What 300b's are you running?

[ditdah]

Anyone have any experience with the KR audio 300b?

Deb

Please buy a pair and give us a comparison to your PSvane tubes. I've been on the fence with both pair..

I'm actually really happy with the EH gold grids. Mind you, I have nothing to compare them to......

The KR tube spec for the 300b WE clones are as follows:

Maximum Plate Voltage      550 V 
 Maximum Plate Current      120 mA 
 Plate Dissipation      50 Watts 
 Va      450 V 
 Ia      100 mA 
 Vg      -90 V 
 Transconductance      6.2 mA/V 
 Amplification Factor      3.9 
 Filament Voltage      5 V (DC/AC) 
 Filament Current      1.2 A 
 
Should I expect these to be acceptable for the BeePre based on the spec?

for comparison...here's the EML 300b specs:

300B Filament Ratings
Filament Voltage   5 Volt (AC or DC)
Tolerance on filament voltage   4%
Filament Current   1.3 Ampere

300B Maximum Conditions  Not possible as operating point
Plate Voltage   450Volt
Plate Current   100mA
Power Output in Class A    17.8 Watt
Plate Dissipation   40Watt (note5)
Grid resistor with Auto Bias   250k Ohm (note4)
Grid resistor with Fixed Bias   50k Ohm (note4)

300B Factory Test conditions
Plate Voltage   300V
Plate Dissipation   17Watt
Plate Current   56mA
Grid Voltage   -58Volt
Plate Impedance   700 Ohm
Amplification Factor   3,9
Transconductance   5.6 mA/V

[Paul Joppa]

...
As regards EMLs, I will let PJ weigh in on that. He has had some discussion with Jac about the requirements of the EML tubes and I'm not sure if they are compatible with the regulation method we use to get the S/N ratio as good as it is.

Jac is naturally disappointed that the design is not at present flexible enough to accommodate his tubes - well, so am I! The mesh especially has such a great reputation that I would totally love to hear it.

As I have told Jac, I will continue to pursue a design revision - as Doc B said, we have had to do quite a bit of research and have wound up with a more complex and less flexible circuit than we would prefer. Jac has a lot of analysis, good ideas, and good advice for designers on his web site, as well.

I do have a research plan for this issue. It will have payoffs (hopefully) for several upcoming products, including the BeePre. I won't promise a payoff any faster than the DAC project, now 2 years and counting, though. If you can predict or schedule the outcome, then it isn't research!

Meanwhile, yes the regulator has enough current headroom for 1.3 amps. I've done some calculations that suggest a certain amount of flexibility, but I'm not ready to specify anything except the stock tube until I have an adequate body of actual measurements - there are too many variables that seem small but may or may not add up.

For those who have good meters (1% accuracy or better), you might measure the voltage at the filament pins (1 and 4) of the 300Bs and report them. I'll accumulate the body of data, which will help me validate my analysis and also assess parts and build variability.

[johnsonad]

Thanks Paul! I'll get you a measurement tomorrow. If 1.3 Amps turns out to be an option I will take EML solid plates over anything else based on only my great experience with his other tubes. 

[corndog71]

I would be curious to hear someone's impression of Valve Art 300Bs in the BeePre.  I've enjoyed their EL34 and EL84 tubes in my dynaco amps. 

[johnsonad]

For those who have good meters (1% accuracy or better), you might measure the voltage at the filament pins (1 and 4) of the 300Bs and report them. I'll accumulate the body of data, which will help me validate my analysis and also assess parts and build variability.

Here are mine:
EH Gold Grid tubes
Line: 120.2
B4: 9.88
B1: 5.04
A4: 9.82
A1: 4.99

edited for poor typing....

[xcortes]

I don't think the line voltage is relevant. Still good for you to see 120v. Mine is so high I can't connect anything directly.

[Paul Joppa]

Thanks, Aaron. I see 0.6% difference in the regulated voltage, 0.6% different and within spec since the voltage-set resistors are 1% parts. The means the voltage loss in the wiring is very small, and the solder joints are good.

The A tube is getting 4.83v, and the B tube is getting 4.44 volts (by subtraction). Both of these might go up as the tube comes to full temperature. You can swap the tubes and see if the difference moves with the tube. If it moves with the tube, then the B tube is drawing more current - 1.36 amps if the 8 ohm resistors are spot on and your measurements are exact. If it doesn't move with the tube, then either the 8 ohm resistors are off a bit (they are supposed to be 1% parts) or there's a questionable connection. Most meters can't measure low resistances with sufficiently reliable accuracy so I don't suggest making that measurement.

[johnsonad]

Thanks Paul. This was with only a minute or two of warm up. B tube was slowly drifting down as it warmed up. I will try it again tomorrow and repost the results. I didn't use stock resistors but Mills 8 Ohm 1% parts.  That doesn't mean they are not off though.

[Paul Joppa]

Hard to go wrong with Mills; good choice. I'm quite interested in any measurements, since we have very little experience with this "in the field" as yet. I was startled at the high current/low voltage at first, but as I thought about it I realized that the filament doesn't come up to full temperature until the plate has had enough time to radiate its heat to the filament - and the filament resistance increases with temperature.

I plan to be at BottleHeadquarters next week, and will try to get some data on Doc B's preamp.

[debk]

Is there a specified amount of warm up time you would like us to have before making measurements so as to not introduce any other variables into the data?

I'll try to get data on mine this weekend.

Deb

[johnsonad]

Today's data with 20 mintues of warm up:

EH Gold Grid tubes
B4: 9.88
B1: 5.02
A4: 9.82
A1: 4.98

After reviewing my notes from yesterday I messed up on the B1 voltage.  The previous thread has been edited. There is very little difference from the intial results after a couple of minutes and at the 20 minute mark.

[Grainger49]

At 5 minutes everything should be settled out.  Then listen for half an hour.  See if the measurements moved.  I doubt they will move by as much as 2%.

[Paul Joppa]

Thanks, Aaron - good data. The tubes seem to be getting 4.84-4.86 volts which is 3% low, well within the usual +/-10% and the more strict +/-5% that premium tubes often specify.

EML specifies +/-4%. For a quick estimate, let's assume your Mills resistors are exact and that your meter is also exact. Then averaging the two tubes, your actual current would be 5.00v/4 ohms, or 1.25 amps. If you increased to 1.3 amps, the resistor drop would be 5.2 volts, leaving 9.85-5.2=4.65 volts which is 7% low - below the EML spec.

As you can calculate, a 56 ohm resistor in parallel with the two 8-ohm resistors would, in this case, give 5.00 volts on the 300B filament. I am not recommending this, at least not yet, and I won't be making any recommendations until we have a greater variety of data on the variability from one build to another. But you can see where I'm headed. I expect this will prove to be better than adjusting the regulated voltage, which is a delicate balance between the available voltage and the heat wasted in the regulator.

[johnsonad]

Thanks Paul.  I'm all ears as this continues to develop. Thank you for giving your time to this pursuit.

[debk]

I just got a pair of KR 300b and am listening to them now.  Compared to the Psvane the KR's sound a little more open/spacious and more accurate. The bass is about the same, but the highs are more clear.   I prefer them to the Psvane.

The only drawback is that the top is a little wider than the Psvane so the lead damper rings don't fit as well.

I am very glad I spent the money to get them.  They are on sale at TubeDepot for $599 a pair

Debra