fatboy135
Well-known member
Hi everybody, does anybody noticed that his 20/20 has a strong channel than the other???? maybe I´m going crazy but today I think that I need to up a bit more the volume of one channel to achieve the other channel volume 
20/20 dismatched ???
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fatboy135
Well-known member
yes I swap the loudspeakers cables and the trouble is still on the same boogie channel
Timbre Wolf
Well-known member
Glad you checked the speaker cabs out. My 20/20 is set with different volumes, to match the stereo left/right volume, but that's just because of the different speaker sensitivity between my two cabs.
Each stereo side uses a different pair of EL84 plus a phase-inverter, and one half of the input tube. Could be a mismatch in gain somewhere in that tube chain.
- T
Each stereo side uses a different pair of EL84 plus a phase-inverter, and one half of the input tube. Could be a mismatch in gain somewhere in that tube chain.
- T
fatboy135
Well-known member
yes i think so, maybe i must to open it and checked with the oscilloscope
After double checking, I don't seem to have that problem with mine. As I mentioned earlier, could be tubes.
How old is your unit? If old enuff it could just be component wear. Some resistors or caps tiring over time and slightly changing values. Happens to all amps over time.
How old is your unit? If old enuff it could just be component wear. Some resistors or caps tiring over time and slightly changing values. Happens to all amps over time.
fatboy135
Well-known member
my unit is 6 years ago... I just repaired the fx loop of my TA, so I have all the tools over my workdesk, I think it´s time to open the 20/20 and converted it into a 10/10 single ended amp
and converted it into a 10/10 single ended amp now that would be something. Fair amount of work though....have to change both OT's, remove the Phase Inverters, run each set of EL84's (2 per side) in parallel.....etc. Could be done.
Be a shame to change such a wonderful amp though. I love the 20/20!
Be a shame to change such a wonderful amp though. I love the 20/20!
Timbre Wolf
Well-known member
+1luigi said:Be a shame to change such a wonderful amp though. I love the 20/20!
Why bother even considering it? The cost of all that work would easily pay for buying a used Lexicon Signature 284.
- T
fatboy135
Well-known member
It is a joke I love the 20/20 on push pull, but it's time to repair it, anyway I prefer to change the tube bias point better than got a pure class A amp.
One question what about install an external switch that allows you to change the bias point from a cold point into a hot point??? of course it implies a bit reduction of power but you can use the cold point for regular playing and the hot point when you go to record.
It is only a few resistors to be installed with the switch you could select what array of resistors you want to use, the resistors that fixes into the cold or the resistors that fixes in a hot way. It is like an external bias select. Anyway it is only a crazy idea
I love the 20/20 on push pull, but it's time to repair it, anyway I prefer to change the tube bias point better than got a pure class A amp.One question what about install an external switch that allows you to change the bias point from a cold point into a hot point??? of course it implies a bit reduction of power but you can use the cold point for regular playing and the hot point when you go to record.
It is only a few resistors to be installed with the switch you could select what array of resistors you want to use, the resistors that fixes into the cold or the resistors that fixes in a hot way. It is like an external bias select. Anyway it is only a crazy idea
Timbre Wolf
Well-known member
Sounds like a more sane, and potentially appealing idea!fatboy135 said:Anyway it is only a crazy idea
- T
fatboy135
Well-known member
hey TW I recomend to you to read " El Quijote" one of the best classical books of spanish literature, you could identificate us in these book.You are Sancho Panza and I´m Don Quijote. Read it is a lovely knights books
Timbre Wolf
Well-known member
And I recommend that you try using a Raytheon 5751 in one of your amps - make sure to get the kind with the "windmill" getter. :lol:fatboy135 said:
- T
fatboy135
Well-known member
did you read my post on rack equipments about a mod for the triaxis??? I replace an op amp with so good results, it could be very useful for people who wants a cleaner TA
Timbre Wolf
Well-known member
I hadn't seen that thread yet. Thanks for bringing it to my attention. I'm glad you've come upon an interesting new mod!fatboy135 said:
- T
fatboy135
Well-known member
yes it is very easy, only replace the ic3 with an ic socket and a better op amp I achieve to get the master 1.5 more in all presets without any distortion, I setup the presets with master 6.5 withput any dinamic voice overload.
