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Post subject: Re: 68 Deluxe Reverb distorts strongly when cranked
Posted: Tue Mar 27, 2018 12:54 am
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ParksideDrive wrote:
Glad you got the thing fixed. Cheaply and easily Sort of...I'd still like to have V1 out, but without that nasty sound. Wondering if I should drop the Bias?



Most Fender vintage amp sound good with 60% bias. I won't go more than 70% , do listening test.
Why 80 %
Bias have nothing to do with V1 in or out , or any other tube.


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Post subject: Re: 68 Deluxe Reverb distorts strongly when cranked
Posted: Tue Mar 27, 2018 12:56 am
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BMW2002Ti wrote:
Get a cheap 12AX7 and put it into V1. In this amp, the second half of V1 shares the cathode resistor and bypass cap with second half of V2. Pulling V1 will not only effect B+ onto V2 anode, but also the idle bias.

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https://robrobinette.com/AB763_Modifications.htm


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Post subject: Re: 68 Deluxe Reverb distorts strongly when cranked
Posted: Tue Mar 27, 2018 9:34 am
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stratele52 wrote:
Bias have nothing to do with V1 in or out , or any other tube.


Are you talking about the bias of the output tubes? Because all tubes, whether input, gain, PI, or output have set bias. Signal tubes tend to use cathode resistors with bypass caps plus grid-to-ground resistors to help set the bias. Usually in Class "A" single-ended mode.

So, pulling V1, which shares the cathode resistor with V2 (second half) will effect V2 performance.

http://www.diyaudio.com/forums/tubes-va ... -bias.html

:mrgreen:


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Post subject: Re: 68 Deluxe Reverb distorts strongly when cranked
Posted: Tue Mar 27, 2018 9:53 am
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I’ll take a reading, but I think in the past: having a tube in V1 will lower the output Tube bias.

And it explains what’s happening here

1) no tube 1. Bias At 19-20 VDC = problem
2) replace tube in V1, bias unchanged from above- no problem.

In other words, the strain (or whatever you want to call it) that is being placed on the amp by not having a tube in V1is causing an issue.

I suspect (though I haven’t tried yet) if I remove the tube from v1 and bias lower, the problem will disappear as well.

Just my theory, if I’m wrong let me know

I’ll post my results


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Post subject: Re: 68 Deluxe Reverb distorts strongly when cranked
Posted: Tue Mar 27, 2018 9:58 am
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Also I love this schem! labelled everything! Thanks BMW

Saved the pic


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Post subject: Re: 68 Deluxe Reverb distorts strongly when cranked
Posted: Wed Mar 28, 2018 9:25 am
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Well, must be some other interaction between V1 and V2, I don't see. But, if the amp is fine with V1 in, leave it in. 12AX7 are cheap.

:mrgreen:


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