04-07-2018, 11:25 AM
I kinda went silent on this while doing some research. One thing's for sure, there is a LOT of reading and there are many different opinions on how to redo these. About 4 different "kits" available, One says this, the other says that, one source gives me the creeps, and so on. It has gotten to the point of total frustration and am about ready to shelf the project.
So - to source my own parts, and following only the major advices that are proven? The main things I see coming up over and over are:
1. Adding screen resistors on the output tubes to prevent arcing and subsequent destruction of said output tube (I understand this)
2. Adding a 10 ohm resistor on the cathode of the output tubes to be able to measure current (I understand this too)
3. Adding some type of bias adjustment to set the current of each output tube, they say the bias is set too hot originally from the factory
4. Dropping the grid resistor on the outputs from 330k to 220K and doubling the coupling caps value (long, educating explanation available)
This is in addition to a full recap of the power supply, any other E-caps and paper caps.
Here is what I see being talked about mostly: With the higher line voltages, this ups the B+, which is hard on caps and tubes. The newer caps have a lower ESR (or whatever) and can cause arcing on the output tubes - thus adding a 100 ohm screen resistor, and with the higher line voltages and B+, the tubes tend to draw more current = short life. The power switch is taxed from the charging of the caps when you hit it, so a current limiter is recommended (or a relay). The new production 7591 tubes are a little different, (lacking quality maybe) from the old production, so again, bias adjustment is needed.
And, the list goes on and on, but this is the basic problem area. I suppose once I get one figured out and working properly, the rest will come easy. If anyone has done this with a kit, has a set of instructions and is willing to share, please PM me. I do have a couple of these to experiment on
At the moment, this is where I am.
So - to source my own parts, and following only the major advices that are proven? The main things I see coming up over and over are:
1. Adding screen resistors on the output tubes to prevent arcing and subsequent destruction of said output tube (I understand this)
2. Adding a 10 ohm resistor on the cathode of the output tubes to be able to measure current (I understand this too)
3. Adding some type of bias adjustment to set the current of each output tube, they say the bias is set too hot originally from the factory
4. Dropping the grid resistor on the outputs from 330k to 220K and doubling the coupling caps value (long, educating explanation available)
This is in addition to a full recap of the power supply, any other E-caps and paper caps.
Here is what I see being talked about mostly: With the higher line voltages, this ups the B+, which is hard on caps and tubes. The newer caps have a lower ESR (or whatever) and can cause arcing on the output tubes - thus adding a 100 ohm screen resistor, and with the higher line voltages and B+, the tubes tend to draw more current = short life. The power switch is taxed from the charging of the caps when you hit it, so a current limiter is recommended (or a relay). The new production 7591 tubes are a little different, (lacking quality maybe) from the old production, so again, bias adjustment is needed.
And, the list goes on and on, but this is the basic problem area. I suppose once I get one figured out and working properly, the rest will come easy. If anyone has done this with a kit, has a set of instructions and is willing to share, please PM me. I do have a couple of these to experiment on
At the moment, this is where I am.
If I could find the place called "Somewhere", I could find "Anything"
Tim
Jesus cried out and said, "Whoever believes in me , believes not in me but in him who sent me" John 12:44