Ok back to the original topic I must so say that I also see that the 20/20 sounds very poor on low volumes, and I must to increase the overall volume to get the classic boogie tone. I put the volumen pots about 9 pm ( 1/4 of the total length of the pot )
Ok back to the original topic I must so say that I also see that the 20/20 sounds very poor on low volumes, and I must to increase the overall volume to get the classic boogie tone. I put the volumen pots about 9 pm ( 1/4 of the total length of the pot )
I found this topic, I know It's old, but I have the same problem with my 20/20. Channel A is normal, but channel B has a lower volume with a little darker tone than channel A.
I checked it to see if last owner did any mod on channel B but it seems untouched. I have changed tubes, I have tested it with another preamp and diferents cabs, but the problem is still there.
Fatboy, Did you repair you mesa 20/20?. Can you tell me your experience? Where did you find the failure? Thanks in advance for any advice.
I checked it to see if last owner did any mod on channel B but it seems untouched. I have changed tubes, I have tested it with another preamp and diferents cabs, but the problem is still there.
Fatboy, Did you repair you mesa 20/20?. Can you tell me your experience? Where did you find the failure? Thanks in advance for any advice.
fatboy135
Well-known member
a bit dismatched is not a seriously problem, for example on my 20/20 the matter was very easy, it becomes with different phase inverter tubes, and that makes me to sound a bit different, only heared if you pay a lot of attention
disassembled
Well-known member
A lot of things come into play like mismatched power tubes, unmatched input tube, different tolerances on the volume pot, etc. Although your amp was designed to pretty much be the same for both channels it probably never will be due to the components inside. I find that when the 20/20 runs out of volume turning the volume knob up doesn't do anything other than add more compression. So just run it wide open
The only thing i recommend is that you retube the amp if you haven't already and make sure that you use balanced 12Ax7's along with a matched quad of power tubes. This is not a problem or defect of the unit unless the control difference is drastic (IE one knob is at 7oclock and the other knob is at 12 oclock to achieve equal volume).
Greg
The only thing i recommend is that you retube the amp if you haven't already and make sure that you use balanced 12Ax7's along with a matched quad of power tubes. This is not a problem or defect of the unit unless the control difference is drastic (IE one knob is at 7oclock and the other knob is at 12 oclock to achieve equal volume).
Greg
Thanks in advance for your responses. I think my problem is not only volume diference ( to match both channels I have to put channel A volume knob to 9 O'clock and channel B to 12 o'clock), is more about character.
Channel A is like a dream, responsive, open, geat cleans throught it and distorted sound greats too, but channel B is more darker and deeper. It sounds ok with heavy distortion, but not for blues or clean work, therefore if I increase volume on channel B over 12 o'clock It becomes almost distorted (like it was closed in a box) without compresion while channel A if fuller and more compressed in a good way).
I retube It last week with mesa tubes, even I interchange tubes between channels without a noticeable change. I'm electronic technician so I thought it would be a filter caps problem or presence problem, so I'm wondering about if your problems with your 20/20 are similimar to mine before go to repair it. Any suggestion about servicing the amp?
Channel A is like a dream, responsive, open, geat cleans throught it and distorted sound greats too, but channel B is more darker and deeper. It sounds ok with heavy distortion, but not for blues or clean work, therefore if I increase volume on channel B over 12 o'clock It becomes almost distorted (like it was closed in a box) without compresion while channel A if fuller and more compressed in a good way).
I retube It last week with mesa tubes, even I interchange tubes between channels without a noticeable change. I'm electronic technician so I thought it would be a filter caps problem or presence problem, so I'm wondering about if your problems with your 20/20 are similimar to mine before go to repair it. Any suggestion about servicing the amp?
