Power Transformer rant

[VacuumVoodoo]

Transformers secondary voltages should be spec'd as "nominal".
This assumes:
nominal 120V on primary side
secondary voltages under nominal resistive load
assuming load regulation of 10% i.e. secondary voltages dropping 10% from idle to nominal load.
Also, the apparatus powered by the transformer should operate correctly under +-10% variation of primary side voltage.
This implies your ripple filter caps hanging on the rectifier must be spec'd at least 30% over idle secondary voltage.
Or make sure the amp is never powered up without all tubes in the sockets.
Power transformers are usually designed for continuous operation under nominal load.
This is a far cry from how a guitar amplifier loads the PT in use. The PT yo-yo'es from idle to full and overload.
And that's a major part of amps dynamic response.

[Phil_S]

Out of curiosity, I pulled up a Hammond spec sheet on one of their guitar replacement transformers. It says that it is for 120v, turns ratio is 5.82, unloaded voltage is 698.6, and loaded voltage is 650 @81mA. Methinks some thought went into this before they built it. The information isn't a secret. Is it that line voltage varies so much or what?

BTW, I'm guessing a real "classic" would have been built for 110V or 115V input, so at least they attempted to live in today's world.

[VacuumVoodoo]

Line voltage is allowed to vary +/-10% relative to nominal 120V i.e 108V to 132V.
An intelligently designed "real classic" transformer would deliver correct "classic" secondary voltages off 120V line. My opinion. Many will certainly have a different opinion.

[ToneMerc]

An intelligently designed "real classic" transformer would deliver correct "classic" secondary voltages off 120V line. My opinion. Many will certainly have a different opinion.

Bingo!, I agree 100%

TM

[johnnyreece]

As much as people may knock Weber, I like that their PT's have two primary taps: One for 120V and another for 125V. I always use the 125, as my wall voltages are smack in the middle (seriously, 122.5). They aren't made in the USA (I don't believe), but it is what it is.

[briane]

As much as people may knock Weber, I like that their PT's have two primary taps:

super secret trick: the only PT's I use.....bout says it all.

[Ken Moon]

I've had good luck with the Hammond 300 series - if the B+ is too high, just get one with a lower output voltage - they have a bunch to choose from. They seem to run cooler than the 200 series and have a handy 50V fixed bias tap. I like a B+ of about 410V on 6V6s.

As to heaters, I always use 0.1 ohm series resistors on each leg, and this usually puts the voltage at 6.3 to 6.5 volts with my 120V+ wall voltage. I like Mills MR12 12W resistors, but that may be overkill.

Antek toroids are another choice, and they also come in a variety of output voltages, and can source a bunch of current.

You can find decent Hammond prices at STFelectronics.com, Fairradio.com, and Angela.com, but each place has a different variety in stock.

[rp]

Almost every PT I use I get custom wound by Heyboer which I will setup for modern voltages, it eliminates the WTF factor. The MCI complaints I have seen across amp building forums, there are several here. TM

When I was in the States this is what I started doing for everything. The up charge is like $20 and worth it to avoid surprises and eating a Hammond 200, etc.

I'm with Ken Moon too, avoid the 200 series PTs and use the Hammond 300 series, the 120V tap works great on the HT and heaters. The 200s are wacked, IIRC the HT ratings are for the stupid vintage 110V taps so you get to choose low HT and 6.3V heaters or spec'd HT and 7.3V heaters, WTF!

In europe the 300 series let you play with 220 and 240 taps which is sweet.

I found a place here near Rome, Raoli, that will wind PT custom for about 100€ and if I can't get find a 300 I want then will use them. Anyway, you all that have problems just use Heyboer custom - affordable and I've never waited more 2 weeks, delivered.

[randalp3000]

I've had great luck with the 200 series Hammonds. All the ones I've used have a tap for 115V and 125V. The cover page for the 200 series all says 115v mains but if you look at the spec sheets they all have 115 and 125v taps. http://www.hammondmfg.com/263.htm, http://www.hammondmfg.com/pdf/EDB272JX.pdf

The only problem I see for my builds is that I don't have enough load for the filaments but I'm still around 6.6vac with a 125v main.

[John_P_WI]

And the 200 series are about 45 to 50 bucks less the last time I looked for a 272 jx vs 372 jx PT.

[rp]

I've had great luck with the 200 series Hammonds. All the ones I've used have a tap for 115V and 125V. The cover page for the 200 series all says 115v mains but if you look at the spec sheets they all have 115 and 125v taps.

Yes, but they don't have dual secondaries. The added 125V is a totally lame solution - IIRC when I used the 125V taps I had much too low an HT than what I planned on when I ordered it. If I used the 115V I got +7V on the heaters. IOW the 115V is completely useless in modern America, why does it even exist, it just adds confusion. the 125V is fine but you need to be familiar with the 200 series and figure out/guess before hand what you are really going to get, otherwise you just bought a $100 paperweight. Who needs that crap. With the 300 series you get what you expected and I love those, but as Hammonds cost as much as Heyboers for $20 extra I would just as soon as custom order exactly what I want, and I can specify them loaded or unloaded too to be really spot on. I've even had Heyboer match mounting holes for me once so no reptile dentistry was needed - can't beat it w/ a stick. Wish it wasn't so expensive to have Heyboer ship to Bumfucco.