1965 Vibrolux Reverb - First Major Project DONE!

[Yeatzee]

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.

UNREAL. What an absolute idiot I am.

I'll try that now, and I believe the intermittent Pt issue is the on switch. I read the voltage when it wasn't turning on and on side of the switch was getting 117v and the other side connected to the Pt was showing 0v. Then suddenly it just turned on after some time sitting.

[Yeatzee]

Bingo negative bias is reading a range of - 44 to - 57 now. Hallelujah. Sluckey if you're in the SoCal area let me buy you a beer!

Now to test the other voltages, followup on that shortly.

[sluckey]

Moving that brown wire may just fix the no sound issue too. But, don't rush it. That bias cap has been severely stressed. It's had a big positive voltage applied to the negative lead. I highly recommend replacing it with a fresh cap. You used a 100µF @ 100V which is fine. If you don't have another, then just put the one you removed back on the board until you can get a new replacement. There was nothing wrong with the old cap.

[Yeatzee]

Moving that brown wire may just fix the no sound issue too. But, don't rush it. That bias cap has been severely stressed. It's had a big positive voltage applied to the negative lead. I highly recommend replacing it with a fresh cap. You used a 100µF @ 100V which is fine. If you don't have another, then just put the one you removed back on the board until you can get a new replacement. There was nothing wrong with the old cap.

It did! After checking voltages I decided to plug in a guitar cable just to see and both channels made a sound. Hurrayyyyy!

I made an order for a new switch as well as a new bias cap and some other resistors I was missing. It's quite late now, but here were the voltages hopefully I wrote it all down correctly!

6l6's - both measured 430v at pins 3 &4 and bias on pins 8 matched what the bias pot showed (-57 or so, I made it the coolest it'd go on the pot).
V6: pin 1 = 183v, 2 = 56v, 3/8 = 90v, 6 = 175v, 7 = 60v
V5: pin 1 = 420v, 2 = -40v, 3 = 0, 6 = 367v, 7 = -40v, 8 = 0
V4: pin 1 = 198v, 2 = 0v, 3/8 = 1.3v, 6 = 189v, 7 = 0v 
V3: pin 1 = 415v, 2/7 = 8.3v, 3/8 = 14.7v, 6 = 0v
V2: pin 1 = 190v, 2 = 0v, 3 = 1.3v, 6 = 198v, 7 = 0v, 8 = 1.4v 
V1: pin 1 = 185v, 2 = 0v, 3 = 1.3v, 6 = 182v, 7 = 0v, 8 = 1.4v 

[sluckey]

6l6's - both measured 430v at pins 3 &4 and bias on pins 8 matched what the bias pot showed (-57 or so, I made it the coolest it'd go on the pot).
V6: pin 1 = 183v, 2 = 56v, 3/8 = 90v, 6 = 175v, 7 = 60v
V5: pin 1 = 420v, 2 = -40v, 3 = 0, 6 = 367v, 7 = -40v, 8 = 0   recheck with tremolo on
V4: pin 1 = 198v, 2 = 0v, 3/8 = 1.3v, 6 = 189v, 7 = 0v 
V3: pin 1 = 415v, 2/7 = 8.3v, 3/8 = 14.7v, 6 = 0v    not good. recheck
V2: pin 1 = 190v, 2 = 0v, 3 = 1.3v, 6 = 198v, 7 = 0v, 8 = 1.4v 
V1: pin 1 = 185v, 2 = 0v, 3 = 1.3v, 6 = 182v, 7 = 0v, 8 = 1.4v

Recheck... V5 is the tremolo tube. Readings are correct for tremolo OFF. Get another set of readings with tremolo ON. Need a footswitch to turn it on. If you don't have a footswitch use a gator clip test lead to connect the VIB footswitch jack tip to chassis.

Recheck... V3 is the reverb driver. Pins 1 and 6 should be tied together and both pins should read 415vdc. Pins 2/7 should be zero volts, and pins 3/8 should be about 8vdc.

All other voltages look normal. Looks like you're about done except for setting bias. I suggest buying two 1Ω 1% 1W resistors to make setting the bias easy. More about setting bias later. Meanwhile read this web page for everything you need to know about setting the bias...

     http://www.duncanamps.com/technical/lvbias.html

[sluckey]

Pic shows how to permanently install 1Ω bias resistors on pin 8 of the 6L6 sockets. I highly recommend doing this...

