cathode biased el34 woes
Moderators: pompeiisneaks, Colossal
cathode biased el34 woes
Hello, I have a Rocket preamp and PI going into a 20 watt PP cathode biased amp with two 6v6.I replaced the 6v6s with EL34s and I love the sound especially with Paul Ruby's treble boost switch.Fat clear bottom and chime! It is plenty loud but past 2 on the volume(not 2:00) it produces terrible blocking distortion and motorboating.It has 1.5k grid stoppers - and a 100ohm 10W resistor with a 40uf to ground from the cathode. Any ideas on getting it running on EL34? Thanks!!
Re: cathode biased el34 woes
More info, please:
Plate voltages on the EL34. The cathode resistor may not be suitable for EL34's. What output tranny? Screen resistor value?
Just trying to help.
D
Plate voltages on the EL34. The cathode resistor may not be suitable for EL34's. What output tranny? Screen resistor value?
Just trying to help.
D
Re: cathode biased el34 woes
Thanks D. I did not have the el34 grounded properly. I have another question though: does each EL34 # 8 pin cathode need it's own resistor to ground or can I use a jumper? I used a jumper but I don't know if that is the correct wayto do it. Thanks again!
Re: cathode biased el34 woes
You can jump the cathodes together and use one resistor for both. It typically will be half of the value required for both, but needs to be able to handle the current/power dictated by the voltage drop across itself.
The plate voltage has a direct influence on the required value of the cathode resistor (bias point).
Good info on biasing amps at: http://www.aikenamps.com/ under tech info/advanced.
HTH D
The plate voltage has a direct influence on the required value of the cathode resistor (bias point).
Good info on biasing amps at: http://www.aikenamps.com/ under tech info/advanced.
HTH D
Re: cathode biased el34 woes
D, with a 250ohm 10W resistor I get 358 on pin 3, I get 353-360 on pins 4 and 6. Pin 8 is 25. Do you think these voltages are OK? Sorry but the Aiken articles are over my head. Thanks!
Re: cathode biased el34 woes
Pretty good.
Assuming the tubes are well matched and you have one cathode resistor.....Here's my math:
Plate voltage is 358 - 25 = 333volts
cathode current is 25v/250ohm = 100ma (50ma per tube)
50ma x 333volts = 16 watts per tube
16 watts / 25 watts (max power output EL34) = 66%
This includes the screen current.
Most people (or maybe just me) would find that a pretty good place to start. Some folks like more, some like less idle current. What do your ears tell you?
HTH, D
Assuming the tubes are well matched and you have one cathode resistor.....Here's my math:
Plate voltage is 358 - 25 = 333volts
cathode current is 25v/250ohm = 100ma (50ma per tube)
50ma x 333volts = 16 watts per tube
16 watts / 25 watts (max power output EL34) = 66%
This includes the screen current.
Most people (or maybe just me) would find that a pretty good place to start. Some folks like more, some like less idle current. What do your ears tell you?
HTH, D
Re: cathode biased el34 woes
D, I think it sounds great and is very quiet. It sounds really good through a Jensen P12r that would probally blow if I really cranked it. I tried it through a Weber California but it lost the zing and it was too bright and tight through a Kendrick Blackframe. I'm playing with the idea of a higher rated Alnico or (maybe a Weber ceramic 30W Blue Dog). Thanks for the tech help on this one!