Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frank57 on October 28, 2011, 07:52:49 am
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Is there an advantage to having a lower B+ in the preamp?
I tried an experiment in this amp with a 2 watt 47k resistor.
It sounds about the same perhaps a little smoother.Would the lower voltages be helpful in any way?
I also tried an experiment with 1k screen resistors.
You get a lot of compression going on so maybe better for the tubes but sound wise I'm not so sure.
Maybe 470 might be a better compromise.
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It depends on what you want. I build BF Deluxe clones with higher B+ to the preamp and PI, actually to SF specs. I get more clean headroom that way. If you want earlier breakup, lower the voltages. Allen Old Flames actually have a "brown" switch on the back that changes a B+ resistor to lower the voltage. The name describes what it does.
Dave
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I did a quick test of it and there is a bit more breakup sooner.
It could be useful if I take a gain stage out, because the Hiwatt phase splitter is keeping things super clean.
One thing I notice is if I increase the screen resistors from 100 to 1k there is a lot of compression going on.
Probably 470 is a better compromise, but it sounds like a way of getting a cathode biased feel to a fixed bias amp.
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Thats why the old tweed amps sound like they do, general lower voltage than the latter higher voltage blackfaces, silverfaces with NFB.
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For reducing B+ noticeably, you really need to change the supply resistor closest to the reservoir cap, and you typically need to increase it by (say) double to make any sort of dent in the B+ voltage.
As to what sort of difference it makes, depends on how you've set up the stage
See attached pic for a comparison of load lines between 250V and 350V B+ in an otherwise identically set up 12AX7 triode. The main resulting difference is in how hard the grid can be driven before clipping. The 250 B+ stage goes into grid current limiting sooner in this example
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The 250 B+ stage goes into grid current limiting sooner in this example
Hi TW!
Have you been reading Merlin's book? ;) (to avoid confustion for anyone, that first LL should read -2.75 for part going towards cut-off) I love his loadline program. Take care
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The 250 B+ stage goes into grid current limiting sooner in this example
Hi TW!
Have you been reading Merlin's book? ;)
Only about a zillion times :)
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I always enjoy re-reading things after some time has passed. There's so much in there to digest it all in one or two readings and remember it all, is nearly impossible.
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OK, you've caught my attention and this is all way over my head :worthy1: I was surfing through the schem library and found duncans blues amp and noticed that the preamp section B+ was running less than 63v volt as the filter caps were only rated at 63v :dontknow: The first stage i can only guess that the B+ is about 40v with plate resistor of 100k and cath of1.2k this is repeated for the next three stages with a 10k dropping resistor between each stage. Is anyone able to comment on or do a LOAD LINE graph on this setup as it is way lower than the usual setup.Thanks