Tube ringing

gdw3 · Post by **gdw3** » Mon May 22, 2006 1:08 pm

I recently got an early 50's Rick M-11 amp. There is a high pitched ringing when the volume is in the middle ranges, and the ring goes away at volume levels of around 9 and above.

Lightly pressing on the 2nd 6V6 tube (RCA) changes and slightly mellows out the ringing. It looks as if the tube sockets are probably not original. No way to know how recent.

Advice?

soundmasterg · Post by **soundmasterg** » Tue May 23, 2006 12:31 pm

One of the preamp tubes is likely microphonic, possibly the one you were playing with there. Change to a new manufacture or NOS tube of the same type and the ringin will probably go away. Good sounding, reliable new brands would be the new Tung-Sol reissue 12ax7, JJ ECC83S and JJ ECC 803S, Sovtek 12ax7LPS, EH 12ax7. Good NOS would be pretty much anything as long as it sounds good.

gdw3 · Post by **gdw3** » Tue May 23, 2006 12:44 pm

Thanks for the advice, Greg.

As I know nothing about tubes, here comes a dumb question: I don't replace it w/ a 6V6? Which of those that you mention would help to break up into overdrive easier (if any)?

And, will the amp require biasing when I do change it?

EDIT: one more question. In listings of the JJ803S tubes, they give a certain price, and then a slightly higher one for "balanced triodes". What does this mean?

Thanks so much for your knowledge and generosity with it.

soundmasterg · Post by **soundmasterg** » Wed May 24, 2006 10:12 am

Hi Gordon. I was sleepy when I posted this last night and didn't notice that it was a 6V6 that was the problem tube. Of course you should replace the bad one with another 6V6 but if the amp uses a pair of them as the output tubes, you should put a new pair in rather than just replacing one. The best 6V6 brand today is the JJ 6V6, which you can get from www.tubesandmore.com. You can get them matched for a little bit more and that is a good idea on your amp.

As far as if the amp requires biasing when you change the power tubes, it depends on if it is fixed bias or cathode bias. I don't know without looking at the amp or at a schematic. If it is cathode bias, in most cases, then just put the tubes in and go. If it is fixed bias you should rebias the amp.

Regarding the 12AX7 tubes....your amp probably uses some of those in addition to the 6V6 tubes, and if so it doesn't hurt to have some spares on hand. A 12AX7 is a double triode tube, meaning that there are two triodes inside the bottle. If you have them matched with each other, then if your amp has a phase inverter that would benefit from matched triodes or parallel gain stages that would benefit then it can help to balance things out. Something like a stereo hi-fi amp might benefit from the matched triodes, but usually in guitar amps, unmatched to a certain extent fattens the sound up and adds harmonic richness to the tone. I wouldn't bother with matched triodes myself. Matching power tubes is a different story and usually in most amps, matching the power tubes results in a sound with more power and less hum. So that is why I suggest to match the 6V6 pair if indeed your amp uses a pair of them.

gdw3 · Post by **gdw3** » Wed May 24, 2006 11:43 am

Thanks for all the info. Here are the tubes for the amp as hand-written on the chassis:
5V3 (has a GT 5Y3 - does this overdrive sooner?), 6V6, 6V6, 6SC7, 6SC7 (all RCA's)

What's the easiest way to know if it is a fixed or cathode bias? Here's a link to a schematic from the Rickenbacker site:
http://www.rickenbacker.com/pdfs/m11.pdf

soundmasterg · Post by **soundmasterg** » Wed May 24, 2006 2:21 pm

The 5Y3 and 5V3 are rectifier tubes that convert the AC coming in from the wall to DC to power the tubes. They do have different performance. Someone probably put the 5Y3 in there because they couldn't find a 5V3, but the schematic does show a 5Y3 as the preferred tube so it could be a misprint also on the chassis chart. You can get one from the site I listed above.

Check this site out...it shows the difference between the tubes if you compare the specs.

http://www.nj7p.org/Tube4.php?tube=5v3
http://www.nj7p.org/Tube4.php?tube=5y3
http://www.nj7p.org/Tube4.php?tube=6v6
http://www.nj7p.org/Tube4.php?tube=6sc7

For 6V6's, none of the others presently made and also many of the NOS stuff can't beat the current JJ 6V6S for performance and sound. Also, the JJ can take up to 550v so in that way alone it is superior to NOS stuff which can fail if you put them in a Fender Deluxe Reverb at 430v.

For the 6SC7's, NOS is the way to go. It costs more but is the best choice. The tubesand more site above sells them for a decent price.

The easiest way to tell if an amp is fixed or cathode biased is usually to see if a resistor is hooked from the cathode of the power tubes to ground. It isn't a foolproof way to tell, but it is good for a start. To really know, you have to know enough about how the circuits work to be able to read the schematic. I've taught myself over the last 10 years from reading books and asking questions, so it can be done. It just takes hard work and lots of desire!

That particular schematic you posted shows that amp to be cathode biased. The 250 ohm resistor and the 25uf 25v cap connected to the cathodes of the power tubes are the cathode bias resistor and the cathode bypass cap. They are used to allow the tube to set it's own bias level so that the cathode always is positive in relation to the grid, which is grounded through the 240k resistors to ground. Incidentally, that schematic has an error with those 240k resistors. One of them should be connected to the grid of the top 6v6 instead of both being connected to the grid of the bottom 6v6.