[EL34]

I have a complete Fender service info in the library
I wrote this up many years ago after repairing hundreds of fenders

7 Pages of fender service info here
https://el34world.com/schematics.htm#Servicing%20fender%20amps

[Yeatzee]

6l6's - both measured 430v at pins 3 &4 and bias on pins 8 matched what the bias pot showed (-57 or so, I made it the coolest it'd go on the pot).
V6: pin 1 = 183v, 2 = 56v, 3/8 = 90v, 6 = 175v, 7 = 60v
V5: pin 1 = 420v, 2 = -40v, 3 = 0, 6 = 367v, 7 = -40v, 8 = 0   recheck with tremolo on
V4: pin 1 = 198v, 2 = 0v, 3/8 = 1.3v, 6 = 189v, 7 = 0v 
V3: pin 1 = 415v, 2/7 = 8.3v, 3/8 = 14.7v, 6 = 0v    not good. recheck
V2: pin 1 = 190v, 2 = 0v, 3 = 1.3v, 6 = 198v, 7 = 0v, 8 = 1.4v 
V1: pin 1 = 185v, 2 = 0v, 3 = 1.3v, 6 = 182v, 7 = 0v, 8 = 1.4v

Recheck... V5 is the tremolo tube. Readings are correct for tremolo OFF. Get another set of readings with tremolo ON. Need a footswitch to turn it on. If you don't have a footswitch use a gator clip test lead to connect the VIB footswitch jack tip to chassis.

Recheck... V3 is the reverb driver. Pins 1 and 6 should be tied together and both pins should read 415vdc. Pins 2/7 should be zero volts, and pins 3/8 should be about 8vdc.

All other voltages look normal. Looks like you're about done except for setting bias. I suggest buying two 1Ω 1% 1W resistors to make setting the bias easy. More about setting bias later. Meanwhile read this web page for everything you need to know about setting the bias...

     http://www.duncanamps.com/technical/lvbias.html

Got it, I'll do that tonight. I invested in a bias probe as well so I wont need to add those resistors.

[Yeatzee]

I have a complete Fender service info in the library
I wrote this up many years ago after repairing hundreds of fenders

7 Pages of fender service info here
https://el34world.com/schematics.htm#Servicing%20fender%20amps

LOVE it!

[sluckey]

I invested in a bias probe as well so I wont need to add those resistors.

Which bias probe? Got a link?

[Yeatzee]

I invested in a bias probe as well so I wont need to add those resistors.

Which bias probe? Got a link?

Eurotubes

[sluckey]

You need two of those. Otherwise it's a PITA to check bias on your amp. Two 1Ω resistors look mighty attractive to me. 

[sluckey]

Have you rechecked these voltages? And?

Recheck... V5 is the tremolo tube. Readings are correct for tremolo OFF. Get another set of readings with tremolo ON. Need a footswitch to turn it on. If you don't have a footswitch use a gator clip test lead to connect the VIB footswitch jack tip to chassis.

Recheck... V3 is the reverb driver. Pins 1 and 6 should be tied together and both pins should read 415vdc. Pins 2/7 should be zero volts, and pins 3/8 should be about 8vdc.

[Yeatzee]

I have to edit these videos which takes forever, which puts the amp on hold. I will be doing that today. Here's part 8, leading up to the voltage test:

Here's everything from the past few days surmised in video form:

Complete with shoutout to the forum and Sluckey for saving the day!

[bmccowan]

I watched parts of your #9 video. Good of you to give the Forum and Sluckey credit for helping.
I also watched one of your earlier videos where you scraped color bands off resistors and painted on color bands to make the resistors look different than what they are. I cringed at that. Even if its only the tolerance bands, I think they should be left as they are. Why confuse/surprise the next owner of the amp? I understand that you are pretty darn focused on having the amp look original, and I consider it a low priority, but I think actual accuracy is more important than faked cosmetic accuracy.

