1965 Vibrolux Reverb - First Major Project DONE!

[Yeatzee]

Hello, new to the forum and fairly new to the fun world of vintage amp repair! I was pointed to this forum as a great place to share this project, and learn some helpful tips and tricks along the way so here I am  Long story short, I decided to dive into the world of vintage amps after dabbling with some nice modern amps for both the sound as well as the ability to keep them running and not rely on someone else for repair work. I started out with a cheap '55 Magnatone I bought off craigslist that was broken to get my feet wet and start learning the basics. Once that was done I worked on a few other amps, and then decided to dive straight off the deep end when a bucket list amp became available locally.

Que, the '65 FEIC Vibrolux Reverb!

Here she is:


Like I did with the Magnatone, I'm going to document everything on here and in video form warts and all for future reference. Feel free to tag along for the journey, advice greatly appreciated!

Here's the playlist:

Some shots from the owner:







Initial amp assessment:

• Previous work is very shoddy, lot of work done but done poorly
• OT is replaced but with a vintage correct part
• Tube chart is Feb 65 which seems to be the latest date on the amp apart from the OT so going to assume that's the right date. FEIC faceplate which is kind of cool, how late into 65 did those show up still?
• 3 prong needed
• Parts that will be ordered include a bunch of electrolytics, blue molded replacements, some resistors that are out of spec, 3 prong power cord, maybe some better tubes if I can find some non-insane pricing, and hopefully that's about it!

[mresistor]

Wow the amp is very clean. It's beautiful. Anxious to watch as your project progresses. 

[Yeatzee]

Wow the amp is very clean. It's beautiful. Anxious to watch as your project progresses.

It is really a beautiful piece! Don't want to screw it up haha.

So first step after the initial inspection, I decided to replace all of the electrolytics in the dog house as well as the two resistors and all of the bus wire. More for peace of mind than anything, though a couple of the caps were installed in a way I really didn't like. Tried to do a clean job, pretty happy with how it came out.

[sluckey]

I wanted to point out an error you made (actually several errors) when measuring resistor values on the circuit board. You cannot always measure a resistor accurately because there may be other resistors in parallel. This will always give a lower reading than expected. It's a good idea to be looking at the schematic, not the layout, when checking resistors in circuit. One example, at 1:55 on the video you are checking the 820Ω NFB resistor and your meter showed 94.6Ω. But if you look at the schematic you will see that there is a 100Ω resistor in parallel. Doing the math shows that 820||100=89.1. So, there's nothing wrong with that 820Ω resistor. Check it when you pull it out of the circuit. There were several other cases where the measured resistance was much lower than the expected value, but no need to point those out. FYI, as carbon comp resistors age their value tends to increase. Rarely, if never, do they decrease in value unless they show signs of severe heat.

Now jump ahead to 4:40. You are talking about replacing a .002µF cap in the reverb circuit with a 300pF cap. According to the schematic that cap should be .003µF. I have occasionally seen .002µF in that location and the reverb works just fine. But a 300pF cap is WAAAAY wrong and will mess up the sound of the reverb. A .003µF is equal to 3000pF, not 300pF.

I didn't watch any more of the video. I would like to make a suggestion... When replacing components, compare the actual as found values with the layout ***AND*** the schematic. And know that sometimes Fender had discrepancies between their schematic and layout.

Nice project BTW. Have fun and take your time. One question... Did you even turn this amp on to evaluate sound, measure voltages, etc.?

[Yeatzee]

I wanted to point out an error you made (actually several errors) when measuring resistor values on the circuit board. You cannot always measure a resistor accurately because there may be other resistors in parallel. This will always give a lower reading than expected. It's a good idea to be looking at the schematic, not the layout, when checking resistors in circuit. One example, at 1:55 on the video you are checking the 820Ω NFB resistor and your meter showed 94.6Ω. But if you look at the schematic you will see that there is a 100Ω resistor in parallel. Doing the math shows that 820||100=89.1. So, there's nothing wrong with that 820Ω resistor. Check it when you pull it out of the circuit. There were several other cases where the measured resistance was much lower than the expected value, but no need to point those out. FYI, as carbon comp resistors age their value tends to increase. Rarely, if never, do they decrease in value unless they show signs of severe heat.

Now jump ahead to 4:40. You are talking about replacing a .002µF cap in the reverb circuit with a 300pF cap. According to the schematic that cap should be .003µF. I have occasionally seen .002µF in that location and the reverb works just fine. But a 300pF cap is WAAAAY wrong and will mess up the sound of the reverb. A .003µF is equal to 3000pF, not 300pF.

I didn't watch any more of the video. I would like to make a suggestion... When replacing components, compare the actual as found values with the layout ***AND*** the schematic. And know that sometimes Fender had discrepancies between their schematic and layout.

Nice project BTW. Have fun and take your time. One question... Did you even turn this amp on to evaluate sound, measure voltages, etc.?

