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]

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,
mike

It'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,
mike

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.
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:



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