If it was me I would insert a 47 to 100 ohm resistor in front of the 470uf cap.
Then I would convert to fixed bias and run each tube about 40ma (approx -33vdc on the grids). Then play with feedback levels. Maybe a CCS in the PI (easy). Actually I have a RFTLYS EL34 amp that I haven't even looked inside yet. I think it is very similar.
Then I would convert to fixed bias and run each tube about 40ma (approx -33vdc on the grids). Then play with feedback levels. Maybe a CCS in the PI (easy). Actually I have a RFTLYS EL34 amp that I haven't even looked inside yet. I think it is very similar.
If it was me I would insert a 47 to 100 ohm resistor in front of the 470uf cap.
A better place to put that resistor is in series with the secondary HV lead to the rectifier bridge. That way it performs the same function but at the same time provides some protection to the PT HV winding should the bridge fail.
Add a Slo Blo fuse on the primary. Plus a current limiting thermistor.🙂
A better place to put that resistor is in series with the secondary HV lead to the rectifier bridge. That way it performs the same function but at the same time provides some protection to the PT HV winding should the bridge fail.
Add a Slo Blo fuse on the primary. Plus a current limiting thermistor.🙂
John. Would I not need two resistors (one on each of the HV leads before the rectifier) ?
I am considering building a CRCRC after the rectifier that'll go like this:
220uF--50R--220uF--50R--470uF
PSUD2 shows very low ripple (compared to stock) and a B+ around 394V.
Does it make more sense to go with?:
50R--rectifier--220uF--50R--470uF
I am considering building a CRCRC after the rectifier that'll go like this:
220uF--50R--220uF--50R--470uF
PSUD2 shows very low ripple (compared to stock) and a B+ around 394V.
Does it make more sense to go with?:
50R--rectifier--220uF--50R--470uF
Heading to the components store later today. Wondering which option in post #63 makes more sense, or something else maybe?
Two resisters are not required, the drop across one side is all that is required.
That comes from Kirchhoff Laws used in electrical networks. The current entering any point must equal the current leaving that point. And the voltage drops around any loop must equal the voltage supplies around the loop.
Both your solutions will result in quite a bit of ripple/hum reduction. However, I think the PT on your amp has been undersized from the get go. Some of the newer materials are able to withstand higher temperatures. But there is a limit.
The stuff I worked on at Ferranti test department came as large, larger & largest, the kind you find in power stations. Most were 3-phase. Those running hottest had spacers of porcelain, the insulation was a kind of glass cloth. They ran very hot. On a 24 hour run I had to go up a ladder & take temperature on top about once an hour. The soles of my shoes got real hot, I think an egg would have cooked. One shift was overnight, that was boring.😀
Try unplugging all the tubes, check if the temp still rises a lot. Could be an internal short. Than be sure to let the HV caps discharge. If a resister to bleed the caps is not installed there needs to be something put in.🙂
Local parts store was hit and miss.
I ended up with a 25w 100 ohm resistor and 4 each 10w 470ohm and 10w 560ohm resistors.
So now my plan for the power supply is back to 220uF--100R--470uF and I am going to do the separate cathode Rs and caps.
Caps are going to have to be ordered...
I ended up with a 25w 100 ohm resistor and 4 each 10w 470ohm and 10w 560ohm resistors.
So now my plan for the power supply is back to 220uF--100R--470uF and I am going to do the separate cathode Rs and caps.
Caps are going to have to be ordered...
I just want to conclude this thread with another thanks to everyone who contributed.
I've finished up all the mods (both power supply filter and the dedicated cathode resistors and bypass caps).
The amp sounds better than it ever did. Some very, very slight noise can be detected with my ear 2 inches from the driver cones. Slightly more in the right channel, but that could be something as simple as a dirty tube pin. None of it is even close to audible when music is playing.
The only goal I didn't achieve was to reduce power transformer heat. I'm just going to ignore it.
I've finished up all the mods (both power supply filter and the dedicated cathode resistors and bypass caps).
The amp sounds better than it ever did. Some very, very slight noise can be detected with my ear 2 inches from the driver cones. Slightly more in the right channel, but that could be something as simple as a dirty tube pin. None of it is even close to audible when music is playing.
The only goal I didn't achieve was to reduce power transformer heat. I'm just going to ignore it.