 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
68.190.153.213
In Reply to: RE: JJ 300B Quality and Reliability? posted by MikeT on September 02, 2008 at 16:25:45
One of the pair of JJ300Bs that I had developed a heater to grid short after about 100 hours of use. I have had better luck with the gold grid EH300Bs that replaced the JJs.
Follow Ups:
Directly- Heated- Triode shorts are extremely rare unless the tube is run at higher than sensible currents and voltages, UNLESS there is shipping damage.Usually, elements are displaced in DHTs by damage done in shipping-- dropping boxes and rough handling of them by carriers. The tubes are packed well, and will almost never be broken, but internal elements will be somewhat displaced-- close to shorting.
For this reason, I buy tubes by one of two ways: (1) In person, using high-magnification equipment to see anything that is displaced-- even slightly. (2) Buying in bulk. If I need say--- 4 power tubes, then I will buy 12 or so, negotiating for a quantity pricing. I expect 4 of these to be good and usable. I DO NOT expect the other 8 to have survived shipping in very good shape. This simply DOES NOT HAPPEN with todays shippers/carriers. Your tubes WILL receive rough handling, and some WILL be internally damaged.
This is NOT brand-sensitive. What we see is "good batches" and "bad batches" from ALL manufacturers. People get good ones and rave about that brand. All that means is THAT box wasn't dropped.
Of course, some brands have better construction. Among modern-produced DHT types, I have found JJ 300B and 2A3 superior to all other current DHT production in being rugged. The "keep" rate on these is about 60%-- which is FAR better than average-- average is about 20-25%.
Smaller tubes with closer-spacing of supporting elements can stand more shipping abuse. Indirectly-heated tubes are MUCH more rugged about being dropped, as their build is tighter.
Is this a reason to avoid JJ DHTs? Certainly NOT. Buy enough of them to assure some good ones, and delight in their performance, and extremely long-life if run right. Right is about 2/3 of rated plate voltages and currents.
---Dennis---
What was the op point you were running the JJ, MikeyB??
Jeff Medwin
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---
| FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
||||||||||||||
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
