Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: j.w.goode on November 14, 2014, 10:04:34 am
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I have some basic questions about the differences between the AB763 and the 5F6A preamp circuits. I am investigating replacing the normal channel 1 on the AB763 with the 5F6A. Everything above the red line on the Hoffman AB763 schematic with the Hoffman 5F6A preamp up to theC8 coupling cap. My thinking is a tweed sound on channel 1 and a blackface on channel 2. I realize it will need another tube but is there any thing else i'm overlooking. Any thoughts?
jw
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That'll work. You don't even have to use an extra triode if you use a MOSFET for the cathode follower tone stack driver.
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That'll work. You don't even have to use an extra triode if you use a MOSFET for the cathode follower tone stack driver.
+1
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That'll work. You don't even have to use an extra triode if you use a MOSFET for the cathode follower tone stack driver.
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+1
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Thanks guys, I'm not exactly sure how to incorporate the mosfet into the circuit so I will probably do another tube. I do have some ather questions.
The 5F6A was origanally the Bassman Amp with 4 tens. What, in the 5f6a circuit makes it a bass amp? and can it still be used as such with the ab763 power section.
jw
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The preamp and speakers made it a bass amp. The AB763 (6L6 versions)power amp into a bass cab will be fine for a bass amp.
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I have some basic questions about the differences between the AB763 and the 5F6A preamp circuits. I am investigating replacing the normal channel 1 on the AB763 with the 5F6A. ... My thinking is a tweed sound on channel 1 and a blackface on channel 2.
You may want to ask yourself, "what is the 'Tweed' sound?" Go play a Bassman amp (reissue or otherwise), and then play a Super Reverb (reissue or otherwise) at same/similar volume settings. Aside from the lack of effects, I'm betting there's not much difference in sound (at least, not as much as you might imagine).
The tweed Deluxe, Princeton and Champ have much more midrange-heavy sounds due to either a lack of tone controls, or a simple tone control circuit that cuts bass or treble but does not impact midrange.
The 5F6A and AB763 amps both have tone circuits which cut midrange heavily and add controls to allow the Bass & Treble to be cut independently (the simple circuit in the small amps reduce bass-cutting and increase treble-cut as you turn the control down, so the actions are not independent).
The other mid-power tweed amps (Super, Pro, Bandmaster) evolved to an altogether different odd-duck tone circuit that's in-between the other types described above.
Having done what you're thinking of doing, I can tell you what you'll probably get: the 5F6-A channel will have a little more midrange and a LOT higher signal level than the AB763 channel, mainly due to the cathode follower driving the tone circuit for less loss.
Or, you could go a different route. If you imagine that a tweed amp should have more midrange and drive than the AB763, you could simply add a much-higher-resistance pot in between the Mid pot and ground. If you increase the resistance between the Mid pot and ground (to say, 250k-500kΩ), mid-cut in the tone circuit drops to almost nothing, and perceived signal level & distortion goes up. Your tone controls will likely have little effect (except maybe the Treble control on high-treble settings) because the way the cut signal has been removed.
This is sometimes proposed as a switch to disconnect the tone circuit from ground (to increase resistance to ground to infinity), or called a Raw control.
I throw this out for you to think about as someone who's had original tweed amps and self-built copies, and found the reality is often different than what you expected after reading a description of them.
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Sorry it took so long to get back I was finally able to get to Guitar Center to compare the bassman and super reverb. They didn't have either. The salesman said Fender has already done that a couple years ago. He showed me an amp called "68 custom deluxe reverb".This amp has the reverb and vibrato on both channels and a bassman tone stack on what would normally be the non reverb / vibrato channel. It sounded really good with subtle difs between the BM and DR side. I looked it up on you tube when I got home, its all ove the place. I can't find a schmatic to it anywhere though.
I was looking to replace the DR channel 1 circuit with the bassmam preamp circuit. If I understand this amp right, I may be able to change out the tone stack. I'm not sure what exactly makes up the tone stack. I know it encompasses the Treble, Bass and Mid pots on the schematic but not sure about all the surounding components. Can someone show me what circuitry is the complete tone stack? It would be greatly appreciated, Thanks.
jerry
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Here are the two tonestacks for comparison. Just replacing the DR TS with the Bassman TS will not make it sound like a Bassman. You really need the entire Bassman preamp.