Wurlitzer 44 amp project

[dsmith9]

I am starting a project with a Wurlitzer Model 44 (or 6144) amplifier I picked up the other day.  2 6SQ7 tubes (pretty similar to 12AX7 I think), 3 6SN7, 1 6SL7, 2 6L6 power tubes, and a 5U4G rectifier.  It's in great shape.  Came with the original Jensen 12" field coil speaker.  All original tubes except the 6L6s.  The reason I bought it is because it came with the complete Service Manual - I've included relevant parts below.  Schematics and circuit explanations for these seem to be hard to come by.

Fired it up.  Tubes all lit up, fairly quiet hum through the speaker, especially considering the Swell Control socket has been jumpered to hard-wire it at max volume.  Voltages generally check out fine, except the Control Tube is not right.  This is the gain stage that turns the amp "on" when an organ key is depressed.  Biased at cutoff when nothing is being played, this tube is supposed to be re-biased into proper operating range when an ogan key is pushed.  Pins 2/6 of the Control Socket are currently jumpered, hardwiring it "on", so the Control Tube should be biased for proper operation.  But I'm reading 0 vdc on the grid, 111 vdc on the plate, and 2.1 vdc on the cathode.  These values don't correspond correctly to either the "on" or "off" condition, so clearly something is wrong.  Just a bad tube?  Unfortunately I don't have another 6SLT to try.  Ah well, I haven't ordered anything from Tube Depot in weeks.

I "plugged in" by connecting a guitar cord to each of the inputs using test cables.  Hum went through the roof, but I assume this is because I had 12" of unshielded cable connecting my guitar to the amp.  I tried playing through both channels, although I did not jumper the Vibrato Control to see that circuit do its thing.  Volume on both channels was very low - 1-2 watt level - and there was no clean headroom.  Heavy distortion.  The amp sounded like a dimed 5E3 through cheap earbuds.  I assume this is because of the improperly biased Control Tube gain stage (?).  I understand the concept of using bias as a switch, but I don't understand this circuit.  I could use some help here. 

From what I've read, the consensus opinion seems to be to strip these amps and build something more useful with the iron.  However, If I can get this volume/headroom problem figured out, I feel like it would be fun to build a working amp with this rig, at least to learn a thing or two and to hear what this vibrato circuit sounds like.   It doesn't seem like that should be a very hard project, but I could definitely use help. 

About me:  First time on the board, as you can see.  I've built 5 amps - a handful of Fenders and a Dumble.  Nothing original - some RobRob type mods, but I'm not yet a tweaker.  I'm learning, but still need lots of guidance.  That's one of the reasons I'd like to play with this.  The amps I've built have been from kits or new parts, so they've been pretty big investments.  They sound great, so I've been reluctant to experiment much.  Since I've got just $50 invested in this, and it sounds like crap, it seems like the perfect playground.

Assuming I can get the volume/headroom issue resolved, here's part 1 of the plan:

- Wire the inputs, (should I treat these as two separate channels, a clean one and a vibrato one?  Do I just ignore the fact that they were originally fed "bass" and "treble" inputs?).  I suppose 2 channels with jumper-ability provides the most flexibility.  The treble/vibrato channel sounds particularly screechy right now.  Although it's stupid to evaluate tone right now, it would be great to get ideas how to tune/voice these channels.

- Wire a Volume pot at the Swell socket (100K from the schematic? seems awfully small.)

- Add a tone control/stack.  Easiest would be a tweed control right after (?) the volume control.  This would replace the current off-board treble bleed attached to the Swell Control.  Am I right in assuming that the Control Tube is also a gain stage ( > unity gain).  If so, I assume it would give me enough gain and input impedance to put a more elaborate tone stack in front of it.  Yes?

- Vibrato Control Socket: Wire 1M pot in place of the Vibrato Depth control.  Wire 500K switched pot and and 800K resistor to ground in place of the Vibrato Speed control.

