According to the schematic in post #18 the left side section of the 6SN7 is passing 2.5 mA, so the cathode voltage would be 2.5 V. But the curves for the 6SN7 show that at a plate voltage of 136.5 V and a bias of -2.5 V, the current would be 8 to 9 mA.
139 V is supposed to be 131 V. The Vgk is -4V.
Sorry for the schematical errors, haven't been home during the weekend so I don't have my original notes available. The currents might be off as well, I only drew a digital copy of the input section before I left home. The voltages in the first post are measured on the actual circuit this Friday.
I will follow up with the fully corrected schematic asap...
Last edited:
Yes, that's all that is really needed. He did use a much larger capacitor initially, but it took too long to charge up.
But the 1uF that replaced it, was too small.
I think in relation to the two 220k resistors even a bad electrolytic with highish ESR would be suffficient. Which doesn't exclude to ask for a good part. Isolation resistance is more important here.
Best regards!
You would not want some signal not shorted by the cap to reach the first triode grid ( positive feedback ) ... Maybe I exagerated a bit , maybe not 🙂 , definitively you can't use a bad cap
Last edited:
Yes, of course a completely dried out 'lytic won't work, but a cap with an ESR of just 1 % of these 220k certainly won't hurt. This would mean an ESR of 2k2, very close to dried out 😉.
Best regards!
Best regards!
Yes, the C needs very low leakage resistance so both grid DC voltages will be equal.
Try around 10uF, at 100VDC or higher.
Today I finally got some time to keep working on the preamp. I picked a decent 4,7µF polyester cap (rated at 200V). Physically very big off course, but a lot better leakage-wise than a similar electrolytic cap. To rule out any new error sources if nothing else...
Now the voltage in the grid circuit is rock solid at 32,79 V after a some warm up-time. I'm really glad that it was such an easy fix. Time to start measuring gain- and phase-margin!
Thank you for all your input! I really learned a bit or two along the way. A little frustrating though, I should've seen the positive feedback-path to the leftmost grid! 🤦♂️
Now the voltage in the grid circuit is rock solid at 32,79 V after a some warm up-time. I'm really glad that it was such an easy fix. Time to start measuring gain- and phase-margin!
Thank you for all your input! I really learned a bit or two along the way. A little frustrating though, I should've seen the positive feedback-path to the leftmost grid! 🤦♂️