Music Man Amps

Let me give you the bad news. You say you put new 6L6 tubes in the amp. Now the bad news....This chassis is set up for EL34/6CA7 tubes. When you look at the tube socket on the left side of the picture, you see the big black resistor off the tube socket, that means that this amp was set up for EL34's. So this is an early production run, because later on they did use 6L6 tubes. Also you can see from the picture you have all the blue Mallory caps. These are all original and 35 yrs. old. and need to be replaced with some new Sprague/Vishay TVA caps. You can get them from Mouser. The caps underneath the amp are the 450 volt/100, you cannot use TVA caps. They are too big. You have to use Illinois Central or something else. Also the cement resistors, look like they have been changed...they look pretty white....check them out. This should get rid of your hissing. Good Luck.

Terry

Sorry about that!  When you said "Bias stabilized at 44mV and 20mV" my brain filled it in as "mA".  I have learned never to use the Factory biasing method as written but to use it as a guide to do it properly.  Measuring voltage across the resistor is not actually measuring quiescent bias current but is only an indicator that relies on the resistors being acccurate, the transistors being matched and the tubes operating properly so it can be misleading because you are looking at a remote "symptom" and not an actual measurement of the tube's plate current.  Tube circuits can have other problems that add or subtract plate current through alternate paths that cannot be seen by measuring at what are essentially transistor-isolated cathode resistors.  I always measure bias current directly so I can spot problems that are invisible to the "factory" method. 

Simply converting "mV across a resistor" into mA with Ohm's Law yields a factory setting range of 6.4mA to 14.1mA.  This range was probably "acceptable" to Music Man for several reasons.... while it is colder than what is sonically desirable, it helps the tubes outlast their warranty period and the range is pretty big because the factory didn't bother to use pre-matched tubes.  With factory settings like these it is no wonder that 40 year old amps can still have their original factory tubes but they can sound better than that if matched and optimized. With matched output tubes you get much closer matching on the bias current (assuming a properly balanced & working amp) which allows a higher (18 to 22mA typical) bias setting with both tubes being more nearly identical.   This is more important now than when the amps were made because the tubes available today are simply not as good as the U.S. made originals.

Not having a local tech does tend to make you try to do it yourself!  It makes it difficult to fix many types of problems because actual troubleshooting will fix more problems quicker and cheaper than guessing what might be wrong and trying to replace parts in the hope that you can fix an issue.  It can be compared to ordering lots of parts to fix an automobile symptom (which can be caused by numerous possible failures).  It would theoretically be possible to spend more than the car (or amp) cost when new and still not fix the problem or cause even more issues because you swapped out good working parts for potentially inferior new ones!

If you are going to modify the amp for using the 6L6 output tubes check out the Factory Service Bulletin regarding a modification that was made to 6L6 amps because of an instability that arose when the factory Sylvania 6L6 tubes were changed and I found this to be true with other brands as well!  I saw this first hand on an HD-150 amp (with an unstable intermittent oscillation problem only visible on an oscilloscope ) and the mod fixed it right up.  This was never necessary on the 6CA7/EL34 models.

Since YOUR amp is an RD, the BIAS is actually driven by the transistors. They are NORMALLY measure by the bias meters by interrupting the stream between the tube and the socket on pin 8 and either measuring it as a voltage, with a resistor or TOTALLY interrupting it and measuring it directly as a CURRENT measurement. That's with a normal(SIC) anode or plate driven amp. The cathode is going to ground in those cases. In your case, your amp is DRIVEN through the cathode so you might as well measure it with the resistors. To be honest with you, I've never even TRIED to measure a cathode driven amp with an interrupter type device so it MIGHT work but why? Maybe Lars has tried it. He's more familiar with RD's.

Measuring most components "in circuit" is problematic at best.  A circuit is a collection of parts all interconnected in various different configurations.  An out-of-spec part or failed component may or may not measure much different than when everything is as it should be.  In order to be sure you making a proper evaluation of a single part you have to lift one or more legs out of circuit to isolate the component enough to make a valid measurement.  Otherwise, you can be deceived and come to the wrong conclusion.

