8k, 8ohm OT with a 6L6 question

[Vacuumdust]

Single ended OT rated at 8k primary to 8 ohm secondary. Obviously designed for a single 6v6 but touted as able to handle a 6L6. So based on “reflected” impedance, if I’m using it with a 6l6 (4K) should I ideally be using a 4 ohm speaker. (4K to 4ohm). So many single ended amps often referred to as “self biasing” boast the ability to swap several power tubes/rectos and obviously with those swaps the speaker stays the same. So obviously the amp works just fine with different combinations of tubes but is most likely only optimized for one. Anyway, this topic has always been just beyond my grasp. So my question remains...using a single 6l6 with this OT, what speaker impedance would be not just nominal, but optimal. Thanks!

[Stevem]

Part of the issue is that your leaving the level of plate voltage factor out of the picture.

In terms of a 6V6 for example The higher the plate voltage above the class A 315 volt spec as it refers to the load line, the higher the primary impedance needs to be.

Basically when you hook up a lower impeadance speaker to a OT tap set up for the normal higher load your then asking , for example a 8 ohm tap winding to handle 30 percent more current then it may be designed to when you apply a 4 ohm speaker on it!

[martin manning]

Say you have a SE set up for 6V6, and it has some plate and screen voltages, as well as a cathode resistor that puts the plate dissipation near 100% (plate current goes with plate and screen voltages to achieve this), and the primary impedance is such that the operating point is approximately centered on the load line. The power supply and the OT are both designed for these conditions.

Now you swap in a 6L6. The power supply should not be asked to deliver much more than the voltage and current at the 6V6 idle conditions, considering both the OT standing current level and the power supply's ability to deliver it. You also have fixed cathode and screen resistors, which will determine the 6L6's operating point, and it will likely settle at a bit higher idle current than the 6V6, and the plate voltage will be reduced a bit due to power supply sag. To keep the operating point centered (as best you can), you should not change the primary impedance. The 6L6 will be operating at less than it's capability, but to fully utilize it you would need both a larger power transformer and a larger OT to handle the increase in idle current.

[Leo_Gnardo]

FWIW I've installed 6L6 (and JJ 5881) into Deluxe Reverbs - biased at 30 mA - get a little extra power but not a lot. As expected, not an ideal load for the tube/transformer but the combination does work and haven't yet had a wrecked OT nor PT. And the owners of those amps are well satisfied with them. One of these Deluxes recently returned for an issue that had nothing to do with the power department. It had been running JJ 5881's over the last 2 1/2 years with almost daily use. A little maintenance look, slight bias adjustment & she's off & running for maybe another couple years. Solid state rectifier in this one, and it clocks in at a clean 30 watts to 8 ohms.

[martin manning]

For push-pull, the centering of the zero-signal operating point is a non-issue. Also, the DR's 6k6 Zpri is a bit closer to what a 5881 or 6L6 would normally use, which should help to keep the screen current under control. A DR with 5881 and SS rectifier sounds like a pretty good way to get just enough extra power in a situation where a stock DR wouldn't cut it.