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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 28, 2012 6:19 am
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Parkside, that resistor in upper right of Slide #1 (soldered between two legs of 100/100 caps) is a 2.2 k-ohm (red-red-red) resistor. And this is the correct one. And in the correct position. My schemo is wrong.

Make sure that the (+) side of those two caps are going directly to ground (just test the resistance between the caps and the chassis). BIAS POWER supplies are really tricky (obviously, by all the boo-boo's I've made --- boy some knucklehead at Fender couldn't have made a more confoosing schemo). :?: :shock: :?:

I would be careful about marking the chassis with (+) and (-) with these two caps. It could lead to even more confoosion.

Good luck!

EDIT: latest & greatest schemo... :x

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 28, 2012 10:27 am
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Hi guys

One of the things that's confusing is the schematic route to ground.

Here's my questions : does it matter which route the caps take to ground?

For example, today's work will be trying to match caps 3,4, and 5 with the schem.

1) caps 3 and 4 are jumpered at the negative, with a black wire leading to the positive of cap 6 (which is jumpered to cap 7 positive) which then picks up another black wire and heads to ground....

In other words neg cap 3 and 4, and both positives of 6 and 7 all head to ground on the same train.

2) cap 5 is separate from all of this (which i hope is a clue) neg has an olive green wire that leads to ground as well.

Conclusion= cap 3, 4, 5 neg all head to ground. Cap 6, 7 pos lead to ground. Cap 5 seems to be separate.

So to return to my question: since all of the negatives of these caps go to ground one way or another, can anything be deduced by following them?

With regards to identifying which caps are what, following the neg seems to tell me "yes...the neg of this cap goes to ground, just like all the others" -in other words it tells me nothing about which part of the schem I'm dealing with.

The goal for now is to be able to say "this cap is THIS on the schem"


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 28, 2012 10:39 am
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Not sure what you are asking, but a proper ground is necessary to complete the electrical path and for minimum noise. Grounding is an entire subject on how to best do it as there are several methods. Right now I suggest you concentrate on making sure that things are grounded and tracing out the signal path (all the voltages) through each component to make sure they go where they are supposed to and through the proper component. Hope that makes sense. :D

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 28, 2012 5:29 pm
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" In other words neg cap 3 and 4, and both positives of 6 and 7 all head to ground on the same train. "

Ok... Look at my latest (Version 5a, LOL!) schematic of the power supplies (both the MAIN and the BIAS). Caps #3 and 4 (the 47mfd/500VDC, correct?) are in the MAIN power supply. And should have the (-) go to ground or a chassis point.

Caps #6 & 7 (100mfd/100VDC) are in the BIAS POWER supply. Remember, this supply HAS TO BE negative VDC. You may want to read up on why this is, in a Fixed-Biased amp. Anyhow, the polar caps (or electrolytic caps) MUST have the (+) go to ground. This is also why the DIODE seems to be reversed in the BIAS SUPPLY. It is feeding half-wave NEGATIVE VDC to this rail.

The reason I split the two PS in my schematic was to show the two circuits (MAIN AND BIAS) separately. And how they relate to each other (esp grounding points) in the amp's layout.

Hang in there! think you are getting the idea. :idea:

Here's Randall Aiken's blurb on the two major types of BIASING:

http://www.aikenamps.com/WhatIsBiasing.htm


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 28, 2012 7:46 pm
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Hey Bmw

I can say for absolute certain (or as a certain as a novice can be) That both the Bias Caps (6 and 7) send positive to ground....They tie in with the negative at cap 4 (which is jumpered to the neg of cap 3)

The negative side is bridged by a resistor. Cap #6 Negative is the blue wire heading to the diode, resistor, brown wire combo. Cap#7 Neg is orange, and heads to "output tubes matching"

Image

Image

Im currently working on drawing the schem for what each cap in the power rail is doing.

Thanks
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Sun Oct 28, 2012 9:57 pm
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" I can say for absolute certain (or as a certain as a novice can be) That both the Bias Caps (6 and 7) send positive to ground....They tie in with the negative at cap 4 (which is jumpered to the neg of cap 3). "

Yup, this is correct. And that resistor tied to the (-) legs or the two 100/100 caps should be 2.2 k-ohms. Correcto?

:D


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 12:10 pm
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Hi Bmw

Yes, That is a 2.2 Resistor on the Negative legs of cap 6 and 7 (the bios 100/100 caps) (red red red silver 2+2x100, 10%=2200 ohm 2.2k)

I haven't lifted any resistor legs and measured yet, as I still want to get my map of where the caps are going completed. Im in the home stretch. Should be done today.

Thanks
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 3:41 pm
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" The bios 100/100 caps. "

Just to be anal, it's BIAS not BIOS. BIOS is the mother board of computers. BIAS means the idle is pre-set value on the tube (or anything), when the tubes are at idle.


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 4:05 pm
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Yep re Bias (that was a typo)

OK. Here are my maps.

The first drawing is just an overview.

