Heater question

[Bob M.]

I'm working on a homebrew that someone didn't get right (after several attempts). My questions are about the heater circuit. I'm working with some tubes that I'm not all that familiar with (EF86, ECF82, EZ81) and others. All tubes have heater connections at pins 4 and 5. The power transformer chosen by the owner does not have a center tap for the heater winding and, I think, the circuit he copied envisioned one side (pin 4) tied to ground. I would prefer to use 2x 100 ohm ground reference resistors. Can I impliment this in the normal fashion: lift pin 4 from ground, connect the 2x 100 ohm resistors to pins 4 and 5 and the other ends of these two resistors to ground?

Also, the heater voltage is a little hot (about 6.9 VAC, measured at 120.5 VAC). Can I use some series resistance to get it around 6.3~6.5 VAC. Should I be using about 100 ohm resistors for this? At what wattage?

Thanks in adv,

Bob M.

[stokes]

If the 6.9 ac volts you are seeing is with no tubes in the amp it will likely come down when the tubes are installed.If you are getting 6.9v with the tubes installed,I personally wouldnt worry about the extra 1/2v.
When you lift the pin 4 ground you must now connect the other side of the heater winding to all the pin 4's.The artificial CT using the 100ohms get connected just as you described.

[Bob M.]

Heater /voltage question

Stokes,

Thanks for your reply.

What would you consider the upper limit for heater voltage for tubes designed to run at 6.3 VAC? If one chose to lower that voltage, what's the best way to accomplish this?

The numbers I quoted on heater voltage were with all tubes installed and the amp up and running a while, having stablized. The voltage delivered in my part of the world has very little swing here, being between 120.8 and 118.3 VAC (but usually within a volt, between 119.5 and 120.3 about 90% of the time), measured daily.

Thanks,

Bob M.

[stokes]

I have seen a few old Fenders with 7v's and didnt seem to have any adverse effect,but I wouldnt want to see higher than that.To use the 2x 100ohm ct set up you have to rewire to the twisted pairs heater set up,see what the volts are after you do that,its possible the volts may come down a bit.