Blackface Deluxe non-reverb servicing

Lowerleftcoast

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In this example, the grid leak is 1M and the effective input impedance is 1.8M without NFB.
I am no expert by any means and I appreciate the opportunity to learn. My ears hear a difference that I don't think is possible with the inference of the writings you have provided.
Anyway, I have no empirical data for my observation. Just my ears, and I will throw in @Wally and @schmee 's ears as well. Can the three of us be any good at hearing changes in sub bass frequencies? We are talking about corner frequencies at or below ~43Hz (with your example .001uF, 3.7M) in these Fender amps. When we install a .01uF the corner will be at ~4.3Hz. How do the three of us hear a difference when we intall a .022uF? Ears can deceive so... ?

So running with what you have brung...
The writings suggest a voltage drop of .56V across 1M grid leak 1. It seems the RC filter is acting much like I think it is, just acting as plain old high pass RC filter, letting low frequencies to ground, (or to be cancelled with NFB after it passes through the ~1M).

Am I wrong headed in that thought? Does the impedance from cathode to plate change the RC filter as the writing suggests?
 

schmee

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I am no expert by any means and I appreciate the opportunity to learn. My ears hear a difference that I don't think is possible with the inference of the writings you have provided.
Anyway, I have no empirical data for my observation. Just my ears, and I will throw in @Wally and @schmee 's ears as well. Can the three of us be any good at hearing changes in sub bass frequencies? We are talking about corner frequencies at or below ~43Hz (with your example .001uF, 3.7M) in these Fender amps. When we install a .01uF the corner will be at ~4.3Hz. How do the three of us hear a difference when we intall a .022uF? Ears can deceive so... ?

So running with what you have brung...
The writings suggest a voltage drop of .56V across 1M grid leak 1. It seems the RC filter is acting much like I think it is, just acting as plain old high pass RC filter, letting low frequencies to ground, (or to be cancelled with NFB after it passes through the ~1M).

Am I wrong headed in that thought? Does the impedance from cathode to plate change the RC filter as the writing suggests?
One thing about caps and changes in amps (or even guitars) is it seems to me it's not readily determined "A/B" ing . But rather playing for some time, different songs, or even for an entire gig or in a band mix that brings things out. Add to that there's the difference of the music room vs a venue. Huge difference! I've had amp builds that I have loved in the music room and took to gig and found nearly unplayable!
 

Ten Over

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So running with what you have brung...
The writings suggest a voltage drop of .56V across 1M grid leak 1. It seems the RC filter is acting much like I think it is, just acting as plain old high pass RC filter, letting low frequencies to ground, (or to be cancelled with NFB after it passes through the ~1M).
The RC filter doesn't let low frequencies to ground. The capacitor's impedance increases with decreased frequency which alters the voltage divider formed by the capacitor and the input impedance. The increase in capacitor impedance results in a reduction of the voltage at the junction of the capacitor and the resistance.
 

Lowerleftcoast

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If you don't trust my math, look what Valve Wizard says about the input impedance of an AC LTPI.
I am not having a problem with your math. I had a read through the AC LTPI article (again). Thanks for directing me to it.

The math shows some very low cutoff frequencies. Where I am having trouble is that the numbers do not match my knowledge and experience with low frequencies and what others and I *hear* when these caps are changed. Now maybe it can be attributed to harmonics of the sub-bass frequencies, IDK, but for me there is some disconnect between the numbers and what I experience. I want an explanation dagnabbit. hah.
 

schmee

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I didn't see an answer to this question in the thread. You want an 18/3 SVT cable and a Heyco SR 5N-4 strain relief.
The 18/3 SVT cable is listed as 1/4" OD, isn't that a bit smaller than Fender normal cable OD? I have used 1/4" power cables in Fender Chassis before but they were not tight.
 

Bob Arbogast

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The 18/3 SVT cable is listed as 1/4" OD, isn't that a bit smaller than Fender normal cable OD? I have used 1/4" power cables in Fender Chassis before but they were not tight.
Yes, that is smaller diameter. In my experience, black panel Deluxe and Deluxe Reverb amps, have a smaller chassis cutout for the power cable. The strain relief I mentioned fits that smaller cutout and the SVT cable.
 

Silverface

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That's an interesting thought, you really think one can hear a difference?
Sorry for dropping in so late!

As noted - it's a can of worms.

But it really depends on the specific amp (cabinet and baffle variables, speaker differences - even with the same brand/model-

How it's optimized by the tech for the player who will use it (not to arbitrary specs, which are a waste of time when modern parts may be installed - tubes installed, how power amp AND preamp tubes are biased; resistor tolerances; transformer spec variables; the voluyme level at which the amp will be run-

The players' instruments, effects, cables-

Input voltage from the power socket-

There are more, but ALL of these can make a difference in how some players can...or can't...hear differences between cap brands, types within the same brand, very minor differences in capacitance and ESR...

An unusual non-amp example: I prefer the "slope" (speed and how quickly - picture a graph - a cap /pot combination may result in the treble rolloff being a straight, linear shape; perfectly even from the pot's "off" to "wide open" position; of a concave or convex curve (either more likely) - some a quick drop and then slow progression to "mud"' or a slow, gradual drop with a sudden drop to "mud".

They are passive caps and can only "subtract treble". They can't ADD any at all!

And I happen to prefer using .047uf "brown turd" caps from Fender amps, that most techs dislike and remove/discard.

Well, I've found them to provide the most controllable - meaning the most linear - frequency drop in most Teles - when used with 500k ohm audio pots and not the to me) useless "treble retention" system of using a cap & resistor to bridge the input and wiper tabs on the volume pot.

And it doesn't matter which pickups I'm using.
 

dannyh

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I found some pics of my '64 before I had it serviced. Kinda hard to tell but it looks like mine did not have those "extra" resistors.

May not be of any help but here are the pics.

IMG_0764.JPG
IMG_0765 (1).JPG
 




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