Vibrolux Reverb Bias?

[mike_mccue]

Hi,
A friend left a Silverface Vibrolux Reverb at the house last week.

Today I powered it up and quickly noticed that the power tubes were running hot.
One was glowing cherry red and was quite unlike the other.

I only know a bit about bias and have only D.I.Y. built simple self biasing single ended amps.

My old tube amp mentor and repair tech friend used to come over and help me with my larger amps but he has moved away so I'm considering getting a compu-bias meter and trying to do some work myself.

The first thing I'm curious about... why would one tube run differently than the other?
Is the Silverface Vibrolux Reverb a bias balance design rather than a bias adjustment?

Any tips or insights? FWIW... I'm very respectful of the safety concerns... I just think under the circumstances it's finally time for me to learn how to do the bias adjust my self.

best regards,
mike

[Amp Kat]

The Compu Bias is a fantastic meter and I highly recommend it. What I would try is swapping the tubes around and making sure it's not the tube giving it up. If the other tube redplates then pull that tube and check pin 5 with your meter with respect to ground and monitor the negative grid bias voltage to see if it's steady or even to low. I would think around -46 to -51 would be close to specs maybe. If it starts dropping towards zero or less negative ,then the coupling cap could be faulty or even the bias cap. I've seen a faulty tube socket do it too.

[Enzo]

Most likely because it is a bad tube. Trade places between the power tubes. if that same tube still gets red hot in the new position, it is a bad tube. If however, it works fine in the new position and now the OTHER tube gets red hot in the same socket as before, then something about that socket's circuit is bad. Most likely either an open grid resistor, loose socket pin, or leaky coupling cap from the PI.

Mis-adjusted bias is way down my list here.

Oh, and what KB said, too.

[mike_mccue]

Thank you both for your well considered recomendations.

I move slow... but I'll keep posting as I troubleshoot.

Thanks again,
mike

[mike_mccue]

I swapped the tubes today and the problem persisits in the original socket position.

I'll make some measurements in the circuit as has been suggested.

If I did order a Compu Bias would the info it can provide be relavant to the immediate troubleshooting required or will it be more useful after I have solved the problems and just want to make final adjustments.

For Example, Is the Pin5 test described above one of the measurements made by the Compu Bias?

thanks again,
mike

[Amp Kat]

I swapped the tubes today and the problem persisits in the original socket position.

I'll make some measurements in the circuit as has been suggested.

If I did order a Compu Bias would the info it can provide be relavant to the immediate troubleshooting required or will it be more useful after I have solved the problems and just want to make final adjustments.

For Example, Is the Pin5 test described above one of the measurements made by the Compu Bias?

thanks again,
mike

Kinda in a way but not really Mike. The Compu Bias is strictly for biasing tubes but it does give you a readout of the plate voltage and the tube current draw in milli amps and the power dissipation also. The way it works is the tube grids are using a negative grid bias voltage to keep the control grid (pin5) negative with respect to the Cathode which is usually tied to ground hence the negative voltage. The less negative going towards zero the more current the tube draws. The more negative going away from zero the less current the tube draws. You have two sides from the peak inverter that are tied to the coupling caps that go to the grid resistors and on to the power tube control grids. This is where the negative grid bias is tied in at and if the coupling cap from the peak inverter are bad it will change the negative grid bias and bring it less negative causing the tube to draw more current and start red plating like yours is doing. So if you monitor this voltage and watch when the tube starts red plating to see if this voltage changes (less negative) or stays constant which would mean it's not the problem and then we'd have to find out what's causing the tube to draw excessive current. Hope that helps

[mike_mccue]

Ok, that will be my next procedure.

Thanks very much,
mike

[mike_mccue]

http://www.harmoniccycle.com/hc/images/guitar/6L6_cherry-red.jpg


Greetings,

I tested the voltage to ground from pin 5 on both sockets.

The socket that causes the tubes to go red has a wildly fluctuating voltage while the other pin 5 reading is stable at -63.3vDC. The fluctuations on the socket in question make it hard to estimate a value... it was moving around fast on my dVOM.

To confirm, I was attached to ground at the ground lug from the wall power within the chassis. At the sockets it seemed the most practical way to connect to pin five was to use one of my probe clamp type attachments connected to the lead from the small ceramic? cap that bridges the socket.