- I assume electrolytic capacitors will all need to be changed.  Could capacitor issues be contributing to the problem?

One other question: can the field coil speaker be used as a regular speaker? (other than the fact it weighs a ton).  If so, I might move the choke into the chassis.  Something about 300 volts running to the speaker makes me just a tad nervous.

Thanks in advance for any ideas, advice, or warnings.  At the very least I hope it's useful for the community to have these documents.  Attached in this post are 2 pics and the schematic.  Next post will have the relevant pages from the service manual.

Cheers
Dan

[dsmith9]

and the service manual.... (wow, tougher than I imagined to get these scanned docs through the attachment gauntlet)


---- UPDATE ----

The pdfs attached below are for only a few of the pages from the Service Manual.  Here are direct links to the full Service Manual and Owner's Manual.  These are also included later in the discussion.  The link names have been updated and copied here for easy access (I couldn't put them in the original post because I had exceeded the character limit on the post - OK, point noted about being long-winded).  The Service Manual has been updated to include clearer schematic and layout pages.

1952 Wurlitzer Model 44 Service Manual: http://bit.ly/W44ServiceManual
1952 Wurlitzer Model 44 Owners Manual: http://bit.ly/W44OwnersManual

[sluckey]

Nice find. I'll be following your progress with interest.

I would go through the power supply and power amp first. Get them working fine. Yes you'll probably need new filter caps. You'll just have to get used to having B+ on the speaker. No way around that. The field coil is not a choke and it cannot be moved to the chassis. The field coil is part of an electro magnet and must remain on the speaker. That speaker has no permanent magnet. If you want to use a modern speaker rather than the FC speaker, you can put a 6KΩ/30W resistor inside the chassis wired to simulate the FC.

Once the PS and PA are working fine, then move on to the preamp circuit.

[dsmith9]

OK, I've made quite a bit of progress.

The Control Tube was a mess.  Someone had made some significant changes that weren't working.  There is a circuit located just below the Swell Control on the schematic, which is confusing to me, but I assume is part of the bias switching function.  Anyway, this had been completely changed - there was no printed circuit and different components - and was now powering the plate instead of R63, after passing the B+ through a bunch of resistors and caps that delivered only 40v to the plate.  I removed this and restored the plate wiring as shown in the schematic.  I also detached R60 from the cathode and voila, voltages checked out and everything works....almost.

It's not as loud as I expected and there's still distortion at louder volumes but I think that may be more a product of the speaker than the amp.

Unfortunately the vibrato function doesn't work.  If I put a scope on the west side of C14 of the oscillator , I get a nice clean signal at the same frequency as the input, regardless of vibrato setting.  The circuit narrative in the service manual describes the oscillator as needing a disturbance, such as a plate supply ripple, to initiate it.  Was that the function of the connection between the Control Tube cathode and the oscillator plate?  Thank you sluckey for posting the excerpt from the Dorf book - it helped me to understand the circuit better.  That book shows the identical circuit to the W44 schematic, but with no disturbance needed.  I think you built an old AC-15 with this vibrato circuit, and it looks like you built it like the Dorf book.  Any ideas?

Anyway, it's cool to be playing through it already.  Not surprisingly, there's lots of resistor hiss, but I can't believe how little hum there is.  When I build amps I spend so much time worrying about lead dress and proper grounding to avoid hum and oscillation, and this thing looks like my dog puked in a box.  Crazy.

-Dan



 

[Willabe]

There's a group on Face Book that might be able to help you out some?

Wurlitzer EP Restoration Group.

[trobbins]

Have you confirmed the low frequency oscillator is working - eg. signal at SW-2 vib depth, and then as fed to the cathodyne "A" triode, and say on R21/R24 (or west side of C14 as you indicate) ?  Are the DC idle voltages ok on triode "C" and "A" (ie. about 70Vdc on R26)?  Are the SW-1 and SW2 contacts ok, and the various high value resistor values?


