Bassman AB165 help needed

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Re: Bassman AB165 help needed

Postby Gasstationman on Thu Jan 07, 2016 1:29 am

Is this the Allen mod?
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There's some Allen mod schematics so I'm not sure what circuit that is the right one. You have blue caps for the power amp if it is.
I have fiddled with an AB165 ending up with the basschannel very similar to the normal. The differences are 470pF treble cap .022uF for mid and bass and 820ohm biasresistor on the inputstage and one tube removed. Much work and the "easiest" solution sounded best. Unfair but true.
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Re: Bassman AB165 help needed

Postby BetterOffShred on Thu Jan 07, 2016 3:23 am

That is the schematic from el34world, but it's been said it's veryvery close to the Allen mod, but with different deep switch caps for the 'modes'. I know after the headaches I've had with my own troubles I created for myself that it may seem a bit foolish to keep messing with this thing.. which is why I'm not going to mess with it till it's working with my initial conversion. But hey I don't want to get rid of it, and I'd like to get the most out of it.

Thanks for looking! And for posting that schematic too :thumbsup:

-Brett
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Re: Bassman AB165 help needed

Postby Dean Jr. on Thu Jan 07, 2016 5:50 pm

My bad...I took that 1969 dating in the first post to be correct...and missed the chassis stamp for the 12th week of '68. +1 on it being an AB165. I still advocate maintaining the 3 gain stages in the Normal channel. People pay huge dollars for Trainwreck Express amps for a reason.....and there is the basis for an express sitting in the that channel. YMMV....
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Re: Bassman AB165 help needed

Postby Gasstationman on Thu Jan 07, 2016 10:30 pm

I had to have a look. Trainwreck Express is a different animal than the Allen Mod. Moderate gain in all stages. No divider network with one channel, high return signal from the feedbackloop , .1uF into PI and no MV. My guess is loud and very good tone. Can one channel and no divider be a big part of the magic?
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Re: Bassman AB165 help needed

Postby BetterOffShred on Fri Jan 08, 2016 3:23 am

Huzzah!
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My components showed up, so I can spend Friday night, and/or Saturday getting it going, and hopefully get some sound out of the old girl once again! (note the delay pedal with no shell)

As for the Allen vs the Trainwreck, I really want the 2 stage gain 'Glassy' sound from the AA864 normal channel. It's one of the main reasons I decided to continue with this mod after learning more about it. I need to read more about the bass channel mods and see if that EL34world schem is for me, but it's looking good so far. I'm planning on juicing up my signal with a Wampler Paisley clone for some nice OD. I really like that pedals tone, and Brad Paisley's playing, but I don't really enjoy his music overall.

Anyway, I'll post some pics hopefully tomorrow night once I get everything In and solid! :)

-Brett
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Re: Bassman AB165 help needed

Postby BetterOffShred on Sun Jan 10, 2016 12:59 am

Well I decided to get the Delay pedal off my desk before I started work on the Bassman, so I braved the cold and fired up the drill press... I painted this case like 4 years ago for a different project, but thought it was kind of funny, so I used it. The delay:
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And then this morning and this afternoon I spent quite a bit of time with the soldering iron, working on finishing the AA864 normal channel on my Bassman.. here it is
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But as I'm sure some of you will notice right away.. I accidentally burned the crap out of one of the .1's :POd: so.. I hope it's not ruined.. Anyone have any experience with this kind of burn on a cap of this nature? It looks pretty deep.. I already ordered another one and some Mallories for the .1 and .047 in the tone stack. Thanks for looking..
Image

-Brett
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Re: Bassman AB165 help needed

Postby Gasstationman on Sun Jan 10, 2016 3:52 pm

Shit happens but it looks fine besides that. Measure the biasvoltage over both 0.1caps before you put powertubes in the amp. You get a red tube if the cap is shorted.
AB165 have .02 coupling caps after the PItube. I don't think you will hear any difference with .02 .047 or .1 caps when you play guitar and probably not with bass either.
I looked at the first pics and saw that the leads to the octalsockets are wrapped and twisted around eachother. Big risk for oscillation and overhearing. Rewire without parallel wires as much as possible. Well spent 15minutes. This one looks ok.
Good luck.
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Re: Bassman AB165 help needed

Postby BetterOffShred on Mon Jan 11, 2016 4:19 am

I sure appreciate you fellas taking the time to look over my stuff and provide valuable input, thanks so much.