Appreciate it! Yeah I intended to redo basically everything so all resistors will be checked out of circuit as well. I don't know enough off hand to identify where they can and can't be read accurately but I figure it's a good start just reading everything so I could make an initial order of parts to get the ball rolling. I ended up actually buying some NOS AB resistors to swap everything that isn't already an AB.

And yep I caught that later with the reverb cap, I was steered to that by someone but obviously it was incorrect. Now I know how to convert micro/nano/pico fareds. Slowly learning!

[Yeatzee]

Part 3 Takeaways:

• Amp had two blue molded's missing from the vibrato circuit. I bought a .1 and then planned to use the .047 death cap to complete the set but didn't know the death cap is a 600v instead of a 400v that would have been there.
• Do I swap the death cap in despite it being bigger (assuming it all fits) or do I take the sole 400v .047 blue molded in-between the two pairs on the normal/vibrato channel and move that to the vibrato channel and put the death cap in place of that one so that the vibrato channel and normal channel both have the proper pairing of blue moldeds? Discussed in the video @ 9:00
• I bought 3 F&T 2 in 1 25uF caps and they are huge. When I mocked it up it (@14:00) it seems like they really wouldn't fit with the other parts side by side not giving enough room. I know the common move is to just swap in the little baby 25uF mod/sprague caps twisting the negative end together but I don't like the look of that (... I know). It looks cheesy to me having these tiny little caps in a place where the large mallory's once were. Can I make them fit? Let's find out...

[mresistor]

A death cap or line filter cap is not necessary once you install a grounded power cord and hook it up properly.  If you do want to keep the ground switch operational then you should replace the death cap with a X1Y2 safety capacitor like this https://www.justradios.com/X1Y2capacitors.html


improved mains wiring
 

[sluckey]

I have a collection of hi-rez photos of a '64 Deluxe Reverb that may be of interest. There are a few slightly different parts values, but otherwise the board is identical to your Vibrolux.

     http://sluckeyamps.com/64DR/index.htm

Also take a look at this short video...

[Yeatzee]

A death cap or line filter cap is not necessary once you install a grounded power cord and hook it up properly.  If you do want to keep the ground switch operational then you should replace the death cap with a X1Y2 safety capacitor like this https://www.justradios.com/X1Y2capacitors.html


improved mains wiring
 

3 prong is the last thing I do, more or less, just because it's annoying to work on the amp with that power cable hanging off the work bench. The plan for the death cap was to reuse it to replace one of the missing blue molded caps, my dilemma was more because it's a 600v and thus larger than what was originally there, should I swap the existing 400v .047 into the vibrato channel position and put the 600v death cap in it's place or? I picked up a .1 off ebay to complete the set either way.

I have a collection of hi-rez photos of a '64 Deluxe Reverb that may be of interest. There are a few slightly different parts values, but otherwise the board is identical to your Vibrolux.

     http://sluckeyamps.com/64DR/index.htm

Also take a look at this short video...

I'll take em!

And great find on that video, can clearly see everything. Wish I had found that sooner! I do wish people showed more of the work vs just skipping to tada it's all done, that's why I'm going to try and just record and show most of the work or at least me starting the work so that it's not some big mystery how it's all done.

[sluckey]

I'll take em! If it's easier, my email is yeatzeeguitar@gmail.com.

Just visit the link I posted and save them. No need for email.

[Yeatzee]

I'll take em! If it's easier, my email is yeatzeeguitar@gmail.com.

Just visit the link I posted and save them. No need for email.

Woops, missed that sorry! Thanks!

[bmccowan]

I do wish people showed more of the work vs just skipping to tada it's all done, that's why I'm going to try and just record and show most of the work or at least me starting the work so that it's not some big mystery how it's all done.

Yeatzee, There is a lot of clear, helpful, direction out there. Both in video: D Lab, Uncle Doug, Fazio, etc. and in print - right here in both Doug's pages and on the Forum, Rob Robinette's site, Sluckey's site, Valve Wizard, etc. You will save yourself a lot of brain ache if you spend some time researching and reading before diving in. Things like measuring resistor values in circuit, and the wattage rating of capacitors are well documented in numerous places.
Great amp - great project - good luck with it and I applaud you documenting the steps.

[Yeatzee]

I do wish people showed more of the work vs just skipping to tada it's all done, that's why I'm going to try and just record and show most of the work or at least me starting the work so that it's not some big mystery how it's all done.

Yeatzee, There is a lot of clear, helpful, direction out there. Both in video: D Lab, Uncle Doug, Fazio, etc. and in print - right here in both Doug's pages and on the Forum, Rob Robinette's site, Sluckey's site, Valve Wizard, etc. You will save yourself a lot of brain ache if you spend some time researching and reading before diving in. Things like measuring resistor values in circuit, and the wattage rating of capacitors are well documented in numerous places.
Great amp - great project - good luck with it and I applaud you documenting the steps.