Unfortunately the 6SL7 only has a mu of 70, which would make oscillation start-up a bit suspect (compared to say using a 12AX7). You may need to add an extra RC stage or trim the RC values - the speed switch should be able to start oscillation.

[dsmith9]

Thanks @trobbins for the great suggestions.  You just missed one - the one my wife always offers first: "Have you gone back and looked at it again, and thought about it carefully before asking for help?"  The problem was that I misread the schematic.  I don't have a switch for Sw1 or Sw2 and I jumpered it incorrectly.  Repeatedly.  So now it works.  It's a pretty warbly, Hawaiian-sounding vibrato - not really to my taste right now.  I need to see if I can slow it down.  I'm assuming if I increase the value of R34 (or add a Pot between it and ground) I can get a slower vibe going.

[trobbins]

R74 is the most 'typical' location for a speed/frequency pot - and yes well worth taking your time about what LFO frequency suits your taste.  It is an effect after all, and so only really worthwhile for certain types of music.

I'm very pleased you have posted the Wurl 44 service info and schem, as that is the original organ that Wurlitzer introduced this form of vibrato circuit, which just predates the other common technique based on Moses' phase angle variation scheme and developed in to a vibrato effect mainly by Magnatone.  And the date of that Issue 2 schematic is Dec 1952, which is before the 1953 date on the page 3/21.  Can I ask if a full scan of the service manual could be made available somehow - as I'd be keen to check for any other dating or interesting info.

Ciao, Tim

[Willabe]

It's a pretty warbly, Hawaiian-sounding vibrato - not really to my taste right now. 

It will sound different when played/mixed in with a band. It's a back round texture.

To slow it down you increase the value of 1 (or 2) of the caps in the oscillator. It changes the time constant, charging up of the cap, of the oscillator R/C network. A larger value cap takes longer to charge up.

Change the value of C40, or C39, or C38, from .01 to .02. Try increasing just 1 of them and see if that's slow enough. Doesn't matter which cap, which ever is the easiest to change.

[dsmith9]

Awesome info guys.  Thanks.  I guess that's the thing about an R-C network - you can change either R or C to get the desired effect. 
Tim, I will scan the rest of the manual and post it in the next day or two.  I also have the user manual if that is of interest to you.  I'll probably put it up on a Google Drive and post the link here.  The only way I can "scan" is to photograph with my phone and the smallest file I can compress to is about 500KB/pg.

[trobbins]

The 'printed circuit' would have been to de-click any key press action, and emulate the ramp up in sound that a pipe organ note would typically exhibit.  Yes definetely something you don't need for a normal guitar amp application.  R60 would have been keeping cathode voltage higher than typically needed for normal audio work - so removing R60 is appropriate, and you may want to bypass R59 now although that increases gain within the feedback loop.

The output stage has global feedback, and the C41 shelf network appears to be the dominant high frequency stability roll-off mechanism.

Although the OT is probably pretty good for low frequency response, you may not need or want that extension - which could be reigned in by dropping C26/27 to more non-hifi values that may help alleviate gross blocking overload.

You may not have enough gain for guitar pickup to the bass input.  Perhaps if you short out R42, R55, C25, and disconnect R43, that would give a simple '12AX7' style triode input stage connecting to the 'output stage'.  You could then disconnect R76, to see what total gain you have available from input to speaker (which you could raise a bit by bypassing R59).

I'd appreciate a scan of the user manual too if not too much hassle.  As you found out, some organs don't yet have a 'public' tech manual.  Google drive seems like a good go - I'd certainly like to link to anything you put up, or can host it myself if ok with you.

[dsmith9]

OK, I'll scan everything I have.  You are more than welcome to copy it or host it wherever you'd like.  I'm sure it will get more use that way.

Everything you suggested makes a ton of sense.  Thanks for explaining the click-supression-swell-sound loop.  That was real head-scratcher.   The prior tech (who clearly worked on this many years ago, because he installed very old-looking carbon comp resistors), had already disconnected the NFB loop.  I haven't reattached it yet.  I'm actually a fan of amps with switchable or variable NFB, so I may go that route.

