Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bnwitt on August 27, 2010, 09:53:36 am
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I just finished converting a Fender 59' Bassman Vintage to a Hoffman version 5F6-A and once again I had to change the main bias dropping resistor to get the bias supply to a range that worked for the amp. The Hoffman layout shows a 1k ohm resistor at the front of the bias circuit and in order to get the range where middle was 70% for several sets of test tubes I had to go with a 2k2 ohm value. I also noticed that the reissue board had a 1k8 ohm in that position. The reissue transformer was producing 79 volts AC out of the PT bias tap. So, that is probably why I had to change the resistor value. This prompted me to check the original schematic against the current one and I see that the original fixed non-adjustable bias voltage was -48 compared to the reissue value of -52. That's a pretty big difference and the reissue obviously runs most tubes pretty cold compared to the original. Of course the more recent "Vintage" version added a bias pot to adjust the voltage so one can get a hotter idle current. I ended up with -50 on the Winged C set to acheive 70% idle current and the 2k2 ohm resistor put that right in the middle of the pot's range.
So, I'm thinking the fixed bias of -52 volts, the sand rectifier, the cheap components and the bundling of wires all together are what contributed to the anemic sound of the orginal reissue. The Vintage model put back the tube rectifier and added the bias pot but kept the pcb and bundled wire so it too pales in comparison to a real 5F6-A or a Hoffman hand wired unit. Everytime I A/B a reissue with a hand wired unit the difference is night and day even to my old ears so it amazes me that Fender keeps doing things this way.
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I think I was one of the first group of people to use the Hoffman board for the Bassman, he made them himself those days. It was a long time ago........I don't even remember how I found out about Doug, I don't think I had a PC yet.
The Reissue that I bought was horrible, I had someone put the board in and the amp was amazing. I think I also had the guy use a 5Y3 rec tube to bring down the voltage. He also changed one channel to the Marshall values 2.7K/.68 on the cathode and .0022 on the coupling cap.
The amp was too loud for those Eminence speakers and those mods tamed the amp and made it very usable. But for the gigs I did it was still too loud.
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"But for the gigs I did it was still too loud."
Tell me about it! My regular gigging amp is a 5F6a preamp and PI with a BF Deluxe power section. Even with 6V6's, it was still too loud. I just changed the rectifier from a 5AR4 to a 5Y3 and converted to cathode bias to bring it down to about 15 watts. It's still plenty loud for any club and has loads of clean headroom. What does Fender do to those 50 watt amps they're making to get them so dead and muffled?!? Any of my 15 watt 6V6 amps just overwhelm the 6L6 Fenders. If I turn up above 3, I get yelled at for being too loud. I don't get it!
Dave
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Yep, no doubt the handwired 5F6-A is mucho loud compared to the reissue. Hopefully it won't be too loud for the owner. He is gigging with one of my 5E3 amps right now. Maybe Fender is intentionally reducing the output power of the reissue?
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Hey BB, What kind of voltage drop did you see with the rectumfrier change as I am considering trying in a BF Deluxe build? Right now I get 435v on the PY and I know that is really pushing the GE 6V6's.
Sorry, I don't mean to hijack the thread....
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I'm doing an LTD to hand-wired conversion right now.I'm also adding VVR for studio use.I stick to the original layouts for mine.
I'll post some pics when it's done.I am also of the opinion that the Reissues were a bit anemic compared to an original.I'm playing this one for a bit to get used to the tone and see what improvement there will be.I'm using NOS paper in oil coupling caps for his one.I like the squishy feel of them in a vintage amp.I did one a few years back and the tone was pretty outstanding.
Should be a cool sound.
Thanks for your post Bnwitt!
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Psycho,
How do you squeeze in a VVR in that tight chassis with an orginal eyelet board?
Barry
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Well the owner picked up the Hoffman 5F6-A last night. He played it for quite some time and I always enjoy hearing a good player through one of my amp builds. He'll be gigging with it tonight. He did have the amp down on 2 or 3 volume wise so it is much louder than what he is used to. He was very impressed with the clean tones. Said they were "woody". I hooked up a Fulltone OCD pedal to the amp and showed him how great the distorted tone could be, but the volume had to go down to 1 :laugh: Man the 5F6-A is an awesome guitar amp when hand wired and in a 4X10 solid pine cabinet. Leo will always be my hero for so many reasons. :grin:
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I'm going to use one of the kits that Skipz Circuits sells.It has a compact board and a pot that I'll install where the fuse holder is now.I'll put the fuse beside the AC cord on the lower end of the chassis.
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Cool. I hadn't seen Skipz Circuits before. I may have to try one of those.