I've watched quite a bit from those youtube channels mentioned, but they all do tend to cut quite a bit of the work out so picking up the technique's can be a bit more difficult. I'm a learn on the job kind of person, the best way for me to learn is to do. Doesn't mean I'm being dumb about it though, I do have some lifelines I'm in talks with along the way to give me more real time advice if I run into any snags. I've also consumed dozens of hours of YT videos and worked on a few other lesser amps to get some reps in.

As far as I can tell, basically 90% of the resistors in the amp can be read in-circuit? Like I said, all non-AB's are being removed and replaced either way more about giving myself a starting point and trying out the new autoranging multimeter (fanncccyyyy lol).

[dwinstonwood]

This is a really cool project. So, go slow and enjoy every bit of it. 

Along with this forum, Merlin Blencowe's (the Valve Wizard) book: Designing Tube Preamps for Guitar and Bass, 2nd edition, has been invaluable to me.

So, what helps me the most is finding the sources that explain why a circuit uses the specific parts and values it does, not just how to successfully assemble them. (For example, it took me a good while to be able to see why certain voltage ratings are important in certain places. In other words, I had to buckle down and learn Ohm's Law)

I'll be tuned in to your project!

[Yeatzee]

This is a really cool project. So, go slow and enjoy every bit of it. 

Along with this forum, Merlin Blencowe's (the Valve Wizard) book: Designing Tube Preamps for Guitar and Bass, 2nd edition, has been invaluable to me.

So, what helps me the most is finding the sources that explain why a circuit uses the specific parts and values it does, not just how to successfully assemble them. (For example, it took me a good while to be able to see why certain voltage ratings are important in certain places. In other words, I had to buckle down and learn Ohm's Law)

I'll be tuned in to your project!

Sweet, thanks for the rec! I'm far more interested in the practical knowledge to keep/maintain vintage amps, but I know I'll have to dive into that stuff more over time.

Here's Part 4:

More or less just a continuation of removing more parts to set myself up better later, and checking resistors more (lifting a leg first). Also try a basic mockup with the giant F&T caps and run into a crowded eyelet that didn't seem to have any space left. Spoiler, I figured out one of the extra wires in there was a leftover from the previous tech that was left there so mystery solved lol.

[sluckey]

Just watched part 8. I've always kinda thought of the Pro Reverb as Fender's experimental "red-haired stepchild" amp. Built from leftover parts. You never know what you may find in one. And most often the actual amp will not match the published schematic.

I'm still waiting for the video when you actually apply power to this amp. You never answered my question... "Did you even turn this amp on to evaluate sound, measure voltages, etc.?"

[Yeatzee]

Just watched part 8. I've always kinda thought of the Pro Reverb as Fender's experimental "red-haired stepchild" amp. Built from leftover parts. You never know what you may find in one. And most often the actual amp will not match the published schematic.

I'm still waiting for the video when you actually apply power to this amp. You never answered my question... "Did you even turn this amp on to evaluate sound, measure voltages, etc.?"

eature=share

I plugged it in for 10min to confirm it worked, then off to the bench it went! Why would I apply power to the amp before it's back together? Am I missing something?

[sluckey]

I plugged it in for 10min to confirm it worked, then off to the bench it went! Why would I apply power to the amp before it's back together? Am I missing something?

That video answers part of my question. So, the amp sounds good and seems to work very well. Does the tremolo work?

Your plan is to restore this amp to original condition, changing a lot of parts that tested out of specs or just because you didn't like the way they looked. Just as you evaluated the sound of the amp, it's equally important to get a baseline of the "before" voltage readings for all tube pins and the filter caps. Having this baseline "may" come in handy in the event that the amp does not perform well after all the work. And since you changed a resistor on the bias pot, knowing what the bias voltage on pin 5 of the 6L6s will allow you to set the bias pot for the same bias voltage as before. I know you have been meticulous in replacing these components, but sometimes collateral damage does occur, no matter how careful you are. Suppose an under board jumper wire comes loose. Sometimes difficult to troubleshoot even for an experienced technician. Having those baseline voltage readings just might make troubleshooting easier.

Taking a post set of voltage readings for the future is also a good idea. When I finish a project I like to record the voltage readings on the schematic. Never know when they might come in handy.

[Yeatzee]

I plugged it in for 10min to confirm it worked, then off to the bench it went! Why would I apply power to the amp before it's back together? Am I missing something?

That video answers part of my question. So, the amp sounds good and seems to work very well. Does the tremolo work?

Your plan is to restore this amp to original condition, changing a lot of parts that tested out of specs or just because you didn't like the way they looked. Just as you evaluated the sound of the amp, it's equally important to get a baseline of the "before" voltage readings for all tube pins and the filter caps. Having this baseline "may" come in handy in the event that the amp does not perform well after all the work. And since you changed a resistor on the bias pot, knowing what the bias voltage on pin 5 of the 6L6s will allow you to set the bias pot for the same bias voltage as before. I know you have been meticulous in replacing these components, but sometimes collateral damage does occur, no matter how careful you are. Suppose an under board jumper wire comes loose. Sometimes difficult to troubleshoot even for an experienced technician. Having those baseline voltage readings just might make troubleshooting easier.

