13GB5 challenges 300B!

조회 수: 6567, 2013-07-08 14:53:29(2013-03-02)

To test R-core OPT, I set up a textbook type 13GB5-4KE8 two stage amp.

My SMPS is designed to connect heater circuits in series  

The pentode section of 4KE8 is connected as a triode. It has a reasonably high mu, 

around 70. The triode section has a lower mu. Since I could not find any other 600ma heater tube, I decided to try this tube. It might work.

There are no large capacitors in the B+ return path. The B+ source impedance of SMPS is low and there is no AC power line ripple noise. Why do I need capacitors there?      . 

I set my output power target at 8 watts with 3V pp input.

(Excess gain if any, goes to NFB)

 

Carefully tweaking I was able to get 1% of THD at 11W.

This means two amp stage have a closely matching 2^nd Harmonic dominating distortion characteristics. When the phase of two stage is reversed, the matching distortions can compensate each other by canceling out 2^nd harmonics thus restoring original wave form at the output.

A few spot checking has verified that the remaining 2^nd Harmonics are very low and the 3^rd Harmonics are dominating.

Please note the picture below. The distorted input wave form (output of driver tube) is to compensate or cancel out the Power stage distortion. 

The purpose of this test is to check the gap setting and power handling capability of the R-core Output transformer. The gap setting , power handling – all OK.

 

For the 13GB5, the data book says Pd max= 17W and Pd sg max = 6W.

On top of that I can go 20%  further without hesitation.  ------ ($1 tube)--

New maximun Pd is : Pd = (17+6) x 1.2 =27.6 W

Thanks to the low rp value of the 13GB5, the plate conversion efficiency could go as high as 40%,

 

Pout = 27.6 x 0.4 = 11.04 W 

Though this amp has very low distortion, the residual distortion components are primarily 3^rd Harmonics. The sound is flavored by the 3rd Harmonics and it approaches that of a low distortion transistor Amp.

Knowing the level of 3rd harmonic is very important  for re- tuning distortion spectrum to create a 2^nd Harmonic rich tube amp. This issue will be separately addressed  later .

During the test, I have noticed

1) Unusually heavy loading effects of 13GB5 to the driver. 

    The power stage is supposed to operate as an A1 class (no grid current).

2) Strangely, the plate DC current increases as the power output increases..

 

                                                      Send question to:  kdkang@gmail.com

**************FYI*************

Q: 박사님 6dB NFB 라면 NFB 후의 이득이 NFB전의 이득에 절반이 되도록 한다는 의미입니까?

6dB (2배)라는것은 상대적인 개념인데 어느곳을 기준으로 6dB인지 잘 모르겠습니다. 
또 실제로는 -6dB 가 되는것이 아닐까 합니다.

A: 이득이 반이돼면 -6db,가되고 +6db면 2배 가됍니다. 
얼밀히 따지면 NFB 6db 면 -6db 를 의미하고 
NFB -6db 는 +6db 로 볼수있습니다. 
보통 +/- 상식으로 알아보고 특별히 기준점을 명시 안하면 측정대상의 전후치를 비교한 것 입니다.