John Broskie's Guide to Tube Circuit Analysis & Design
 

13 May 2006

High-Octal Power


Aikido PCBs and tube sockets available at the GlassWare Yahoo Store

New and old stuff
The newest printed circuit board added to Aikido line up is a mono board for octal triodes, such as the 6AS7, 6BL7, 6BX7, 6SL7, 6SN7, 12SL7, 12SN7, 12SX7, 5691, 5692, 6080, B65, ECC32, ECC33… The boards are the same size as the 9-pin mono Aikido boards (4″ by 6″) and made from the same thick (0.093″) PCB stock and heavy copper (2oz) traces as the stereo boards. The boards hold a single power-supply-bypass capacitor and the Aikido-power-supply capacitor and two-resistor voltage divider resistors that sample the power supply noise and relay a portion to the output tube’s bottom triode. Like the stereo boards, the mono octal boards also hold two output coupling capacitors and provide a jumper to hardwire the two outputs together. These boards sell for $24 each. If you need five of them, then the price is $99 for five of them. (Yes, there those running five channel; I have already sold a few sets of five 9-pin Aikido mono PCBs.)

Since these boards follow the same rules as the 9-pin mono boards, please read the previous blog entry for more information.

 

Aikido tubes: results and values
I keep getting requests for more help in selecting tubes and cathode resistor values for the Aikido line stage amplifier. As many of you can guess, I am not about to say that one tube or one set of resistor values is perfect. (What separates me from so many other tube gurus, aside from my not considering myself to be a guru, is my steadfast refusal to believe in one tube, one resistor value, one topology, one capacitor type…) So, instead of one quintessential archetype, I’ll give many design examples. I have compiled a table of tubes and values and results.

