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Post subject: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 11:54 am
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Hi all,
I'm helping a friends work on his 66 twin that has a volume problem.
Even with a guitar volume set to ten, there is no sound from the amp until the amps volume control is almost to 3.
This is on both inputs. On my twin, if the volume is at 3 the neighbors are complaining.

He had a cap job done and put new 6L6EH tubes in it about a year ago.

Anyone come across this problem?
Bob


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 1:09 pm
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Check connections from amp to speakers. Bad jack & frayed wires. Move another, known good 12AT7 into the phase inverter position (V6).

Use some contact cleaner in the standby switch. Work a few times with amp off.

See if these suggestions help. If not, post back. Good luck!


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:01 pm
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Thanks BMW,
I tried a different speaker cable and cleaned the bypass switch and speaker jack.
Still no sound until 3 on both volume pots.
The 6L6's have 461V at the plates and the PI plates have 242V so that seems ok.
I even tried a different 2x12 speaker cabinet in case it was the speakers.
Stumped!


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:01 pm
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Use a good guitar cable .


Did he do some job on the amp just before the volume drop or one day volume drop by itself ?

Check if all tubes are seated well. Clean all tubes contacts.

New 6L6's one year ago with bias check and adjust or no ? No bias = 50% ( ? ) chance of short tube life .

Low volume ; as BMW2002Ti write replace 12AT7 it is the only tubes with 6L6's are used on both channels. 6L6's could be a issue too.


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:03 pm
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Speakers are connect on the good output jack ?


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:22 pm
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Thanks stratele52,
I forgot to mention that I did try a different 12AT7 with no luck.
Cleaned tube sockets.
tried a different patch cord.

I checked the bias which was 35ma on one 6L6 and between 30 & 32 on the others.
I tried adjusting the highest reading 6L6 to 45ma but no change
Bob.


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:45 pm
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There is something we forget or you do wrong or forgot to tell .

Tubes are good; did you try the right 12AT7 the Phase inverter ? Are you sure your 12AT7's are good ?

Patch cable ; you don't know if they are good so you need to try many ?

Bias tell 6L6 's are good enough.

Voltage is OK at 461 V ; on each tube ( 6L6 ) ?

Replace all 12AX7 on one channel and do test

What about speaker output jack ?

Are you sure guitar work well ? Try on other amp .

Check tube heater voltage on each tubes


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:53 pm
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Check all resistors on the Phase inverter circuit . Bad resistor = low volume or distortion


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:55 pm
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Yeah, idle bias on the 6L6GC prolly won't effect the volume, unless it was near zero mA. Your plate voltage seems ok. Normal range of bias for healthy 6L6GC, is around 36-40mA with a plate voltage of 460-475VDC.

So... if you are getting 30+ mA off each 6L6GC and changing the 12AT7 phase inverter didn't help... check the wiring between the output tranny's secondary and the speakers. Check the primary leads from OPT to the 6L6GC's.

Must be a connection issue. You didn't put an optional MUTE switch in it, did you?


8^)


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 4:59 pm
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stratele52 wrote:
There is something we forget or you do wrong or forgot to tell .

Tubes are good; did you try the right 12AT7 the Phase inverter ? Are you sure your 12AT7's are good ?

Patch cable ; you don't know if they are good so you need to try many ?

Bias tell 6L6 's are good enough.

Voltage is OK at 461 V ; on each tube ( 6L6 ) ?

Replace all 12AX7 on one channel and do test

What about speaker output jack ?

Are you sure guitar work well ? Try on other amp .

Check tube heater voltage on each tubes



Checked all the above.

Check the resistors! great idea, I didn't think of that. Will do that later this evening.
Thanks


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Post subject: Re: 66 Twin Volume Problem
Posted: Wed Oct 08, 2014 5:05 pm
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BMW2002Ti wrote:
Yeah, idle bias on the 6L6GC prolly won't effect the volume, unless it was near zero mA. Your plate voltage seems ok. Normal range of bias for healthy 6L6GC, is around 36-40mA with a plate voltage of 460-475VDC.

So... if you are getting 30+ mA off each 6L6GC and changing the 12AT7 phase inverter didn't help... check the wiring between the output tranny's secondary and the speakers. Check the primary leads from OPT to the 6L6GC's.

Must be a connection issue. You didn't put an optional MUTE switch in it, did you?


8^)


Thanks BMW.

No MUTE switch.
I will dig deeper into the PI circuit and do a point to point check in the entire output section this evening and tomorrow.


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Post subject: Re: 66 Twin Volume Problem
Posted: Thu Oct 09, 2014 1:05 am
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Volume pot issue ? On both channel ?


I fix a Fender amp with low volume on one channel . The volume pot value was out of range , too low in value .

Are you sure knobs are going to or stop at " 0 " not to " -3 " :lol:

+1 BMW , any mods on the amp ? Any pictures from inside


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Post subject: Re: 66 Twin Volume Problem
Posted: Thu Oct 09, 2014 12:44 pm
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Yea, does 3 have the loudness or a normal 3? What about higher, does the volume increase normally after 3?


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Post subject: Re: 66 Twin Volume Problem
Posted: Thu Oct 09, 2014 4:39 pm
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:D :D
Well I checked all the resistors and they were all within specs.
I sprayed all the pots with cleaner & there was no change so decided to opened them up and sprayed & cleaned them with Deoxit & a q-tip (they were a mess).

You can now hear the amp below the 3 setting. I'm surprised at the difference between spraying the cleaner in the pots vs actually cleaning them made.
Also, my friend says he never noticed this volume problem before because he always plays with the volume around 5 or 6 ( might be why he always says "what?".

Thanks for all the great feedback everyone.
By trying the different solutions that are given it helps tube noobies like myself better understand what's going on in these circuits.
Bob


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Post subject: Re: 66 Twin Volume Problem
Posted: Thu Oct 09, 2014 5:02 pm
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Damn 5 or 6 on a Twin, respect :) My Bandmaster at 3 starts rattling the windows already. I need a bigger room. Glad it turned out to be easy fix.


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