Could you elaborate a little over what you mean on the octals? I know you mean the 6l6s, but I'm not sure what you mean otherwise. The wires off the 6l6s are certainly not neat and tidy
. Maybe I should try to clean that up a little?

-Brett
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Re: Bassman AB165 help needed

Postby pdf64 on Mon Jan 11, 2016 1:29 pm

Gasstationman wrote:Shit happens but it looks fine besides that. Measure the biasvoltage over both 0.1caps before you put powertubes in the amp. You get a red tube if the cap is shorted.
AB165 have .02 coupling caps after the PItube. I don't think you will hear any difference with .02 .047 or .1 caps when you play guitar and probably not with bass either.
I looked at the first pics and saw that the leads to the octalsockets are wrapped and twisted around eachother. Big risk for oscillation and overhearing. Rewire without parallel wires as much as possible. Well spent 15minutes. This one looks ok.
Good luck.
Image


Ha, how interesting, I disagree on every point!
That's a terrible place to risk a dodgy cap. OK if it shorts (now or at a later date) then the power tube will immediately red plate and hopefully the line fuse blow before damage is done, most pertinently to the PT and OT.
But it may just get leaky and gradually cook the amp, to the point where all the iron gets damaged. Bear in mind that a slow blow mains fuse, which is all there is with such a Fender, may take many minutes, perhaps even hours, to blow, even at twice its rated current.
Slow blow fuses are by nature slow to respond to short type fault current, but that means they take forever to stop heavy current flow that's not much more than their rating.
Adding a fuse for the HT winding is a great use for the ground switch hole in the back panel.

Smaller coupling caps may not sound much different while the amp is used in its linear range, but NMV tend to get overdriven, with the power tube control grid being the first point of the amp to clip, and the last to leave clipping. Bias shift (leading to blocking distortion) is therefore a big issue, and smaller coupling caps will tend to mitigate that.

Regarding lead dress, capacitance connects everything to everything else; it's the job of screening to minimise that but it's impossible to eliminate it, which is where lead dress can help to assist good coupling (eg to ground) and minimise bad coupling (eg positive feedback).
Twisting the signal conductors of balanced system will achieve both, as it should act to to minimise the radiated EM field, and should act to roll off super audio frequencies, thereby further preventing them from coupling back to sensitive earlier stages.
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Re: Bassman AB165 help needed

Postby Gasstationman on Mon Jan 11, 2016 9:42 pm

Brett you're right, octals are sockets for the powertubes.
Good luck with the rebuild.


Sorry pdf64, you are at the wrong forum, this is for music and tube amplifiers. You are welcome to join this forum instead.
Enjoy.
http://www.mig-welding.co.uk/forum/thre ... rol.57791/
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Re: Bassman AB165 help needed

Postby pdf64 on Tue Jan 12, 2016 8:51 am

Meow - I salute your humour!
By strange coincidence a friend gave me a treadmill control board to fault find at the weekend (treadmill motors being the topic of the linked thread).

WRT tube amps, just to note that neat and tidy wiring can often run counter to good lead dress.
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Re: Bassman AB165 help needed

Postby Gasstationman on Tue Jan 12, 2016 10:13 pm

That was actually a very funny coincidence.
It must have been a sign from the guitar gods.

It's hard to tell sometimes. Here is an oscillator.
Image
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Re: Bassman AB165 help needed

Postby pdf64 on Tue Jan 12, 2016 10:41 pm

Those OT wires to the plates look rather tight!
But I can't see any control grid stoppers, which I thought all the post tweed Bassman models had as stock?
Fitting those would be my first line of attack.
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Re: Bassman AB165 help needed

Postby pdf64 on Wed Jan 13, 2016 9:10 am

The model identity of the amp guts pictured has been bugging me, specifically the combination of 0.1uF coupling caps to the power tube control grids, no control grid stoppers, tube rectifier and 100pF stability cap across the LTP plates.
I had been trawling through BF schematics, as with the tube socket placement it wasn't a tweed, but forgot that the brown 6** series had the same chassis layout as BFs.
I think it may be a 6G11 Vibrolux?
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Re: Bassman AB165 help needed

Postby Gasstationman on Wed Jan 13, 2016 10:30 pm

It is a Brown Fender Pro that was back at the factory in June 1963 to get a 125P5B PT and tube rectifier. Probably to get rid of the very hard sound caused by the oscillation. It's ok now. No blockers and 100pF cap is original. The screengrid resistors are changed.
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