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Hey everyone,
So I just finished my first actual from the ground up amp build. It's a cross between a Plexi 50 Bass and Lead and
a JTM50 with a tube rectifier. My plan was to use a DPDT switch to go from solid state to/from rec tube. Now have any of you guys actually done this? I found a drawing layout here and there on how to do it but looked sketchy (pun intended) at best!
I went ahead and used the solid state rectifier (as a plexi50 bass/lead) and put the tube rec switch thing on hold for now. I want to get more info on the best way to configure it taking into consideration the bias adjustment and voltage drop etc.. With that said....I am very happy! It sounds great, totally mean and ballsy. I've actually never played a real(or even a clone of one) Plexi 50 or JTM for that matter before so this sound was new me. Now I understand what all the fuss is about over this particular amp! This was a long time coming. Plus, it works with minimal troubleshooting and sounds damn amazing. Power tubes seem happy but I haven't verified voltage readings yet but will do that soon. I will keep you all posted.
I'm going to use this method to adjust the bias on power tubes:
1) If not using a bias probe, you need to have a 1 ohm resistor on each power tube socket (one end of resistor to ground; the other end tied to pins 1 & 8 ) Use a 1% tolerance resistor.
2) Turn power on and let tubes warm up. Switch standby to "on".
3) *Be very careful on this step (high voltages)*
Set your meter to the highest DC volts (600 or higher) and put the common (ground) lead in one of the chassis mounting nuts or alligator clip it. With one hand behind your back for safety, touch the positive lead to pin 3 (plate) of one of the power tubes. This is your plate or "B+" voltage. Write it down.
4) EL34's are 25 watts max and you want to set the bias to be between 60% and 70% maximum dissipation watts. (60% = 15 watts; 70% = 17.5 watts) Divide your desired wattage by your plate voltage from step 3. (example: 15/450 = .033 or 33mV) This is your desired bias voltage (probably between 30 and 40 mV).
5) Set your meter to lowest DCV setting (200mV or less) and touch the ground lead to the ground side of the 1 ohm resistor and the positive lead to the pin 1 side. Do this on all power tubes and adjust the tube with the highest value to your desired bias voltage from step 4.
Plate voltage will change a little when adjusting the bias so you may need to take another reading from pin 3 and adjust your equation accordingly and repeat steps 4 & 5.
Does this method sit well with you all? It is probably the best way to go for a dingdong like myself!
...will post better pics soon...
So I just finished my first actual from the ground up amp build. It's a cross between a Plexi 50 Bass and Lead and
a JTM50 with a tube rectifier. My plan was to use a DPDT switch to go from solid state to/from rec tube. Now have any of you guys actually done this? I found a drawing layout here and there on how to do it but looked sketchy (pun intended) at best! I went ahead and used the solid state rectifier (as a plexi50 bass/lead) and put the tube rec switch thing on hold for now. I want to get more info on the best way to configure it taking into consideration the bias adjustment and voltage drop etc.. With that said....I am very happy! It sounds great, totally mean and ballsy. I've actually never played a real(or even a clone of one) Plexi 50 or JTM for that matter before so this sound was new me. Now I understand what all the fuss is about over this particular amp! This was a long time coming. Plus, it works with minimal troubleshooting and sounds damn amazing. Power tubes seem happy but I haven't verified voltage readings yet but will do that soon. I will keep you all posted.
I'm going to use this method to adjust the bias on power tubes:
1) If not using a bias probe, you need to have a 1 ohm resistor on each power tube socket (one end of resistor to ground; the other end tied to pins 1 & 8 ) Use a 1% tolerance resistor.
2) Turn power on and let tubes warm up. Switch standby to "on".
3) *Be very careful on this step (high voltages)*
Set your meter to the highest DC volts (600 or higher) and put the common (ground) lead in one of the chassis mounting nuts or alligator clip it. With one hand behind your back for safety, touch the positive lead to pin 3 (plate) of one of the power tubes. This is your plate or "B+" voltage. Write it down.
4) EL34's are 25 watts max and you want to set the bias to be between 60% and 70% maximum dissipation watts. (60% = 15 watts; 70% = 17.5 watts) Divide your desired wattage by your plate voltage from step 3. (example: 15/450 = .033 or 33mV) This is your desired bias voltage (probably between 30 and 40 mV).
5) Set your meter to lowest DCV setting (200mV or less) and touch the ground lead to the ground side of the 1 ohm resistor and the positive lead to the pin 1 side. Do this on all power tubes and adjust the tube with the highest value to your desired bias voltage from step 4.
Plate voltage will change a little when adjusting the bias so you may need to take another reading from pin 3 and adjust your equation accordingly and repeat steps 4 & 5.
Does this method sit well with you all? It is probably the best way to go for a dingdong like myself!
...will post better pics soon...
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