[Yeatzee]

I watched parts of your #9 video. Good of you to give the Forum and Sluckey credit for helping.
I also watched one of your earlier videos where you scraped color bands off resistors and painted on color bands to make the resistors look different than what they are. I cringed at that. Even if its only the tolerance bands, I think they should be left as they are. Why confuse/surprise the next owner of the amp? I understand that you are pretty darn focused on having the amp look original, and I consider it a low priority, but I think actual accuracy is more important than faked cosmetic accuracy.

Disagree, but that's fine! Only one swapped resistor was a silver band in a gold band location, and it measured within 5% as I recall. The rest are gold band turned silver, so they actually are closer to the correct spec than you'd expect with a silver. I don't really follow how that's a bad thing but to each their own :)

Edit: just checked, the 82k gold swap measured 83k. So within 1% actually

[sluckey]

That gray power cord stands out like a sore thumb!   

[bmccowan]

I think maybe some extremely rare blackface amps came with gray power cords. Few of us will ever be lucky enough to see one

[Yeatzee]

I think maybe some extremely rare blackface amps came with gray power cords. Few of us will ever be lucky enough to see one

Yep...



1) I like the look.
2) My 1964 Fender spring tank also has a Grey power cord and I wanted this to match.

[bmccowan]

So my attempts at sarcastic humor are dashed!

[acheld]

I'd be surprised if that gray cord is original.   Old vinyl don't look like that.

I could be wrong, of course.  And frequently am.  Still, I'd be surprised.

[Yeatzee]

Have you rechecked these voltages? And?

Recheck... V5 is the tremolo tube. Readings are correct for tremolo OFF. Get another set of readings with tremolo ON. Need a footswitch to turn it on. If you don't have a footswitch use a gator clip test lead to connect the VIB footswitch jack tip to chassis.

Recheck... V3 is the reverb driver. Pins 1 and 6 should be tied together and both pins should read 415vdc. Pins 2/7 should be zero volts, and pins 3/8 should be about 8vdc.

V3 measured exactly as you said it should when I measured today.

V5 with Trem rca jack shorted so it works I got the following. Hard to tell exactly because they were obviously going up and down so it's the best I could tell.

1 - 330v to 150v
2 - 0.9ish to 1.4ish
3 - 1.6ish to 2.3ish
6 - 316 to 350ish
7 - 2.9 to - 3.4ish
8 - 18.5ish

When connected to pin 6 the awful Trem tick went away for whatever that's worth. It's quite loud and present even with the Trem settings both to 1 on the pots. Any ideas there? I've heard of adding a cap as well as just moving some wires around. I tried chopsticking wires to see if shifting them anywhere changed anything but it didn't. I also changed the V5 tube and that didn't help.

I also checked bias, what am I missing as far as why I'd need two bias probes? I've only ever used the probe one other time but it seems pretty straight forward quick and easy to me 

With some old rca 6L6GC's and 120v wall voltage I'm getting 419v plate voltage and 34 plate current on one tube / socket, and the other read 421v plate voltage 31 plate current. I kept the probe in the same socket and swapped tubes, then did the same in the other socket. The difference between the two sockets was always 2v more plate voltage on one sockets measurements that's it. That was with the bias pot maxed out, the other direction I got colder readings. Using the Robinette calculate that puts me at about 47% dissipation and 44% dissipation between the two tubes. I was hoping for 50-60% or so. So that tells me the resistor on the bias pot needs to be swapped, any recommendations for what value to start with and try?

[sluckey]

V5 with Trem rca jack shorted so it works I got the following. Hard to tell exactly because they were obviously going up and down so it's the best I could tell.

The fact that the voltages are changing is all I needed to know.

When connected to pin 6 the awful Trem tick went away for whatever that's worth. It's quite loud and present even with the Trem settings both to 1 on the pots. Any ideas there? I've heard of adding a cap as well as just moving some wires around. I tried chopsticking wires to see if shifting them anywhere changed anything but it didn't. I also changed the V5 tube and that didn't help.

This is a well documented issue with Fender tremolo. I attached the Fender service bulletin that explains the "fix". Basically install a cap on the board and move some wires around. This does not work for all amps! Do a google search for "fender tremolo ticking fix" for lot's if ideas.

