2nd Harmonics Rich 5AQ5 PP Amp

조회 수: 2504, 2013-03-24 14:53:57(2013-03-23)

 

My test circuit should produce 10W of power in a PP topology. 
6V6 PP - P/K split resistor inverter -voltage Amp with a mu of 70 will do the job.

After a long search, I settled with a 9-pin 6LF8 compactron. When the pentode section is connected

 in triode mode it has mu = 25 and rp = 5 Kohm (checked with my curve tracer). 

It will work fine for the phase inverter.

The other triode in the same tube has mu of  70 which is adequate  with a little to spare for  the NFB. 

Two 6V6s were replaced with 5AQ5s in order to make all 600 ma heater system.  (My SMPS requirements)

2 separate variable “C” supply are used for PP tube DC balance. 
Because the OPT core which is under test has no gap, balancing DC is important.  

No attempts are made to match the 5AQ5s used.

 

Every body is talking/selling “Matched pair” tubes. 
For  matching power tubes,  entire operating range should be considered.

One spot matching does not help much.
 
During THD testing, The THD level stayed high, much higher than I expected.

Though it stayed below 1%. I checked the measuring equipments. They are all OK.
The bias were set at Ip=45 ma on both tubes. 
I suspected the the high THD is primarily 2nd Harmonics.

There are two sources: 

(1) the PP circuit is not cancelling out  its own 2nd Harmonics and

(2) the signal feeding the phase inverter has high level of 2nd Harmonics.
The inverter itself has no gain. It is a 100% NFB amp stage.

I think the inverter stage, at least, at 1W power level is very linear.  

 

I measured 2nd and 3rd Harmonics at the out put terminals. 
Sure enough, the 2nd Harmonic is dominating.

At 12W output, the 2nd /3rd Harmonic ratio is 8.

At 0.2-1W it is 3-5 
Since the THD is less than 1%,  the harmonics spectrum may not play much

on the “How it sounds”

Anyway, this is an easy way to make 2nd Harmonic Rich Amplifier..  

 

R-core SE 용 OPT 를 Gap을 없에고 Push Pull 회로를 만드러서 Test 해 보았습니다.

THD 1% 이내에서  2차 고조파가 많이 나오는 Amp 가 쉽게 만드러 졌습니다.

그러나 THD 가 0.5% 내외라서 2nd 다 3rd 다 해 보았자 일반 사람들에게는 모두 비슷한 소리로 듣길 것입니다.