65 Twin Reverb RI - Hand-wired conversion

[sjwood3]

Working on converting a 65 Twin Reverb Reissue to hand-wired.  Any chance there's a picture or schematic of how a hand-wired 65 Reissue Twin Reverb transformer should look?   I've looked at the service manual for the 65 RI, but not sure how that transformer interplays with the old Fender schematic and layout.  https://www.zikinf.com/manuels/fender-65-twin-reverb-service-manual-en-44415.pdf I know the newer PT doesn't have a center tap and that I'll need a bridge rectifier.   I've got it all wired up except I'm stumped on the bridge rectifier part. I've got the entire amp wired up except for this part.    I can't find a data sheet on this transformer either A037610.   Any help is appreciated!

For reference I found this.  Pretty helpful discussion. 
https://el34world.com/Forum/index.php?topic=9880.msg89973#msg89973

[shooter]

not sure, I'm missing something;

the re-issue shows a bridge, wire it like that, DO NOT use a center tap

also confused;
you're doing it as a re-issue, or just using iron and building a generic ab763 

[sjwood3]

I am using the 65 twin reverb  RI irons in an AB763 Twin Reverb Blackface rebuild.    I've added RCA jacks, instead of the footswitch.  I have the 100ohm resistors instead of the pot.  The whole bit is identical to the blackface version except for the PT.   Followed the layout and schematic to the T, but the PT is different from the old one.   

[sluckey]

Wire the FWB rectifier exactly as shown on the RI schematic. Also, the RI transformer has a separate bias winding (two brown wires. Connect one brown to ground and the other to the bias diode. Use the RI bias circuit rather than the original AB763 circuit. I would use the hum pot rather than the two 100Ω resistors.

[sjwood3]

I sincerely appreciate the advice!   

[sjwood3]

Use the RI bias circuit rather than the original AB763 circuit.

Ok, I can install the FWB....seems pretty straight forward on schematic.   I get the older style bias circuit. Just having trouble visualizing the layout in my head.  I'm currently starring at the two schematics to note the differences. 

Also, why use the RI bias over the AB763? I intend to follow your advice on that.  Just curious. Does it have to do with the lack of center tap on the PT?  Is the AB763 incompatible or is the RI just superior for this applicaiton?

[shooter]

why use the RI bias

the bias was "designed" for the RI PT, using the "old" bias circuit would require you to "re-design" to work with the RI tranny.

[2deaf]

Also, why use the RI bias over the AB763? I intend to follow your advice on that.  Just curious. Does it have to do with the lack of center tap on the PT?  Is the AB763 incompatible or is the RI just superior for this applicaiton?

The AB763 bias circuit will work with the reissue bias winding.  I would up the capacitor to 100uF/100V if you take this path.

The RI bias circuit has a much lower ripple voltage as compared to the AB763.  Poorly matched output tubes will have less idle hum with the RI bias circuit.

The RI bias circuit will cause your output tubes to fry in the event that the wiper of the 10K pot fails.  To prevent this, install a 2.2M resistor between the wiper and the 3.3K resistor.  Or a 1M.  Or a 3.3M.  Or a 4.7M.  The AB763 bias circuit has the same design flaw. 

[sjwood3]

Also, why use the RI bias over the AB763? I intend to follow your advice on that.  Just curious. Does it have to do with the lack of center tap on the PT?  Is the AB763 incompatible or is the RI just superior for this applicaiton?

The AB763 bias circuit will work with the reissue bias winding.  I would up the capacitor to 100uF/100V if you take this path.

The RI bias circuit has a much lower ripple voltage as compared to the AB763.  Poorly matched output tubes will have less idle hum with the RI bias circuit.

The RI bias circuit will cause your output tubes to fry in the event that the wiper of the 10K pot fails.  To prevent this, install a 2.2M resistor between the wiper and the 3.3K resistor.  Or a 1M.  Or a 3.3M.  Or a 4.7M.  The AB763 bias circuit has the same design flaw.

