Pilot 245-A Integrated Tube Amp B Channel Issues Looking for Advice

[njm]

Hi all,

So I recently purchased this Integrated Amp, and I must say, I am shocked at how good it sounds. What a beautiful machine. However, after listening for a few days, I noticed that the B channel was lagging behind the A in volume. I cleaned the potentiometers on the balance, volume, input and speaker selection knobs, and it seemed to clear the issue up, maybe (I'm not really sure, because I turned it off, let it sit overnight, performed said cleaning, and then fired it up, and it seemed to be playing equally out of both channels. The cleaning may not have had anything to do with it). In any case, I flipped the tubes from the B to A channel, one set at a time until all had been swapped, and no change. I then noticed that the 12AU7A tube, which is the only other tube along with the rectifier tube that isn't part of an A/B set, had one filament that would periodically flicker, and that was glowing slightly less than the other filament. On the photofact schematics this tube is listed as both an A and B Amplifier tube, so I'm wondering if this tube is the culprit. I already ordered an NOS replacement tube, but if that doesn't solve the issue, does anybody know, probabilistically what I'm looking at here in terms of repair? Is there a common source for this problem? I've done a bit of research and was only able to pull up a thread or two on this, so I was hoping somebody out there might provide some insight. Thanks in advance!

 

[primosounds]

Hi all,

So I recently purchased this Integrated Amp, and I must say, I am shocked at how good it sounds. What a beautiful machine. However, after listening for a few days, I noticed that the B channel was lagging behind the A in volume. I cleaned the potentiometers on the balance, volume, input and speaker selection knobs, and it seemed to clear the issue up, maybe (I'm not really sure, because I turned it off, let it sit overnight, performed said cleaning, and then fired it up, and it seemed to be playing equally out of both channels. The cleaning may not have had anything to do with it). In any case, I flipped the tubes from the B to A channel, one set at a time until all had been swapped, and no change. I then noticed that the 12AU7A tube, which is the only other tube along with the rectifier tube that isn't part of an A/B set, had one filament that would periodically flicker, and that was glowing slightly less than the other filament. On the photofact schematics this tube is listed as both an A and B Amplifier tube, so I'm wondering if this tube is the culprit. I already ordered an NOS replacement tube, but if that doesn't solve the issue, does anybody know, probabilistically what I'm looking at here in terms of repair? Is there a common source for this problem? I've done a bit of research and was only able to pull up a thread or two on this, so I was hoping somebody out there might provide some insight. Thanks in advance!

Since the amp has old tubes it is always a good idea to put new ones in. Cleaning the pots is also a good idea. You also need to clean the slide switches as frequently the signal chain runs through the slide switches too. If after doing all that you still have a problem you might want to clean the tube sockets and retension the pins.

 

[njm]

Thanks for the reply! That makes sense, although there are no slide switches that I'm aware of on this model, if you're referring to what I think you are here. I did clean the tube sockets, but only by spraying a little deoxit in there. I didn't know if it was a good idea to be sticking anything down into those tiny holes. Otherwise, is retensioning the pins something a greenhorn can do? I've never heard of this. Lastly, do you think the problem could be down-circuit somewhere? In a capacitor or resistor, etc.?

 

[Retrovert]

I just helped a member diagnose this exact problem.

The first thing to check is the controls.

For the volume control and the balance control—as well as the other controls which are not causing this problem but will cause other imbalances—measure the resistance of both halves of the potentiometer at various positions. Many times when an amplifier has sat for years, even decades, the wiper sits in one position and builds up corrosion in the form of oxide and sulfide. The resistance element may have various materials, conductive or non-conductive, migrating to the surface and interfering with the resistance or the wiper.

You may find the controls out by tens of percent which readily accounts for the problem.

After cleaning with the appropriate cleaner, the potentiometer should be lubricated with a good silicone grease as this was what was originally used, and it prevents wear of the resistance element. The sprays are not suitable for this purpose, as the lightweight nature, and creep for silicone oil, is not lasting. Mixer rebuilders complain about the lack of durability of the sprays as well as the creep.

 

[njm]

Thanks for the response. Do you think Deoxit spray is okay for the cleaning? Or should I use a different product? You know, as an aside, I did notice the on the balance pot, which is set up so that there are two pots, one in front of the other, the rear pot is a tarnished black color (not entirely black, but darker than all the other pots on the controls), I didn't think much of it, but does this suggest anything to you?

 

[primosounds]

Thanks for the response. Do you think Deoxit spray is okay for the cleaning? Or should I use a different product? You know, as an aside, I did notice the on the balance pot, which is set up so that there are two pots, one in front of the other, the rear pot is a tarnished black color (not entirely black, but darker than all the other pots on the controls), I didn't think much of it, but does this suggest anything to you?

