I was noodling around on the bench, and tried out a tube type that far exceeded
the PT's ma. rating, I had found published data for the device that supported
the loading provided by the OPT and thought might as well try it out.
After a bit of tweaking, to where the rig sounded nice, I took voltages.
The resulting bias was around 50% of the power supply's published ma..
Also, incidentally, the total plate dissipation matched the PT advertised
rating in watts, this was a surprise coincidence that got me to thinking.
The old adage about an amp being a power supply attached to a speaker.
It seems more valid to choose and bias the device (power tube) in
accordance with the design limitations of the power supply.
Any arguments for or against this rational?
Another Bias
Moderators: pompeiisneaks, Colossal
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Another Bias
lazymaryamps
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Super_Reverb
- Posts: 188
- Joined: Tue Dec 21, 2010 6:28 am
- Location: Indianapolis, USA
Re: Another Bias
Well.... It depends what your goal is with the amp and with your hobby.
Maybe a guy wants to max out the OT or perhaps slightly/significantly overdrive it to get saturation nonlinearities.
If a guy wanted a stiff amp with a lot of punch, he might choose a P/T with 50-100% more HT current capability: the lower resistance 2ndary windings would cause less IR drop and depending on filter caps and rectifier could give him a punchy feel with very little sag.
Or maybe you have OTs or PTs around the shop and decide you'd like to be the next boutique amp guru and build a prototype over the weekend.
Commercial considerations aside, If the amp you're designing/building sounds good to you, then it's a good amp, right?
For my money, I decide on the output device(s), choose the OT depending on output tube capability, desired output power, and my ultimate use for the amp. Finally, I choose the PT voltage and current capability based on desired output power and plate/screen voltage capability, but always with significant current design margin because I want the output tubes and OT (and speakers) to be the star of the show. I can dial in the envelope with rectifier and filter cap design. My preamp designs are simple, typically clean with enough gain to drive the outputs hard enough to get mojo tone.
cheers,
rob
Maybe a guy wants to max out the OT or perhaps slightly/significantly overdrive it to get saturation nonlinearities.
If a guy wanted a stiff amp with a lot of punch, he might choose a P/T with 50-100% more HT current capability: the lower resistance 2ndary windings would cause less IR drop and depending on filter caps and rectifier could give him a punchy feel with very little sag.
Or maybe you have OTs or PTs around the shop and decide you'd like to be the next boutique amp guru and build a prototype over the weekend.
Commercial considerations aside, If the amp you're designing/building sounds good to you, then it's a good amp, right?
For my money, I decide on the output device(s), choose the OT depending on output tube capability, desired output power, and my ultimate use for the amp. Finally, I choose the PT voltage and current capability based on desired output power and plate/screen voltage capability, but always with significant current design margin because I want the output tubes and OT (and speakers) to be the star of the show. I can dial in the envelope with rectifier and filter cap design. My preamp designs are simple, typically clean with enough gain to drive the outputs hard enough to get mojo tone.
cheers,
rob
-
Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: Another Bias
Playing with the notion on paper, I arranged a half dozen output types
by max plate dissipation, the 70% mark, 50% and 40% for giggles.
Turns out to be: EL84, 6V6, EL84, 6L6, KT88. and GE 6550.
In ascending order, I grabbed random design center and max figures etc...
The 50% mark on the PT sits around 465v .055, about 25w total plate
dissipation for a pair of tubes on a PT rated to 120ma.
with this circuit and 100% plate dissipation you can use El84
or....
100% EL84/6V6
70% 6V6
50% EL34/6L6
40% 6L6/KT88
The Xfomers are ment for 6V6 and it shows, you can see how assumptions
lead to a rule of thumb.
even at a bias of 40%, large power tubes sound very sweet with the right loading.
And such conservatism leads to long tube life... Interesting argument.
I still like cooking toast over a quad of 6V6, cathode bias to within an inch of their life.
by max plate dissipation, the 70% mark, 50% and 40% for giggles.
Turns out to be: EL84, 6V6, EL84, 6L6, KT88. and GE 6550.
In ascending order, I grabbed random design center and max figures etc...
The 50% mark on the PT sits around 465v .055, about 25w total plate
dissipation for a pair of tubes on a PT rated to 120ma.
with this circuit and 100% plate dissipation you can use El84
or....
100% EL84/6V6
70% 6V6
50% EL34/6L6
40% 6L6/KT88
The Xfomers are ment for 6V6 and it shows, you can see how assumptions
lead to a rule of thumb.
even at a bias of 40%, large power tubes sound very sweet with the right loading.
And such conservatism leads to long tube life... Interesting argument.
I still like cooking toast over a quad of 6V6, cathode bias to within an inch of their life.
lazymaryamps