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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Tue Sep 17, 2019 5:55 pm
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I do not like Tungsol reissued 12AX7 (Russian). Their bottles are a bit too fat (not much clearance for OEM heat shields). And the pins are a little fat (IIRC). Tone is not good, so modifying tube shield and socket is prolly not worth it. As modern 12AX7 tubes go, the Gold Lion B759/ECC83 is better. IMHO.

https://tubedepot.com/products/genalex- ... gL-4_D_BwE


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Sep 20, 2019 11:02 am
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I changed out that hundred K resistor, now I have no bright switch on channel 2 and even though I can hear the faint pulsing of the tremolo including the speakers I have no tremolo for the amplified single of the guitar.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Sep 20, 2019 1:04 pm
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Do you feel fairly okay dealing with 400-500 VDC? I think you, or a good tech, need to do a voltage check with the unit at idle. Compare your results with the nominal voltages on the schematic and layout diagram.

Tap around the circuit with a wooden or plastic probe and see if the voltage jumps. Check for signs of poor soldering or intermittent shorts. Bad grounding points. Improper contact to the chassis, from the circuit board.

Just my 2¢ worth. Good luck!

:mrgreen:


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Sep 20, 2019 3:05 pm
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royb3988 wrote:
I also noticed that when I turned the amp on this morning...it had been off since 11PM last night, the Tremolo went almost 60 seconds after I hit the foot switch and had not engaged...as soon as I touched Tube V3 the Tremolo kicked in. I will check that tube and tube socket as well. Again, when I hit the foot switch I could faintly hear the pulsing of the Tremolo...then when I touched the tube it came on as it should.
RB3988


To me, that's a huge clue as to what's going on.

dcgillespie wrote:
Don't ya just hate intermittents? Dirty or bad tube socket is always a possibility with weird goings on.

Dave

In my opinion, you have a dirty or loose connection IN V3. It doesn't mean that the V3 tube socket is loose, it means that one or more of the connections in the V3 tube socket is dirty or loose if that makes any sense . If you can replicate the sympton where the tremolo doesn't work until you wiggle the connection of V3, then I think your problem lies there.

You have a 52 year old amp. It may seem like it's impossible to fix, but it's not. Forget the bright switch for now, and concentrate on the tremolo.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Sep 20, 2019 7:18 pm
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I think its time you coughed up a few bucks and quit chasing the tail!!!
Replace "all" the electolytics as previously suggested, this will help you maintain constant supply voltages to the tubes.

+1 to BMW
Sure you could trouble shoot the system hot now as BMW suggests, but you will be testing old caps that are likely weak/unstable. That's fine a hot test "is" needed to validate other components w/in each of the circuits. Take very good notes, IE. pin by pin & tube by tube starting at the power transformer and going all the way to V1. You'll need to test all the AC and the DC voltages, lamp power, heater filaments. If you find say a discrepancy at the power transformer voltages, you'll need to "correct it then" before moving to the next stage- Output, PI, Tremolo, Reverb, Input.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Sat Sep 21, 2019 9:33 pm
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[url]Image[/url]

That big curved yellow wire that runs parallel to the green/blue wire just above that "bat wing" resistor had come loose from the solder joint, it's soldered back in place now and so far there is a short delay in the Tremolo engaging, maybe just 4 or 5 seconds...if it's not any longer tomorrow morning I will live with it. Bright switch is back! Also...while that wire was disconnected, channel 1 was as loud as any Marshall I have ever heard. Any setting of volume above 2 or 3 was deafening! Channel 2 was almost nonexistent. It's all good now. Question: does the bias setting of the output tubes in any way affect the Tremolo circuit?

Otherwise the amp sounds great!

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Sun Sep 22, 2019 10:01 pm
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The bias-supply current has no effect whatsoever on the tremolo circuit.

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 1967 DUAL (in block letters) Showman
Posted: Tue Oct 01, 2019 9:44 am
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On the schematic there is a 2.2M resistor between the foot pedal and a white wire that goes to the area in a rectangle at the top left of the diagram...I disconnected one end, measured it and got a reading of 2.42M, could that resistor be a culprit?


RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Tue Oct 01, 2019 6:37 pm
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royb3988 wrote:
On the schematic there is a 2.2M resistor between the foot pedal and a white wire that goes to the area in a rectangle at the top left of the diagram...I disconnected one end, measured it and got a reading of 2.42M, could that resistor be a culprit?

That's (barely) within 10%.
Keep in mind that if the resistor is banded silver for 10%, it will never read 5% or better, otherwise it would have been banded gold and sold as a 5%..
1.98 to 2.09m or 2.31 to 2.42 would be the expected values for a 2.2m 10% resistor.

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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 02, 2019 9:06 am
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Good info, thanks.

That resistor seems to be between the bias control pot and the foot switch...on suggestion from a tech online, I changed the resistor to a 1.6M ohm 2% (he recommended a 1.5M but the supply house had none so I got what they had) waited 12 hours and turned on the amp just now. The Tremolo engages immediately when I hit the foot switch...go figure. Amp is now off...if it works properly 12 hours from now I will figure that somehow it is fixed...though I do not know why or how.

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Thu Oct 03, 2019 9:52 am
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The tremolo issue is fixed! While it appears the last suggestion about the 2.2M ohm resistor was the solution, I think it was the collaborative input from everyone in this thread.

Thanks to one and all!

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Oct 04, 2019 11:07 am
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Before you get too giddy, reducing the 2.2M to 1.6M and getting it to work is an alarm bell to me, not a victory dance.
It indicates that your bias supply is not robust enough to run the oscillator properly. The mod covers the underlying problem, but its critical to the power tubes that the bias supply is stable.
The motorboating you heard could easily be attributed to the bias supply collapsing as it tries to run the oscillator. That is the main load for the supply. The motorboating would be coupled to the power tube grids through the bias supply.
If reflowing the bias diode or capacitor helped, I would replace both. Maybe even the 470 ohm resistor, just because.


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Fri Oct 04, 2019 4:04 pm
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TimsAudio wrote:
Before you get too giddy, reducing the 2.2M to 1.6M and getting it to work is an alarm bell to me, not a victory dance.
It indicates that your bias supply is not robust enough to run the oscillator properly. The mod covers the underlying problem, but its critical to the power tubes that the bias supply is stable.
The motorboating you heard could easily be attributed to the bias supply collapsing as it tries to run the oscillator. That is the main load for the supply. The motorboating would be coupled to the power tube grids through the bias supply.
If reflowing the bias diode or capacitor helped, I would replace both. Maybe even the 470 ohm resistor, just because.


+1

This scenario is quite possible, and replacing those components in the bias supply will ensure that the symptoms do not recur. I'd also replace that 27KΩ resistor as well (I'm assuming a fresh bias-supply cap was installed when the P/S was overhauled). Replace the diode with a UF4003 or UF4004 type.

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 1967 DUAL (in block letters) Showman
Posted: Sat Oct 05, 2019 10:46 am
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No victory dance, yet. When my order of 2.2M ohm resistors shows up I will replace the 1.6M...I only purchased it because that was all that was available locally.
From a contributor to this thread... "The bias-supply current has no effect whatsoever on the tremolo circuit." Does it or does it not? Also...until my order of resistors shows up, is there any potential damage to the amp...circuitry wise, fire hazard, safety issue etc. that could result from using the 1.6M ohm resistor in the interim?

More later.

RB3988


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Post subject: Re: Fender 1967 DUAL (in block letters) Showman
Posted: Wed Oct 09, 2019 7:02 am
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royb3988 wrote:
From a contributor to this thread... "The bias-supply current has no effect whatsoever on the tremolo circuit." Does it or does it not?


Do you know how to READ a schematic diagram?

Arjay

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