The only thing I don't understand is your suggestion to disconnect R43.  I thought that was the mixing resistor for the Treble channel.  If I disconnect that, don't I lose that channel?  I was actually thinking about jumpering the inputs and putting a mixing pot in place of R42 and R43 (remember, I'm really just experimenting here).

Also, do you know where I can go to learn more about the phase inverter that's used all over this amp.  It's like a cross between a catodyne and half a long-tail-pair.  I can't find it described anywhere.

Also, I'd like to try using this with a regular speaker to see how much distortion the original speaker is adding.  To test this, can I use the field coil on the current speaker (pin 6) just for its resistive value, and hook a regular speaker to pin 3?

Finally, are there components I should be "worried" about?  Replacing those cap cans is going to be a bitch, from sourcing to chassis mod to wiring.  I'm not sure how much I want to put into this thing.  It may be destined for organ donation. (no pun intended)

Thanks again

[sluckey]

Your PI is a straight up cathodyne. Read about it here...

     http://www.valvewizard.co.uk/cathodyne.html

You can use a regular speaker as you described.

You may consider using individual axial electrolytics rather than cans.

[PRR]

> the phase inverter that's used all over this amp.  It's like a cross between a catodyne and half a long-tail-pair.  I can't find it described anywhere.

What he said. Although drawn three different ways with a couple value changes, it's a plain cathodyne.

[dsmith9]

You guys are awesome.  I have built a couple of 5E3's, so I thought catodynes were always of the "simple" type shown at the top of Merlin's page.  Had I read all the way through.....  (Please see my note above about wife's advise.)  Thanks Steve, I was already starting to plot spots in the chassis to stash caps.  Maybe a vertically mounted cap board situated by the PT after I pull out the Tone Cab Switch and the useless "click" loop.  Also, I suppose I can replace C43C and C43D with a single 47uF cap that's waiting for a home.
I'll post pics as proceed with elective surgery.

[PRR]

> I thought catodynes were always of the "simple" type shown at the top of Merlin's page.  Had I read all the way...

Someone here is building a boat. Say someone else never saw a boat. How to tell them?

A boat is basically a hull. Throw it in the water, you got a boat.

But picnickers and lobstermen need floors, roof, engines or masts. Maybe not as big as shown, but it's a sweet image.

You think your boat is finished and go out to sea. Sea-cops bust you for not having numbers and papers, criticize your lights and fire safety; meanwhile the boat is sinking from slow leaks. And tax-office sends nastygrams.

Merlin top shows the core AC/audio action but neglects DC bias and blocking. It's "just the hull".

[dsmith9]

OK Gang, as promised, here is the full user manual scanned into a pdf.  It's 10MB, so I can't post it here.  Here's a bit.ly that redirects to a shared Google Drive pdf file.  I'm sure there's a better way and a better place to put this.  I leave that up to you.  Let me know if you'd like any of the pages re-shot for any reason or need anything else.  I'll scan and post the user manual tomorrow.  I need to do it when I still have natural light.
Cheers.

1952 Wurtizer Model 44 Service Manual
https://bit.ly/2Xt8faG

[trobbins]

The only thing I don't understand is your suggestion to disconnect R43.  I thought that was the mixing resistor for the Treble channel.  If I disconnect that, don't I lose that channel?  I was actually thinking about jumpering the inputs and putting a mixing pot in place of R42 and R43 (remember, I'm really just experimenting here).

Disconnecting R43 is aimed at just checking the base level gain from input to speaker, using your guitar, to determine whether you need an extra gain stage added in for guitar type application.  The sensitivity of the speaker will likely be a big influence for this assessment.

It may be easier to aim for using the 'two inputs' as separate inputs that are often seen on a guitar amp - eg. the bass input could be a 'normal' input, and the treble input could be modified to become a vibrato input.  In a typical guitar circuit, two inputs are often 'mixed', as per R42-R43, but often with higher resistance values so as to better isolate the non-working channel from adding noise.  In older times, many a band would plug two guitars into the amp if it had multiple inputs as they often only had one amp/speaker for the band.

