1979 Vibrolux Reverb repair...

[SleepLess]

Hi!
A guy dropped his 1979 Vibrolux Reverb at my place yesterday. He said that the sound is really ugly. I said "OK, let's fire it up" and indeed it was ugly: distortion at low volume (3/10) whatever the channel and intermittent volume cuts. It was way under the normal volume for such an amp as well. He also said that it had been blackfaced. The guy also added that it worked really well before and then started working like it does now, but that it was a gradual decrease over some weeks, not an overnight change.

I opened it up today and I have found some strange stuff.
- One speaker is a C10Q, the other one is a C12Q but looks as big as the 10" one (mislabeling?). Both rated at 8 ohms.
- The PT has been changed for an unknown brand.
- I noticed some strange stuff going on around the bias supply and bias board. Here are two pics:





The 220K resistors that normally go to the bias pot are 100K instead and one is located off the board (hidden by the black cable you can see on the pic).
The bias pot ground lug is not grounded but goes to the bias board through a resistor.
There's a 10uF/160V cap soldered to the bias pot wiper (weird?)
The bias board cap is a 10uF/160V as well...

Then I took some measures and I have 454V B+ which is pretty spot on. Then I measured bias for V8 with my Hoffman bias checker. Damn: 1.7mA!!!!!
I then decided to turn the pot and guess what: turning it either way doesn't change a thing!!! The value remains between 1.5mA and 1.7mA.

So I have decided to change the two 100K resistors for two 220K and get rid of the 10uF/160V cap going to the pot. Then I have a spare 50uF/50V cap to change the bias board cap with. I would rather install a 50uF/100V there but I don't have any right now.

With those changes I hope the bias pot will work. I'm pretty sure the amp sounds crappy because of that low bias...
Thanks for shooting your own ideas here!!!

[sluckey]

Have you changed the output tubes? Symptoms sound like dying tubes.

Before declaring the bias circuit faulty I'd check the negative bias voltage on pin 5 of each output tube. If it's approx -40v to -50v, the bias supply is good.

[TubeGeek]

Yeah and I'd go through the circuit and confirm that it is indeed correctly "blackfaced".  Hopefully things straighten out once you confirm things are correct.

[SleepLess]

Have you changed the output tubes? Symptoms sound like dying tubes.

Before declaring the bias circuit faulty I'd check the negative bias voltage on pin 5 of each output tube. If it's approx -40v to -50v, the bias supply is good.

The output tubes are brand new... And the bias pot is not acting at all... I don't have any 6L6s at home besides this pair... All I have is KT66s and EL34s.
But I'll check the negative bias voltage on pins 5 you're right!
Thanks!

[SleepLess]

Alright:
Negative bias voltage (with both power tubes pulled out) is -70.4V. Turning the bias pot is not changing anything to the negative value...

[sluckey]

Sounds like your hunch was right. -70v has got those tubes biased verrry cold. And the pot should vary that negative voltage.

For sure though, this amp ain't been "blackfaced" correctly. You may just have to verify everything.

[plexi50]

Doesnt look blackfaced in the bias section

[SleepLess]

Doesnt look blackfaced in the bias section

That resistor is 100K (should be 220K). The one that should be on the left is missing on the board and is actually hidden between the black cable and it's a 100K as well instead of 220K... µIt goes from the upper eyelet to the pot.
I don't know who modded it but it's hell of a bad job... 

[phsyconoodler]

It's not even remotely 'blackfaced'. You need to get yourself a tube tester right off the bat,or at the very least have tubes on hand if you are going to repair stuff.
  And get familiar with silverface and blackface topology so you can spot half-assed mods like what's in that amp.

[plexi50]

Just grab a schematic that is early vibrolux reverb and follow the layout. Get rid of bias as it now is and redo it match the schematic or layout. And yeah you go to have a tube tester. Your working blind without one

[sluckey]

Negative bias voltage (with both power tubes pulled out) is -70.4V. Turning the bias pot is not changing anything to the negative value...

I just want to hit this one last time because it's still not clear to me. You did measure that -70.4v directly on pin 5 of each 6L6, right?

[SleepLess]

Negative bias voltage (with both power tubes pulled out) is -70.4V. Turning the bias pot is not changing anything to the negative value...