Taking a post set of voltage readings for the future is also a good idea. When I finish a project I like to record the voltage readings on the schematic. Never know when they might come in handy.

Got it, good to know! I have no context to really compare it to from before (first blackface, first vibrolux), but overall it sounded fairly healthy. Reverb seemed over the top but that was about it, like I said only played it for a few minutes during a lunch break because I knew what the end goal was :)

What areas would you recommend I specifically note voltages? Just grab voltage at all of the points on the layout drawing that shows voltage and go from there? I do have a variac on the bench, with the bias I know it'll be interesting to see if the stock value will work but I'd like to start there. I don't intend to bias it for variac use though, as I'd like to play it out in the band once it's 100% good to go. Not ideal I know, but that's the difference between a bedroom princess and a tool. I want it to be tool :)

Part 5 is here:

In it I fix the aptly pointed out incorrect reverb cap with one I found on Ebay, and made some good headway through the terribly installed flying resistors all over the board. Fun stuff!

[sluckey]

What areas would you recommend I specifically note voltages?

See my previous post.

with the bias I know it'll be interesting to see if the stock value will work but I'd like to start there.

If you had a voltage chart you would already have a good starting point. 

I'd like to play it out in the band once it's 100% good to go. Not ideal I know, but that's the difference between a bedroom princess and a tool. I want it to be tool :)

After the obsessive parts replacement I was sure this would be on a shelf behind glass.      I've got a lot of stuff I call "tools" (I.m kinda obsessive about proper tools and test equipment), but I've never put dirty engine grime on a cotton swab to rub onto a screw head that I thought was too shiny. Now that is OBSESSIVE!   

Just funning. You're doing a great job and you're learning some stuff too. You can apply all that knowledge on that silverface Pro Reverb.

Oh, BTW, you can buy those original Mallory dual caps for the cathode bypass caps on eBay. Lot's of people do that, gut them and stuff two modern caps inside. Sure looks better than those big ole F&Ts. People do the same for the Mallory caps under the doghouse.

[Yeatzee]

Oof, plugged it into the current limiter and variac brought up the voltage get pilot light on but no output whatsoever. 

This is where I earn my stripes I guess! Alright let's hear it, what's the proper order of things to check to diagnose this? No tubes as well?

I took some photo's of the board, incase any of you guys are able to pick something out visually.

https://drive.google.com/drive/folders/1-RMAySnMbrXTslfIYkMV-4gbJiCUs8MI?usp=sharing

[Voxbox]

Go round all the valve bases and record the voltages on every pin. Also check all the power supply nodes.
Then post them here.


A scope is really handy for tracing audio too.


I can only echo what Sluckey said about recording the voltages before commencing. Sorry!! It's a principle I apply on every job I do. Then I can see where I've screwed up  Or where needs attention.


Once you get it going, which I'm sure you will, do check for any voltages present on the circuit board on the actual board itself. (not the terminals or eyelets) Voltages may be more noticeable near high voltage points. There should be zero volts everywhere on the board.


The reason is that some of these old Fenders have conductive boards due to being stored in a less than dry environment. It may be more of an issue here in the soggy UK than sunny California.....


I've rebuilt several Fender amps because of this, and the difference in performance and noise levels is remarkable. The amps just come alive when the conductive board is changed out.
The damp in the boards that causes the conductivity is really hard to get rid of. Replacement with a fibreglass board is the best way to go.


I dont want to cause concern, but its definitely worth checking out in any old amp!


Old Marshalls and Voxes dont have the issue due to the paxolin boards used.


All the best and looking forward to the next episode.


Cheers, VB

[bmccowan]

no output whatsoever

Bummer - but it worked before and you will get it working again. I do not disagree with anything VB said, and you will get continued expert help from Sluckey, I am sure.
In the meantime - I suggest you follow a standard startup/troubleshooting procedure.
If you go to the top of the Tube Amp Building - Tweaks - Repairs board you will see two threads - Before Posting Amp Problems... and How to Double Check Your Amp Wiring...
There is great advice in both of those threads - and also links to other troubleshooting guides.
You've been doing nice, neat work, but as Sluckey said it is easy to have something go wrong. Good luck.

[mresistor]

One thing I see so far is that the courtesy outlet is completely bypassed, nothing connected to the hot side. I think the power input cord wiring may be not correct.

[Yeatzee]

One thing I see so far is that the courtesy outlet is completely bypassed, nothing connected to the hot side. I think the power input cord wiring may be not correct.