Also, I'd like to try using this with a regular speaker to see how much distortion the original speaker is adding.  To test this, can I use the field coil on the current speaker (pin 6) just for its resistive value, and hook a regular speaker to pin 3?

Yes that would be fine for temporary testing.  The field coil appears to load B+ by about 50mA, which is significant.  One option is to remove that field coil load and let B+ rise higher, and modify the R84 to drop more voltage and may need to tweak cathode bias to keep 6L6G dissipation ok.  Another option is to upgrade to 6L6GC, and modify the cathode bias to suit a higher idle current, and load the B+ down more that way.  These possible changes would blend in with a myriad of changes one could dream up with the existing amp circuit as it is.

If your aim is as a guitar amp, then I'd recommend you try and patch over to a typical 'guitar' speaker as the 'regular' speaker to get a good feel for actual loudness/cut-through when trying to compete with a drum kit   

Finally, are there components I should be "worried" about?  Replacing those cap cans is going to be a bitch, from sourcing to chassis mod to wiring.

It can be much easier, and achieve a lower noise floor, to use discrete electrolytic caps located and soldered directly to where they achieve a local star ground point and B+ point.  Eg. C34C negative would connect directly to HT CT tap (CT tap would then go to chassis directly and not via speaker connector). C43D would connect directly to R63/R79 B+ node, and ground ends of R78/R57.  C2A would connect directly across R4/R6 node and input valve cathode ground nodes. etc.

[dsmith9]

Thanks again for the ongoing education. 

It finally occurred to me to just hook a regular speaker up to the aux spkr connections and use the Tone Cabinet switch to route the OT output to that speaker, while leaving the primary speaker in place to serve as a current dump (duh, this is how it was designed).  What I discovered was very encouraging.  I used a moderately efficient British-voiced 12" ET65.  Wow.  The amp was now very loud with a nice breakup at higher volumes.  Definitely too dark, though.  The real surprise was when I brought the Field Coil speaker into play.  That speaker is super bright, almost tinny and very inefficient.  But if I set the Tone Cab control at 25% ET-65 and 75% FC, the effect was astonishing.  Most of the volume still came from the ET-65 with the FC adding a really nice top end.  Not chimey, but not brittle either.  It's by no means a perfect tone, but has me excited to see what I can make this thing do, and motivated to go ahead with the fairly extensive overhaul it deserves.

So I will order me a bunch of filter caps and start to rip this baby apart.  I am also envisioning a nice natural wood cabinet for it, with a matching cab for the speaker.

I will report progress as we go.

Cheers, and, as promised, here is the owner's manual.  You will find it particularly useful if you want to know how to play Dying Cowboy. 

1952 Wurlitzer Model 44 Owner's Manual
https://bit.ly/2TpEA39

[dsmith9]

Can anyone offer some help on the power line?  Since I'm replacing the can caps with axial caps, I have the opportunity to rethink cap placement, and maybe the larger picture.  The amp has 9 B+ voltages!  One is used to create a B+ that is used off board for another part of the organ, so that comes out.  Which still leaves 8 separately filtered voltages.  Isn't that - let me find the word, oh yes - insane?  Here is a table of the B+ values and assignments:

         
B+   Source   Resister Filter Cap   Voltage           Feeds   
B+      V9                                    320                           OT CT
B+1     B+0     100R         30uF     317           V7, V8       Power Tubes
B+2     B+1      4.7K         20uF     275           V3A, V6B   Gain, Power PI
B+3     B+2      4.7K         20uF     250           V4B, V5     Vib Entry PI, Vib Mixer
B+4     B+3      4.7K         30uF     240           V1, V2       Input 1, Input 2
B+5     B+1      4.7K         20uF     280           V4A           Vib LF Filter
B+6     B+5      4.7K         20uF     260           V6A           Vib Mixer PI
B+7     B+6      4.7K         20uF     255           V3B           Vib Oscillator