Tube
mu
rp
Rk
Ik(mA)
B+
R15
R16
Input Gain
Input Gain dBs
Output Gain
Output in dBs
Zo
6AQ8
57
9700
100
10.0
300
93220
100k
28.1
29.0
0.97
-0.24
248
6AS7
2.23
234
55
100.0
100
5437
100k
1.1
0.9
0.60
-4.47
95
6AS7
2
310
205
75.0
150
0
100k
1.0
0.0
0.61
-4.28
220
6AS7
1.87
441
530
50.0
200
0
100k
0.9
-0.6
0.61
-4.24
456
6BK7
43
4600
200
10.0
300
91111
100k
21.2
26.5
0.97
-0.27
279
6BL7
14.80
3140
196
10.0
150
76190
100k
7.4
17.3
0.91
-0.83
343
6BL7
15.40
2470
94
20.0
200
77011
100k
7.7
17.7
0.91
-0.86
219
6BL7
15.40
2540
165
20.0
250
77011
100k
7.7
17.7
0.91
-0.79
283
6BL7
15.90
2200
114
30.0
300
77654
100k
7.9
18.0
0.91
-0.79
219
6BQ7
38.00
5900
191
10.0
300
90000
100k
18.7
25.5
0.96
-0.32
311
6BS8
36.00
5000
220
10.0
300
89474
100k
17.8
25.0
0.96
-0.33
321
6BX7
8.96
1760
267
10.0
100
63504
100k
4.5
13.0
0.87
-1.24
370
6BX7
9.44
1420
182
20.0
150
65035
100k
4.7
13.5
0.87
-1.21
273
6BX7
9.80
1270
158
30.0
200
66102
100k
4.9
13.8
0.87
-1.16
239
6BX7
10.10
1170
147
40.0
250
66942
100k
5.0
14.0
0.88
-1.13
220
6BX7
9.52
1730
542
20.0
300
65278
100k
4.7
13.5
0.89
-1.04
565
6CG7
20.50
10200
583
3.0
150
82222
100k
10.0
20.0
0.93
-0.59
827
6CG7
21.10
8960
397
5.0
200
82684
100k
10.4
20.3
0.93
-0.59
657
6CG7
21.00
9250
626
5.0
250
82609
100k
10.3
20.2
0.94
-0.56
820
6CG7
20.80
9840
1000
4.5
300
82456
100k
10.1
20.1
0.94
-0.53
1063
6CG7
21.40
8370
470
7.3
300
82906
100k
10.5
20.4
0.94
-0.56
686
6CG7
21.90
7530
243
10.0
300
83264
100k
10.8
20.7
0.93
-0.60
489
6CG7
21.80
7680
352
10.0
350
83193
100k
10.7
20.6
0.94
-0.57
576
6DJ8
30.20
3670
182
5.0
100
87578
100k
15.0
23.5
0.96
-0.39
273
6DJ8
30.70
2870
124
10.0
150
87768
100k
15.2
23.7
0.96
-0.39
199
6DJ8
30.00
2960
205
10.0
200
87500
100k
14.9
23.4
0.96
-0.37
274
6DJ8
29.60
3060
291
10.0
250
87342
100k
14.6
23.3
0.96
-0.36
350
6DJ8
28.60
3980
673
5.0
250
86928
100k
14.0
22.9
0.96
-0.35
667
6DJ8
28.30
4080
845
5.0
300
86799
100k
13.8
22.8
0.96
-0.34
787
6DJ8
28.90
3400
481
8.0
300
87055
100k
14.2
23.0
0.96
-0.35
511
6FQ7
See 6CG7
6GM8
14.00
3400
187
2.0
24
75000
100k
7.0
16.8
0.90
-0.90
357
6H30
15.40
1140
69
20.0
100
77011
100k
7.7
17.7
0.91
-0.80
127
6H30
15.9
1040
74
30.0
150
76471
100k
7.9
18.0
0.92
-0.75
124
6H30
15.40
1310
221
20.0
200
90431
100k
7.7
17.7
0.92
-0.68
267
6H30
15.40
1380
294
20.0
250
89474
100k
7.7
17.7
0.93
-0.66
330
6H30
15.00
1670
530
15.0
300
89189
100k
7.4
17.4
0.93
-0.65
528
6N1P
39.8
12200
328
3.0
200
89189
100k
19.4
25.8
0.96
-0.32
539
6N1P
36.00
9480
221
5.0
250
75000
100k
17.7
25.0
0.96
-0.36
422
6N1P
35.00
956
642
5.0
300
94444
100k
17.1
24.7
0.97
-0.25
569
6N27P
14.00
3400
187
2.0
24
75000
100k
7.0
16.8
0.90
-0.90
357
6SL7
70.00
43000
1000
1.3
300
94444
100k
31.4
29.9
0.98
-0.17
1174
6SN7
20.50
10200
583
3.0
150
82222
100k
10.0
20.0
0.93
-0.59
827
6SN7
21.10
8960
397
5.0
200
82684
100k
10.4
20.3
0.93
-0.59
657
6SN7
21.00
9250
626
5.0
250
82609
100k
10.3
20.2
0.94
-0.56
820
6SN7
21.90
7530
243
10.0
300
83264
100k
10.8
20.7
0.93
-0.60
489
6SN7
21.10
9000
680
5.8
300
82684
100k
10.3
20.3
0.94
-0.54
846
6SN7
21.40
8360
470
7.2
300
82906
6SN7
20.80
9840
1000
4.5
300
82456
100k
10.1
20.1
0.94
-0.53
1063
9AQ8
See 6AQ8
12AT7
60.00
15000
270
3.7
200
93548
100k
29.1
29.3
0.98
-0.21
457
12AU7
17.00
9560
427
2.5
100
78947
100k
8.4
18.4
0.92
-0.75
757
12AU7
16.60
9570
741
3.0
150
78495
100k
8.1
18.2
0.92
-0.71
959
12AU7
16.70
9130
768
4.0
200
78610
100k
8.2
18.2
0.92
-0.69
959
12AU7
17.90
7440
336
8.0
250
79899
100k
8.8
18.9
0.92
-0.71
601
12AU7
18.10
7120
328
10.0
300
80100
100k
8.9
19.0
0.92
-0.70
581
12AV7
37.00
6100
120
9.0
200
89744
100k
18.3
25.3
0.96
-0.36
258
12AV7
41.00
4800
56
18.0
300
90698
20.4
26.2
0.96
-0.35
160
12AZ7
See 12AT7
12AX7
100.00
80000
2000
0.5
200
96078
100k
39.0
31.8
0.99
-0.11
1719
12AX7
100.00
62500
1100
1.0
300
96078
100k
42.6
32.6
0.99
-0.12
1238
12B4
6.50
1050
138
15.0
80
52941
100k
3.2
10.2
0.82
-1.77
233
12B4
6.54
956
138
20.0
100
53162
100k
3.3
10.3
0.82
-1.74
222
12B4
6.80
736
96
40.0
150
54545
100k
3.4
10.6
0.82
-1.70
163
12B4
6.70
770
178
40.0
200
54023
100k
3.3
10.5
0.83
-1.58
233
12B4
6.46
925
383
30.0
250
52719
100k
3.2
10.2
0.84
-1.50
407
12B4
6.36
997
499
30.0
300
52153
100k
3.2
10.0
0.84
-1.49
497
12BH7
16.10
5480
340
4.0
100
77901
100k
8.0
18.0
0.92
-0.76
549
12BH7
15.70
6090
706
4.0
150
77401
100k
7.7
17.7
0.92
-0.71
826
12BH7
15.90
6140
787
5.0
200
77654
100k
7.8
17.8
0.92
-0.68
877
12BH7
17.40
4870
383
10.0
250
79381
100k
8.6
18.7
0.93
-0.67
541
12BH7
18.40
4300
267
15.0
300
80392
100k
9.1
19.2
0.93
-0.65
422
12BZ7
100.00
31800
300
96078
100k
48.5
33.7
0.98
-0.17
292
12DJ8
See 6DJ8
12FQ7
See 6SN7
12SL7
See 6SL7
12SN7
See 6SN7
12SX7
21.20
8750
218
5.0
80
82759
100k
10.5
20.4
0.93
-0.64
519
5687
18.10
1760
37
24.0
150
80100
100k
9.0
19.1
0.91
-0.78
119
5687
17.50
1970
132
20.0
200
79487
100k
8.7
18.8
0.92
-0.68
216
5687
17.40
2060
198
20.0
250
79381
100k
8.7
18.7
0.93
-0.65
276
5687
16.90
2440
397
15.0
300
78836
100k
8.4
18.5
0.93
-0.62
455
5691
See 6SL7
5692
See 6SN7
5751
70.00
58000
1250
0.8
200
94444
100k
30.5
29.7
0.98
-0.17
1407
5963
21.00
6600
200
10.0
250
82609
100k
10.4
20.3
0.93
-0.63
433
5965
47.00
7250
220
8.2
300
91837
100k
23.1
27.3
0.97
-0.26
337
6072
44.00
25000
1250
2.0
300
91304
100k
20.3
26.2
0.97
-0.25
1272
6080
See 6AS7
7119
21.70
2390
324
15.0
300
83122
100k
10.7
20.6
0.95
-0.48
377
B65
See 6SN7
ECC32
See 6SN7
ECC33
35.00
9700
89189
100k
17.3
24.8
0.95
-0.48
248
ECC81
See 12AT7
ECC82
See 12AU7
ECC83
See 12AX7
ECC85
See 6AQ8
ECC86
See 6GM8
ECC88
See 6DJ8

