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Post subject: SRSF AB763 - revisited - crummy sound on low strings
Posted: Sun Nov 04, 2012 4:19 pm
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Hi All,

Been out of commission for a while due to conflicting demands :-(

Did a recap and some resistor replacements on my Silverface AB763 Super Reverb last year and then got busy and project has been on hold. Resurrected the project this past week and found some fartty-type distortion on low strings while high strings nice and clean - both tremolo and normal channels affected. Also getting some funky oscillation on reverb once setting gets past 6. So I measured some voltages as per below. Any guidance you can give is greatly appreciated. Where should I start?

Chris


Super Reverb Voltages with no Power Tubes in place

Bias (Pin 5 of each PT): -52 vdc, -52 vdc


Tx Reds: 366 vac
Bias Pot middle tab: -63.6 vdc

Using Dual Bias Probe I get a very low reading of .012/.014 mv

PT1
4: 477 vdc
3: 477 vdc

PT2
3: 477 vdc
4: 477 vdc

12AT7 (1)
6: 249vdc
1: 265 vdc
3: 99 vdc

12AX7:
1: 387 vdc
3: .001 vdc
6: 336 vdc

7025 (1)
1: 274 vdc
3: 2.31 vdc
6: 277 vdc

12AT7 (2)
1: 468 vdc
8: .46 vdc

7025(2)
1: 275 vdc
6: 278 vdc
8: 2.23 vdc

7025(3)
1: 283 vdc
3: 2.3 vdc
6: 284 vdc
8: 2.23 vdc

DD: 442 vdc

Red: 494 vdc
Yellow: 493 vdc


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Sun Nov 04, 2012 5:58 pm
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Best to show (or post) which side of which 12AX7/7025 or 12AT7 you are getting those readings. Maybe PhotoShop the numbers onto the schematic?

Also, you may need to look at the coupling caps. Maybe a leaky one. Esp in gain and PI stages.

http://www.webphix.com/schematic%20heav ... _schem.pdf

Some drawing software and word processors have ways to convert PDF files to one you can edit. FYI...


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Sun Nov 04, 2012 9:26 pm
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BMW2002Ti wrote:
...Also, you may need to look at the coupling caps. Maybe a leaky one...

+1

If the numbers represent pin numbers for the preamp tubes, I see several readings that are way off, including Cathode voltages on the 12AT7s. You should have all tubes installed to take these readings.

It's difficult to understand what is what, you seem to have many terms wrong, and not clear, such as "PT", when probably you actually mean "OT" as you won't have DC on the PT, or do you mean "power tube"? They are output tubes. The PT is the power transformer, and has the input voltage applied to it, and OT is the output transformer.

The bias probe readings are worthless with no output tubes installed. Install the output tubes for the OT primary DC measurement and bias current measurement.

WTF?

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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Sun Nov 04, 2012 9:52 pm
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Don't think Task's efforts are too far off the mark! A little clarification would help. But we rarely get such good specifics. Hey Task, we need to know the readings with and without the tubes installed. Otherwise, your approach really helps the diagnosis. Thanks, Art

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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 4:42 am
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Hi shimmilou,

Apologies. PT = OT, or 6L6s.

Al measurements with tubes in, except for two right side preamp tubes , in layout view, since the pop that occurred when i touched the hot pins nearly caused the speaker cones to deafen me and threatened to kick me in the package :-(

shimmilou wrote:
BMW2002Ti wrote:
...Also, you may need to look at the coupling caps. Maybe a leaky one...

+1

If the numbers represent pin numbers for the preamp tubes, I see several readings that are way off, including Cathode voltages on the 12AT7s. You should have all tubes installed to take these readings.

It's difficult to understand what is what, you seem to have many terms wrong, and not clear, such as "PT", when probably you actually mean "OT" as you won't have DC on the PT, or do you mean "power tube"? They are output tubes. The PT is the power transformer, and has the input voltage applied to it, and OT is the output transformer.

The bias probe readings are worthless with no output tubes installed. Install the output tubes for the OT primary DC measurement and bias current measurement.

WTF?


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 4:46 am
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Thanks Art,

I will do measurements for both tubes in place, and tubes out tonight. As mentioned in response to shimmilou, getting tube-in readings on two right hand preamps tubes caused a dramatically loud pop that i am not sure i want do redo. But would i be right in suspecing the pop is also symptomatic of some coupling failure? I seem to remember from my previous electronic days, that such pops were indicatove of creeping dc across stages.

