help spec'ing transformers please

[Randall]

Me again. I could use some help choosing transformers for an Ampeg B-15NC build. I would like to give Magnetic Components a try. I inquired and got a response from the engineer who drew up the drawings. (I like that!). He says they don't have exact replacements, but offered me these.

PT: http://www.classictone.net/40-18094.pdf
OT: http://www.classictone.net/40-18064.pdf
B-15NC: The Unofficial Ampeg Page

The PT with a 5AR4 shows B+ of 422v, where the schematic shows 440v. Is this enough of a difference to make a difference? I think this would have me expect to see voltages about 5% lower than called for, but if going by the Fender +/- 20% standard, shouldn't this be OK? And I plan to only have one channel, eliminating a 6SL7 tube, will this put less of a load on the PS and help to bring voltages up a bit?

The OT, I have no real idea how to spec the primary. If I decide to run an extension jack, which would be the prefered choice 4 or 8 ohm tap, and why? I notice Ampeg used 8 and 16 ohm.

On the schematic, the K on the output cathodes, does that go to the K on the hum balance? If so, is that a good idea, or should I just use the CT on the filament winding?

Thanks guys.

[Randall]

Since this generated no responses I took a chance and ordered the above models from Triode with some help from Mark at Magnetic Components. I still would be glad to get some advice on my lower voltage question, as well as the hum balance control wiring.

[Tom Phillips]

1) I have a strong opinion that 422V vs. 440V B+ will not make a perceptible difference in your amp's performance.
2) Yes. The markings on the schematic do mean that the wiper on the hum balance pot is connected to the power tube cathodes. It's difficult to predict if that connection or connecting to the heater center tap will give the lowest hum level. Many times there is not much difference. You could try both. Most people would probably choose the cathode connection.
Cheers,
Tom

PS. Ya gonna put a standby switch in that new build or do without?

[Randall]

Thank you Tom. I was actually wondering whether to use the hum balance as drawn or ground the CT, as my thinking was the hum balance circuit was made because the trafo had no CT. Is this flawed?

And I am still undecided about the standby switch. I am digesting the thread on that subject. I'm not overly concerned with tube life, seems to me that is a perfect world argument. There are many factors effecting that. I'm just used to a standby switch, and I like the idea of switching off B+ during breaks. But I don't have a water tight reason for one. But you are right, it would be more old school without one, and putting the power switch back side.

[Tom Phillips]

Randall,
Using the hombalance port is considered a step up in capability to reduce the heater circuit induced hum as opposed to a simple grounded heater CT. It may or may not allow lower hum to be dialed in. It depends upon the exact tubes you have installed. If you have the parts and the space for it then I would use the hum balance pot.
Tom