Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on March 09, 2021, 03:50:43 pm
-
I have one on the bench and it's been modded with an extra tube in the normal channel. The owner said he bought it like that (25 years ago) and it had KT66's in it. At some point he had it serviced and they had undone some of the mods. He brought it to me and has given me free reign to "just make it sound good".
I'm tempted to restore it to original with a few sensible tweaks but I'm unfamiliar with the TFL5005 model. Can somebody shed some light on what it is? It seems like an odd naming convention for a Fender amp.
-
https://el34world.com/charts/Schematics/files/Fender/Fender_bandmaster_reverb_aa1069_schem.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_bandmaster_reverb_aa1069_schem.pdf)
It's an AA1069 Bandmaster Reverb.
They also made one without reverb and it was designated TFC-5005.
-
http://ampwares.com/amplifiers/fender/bandmaster/fender-bandmaster-reverb/
-
These amps are super easy to convert to AB763 specs and will yield great results in doing so. At one point these things were dirt cheap and make great blackface conversions. The original circuit is pretty meh.
-
These amps are super easy to convert to AB763 specs and will yield great results in doing so. At one point these things were dirt cheap and make great blackface conversions. The original circuit is pretty meh.
You are 100% right.
And what is the look of the circuit right now with all mods :w2:
-
I'm going to propose doing just that-restoring to AB763 and then taking it from there.
-
Be prepared for weirdness - SF era 022848 OTs have a rep for causing instability.
I suggest to put it back stock (but adding a better bias adjustment); it will accommodate being overdriven more gracefully than if reverted to AB763 (better bias shift mitigation).
-
pdf64-Care to elaborate on a "better bias adjustment"?
-
Do you want to retain the bias balance feature? ie have an additional trimmer for overall bias?
-
Ah, so it seems the original ckt only had a "fixed" fixed bias and the bias pot was actually a balance pot. RobRob shows how to add the 25k pot to make it a true bias adj. ckt. I assume this is what you're referring to?
-
Yes, a 25k pot sounds an excessive value though.
-
I have one on the bench and it's been modded with an extra tube in the normal channel. The owner said he bought it like that (25 years ago) and it had KT66's in it. At some point he had it serviced and they had undone some of the mods. He brought it to me and has given me free reign to "just make it sound good".
I'm tempted to restore it to original with a few sensible tweaks but I'm unfamiliar with the TFL5005 model. Can somebody shed some light on what it is? It seems like an odd naming convention for a Fender amp.
TFL 5005D? AA768 was in a bunch of them. Some gut shots might help determine what you have.
-
Personally I like to completely blackface the output stage, including the bias circuit, and replace the OT with one from a blackface Bassman. If it isn't a '68 or '69 you probably need to get rid of the brown turd or blue blob coupling/signal capacitors.
-
... If it isn't a '68 or '69 you probably need to get rid of the brown turd or blue blob coupling/signal capacitors.
What’s the necessity, they’re typically excellent caps, almost invariably absolutely fine?
I see very few BF amps, but of those, most seem to have a 0.1uF brown radial film type coupling cap in their reverb channel, usually between the 3rd stage anode and the trem intensity / channel mixer resistor. Or a ceramic, which due to its value and voltage inevitably must be a low spec class 2 type.
That coupling cap might be the most heavily loaded one in the amp, due to the high signal level and the 50k load of the pot, so if it had shortcomings, they might be expected to be made readily apparent.
Yet BF amps and their blue molded caps are regarded as the holy grail by the ‘awesome tone’ aficionados, it seems they’re happy to ignore the other cap types in that scenario.
The magical status attributed to the blue molded caps seems somewhat irrational to me, given that ceramic and brown radial film cap types are also used alongside them for signal coupling / filtering in BFs :dontknow:
-
Pdf64, After '69 they most likely won't have the blue molded caps they will have either the brown turds or blue blobs and I have never heard a BR with those caps which I thought sounded very good. You may like them, I'm just giving my opinion based on my experience. If I remember correctly the cap from the vibrato intensity pot is not a signal carrying cap, it actually shorts out the signal, but I could be mistaken.
-
If I remember correctly the cap from the vibrato intensity pot is not a signal carrying cap, it actually shorts out the signal, but I could be mistaken.
It's definitely a coupling cap.
-
This one has brown turds. But since this amp has been severely modded, it's probably moot. The owner got back to me and said he just wants it to sound like a Marshall with that '70's crunch. Apparently at one point, this amp sounded good (like that) but it was worked on later and half of the "good mod" was butchered.