Here's where I go over how I did the 3 prong (skip ahead to like 21:20 or so):

Chassis ground, neutral connected to courtesy Outlet just as a nice clean way to connect to one of the PT leads (wasn't looking to maintain it), and then hot to the fuse tip with a wire going from the other fuse connection point to thr switch. Then the other black PT lead going to the  remaining spot on the switch.

[sluckey]

There are two models of the VR (Vibrolux Reverb)... aa270 and aa964. Which layout have you been using?

I spotted an incorrect resistor value. It's not causing your symptom but should be corrected. See attached pic.

I cannot see the resistors under those huge F&T dual caps but need to verify values. To do that, with power off, measure resistance in reference to chassis for these tube pins... V1-3, V1-8, V2-3, V2-8, V3-3, V3-8, V4-3, V4-8, V5-3, V5-8, V6-3, and V6-8. Report measurements.

[Yeatzee]

no output whatsoever

Bummer - but it worked before and you will get it working again. I do not disagree with anything VB said, and you will get continued expert help from Sluckey, I am sure.
In the meantime - I suggest you follow a standard startup/troubleshooting procedure.
If you go to the top of the Tube Amp Building - Tweaks - Repairs board you will see two threads - Before Posting Amp Problems... and How to Double Check Your Amp Wiring...
There is great advice in both of those threads - and also links to other troubleshooting guides.
You've been doing nice, neat work, but as Sluckey said it is easy to have something go wrong. Good luck.

Woops, should have checked the stickies! Doing that now!

[Yeatzee]

There are two models of the VR (Vibrolux Reverb)... aa270 and aa964. Which layout have you been using?

I spotted an incorrect resistor value. It's not causing your symptom but should be corrected. See attached pic.

I cannot see the resistors under those huge F&T dual caps but need to verify values. To do that, with power off, measure resistance in reference to chassis for these tube pins... V1-3, V1-8, V2-3, V2-8, V3-3, V3-8, V4-3, V4-8, V5-3, V5-8, V6-3, and V6-8. Report measurements.

It's the weird one:



I was told to rely on the AA964 layout so that's what I've been using. I guess I can review my old footage / photo's and make sure I did everything as it was before.

Yeah I've got a replacement resistor on the way (there were no 470k NOS AB's when I looked), but from what I understood that wasn't going to cause any major issues just change the tone a touch. It's the closest I could find at the time. I'll do that after my meeting this morning and report back. I do show the installation of every one of those resistors in this video though, and checked each and every one with the MM before and after install to make sure I had it correct. I even did a check mark next to each resistor as I went on the schematic to make sure I didn't miss any. Tried to be thorough but obviously failed somewhere!

[sluckey]

Replacing that 510K is very low priority. All that did was limit the max reverb a little bit. These amps have plenty of reverb to spare.

I still want those resistance readings and don't just put your probes across the resistor. Do it exactly as I suggested. This will also verify that the resistors have good connection all the way to the tube pins.

When I have those resistor measurements, I'll outline exactly how to measure the voltages I mentioned earlier. This is where we are likely to find some clues as to what's wrong.

[Yeatzee]

There are two models of the VR (Vibrolux Reverb)... aa270 and aa964. Which layout have you been using?

I spotted an incorrect resistor value. It's not causing your symptom but should be corrected. See attached pic.

I cannot see the resistors under those huge F&T dual caps but need to verify values. To do that, with power off, measure resistance in reference to chassis for these tube pins... V1-3, V1-8, V2-3, V2-8, V3-3, V3-8, V4-3, V4-8, V5-3, V5-8, V6-3, and V6-8. Report measurements.

• V1-3: 1.5k
• V1-8: 850ohm
• V2-3: 1.5k
• V2-8: 840
• V3-3 & 8 (jumpered): 2.3k
• V4-3 & 8 (jumpered): 805
• V5-3: 2.8k
• V5-8: 100k
• V6-3 & 8 (jumpered): 29k

[Yeatzee]

Replacing that 510K is very low priority. All that did was limit the max reverb a little bit. These amps have plenty of reverb to spare.

I still want those resistance readings and don't just put your probes across the resistor. Do it exactly as I suggested. This will also verify that the resistors have good connection all the way to the tube pins.

When I have those resistor measurements, I'll outline exactly how to measure the voltages I mentioned earlier. This is where we are likely to find some clues as to what's wrong.

Woops looks like we posted at the same time. Done, see above! Chassis as the ground reference for those measurements. Good to know about the resistor and what that one affects!

[sluckey]

Resistance reading look good except the last one. That 29K should be about 22.5K. Not a showstopper but is more important that that reverb resistor. You put a 27K where a 22K is called for. See attached pic...

[Yeatzee]

Resistance reading look good except the last one. That 29K should be about 22.5K. Not a showstopper but is more important that that reverb resistor. You put a 27K where a 22K is called for. See attached pic...

Noted, I believe that was also one I couldn't/didn't get the right value for but figured it was close enough to do a test. I need to find my notes. There was also one resistor on the bass pot that I didn't have the right value for so I kept the replacement resistor that was there already and then the 470k we discussed. Those were the only ones that weren't exact to specs but close. So for voltages, should I just start checking along with the schematic then? No tubes in?