I use contact cleaner, then move the knob back and forth a few time then might follow up with another squirt of c.cleaner if the overflow looks at all dirty. You can put a white napkin under the pot or switch to catch the exudate and check for dirt. Then after that follow with Caig.
Maybe tobacco stains or heat stains can discolor metal. I don't think one should use Caig on the tube sockets since they leave a residue which may cause you grief down the line. I just use contact cleaner with a "flossing brush" from the drugstore. Retensioning the pins involves using a small insulated jewelers screwdriver and carefully pushing the two halves of the stamped metal pin claspers in the socket closer together. But you can check the condition of these by using some high magnification glasses and a bright light. If they are nice and shiny you probably don't need to clean just retension the pins. I just do this as a matter of course for rehabbing old tube amps.

 

[Retrovert]

Just to expand on what Primo has written, because not all contact cleaners are alike; some contain the dreaded acetone.

The resistance element is usually some polymer. So any plastic solvent, such as acetone, can damage or destroy the surface, as the plastic will be disolved. Denatured ethyl alcohol is normally used. Most "contact cleaner" is just alcohol and propellant.

Caig makes a few specific products, if you insist on a Caig product, for this purpose. Moving Contact Cleaner is one. Be sure to read the label and MSDS on the product.

DeOxIt is nothing fancy, BTW, being a simple corrosion remover for metal complexes. The basic recipe for such products is:

* Strong base (ammonium hydroxide) to break up the oxide or sulfide layer. (In a silver contact the corrosion is a combination of oxide, which is conductive, and sulfide, which is not.)

* Surfactant/soap to make the metal complex soluble. This is typically oleic acid, same stuff in olive oil and what our fat cells store.

* A carrier to remove the metal oxides, usually naptha, dedorized kerosene (aka lighter fluid), or in DeOxIT a non-flammable fluorocarbon of some sort as I recall. If any readers are old enough to remember the original Brasso which stank of petrolenum, that was the kerosene.

* Optionally EDTA as a corrosion inhibitor.​

This is not the best product to clean a resistance element, but it does work on the wiper.

Any of those products will remove the lubricant, creating the potential for abrasive wearing out of the element. Silicone grease works better than spray lubricants.

 

[njm]

Just to expand on what Primo has written, because not all contact cleaners are alike; some contain the dreaded acetone.

The resistance element is usually some polymer. So any plastic solvent, such as acetone, can damage or destroy the surface, as the plastic will be disolved. Denatured ethyl alcohol is normally used. Most "contact cleaner" is just alcohol and propellant.

Caig makes a few specific products, if you insist on a Caig product, for this purpose. Moving Contact Cleaner is one. Be sure to read the label and MSDS on the product.

DeOxIt is nothing fancy, BTW, being a simple corrosion remover for metal complexes. The basic recipe for such products is:

* Strong base (ammonium hydroxide) to break up the oxide or sulfide layer. (In a silver contact the corrosion is a combination of oxide, which is conductive, and sulfide, which is not.)

* Surfactant/soap to make the metal complex soluble. This is typically oleic acid, same stuff in olive oil and what our fat cells store.

* A carrier to remove the metal oxides, usually naptha, dedorized kerosene (aka lighter fluid), or in DeOxIT a non-flammable fluorocarbon of some sort as I recall. If any readers are old enough to remember the original Brasso which stank of petrolenum, that was the kerosene.

* Optionally EDTA as a corrosion inhibitor.​

This is not the best product to clean a resistance element, but it does work on the wiper.

Any of those products will remove the lubricant, creating the potential for abrasive wearing out of the element. Silicone grease works better than spray lubricants.

Perfect. Thanks so much for the thorough explanation. I'm on my way to pick up some silicone grease and contact cleaner from Lowes. I'm hoping this resolves the issue. If not, I've contacted Craig Otsby over at nosvalves.com (I read that he does good work on vintage tube amplifiers), and he quoted me around $450 for complete electrical refurb, which seems reasonable for parts and labor. Does anyone by chance know Craig, or have any experience working with him? I'm assuming he does good work, again, based on what I've read, just curious if anyone here had heard of him etc.

 

[Retrovert]

Perfect. Thanks so much for the thorough explanation. I'm on my way to pick up some silicone grease and contact cleaner from Lowes. I'm hoping this resolves the issue.

You're welcome.

If not, I've contacted [...] and he quoted me around $450 for complete electrical refurb, which seems reasonable for parts and labor.

As a general rule I suggest always obtaining a written description of what a rebuild includes, and a guarantee that unless it properly works when returned (specify what this means) you are under no obligation to pay. Does it include all resistors and capacitors? New wiring? New potentiometers? New tube sockets? Cleaning and polishing of the chassis? New panel connectors?

You might want to specify replacing every single resistor with metal film, as the carbon composites will be out of spec, often by as much as several hundred percent, and all of the ceramic capacitors and electrolytics with film. This way the amplifier will function perfectly in spec.