The phase inverter tube is the 6SC7 that is closest to the power tubes, and it is a paraphase style phase inverter, which sounds cool for leads and gives good drive but because of unbalanced signals from the two sides of the tube, it often is very hard to get a good useable clean sound with this style of phase inverter.

The other 6SC7 is setup as in input amplifier for your guitar signal, but both sides of it are running in grid leak bias, which is a noisy way to get amplification. It can sound cool and work well, but the input caps should be replaced if they are old, and this setup also does not work well with a too high of an input signal, like if you used an overdrive pedal for instance. The input caps would be the two .05uf caps that are hooked to the grids of that 6SC7. Some early Fender amps used the grid leak bias setup before they changed to a more common grounded grid setup.

soundmasterg · Post by **soundmasterg** » Wed May 24, 2006 2:22 pm

Ran out of room with the message size limit...thats a first for me!

With that amp, you can just get new tubes and change them, with no need to set the bias. It will set itself, as long as the cathode bias resistor is in spec. You can measure across the 250 ohm resistor with a meter as long as the amp is off and the caps have been discharged. It should read within 10% of it's 250 ohm value. You should have the 6v6 tubes out of the amp when you measure this to get an accurate reading. If you don't know how to drain the voltage out of the caps safely then you should not try this as you coudl shock and possibly kill yourself. Please be careful!

Also, if they have not been replaced yet, all of the electrolytic caps in the amp should be replaced, and many of the signal caps, especially if they are paper types, shoudl also be replaced. You should take this to a qualified amp repair person if you don't know how to do this yourself. The electrolytic caps have a chemical inside them that allows them to function, but over time it dries out which causes the caps to fail. The usual expected lifespan on electrolytic caps is 10 years or so with performance meeting published specs, and today's caps are greatly superior to caps made back then. Paper caps also fail over time as the paper degrades and the caps can leak DC voltages, which throws off the operating points of following stages and causes malfunctions. The whistling noise you are getting could be a bad tube, but it could also be any of these caps, or drifted resistors. If the amp has not been overhauled yet, it should be to make sure that it will work correctly. A good tech can do this without butchering the amp and causing it's value to erode, although some parts do have to be replaced usually. It is like routine maintainence on your car in a way.

soundmasterg · Post by **soundmasterg** » Wed May 24, 2006 2:24 pm

Woops, just a correction. I meant to type that the JJ 6V6S can take up to 500v, and not 550v.

gdw3 · Post by **gdw3** » Wed May 24, 2006 3:57 pm

Wow, that is so informative and helpful! Thanks a million!!!

Regarding your comment on the phase inverter tube, this amp has no trouble at all making beautifully clean tones, so I guess I got lucky!

I think the amp had been serviced, somewhat sloppily, in the past. I'll look again more closely, now that you've given me a better idea what to look for, but I don't think there are any of the paper caps left. Thanks again.

soundmasterg · Post by **soundmasterg** » Thu May 25, 2006 9:41 am

What I meant about the phase inverter circuit is that as compared to a cathodyne or a long tailed pair arrangement, the paraphase as in your amp doesn't do clean tones as nicely as the others. The cathodyne design was used in Sunns, Oranges, Hiwatts, some Silvertones, some Fenders, some Gibsons, some Ampegs, and a lot of hi fi stuff too. The long-tailed pair is used in almost all Marshalls, most Fenders, most Voxes, most Mesa-Boogies, etc. That doesn't mean that the paraphase setup can't do clean...just that it doesn't do it as well as other designs. Usually, in order to get good cleans with a paraphase, the overdrives aren't as good and it is more of a compromise to use that circuit than to use the others. The benefit of the paraphase circuit is that when it does overdrive it is usually harmonically richer sounding and thicker sounding than the other designs. A good use of the paraphase setup is for a harp amp or a dirty blues amp, but of course the rest of the amp has to be designed accordingly for that need.

If the paper caps are gone, then that will be a good thing for reliability.

greymax1 · Post by **greymax1** » Fri Feb 09, 2007 5:41 am

Hi, I am new to this board. I have acquired an M11 and not sure of its lineage. I know its simular to the Fender champ. The schematic posted on the Rickenbacker site is incomplete and has errors in the inputs to the power amp and the volume control is a single. This same schematic is everywhere I have looked and no one has mention the errors. My M11 came to me in sad shape. The power transformer had been replaced and tubes were missing. I have recaped and replaced the speaker with a Jensen. Can anyone provide a source for the correct schematic?

Front of my M11
ric1.bmp (10.5 k)

doctorwho · Post by **doctorwho** » Fri Feb 09, 2007 11:14 am

I'm not an amp expert, but here is the picture:

The resident experts should be able to help you. Are you sure that the circuit wasn't changed when the power transformer was replaced?

greymax1 · Post by **greymax1** » Fri Feb 09, 2007 8:57 pm

I have no way of knowing what was changed. Looks like things exploded inside (caps maybe). The first preamp was paralleled into the phase amp. Not sure if that's right, but that's the only way I see for the single volume control to work for now. There is no way that I know of to split the 6SC7 common cathode _ pin 6. I had though that a 6SL7 could do it, but thats not original. The only schematic I have is the Rickenbacker Site one, copywrite 1997. I am surprised no one has questioned the errors until now.

beatlefreak · Post by **beatlefreak** » Sat Feb 10, 2007 10:42 am

It could also be possible that the schematic is correct, and your amp was modded somewhere along the way.