I also checked bias, what am I missing as far as why I'd need two bias probes? I've only ever used the probe one other time but it seems pretty straight forward quick and easy to me 

With two bias probes you can quickly see both tubes at the same time without having to check one tube, turn the amp off, move the probe, turn the amp on, check the other tube, adjust the bias pot, turn the amp off, move the probe, turn the amp on, recheck the first tube, etc, etc.

Using the Robinette calculate that puts me at about 47% dissipation and 44% dissipation between the two tubes. I was hoping for 50-60% or so. So that tells me the resistor on the bias pot needs to be swapped, any recommendations for what value to start with and try?

You need to decrease the size of the 27K resistor on the pot. Replace with a 22K and recheck the bias. If still not high enough dissipation, replace with an 18K and recheck the bias. Still not high enough dissipation, replace with a 15K. You should not have to go lower than 15K. Does having two bias probes or two 1Ω resistors make sense now?

So, now you know why someone had put a smaller resistor on the pot. Do you recall what the resistor value was before you replaced it? If so, use that same value as a starting point.

EDIT... Looking at video #1 I see that resistor was indeed a 15K. So, use that as a starting point.

[sluckey]

Q... Why is the bias of your amp so cold with the stock Fender resistor value?

A... The line voltage was considerably lower back in 1965. Today it's not uncommon to see 120V to 125V. This causes all the voltages in the amp to be higher. With a higher negative bias voltage (-50v is higher than -40v) the bias current will be lower, causing the amp to run cooler. The fix is to lower the value of that 27K to compensate for today's higher line voltage.

[sluckey]

Tremolo tick fix...

[mresistor]

I'd be surprised if that gray cord is original.   Old vinyl don't look like that.
I could be wrong, of course.  And frequently am.  Still, I'd be surprised.

The latest 76 Twin I worked on came with the ultra rare orange power cord.   

[Yeatzee]

Hmmm, no reverb now 

Reverb worked when I tested it before working on the amp, and I tested the tank w/ it's rca cables into a different fender and it worked perfectly fine so it's something with the amp side not the tank side. What should I be reading on the multimeter at the RCA Jacks on the amp side for the reverb input and output?

Edit: solved. I swapped tubes in V3 didn't help. V4 had a what I thought to be new JJ but I swapped it with another tube and the verb came on nice and strong. Oddly enough that JJ works seemingly fine in V1, but V3 no reverb. Odd!

[sluckey]

Hmmm, no reverb now 

Edit: solved. I swapped tubes in V3 didn't help. V4 had a what I thought to be new JJ but I swapped it with another tube and the verb came on nice and strong. Oddly enough that JJ works seemingly fine in V1, but V3 no reverb. Odd!

Could be a bad connection with the socket such as dirty pins or maybe a bad solder connection. Clean the socket pins with some electronics contact cleaner. Spray and then repeatedly insert/remove a tube several times to wipe the pins. Also retouch all the solder on the socket. Then put the "bad" tube back in the V4 socket. It just may work fine now.

[Yeatzee]

Hmmm, no reverb now 

Edit: solved. I swapped tubes in V3 didn't help. V4 had a what I thought to be new JJ but I swapped it with another tube and the verb came on nice and strong. Oddly enough that JJ works seemingly fine in V1, but V3 no reverb. Odd!

Could be a bad connection with the socket such as dirty pins or maybe a bad solder connection. Clean the socket pins with some electronics contact cleaner. Spray and then repeatedly insert/remove a tube several times to wipe the pins. Also retouch all the solder on the socket. Then put the "bad" tube back in the V4 socket. It just may work fine now.

Yeah I thought so too, but this was after cleaning the pins doing that method and re-hitting the solder joints. It's 100% repeatable, the other tubes I put in V4 the reverb works except with that one JJ. So strange!

I tossed the chassis into the cab for a quick test as a complete unit and it sounds DIVINE. So excited! The tremolo tick is also basically gone in the cab, must be because of the bit of shielding when it's in the cab? Super excited! I've got some more resistors still in transit, the ones mentioned earlier in the thread where the values are off a bit, but other than that seems to be just about ready to go.

[sluckey]

Yeah I thought so too, but this was after cleaning the pins doing that method and re-hitting the solder joints. It's 100% repeatable, the other tubes I put in V4 the reverb works except with that one JJ. So strange!