Can someone guide me to the next step?

thanks,
mike

[mike_mccue]

"If it starts dropping towards zero or less negative ,then the coupling cap could be faulty or even the bias cap. I've seen a faulty tube socket do it too."


I guess it's time to start replacing caps?


thanks,
mike

[Alex R]

Intermittent bias supply is often caused by occasional arcing inside the smoothing cap (or caps) that goes positive-to-ground in the bias circuit, yes. Medium sized electrolytic can/cans.

BUT... the fact that it's only happening on one side means that the prime suspects are the limited number of components that serve that one side only, none of which are caps. There will be a pair of feed resistors, often 100k or 220k, splitting the bias supply between the two push-pull sides, one of which may have a crack hidden inside and be opening intermittently and arcing. There may be a problem with the socket's pin 5 solder or the pin connector itself, sometimes the solder cracks or they connector breaks inside the socket.

You can run the amp with the chassis out and the power tubes pulled, and just use the voltmeter to see how far back into the circuit the intermittent bias supply goes from that pin 5. If it's stable voltage one side of a component and dodgy the other side, you've found your problem.

This is a nice little problem to cut your troubleshooting teeth on; I think you'll find it wth a voltmeter.

[mike_mccue]

Thanks Alex,

I'll give that a look before replacing anything.

best regards,
mike

[Bruce / Mission Amps]

Thanks Alex,

I'll give that a look before replacing anything.

best regards,
mikeIt's most likely just a bad power tube or socket.
The wildly variable voltage is mostly due to the tube freaking out in the socket... so don't go chasing a red herring thinking suddenly you have bad coupling caps or other things that "could" go bad. 90% of the time it is just the power tube or the socket itself.
Get ready to replace the power tubes, but before inserting them, check your voltages on the sockets and see if they are now stable without the power tubes inserted... especially the bad one.
If not then you might have to clean and retension the lugs in the tube socket... do this from the tube side.

[mike_mccue]

Thanks Bruce,

I think the tube isn't the cause because I already swapped them and the problem seems associated with the socket not the tube.

I do anticipate that the tubes are probably no longer matched or at their best.

I'm gona check the voltages at various points (with the tubes removed as suggested) tommorow

best,
mike

[Bruce / Mission Amps]

Thanks Bruce,

I think the tube isn't the cause because I already swapped them and the problem seems associated with the socket not the tube.

I do anticipate that the tubes are probably no longer matched or at their best.

I'm gona check the voltages at various points (with the tubes removed as suggested) tommorow

best,
mikeI find squirting a little Deoxit in the tube socket holes and then pushing the tube in and out, with a pin wipe from a clean rag each time, can drag some junk out.
Re tensioning the lugs is probably something you really should do after that though.
I use a small 1/16" diameter piece of stainless steel, sharpened to a dull point with a flat filed off on wide side to do that.
An old, very tiny screwdriver can be made into a tool like that.

[mike_mccue]

Ok,

Today I use a small amount of spray cleaner on the tubes bases and did the in and out several times.

Everything seems clean... but I'm not real clear on retensioning the pin sockets. Do you just use the small tool you described as a pry bar to shove the walls of the socket inwards?

I did measure voltages in the amp today with the tubes removed, the speakers connected, and the amp powered up:

http://www.harmoniccycle.com/hc/images/guitar/vibrolux_rev_01.gif

Basically the grid bias on one socket seems stable and then suddenly drops to something like -25vDC.

The voltage on the other trace to the socket seems eratic but often times seems to just end up a OvDC.

After a while the voltage will return to the expected levels... especially after I poke around the Capacitor by the diode board and the other large one that connects to ground from the bias pot.


Any ideas?

thanks,
mike

[Alex R]

The fault goes further back than the socket. I would change the 50/70 caps first, then the pot if that doesn't work. 100uF/100v axial caps are widely available and do the job well. I truth I'd probably just renew everything in the bias circuit to make sure - the 50/70 caps, the 1k, the diode, the 15K on the pot, the pot, the 2 x 100k - can't be too careful with bias.

[mike_mccue]

Thanks,

At the risk of expecting too much:

Is it reasonable to think that a problem so far "up stream" would cause varying effects out at the tubes?

In other words it confuses me that both tubes are not similarly impacted by a problem that might be shared.

Not trying to argue... just a sincere student type question?

best regards,
mike

[Alex R]

Yes, problems way back in the bias supply will certainly affect the bias at the tubes.

