Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hiegdk on September 30, 2023, 07:56:25 pm
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Hi all, I've seen lots of examples of AB763 builds that omit the normal channel but haven't come across any that omit the vibrato/reverb channel instead. I'm interested in building a AB763 Twin Reverb head with just the normal channel and was wondering if anyone here has done something like that before. I have a '66 TR that's in amazing shape sounds great - so I have the vibrato channel sound covered when I need it - just looking to build something a little more portable that I can pair with some of my cabs for different vibes. I'm hoping to build it in a smaller chassis than a typical TR as a head version of a standard TR won't fit on my primary cab, a Dr. Z Backline 4x10.
Appreciate any advice or wisdom this community has to share!
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Rob Robinette's Blackvibe (https://robrobinette.com/RR763_BLACKVIBE.htm) is exactly what you are describing.
AB763, no reverb, no trem & is a sorted out design.
It should fit in quite a small package.
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http://sluckeyamps.com/misc/AB_Lite_II.pdf
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Awesome, thanks for the quick replies! I assume modifying either of those designs to use solid state rectification and a 4x6L6 output section like the Twin Reverb would be fairly trivial...
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I wouldn't use the term trivial, but it is straightforward.
You do have to choose the correct PT, OT, and provide space for the extra tube (of course).
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Yeah, I was planning on getting the iron from Mercury. I'm also trying to determine if there's an appropriate ready-made chassis available somewhere or if I'm going to have to start with a blank chassis and drill it out myself.
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MM's prices are out of whack. I've had great luck with Hammond iron from HawkUSA (great prices, fast shipping.
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Yeah, I was planning on getting the iron from Mercury. I'm also trying to determine if there's an appropriate ready-made chassis available somewhere or if I'm going to have to start with a blank chassis and drill it out myself.
This chassis may work - I bought one - well made. https://www.tedweber.com/6mxch/ (https://www.tedweber.com/6mxch/)
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Getting one specific to your stripped-down circuit might be a stretch, but maybe something that came drilled for a tube rectifier would come close, as you would only need to add one 8-pin tube socket hole (using a SS rectifier)
A Princeton Reverb would work for the board but might be too small for the transformer set.
I have found a nice in-between size is a Blackface Bassman chassis. Shorter than a 2-channel AB763 but it still has the big amp front-to-back depth.
I have bought several chassis (all blanks) from Zach Hunter on eBay & I have been happy with them. https://www.ebay.com/str/zhmusicparts
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Oh a whim I picked up a BF Princeton chassis from mojotone to see if I can make everything fit. I picked up some iron from Hammond, (290FX PT, 1760W OT, and
194B choke). As mentioned earlier in the thread I want to build a Blackvibe/Twin Reverb hybrid.
I know cramming that power and output section into this chassis is a little silly, but I'd really like to end up with a fairly compact head. Any advice on how I might accomplish this safely and reasonably? Considerations for modifying the chassis and/or placement of transformers? Has anyone every done this before? Should I really be looking at a larger chassis?
Attached a photo of the iron haphazardly placed on the chassis for reference.
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That project will easily fit your PR chassis. Heck it would fit Hoffman's Blank Aluminum Chassis 17 x 6.5 x 2.5. I would make a simple change to your proposed layout... Move the right octal socket to the left and put it in the hole where the OT secondary leads are presently located. Then put the choke in the spot that octal socket was located, ie, choke next to the PT.
Do you have a board layout?
Look what I built in a smaller 12.5 x 6.5 x 2.5 chassis... http://sluckeyamps.com/phoenix/phoenix.htm
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And, with sluckey's guidance, I also fit a Fender 6L6 circuit into the Hoffman Stout chassis; my Musing/AA864 40:
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That project will easily fit your PR chassis. Heck it would fit Hoffman's Blank Aluminum Chassis 17 x 6.5 x 2.5. I would make a simple change to your proposed layout... Move the right octal socket to the left and put it in the hole where the OT secondary leads are presently located. Then put the choke in the spot that octal socket was located, ie, choke next to the PT.
Do you have a board layout?
Look what I built in a smaller 12.5 x 6.5 x 2.5 chassis... http://sluckeyamps.com/phoenix/phoenix.htm
Thanks sluckey! I've ordered a BlackVibe eyelet board and a Bias Board 2 from Hoffman.
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AB
763 Normal Channel Only ?
(https://i.imgur.com/NMMY7WW.png)
Sorry I wasn't able to resist
:l2: :l2: :l2:
Franco
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That project will easily fit your PR chassis. Heck it would fit Hoffman's Blank Aluminum Chassis 17 x 6.5 x 2.5. I would make a simple change to your proposed layout... Move the right octal socket to the left and put it in the hole where the OT secondary leads are presently located. Then put the choke in the spot that octal socket was located, ie, choke next to the PT.
Do you have a board layout?
Look what I built in a smaller 12.5 x 6.5 x 2.5 chassis... http://sluckeyamps.com/phoenix/phoenix.htm
Something like the attached?
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yes
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Would this amp - Blackvibe with Twin Reverb power section - need a standby? I know some amps do and some don't based on inrush current, etc... Is the standby on these higher power SS rectified AB763 circuits a glorified mute switch, or is it serving a vital function to protect the rest of the amp from a big spike when power on?
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I don't normally include standby switches in my builds.
See: http://www.valvewizard.co.uk/standby.html (http://www.valvewizard.co.uk/standby.html)
If you're worried about pops when plugging in or unplugging instruments, you can:
1) Use SilentPlug (Neutrik) plugs on the instrument side -- highly recommended. Or,
2) Partially pull the plug from the amp -- the tip is disconnected, but the ground path is intact -- and then you can unplug your instrument without a pop. Reverse the process for plugging in. Works great, little fuss.
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Thanks acheld, great link.
Another question... if I only have a 1/4W 47r resistor for the NFB tail will that work OK? Using 1/2W or larger everywhere else.
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.. Is the standby on these higher power SS rectified AB763 circuits a glorified mute switch, or is it serving a vital function to protect the rest of the amp from a big spike when power on?
Standby here can be seen as being a very crude mute switch. I suppose that in its favour, it may save a bit of valve life whilst in the muted, standby mode.
I recall some of the recent Fender Bassbreaker models have an audio mute switch (labelled mute), rather than standby.
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It's a bit of a dog's breakfast but it works! There a some things I'd do differently if I were to do this over. The Princeton chassis was a bit annoying to work in, mainly because the oversized PT ended up a little too close to the lamp (my fault). I place the board a little too close to the front of the amp - it fits, but just barely. The wiring could be neater too. It is working though, and it sounds good! Experimented with a PPIMV and an adjustable bright cap, decided to ditch both so I know have a superfluous pot.
Also, the labels on those large F&T cabs seemed to rub off with very minimal handling.
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If it sounds great, that's all that matters! Once you put this in a cabinet, no-one else will see your wiring . . . it'll be a dog's dessert!
the labels on those large F&T cabs seemed to rub off with very minimal handling.
The beauty of F&T caps is that you won't need to handle them ever again. I've never had one fail! I'm sure they do, but those caps are the bees knees (or whatever!).
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Thought I'd follow up with the final product. I have a neighbour who builds cabinets so I got him to whip up this head cab for me. Got slightly customized faceplates from Mojotone. Ended up removing the PPIMV so the Master knob is a placebo now - perfect for when the sound guy demands I turn down :icon_biggrin:. Might take some better photos later.
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Looks nice!
I always like to see the follow-ups, thanks for posting.
PS Didn't realize Mojo was offering custom faceplates again.