My first mesuring with chinese 12au7 from my desk and a pair of 6550. I found it a little "low" at many measuring point.
R70 is 0.27 Ohm (against 0.47 stock value), D7 is DF210G (bridge rectifier).
R70 is 0.27 Ohm (against 0.47 stock value), D7 is DF210G (bridge rectifier).
| Measuring Point | Reference Value | Measuerd DC Value | |||
1 | 2 | 3.2 | 3.7 | 2.8 | 3.2 |
3 | 4 | 3.2 | 3.7 | 2.3 | 3.3 |
5 | 6 | 100 | 120 | 106.9 | 96.1 |
7 | 8 | 100 | 120 | 106.1 | 94.5 |
9 | 10 | 195 | 220 | 186.8 | 186.8 |
11 | 12 | 1 | 1.2 | 1.1 | 1.1 |
13 | 14 | -12 | -24 | -17.7 | -17.6 |
15 | 16 | 265 | 290 | 251 | 251 |
17 | 18 | 0.4 | 0.55 | 0.47 | 0.47 |
19 | 20 | 255 | 280 | 231 | 232 |
21 | 22 | 250 | 275 | 235 | 232 |
23 | 24 | 255 | 280 | 243 | 245 |
25 | 270 | 300 | 260 | ||
26 | -29 | -35 | -29.1 | ||
27 | -30 | -36 | -30.1 | ||
28 | 5.6 | 6.4 | 5.3 | ||
29 | 6 | 6.8 | 5.5 |
I checked with other two measuring tool, it is accurate. The house has 3 phase, it looks this room (where I build the amp) is on that phase which is low, other rooms have 233V and above.
Yesterday I rolled my 6550C to a pair of 807 tubes. The amp was used with speakers with moderate volume. I did not notice any abnormal. It was absolutely good sound.
Today I used it again and after a cca. 3/4 hour I have heard a strange "effect" from my right speaker: it was such as "distortion". I came to the left speaker and there was a much more strange effect: like the sound was "looped, echoed". The led on the faceplate stayed blue but I switched off the amp.
After some time I swapped the tubes to my 6L6 and played music ~2 hour long without any "problem".
Today I used it again and after a cca. 3/4 hour I have heard a strange "effect" from my right speaker: it was such as "distortion". I came to the left speaker and there was a much more strange effect: like the sound was "looped, echoed". The led on the faceplate stayed blue but I switched off the amp.
After some time I swapped the tubes to my 6L6 and played music ~2 hour long without any "problem".
Hello Victor!
Do you mean "cca"? It is my fault, I just tried to express the time. Cca was just this: "circa" .
Yes, I have got it.
The driver tubes were 12au7 and the power tubes were the mentioned 807 when the strange echo effect appiered.
After nearly 50 hours of listening to music, I am completely satisfied and happy with the resulting sound.
Only with the 807 tubes did I experience an audibly disturbing distortion at high volume, but I suspect that the tubes may be bad, although they sound great at low & normal volume.
What piques my consciousness, however, is the low heater voltage values in my previously posted table. I strongly assume that these two values "must" be around 6V, but the 5.3V and 5.5V values measured with 6550C and 12AU7 pairs seem far to me.
Only with the 807 tubes did I experience an audibly disturbing distortion at high volume, but I suspect that the tubes may be bad, although they sound great at low & normal volume.
What piques my consciousness, however, is the low heater voltage values in my previously posted table. I strongly assume that these two values "must" be around 6V, but the 5.3V and 5.5V values measured with 6550C and 12AU7 pairs seem far to me.
Heater voltage for these tubes is 6.3V +/- 10% so any value below 5.6V is lower than normal and may affect the lifetime of the tubes.
The question did not let me rest, especially after your answer, so I took the measurements at the place of use. I got values of 6.28V(position 28) and 6.4V(position 29). It is true that the 807 tubes were in it and not the 6550C ones. Well, I also measured 232V in the outlet here. Now everything seems to be fine, not just based on the sound (except for the distortion of the 807s at high volume).
Thank you Ferenc once again!
Is there a way to increase the Heater Voltage? I replaced the 3300uF capacitors with United Chemi-Con 16V 8200uF (this was in stock at vendor and they just fit), the original 1510 rectifier of position D7 was replaced with Comchip DF210-G, while the PTC6 was replaced with the recommended Littlefuse RHEF400-2.
I have a good guess that the secondary of the transformer supplies 6V AC, this is rectified with a multiplier of 1.441, but due to the Vf voltage forward drop value of the rectifier, say with the above DF210 rectifier ~2x1.1V drops from the final DC voltage and so remains at a theoretical level of ~6.4V DC, if the input voltage is 230V.
I noticed that during the measurements, if the input mains voltage changes (which is far from constant), the values measured in points 28-29 also jump between 6.16V and 5.8V.
I have a good guess that the secondary of the transformer supplies 6V AC, this is rectified with a multiplier of 1.441, but due to the Vf voltage forward drop value of the rectifier, say with the above DF210 rectifier ~2x1.1V drops from the final DC voltage and so remains at a theoretical level of ~6.4V DC, if the input voltage is 230V.
I noticed that during the measurements, if the input mains voltage changes (which is far from constant), the values measured in points 28-29 also jump between 6.16V and 5.8V.