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76.0.20.186
In Reply to: RE: JJ 300B Quality and Reliability? posted by mikeyb on September 02, 2008 at 20:22:44
What was the op point you were running the JJ, MikeyB??
Jeff Medwin
Follow Ups:
Vp = 425 Volts, Ik = 70 mA, Pdiss = 30 Watts. Fixed bias. Push-Pull. Lundahl 1620 output transformer set for 6K P-P.
In truth, the operating point shifts about +/- 10% with line voltage because the bias supply is regulated and the B+ supply is not.
One of these days I'll fix that.
Hi, Mikeyb.
Please forgive my use of too many CAPS! I'll try to talk better next time.
Here's something that the JJ 300B will really take a shine to: Plate voltage across tube: 375. Plate current: 53 ma. Adjust bias to get it.
You can expect far greater dynamics and wider, much cleaner bandwidth.
If you also provide low-DCR power to the tube and its driver, and use minimal sized chokes and capacitors-- of very high quality-- watch out!
You'll start playing World-Class.
---Dennis---
I'll ignore the low DCR reference. By the way, the driver supply (+250) is regulated, with a low dynamic impedance.
Here is my concern about runing a 300B at only 53 mA. To do that, you need to have a relatively high negative grid voltage, which moves the operating point toward 'pinch off'. Not 'centered'. So you will be in danger of running into class AB rather than remain in class A, and in a less linear region.
Correct, but not relevant unless you intend to run the amp at max. power output-- which would imply using very inferior (under 95db/watt) speakers.
If you are trying to do this, then drop the plate voltage into a more normal range.
Let's take a look at two operating points for the 300B. I am using JJ tube data and also data from Western Electric, so as you read this, kind of interpolate between the two sources of information. This will help you to visualize what's going on.
Let's consider operating point number one: Here, we are running the tube at 375 VDC across it, and 53 ma. of plate current. This combo shows one of the least total distortion ratings in the old W.E. data-- a kind of interpolation between reduction of both 2nd and 3rd harmonics. I take a hard look at 3rd, especially.
This op. point-- 375 VDC across the tube, and 53 ma. plate current is both low-distortion and linear as long as you don't push the amp beyond about 5 watts. This combo must run into a 5K output transformer in order to work, and you can get about 6.4 watts out of it. But, you DON'T try to get above about 5 watts. Simply choose your driver to limit the amps signal output to 5 watts.
The other op. point is also one of the most linear and low-distortion for the 300B. Here, we are going to run the tube with 250 VDC across it, at 60 ma. plate current. This is to be run into a 2.7K output transformer. This is the setup I like. You can get 4 watts out of this. This amp can be pushed much harder and still maintain its composure. Note that the grid bias is much closer to your idea.... around (minus) -46 volts or so.
What would be the applications for these amps? The first one would be used with very high-Eff. speakers, and would sound big, bold, terrific, and would have great "fun" or "jump-factor". But only on the right speaker.
The second one would operate well on anything it could drive, and would sound accurate and linear, and be well- composed.
Pull-up the tube data on these for more help in following all this.
There are many things you can do to get music to sound great in your own setting. You can use the rules you actually need, and not use the ones that don't apply to your situation.
The new JJ 2A3-40 is now the "300B" of choice-- its filament voltage is a huge advantage, and the 2A3 set-up grid structure and amp-factor are also better than any 300B. This new tube greatly outperforms 300B tubes.
---Dennis---
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