I just want to hit this one last time because it's still not clear to me. You did measure that -70.4v directly on pin 5 of each 6L6, right?

Yes I did. Amp fully ON with power tubes pulled out...

[SleepLess]

Just grab a schematic that is early vibrolux reverb and follow the layout. Get rid of bias as it now is and redo it match the schematic or layout. And yeah you go to have a tube tester. Your working blind without one

I will change it. The thing is that I need to decide on what blackface schematic I need to follow. I have downloaded two schematics from ampwares and I basically have the choice between AB568 and AA964 and both have some big differences, particularly around the bias area. The AB568 schem points to 100K bias feed resistors with a tricky 10K resistors assembly whereas AA964 has the more common 220K resistors...

So which one do I pick? AB568 or AA964? 

[SleepLess]

Here's what I've found after some online research:
AA964 is from September 1964
AB568 is from sometime after May 1968
AA270 is from February 1970

So I'm going for AA964...
Thanks!

[SleepLess]

I have changed a few things around the bias area. The 100K resistors are out and new 220K are in. Bias pot is wired correctly to the board and I have a 27KΩ resistor back on it. Bias board resistor is back to a 470Ω value. The bias board cap is now a 50uF 100V instead of the 10uF 160V.

Ladies (if there's any here...!) and gentlemen the bias pot is now active!!! I can turn it from -45V to -58V. Way better!
I can't plug any guitar tonight as it's already too late but I will tomorrow.

One more thing:
I have circled in red three resistors that are way out of the blackface values. I should have two 1MΩ resistors on the side and one 470Ω in the middle. Here I have two 220KΩ and one 1MΩ in the middle.



Have you guys any idea about what these resistors do? Do you suggest I change those three as well???
Thanks, I'll keep you posted tomorrow about this amp!

[plexi50]

I have changed a few things around the bias area. The 100K resistors are out and new 220K are in. Bias pot is wired correctly to the board and I have a 27KΩ resistor back on it. Bias board resistor is back to a 470Ω value. The bias board cap is now a 50uF 100V instead of the 10uF 160V.

Ladies (if there's any here...!) and gentlemen the bias pot is now active!!! I can turn it from -45V to -58V. Way better!
I can't plug any guitar tonight as it's already too late but I will tomorrow.

One more thing:
I have circled in red three resistors that are way out of the blackface values. I should have two 1MΩ resistors on the side and one 470Ω in the middle. Here I have two 220KΩ and one 1MΩ in the middle.



Have you guys any idea about what these resistors do? Do you suggest I change those three as well???
Thanks, I'll keep you posted tomorrow about this amp!

Well it's like this. They used a lot of different part values in the later silver face amps. Use the AA964 layout and you will have a great sounding blackface amp. Change both of the 220K's to 1Meg & 470 ohm. Also make sure the tail resistor is 22K,feedback 820 ohm and presence (100 ohm)

[Willabe]

the bias pot is now active!!! I can turn it from -45V to -58V. Way better!

That might not be enough swing for the bias.


              Brad     

[SleepLess]

Hi there everyone!
So I have just fired it up and it works really well. - B+ is 454V and I biased V8 at 40mA. The pair is a little mismatched since I have V7 at 30mA.
There's one strange thing going on though:

The amp sounds good but I played through it leaving the Hoffman bias checker engaged and both tubes have a varying current: with low volume on the guitar I still get about 42/43mA but when I turn the guitar up to 10 the bias goes up to 60mA in V8 (with constant variations, it doesn't stay up there all the time). I did the same with the bias checker in V7 and the bias varies between 32mA and 50mA. Is this normal??? What would be the cause for that variation???

I'm off to changing the 1M/470/1M resistors now...
Thanks!

[SleepLess]

OK, three resistors changed and I have spotted other strange things:

- The Orange Drop cap on the board should be .01uF as per blackface values and it's actually a .1uF!
- There's a small cap going from V5 pin 3 to the vibrato pedal ground lug. It's not on the schematic. Can I get rid of it? I wonder waht purpose it would serve.



All in all this amp has been modded but strangely. There are some added components that I can't fathom... Why a cap on the bias pot? Why was the bais pot wired so that it couldn't work? Why that cap on V5 pin 3?

