Yes you will see lower voltage with cathode bias because the amp is pulling. More current.
Sounds like you are maxing out your ppt
Cathode/Fixed bias switch for half power
Moderators: pompeiisneaks, Colossal
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gingertube
- Posts: 531
- Joined: Mon Nov 14, 2011 2:29 am
- Location: Adelaide, South Oz
Re: Cathode/Fixed bias switch for half power
How much current the amp draws in cathode bias depend on your choice of the cathode bias resistor value.
I built a London Power Standard with a quad of 6V6, each output pair was switchable between triode and pentode mode and between cathode bias and fixed bias. It included power scaling of just the output stage (preamp left at full voltage).
Like posters above I found that I rarely use the Triode/Pentode Switch prefering to run Pentode Mode (actually I occassionally run one pair in triode mode with the other pair in pentode mode BUT never with both pairs in triode mode)
I used a common (to a push pull pair of output tubes) cathode resistor for the cathode bias mode with the resistor value carefully chosen (470R for a pair of 6V6) to give exactly the same idle current as in fixed bias. I tried it with and without a cathode bypass capacitor. With a cathode bypass cap there was very little sonic difference to pick between cathode and fixed bias making the whole switch thing a waste of time - so I deleted the bypass cap to give a different sound.
Buried in the above posts are some points about Amp and Speaker interaction on which we could write a 3 or 4 page essay. The critcal points here are:
Pentode Mode Output Impedance is about 15 times Triode Mode Output Impedance.
Pentode Mode Output Impedance is 5 to 6 times Ultralinear Mode Output Impedance.
So speaker electrical damping will be VERY different.
Output Impedance in a Class AB Amp actually increases by X4 as the signal peak passes the point where one tube cuts off. This happens at lower volume (lower power) in a cold biased amp. This can be set to give loose initial attack (high peaks) with a tighter sustain of a note or chord. Note at least one modern Fender Amp with the output tubes biased at 7 Watts dissipation - this could only have been done to take advantage of this.
Hope this is of help.
Cheers,
Ian
I built a London Power Standard with a quad of 6V6, each output pair was switchable between triode and pentode mode and between cathode bias and fixed bias. It included power scaling of just the output stage (preamp left at full voltage).
Like posters above I found that I rarely use the Triode/Pentode Switch prefering to run Pentode Mode (actually I occassionally run one pair in triode mode with the other pair in pentode mode BUT never with both pairs in triode mode)
I used a common (to a push pull pair of output tubes) cathode resistor for the cathode bias mode with the resistor value carefully chosen (470R for a pair of 6V6) to give exactly the same idle current as in fixed bias. I tried it with and without a cathode bypass capacitor. With a cathode bypass cap there was very little sonic difference to pick between cathode and fixed bias making the whole switch thing a waste of time - so I deleted the bypass cap to give a different sound.
Buried in the above posts are some points about Amp and Speaker interaction on which we could write a 3 or 4 page essay. The critcal points here are:
Pentode Mode Output Impedance is about 15 times Triode Mode Output Impedance.
Pentode Mode Output Impedance is 5 to 6 times Ultralinear Mode Output Impedance.
So speaker electrical damping will be VERY different.
Output Impedance in a Class AB Amp actually increases by X4 as the signal peak passes the point where one tube cuts off. This happens at lower volume (lower power) in a cold biased amp. This can be set to give loose initial attack (high peaks) with a tighter sustain of a note or chord. Note at least one modern Fender Amp with the output tubes biased at 7 Watts dissipation - this could only have been done to take advantage of this.
Hope this is of help.
Cheers,
Ian
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Reeltarded
- Posts: 10189
- Joined: Sat Feb 14, 2009 4:38 am
- Location: GA USA
Re: Cathode/Fixed bias switch for half power
Please keep talking, Ian. I could read all night!
Great stuff!
Great stuff!
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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funkgang49
- Posts: 130
- Joined: Sun Mar 25, 2012 4:36 pm
- Location: Northern New Jersey
Re: Cathode/Fixed bias switch for half power
Ditto! Tell me more. Great stuff!Please keep talking, Ian. I could read all night!
Great stuff!