Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bdimb on July 30, 2013, 12:49:39 pm
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Background:
Count me in as one of those guys whose life doesn't allow turning my amp up. The 5E3 I built years ago sounds fine but I now notice crappy bass and Can't rule out it has been there a long long time. Tried changing the coupling caps and upping filter cap size, but that didn't do it.
Getting closer to the topic:
So, when I went back to square one and retraced my build, I find that Mr. H originally specified 25W OT and I used a 15W Hammond. While I doubt that is the root cause, I though I would pose the question; is the larger OT preferred with a Hoffman circuit?
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What values of caps did you use and where in the circuit did you use them? Bypass and Coupling. What value filter caps did you go to?
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Changed coupling cap from .1 to .022
First filter cap from 16uf to 20uf
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If money isn't an issue , I would probably would not choose the universal 15W Hammond. On a recent 5E3 build of mine , I chose an Allen Amps Deluxe Reverb upgrade OT. But , your speakers and cabinet size all shape the sound too. Is the bass farting out ... then there are mods all over this forum to correct that... I use a .047 cap on channel two , instead of .1. Also , I'm using a GZ34 rectifier with a 40uf filter cap.. no crappy bass.
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Changed coupling cap from .1 to .022
The 5E3 has four .1 coupling caps. Did you change all four or just one? Im sure you changed all the tubes with known good tubes before changing any caps? Did you use the Hoffman layout when you built the amp?
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Try changing the cathode bypass on the 2 preamp tubes down to say ~1mfd or less. For now just lift the ground end of them, 1 at a time and see if that helps. It will give you an idea of how it affects the amps sound.
What speaker do you have in it right now? Speaker will make a huge difference.
Those 2 things can cause bad sounding bass even with an upgraded OT.
Brad :icon_biggrin:
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Thanks for the help guys, here's some answers:
As for coupling caps, it has been a while but I am fairly sure I did not change four; only the two coming from the volume pot wipers to pins 1 and 6 of 12AY7. (here's where I reveal that I got most of my tube amp learning from the interwebs, I hope to heck these are the coupling cpas!)
I am also thinking that I did go higher in that first stage of filtering, possibly to 40 something uf, and that the 20uf is a replacement of the 40uf.
The board was assembled at Doug's shop, so it is THEE Hoffman layout so to speak.
Had a old Ceramic Jensen, now a Weber Alnico 12A125.
Money is an enormous issue, I may have to sell my Princeton to pay for this project.
Thanks again guys, looking forward to getting this amp tuned up right.
Bob
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Yes, those are two of the four .1 coupling caps. Before you replace those other two, do the test Willabe suggested with the bypass caps on the 12AY7 cathodes connected to pin 3 and pin 8. I have that Weber speaker in my 5E3, great speaker for that amp.
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You guys are awesome; will follow the recommendations and report.
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Hammond tends to overbuild their OT's from what I've seen. I had a Hammond 25w OT that was very close to the size of a stock Marshall 50w OT.
Just a schematic to show some of the comments the guys have made for your consideration. Increasing the power tubes cathode cap from 25uf up to 47uf, 100uf or more may be a help also.
The preamp cathode caps can be even lower. I'd possibly try a 2.2uf on the second gain stage cathode with a 4.7uf on the first one.
with respect, Tubenit
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Tubenit,
That schematic is a huge help!!! :worthy1: I am really excited about working on this amp.
Long story short, I had been using the 5E3 to jam with a piano player but the bass response became a problem quickly.
I picked up a Marshall (MGX50) mainly because its was cheap and has so-so to decent clean sounds, but I lost all the nuances in my picking in that preamp. I started to miss the glass immediately.
I will be placing an order for components today and if it is of any interest, will document results.
I owe you all a beer, coffee, donuts, etc..
Much thanks all way round,
Bob
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Well, I followed Tubenits schematic and all the great advice and did a shotgun approach to replacement/upgrades; I am claiming 90% improvement! Next up is to upgrade the original particle board baffle as well (the bass issue was present using other cabs and speakers, I do believe that it was mostly the caps I was hearing).
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Cool, I am glad that was useful to you and I appreciate hearing the result of your efforts! Thanks for sharing how it worked out.
with respect, Tubenit