B+ = 378V, Vk = 26.7V


125V tap wired, 122V applied:
Yellow =5.2V
Green =6.45V
Red =610V
B+ = 348V, Vk =24V

125V tap wired, 125V applied:
Yellow = 5.28V
Green =6.6V
Red =622V
B+ = 356V, Vk = 25V

Then, to check if the 6.3V heater winding would drop with more loading, I added an extra power resistor across it for a total of about 3.8A loading, difference was less than .1V.

[echoride]

Yes, measure your filament voltage first and if it is too high then there are fixes. But 6.1 to 6.5 is fine. If you want closer than that you need DC regulated heater line and will be not more than 60% efficient, plus a lot of parts.