Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MikePR on August 19, 2015, 05:10:01 am
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Hi i am new to the forum. Just got this 1993 Fender 410 Super Amp used and decide to modify it to be a single channel reverb one with no footswitch for reverb nor effect loop. Here included are the original schematic, the edited version i did plus two pre amp and power amp section ones that i made from my edited version. This is my first built and i think i did my homework. I think i am at a point of saturation. Just changed the resistor based fixed bias (shown on R33 in original schematic) to an adjustable one with a 25kl pot plus a 15k resistor. Also changed the diodes to 1N4007's. Would like somebody to check it out and tell me if i am in the correct track. I am not sure about the bias section of it with all the extra caps either. Thx
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modded
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final product...
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While you're waiting for a guru to respond, a suggestion for your bias. Replace the original R104 (33K) with your 25KL pot. in series with a 25K resistor. Hook one end of the pot. to ground and also hook the wiper to ground.
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Welcome!
I do not see why you kept the +/-16V supply. That's probably for chips and switching, which you have chopped away.
When you do that, be sure to keep the ground (conceptually at PT Brn CT, but implemented at the far end of the +/-16V supply).
Your R51 bias pot will NOT work. It is in series with an infinite resistance (6V6 grids). Please look at known-good bias trim schemes.
The markings on your Brown leads make no sense. They seem to say DC voltage where is only AC voltage, 24-0-24 (48VCT).
If you truly forever lose the +/-16V supply, there is a *wee* bit simpler way to use the brown leads to get the -72VDC which you will divide-down to your desired bias. But it doesn't save much parts so I guess it is betetr to stick with the way it is.
R26 R27 seem odd. Yes, that seems to be on Fender's blurred plan. But one resistor will do here. If R26 has to be 3 Watts, then R27 has to be 1.7 Watts (and R32 R33 even more!); however some thumb-twiddle suggests R26 is throwing 0.06 Watts. And thinking over similar long-tails I'd expect R26+R27 to be 15K-50K, not 11K.
The series 4.7u+22u caps in the tail of the tail are also odd, though this *might* skew the bass distortion (unpredictably).
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R26 R27 seem odd. Yes, that seems to be on Fender's blurred plan. But one resistor will do here. If R26 has to be 3 Watts, then R27 has to be 1.7 Watts (and R32 R33 even more!); however some thumb-twiddle suggests R26 is throwing 0.06 Watts. And thinking over similar long-tails I'd expect R26+R27 to be 15K-50K, not 11K.
The series 4.7u+22u caps in the tail of the tail are also odd, though this *might* skew the bass distortion (unpredictably).
Looks like Wilkens uses R26 and R27 to fine tune the PI in conjunction with R61 and R62 in order to achieve an output that has exactly the same magnitude on both sides.
C53 and C54 along with R115 are about the same as having your presence control almost off. It's not going to do anything until you get down to around 200Hz and even at 82Hz it's going to be real subtle.
In both cases, it looks like Wilkens uses combinations of readily available values to get an oddball value that achieves a level of precision that is totally unnecessary.
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Thx for the input guys. Have homework to do then.
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Here is where i got the Brown numbers.