I would roll the pins and throw it in the trash so it could never cause any more grief.

What are you doing about the bias?

[Yeatzee]

Yeah I thought so too, but this was after cleaning the pins doing that method and re-hitting the solder joints. It's 100% repeatable, the other tubes I put in V4 the reverb works except with that one JJ. So strange!

I would roll the pins and throw it in the trash so it could never cause any more grief.

What are you doing about the bias?

I got one of those circuit boxes with variable resistors in it so you can experiment with values for Christmas this past year and completely forgot about it because I haven't had a use for it yet. I'm going to try and use that to find the right value resistor I want in the bias circuit. Unfortunately the closest resistor I have to what I'll likely need is a 22k which probably won't get me quite there. Buying onesy twosy resistors is quite a pain / expensive!

[sluckey]

Did you keep the 15K that you removed? If so put it back on the pot until you need to order enough parts to justify the shipping charges. Or, if you have a 33K put that in parallel to the 27K. That would make the total resistance 14.85Ω.

[mresistor]

Buying onesy twosy resistors is quite a pain / expensive!

If you want to do that then buy them from ebay.

[acheld]

I got one of those circuit boxes with variable resistors in it

So did I about eight years ago.  I have never used it -- thought I would -- but no.

Best thing to do is to make a big purchase on Mouser or Digi-key -- look up E-24, E-48, E-96 series (see pic) and buy 20 of each value per decade.

Nice thing about Mouser is that they will come in small bags that you can sort in a box to keep at hand.

Maybe you've already done this . . .

[bmccowan]

That's what I do, but its kind of a waste of $ when you can simply scrape off the color bands you don't want and paint on whatever you do want. Easy peazy.
"It's better to look good than feel good," Fernando Lamas

[Yeatzee]

That's what I do, but its kind of a waste of $ when you can simply scrape off the color bands you don't want and paint on whatever you do want. Easy peazy.
"It's better to look good than feel good," Fernando Lamas

Good one! 

Swapped the bias cap, the last remaining resistors that were not right, and put a 22k on the bias pot to see how close that got me. That brought it to nearly 60% plate dissipation, which I'm pretty happy with.

[bmccowan]

Glad you can take a little ribbing.
On a serious note, if your 60% is at the extreme end of the bias pot range, you might want to keep tweaking a bit so that you have some easy adjustment range at hand.

[sluckey]

Here's a little tip (literally)...

Use some probe tip covers (even heatshrink tubing works) when checking voltages on tube sockets to minimize the chances of accidentally shorting two adjacent pins together.

[Yeatzee]

Glad you can take a little ribbing.
On a serious note, if your 60% is at the extreme end of the bias pot range, you might want to keep tweaking a bit so that you have some easy adjustment range at hand.

Yeah for sure, but I'm going to wait until I need to make an order for another amp and get a 15k CC then so I'm not spending $10 on a resistor and delaying the series a week. 50-60% was the range I was shooting for to start at so we're just about on the money with both sets of 6l6 tubes I have available. Short term good, long term will be swapped :)

[Yeatzee]

Here's a little tip (literally)...

Use some probe tip covers (even heatshrink tubing works) when checking voltages on tube sockets to minimize the chances of accidentally shorting two adjacent pins together.

Haha I literally did that right after testing the tube pins with heat shrink. Made it way more difficult to measure stuff afterwards though so Im going to cut it and leave a bit more workable space. I need to get another set of the little hook leads though, the black one's little plastic chin broke.

[sluckey]

I just use insulation from 12AWG house wire on my Fluke probes. Easy slip on/off, snug fit, and easy to lose. But I have plenty of replacements.   

[Yeatzee]

More voltage tests and initial bias + troubleshooting reverb.

Then:

Swapping the last of the out of spec resistors and replacing the bias cap in this one. Bias with 22k is now just under 60% dissipation.

[Yeatzee]

Working on the final video of the series now!

[Yeatzee]

And just like that, we're done!

Thanks for tagging along and helping me out along the way. Playing demo is about halfway in the video (timestamped)

[bmccowan]

Congrats - always a good feeling.

[Yeatzee]

Congrats - always a good feeling.

Thanks! Definitely great to finally hear it in all its glory.