I was thinking of building a 5E3 Tweed Deluxe clone with a friend who asked me to help him with a first time build. He didn't mind dropping the interactive tone controls and so I thought we could make use of the spare half of a 12AY7 to use as a buffer or gain stage for an EF86 pentode preamp channel. He also liked the idea of switchable cathode/fixed bias and switchable negative feedback so I've drawn a schematic for the design.
The EF86 channel goes EF86 - 1M volume - 1/2 12AY7 cathode follower - James treble/bass. Both channels use 220K mixing resistors and there are other changes to the power supply such as DC heaters for the preamp tubes.
Does anyone have any thoughts about this circuit? Would the 12AY7 be better implemented as a conventional gain stage rather than a cathode follower?
All help and advise would be much appreciated 🙂
The EF86 channel goes EF86 - 1M volume - 1/2 12AY7 cathode follower - James treble/bass. Both channels use 220K mixing resistors and there are other changes to the power supply such as DC heaters for the preamp tubes.
Does anyone have any thoughts about this circuit? Would the 12AY7 be better implemented as a conventional gain stage rather than a cathode follower?
All help and advise would be much appreciated 🙂
Attachments
You do _NOT_ want 200V DC on your PreOut/PowIn jack!!
You also have significant DC flowing from V2a plate down through the bass-pot _wiper_ to ground, which will be very scratchy until the pot just quits.
V1b will not bias-up as expected with the 1meg pot hanging on a grid that wants to be at high DC voltage.
You can't usefully take negative feedback over many stages and volume/tone controls. This forces re-design of the whole scheme.
Your 6V heater winding is "grounded" at two *different* places: in the AC CT and after the DC rectifier. They can not both be ground. I have seen stuff like this in commercial gear (though maybe with 100r resistors instead of a hard tap). There isn't any reason to "ground" the 6V6 end of things, just so long as they don't float to infinity. Also I think any 1-stage DC filter will have enough small ripple to be "worse" than good AC wiring; I'd use C-R-C filtering to keep the high harmonics of 100/120Hz way down.
Minor: C1 and C6 will POP when switched. You want 10K-100K across the switch contacts when open to keep the cap charged-up when not in use. Also I'd put the switch in the ground end of the leg just to reduce wiring (you'll have a ground on the panel the switch is on).
Your *other* "C1" (or are there more?) may pop when switched but being in a high-level push-pull stage it won't throw your speaker cone across the room.
Gain staging: your upper path is conventional, and we know that works. The lower path is a pentode: more gain than a triode, less than 2 triodes cascaded. If the V1b stage is rigged for gain, you may have "too much" gain in this path..... hmmm, the James tone network throws-out a lot of gain so you may be OK.
You also have significant DC flowing from V2a plate down through the bass-pot _wiper_ to ground, which will be very scratchy until the pot just quits.
V1b will not bias-up as expected with the 1meg pot hanging on a grid that wants to be at high DC voltage.
You can't usefully take negative feedback over many stages and volume/tone controls. This forces re-design of the whole scheme.
Your 6V heater winding is "grounded" at two *different* places: in the AC CT and after the DC rectifier. They can not both be ground. I have seen stuff like this in commercial gear (though maybe with 100r resistors instead of a hard tap). There isn't any reason to "ground" the 6V6 end of things, just so long as they don't float to infinity. Also I think any 1-stage DC filter will have enough small ripple to be "worse" than good AC wiring; I'd use C-R-C filtering to keep the high harmonics of 100/120Hz way down.
Minor: C1 and C6 will POP when switched. You want 10K-100K across the switch contacts when open to keep the cap charged-up when not in use. Also I'd put the switch in the ground end of the leg just to reduce wiring (you'll have a ground on the panel the switch is on).
Your *other* "C1" (or are there more?) may pop when switched but being in a high-level push-pull stage it won't throw your speaker cone across the room.
Gain staging: your upper path is conventional, and we know that works. The lower path is a pentode: more gain than a triode, less than 2 triodes cascaded. If the V1b stage is rigged for gain, you may have "too much" gain in this path..... hmmm, the James tone network throws-out a lot of gain so you may be OK.
Hey there, thanks for the response and all the advice! I've addressed each of your points below...
- the little X means those wires cross each other and aren't connected (if that's what you mean?). Or should I move coupling cap C4 to before mixing resistor R25? I've done that and put an extra coupling cap in for the power amp in.
- not sure what you mean by this, do you mean the preceding 'tone' control? Or are you talking about channel 2?
- this is interesting, should I put a coupling capacitor before R27 to block this?