Does it have the same solution?  I have the AB763 Bias circuit installed, but I immediately realized the difference when I started testing.  How would you implement this?

[sluckey]

Just put a 2.2M resistor directly on the bias pot, one lead to the wiper, the other lead to the "hot" side of the pot.

[sjwood3]

Is this a correct representation of what you're saying?   Not in anyway am I proficient with this DIYLC program.  haha.  27K is soldered to the top of the pot.  Keeping in mind I'm going with the non RI bias circuit and I've installed a 100/100 cap in the bias board as well. 

[shooter]

that's how I'm reading it;

here's Dougs file, open, copy, paste in your project, play with mice ears to fiddle data for part

[sjwood3]

So I powered up my Twin Reverb amp, voltages all check out.  Corrected the horrible positive feedback squeal from the output transformer, added the 2.2M resistor to the bias pot, got 100/100V Bias cap installed.    The rectifier works as expected.    All the tubes light up.

Here's my next issue.   I plugged it into a speaker, reverb tank, etc.   Noise changes with volume, treb, mids, and bass, but no sound from a guitar on either channel.    My guess is that something past where the channels converge, there is something blocking the signal or grounded improperly?     I was, however, able to produce a signal in both inputs from the functional generator on my phone.    Is this a grounding issue?   I'm not completely proficient with my oscilloscope yet, but I'm guessing this is my chance!   This one has been an adventure.   Almost there!  Curious to hear some positive feedback (amp pun). 

[shooter]

guessing this is my chance!

yesser it is 

start with a signal source into the scope
play with the knobs, read tutorials, get the signal stable, easily "readable"
now set the source signal amplitude to ~~~~ 150mVAC p-p

MAKE SURE YOU HAVE SCOPE SET FOR AC COUPLED

once you got that it's just plug in the signal like it was a guitar,
probe the 1st point the signal gets to a tube, typically V1A grid
It should be ~~ the same look and size as straight into the scope

set the probe to 10X

grab schematic and probe each tubes plate till you get to the PI
you might need to set gains n volumes, you goal is to keep the signal "clean" - not clipped or over driven - for now.

If you get that far and had fun, stop back 

[sjwood3]

OK.   I'm stumped: tubes are new and the right ones, got a good signal at the speaker jack and input jack on my scope.   Nice stable sin wave...still no sound.   I'm using a functional generator app on my phone with a headphone jack converted to 1/4inch jack for the source input.   I'm also using a dummy load suitable for the output.  WHen I plug into speakers, I get output noise floor, but no guitar signal.  I've used different guitar and cable combos.   Obviously I'm wrong somewhere or it would work.  haha.  Open to troubleshooting ideas??? 

[shooter]

ok, go back to your sig source and speaker dummyload and verify you still have the sin wave;

now plug in guitar and strum, see yourself playing on the scope?
IF so;
how many speaker jacks do you have?
if more than one, make it just 1 (for now)
make sure you have a good ground (based on your meter) back to the main ground lug
plug in good speaker n test

[sjwood3]

[THis post has been updated with new results]

Solution!   I think.   Grounded speaker jack properly.  And, as predicted, I did something wrong.  1 Ohm resistors at the inputs instead of 1 M!    I know.     Just need to order the resistors and see if my woes are over!

[shooter]

I know.   

add up all the parts you got correct, divide into the 2 that are wrong and you're still at A- 

[sjwood3]

Ok, all inputs work!!!!   One problem persists.   When I increase the intensity pot, it decreases the overall volume of the amp on channel two.  It sounds great otherwise!   

[shooter]

not sure... follow the intensity to a 220k.
put a 220k in parallel (gator clips encouraged :) with "that" 220k and see if you went the right way tor'd happy 

[2deaf]

Ok, all inputs work!!!!   One problem persists.   When I increase the intensity pot, it decreases the overall volume of the amp on channel two.  It sounds great otherwise!

