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"I have a TH30 here and two of the four EL84 operate at lower level than the other two. I have used a new quartet of EL84 TAD and I have measured the intensity directly through the plate in each one. There is an appreciable difference (8 or 10 mA approximately)".
I correct it: the EL84 quartet did not have a good matching. For some reason
I left the amp to examine it again and I just did it now.
The voltage measurements at the cathodes were 10.8 and 10.7V (in pairs, perfect) but on the plates were:
1. 30.8mA
2. 27.2mA
3. 31.0mA
4. 37.0mA
It was between tubes 2 and 4 where I saw those 10mA difference. Not between pairs. Sorry.
I have taken those same tubes and measured them in a Classic 30 (fixed bias) through the plates. The measures were:
1. 23.8mA
2. 22.0mA
3. 25.1mA
4. 31.6mA (!)
Itīs definitely a unmatched quartet that I will have to return to the distributor.
I installed another new quartet from TAD and the cathode voltage was: 10.6V in both pairs. On the plates were:
1. 30.7mA
2. 29.7mA
3. 31.2mA
4. 32.0mA
Finally: each cathode sees 75 Ohms (150 Ohms resistor assigned to each pair) and the screen grid resistors are all identical (470 Ohms, measured from node B to pin 9 in all tubes).
In the 2-tube mode, the 100K resistance appears between the cathodes of the inactive pair and ground (disconnected). Everything matches the schematic.
I correct it: the EL84 quartet did not have a good matching. For some reason
I left the amp to examine it again and I just did it now.The voltage measurements at the cathodes were 10.8 and 10.7V (in pairs, perfect) but on the plates were:
1. 30.8mA
2. 27.2mA
3. 31.0mA
4. 37.0mA
It was between tubes 2 and 4 where I saw those 10mA difference. Not between pairs. Sorry.
I have taken those same tubes and measured them in a Classic 30 (fixed bias) through the plates. The measures were:
1. 23.8mA
2. 22.0mA
3. 25.1mA
4. 31.6mA (!)
Itīs definitely a unmatched quartet that I will have to return to the distributor.
I installed another new quartet from TAD and the cathode voltage was: 10.6V in both pairs. On the plates were:
1. 30.7mA
2. 29.7mA
3. 31.2mA
4. 32.0mA
Finally: each cathode sees 75 Ohms (150 Ohms resistor assigned to each pair) and the screen grid resistors are all identical (470 Ohms, measured from node B to pin 9 in all tubes).
In the 2-tube mode, the 100K resistance appears between the cathodes of the inactive pair and ground (disconnected). Everything matches the schematic.
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