1st post from new member at Fort Knox KY

[PRR]

There is also "history". Just before 6V6 appeared, many-many good radios ran 2*6F6 with 250V and a 10KCT OT. The 6V6 specs for 250V show what you will get if you drop 6V6 into your 6F6 radio production (and re-bias). The 285V condition shows that if you can find another 35V *and* change the OT spec, you can bump your 10W job into a 14W job; but many 2*6V6 rigs were run on the lower conditions (10W used to be enuff for about anybody).

[Paul1453]

> 6AQ5 Class A at 180V = 5500 and 250V = 5000.

Those are ONE TUBE conditions.

Not easily comparable to 2-tube push-pull.

Within the 2-tube suggestions, there isn't a "right value" of load, merely a "best value" for the Voltage and Current which you run at, and how hard you want to stress the tubes.

For some tubes the "optimum" changes from 5KCT at 500V to 20KCT at 1,000V. Twice the voltage, better be half the current, so load should be FOUR times higher. (The "small" tubes we usually run, even unto 6550, don't have such a huge range of happy voltages and currents; you will still see this trend.)

Right, and the 6AQ5 Class AB would be 2 tubes in P/P configuration correct?  The data sheet recommends 10000 for AB operation at 250V.  I was wondering if this 25% increase in recommended Load would be an issue when trying to use my current P/P OT which has approximately 8000 for it's load at 8 ohms?  I don't think the 6AQ5's would burn up the OT, but might burn themselves out with the lighter load.  I plan on trying this out this weekend.  After all these are all salvaged parts and intended to be used to help me learn and experiment before I try to build a real keeper amp.

When I just swapped the 6AQ5 into my SE Champ prototype, they did not like going above the 250V unlike the 6V6.  I went to 265V and the sound got worse, and down to about 220V the sound seemed about the same for the 6AQ5.  6V6's were running at 380 on that design and I didn't try to push that at all, mostly because it was approaching the limits on my adjustable bench PS.

[HotBluePlates]

More beginner questions, I hope you don't mind.
I've got a center tap OT that came out of a Galaxy Projector Amplifier.  I've got the schematic and it shows it was a EL84 PP output circuit with an 8 ohm speaker jack.  So I did the voltage ratio thing.  6.6 VAC on the Secondary got 213 VAC across the full Primary.  That gives me 32:1 ratio, squared = 1024, times 8 =  8192.  So I look at the data sheets for the 6BQ5, 6AQ5, and 6V6.

All 3 tubes are 12w plate dissipation (except for the later 6V6GTA), and you will find they all commonly have 8kΩ primary impedance for push-pull operation of 2 tubes (then again, there's the Deluxe Reverb which uses a lower load impedance for a little more output power).

Better yet, copy the Galaxy arrangement, including plate and screen voltage, and the circuit will be turn-key. Except that the EL84 can use less bias voltage to idle at the same current, so you will see a lower bias voltage (or smaller cathode resistor, if used). EL84 might use 100-150Ω for a pair where 6V6 might use 250-330Ω in the same circuit.