The table above lists many triodes suitable for an Aikido amplifier (including a few that won't work with the PCBs, like the 12B4). The same tube is listed with different B+ voltages and cathode resistor values. Two gains are listed: the first is the gain the tube realizes in the input position in the Aikido; the second is the gain of the tube in the output stage. To calculate the final gain we must multiply the two voltage gains together (or add the gain in dBs together). For example, given an Aikido line amplifier with a B+ voltage of 300V, and a 6CG7 input tube with cathode resistors of 1k, and a 6DJ8 output tube with cathode resistors of 481 ohms, the final voltage gain equals 10.1 from the 6CG7 against the 0.96 gain of the 6DJ8, with a product of 9.7. or, working with dB instead, 20.1dB plus -.35dB, for a total of 19.75dB. (Aren’t decibels great?) If you have additional data, send it in and I’ll add to the list.

 

Aikido and 300B amplifiers
Once again I was overly pessimistic. Many want to build a single-ended, 300B-based amplifier that uses the Aikido amplifier as the front end. My guess was that the 300B would require too much voltage swing from the Aikido stage. Well, I now think that I was wrong. I performed a quick experiment: I threw together an Aikido amplifier (nothing like having a stack of PCBs to play with) that used a 12AX7 as the input tube and a 12AU7 as the output tube. I then fed the Aikido a 1kHz signal. At 1V out, the Aikido is distortionless, at least as far as my eye can tell (I couldn’t use my FFT setup and I believe the eye can see down to about 3% distortion with sinewaves). I then drove the 12AX7 to a 70V output. I expected to see grid conduction and obvious distortion; in fact, I saw neither. It still looked mighty clean and the cathode still had a 0.25V positive advantage over the grid.

Now for some “howevers.” At first I used a B+ voltage of 400V, which made me nervous, so I then lowered the B+ voltage to 300V and I could see a hint of distortion, maybe as much as 1%. I found that the two safety resistors, R5 and R6, were dragging down the gain too much, so I increased their value to 10M. At first I used 1k cathode resistors for the 12AX7, but they had to be upped to 2k to get the best results. I forgot to write down the input signal level, but I remember it being about 1.7Vpk, which is a tad too high for passive line stages, but would be no problem for any active line stage. Below is the experimental circuit.


Now all that is needed is a 300B output stage. I would use two 300Bs in parallel with a B+ voltage of 440Vdc and a one-electron UBT1 output transformer. Why? Using two output tubes means more watts and allows me to use the UBT1 transformer, which I already own. (Actually, the UBT1 is a great transformer, as it has a low winding ratio that allows it to pass an undistorted signal. Usually the higher the winding ratio, the more headaches.) I know that many readers do not approve of parallel output tubes, so here is a single 300B version. 


 

Next time
I hope to cover unity-gain power buffers for driving speakers.

 

 

//JRB

 

www.tubecad.com           Copyright © 1999-2006 GlassWare           All Rights Reserved