Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RicharD on September 22, 2011, 09:57:08 pm
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... and I'm messing with LoWatt amps, I decided to design a little bass practice amp. Of course as per the norm, I'm working with parts on hand. The OT is actually a 1200:4 but I'm treating it like it's a 2400:8. The PT is 330-0-330. I typically shy away from tube rectifiers but my stash of electrolytic caps is dwindling so I wanted a power supply where my available 500V caps could be used vs. building a crazy bank using 100/450V caps which I have plenty of. I ran with the 6SL7 kind of as an after thought in lieu of a 12AX7. The first version did use a 12AX7 with 2 gain stages but it was way too distorted for bass, which in itself was an after thought. I decided to use a single gain stage and maximize it for gain. The cathode follower really doesn't do anything beneficial as far as I can tell. I had thought about using the other triode to drive a line out, but this isn't a stage amp + it seem wasteful to deploy a balanced out transformer into such a keep it simple stupid amp. The KT88 is biased a little conservatively but that is somewhat intentional/accidental. The curve sheet I worked from has the screen voltage at 300V. I thought I'd be biasing for about 27V. I'm still quite green at designing output stages. This is actually only about the 3rd tetrode circuit I've designed from scratch vs. copy catting Leo or somebody else.
It sounds nice and full but quieter than I was expecting, which is good in this case. I was able to run it full throttle w/o getting "the look" from my wife. I bet my neighbors are glad too.
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wow! 200uF 1st filter? that's a lot of filter for 5U4GB, hell, any tube recto for that matter; i'm surprised it's not sputtering - i guess that a relatively low and constant current draw and a semi-anemic PT is keeping it happy?
peace.
--DL
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slam it with SSR - you won't kill KT88 with 440V, even in class A1. if it's the OT i'm thinking you're using, it won't care either.
peace.
--DL
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PSUD2 gets upset at 300uF with a 5U4GB. I thought 200uF was pushing it too, but I get no errors. I ran it for a solid hour. With SS recto, unloaded voltage is >500V. I don't want to use a dozen 100/450 caps.
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Use two in series like Fender amps.Then dump the 5U4 and use 470uf/350v caps in series.More low end thump for the buck.
Especially for bass.Pour the voltage to that KT88 and keep the preamp plates high.
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To get the ripple down to less than 10V, I need 600uF. Using the 100/450's I have on hand, I'd need 12 of them. That might make me open the box of them I have stashed out. I dunno.... This is simply an experiment at this stage anyway.
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Put a pi filter before the plate supply.
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Put a pi filter before the plate supply.
+1 and go SSR. :icon_biggrin: