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Post subject: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Wed Apr 15, 2015 4:13 pm
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Hi everyone!

I have a little problem here with my 1966 Fender standalone reverb unit. When I have the mixer knob at 1 I have a perfect dry guitar sound, almost like a bypass-thing. But when I turn up the mixer knob (no matter what settings the other two knobs are at) my guitar sound gets muffled, like if the mixer knob functions as a high cut. So at 10 the reverb is really beautiful, but the dried signal that gets mixed with it is really muffled.
Another test I did to confirm this issue was to set the dwell knob at 1, so that I don't have any input in the reverb and thus no reverb and checked that mixer knob again. At 1 it's perfect, at 10 it sounds very muffled (and dry since I have the dwell knob at 1).

So it's like the mixer knob not only functions as a mixer, but also as some kind of treble knob. 1=nice and clean, 10=muffled, hi-cut.

Does anyone know if this normal? If not, what could be the problem here?


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Wed Apr 15, 2015 4:28 pm
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Definitely abnormal behavior.

What affect does the tone knob have?

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Wed Apr 15, 2015 4:41 pm
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Did you try another 12AX7 (7025)?

Check the 250k-ohm linear mixer pot. Try cleaning it with Fader Lube or some such pot cleaner. You may need to replace it.

Check voltages on the lower half of this tube, if off... Check the 2.2meg-ohm & 100k-ohm resistors, plus 0.5mfd and 0.1 mfd coupling caps tied to the lower half of the 12AX7. Prolly best to just replace them, if you've already ruled-out the tube and pot.


http://bmamps.com/Schematics/fender/rev ... _schem.pdf


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Thu Apr 16, 2015 8:21 am
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@Aray: the tone knob doesn't affect the mixer pot problem. Makes sense, as the tone knob only affects the wet reverb signal.

@BMW: yes I already changed the 7025 with several others that I know are good, but the issue remains. I'll get into the pot and other surounding components. Maybe the 250pF cap failed?


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Thu Apr 16, 2015 9:00 am
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Yes, it could be the 250 pico-farad cap. The stock ceramic rarely fail, but it has happened. You may want to replace this cap with a silver mica type. Same mfd, at least 600VDC rating. 1KV is better.

Check all solder points, too. Could be a cold connection.

Good luck!


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Fri Apr 17, 2015 10:08 am
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Thanks for all the suggestions, but before I start ordering components just to try a few things, I'd really like to be sure of what is wrong with this unit.
Anyone who really know what it could be? Anyone who had this problem before?

Already chopstick-tested all solder joints, but couldn't find a cold joint...


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Fri Apr 17, 2015 1:03 pm
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First off, are you comfortable measuring hundreds of volts in a live amp? Do you have a secure bench to work on the amp, out of the cab? Need to use only one hand to measure the points. DON'T put other hand on the chassis to keep the amp steady.

Best way to check is with a DVM. Check the voltages you get, compared to the schematic. Check voltages at B+ points. The high VDC at the second and third 40mfd/450VDC cap. Check the voltages around each tube.

Check all voltage points listed on the layout portion of the schematic.

Then, check whether the coupling caps are leaking VDC. You noticed how one side of the coupling cap usually has hundreds of VDC. The other should be very low voltage. Anything above 1 VDC usually means a bad coupling cap.

With the unit off and residual voltage drained, measure the resistance of the resistors mentioned in above post. You need to lift one leg of the resistor before measuring. Measure the resistance of the middle leg of the mixer pot to each side terminal --- through the motion of the shaft. Should get a nice linear resistance range from mid-to-end, for each side terminal.


Lastly, what brand 6K6GT are you running?


Good luck! :)


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Wed Sep 09, 2015 3:38 am
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Ok, it took me some time to get back to my Fender reverb, but last week I finally found some time to get into it and order some parts. In short, the problem is: with DWELL at 1 and TONE at any position the tone of my guitar gets muddy when I turn the MIXER up from 1 to 10. This is very easy to hear, as DWELL is at 1 and thus the signal is 'dry'. MIXER at 1: guitar sounds great, MIXER at 10 guitar sounds muddy (less high end).
Here's what I already tried:
- tried different cables -> no results
- tried with different amps (I'm running the unit with a 1966 Fender Champ as well as a 1966 Princeton) -> no results
- tried different 12AT7 and 7025/12AX7 tubes -> no results
- changed all blue molded .1MFD caps -> no results
- changed all electrolytics -> no results
- changed the 250pF and 10nF caps -> no results
- changed the 250k MIXER pot -> no results
- changed the .05MFD cap -> no results
- measured all voltages at the tubes, all good
- checked for cold/dry solder joints -> no results
- tried an other reverb tank with same specs -> no results

So, as one can see, I've tried almost everything, and still no results. Now, before I go on with this, can I ask a favor of every person who owns an old Fender 6G15 unit? With DWELL at 1 (so your signal is 'dry') and TONE at any position, does the guitar sound change when you turn the MIXER up from 1 to 10? Does the guitar gets muddier when you turn the MIXER? I'm not talking about super muddy, but the guitar sound clearly looses high end with MIXER at 10.
I just want to make sure whether this is normal or not. Maybe this issue is normal with old Fender 6G15 units? Before I go one, I need to know for sure I'm not trying to fix a problem that is not really a problem (eventhough I experience it as a problem).

All help most welcome. Thanks!


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Post subject: Re: Fender 6G15 Reverb Unit - tone problem with mixer pot
Posted: Wed Sep 09, 2015 7:00 am
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bran9206, Sorry to hear about this issue. You prolly already know that the Reverb Unit divides the signal into two. One half is sent through the reverb tank, the other is bypassed to the output. Mixer at "1" = mostly bypassed. Mixer at "10" = mostly through reverb. The issue is with the reverb portion of the circuit.

http://bmamps.com/Schematics/fender/rev ... _schem.pdf


I have never had issues with a muddy tone, no matter where the mixer knob is set.


1.) You happen to have a recording that you can post, demonstrating this muddy tone? It may help the diagnosis.

2.) It sounds like you may have one of the tubes being driven out of the proper range, too early in the volume and/or you still have a bad coupling cap somewhere in the reverb section.

a.) Check the cathode-tied resistors on the 12AT7 and 6K6GT. Replace if they are out-of-spec by more than 15%. I like using a 5-watt wire-wound, 820-ohm resistor on the 6K6GT. Puts the tube in a slightly better idle point (IMHO). 1k-ohm will be fine.

b.) Replace the bypass caps on all of the cathodes. Esp the one on the 6K6GT. This one tends to dry out, due to being next to that 1k-ohm, 2-watt resistor on the 6K6GT. Separate the two as much as possible, to keep heat away from the cap.

c.) There is a 0.002 mfd/400VDC coupling cap feeding the grid of the 6K6GT. May want to check if this one is passing DC. Check the 220k-ohm grid-to-ground resistor on the 6K6GT. Very important that this one isn't way out of spec. Like greater than 300k-ohms. As 220k-ohm is about the limit for a 6K6GT. You can try a 180k-ohm G2G. This will better dump excess signal current to ground and keep the 6K6GT from overloading.



3.) Try a 12AX7 & 12AU7 in the input section (12AT7 position). The change in mu may help determine if this is the section overloading or distorting. Versus the 6K6GT or 12AX7 (7025) positions.

4.) How good is the tank? Do you have another one you can try?

5.) Is the 6K6GT a strong tube? Do have another you can try? You need a strong testing 6K6GT. It makes a big difference in this unit. I like NOS RCA or Sylvania black plates. They are still relatively cheap.

6.) Did you replace the main filter caps?

7.) Put a 1-meg ohm (or even 470k-ohm) resistor from grid-to-ground (pin #2 to ground) on the 7025 (12AX7) output tube. This help stabilize the idle and keeps the 12AX7 from being overloaded & running away.


HTH! Keep us posted.


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