Tube "sweet spot"

[joncar]

Coming across forum reference to a tube's "sweet spot" and "dialing in the sweet spot", I would appreciate it if someone might help me further my understanding about tubes. What is the sweet spot?

[soundbrigade]

Have no clue, but sweet spot uses to be the spot in front of your speakers where listening conditions are PERFECT.

Could a tubical sweet spot be a point in the Ua/Ia-characteristics where the tube works most optimal??

[Geek]

Many tubes have a bias point where they sound best by consensus of ears, rather than test equipment.

Cheers!

[joncar]

Thanks to you both for your replies and information. If I am getting this right, the sweet spot is a certain combination of voltage and current. And that it may or may not correspond exactly to the designer's circuit parameters--which I assume would include consideration of other things like tube life, safety, etc.. Does the sweet spot change as a tube ages and do individual tubes within a type (e.g.,6AV5GA's in my particular case) have different sweet spots? Is changing the cathode resistance the way you find the bias current that gets you to the sweet spot? Does measurement enter into the process of finding the sweet spot at all or is it strictly by ear?

I'd appreciate any further info on this.
Thanks again.

[Geek]

Hi,

Does the sweet spot change as a tube ages and do individual tubes within a type (e.g.,6AV5GA's in my particular case) have different sweet spots?

In my experience with my spud amp (6FD7 daily runner affectionately called Baby Amp, a 2.5W/ch. SET), no, the sweet spot is the same regardless of age or brand.

With my ST-70, I found the EL34 sweet spot at 39mA/tube, despite the original spec of 50mA/tube. I have less experience rolling brands with these (EL34's are more expensive than 6FD7's in my spud), but it seems the sweet spot remains consistent with two brands and over two years.

Cheers!

[joncar]

Thanks Gregg. Is finding the sweet spot a matter of trying different output tube cathode resistors? Is it practical to use a variable resistor in that position to do so?

Off topic, I see you live in an area where I spent many wonderful summers boating with my young family--Maple Bay, Ganges, Montague Harbor, et. al.

[Geek]

Hi,

You can use different cathode resistors for self-biased power tubes, yes.

I breadboard the circuit and used fixed bias to find the sweet current and then use the appropriate resistor.

I find it tends to prove the rule of thumb for class-A, set the plate dissipation to 95% and for AB, 70%. But I haven't tested *all* the tubes used for audio

Heh, y'know what? Despite all the scenery and summer things to do around here, I have very rarely ventured to do so

I'm 43 and haven't been to Cultus Lake, 20 mins away, since I was 6!

Cheers!

[EWBrown]

6AV5GA "sweet spot" is around 300V P-K and 66 mA, or 20 W PD, with a 3K load - the 11 watts published rating is overly cautious and conservative, more applicable to to its original TV horizontal deflection purposes. For Class A audio, 20W is good, it depends on the plate structure and manufacturer. I find that DuMonts (actually re-branded Sylvanias) and Sylvanias are the most robust, some of teh RCAs have small "wimpy" plate structures.

6EM7/6FD7/13EM7 230-240v p-k AND 37-40 Ma is their sweet spot for best soun d quality.

THis is determined when the tube is operating in the most linear part of the plate curves.

/ed B

[Geek]

6EM7/6FD7/13EM7 230-240v p-k AND 37-40 Ma is their sweet spot for best soun d quality.

Yup!

And their gain-triode sections at 200V Va at ~1mA... running them B+/2 makes them a little stale.

Cheers!

[joncar]

Thanks Ed and Gregg--I re-measured the 6AV5GA voltages in my amp. I measure 309vdc plate to ground, 50vdc cathode to ground. I am using the stock GetSetGo circuit (except for the 100ohm screen to plate resistor) so the 3 2.7k cathode resistors equals 8.1k. If I am doing this right, that gives me 259 P-K volts at about 62ma. Is that correct? Inputs are shorted, 8ohm resistors speaker loads. Output transformer is Edcor 5K to 8ohm.

How can I get to the sweet spot from here?
Thanks for any guidance.

[EWBrown]

That seems correct, 50V across 900 ohms is 55.5 mA cathode current, swich is right for the G*S*G circuit. If you use a solid state rectifier in place of teh 6AX5 rectifier, the B+ and current will increase by about 10%.

This will deliver a bit more output power, not a major difference, overal, perhaps an exta half watt.l.

I tried this in my first 6AV5GA prototype, I used UF4007s, each with a 50 ohm, 5W resistor in series with the anode, and the cathodes joined together. Put it al in an empty octal tube base, to make it pluggable, for comparison.

/ed B

Next experiment, a 6GE5 SET amp, these are relatively cheap, TV horizontal sweep tubes, in a 12 pin compactron package.
From my preliminary "static" testing, these tubes behave just like 6B4G, using SE-CAD, but then they have a bit more power handling capability, and higher voltage and current tolerance than 6B4Gs. Still a work in progress...

[joncar]

Thanks Ed--in an earlier post you indicated that the sweet spot for the 6AV5GA is "about 300volts p-k and 66ma into a 3K load". Would these figures be the same into a 5K load?

[msmpe]

6AV5GA "sweet spot" is around 300V P-K and 66 mA, or 20 W PD, with a 3K load

I agree with Ed on the 6AV5GAs. I'm running B+ at measured 309VDC with the original PST which will be replaced in a month. Current draw per my Heathkit PS2 is 68mA and the amp is biased for that. I'm using an 8W-5K-8ohm OPT. My VA isn't the best but seems to work. It is a 12AX7 with gain around 40 and unbypassed. If I had more room on the chassis, I would have experimented with the VA a little more (VA-CF or UL VA cascode). I will do some tube rolling in the VA over the next few weeks.

The 6AV5 is a good tube.