Any thoughts on this?  Can I move stuff around and simplify?  Does the Vibrato circuit need to be isolated?  Does each half-tube at the end of the string need its own B+?   From a layout perspective, I would like to move V3A onto B+5, V6A onto B+2, V3A onto B+5.  This might require some adjustment to Rp's, but nothing crazy.  Right now the grounding is pretty much all over the place, so this would not only mean fewer power lines running all over the chassis, but also an opportunity to ground tubes closer to their source caps.

Just to recap the architecture.  Channel 1 goes through an initial gain stage, volume control, second gain stage, PI, and Power Tubes.  Channel 2 goes through gain stage, through long Vibrato circuit, and rejoins before the volume control.

Also a question on grounding. Currently the HT wires are grounded with an 180R resistor and a 80R resistor at the end of the line (tube V1).  I've always set the artificial center tap with two 100R resistors at the other end, usually off the lamp.  Does this matter?  Why use 2 different values?

Thanks






 

[trobbins]

Wurl do appear to be concerned about the supply filtering of the various vibrato sections.  Perhaps related to high ESR from such vintage caps available at the time, and perhaps to constrain any motorboating or interaction between sections or the distortion of the audible signal due to ripple of the B+ from the really low frequency (5-6Hz).

B+ filtering can be less of a concern if two sections powered from the same B+ have opposite phasing, such that the net current going in/out of the filter cap is closer to zero.

LFO "C" plate rising causes LFO PI "A" stage to sink more current, so could combine C15A and C (remove R66).

You could try and short R65, to just use one filter cap for "B" as well.

Yes the 180/82 values for the humdinger appear a bit odd.  It is likely to be related to the total wiring/layout structure of the organ introducing a consistent level of hum that can be partly neutralised by that resistor setting.  Many amps using a humdinger pot that can sometimes achieve a 'tuned' hum minimima for the given arrangement of wiring and valves - sometimes that depends on the valves themselves, and so the humdinger may require tuning if valves are swapped out.  You may benefit from using a 100-250 ohm pot in that location - I typically use a 220 ohm 10 turn 0.6W trimpot as I have them available.  If hum is noticeable, and due to valve heater-cathode leakage, then you may get some benefit from elevating the humdinger reference voltage from 0V to something like the 20V output stage cathode level, or another contrived level.

Thanks for the scans 

[dsmith9]

Things are starting to take shape.  I took out most of the old wiring, the power filter line, a bunch of components, and have relocated some of the wall-mounted component strings.  It's a lot cleaner in there now.  I left the cap cans in for aesthetic reasons, but I clipped and folded down the leads so I can use the space.  I can't believe it, but I'm starting to know my way around the insane point-to-point layout. Notice all the little wire pig-tails, left in so I don't forget that something needs to get attached there.  The side facing the bottom will be the front panel.  I've laid out the front and back, but haven't drilled yet.  Parts are on their way.

I've also re-designed the Vibrato circuit and layout to allow variable speed and depth control and a vibrato/tremelo/off switch, and, of course to locate all the controls on the front panel.  I've included the modified schematic below if anyone wants to desk-check me.  A huge shout out to Sluckey, who figured this circuit out and documented it superbly in his AC-15 build.

Working off of paper schematics and layouts is a pain.  I've avoided re-keying the schematic, because I'm not really changing all that much, although I probably will eventually just to document what I have.  There's no way I'm re-doing the layout.  I'm not sure the best way to document changes to the layout, especially since that's really the more important document.  I've got lots of different marked up copies.  Is this what it was like in the old days?

One more thing.  I took a new picture of the schematic that I think is clearer.  I've put it in the original post if you want to grab a copy.

