Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: alathIN on April 11, 2020, 03:17:52 pm
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This project is based on Rob Robinette's micro AB763 schematic with the following differences:
1 - reverb, which Rob deleted, added back in
2 - second preamp channel based on Matchless EF86 circuit
My main questions are about the reverb circuit and how/where to bring in the channel II signal
Note: the inline image didn't work well. Attached as a .sch file if you have that program, jpeg if you don't.
Reverb:
- this is adapted from a Merlin reverb driver circuit
- not sure about how I handled the dwell control and screen resistor
- I already have the standard reverb transformer and a tank that are compatible with AB763s - but not sure they'll work with this driver circuit. I just built a Weber ReVibe with a 6k6 reverb driver and I love the sound - but it used a much bigger/beefier reverb transformer than the standard AB763 reverb transformer. I don't even know what specs to look at / calculate to know if this needs to change
Channel II
- This is based on the Channel I from my Trinity TC-15, which sounds great to me - but would be nice to have in a less-loud amp. Screen and plate resistor values tweaked for a bit more clean headroom
- Not sure where/how to bring this signal in. If I do it like the "normal" channel on an AB763, Channel II won't have reverb. I'd like to have reverb work on this if possible - but not sure where it would tap in or what kind of coupling would be needed
Appreciation in advance for any help/advice/suggestions.
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I think I have found answers to a couple of my questions.
Rob Robinette has a "Normal Channel Reverb Mod" which allows both channels of an AB763 to use the same reverb circuit https://robrobinette.com/AB763_Modifications.htm#robrob_Normal_Channel_Reverb_Mod
I tried to adapt this mod into my schematic (updated version attached below)
I suspect I will have to adjust the value of the mixing resistor on Channel II because on my TC-15, the EF-86 preamp channel is quite noticeably louder than the TC-15's other channel based on the Vox top boost preamp.
Also, it looks like the usual Fender 022921 reverb transformer won't work with my EL84 reverb driver. The Vibro-King amp used an EL84 reverb driver very similar to mine, and its primary impedance is 5k whereas the 022921's is variously listed as 15k or 25k.
Fortunately Hammond makes a replacement Vibro-King transformer.
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> Fender 022921 reverb transformer won't work with my EL84 reverb driver. The Vibro-King amp used an EL84 reverb driver very similar to mine, and its primary impedance is 5k whereas the 022921's is variously listed as 15k or 25k.
How much drive Power do you really need?
6F6/6V6/EL84 will make "most" power near 5k load; but do you really need 5 Watts into an eraser-size tank coil? The unitized Fender Reverbs got away with a whole 12AT7 working ~~15k for maybe 1 Watt. An EL84 loaded in 15K may be 2-3 Watts which seems ample.
Use what is in hand.
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> Fender 022921 reverb transformer won't work with my EL84 reverb driver. The Vibro-King amp used an EL84 reverb driver very similar to mine, and its primary impedance is 5k whereas the 022921's is variously listed as 15k or 25k.
How much drive Power do you really need?
6F6/6V6/EL84 will make "most" power near 5k load; but do you really need 5 Watts into an eraser-size tank coil? The unitized Fender Reverbs got away with a whole 12AT7 working ~~15k for maybe 1 Watt. An EL84 loaded in 15K may be 2-3 Watts which seems ample.
Use what is in hand.
This is probably good advice - but I already ordered the 5k primary transformer and sometimes I'm just compelled to try stuff.
Of course the Fender amps with the 12AT7 driver are rightly famous for great reverb sounds.
I recently built a Weber ReVibe and was blown away by the glorious reverb sound. I (perhaps falsely) attributed this to the 6k6 reverb driver and lower impedance transformer.
One thing that has always bothered me - probably irrationally - is the 3.3M mix/balance resistor on the dry signal of the usual Fender reverb circuit. I hate the idea of attenuating the dry signal to balance a weak reverb output. If I'm reading the ReVibe schematic correctly, the dry signal is attenuated with a 330k resistor. I attributed this to the more powerful reverb driver but I realize there may be other factors at work.
I've heard good things about Ampeg and Vibro King reverb that use EL84 (or similar) drivers so was just curious to give it a try.