That pot is the famous silverface bias balance pot. Many people like to rewire these for a blackface bias adjust control, but I like them because you can use them to balance any old pair of tubes and dial out hum. You can adjust the bias level if you want to by changing the 15K resistor soldered to the pot.

Anyhow, the fact that one side is affected more than another is possibly traceable back to the pot, which actually distributes and adjusts the bias voltage between the two sides. Pots are mechanical in part and so prone to failure. Some problem with one side of the track maybe.

But because the problem seems to go back beyond the pot I'm thinking that maybe one of the caps is at fault. The mere fact that the bias pot presents different resistances between the supply and either side of the power amp could be responsible for the imbalance you're observing - in a bias circuit small resistance changes can have large effects on voltage.

So it's not as obvious as it at first seems (and as it at first did to me!) that the pot is at fault. Anyhow, bias voltage supply is crucial, and the parts involved are few and cheap, so I'd always think of replacing the lot.

[Bruce / Mission Amps]

Thanks,

At the risk of expecting too much:

Is it reasonable to think that a problem so far "up stream" would cause varying effects out at the tubes?

In other words it confuses me that both tubes are not similarly impacted by a problem that might be shared.

Not trying to argue... just a sincere student type question?

best regards,
mikeUnplug the amp and pull out the power tubes.
Replace both bias supply filter caps...make sure the positive end is grounded.
Spray some of your pot cleaner in the bias balancing pot and swish the control wiper around full rotation left and right a bunch of times and then set the voltage on the power tube socket's, lugs 5, so that there is the same negative voltage on each lug with no tubes installed.
If you are able to do that and you are SURE your power tubes are OK, then the sockets are bad and need cleaning and retensioning or, replacement.

[mike_mccue]

Thanks Guys,
I'll get back to you as soon as I purchase and install the caps.

best regards,
mike

[Enzo]

Yes, wedge the sharp object between the socket pin and the side of its hole to shove it inwards. I sometims use a jewelers screwdriver. I have a pointy tool sorta like the dentist uses I sometimes use. One of those large size safety pins like for diapers also works.

Check this out, I use it all the time. Most any auto parts store should have them.

http://www.sjdiscounttools.com/kdt2554.html

[Dave Curtis, dB AudioTech]

I find squirting a little Deoxit in the tube socket holes and then pushing the tube in and out, with a pin wipe from a clean rag each time, can drag some junk out.


IMHO,

I'm hesitant at using Deoxit on tube pins, especially power tubes that run base-up. You may very well encounter heat that Deoxit isn't rated for (I don't know that rating for the current product), and turn it to varnish. I flush the socket with a non-residue cleaner, using the tube as you describe to help clean the socket while the stuff dries off, and then use a ProGold GxL pen (older 400 deg. C formula) on the pins.

[mike_mccue]

Thanks guys,

I installed the 2 100uf 100v negative bias capacitors. I used some Sprague Atoms but I also bought some Xicon hi Temps. I had finished with the Sprague install before learning that Xicons are considered a good swap.

old:

http://harmoniccycle.com/hc/images/guitar/VXR/VibroluxReverb_negbiasOLD.jpg

new:

http://harmoniccycle.com/hc/images/guitar/VXR/VibroluxReverb_negbiasNEW.jpg

In any event, I tensioned the socket pins as described and recommended.

I powered up without tubes and measured the voltages.

I'm starting to think my meter is getting old... I swapped batteries with a fresh one... but the old turned out to be strong. What I'm noticing is that the meter seems sluggish.

The voltages at the new caps seemed very stable.

The voltage at pin 5 of the socket near the rectifier tube is -61.3vDC and rock solid stable.

The voltage at pin 5 of the socket near the driver tube seems to stabilize after taking a while to increase. That's what makes me question my meter... it seems sluggish, but on this socket more so than on other measurements.


Once I felt like I could measure the stable voltages (albiet I wait for the *problem* socket to stablize each time I measure) I tried to balance them with the adjust pot. I found that I had to twist the pot to it's limit yet the closest to a match that I coul set was -61.3vDC on the good socket and -60.1vDC on the bad socket.



OK,

So I installed the tubes and powered up, made observations, and powered down. I swaped the tube positions and repeated.

The red plating only occurs on the *problem* socket.

I have other 6L6 tubes but hesitate to expose them to the red plate until I sort this out further.