[dsmith9]

One more thing.  This is the first time I've worked on a vintage piece of equipment.  It's almost impossible to unsolder components, since their leads are double wrapped through their destinations, so I've taken to clipping close and trying to open up enough space with a solder sucker to get a new wire in there when the time comes.  Leads and terminal strips are dirty - with that kind of greasy dirt.  Should I be trying to clean them with something?  I'm using flux liberally, and I can sand old leads lightly, but I don't really have a way of cleaning loaded terminal strip terminals.  The octal preamp sockets are nice, because they usually have a clear hole on each pin.

[shooter]

I've taken to clipping close

I like to leave the lead long, I use a small needle-point pliers, bite down, twist 180. now use a solder gun, click trigger, wait 10sec, then "wedge" it just below the loopy wire, once you get flow, push your needle "into" the hole.  the solder will cool, but you should have something on "the other side" to bight on, bite down, heat.......
takes time, but you will like it better   

I like a good resin solder wick.
also re-heating old solder makes it brittle, keep re-flowing a small amount of new as you work the wire pieces out.  helps keep the posts wetted for new work.

[PRR]

Yes, thanks for the scans. I know they will go to good use. (Thanks to Tim too.)

> Wurl do appear to be concerned about the supply filtering of the various vibrato sections.  Perhaps related to high ESR from such vintage caps available

Guitar amps come from good radios.... they were not expected to last forever.

Organs historically "last forever". The tooter JS Bach played on in 1703 is still in service. Some early Hammonds appear to be reaching for that lifespan. Generally an organ dealer wants to keep each customer for a lifetime (there aren't so many organ buyers!) so you design for low trouble. Guitarists are more likely to trade-up or buy more amps, are less sensitive to service life. Also Wurl had a *long* history and high reputation, unlike some man named Leo or Jim.

[dsmith9]

You're not kidding a out reliability. The way these components are connected, I'm pretty sure this whole amp would have worked without solder.

Another grounding question.  Say tube stage A feeds tube stage B, and each has its own B+, is it better to ground the stage B grid leak resistor with the stage B ground or the stage A ground?  (I'm guessing A)

[trobbins]

I'm for B - that is the path for typical grid current, and that stage B 0V is the reference node for voltage gain of stage B grid signal (or noise or whatever else can couple to the grid).

Yes stage A AC current through the coupling cap would then have to flow back to stage A 0V node, as well as dc related charging current of the coupling cap (including if coupling cap current flowed due to the grid clipping).

[PRR]

> is it better to ground the stage B grid leak resistor with the stage B ground or the stage A ground?

A pure grid-leak, it hardly matters. The value is very high, so less important where it goes to.

A Volume pot (which may also be a gridleak) becomes critical, because if you turn it to zero you are directly listening to whatever it calls "ground". I would normally have it go to next-stage cathode network. Then when full-down the next-stage grid-cathode gets zero voltage difference.

[dsmith9]

Parts are here.  Layout is done.  Ready to go.

Wiring without causing problems is going to be a challenge.    Adding vibrato controls, relocating side wall controls so that I can mount this in a cabinet, and moving all the controls to the face - well, let's just say this won't be a point-to-point amp any longer.  I was kind of stuck keeping most of the filter caps together, so there are B+ lines all over, just like before.  There are still many ground points to the chassis (15 if I remember right), but I've tried to do some intelligent consolidation, which means there are now ground wires.  Plus the new signal wires.  Right now I'm only planning to use shielded cable from the volume to the grid of the Gain tube.  Should I also use shielded cable on the vibrato wires?  (I'm thinking to protect other wires from the low-frequency oscillator signal.)

[scrimpus]

I just converted one of these last year now schematics finally pop up. This was not an easy circuit to follow without one.

[dsmith9]

Very impressive. I can't imagine taking this on without a schematic. How did it sound when you were done? And what's that "Edge" control?

[scrimpus]

It sounds great with the stock speaker but very dark with anything else so it still needs a bit of re-voicing. Usually I would have re-stuffed the old cap cans by now but they seem to still work just fine somehow with very little noise.The edge control is just a presence control I stuck in the NFB loop.