Chris

aclempoppi wrote:
Don't think Task's efforts are too far off the mark! A little clarification would help. But we rarely get such good specifics. Hey Task, we need to know the readings with and without the tubes installed. Otherwise, your approach really helps the diagnosis. Thanks, Art


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 5:22 am
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It may be "blocking distortion", check for shorted screen/grid resistors.

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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 6:24 am
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Hi Denitronik,

Thanks for your input. Those are the 1500 and 470 ohm resistors on each 6l6, correct?

Specs for these are 1W for the 470s and fireproof I think, yes? The 1500s look to be 1/2 watters. Should tolerance be tight on theseor are +- 10% sufficient?

Chris

denitronik wrote:
It may be "blocking distortion", check for shorted screen/grid resistors.


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 11:58 am
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It is Blocking Distortion. But there can be several reasons for this (besides bad resistors).

The 1500-ohm grid stoppers (on the contral grids of the 6L6GC) are correct. the 470-ohm resistor on the screen grid are correct, also. And yes, heat resistant, flame-proof are the way to go. As these resistors are solderd right onto the 6L6GC socket tangs. Directly under those 6L6GCs.

BUT...

Rarely do these resistors cause a blocking distortion "farting tone" you describe. When they are out-of-spec, this can lead to an unstable amp and blown fuses and output tubes. But, not usual "farting tones." As a last step, when all else fails --- people have been known to increase the grid stopper resistance (from 1500 to say, 2500 or more ohms). All this does is reduce the input signal voltage onto the 6L6GC. Kinda a sledgehammer effect to solve an overall design issue. In most BF Fenders, usually an isolated problem in one section is causing this distortion. A little detective work is needed.

Blocking distortion, in my experience:

1.) Way too hot idle bias on the 6L6GC.

2.) Improper plate voltage on the input, gain, tone, EFX, or phase inverter section tubes. (12AX7, 7025, or 12AT7)

3.) Wrong bypass caps on the above tubes. Esp too high mfd value. These are the electrolytic caps that parallel the cathode-to-ground resistors on certain triode section of the 12Ax7, 7025, or 12AT7. They are usually 25mfd/25VDC in most Fender BF amps.

4,) Leaky coupling caps. Putting a lot of DC voltage onto the next section's grid. These caps must block or stop as much DC voltage, as possible. Allowing only signal VAC to pass.

Notice how the caps are tied to the anode of one section (High DC voltage). AND the other side of the cap to the grid section of the next tube? If this cap leaks too much DC it causes the next section to run too "hot." To the point of causing too much low frequency passing and "farting" tone. Even can lead to damage to that section.

That's why the voltage reading on each specific site is important. For instance, those 0.1mfd coupling caps from the 12AT7 phase inverter to the 6L6GC should have around +230VDC on one side and NEGATIVE DC on the other (part of bias circuitry). If not, you risk bad distortion and burnt 6L6GCs.

http://www.webphix.com/schematic%20heav ... _schem.pdf

BTW: I like using the LAYOUT drawing when doing readings. Insures that I'm reading the correct points.

5.) Improper or out-of-spec cathode-to-ground resistor(s) on thsoe 12AX7, 7025, or 12AT7 tubes. Which leads to too hot idle bias = blocking D.

What works nice, is a tube per tube check of each section. Pull all tubes, except the rectifier. Now, read voltages. They should be higher (around 20-25%) at each read point. This is ok, as no tubes = less current draw = higher read voltages. Amp at idle (volume(s) at zero, no input, speakers connected.

Any weird readings? Like points D reads the same as point B (B should be ~50VDC lower)? Suspect the power supply section. You must have each point read correctly RELATIVE to each other. That is, the read point at the standby switch > After TR1 > point B > point C > point D. The actual voltage maybe higher than what is posted in the schematic. Should be within 20-25% range. BUT, the order in drop of voltages at each of these points should be the same. If not, you must correct this first.

If ok...

TURN OF AMP & put in the first (INPUT, V1) 12AX7, 7025. Relight amp...

Read the first half (or first triode section of V1) of the 12AX7, 7025. Read the plate VDC (~230VDC), cathode to ground VDC (~2.3VDC), and DC voltage AFTER the coupling cap. Which is the case of the AB763 Super Reverb, is after the tone stack. This point (to Volume control) should be in mV range --- amp at idle, no input, speakers connected.

All ok? Go to the next section (second half or triode section of the same input tube). Plate should read around +230VDC. Cathode to ground around 2.1 VDC. And the point past the 0.1mfd coupling cap should be in mV range.

All ok? Pull this tube and inssert the next 12AX7, 7025 (input of the Vibrato section). Do the same measurments. All ok?