[sluckey]

First, plug the amp straight into the wall with no lamp limiter. Put the amp in standby mode so there will be no high B+ voltages. Plug in all tubes. Turn the power on. Are all filaments lit? Little tubes can be hard to tell so turn off lights if needed. Measure the AC filament voltage by connecting one probe to pin 2 of a 6L6 and the other probe to pin 7 of the same tube. Should be about 6.3VAC. Now connect the black probe to chassis. All the rest of the voltage readings will be referenced to chassis. Measure the AC voltage on pins 4 and 6 of the rectifier tube (BTW a 5AR4 is the same as a GZ34). Should be 300VAC or more and each pin should read the same. Next, measure the AC voltage at the point on the little bias board where the PT red/blue wire connects to the 470Ω resistor. Should be about 50VAC. This completes the AC voltage checks so switch your meter to dc volts for the rest of the readings. BTW, you should be writing down all these readings and post them when done.

Connect your meter to pin 5 of a 6L6. Looking for about -50VDC. Repeat for the other 6L6. This is your bias voltage. Turn the bias pot to each end and record the high and low voltages. Then set the bias pot for the most negative voltage on pin 5. We'll set the bias pot properly later.

Now for the B+ voltages. BE CAREFUL! Remove the dog house cover. Turn the standby switch to operate mode. Measure the DC voltage on the positive end of each filter cap. Now move on to the big tubes. For each 6L6, measure the DC volts on pins 3, 4, 8. Pin 8 should be zero, but check it anyway.

Now for the little tubes. Measure the voltage on pins 1, 2, 3, 6, 7, and 8 for every little tube. Some of these voltages will be zero. Thats OK. Record those too.

Don't be switching the amp to standby or off between readings. Leave the amp ON and be careful. You have steady hands and those little grabber probes you have should make this easy. Just don't short two adjacent pins together when movinf the probes.

That's enough for now. Post all these readings. Hopefully they will reveal a clue as to what's wrong.

[Yeatzee]

First clue, I brought it up on the variac / current limiter to 117 vac with no tubes and speaker cable connected and the pilot light did not light up this time. I checked the voltage on the fuse / switch and everything read 117v. Checked the power transformer black wire connections and they also read the same. So I turned it off, dreading the worst that somehow the PT died doing nothing. I then decided to turn it back on and check the voltages again and suddenly after a minute or so the bulb started to light up and the heaters were getting about 3.1ish vac each with chassis ground so I think that means it's about correct..? No tubes in I checked the rectifier connections and on pins 4 and 6 show 330VAC. Pin 2 showed 5vac and -11.3vdc.

Red wire to bias circuit measured 47vac.

I've got to run to a meeting so I'll be able to do the rest later. Going to be driving me crazy in the meantime

[Yeatzee]

First, plug the amp straight into the wall with no lamp limiter. Put the amp in standby mode so there will be no high B+ voltages. Plug in all tubes. Turn the power on. Are all filaments lit? Little tubes can be hard to tell so turn off lights if needed. Measure the AC filament voltage by connecting one probe to pin 2 of a 6L6 and the other probe to pin 7 of the same tube. Should be about 6.3VAC. Now connect the black probe to chassis. All the rest of the voltage readings will be referenced to chassis. Measure the AC voltage on pins 4 and 6 of the rectifier tube (BTW a 5AR4 is the same as a GZ34). Should be 300VAC or more and each pin should read the same. Next, measure the AC voltage at the point on the little bias board where the PT red/blue wire connects to the 470Ω resistor. Should be about 50VAC. This completes the AC voltage checks so switch your meter to dc volts for the rest of the readings. BTW, you should be writing down all these readings and post them when done.

Connect your meter to pin 5 of a 6L6. Looking for about -50VDC. Repeat for the other 6L6. This is your bias voltage. Turn the bias pot to each end and record the high and low voltages. Then set the bias pot for the most negative voltage on pin 5. We'll set the bias pot properly later.

Now for the B+ voltages. BE CAREFUL! Remove the dog house cover. Turn the standby switch to operate mode. Measure the DC voltage on the positive end of each filter cap. Now move on to the big tubes. For each 6L6, measure the DC volts on pins 3, 4, 8. Pin 8 should be zero, but check it anyway.

Now for the little tubes. Measure the voltage on pins 1, 2, 3, 6, 7, and 8 for every little tube. Some of these voltages will be zero. Thats OK. Record those too.

Don't be switching the amp to standby or off between readings. Leave the amp ON and be careful. You have steady hands and those little grabber probes you have should make this easy. Just don't short two adjacent pins together when movinf the probes.

That's enough for now. Post all these readings. Hopefully they will reveal a clue as to what's wrong.

Keep just missing eachother lol. Got it, I'll do the remaining bits and redo without the limiter later today.

