Building Started...

[Grainger49]

   .  .  .    Blumenstein Audio does not publish any sound pressure vs power data. They conservatively spec the Orca sensitivity at 89dB at 1Watt and 1Meter.  .  .  .   

I'm running my Orcas on "3.5 Watt" (quotes because of Paul) Paramours.  I'm in front of them now, my RS meter says they are playing at the mid 80sdBA.  Much louder than I want to listen.  I do think that Blumenstein is conservative.  Maybe more like 83-85dBW.  In the 90dBA range OSHA requires ear protection.  I require no one to wear ear plugs in the listening room.

[rlyach]

Paul,

Thanks for all the information. I feel better about deciding to use 2A3's. If you actually get closer to 4 Watts out of the 2A3's then the SPL difference is actually closer to 4db instead of 3  .  Incidentally, While I was calculating my load lines for the respective tubes I think I came across another advantage of the parafeed design. The musical impedance that the tube sees is a sum of the resistance of output transformer, the reactance of the output transformer and the reactance of the parafeed capacitor. Since the reactance of the cap is 1/(2*pi*f*C) and the reactance of the output transformer is (2*pi*f*L), the parafeed cap's impedance will decrease with frequency and the output transformer impedance will increase with frequency. These seem to act together to give a more stable tube load line over frequency. Maybe I am over analyzing all this. I will use my LCR meter on the various components tonight and see if the numbers add up.

Grainger,

Thanks for the info on the Orcas. I assume the RS meter is a Radio Shack sound pressure meter. How far was the meter away from the speakers when you made the measurement? This might give us some idea as to the effeciency of the Orca's in a listening environment.

[Grainger49]

Randy,

Yes, a vintage Radio Shack analog SPL meter.  It was about 12' from the speakers.  Let's see if I have a hosted image of the Orcas from my listening La-Z-Boy?

Yup:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi244.photobucket.com%2Falbums%2Fgg7%2FGrainger49%2FIMG_1337_zps71c528a6.jpg&hash=0bbd396951a21875172387a27b35e53cb5e9ffb9)

Short legs and short fat (wide) feet.  My ears are at least 12' from the Orcas and as you see they are way away from any reflecting surfaces.

[rlyach]

Wow... That calculates to 95 db at 1 meter. If we knew the power level you were listening to we could actually calculate efficiency. We could roughly get that number if we knew the attenuation setting and the max power of the amp. Very nice system and listening setup by the way...

[Doc B.]

You can't do that kind of 1 meter vs. 4 meters calculation based upon an in-room measurement. You have to take the driver outside or into an anechoic chamber to get values that you can rely upon. In room readings will be higher at a distance than anechoic measurements. PJ can give you the rundown regarding the reverberant field, etc. in remarkable detail. Of course this is a good thing when you are using low power amps and medium sensitivity speakers.

[Paul Joppa]

Randy, you are quite right about the load impedance. A well-chosen parafeed capacitor/plate choke combination keeps the load impedance seen by the tube close to resistive, to a much lower frequency than would be the case with a series feed transformer of the same inductance. This is in fact how I arrived at the rule of thumb for parafeed capacitance (2*L/(R*R)). It permits greater power and lower distortion in the deepest bass. Of course, it also assumes a resistive speaker impedance at those frequencies - not always an accurate assumption!

[rlyach]

My Goldpoint attenuator came in so I can continue my build. Unfortunately my Orcas will not be here for at least another week, so I won't be able to enjoy the system until then. Here is the final plot of attenuator data. The Goldpoint has virtually ZERO tracking error. In fact the resistors matched so well that the largest mismatch error was only .004db.

[drewh1]

Hi Randy - I am waiting for my kit and have appreciated reading about your progress.

Do you mind telling me what value you ordered for the Goldpoint Stepped attenuator?  I would like to get one and some knobs but am not such a newbie to this, I am not sure how to calculate the startgin value.

Thanks,

drew.

[rlyach]

Drew,

I just ordered the same value that the kit shipped with, 100K. I also ordered the smallest (1.1 inch) knobs. I think they look great. By the way, I finished my build today and everything works. I only had a pair of cheap RCA 5" two way speakers to play it through until my new Orca's arrive, but even with these inefficient $50 speakers, it sounded amazing. I will post a picture of the top side once my Orca's and the base arrive from Blumenstein Audio. Here is the underside. You can see some of the upgrades, including caps, resistors, the volume pot, and selection switch. I could not be happier with this amp. One thing that surprised me was my hum adjustment actually read 0.0mV on both channels. No hum at all.

[drewh1]

Nice Job, looks great and nice to hear it worked the first time. - I'll likely get the GP before I get the kit but wait on Cap upgrades. I want to see If I can actually hear a difference in the caps after I have listened for awhile.

I also appreciate hearing that it sounded good with your cheapie speakers, I have no worries this amp will drive my Green Mountain's adequately.

What do you have planned next?

[Grainger49]

Randy,

That is beautiful work.  It is so neat it looks like a military piece of equipment.  It makes my first Bottlehead build look awful. 

I took my time and built my Eros over a week's time.  It got a full treatment of heat shrinked STP ends with different colors for right and left.  I built stock to begin with.  Now I have trouble taking it out of the system to put in some upgrades I bought.  I just love listening to it too much.

[rlyach]

Thanks for the compliments on the build. The instructions deserve most of the credit. I just took my time. I also wanted to mention that the voltages came in very close to the expected values. The 2A3 cathodes were both right at 61 volts. The 2A3 plates were both exactly 360V. The B+ was 388V on both left and right channels. I also noticed that the plate voltages of the triodes in the 12AT7 did not match exactly. One was 214V and the other was 207V. I suspect the difference is because the 300K bias resistors in the C4S board did not match exactly. This wont matter though as the flat load line will keep the tubes output swing the same, and since they are AC coupled to the power stage the DC difference will not matter (correct me if I am wrong). It is very gratifying to see that the voltages matched the calculations and the simulations I ran using the published plate curves for the JJ tubes. Thanks again for all the encouragement. I think I may try and build the Toad guitar amp next. I thought about building speakers but I am not much of a wood worker.

[Paul Birkeland]

One was 214V and the other was 207V. I suspect the difference is because the 300K bias resistors in the C4S board did not match exactly.

That voltage difference is a matter of different emission from each triode of the 12AT7.

You could alter the 300K resistors to be 250K-350K and you'll still see the same plate voltages you have now.

[Jim R.]

Randy,

Glad you got it done and  it sounds good so far.  Just wait until you get those Orcas, and by the time the Orcas have their full break-in time, the amp too should be well seasoned and I'm predicting you're going to be amazed at what  you hear.  The JJs are really great tubes and you have to spend a whole lot more to better them, and even then, no telling how much of an improvement it may be to your ears.  Plus they're built like tanks and are run well under their dissipation limits so they should last a very long time.

-- Jim

[rlyach]

Thanks for the info on the 12AT7. I wonder if you can get one that has matched triodes? Yes, the JJ's are very nice tubes. The glass is very thick and the tubes look nice as well. One more piece of data. I used the min/max function of my DMM to measure the peak output voltage into my 8 ohm speakers at full volume. I measured 5.93V p-p. That calculates to 4.39 Watts of peak power from this amp. I realize this is a simplistic way of looking at it but it does match my simulation and hand calculations very well. I may get ambitious and hook up a sine wave generator to the input and measure the output for power and distortion. Thanks again for all the help understanding the inner workings of this amp. I have really enjoyed the building process.