Pull this tube and put in the 12AT7 of the phase inverter. Do same readings. Repeat this process with the 12AT7 and 12AX7/7025s of the Reverb and Vibrato section.

You can do this type of voltage checking because all of the tubes are powered via parallel connections to the power supply. Including the heater circuit. This makes isolating the problem section in the amp a lot easier.

HTH! Keep us posted. :mrgreen:


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 4:44 pm
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Thanks BMW,

I have my work cut out for me :-)

I understand all of your points below except for the comments about half of the 12AX7 and 7025. In terms of pins, what does this mean?

Chris


BMW2002Ti wrote:
It is Blocking Distortion. But there can be several reasons for this (besides bad resistors).

The 1500-ohm grid stoppers (on the contral grids of the 6L6GC) are correct. the 470-ohm resistor on the screen grid are correct, also. And yes, heat resistant, flame-proof are the way to go. As these resistors are solderd right onto the 6L6GC socket tangs. Directly under those 6L6GCs.

BUT...

Rarely do these resistors cause a blocking distortion "farting tone" you describe. When they are out-of-spec, this can lead to an unstable amp and blown fuses and output tubes. But, not usual "farting tones." As a last step, when all else fails --- people have been known to increase the grid stopper resistance (from 1500 to say, 2500 or more ohms). All this does is reduce the input signal voltage onto the 6L6GC. Kinda a sledgehammer effect to solve an overall design issue. In most BF Fenders, usually an isolated problem in one section is causing this distortion. A little detective work is needed.

Blocking distortion, in my experience:

1.) Way too hot idle bias on the 6L6GC.

2.) Improper plate voltage on the input, gain, tone, EFX, or phase inverter section tubes. (12AX7, 7025, or 12AT7)

3.) Wrong bypass caps on the above tubes. Esp too high mfd value. These are the electrolytic caps that parallel the cathode-to-ground resistors on certain triode section of the 12Ax7, 7025, or 12AT7. They are usually 25mfd/25VDC in most Fender BF amps.

4,) Leaky coupling caps. Putting a lot of DC voltage onto the next section's grid. These caps must block or stop as much DC voltage, as possible. Allowing only signal VAC to pass.

Notice how the caps are tied to the anode of one section (High DC voltage). AND the other side of the cap to the grid section of the next tube? If this cap leaks too much DC it causes the next section to run too "hot." To the point of causing too much low frequency passing and "farting" tone. Even can lead to damage to that section.

That's why the voltage reading on each specific site is important. For instance, those 0.1mfd coupling caps from the 12AT7 phase inverter to the 6L6GC should have around +230VDC on one side and NEGATIVE DC on the other (part of bias circuitry). If not, you risk bad distortion and burnt 6L6GCs.

http://www.webphix.com/schematic%20heav ... _schem.pdf

BTW: I like using the LAYOUT drawing when doing readings. Insures that I'm reading the correct points.

5.) Improper or out-of-spec cathode-to-ground resistor(s) on thsoe 12AX7, 7025, or 12AT7 tubes. Which leads to too hot idle bias = blocking D.

What works nice, is a tube per tube check of each section. Pull all tubes, except the rectifier. Now, read voltages. They should be higher (around 20-25%) at each read point. This is ok, as no tubes = less current draw = higher read voltages. Amp at idle (volume(s) at zero, no input, speakers connected.

Any weird readings? Like points D reads the same as point B (B should be ~50VDC lower)? Suspect the power supply section. You must have each point read correctly RELATIVE to each other. That is, the read point at the standby switch > After TR1 > point B > point C > point D. The actual voltage maybe higher than what is posted in the schematic. Should be within 20-25% range. BUT, the order in drop of voltages at each of these points should be the same. If not, you must correct this first.

If ok...

TURN OF AMP & put in the first (INPUT, V1) 12AX7, 7025. Relight amp...

Read the first half (or first triode section of V1) of the 12AX7, 7025. Read the plate VDC (~230VDC), cathode to ground VDC (~2.3VDC), and DC voltage AFTER the coupling cap. Which is the case of the AB763 Super Reverb, is after the tone stack. This point (to Volume control) should be in mV range --- amp at idle, no input, speakers connected.

All ok? Go to the next section (second half or triode section of the same input tube). Plate should read around +230VDC. Cathode to ground around 2.1 VDC. And the point past the 0.1mfd coupling cap should be in mV range.

All ok? Pull this tube and inssert the next 12AX7, 7025 (input of the Vibrato section). Do the same measurments. All ok?

Pull this tube and put in the 12AT7 of the phase inverter. Do same readings. Repeat this process with the 12AT7 and 12AX7/7025s of the Reverb and Vibrato section.