[sluckey]

First clue, I brought it up on the variac / current limiter to 117 vac

Get rid of the current limiter. It has served it's purpose. It will also mess up your voltage readings. Using a variac is OK. I like to set my variac to 120VAC when I'm checking voltages on any amp. I also note that on the schematic. That way, 10 years later I can set it to 120 and expect to see the same voltages as before (if the amp is working properly).

[Yeatzee]

First, plug the amp straight into the wall with no lamp limiter. Put the amp in standby mode so there will be no high B+ voltages. Plug in all tubes. Turn the power on. Are all filaments lit? Little tubes can be hard to tell so turn off lights if needed. Measure the AC filament voltage by connecting one probe to pin 2 of a 6L6 and the other probe to pin 7 of the same tube. Should be about 6.3VAC. Now connect the black probe to chassis. All the rest of the voltage readings will be referenced to chassis. Measure the AC voltage on pins 4 and 6 of the rectifier tube (BTW a 5AR4 is the same as a GZ34). Should be 300VAC or more and each pin should read the same. Next, measure the AC voltage at the point on the little bias board where the PT red/blue wire connects to the 470Ω resistor. Should be about 50VAC. This completes the AC voltage checks so switch your meter to dc volts for the rest of the readings. BTW, you should be writing down all these readings and post them when done.

Connect your meter to pin 5 of a 6L6. Looking for about -50VDC. Repeat for the other 6L6. This is your bias voltage. Turn the bias pot to each end and record the high and low voltages. Then set the bias pot for the most negative voltage on pin 5. We'll set the bias pot properly later.

Now for the B+ voltages. BE CAREFUL! Remove the dog house cover. Turn the standby switch to operate mode. Measure the DC voltage on the positive end of each filter cap. Now move on to the big tubes. For each 6L6, measure the DC volts on pins 3, 4, 8. Pin 8 should be zero, but check it anyway.

Now for the little tubes. Measure the voltage on pins 1, 2, 3, 6, 7, and 8 for every little tube. Some of these voltages will be zero. Thats OK. Record those too.

Don't be switching the amp to standby or off between readings. Leave the amp ON and be careful. You have steady hands and those little grabber probes you have should make this easy. Just don't short two adjacent pins together when movinf the probes.

That's enough for now. Post all these readings. Hopefully they will reveal a clue as to what's wrong.

Without the current limiter in chain, bringing up the voltage gradually on the variac the pilot light always comes on but every once in a while it seems to be delayed like it's warming up or something and needs a sec. That's just an aside, but worth noting.

I started to do what you outlined but the tubes felt and smelled unusually hot, I couldn't see redplating but it wasn't a good smell so I decided to remove all tubes but keep the rectifier in there to read voltages.

Once up to 117vac I checked all of the PT leads and everything seemed ok. Black PT lead connected to the switch read 117v, the other black lead connected to the neutral of the 3 prong read basically 0. Red leads off the PT to Rectifier at pins 4 & 6 read 330vac. Yellow leads off the PT to rectifier at pins 2 and 8 read 452vdc. Green leads to heaters / lamp read 6.5vac. Red/Blue lead to bias circuit reads 47vac.

After checking the PT I checked the bias because of the tube smell / heat issue, and on the middle wiper of the bias pot it reads -15vdc. Schematic says it should read -42vdc so I think there's at least one major issue. I checked the 470hm 1 watt resistor which appears to be original, and it read 473ohm so that's fine. I then checked the diode which also looks original and that's where I noticed the first major issue. Ive never measured a diode before, but as I understand it when measuring I should get an ohm rating with the probe leads one way and no ohm reading if they're swapped. Well the diode read about 30kohm with the leads either way so it seems to no longer be functioning. I have some 1n4007 diodes from a different project laying around, I checked them with my multimeter and they do show no reading one direction, then 5mohm the other way. On the layout drawing it doesn't give any details about what the diode should be so I need to figure out if the diodes I have can work or if I need to order something different.

What would a failed diode in the bias circuit do to things? I haven't grasped how the bias circuit works yet.

Edit: found this excellent article on the site, very helpful: https://el34world.com/charts/Biascircuits.htm

So the diodes I have on hand are 1n4007 1000v 1a according to the specs on my order sheet. Is that find for the bias circuit? It's only 50v roughly so I'm guessing it's probably more than fine, but it does look quite small next to the original in there so want to be sure!

[Esquirefreak]

1N4007 is fine. I usually use UF4007. For a HT rectifier I like the UF5408.

Make sure that you've got at least -50VDC on the output of the bias pot (and on pin 5 of the power tube sockets as well) before putting in new tubes. A good routine/starting point is to have "too much" negative voltage before biasing properly.

Confirm that the new diode is good and you have enough negative voltage at the other end of it (and on pin 5 of the power tube sockets). If the amount of negative voltage is too low, the shunt/bleeder resistor on the bias pot may be too small. I think around 22k is pretty close.

