2a3 Operating Points

[ebag4]

What are the operating points for the Stereomour in 2a3 mode?  The reason I ask is because I read that the JJ 2a3-40 tube sounds best at 245V and 43mA. 

Thanks,
Ed

[Paul Joppa]

Sounds like Jeff Medwin's operating point. I never know when he's pulling our collective legs ... :^)

Stereomour nominally operates at 300v plate to cathode, 50mA current, and 4000 ohms load impedance. In my opinion, all three numbers define an operating point, and using just two is quite meaningless. Not to mention that it is difficult - nearly impossible in fact - to separate the sound of an output stage from the sound of the driver stage, and the sound of how those two interact.

[ebag4]

Thanks Paul.  I actually read it on the Tube Asylum.  Here is the comment made by Drlowmu:

Dennis finds these tubes run great, and sound wonderful at about 245 P-K and about 43 mA of current. I don't think he'd ever run them higher than 285 VDC. 245 VDC at 43 mills is 10.53 watts dissipation. And its glorious there with the amp adjustments he has made!!

And I just read the signature, apparently drlowmu is Jeff Medwin, good call!

Here is the link just in case anyone is interested: http://www.audioasylum.com/forums/tubediy/messages/14/148895.html

Best,
Ed

[InfernoSTi]

Ed, as you already know, I am VERY happy with the JJ 2A3-40s at the stock operating points.  I think they will last forever, too.  For $210/matched pair from Eurotube, you just can't go wrong.  I have read all the various threads on AA but settled on stock and rolling caps as the way to go. YMMV

John

P.S. I have some black plate RCAs and a set of single plates but I just like the JJ 2A3-40s better.  Really...

[Jim R.]

Ed,

And to add to what John said, I too really like the JJ 2a3-40s and liked them in every 2a3 amp I've had.

Glad to hear you're liking the amp and that things arestarting to setetle down a bit.  The sovteks are really quite decent tubess, but the JJs are just smoother, more refined, more dynamic, and are better at the extremes.  I'm not going to say they're better than the EMLs, but I have yet to hear the EML 2a3s in one of my amps.

-- Jim

[ebag4]

Thanks John/Jim,
The amp is really starting to sound good.  I have a pair of mundorf Silver/Oil .1 uF caps on the way from Hong Kong, should be here in the next day or so.  I have rolled a JJ ECC81 in and it sounds very good.  I believe I have decided on the JJ 2a3-40s, I simply have read nothing but great things about them.  BTW, I ordered the ECC81 from Eurotubes last Friday and it arrived here on Tuesday, not too shabby for a holiday weekend.

This amp has improved my system in the bass, the issue is my speakers are on SS servo amps from 200 Hz and down, I have no idea why this area has improvd but it is very obvious to me the improvement is real and significant.  I don't understand why unless the load this amp puts on my Dodd Buffer is more benign than other amps I have used.

Once I get the Mundorf's in to replace the stock coupling caps I will probably go ahead and order the JJs.

I am really starting to enjoy this amp!

Best,
Ed

[Paul Joppa]

Forgot to mention that you can run a 2A3 in 45 mode with the Stereomour; that would be 275v at 35mA into 4K ohms. I don't know what Dennis and Jeff may have tried, but I am guessing that their experiments were done with a 2500 or possibly 3000 ohm transformer. To correct for that, you would run a slightly higher voltage and lower current if you had a 4K transformer - so I believe this would be very close to the point that Jeff mentions. And it's an easy experiment to do!

[ebag4]

Paul, thanks for that.  I am considering installing a switch to go between 45 and 2a3 operation.  It appears I can do this with 4PDT and a 4PST so I will be looking at 8PDT switches and/or relays to do this with.  What are the contact ratings I should be considering?  4 of the contacts are switching in and out 5 watts resistors so I assume I need some current capacity there, what about the choke current?

Also, if I needed to pick up some form of power to drive a relay with what would you recommend, 120VAC from the IEC or another voltage from somewhere else.  If I end up using relays they will probably be located near the center of the chassis, I don't want to introduce additional noise.

Thanks,
Ed

[Paul Joppa]

I do not recommend switching these components while power is on. The potential for transient voltages that may damage parts, or produce large impulses that may damage your speakers (or your hearing!), is just too great.

Specifying the switches for live switching would also a fairly complicated engineering task, due again to the nature of the transient voltages and currents - note that all the changes involve reactive components, chokes or capacitors. I have not done that engineering.

[ebag4]

I do not recommend switching these components while power is on. The potential for transient voltages that may damage parts, or produce large impulses that may damage your speakers (or your hearing!), is just too great.

Specifying the switches for live switching would also a fairly complicated engineering task, due again to the nature of the transient voltages and currents - note that all the changes involve reactive components, chokes or capacitors. I have not done that engineering.

I agree, I would not want to use the switch as an AB type switch, I envisioned switching prior to turning the unit on but I see that if I powered the relay with anything other than 120VAC directly from the IEC the unit would have to be on.  I see your point and understand your hesitance to make a recommendation.

Best,
Ed

[Grainger49]

Thanks John/Jim,

   .  .  .   BTW, I ordered the ECC81 from Eurotubes last Friday and it arrived here on Tuesday, not too shabby for a holiday weekend.

Best,

Ed

Ed,

This is Mr. Eurotubes.  Anyone with this much hair and appreciation for good T-Shirts has to be cool.

[ebag4]

Thanks John/Jim,

   .  .  .   BTW, I ordered the ECC81 from Eurotubes last Friday and it arrived here on Tuesday, not too shabby for a holiday weekend.

Best,

Ed

Ed,

This is Mr. Eurotubes.  Anyone with this much hair and appreciation for good T-Shirts has to be cool.

Thanks for the pic Grainger.

Well he is certainly cool enough to get some of my return business, I placed my order for a pair of JJ 2a3-40s today.  Can't wait to hear them in the Stereomour!!

Best,
Ed

[InfernoSTi]

Ed,

I hope they are all to you that they are to Jim and I.  I can't wait to hear your impressions of the JJ 2A3-40 (and again after 250 hours)!!!!

John

BTW, I've only spoken with Mr. Eurotubes once but can assure you his cool...very nice guy and helpful. 

[Jim R.]

A question for Paul J:

Ok, sorry, I'm playing what-if games again and kind of thinking out loud, but I wonder if the tfa-2004 jr would fit on the stereomour chassis, (or under it as the case may be), and if so, what would be the new operating point for the 3.3k impedance OPTs?

Thanks,

Jim

[Paul Joppa]

The TFA-2004Jr will not fit on the Stereomour chassis plate.

The subject of operating points for triode power stages is too big for a post, but I've posted my conclusions before; here they are again:

First, you need to understand three resistances:

1) plate resistance, rp. This varies inversely with the cube root of the DC current, so unless you match the operating point in the tube manual, you want have to correct it slightly.

2) "beam resistance" Rb - this is the plate to cathode voltage, divided by the plate current. (I used to call this "operating resistance" or "equivalent resistance" until I found the correct term.)

3) Load impedance RL; I calculate an "optimum" value RLopt. Most amplifiers use an impedance less than this; the compromises are small if you go as low as 70% of RLopt. I prefer to keep the full value because it provides a margin for speaker impedance variations and limited output transformer and/or plate choke inductance.

OK, then Rb = 2.38*rp + RLopt. From this you can deduce (with some algebra) that the plate to cathode voltage will be the square root of the quantity (Rb*PD), where PD is the plate dissipation. And of course the plate current is the dissipation divided by the voltage.