I would like advice on the limiter channel of my amp. It seems that the distortion has been getting more "mushy" as of late. I just had it re-capped and that helped but, it still doesn't seem right. I used to run the gain at about 6 or so and now am running it at 4 to 4.5. I gig regularly with this amp and want to restore it to its former glory. Thanks for any help.
Thu, 10/15/2015 - 14:13
#1
G Mac
Re-Cap?
Did the re-cap job include replacing C10 (2uF 25V)? That electrolytic cap stabilizes the cathode bias voltage on the 1st half of the 12AX7 and a bad cap will degrade the signal going to the Limiter (2nd half of 12AX7). Also verify that the +/-16V and +/-8V power supply voltages are in spec in case D10 and/or D11 zener diode(s) has failed. -mgriffin
Thanks!
To mgriffin155...Thanks for your reply! You may have guessed from my original post that I am electronically illiterate however, I am trying to learn what I can. I looked at my repair receipt and the 2uF 25V cap is not listed. My repairman was working w/o a schematic and generally only works on pre-1978, point to point wired amps but was gracious enough to try to fix my issue(s). I sent him the schematics and hopefully, we can find the solution. He did mention that he was unable to locate one of the caps( I'm assuming it was the one in question) but that the one that was there was the only one that was holding a charge so, he left it in. My current questions are;(1) Do you know where I can get the elusive cap? and (2) Do you have any advice that I can share with my repairman about working on MM amps? I have seen that you post often and you seem to possess great knowledge so, I hope you will be willing to help me(us) in my(our) quest. Thank You again for your time and help!
Cap Source
Mouser is a good source. They have 2.2uF 25V 10% caps. http://www.mouser.com/ProductDetail/Vishay-Sprague/173D225X9025VE3/?qs=s... The slightly higher cap value will not hurt your amp. Have your repair tech check Mouser.com in case he wants to order other parts to save on shipping costs. Disclaimer: I can't guarantee changing that cap will fix the limiter issue, but it's where I'd start. Also, have him check the zener diodes I mentioned and change them out if the voltage readings are off. Good Luck. -mgriffin
Thanks some more!
To mgriffin155-Thank you for the link to Mouser! Just dropped my amp off this afternoon and am excited to find out what results will occur! These amps are such jewels! My amp tech said that he appreciates your input as well. I'll let you know how the repair goes if you like; 'til then, Thanks Much!
RD112-50
To further this thread for future info:
Picked up your amp Gregg, the schematics and the comments from this thread!
Previously I had changed the electrolytics on this amp EXCEPT what now proves to be C35 - 2uf/350v. As you said above, that cap did test good (using a Mercury 1400 in-circuit cap tester) and I don't think it is in any audio path to boot, beyond its filtering use for V1 plate.?
Having received the amp back, further inspection does indeed show C10 (2uf/25v), mounted across V1. It tests good at approx. 3uf, (which is quite typical for cathode bypass caps as they rarely fail or go way out of spec due to the low voltage it receives). Not to mention that caps of that day were sold with specs of -20% to +50%. Meaning it could measure anywhere from 1.6uf to 3uf and still be considered "in spec".
Both Diodes at D10 and 11 test good as well.
Using an Orange VT-1000 tube Tester your Groove tube 12AX7 preamp tube showed 7 on one triode and 9 on the other.
While the tube isn't necessarily classed as weak, the triodes are mismatched and 7 is a bit low for my tastes. I have installed a new JJ that tests at 10 on both sides.
Now as for the perception of "mushy"!! I can't really tell. I think the low end is a bit "loose" at shop volume, using a Strat in the first 3 switch settings. However bridge pickup alone is as one would expect. Bright ! I also noticed the settings the amp arrived with had the Treble at less than 5, bass up around 7, and the brite switch OFF, so I would assume that is where you like it? But "mushy" is a descriptive, perception in the ear of the listener that I can't gauge in the shop.
At any rate trying to tie that with what you say above re: that you used to set the gain at 6 and now only get to 4.5..... I would say that it isn't unusual at all for your typical settings to change(even dramatically!) after a fresh set of caps are installed.
I am going to replace the C10 cathode cap just for giggles and see if indeed that "clears the throat" of the Limiter side...right now and strictly personally.... I would simply turn the treble up and bass down for a little more clarity.
I'll check back in after....
Good Follow-up
Hello Mr. Sound, Thanks for checking out the 2uF cap and zeners and the tube gain match. You did all of the right things. You are also correct that "Mushy" is a very subjective term, but sometimes one must go with what one "heard". Changing out the big power supply caps and new 12AX7 did the most good and it probably sounds much more like it did when brand new. While you have the amp apart, verify the speaker is in good condition as they sometimes get flabby with age. Just like us old farts. Anyway, good job and I hope that Gregg is happy. -mgriffin
Replying to #'s 6 & 7
To recycled_sound...Thanks for all your effort and expertise; without which I would be in a world of trouble! I truly appreciate your patience and willingness to help! I can't wait to hear what it sounds like!
To mgriffin155...I feel that your help has also been of tremendous value; Thanks!
Finally to you both; regardless of the end result, I am happy that you have endeavored to assist me and am eternally grateful! -Gregg
The final note
I got my amp back from Recycled Sound and it is doing great! With the replacement of the caps and the addition of a balanced 12AX7, it seems to have done the trick. I have had to search for the "sweet spot" as the amp has changed a bit but, overall I am very pleased! Thanks again to all who assisted me! Music Man is the best!