Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tsquared on February 05, 2015, 11:41:12 am
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I've been trying to get this sounding better. It sounds pretty crappy, even after a complete recap and cleanup. It has no low end to speak of (even without the little choke stuck between the speakers.) I see pretty small values in the coupling caps .01, even on the PI.
My plan is to put in a Pro Reverb output tranny I have from an upgrade, and wire the two 8 ohm celestions for the needed 4 ohms on the tranny, lose the crazy choke bullshit, and maybe up the PI coupling caps. The original speakers are weak and full of rips, so I already swapped them out. Oh... and maybe stick in a GZ34 rectifier, rather than the 5y3GT in there now.
Any thoughts from wiser men than I???
Thanks,
Tod
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Your ideas sound good. In addition, consider putting a 50-100µF bypass cap on the cathodes of the output tubes.
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Why did they put a choke across the OT secondary?
For a built in compressor to keep the OT from burning up?
Brad :dontknow:
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ditch the choke.
cathodyne: add a grid stopper @ V2B. insert a 220K-470k between R26 & pin7. 220K is not a effective as 470k, but 470K may shave off more HF then desired.
.01uF caps with 560K load has f-3dB @ 25Hz - not the problem. if you change grid-leaks R31 & R32 to lower value, say 220K, then .01uF has a roll of approx. 2x that or ~58Hz. IOW, that's not the trouble here.
ditch the crappy valco reverb: put a 12AX7 in that hole and add a driver stage in front of each tone stack. rework the tone stacks to bassman or AB763 TMB, for fun, voice each channel's tone stack differently. if you do this, split off another PS tap V1A&V1B.
insert some NFB: inject under V2A use 47R under C12&R24 with 2K-4K NFB R: experiment. what you actually need depends on the impedance of the OT secondary tap - in this case it's 8ohms.
--pete
bump B+. make R30 ~10K and R29 ~10-15K.
as you say - ditch the 5Y3, use 5V4A or 5AR4. try a SS plug too.
OT: replacing it would probably be a big plus.
if it were mine, i'd gut it and build something more to my liking or unload it as-is.