Measuring the plate current of a tube directly is the only way to catch every potential source of current.  An output tube has many more connections than just a cathode and plate.  If any of these elements get grounded or shorted or connected to a leaky or failed component then additional current sources or current sinks will be created and many of these will not be visible by simply looking at the source resistor at the cathode.  You can have that Cathode resistor out of spec (or bad) and adjust the bias to be erroneous because the measurement is based on that in-circuit resistor. 

You can measure that "proper" voltage across that 40 year old cathode resistor and not even see ANY plate current under a variety of possible conditions!  Conversely, it is also possible to see no voltage across the cathode resistor and still see an output tube "red-plating" as plate current leaks in from another tube element or failed component.  You simply cannot trust that resistor measurement bias method under all conditions so it makes sense to use a bias probe so you see the "bigger picture" and get the bias current adjusted correctly or determine if there is another problem.

Using a bias probe puts the precision of the measurement squarely on an outside 1% resistor that is a part of the test gear that is not subjected to constant use and accumulated thermal deterioration placed on the in-circuit "cathode" resistor & transistor.  It also places the point of measurement where it can "see" the big picture (at the plate) where the B+ source is applied.

I have chased my own tail a time or two (more times than I can count, actually) relying on in-circuit readings that are determined solely by in-circuit components.  Not everything is as it seems unless you can isolate and introduce a known trusted element that does not live in the circuit being tested to act as a "reality check".  That is what a good piece of test gear properly employed (like a bias probe) can do for you.

This is just my opinion based on observation and experiences.  Since troubleshooting bias and output tube issues centers on the circuitry that works the hardest and hottest in an amp, I have found it best not to rely on assuming that everything matches the schematic.  It is one way to go that I have found that cuts through a lot of potential problems and avoids wasting a lot of time based on erroneous information.

I wasn't trying to tell you what to do, just point out some of the pitfalls of approaching an issue with a shotgun!  LOL!

"Restoring to new" by changing a lot of parts isn't quite what you might think it is.  If your goal is just proper operation of the amp you can do it but "restoring" and amp to original condition and reliability is much tougher.  Many components just are not made as well as the factory originals (tubes and filter caps come to mind).  Really fine filter caps (or NOS Tubes) are extremely expensive now days (much more expensive than a bias probe) and most of those commonly available at reasonable prices are just not made to the same specs or quality level as the originals.  Also, carbon comp resistors are getting increasingly more difficult to find and carbon-film or metal film resistors are not the right components to use in many tube amp circuits.  These new parts may not only not work as well in some circuits but may also become additional sources of failure!

The 1% 1-ohm resistors would have to be inserted in series with the plate of the tube in order to make the measurement and this can be dangerous and very time consuming if you simply unsolder the the plate wire and solder the 1-ohm resistors in.  You won't want to leave them in there (or you would have to have beefier ones because the entire current of the tube is flowing through these when the amp is operating and being in there all the time negates the reason for having them a part of a bias probe so they don't deteriorate while they are not in use) and you will have to ensure that the high-voltage B+ is drained before inserting and removing the resistors. 

This is why a bias probe adaptor for your meter is a much smarter way to go.  It costs only about as much as a single output tube, doesn't requiring re-wiring or modifying your amp to use and is a huge enhancement in safety (think Life Insurance Policy) when working with 700Vdc!  You don't need an expensive one with its own meter display.  It is a very sound investment if you are going to work on your own amps. IMHO

Yes, Sprague, F&T and TAD are good choices.  Unfortunately, there used to be many more choices at much lower prices than today.  Electrolytic capacitors are the one component you can't normally stockpile for decades because similarly to batteries, they can chemically deteriorate, dry out or leak over time while sitting on the shelf.

Don't pass up Illinois caps just 'cause they are cheaper. I've put a LOT of them in and had no failures. They are used in a lot of OEM installations too. Along with Panasonic. Just sayin.....