Sometimes the neon lamp will stay lit if V5B isn't firmly in cutoff and the intensity control will become a volume control when the tremolo is off.  Check the voltage on V5 pin 7 when the tremolo is switched off by the pedal.

[sjwood3]

Sometimes the neon lamp will stay lit if V5B isn't firmly in cutoff.......Check the voltage on V5 pin 7
[/quote]

What is cut off?  If not in cut off how do I make that firmly happen?  I dont have a foot switch yet. For it. I do have a super reverb that I can borrow one from.

Pin voltage at V57 was 0

I did not that my voltages for this tube are slightly off from the original ab763.  But could that be related to the rectifier change I made?  It has a full wave rectifier because of the reissue transformer. 

All V5 voltages:
1-277
2-3.4mV
3-2.3
4-6.3
5-6.3
6-405
7-3.3mV
8-5.38
9- 6.3

[2deaf]

What is cut off?  If not in cut off how do I make that firmly happen?  I dont have a foot switch yet. For it. I do have a super reverb that I can borrow one from.

Pin voltage at V57 was 0

Cut off is when there is essentially no current flowing through the tube.  If you can get the current low enough, the neon lamp will not stay lit.  The AB763 does this by applying the negative bias voltage to the grid.  There should be a healthy negative voltage on the grids with no footswitch or with a footswitch that makes the tremolo stop.

The negative bias voltage is supplied to the grids of the tremolo tube via a 2.2M resistor.  There is no voltage drop across the 2.2M resistor and the 1M resistors going to the grids when the tremolo is off because there is no current flowing in that circuit, so the grids are at the negative bias voltage.  Your meter will read a different negative voltage if everything is functioning properly, but it shouldn't read zero.

What are the voltages to ground on each side of the 2.2M resistor?   

 

[sjwood3]

Ok.  I had the negative bias wire and the wire that went to the intensity pot backwards.  I now have negative voltage on pin 2 @-50 and pin 7 @-46.  Still no tremolo effect.   

Update V5 voltages

1-450
2- (-49) neg
3- 0
4,5 - 6.3
6-407
7-(-45)neg
8-0
9-6.3

[sluckey]

Still no tremolo effect.

You have no tremolo because of the high negative voltage on V5 pins 2 and 7. You must enable the tremolo with a footswitch, or a shorted RCA plug, or a gator clip lead connected between chassis and the center of the RCA phono jack. Once that jack is grounded the tremolo will work (assuming no other wiring errors).

[sjwood3]

Shorted to ground as described and the 2/7 pins went to about neg 17 each.  A little less on pin 7 .  No trem engaged.  I'm still hunting for more wiring errors I may have made....so close! 

[sluckey]

When you ground the trem jack the voltage at V5 pins 2 and 7 should go to ZERO. Something still not right. Post a hi-rez pic. Maybe we can spot something.

[sjwood3]

When you ground the trem jack the voltage at V5 pins 2 and 7 should go to ZERO. Something still not right. Post a hi-rez pic. Maybe we can spot something.

Here's a Google photos link to the current state of affairs.  Should be pretty high resolution there. 

https://photos.app.goo.gl/kMEeyY3q9gQcSkZt8

[2deaf]

The yellow wire from the RCA jack should connect to the 2.2M/1M/1M junction like that dotted line shows.  I can't tell in the pictures where that yellow wire connects.

[sluckey]

see attached pic...

[sjwood3]

I added a pic of the underside of the board to the album.  It was pre populated when I received it.   

[2deaf]

Yellow wires are reversed.  The one going to the RCA jack should go to V5A and the one going to V5A should go to the RCA jack.

[sjwood3]

Great success.  Need to work out the thumping with the speed past 4, but it's there!  .  Seems to be a commonly address issue here.  Likely coming from high voltage in the power supply?  Thanks guys! 

[sjwood3]

Final update.   It works!  Reverb and trem sound great.    I sincerely appreciate all the help.