Harmonic Twin - A Twin Reverb 65' Reissue Rebuild

[JayCobie]

I've been wanting to rebuild my Fender Twin Reverb 65' Reissue for a while now. First it needed new power tube sockets, and then I wanted some more extensive mods, and finally I just decided to rebuild it with Harmonic Tremolo.

My tendency toward feature-creep through the process has possibly added too many features, but I think it's manageable. I'm getting closer every day to it finally becoming the Harmonic Twin I want.

Main feature changes from stock AB763:
-Just one channel
-New tonestack
-Harmonic Tremolo
-Bias balance and bias measuring points on back
-PPIMV
-Dwell control on reverb input
-Presence control instead of fixed NFB
-Protection resistor on OT secondary

I would appreciate some feedback on my current work-in-progress schematic. Namely:
-My bias schematic is lifted directly from Sluckey's 1971 Sunn Sceptre schematic, but I'm not sure if component values need tweaking for this amp.
-I'm unsure if the input to V3B needs work or if I should avoid the 220k grid stopper/mixing resistor?
-My placement of the standby switch is a result of reading articles on Merlin Blencowe's website, but I'm unsure if it would be better placed elsewhere.
-Any other suggestions?

I know the tremolo bypass will cause pops because of the voltage differences, but I'm fine with that. I have enough components to deal with already.

[vampwizzard]

Generally: tt may take some value experimentation unless you want to model the amp in spice.

re Dual Bias: It appears youve already changed values on your dual bias circuit to match your PT/match Robrob's circuit. The sweep range is adjustable with the bias resistor (the 18K's).

[2deaf]

That seems like a lot of NFB with that 22K resistor.  Maybe 56K or 68K would be better.

The 270r resistor across the OT secondary needs to be 5W minimum.

The bias circuit has a huge range which will make the adjustment coarse around the desired bias voltage.  Maybe change the 1K to 4.7K, the pots to 25K, and the 18K's to 47K's. 

[JayCobie]

I just realized the Sunn Sceptre schematic from Sluckey designated 10k trimpots, not 50k's like I had.

Here is a new version of the schematic with some tweaks based on feedback so far. I also changed more values in the NFB based on Rob's suggestions here, but the final values will probably be massaged more when it's built: https://el34world.com/Forum/index.php?topic=12099.0

[PRR]

When not engaged, the tremolo circuit will distort on the 5X bigger signal it gets, and that may corrupt the straight signal.

And it is not necessary. The trem can be non-pop, non-corrupt, and use a cheaper switch; though it may need an extra cap.

[JayCobie]

That is a very good suggestion. Not sure why I stuck to the other switching method. I got plenty of spare coupling caps.

Added to schematic, and cleaned up the schematic a little as well.

I'm just waiting on my final shipment of caps from Germany for this project, then I can plan component layouts on my generic tagboards and see if I've underestimated the space available in the chassis or not.

[sluckey]

My bias schematic is lifted directly from Sluckey's 1971 Sunn Sceptre schematic, but I'm not sure if component values need tweaking for this amp.

I would plan on having to tweak the bias circuit. I would shoot for an adjustable range of -40v to -60v. This can be done safely with no tubes plugged in.

V4B has lost it's grid connection in this latest revision.

[ac427v]

Presence pot needs a 4k7 resistor in parallel

[2deaf]

I would plan on having to tweak the bias circuit. I would shoot for an adjustable range of -40v to -60v.

My suggested bias circuit calculates out on paper as -37.5V to -57.5V.  I am reasonably confident that this circuit will work in this amp right out of the box, but the only way I would be totally certain would be if I had this amp on my bench and I put my circuit in it.

Replacing the 47K resistors with 56K's changes the calculated range to -40.3V to -58.2V, which is closer to a center of -50V.  I didn't suggest this because I suspect -48V is more likely to idle middle-of-the-road 6L6's at 70%.     

   

[2deaf]

There is a lack of consistency in the use of dots to indicate wires that are connected.

The 56K NFB resistor is good with a 5K presence pot, but no good with a 25K presence pot.

The 100nF presence capacitor would typically be connected from ground to the wiper.  There will be DC on the presence pot so that it will be scratchy when you adjust it.  If this bothers you, there are other ways of doing it. 

[PRR]

Presence pot needs a 4k7 resistor in parallel

I don't think so; but something IS wrong there. There are about 3 ways to skin this cat but I don't think this does the job right. Re-plagiarize.

[sluckey]

Here's the presence circuit for the 59 Bassman Reissue...

[PRR]

Pot and cap in series? 

[sluckey]

Yep. Probably stolen from the later Marshalls. Payback time!   

https://el34world.com/charts/Schematics/files/Marshall/Marshall_1959_jcm800_super_lead_power_amp_100watt_year_1988.gif

[JayCobie]

There is a lack of consistency in the use of dots to indicate wires that are connected.

That is a side-effect of the lackluster program I'm using to draw this schematic. It's just the online editor on https://www.schematics.com/
Other programs were driving me up the wall with their learning curve, but this one is quick, simple, and free, though it is lacking in basic stuff like jumping wires easily. I tried compensating for that in this newest iteration.

I think lifting the bog-standard presence with the cap in series with the pot will be fine.

[JayCobie]

I updated the schematic with 2 minor fixes, removed unnecessary 220k resistor before V3B and added 1M resistor from grid to ground on V4B.
Fixed and updated the footswitch wiring. Reverb now goes to ring and Tremolo to Tip, as in the original reissue.

Since I had an unused hole on the front panel (and because the footswitch jack is going on the front panel in my build), I added a switched tip connection on the footswitch jack, intended for an on/off switch for the tremolo. Since the on/off switch goes through the switched tip connection, plugging in a footswitch will override this tremolo on/off switch in favor of the footswitch.
I don't forsee myself always having the footswitch plugged in, so why not make more use of the front panel?