You can do this type of voltage checking because all of the tubes are powered via parallel connections to the power supply. Including the heater circuit. This makes isolating the problem section in the amp a lot easier.

HTH! Keep us posted. :mrgreen:


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 5:12 pm
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taskerc,

Ok... The 12AX7 and 12AT7 are two triode sections built into one tube. For the 12AX7 pinouts:

Image

T1 = triode section one

T2 = triode section two.

K = kathode (cathode in English)

G = control grid

P = anode plate

H = heater lines. For most dual triode 9-pin miniature tubes, the heater lines are in parallel (pins 4&5 tied together). And pin 9 is grounded.

If you look at the schematic, notice how some of the these tubes use one triode section for one function (1/2 of the dual triode). And the other half, for another function?

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Mon Nov 05, 2012 6:09 pm
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Thanks - now I get it.

Chris

BMW2002Ti wrote:
taskerc,

Ok... The 12AX7 and 12AT7 are two triode sections built into one tube. For the 12AX7 pinouts:

Image

T1 = triode section one

T2 = triode section two.

K = kathode (cathode in English)

G = control grid

P = anode plate

H = heater lines. For most dual triode 9-pin miniature tubes, the heater lines are in parallel (pins 4&5 tied together). And pin 9 is grounded.

If you look at the schematic, notice how some of the these tubes use one triode section for one function (1/2 of the dual triode). And the other half, for another function?

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Tue Nov 06, 2012 8:19 pm
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Finally got measurements done, although not sure if I did coupling cap measurements correctly.

This folder has PDFs of layout diagrams for each tube scenario with measurements overlaid.

https://drive.google.com/?tab=mo&authus ... 0NOVVg3ckk

The drawing (V5) that corresponds to the measurements for V5 (the last 12AX7) is the only one with some wonky numbers.

For example, on the drawing, pin volts should be as follows:

Pin1: 280V
Pin2: ??
Pin3: 2.5v
Pin4/5: ??
Pin6: 390v
Pin7: ??
Pin8: 17.0v

My results are:
Pin1: 418V
Pin2: -15.01
Pin3: 0v
Pin4/5: 0v
Pin6: 372v
Pin7: -14.8
Pin8: 0v

Something looks wrong here.

Otherwise other "only tube X" scenarios look OK to me.

Am I crazy? Have I understood?

Chris

taskerc wrote:
Thanks - now I get it.

Chris

BMW2002Ti wrote:
taskerc,

Ok... The 12AX7 and 12AT7 are two triode sections built into one tube. For the 12AX7 pinouts:

Image

T1 = triode section one

T2 = triode section two.

K = kathode (cathode in English)

G = control grid

P = anode plate

H = heater lines. For most dual triode 9-pin miniature tubes, the heater lines are in parallel (pins 4&5 tied together). And pin 9 is grounded.

If you look at the schematic, notice how some of the these tubes use one triode section for one function (1/2 of the dual triode). And the other half, for another function?

http://www.webphix.com/schematic%20heav ... _schem.pdf


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Tue Nov 06, 2012 9:24 pm
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This is the vibrato tube, it won't be conducting unless the vibrato is in use. The only reading that really looks wrong is the heater voltage, which will be an AC voltage on pins 4/5 to 9, and you should have approx 6.3 VAC. You might have to read from ground to pins 4/5, approx 3.2 VAC. Does the tube glow? If so, that tube, and your readings are likely OK. I doubt if the vibrato tube or circuit is causing the unpleasant sound.

Are you sure that all preamp tubes are good?

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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Tue Nov 06, 2012 11:35 pm
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taskerc, I agree with Shimmy. You should be able to pull V5 (vibrato tube), run a guitar into the NORMAL input and get tone (all other tubes in). If it's still funky... it's not the vibrato section or tube.

I'm starting to think you have a leaky coupling cap. None of the input/tone stage/phase inverter control grids (pins 2 and 7) had high readings?

Just looking at the amp, does it look like any coupling cap were replaced? Are they all in OEM mfd and voltage range?

And all of the bypass caps on the 12AX7 and 12AT7 cathode-to-ground were within normal OEM values?

And you had OK range of idle bias on those 6L6GC?

One last thing, can you sub another known, good speaker(s)? Just to rule out something funky with them. You may have already done this. :D

If all this fails, you may have to go section-by-section from the input to the output to find this problem. This can be done with a DVM. But, is easier with a signal generator and scope (read: good tech).

Sorry. I hope its a coupling cap or out-of-spec resistor or something else which is simple.


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