Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Underwood on March 19, 2015, 09:53:28 am
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:BangHead:
Hello,
I have just tried to get my 6g12 Fender concert amp correct to the schematic on the volume,bass and treble controls. I put all the values as to the 6G12 SCHEMATIC. It actually sounds great now, but the treble can be all the way down and not cut much treble. The bass does not cut much either. From 1-3 is usable and after that it is full on bass until all the way up it cuts out sound completely. I just found a schematic/layout of a 6G8 twin and it had the same cap/resistor combo as I originally had, that I thought was wrong. Obviously this was a transitional experimental period at Fender. The problem is that the pots are 1meg for bass, 350k for treble on the twin schematic, which is probably what mine are. I am going to check and make sure today. The schematic that I changed it to for the 6G12 Concert calls for 250k for both. I can always put it back to what it was before, but I really like how it sounds. I just want to get the bass and treble to be correct(usable). In the pictures, the first is the 6G12 concert and the other is the 6G8 twin. The only other problem is that the treble was not exactly wired like either one. It had a cap that said 210m on it and not the .00025 and .005. :dontknow: :dontknow:
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I forgot to mention that my treble pots have the 70K tap like on the twin schematic. I presently do not have anything attached to it. I have the bass connected to the treble pot from left terminal to left terminal like in the Concert layout.
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I don't really have an idea how those tone circuits should sound or feel.
But, if you are going to wire it IAW the 6G12 schematic you really need to use a 500K-L volume pot, a 250K-L treble pot and a 250K-A bass pot. Using wrong value pots or pots with a different taper will very likely throw the feel and sound of the pots off.
I do know how the AB763 tone stack sounds and feels. It gives a very positive control over bass and treble and it has a scooped mids sound also. Maybe that's an option unless you want to keep the circuit true to the 6G12 schematic.
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First, the "210" cap is most likely a 250 pf (aka .00025) and there isn't much difference between those values...it's just a different way or marking the cap value.
Beyond that, you have something of a quandary. First, check your pot values & note them.
" but the treble can be all the way down and not cut much treble. <---not right
The bass does not cut much either. From 1-3 is usable and after that it is full on bass until all the way up <--most AB763 blackface Fenders, you really can't turn the bass up much past 4 without muddying things quite a bit. <-----not wrong
it cuts out sound completely. <----not right but all you have to do is not to turn your bass all the way up.
I am not sure what I would suggest. You are clearly talking about de-stockifying the amp and I think you said earlier you did not wish to do this. It may well impact value. As is, however it works, you say it looks stock stock stock, yes? (OR do I have you confused w/another poster?)
Maybe you should sell the amp for $2750 and buy two smaller amps and put $1.5K In the bank
Maybe you should make one channel AB763 = 250K / 250K / 1 Meg treb / bass /volume
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Maybe for now I will put it back to original on the Vibrato channel and keep it the way it is on the normal channel. Would that hurt anything? Then I have the option of putting it back completely if for some reason I did sell it. Don't want to sell it though.
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While it was stock, it never had the .005 cap on the treble. Isn't that like 5000pF? Wouldn't that change the sound alot if I put that on there? Maybe it was not on it for a reason? This was stock from the factory before I did the electrolytics on it.
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That Twin tone stack with the tapped Treble pot is an entirely different animal from your Concert. I suspect that the Twin stack came shortly afterward because it's even closer to the classic Blackface tone stack from the AB763 etc. I did build a Super Reverb with a tone stack like that Twin. It was pretty cool and not as mid-scooped as the Blackface tone controls IIRC.
http://el34world.com/charts/Schematics/files/fender/Fender_CONCERT_6G12.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_CONCERT_6G12.pdf)
You don't want a 5,000 pf treble cap in place of the 250pf cap shown.
BTW cap value conversion chart here: http://el34world.com/charts/Charts.htm (http://el34world.com/charts/Charts.htm)
Is that circuit really using 3 triodes for the tremolo? That's wild, and I've never seen tremolo inserted like that.
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Mine is the earlier 5 preamp tube version not the six. It uses 2 for the tremolo and the the other uses 2 1/2 I think. As for the .005 cap, it is on the schematic along with a .00025 cap. Which I think would be 5000pF and 250pF. The twin schematic seems to be exactly what my amp came with stock on my tone controls. Except mine did not have the .005 cap. What would the sound difference be with a .005 cap on there also. It was on the left terminal and grounded to the pot on the schematic. I think my left terminal was just grounded to the pot with no cap. The .ooo25 is on the other side attached to a resistor coming from the right bass terminal. They connect to the brown wire going to the board/ preamp pin 1.
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Looks to me like you have a .01uf between the treble pot and ground instead of a .005uf, a 220K instead of 180K between the bass pot and the treble cap, and a 210pf treble cap instead of a 250pf.
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Yes that is how it was originally. I thought I would change it to the 6G12 schematic, but I realized the pots are different. I have changed everything back except on the normal channel I used a 100K and not a 180K(it came with a 220K from the factory)220K on the vibrato channel. I also did not use any cap in place of the .01 on the treble, maybe I will pot a .005 on or maybe leave it alone.What do you think? It also came stock with the 210pf and I left that alone on the vibrato channel, but I put a 250pF on the normal channel.