Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: waldner on April 14, 2021, 05:10:58 pm
-
I'm building a '63 Brownface Vibroverb clone and thought it might make sense to incorporate some changes as I build it. From what I can tell, the normal channel is less useful than the bright channel- much like on the Blackface amps.
Can anyone who is better at reading schematics and converting them to layouts help recommend some parts changes to do something different with the normal channel? Also any other changes you might make to improve anything? Better, deeper tremolo? Reverb improvement? Or leave those as is?
Thanks!
Here are links to the 6G16 Schematic and Layout:
https://www.mojotone.com/Amp%20Kit%20Schematics%20and%20Wiring%20Diagrams/Brownface/Vibroverb_6G16_WD.pdf?vid=wzfIDMvQAiXxIBqi&shipmeth=50031&chrole=17&sj=p1lcpd5ZK5ZHq6993QdGc9t1e%3B1618427618%3B777049000&ck=07JUgsvQAiTxIHjW&promocode=&cktime=184548&gc=clear&cart=1483068&promocodeaction=overwrite (https://www.mojotone.com/Amp%20Kit%20Schematics%20and%20Wiring%20Diagrams/Brownface/Vibroverb_6G16_WD.pdf?vid=wzfIDMvQAiXxIBqi&shipmeth=50031&chrole=17&sj=p1lcpd5ZK5ZHq6993QdGc9t1e%3B1618427618%3B777049000&ck=07JUgsvQAiTxIHjW&promocode=&cktime=184548&gc=clear&cart=1483068&promocodeaction=overwrite)
https://www.mojotone.com/Amp%20Kit%20Schematics%20and%20Wiring%20Diagrams/Brownface/Vibroverb_6G16_SCH.pdf?vid=wzfIDMvQAiXxIBqi&shipmeth=50031&chrole=17&sj=p1lcpd5ZK5ZHq6993QdGc9t1e%3B1618427618%3B777049000&ck=07JUgsvQAiTxIHjW&promocode=&cktime=184548&gc=clear&cart=1483068&promocodeaction=overwrite (https://www.mojotone.com/Amp%20Kit%20Schematics%20and%20Wiring%20Diagrams/Brownface/Vibroverb_6G16_SCH.pdf?vid=wzfIDMvQAiXxIBqi&shipmeth=50031&chrole=17&sj=p1lcpd5ZK5ZHq6993QdGc9t1e%3B1618427618%3B777049000&ck=07JUgsvQAiTxIHjW&promocode=&cktime=184548&gc=clear&cart=1483068&promocodeaction=overwrite)
-
build as is, then after a week or so of playing, you'll be better informed on what needs tweaked
-
I would build as is and then mod later, but I've done that many times. I have so many amps (kind of an addict/amp hoarder) and already know that I'll want the change the normal channel to do something different. I think you may be right about the tremolo and reverb though as they are characteristic of the brown era sound and worth trying as is.
In all of the demo videos and clips, everyone uses the bright channel only. Seems just like with the AB763 amps where you might as well change it up a bit for the 'unused' channel. I'm not sure if the reverb and tremolo work on the normal channel in the stock build? I'm guessing not? It might be nice to make them work for both channels?
-
I used the 5F6A tone stack cap values on the normal channel of my '67 Bandmaster. Sounds a little different but I'm not really impressed. Easy to do.
The reverb only works on the bright channel but the Tremolo works on both channels since it wobbles the grids of the output tubes. You will most likely have a difficult time getting good tremolo and a satisfactory bias setting when using 6L6s. There a trem mod sheet somewhere in Hoffman's library that deals with getting a stronger tremolo. Not easy to find but the search will build character. :icon_biggrin:
-
The reverb only works on the bright channel but the Tremolo works on both channels since it wobbles the grids of the output tubes. You will most likely have a difficult time getting good tremolo and a satisfactory bias setting when using 6L6s. There a trem mod sheet somewhere in Hoffman's library that deals with getting a stronger tremolo. Not easy to find but the search will build character. :icon_biggrin:
Is this the page you are talking about? https://el34world.com/Hoffman/files/Hoffman_AB763_Mods.pdf (https://el34world.com/Hoffman/files/Hoffman_AB763_Mods.pdf)
Also it looks like the schematic I linked above from Mojotone has the tremolo applied to a 12AX7. Maybe they like that method better than affecting the bias of the output tubes?
-
That's it! How did you find it so fast? I can never find it unless I'm looking for something else. :l2:
-
Ha! I started searching immediately. See my question just above about the Mojo schematic which looks like tremolo is being accomplished with a 12AX7 and may not need much ‘fixing’?
-
Also it looks like the schematic I linked above from Mojotone has the tremolo applied to a 12AX7. Maybe they like that method better than affecting the bias of the output tubes?
No. The Mojo trem osc is connected to a cathode follower which in turn wobbles the bias of the output tubes. The Mojo 6G16 and the Hoffman AB763 trem circuits are identical. That's because Hoffman's AB763 is not a pure breed. His AB763 used the 6G16 bias vary tremolo, rather than the optocoupler used in a real Fender AB763.
-
Ok, I see. So the good news is your mods page I found can help increase the trem if it's weak?
-
Yes. The LED mod is the most effective.
-
I've had the custom vibrolux reverb, which is basically a hotrod 63 vibroverb. That trem was amazing!!! And so amazing I converted my opto trem vibro king into bias vary trem.. both 6L6 amps. No amp has deeper trem than those..
And at any bias setting it's strong. I think it was tubeswell that helped me on that one. It took some cap tweaking to get it deep and right
-
Cool. Good to know it has the potential to be deep and swampy.