voltage readings of my latest JTM45/100

[Dom]

Hi guys,

I've just fired up my latest build and here are the voltage readings. I guess I've some odd voltages on V3:
V1: (1)246 (3)1.95 (6)234.5 (8)1.95
V2: (1)190 (3)1.37 (6)350 (7)190 (8)192
V3: (1)227 (2)45 (3)59 (6)215 (7)44 (8)59
heater voltage:6.35 (V1-4), 6.37 (V5), 6.39 (V6), 6.4 (V/)V

V1 is Tungsram, V2 Mullard I61, V3 Tele SP. Also tried to swapp tubes on V3 but the voltages didn't change so much.

230V tap (wall voltage: 226.5)

Power tubes are TAD KT66:
V4: Pin3: 498V, Pin4: 498V, Pin5: -59V, Pin6: 499
V5: Pin3: 498V, Pin4: 498.5V, Pin5: -59V, Pin6: 498
V6: Pin3: 496V, Pin4: 498V, Pin5: -59V, Pin6: 499
V7: Pin3: 497V, Pin4: 498V, Pin5: -59V, Pin6: 498
Biased at 31mA for the highest tube

Anyway, I've plugged it into my speaker cab. So far so good, no strange hiss, no much noise.
Advice for V3 voltages appreciated.

Cheers

Dom

[Ydna]

What parts did you use for the amp? Chassis, Headbox, and Transformers?

[Roe]

Hi guys,

I've just fired up my latest build and here are the voltage readings. I guess I've some odd voltages on V3:
V1: (1)246 (3)1.95 (6)234.5 (8)1.95
V2: (1)190 (3)1.37 (6)350 (7)190 (8)192
V3: (1)227 (2)45 (3)59 (6)215 (7)44 (8)59
heater voltage:6.35 (V1-4), 6.37 (V5), 6.39 (V6), 6.4 (V/)V

V1 is Tungsram, V2 Mullard I61, V3 Tele SP. Also tried to swapp tubes on V3 but the voltages didn't change so much.

230V tap (wall voltage: 226.5)

Power tubes are TAD KT66:
V4: Pin3: 498V, Pin4: 498V, Pin5: -59V, Pin6: 499
V5: Pin3: 498V, Pin4: 498.5V, Pin5: -59V, Pin6: 498
V6: Pin3: 496V, Pin4: 498V, Pin5: -59V, Pin6: 499
V7: Pin3: 497V, Pin4: 498V, Pin5: -59V, Pin6: 498
Biased at 31mA for the highest tube

Anyway, I've plugged it into my speaker cab. So far so good, no strange hiss, no much noise.
Advice for V3 voltages appreciated.

Cheers

Dom

looks good except v3 which is a little low. recheck wiring of the pi, esp. plate resistors and katode (470). are you using a 8k2 or a 10k voltage drop resistor between pi and screens?

[Dom]

Ok, I've made some measurements. I've checked this amp against my first JTM45/100 and found some odd differences when measuring the PI resistors without desoldering them:
The 100K PI resistor has 92K and the 82K has read 97K.
Before assembling the parts I've double checked every part so that they're close to nominal specs.

Here's a comparison chart between the two amps :

PI filter: 432V (amp 1) ; 426V (amp2)

Dropping resistor: 79V; 8.55K (amp1); 86V ; 8.3K (amp2)
(measured across the resistor)

100K PI R: 146V ; 110K (amp1) ; 187V ; 92K (amp2)
82K PI R: 153V; 80K (amp1); 179V; 97K (amp2)

220K bias resistors: 0.002-0.005V (amp1); 0.08-0.09V (amp2) BOTH WITHOUT POWER TUBES!

10K resistor : 10.4K (amp1); 9.8K (amp2)
470 cathode R: 510 amp1; 465 (amp2)

V3 voltage chart of amp #1:
Pin1: 278V, Pin2: 31.5V; Pin3: 50.7V; Pin6: 286V; Pin7: 33.5V; Pin8: 50.7V
Both amps were tested w/o power tubes, screen filter voltage was 515V on both, and with tubes both have 498-500V.

Any ideas? Perhaps leaking coupling caps ?

Regards

Dominik

[neikeel]

I would not be too troubled .

The readings are not so far out and could be down to quality of solder joints, different solder, slight variation in the resistors, as you have documented.

[Dom]

hi guys

the voltages of V3 are in specs now.
Culprit was a faulty 47pF PI cap.
Many thanks to Brian Wallace for taking time for his 'tele diagnosis'

Cheers
Dom