If you also have any idea about the varying bias...
Thanks!

[plexi50]

Cap on bias regulates and keeps the bias voltage steady. The varying current draw can be power tubes that are bad,abused and so on. The varying bias can also be the blue coupling caps leaking. Those are pretty good old caps and dont go bad all that often but it does happen. Got to have a tube tester or at least known good tubes. Pull your power tubes and just check the grid voltages of pin #5 of the power tube sockets. If they both read the same then you can possably  rule out the coupling caps as being bad.

Bias is suppose to swing up and down as you play. At idle with no signal the bias will be steady and reamin so

[SleepLess]

Bias is suppose to swing up and down as you play. At idle with no signal the bias will be steady and reamin so

OK, so it's all good. Thanks!

[plexi50]

Adjust to -42mv on the grids ,pin 5, of the power tubes. The plate voltage you want to see is near 410 + or - a tad. Where are you getting 454VDC from? You say 454VDC B+. What is the voltage on the plates of each power tube pin 3?

Edit. Adjust to -42 Volts.

[plexi50]

Edit. Adjust to -42 Volts.

[SleepLess]

Pins 5 are at -42V and B+ is at 412V. It was at 454V before I changed anything...
Thanks a lot!

[plexi50]

Pins 5 are at -42V and B+ is at 412V. It was at 454V before I changed anything...
Thanks a lot!

Great! Your done.

[SleepLess]

Great! Your done.

Apparently not... The amp was returned to me earlier this week for a repair after having worked great for two weeks...: the amp lits up OK but within twenty seconds of playing the 2A fuse blows up and the amp stops...

It's Sunday and I have only one spare 2A fuse... Can you advise on what to test first? Obviously I'll go to the store tomorrow and buy some spare 2A fuses but I'd like to make some progress in the repair today as I have some time ahead of me...

Probably worth noting is that the ground switch was replaced by a 1A fuse holder. That fuse doesn't blow. Only the 2A fuse does. Remember I'm in the EU so the amp works at 230V/240V.
Thanks a lot!

[plexi50]

I dont recall power supply caps being mentioned in this thread. Are they new? Lift one end of each cap and test them each with an anolog meter in ohms setting. Of course be careful. Pics of caps under doghouse?

[SleepLess]

I dont recall power supply caps being mentioned in this thread. Are they new? Lift one end of each cap and test them each with an anolog meter in ohms setting. Of course be careful. Pics of caps under doghouse?

They were all changed by a previous tech. Brand new F&T caps...
There's one thing I noted when I first lit it up one hour ago (replaced a 2A fuse) with the 6L6s pulled out: I had a 0V negative bias voltage on pins 5 of the 6L6s for a good minute. Then I measured again after a while and I had -45V. The time lapse before negative bias voltage arrives may be the cause for the fuse to blow. Unfortunately the amp hasn't done this again. I now get instant negative bias voltage as soon as it's on... I measured the two 1.5K resistors and they're good.

 

[sluckey]

I had a 0V negative bias voltage on pins 5 of the 6L6s for a good minute. Then I measured again after a while and I had -45V. The time lapse before negative bias voltage arrives may be the cause for the fuse to blow.

That's a good possibility. Does the fuse blow with the tubes in AND -45v on pins 5?

I'd connect my meter to pin 5 (tubes out) to monitor the -45v and chopstick the entire bias circuit. Looking for a possible loose/intermittent connection, bad solder joint, etc. Try to make the bias voltage drop out.

Remember, you heavily modified the bias circuit. You always have to be suspicious and ask yourself "What has been done to the amp recently?".

[SleepLess]

Well it may have been the solder joint on the bias board. I reflowed it but the chopstick didn't show anything. The amp sounds great. No more popping and I played it for 15 minutes on 5. 

[plexi50]

I had a bad bias pot a year ago and the only reason i found that problem with the bias quickly was that i saw a tiny spark emmit from the pots wiper terminal. Maybe your pot is bad or worn out.

[SleepLess]

I had a bad bias pot a year ago and the only reason i found that problem with the bias quickly was that i saw a tiny spark emmit from the pots wiper terminal. Maybe your pot is bad or worn out.

Mmm... Thanks for your input. If it ever gets back to me again, I'll look at this first!