- I put this in to implement this idea: 5E3 Mods
I can remove it if you think it will not work, or could I run the feedback to the phase inverter instead?
- I've removed the centre tap. What values would you suggest for a C-R-C filter. And am I going to be OK running a cathode follower with a DC heater? The 12AY7 can take u to 180V Grid-Cathode.
- the switches on the back are only to be used in standby mode, but I've implemented that anyway
- they use a similar on-off-on fixedbias-standby-cathodebias switch in the Heritage B-15 and it seems to work well
I've also changed the inputs and added a channel selector switch, and adjusted the resistor values to get grid stoppers.
Let me know your thoughts!
- the little X means those wires cross each other and aren't connected (if that's what you mean?). Or should I move coupling cap C4 to before mixing resistor R25? I've done that and put an extra coupling cap in for the power amp in.
- not sure what you mean by this, do you mean the preceding 'tone' control? Or are you talking about channel 2?
- this is interesting, should I put a coupling capacitor before R27 to block this?
- I put this in to implement this idea: 5E3 Mods
I can remove it if you think it will not work, or could I run the feedback to the phase inverter instead?
- I've removed the centre tap. What values would you suggest for a C-R-C filter. And am I going to be OK running a cathode follower with a DC heater? The 12AY7 can take u to 180V Grid-Cathode.
- the switches on the back are only to be used in standby mode, but I've implemented that anyway
- they use a similar on-off-on fixedbias-standby-cathodebias switch in the Heritage B-15 and it seems to work well
I've also changed the inputs and added a channel selector switch, and adjusted the resistor values to get grid stoppers.
Let me know your thoughts!
Attachments
Rob Robinette's page has too much stuff on it. But the "Switched Negative Feedback" plan *omits* the necessary preamp for clarity, and does not run NFB around any Volume or Tone control; it runs output to the gain-stage before the split-load and the power tubes.
You have divided the power-amp's gain stage from the power amp. The "inverter" does not have enough gain to give effective NFB.
But what the heck. Try it.
Do note that the 56K:1.5K divider tries to force the total amp gain, guitar to speaker, to about 40. We'd like near full output with 20mV from guitar. 20mV*40= 0.8V, or less than a tenth Watt! If this actually happened (it would squeal bad first) you would have to strum VERY hard to play normal loudness.
Yes, I got the "X marks a jump", though I did not know we needed yet another convention for crossovers.
You have divided the power-amp's gain stage from the power amp. The "inverter" does not have enough gain to give effective NFB.
But what the heck. Try it.
Do note that the 56K:1.5K divider tries to force the total amp gain, guitar to speaker, to about 40. We'd like near full output with 20mV from guitar. 20mV*40= 0.8V, or less than a tenth Watt! If this actually happened (it would squeal bad first) you would have to strum VERY hard to play normal loudness.
Yes, I got the "X marks a jump", though I did not know we needed yet another convention for crossovers.
Yes looks like I misread the feedback design, I've corrected that now feeding it into the second triode stages which are post-volume and tone.
I've also had a rethink of the EF86 channel, I think the AC cathode follower was over-complicating it and wasn't a good gain structure, so I've turned it into a conventional 12AY7 triode gain stage. It now goes EF86 - James tone stack - volume - 1/2 12AY7 triode gain - 5E3 1/2 12AX7 cathodyne inverter.
I know that the high output impedance from EF86 won't load very well into James tone stack, but I was hoping the 12AY7 gain recovery would be sufficient for the 1/2 12AX7 cathodyne inverter. I can tweak the bright cap to try to balance out any frequency response changes, and I've designed in a tone bypass switch. It's rather like the Route 66 but using a 1/2 12AY7 into 1/2 12AX7 cathodyne inverter instead of a long tailed pair.
Schematic attached, all help and feedback much appreciated 🙂
I've also had a rethink of the EF86 channel, I think the AC cathode follower was over-complicating it and wasn't a good gain structure, so I've turned it into a conventional 12AY7 triode gain stage. It now goes EF86 - James tone stack - volume - 1/2 12AY7 triode gain - 5E3 1/2 12AX7 cathodyne inverter.
I know that the high output impedance from EF86 won't load very well into James tone stack, but I was hoping the 12AY7 gain recovery would be sufficient for the 1/2 12AX7 cathodyne inverter. I can tweak the bright cap to try to balance out any frequency response changes, and I've designed in a tone bypass switch. It's rather like the Route 66 but using a 1/2 12AY7 into 1/2 12AX7 cathodyne inverter instead of a long tailed pair.
Schematic attached, all help and feedback much appreciated 🙂
Attachments
- Status
- Not open for further replies.