Higher value resistor = more negative voltage. More negative voltage = cooler bias. When you turn the bias pot clockwise you bleed negative voltage to ground, making the grids of the power tubes less negative (or more positive). More positive grids on the tubes allows more current to flow, thus causing them to run hotter.

/Max

[sluckey]

You're not following my instructions! You should be in standby mode at this time! I laid out a very specific, logical procedure to check this amp. Please follow exactly.

Well the diode read about 30kohm with the leads either way so it seems to no longer be functioning.

I doubt that diode is bad. You must disconnect one lead to properly test that diode.

Read your meter manual and learn how to use the diode check function.

BTW if you will measure the voltage on pin 5 of the 6L6s (as I told you) that will tell you if the diode (and the entire bias circuit) is working properly. Should have about -50vdc and it should be adjustable with the bias pot.

Here's a quote from your meter manual...

[Yeatzee]

You're not following my instructions! You should be in standby mode at this time! I laid out a very specific, logical procedure to check this amp. Please follow exactly.

Well the diode read about 30kohm with the leads either way so it seems to no longer be functioning.

I doubt that diode is bad. You must disconnect one lead to properly test that diode.

Read your meter manual and learn how to use the diode check function.

BTW if you will measure the voltage on pin 5 of the 6L6s (as I told you) that will tell you if the diode (and the entire bias circuit) is working properly. Should have about -50vdc and it should be adjustable with the bias pot.

Here's a quote from your meter manual...

It was by accident that it was out of standby. I've never owned or played for any extended time a fender with a standby switch, on the back plate it says standby at the top exactly where it says "on" with the power switch. So I followed instructions literally, amp set to on and standby switch set to standby.... Lol.

If you doubt the diode is bad, what would explain the negative voltage issue? The resistor on the bias pot was swapped with a NOS AB that measured correctly. Turning the bias pot does change the bias, but it's max negative voltage is as stated around -10v or so. The other direction it goes positive.

Edit: and as a side affect of having it out of standby by accident I did get to hear hum out of the speakers so there is output at some level, which gives me some hope.

[Esquirefreak]

Might be an idea to check the coupling caps going to the power tubes. If they leak, one can lose negative voltage.

EDIT: Or disconnect the output of the bias pot (going to the main board) and see if the voltage readings look more correct right there. If so, the issue is either on the main board or on the sockets.

/Max

[Yeatzee]

Might be an idea to check the coupling caps going to the power tubes. If they leak, one can lose negative voltage.

/Max

That wouldn't affect bias readings at the bias pot though right? If I understand correctly that should be the place with the most negative voltage but it's way off the schematic right now. If the resistor reads fine the bias cap is a new f&t and the pot does change the bias value when turned that would leave me with what possibilities outside of a bad diode? Bad ground?

[Esquirefreak]

You should have max negative voltage at the input of the pot.

/Max

[sluckey]

The other direction it goes positive.

You never mentioned that until just now. If you had put the amp in standby MODE as instructed then there would not likely be any positive voltage in the amp. The bias voltage should be the same at the pot wiper as it is on pin 5 of the 6L6s. Best to check it at the tube socket. BTW, I don't like the way Fender labeled the standby switch. It would be more intuitive if they labeled it operate (up) and standby (down). However, everyone (including you now) that has a Fender amp from this era knows that both switches up means play or operate. No one looks at the back of the amp when flipping the power or STBY switches.

Anyhow, properly check the diode and replace if faulty. The banded end of the 1N4007 connects to the 470Ω/1W resistor. Don't put the amp in OPERATE mode until you can measure about -50vdc on pin 5 of the 6L6s. This voltage must be correct before proceeding with the other voltage checks.

[sluckey]

There should be an underboard jumper between the bias diode and cap. Is there? Do you measure zero ohms between those two eyelets?

[mresistor]

https://www.mylespaul.com/threads/1965-vibrolux-reverb-restoration-repair-project.479009/

[Yeatzee]

There should be an underboard jumper between the bias diode and cap. Is there? Do you measure zero ohms between those two eyelets?

The lead is there, zero ohms and continuity check worked as well between the two eyelets. I lifted a leg of the diode and it checked fine on the MM so that's not it. The bias pot resistor measures 25k so that doesn't seem to be the issue either. What else could it be in the bias circuit to cause it to be so off? That just leaves the bias cap and the grounds right?

The bias circuit chassis ground looks solid, and checked out with the continuity test.

[Yeatzee]

https://www.mylespaul.com/threads/1965-vibrolux-reverb-restoration-repair-project.479009/

..... What?

[sluckey]

REMOVE THE GZ34 RECTIFIER TUBE AND DON'T PUT IT BACK IN UNTIL THE BIAS ISSUE IS RESOLVED.

Now look at the attached pic and move that brown wire. That brown wire has B+ on it and you have it connected to the bias circuit. This should fix the bias issue.