I picked up an AB165 for a reasonable price last year. I am working through it, still waiting on plate resistors to arrive and power tube sockets (one has a bit of a chunk out of it). It sounds really nice except for some crackling and hum I am tracking down. I have not converted the bias yet. I am on the fence because it does seem to help quiet the hum. There are two wires grounded from the PT. Am I correct in assuming that one of these is the center tap for the heater supply?
I am considering modding one of the channels to for a higher gain option. Has anyone done BillyZ's Longhorn Mod? BillyZ, if you are out there, the schematic you posted was a few years back. Are there any refinements or new findings since then? Does anyone have other options for a high(er) gain channel? I know that with the topology, you can't really build a Marshall per se, but I would like to have one channel with some real balls to it.
AB165 Longhorn Mod and other questions
Moderators: pompeiisneaks, Colossal
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Gibsonman63
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BetterOffShred
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Re: AB165 Longhorn Mod and other questions
I've been looking at the 'Hyperon' mod, which I believe has schematics available with a Google search. It basically adds another gain stage to the bass channel. Not sure how much balls this will add based off the original circuit, but I have all the parts so I think that may be a summer project. It's worth taking a look at maybe.
-Brett
-Brett
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Stevem
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Re: AB165 Longhorn Mod and other questions
Um, before you start to Mod it you need to get it working right as is or you will be falling into your own trap and pulling your hair out!
Have the power supply filters been changed out yet, and in petucular the bias filter Fender uses is not rated at enough voltage for what the PT applys to it, so it needs to be up rated!
Get the basics up to snuff, then move on!!
Have the power supply filters been changed out yet, and in petucular the bias filter Fender uses is not rated at enough voltage for what the PT applys to it, so it needs to be up rated!
Get the basics up to snuff, then move on!!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Gibsonman63
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Re: AB165 Longhorn Mod and other questions
Agreed Stevem.
I have changed the filter caps, cathode bypass caps, bias cap (100uF/100v), upgraded to a three prong power cable, removed the death cap and ripped out all of the previously poorly executed mods. Yesterday my shipment of plate resistors arrived and that pretty much fixed the rest. The Bass channel runs a little quieter than the normal channel in terms of noise, but both are now tolerable and I have not done any tube rolling yet. Next will be updating the bias to fixed bias adjust and swapping out the power tube sockets and resistors soldered to them, then I should be ready to work on a high gain channel for it.
I have changed the filter caps, cathode bypass caps, bias cap (100uF/100v), upgraded to a three prong power cable, removed the death cap and ripped out all of the previously poorly executed mods. Yesterday my shipment of plate resistors arrived and that pretty much fixed the rest. The Bass channel runs a little quieter than the normal channel in terms of noise, but both are now tolerable and I have not done any tube rolling yet. Next will be updating the bias to fixed bias adjust and swapping out the power tube sockets and resistors soldered to them, then I should be ready to work on a high gain channel for it.
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Stevem
- Posts: 4750
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: AB165 Longhorn Mod and other questions
You should think about using a pentode first gain stage as due to there screen you adjust things to get compression/ sustaine without clipping, which is a big plus!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: AB165 Longhorn Mod and other questions
I did not see your post . I retired last year and have not been too active on this forum.
I do have a few refinements . The lead dress is critical especially on the normal channel after the mod.
Definitely use a 500pf coupling cap to the phase invertor.
I prefer polystyrene but use what you got.
I'll dig into my notes and get back to you.
Z
I do have a few refinements . The lead dress is critical especially on the normal channel after the mod.
Definitely use a 500pf coupling cap to the phase invertor.
I prefer polystyrene but use what you got.
I'll dig into my notes and get back to you.
Z