Is the merit of replacing the sockets related to the fact that you're likely to also replace the coupling caps and small resistors?

thanks for any comments you might share,
mike

[52 Bill]

I can't see what's been done in the photos, but there have been some changes made to the bias pot wiring. Can you tell us what is going on there? Where does the wiper go to? Are there 2 resistors in series to ground?

[mike_mccue]

thanks for taking a close look.

Yes that's 2 resistors in series, a legacy repair/mod I suppose, going from the pot's center tap to ground.

Each resistor seems to measure apprx 8k (Grey, Orange, Green?) but when I measure the series it says 0.6k... so I'm gonna replace that assembly right now with a 15k as spec'd.


http://harmoniccycle.com/hc/images/guitar/VXR/VXR_negbiasPOT.jpg


The other 2 taps seem to go to the board and the 100kOhm resistors where I'd expect.

After replacing the 15k resistor I was better able to balance the neg bias voltage at pin 5 of either socket at apprx -61.2vDC.

But the what ever tube I place in the socket near the driver tube goes cherry red within a few minutes of power up.

I'm still having difficulty measuring the voltage on the problem socket... it's sluggish. The measurements on the good socket seem stable and I did some B+ tracing and the meter seemed responsive.

Is that a possible clue about where the problem with the circuit to the problem socket may be?


I need to get some 2000pf caps and then I guess I'll replace the two sockets.

In the meantime I'm still eager to hear suggestions.

best regards,
mike

I guess it's time to install new sockets and components?

any suggestions?

[52 Bill]

After replacing the 15k resistor I was better able to balance the neg bias voltage at pin 5 of either socket at apprx -61.2vDC.
Ok the bias supply seems to be ok, but something is still pulling down the grid voltage on the one output tube.

Pull out the red-plating tube, and measure the voltage at pin 5. If the -61 volts does not read correctly, turn off the amp and lift the end of the 0.1 PI cap. Turn on the amp and see if the cap is leaking any voltage. If it is replace both the PI caps and check the amp again.

If the PI cap is ok, then try lifting the end of the 2000pF cap at pin 5. Does this fix the voltage here?

Hope this helps.

[Bruce / Mission Amps]

Ok the bias supply seems to be ok, but something is still pulling down the grid voltage on the one output tube.

Pull out the red-plating tube, and measure the voltage at pin 5. If the -61 volts does not read correctly, turn off the amp and lift the end of the 0.1 PI cap. Turn on the amp and see if the cap is leaking any voltage. If it is replace both the PI caps and check the amp again.

If the PI cap is ok, then try lifting the end of the 2000pF cap at pin 5. Does this fix the voltage here?

Hope this helps.I'd be looking at removing the ceramic caps on lugs 5.

[mike_mccue]

Thanks to both of you.

I'll try both things.

Are the ceramic disks on pin5 just the after thought caps put on to squelch squeal that resulted from less careful construction during the mass production runs?

Can you just pull them and see if the amps runs without running away?

best regards,
mike

[mike_mccue]

Great News,

I pulled both 2000pf supression caps and took measurments. The voltage had dropped a bit to apprx -60.0 but both pin 5 measurments were quick and easy to do. They readings indicated the voltage was solid and stable.

The bias trim seemed to be even more capable of helping set the voltage to be nearly identical at each power tube socket.


The amp sounds great. I've never heard it before... now I can get to the fun of playing it for a while.

I may suggest a cap job for good measure.


I can't express my gratitude for the help adequately, THANKS.

feel free to add any suggestions or tips.

best regards,
mike

[52 Bill]

Glad to hear the amp is up and running now.

As you had noted earlier, the bias control is really a bias balance control, used by Fender to "match" output tubes. Now would be a good time to actually set the output tube bias based on current draw, to see what the amp can really sound like with a new set of properly biased output tubes.

You had mentioned looking into a bias probe of some kind, did you ever purchase one?

[mike_mccue]

Thanks Bill,

I had renewed interest in a Compu Bias meter and almost purchased one today but Then I got distracted and ordered/purchased a new guitar... :-)

I will order a Compu Bias at my earliest oppurtunity.

This amp is cleaner and louder than I've become acustomed too.

In the meantime I'm looking at schematics in an attempt to better understand the difference in Fender Bias adjust, and Bias balance circuits.

best regards,
mike

[TD_Madden]

Those are excellent amps...sorry I just sold my '75.... :(