Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: W5FH on July 30, 2024, 08:41:27 am
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Hello-
I am getting drawings ready for a '63 Vibroverb 6G16 clone build as I am eager to try a reverb circuit using the split plate load resistors and to see how the brownface tone works out. I have a few questions if readers have time for giving me some advice/opinions. Attached are the Fender '63 6G16 schematic and the MOJO version of it schematic. I also am referencing the Fender '63 Vibroverb re-issue schematic for my build. I will say that in my build I am going to keep voltages very close to original '63 Fender 6G16. I will be building on a Super Reverb size chassis housed in 2x10 custom Vibroverb cabinet. The output tubes are 6L6GC going into a nice quality heavy-core (2" stack) interleaved 4K to 4,8,16 ohm transformer. I might be alienating some purists of Vibroverb tone here with use of this heavy core output but in my research of '63 Vibroverb I kept coming across recommendations of replacing stock output with heavier core unit, such as from Bassman.
1. I am going to build this one as a single (bright) channel only. Since the MOJO schematic is more legible I will refer to it. The preamp output is taken from junction of 1 meg resistor (from V4-B plate) and 220K resistor (from V1-B plate). Since I will not have the normal channel should I include the 220K resistor and ground the end of it that went to V1-B plate? I am thinking I should do this as in original circuit with both channels when using the bright channel the 220K resistor is dropping some of the audio signal to ground through power supply. Actually, I am thinking I should use a 430K resistor in place of 220K to ground as the plate load resistor of V1-B would be part of signal path looking to power supply ground.
2. I am curious about choice of parallel reverb driver tube. I plan to use the Edcor CXSE5-25K-8 as reverb driver transformer. I like it as it is a good quality gapped core 25K to 8 ohm 5 watt unit; I use it with 12AT7 driver. I cannot find specs on original reverb driver transformer that Fender used with parallel 12AX7, the MOJO unit supplied in kit for parallel 12AX7 driver is 29K primary looking into 8 ohm output. I realize the 12AX7 has higher gain than the 12AT7. I am looking for commentary here on how I should configure the reverb driver circuit. Should I strive to stay with original 12AX7?
3. I am going to start out with the original style tone stack, using the 350K with 70K tap treble pot. There are differences in component values between all schematics that use this style tonestack. I have been looking at plots on the Tone Stack Calculator graphic. If anyone has a suggestion for component values here I am all ears. (Am leaving a spot on front panel for Mid pot just in case, but this is not true to the brownface tone spirit!)
Thanks all,
Byron
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1. For single-channel adaptation for this amp, take a look at the Allen Encore schematic. That’s what I’d do.
2. Using a 12AT7 reverb driver with a BF reverb transformer gets my vote.
3. No preference.
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Looks like none of the 6G _ _ series had a mid pot. But I'd put 1 in anyway.
I love having a mid pot. In place of the 6K8 TS tail R, I'd either go with a 25KL pot standing on the 6K8 tail R, so that the mids are always at least what stock was, you only can increase them.
Or just go with a 25KL pot in place of the 6K8 tail R.
Some go up to a 50KL but that's a lot of mids added back in.
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Thank you'll very much for the suggestions!
Byron