It doesn't NEED biasing but can be biased.Originally Posted by AndrewG
Mine arrives tomorrow. Will do a pictorial about biasing it at the weekend.
K
I think it also went on to say that the valves wouldn't run either hot or cold if it were not biased. This implies that I can confidently swap valves without impairing the amp in any way. True? If not, how would biasing help if the valves aren't being over-stressed?
Cheers for the info Kev. Much appreciated.
Well - if you're sticking new 34's in it I wouldn't bother, but I'm putting 6V6's in mine so it'll need tweaked.
K
In the general run of equipment that used valve op stages, radios, tv's, record players etc, the valves were all cathode biased and the other name for this is "autobias".
This meant that a replacement valve "sorted itself out" and no adjustement was necessary or provided. This system worked perfectly because...
1)The valves were operated at their design center values, not max anode wattage and certainly never (IMEXP) above it!
2)Valves had their characteristics tightly controlled and one PLC84 worked bang on anode current for any other, every time, regardless of maker/sample and for years and *&^%ing years!
1)Guitar amp are almost all cathode biased to the limit of anode dissipation (I think most techs would agree?) and some brands/samples will fall out of the loop and "red plate" whilst others will just stay the nice side of destruction. And ...
2) Valves are simply not what they were!
But for those that want to fiddle/experiment fine, for those that know what they are at. For the noob, 10% either way for anode current ain't gonna make much difference if any, to the sound and for FIXED biased amps Ia goes all over the shop with mains volts anyway!
Dave.
"if you're sticking new 34's in it I wouldn't bother"
And I wouldn't bother with "new" 34's! Amps have only been around 5 bloody minutes, what's the matter with everyone!
Those valves are running WELL under max power (unlike EL84's in a 15watter) a good set will last ages!
Dave.
Because as you said in your previous post people like to fiddle and tweak and try different valves.
"Because as you said in your previous post people like to fiddle and tweak and try different valves."
And do we ever get to hear dot wav one of any results of this tweakery?
We do not!
Heh! I DID say "for those that know what they are at"!!!
Dave.
So - definitive answer - if I were to by some replacement valves of the same type (but not necessarily same brand) for my HT20 head, when I came to replace the ones that came in it, would I need to get the amp biased?
Thats why I stick with cathode biased EL84s. Being a mechanical engineer I like to hit things with a hammer to make them work properly. Not good with objects made with glass.
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