The next 4 slides show a drawing on the left, and the match of the schem on the right. The drawings at left, are what physically exists in the amp (right or wrong.)

Im hoping once everything I have drawn is certified as 'correct' I can proceed to begin measuring the resistors, and posting my findings. If anything seems wrong, let me know. I can back up almost everything I am asserting with photographs of the power rail and Chassis.

Thanks again
Dave

P.s. Everything makes perfect sense, except the connection between the 2 'sandbox' resistors in slide 2...trying to match that to the schematic makes me a bit dizzy. But, If Im reading correctly the 'mystery resitor' could be the 30k 20w shown in the schematic on slide 3, near letter A?

Image

Image

Image

Image

Image


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 5:08 pm
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Really good job! Seems all correct. One thing (only a suggestion) --- but the convention usually has the (+) rail at the top and the (-) or ground at the bottom of these sort of diagrams. RED = hot or (+) and BLACK = chassis ground. GREEN = outlet ground. It just makes it easier to diagnose, at a later date.

As long as you can follow the diagrams, that's all that matters.

This is a difficult amp, in the sense that the power supplies (both BIAS and POWER) are not even separated in the layout. Some components are under the hood. Some are place here-&-there on the chassis. Etc... Not your typical "Leo" layout, which is simple to follow.

As long as you use continuity test (resistance test with DVM) to test the connections, before and after replacement you should be ok.

I recommend using a Variac or the "light bulb current limiter" on first fire up. This is the way I would do it. First fire up is normally where mis-wiring shows up. And either one can save you a LOT of anxiety & damage.

Again, link to do-it-yourself light bulb, current-limiter:

http://antiqueradio.org/dimbulb.htm


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 7:14 pm
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OK. Im going to build a dim light tester tomorrow.

Earlier Bmw said "what's going on with the Dale 'Made in Mexico' Resistor"


...One of the things Tech 1 did, was change the socket of the 6L6 power tubes at V8 (it was loose and the tube could easily fall out) I checked the schem and it specifically calls for a 470 ohm, 1 Watt resistor on all power tubes.

He installed a 470 ohm 5 Watt resistor...sandbox cement, not carbon comp. (photo below.)

Will this affect tone?

V7, V9, V10 all have the 470k 1 Watt carbon comps. And, all four (including V8) have a 1500 ohm carbon comp.

Image

Before I change anything, I want to know everything he changed. The amp had some minor issues before he worked on, but overall sounded good...It hasn't since

So far I know he changed the caps, with 2 way out of spec (and the bias 100/100 caps are AI-Elko?? which Ive never heard of) And this resistor is rated 4 watts higher than it should be...

Thanks
Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 7:20 pm
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This is the screen grid resistor. Really don't need a 5-watt wire wound for this function. But, it won't hurt.

It should not affect the tone.


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 8:34 pm
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BMW2002Ti wrote:
This is the screen grid resistor. Really don't need a 5-watt wire wound for this function. But, it won't hurt.


:shock:

Those grid resistors are spec'ed to one watt for a purpose -- to protect the amp in the event of a runaway tube.

Arjay

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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 8:53 pm
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I should change it back then?

http://www.tubeampdoctor.com/en/shop_Re ... piece_1096

I would really like to return the amp to spec. I'm actually a little annoyed that I can't buy 80uf/75v Capacitors, and have to rely on 100/100. And my strongest suspicions (prior to testing the resistors) are with regards to the 2x 500v/47uf, that ought to be 500v/20uf

As I currently read Dave Hunter's thoughts on components "it isn't simply 'bigger is better.' For one thing, tube rectifiers are only happy with so much filtering in the first position. ....over filtering can sometimes choke and tighten up an amp that might sound more dynamic and open with lower value caps in the power supply..."

Doesn't really describe my problem....But after all my digging, I can't escape Occam's Razor (the simplest theory is usually correct.) I had a tech, that didn't even know to make decent mechanical connections on the filter caps (see photos where tech 2 made butt joints, to repair tech 1's work), change a bunch of parts, most of which are not correct, and its never been right since...

I believe it to be far quieter than it should be, and it distorts the low frequencies. (Youtube clip)

Having said that, I am enjoying the process of figuring out all the details, and you guys have been great...so I'll keep digging. There has to be an answer.

Dave


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Post subject: Re: Fender Twin Reverb Silverface Trouble
Posted: Mon Oct 29, 2012 8:57 pm
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Arjay, please check your email. I believe that these screen resistors really don't act like fuses.

Thanks!

:D


Parkside Drive, no, I believe wire-wound are fine here. If you want to go back to original --- use metal-oxide that are heat-resistant. As they are usually placed between socket tangs UNDER the 6L6GCs. 1-watt will do fine.

The grid stopper resistors (also usually soldered between different socket tangs of the 6L6GC) must be proper wattage. This is to avoid excessive current flow, in the event of an unstable amp exhibiting parasitic oscillations. A good heat resistant metal-oxie 1/2-watt will do fine. Yours is a 1500-ohm resistor on the control grid.

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