Hello Have 130 410HD 2275 chassis dating back to 1978
I know that there are multiple posts about tubes red plating posted here, but I feel stuck and I am not sure why I still have 2 of 6CA7 tubes red plating from time to time.
let me start from the beginning and explain what changes were done to the amp.
Amp has 4 original eminence speakers connected correctly.
Original Phillips 6ca7 tubes were removed and replaced with matched quad JJ 6ca7 also phase inverter tube was replaced.
The bias circuit diodes were updated to 5W and trim pot was replaced because it was malfunctioning.
10ohm cathode bias resistors were removed and 4 individual (dale) resistors are installed, one for each tube (total resistance in parallel is correct).
All electrolytic were replaced with high quality caps.
I have used F&tT 20uf and 100uF capacitors.
The rest of the amp circuit is all original.
The bias is set correctly and and the amp works wonderful for most of the time.
Eventually 2 tubes start to glow hot and mild hum appears.
Is it possible that the tubes are not able to handle 725V or is there something else that I need to check.
I am willing to install 4 vintage USA made tube if that is the issue, but I feel that I have missed something and that the problem still exists regardless of tubes.
Any feedback is much appreciated.
Thanks!
Fri, 09/10/2021 - 17:51
#1
First off, let's go with the
First off, let's go with the comment "biased correctly". Not slamming here but to what specs did you bias it to? If you used that tired old "1/2 volt at point Y" crap, that won't cut it in today's world. I suggest biasing with 1 watt resistors on each tube and measure the voltage across EACH tube separately. That way, you can find out which tubes are really running hot. What was the control voltage going to EACH tube? Were they all equal? Lastly, bias them cooler. With that amount of voltage at the tube, you can bias them at 20-21mA and be just fine. THEN see if they red plate. Mike.
PS- which two are getting hot?
Hi Mike,
Hi Mike,
Thanks for the reply.
I have set it up to 0.48V(following the MM schematic recommendation) measuring across individual cathode bias resistor on all 4 tubes, which is the same method as measuring at Y point.
Control voltage going to each tube is exactly -28.7V on all 4 tubes.
I checked the bias current and it is at 25mA.
I will turn it down to 21mA as you suggested and see if the 2 tubes next to each other (all the way toward the left side of the chassis are still getting hot.
Except for the JJ 6CA7 are there any other brand that you recommend?
Thank again for your suggestion and time.
Regards,
Boris
Not really, but I haven't
Not really, but I haven't been keeping up with the latest trends anyway. I have no troubles with JJ's but I'm not playing out anymore. The JJ's that are in my 2-10-65 have been in there for a LONG time ( probably 5 years) but they don't get used much. Sovteks's are usually robust but I never like the sound of them.. REAL svetlana's were good too but don't exist anymore from what i remember. The bottom line is, today's tubes are a crap shoot no matter what you buy since they are limited production and very little research. You say that you set it to 25mA. How and where did you measure it?
Sounds good, i think I will
Sounds good, i think I will get the another set of JJ tubes just in case I need to replace them.
Today I adjusted the bias down to 21mA and so far it seems fine, but I haven't played it much.
Red plating usually happened after playing loud at volume at 7 and master volume at 7, if I play the amp at lower volumes Iit seems to be stable, but I can't say for sure.
I have 1 tube adapter socket that is connected to the multimeter and thats how I adjusted the bias to 21mA. The first time I set the bias I didn't have the adapter so I just used the 0.5V at point Y.
Thank you for the feedback :)
AH. Gotcha.
AH. Gotcha.