Smoking Bias Resistor

[tommyjr98@gmail.com]

This is my first post and I am a novice with amps so I hope I am posting correctly. 

The amp is a single channel Fargen Blackbird (AB763 clone)with reverb and tremolo.  The problem:  when main power is turned on (standby off and no tubes installed) the circled bias resistor starts smoking quickly.  The transformer makes a soft buzzing sound as well.  How should I proceed with troubleshooting this problem? 

Thanks in advance!!!

[tubeswell]

Check the polarity of the bias supply cap. It should be +ve ground. If it’s been put in the wrong way round, the cap would short, leaving the Bias supply resistor shorting to ground (Causing it to burn). That’s one possibility anyway. (Or the bias supply diode could be the wrong way around?- causing a similar End Result) (or the resistor is shorted to ground from bad wiring?)

[sluckey]

Got a schematic? What is the value of the burnt resistor? How much ac voltage is connected to it?

Has the amp ever worked? Check the bias diode and bias cap.

[tommyjr98@gmail.com]

Thanks for the quick response!  I don't have an exact schematic for this amp.  However, I have analyzed the schematic to an AB763 Deluxe Reverb and this is appears to be almost identical to the vibrato channel.  Only differences are a midrange pot and a few component values.

The burnt resistor is a 470 ohm 2 watt.

The amp has worked up till recently.  I just replaced all the components in the bias circuit because that is what I thought the problem was. 

To get the voltage, how would you suggest I do it?  Should i alligator clip the leads fire it up and hope to get a reading then shut down again quickly?  I'm just worried about damaging other components.

[tommyjr98@gmail.com]

Check the polarity of the bias supply cap. It should be +ve ground. If it’s been put in the wrong way round, the cap would short, leaving the Bias supply resistor shorting to ground (Causing it to burn). That’s one possibility anyway. (Or the bias supply diode could be the wrong way around?- causing a similar End Result) (or the resistor is shorted to ground from bad wiring?)

The + end of the cap does go to ground.  The diode is also oriented correctly too.

[mresistor]

To get the voltage,,   I think I might disconnect the bias v lead to the junction of the 220k  grid leak resistors and then measure the voltage on the resistor. This might tell you where the short is occuring too. When you say tubes pulled does that mean all the tubes  or just the power tubes.

[tubeswell]

Check the polarity of the bias supply cap. It should be +ve ground. If it’s been put in the wrong way round, the cap would short, leaving the Bias supply resistor shorting to ground (Causing it to burn). That’s one possibility anyway. (Or the bias supply diode could be the wrong way around?- causing a similar End Result) (or the resistor is shorted to ground from bad wiring?)

The + end of the cap does go to ground.  The diode is also oriented correctly too.

So the banded end of the diode is pointing to the PT's bias supply winding?


And which PT winding have you sourced the bias supply from? the high tension winding? or a lower voltage bias supply winding?

[shooter]

+1
the schematic is correct the + - at the diode isn't.  the diode should only pass the negative have cycle

[tommyjr98@gmail.com]

To get the voltage,,   I think I might disconnect the bias v lead to the junction of the 220k  grid leak resistors and then measure the voltage on the resistor. This might tell you where the short is occuring too. When you say tubes pulled does that mean all the tubes  or just the power tubes.

I pulled out all the tubes.  My rectifier tube was arcing and I thought the problem was the bias circuit.  Before I destroy any other tubes, I gotta find this short.  I will try your suggestion and measure the voltage at the resistor after disconnecting the bias lead.  Thanks!

[tommyjr98@gmail.com]

Check the polarity of the bias supply cap. It should be +ve ground. If it’s been put in the wrong way round, the cap would short, leaving the Bias supply resistor shorting to ground (Causing it to burn). That’s one possibility anyway. (Or the bias supply diode could be the wrong way around?- causing a similar End Result) (or the resistor is shorted to ground from bad wiring?)

The + end of the cap does go to ground.  The diode is also oriented correctly too.

So the banded end of the diode is pointing to the PT's bias supply winding?

Yes, the banded part is facing the transformer.

And which PT winding have you sourced the bias supply from? the high tension winding? or a lower voltage bias supply winding?

I have not changed any of the PT wiring since it was in a working state.  But to answer you question, I do not know whether it is the high tension winding or lower voltage bias supply winding. 

[2deaf]

How about some resistance readings before you turn that thing on again.

[tommyjr98@gmail.com]

To get the voltage,,   I think I might disconnect the bias v lead to the junction of the 220k  grid leak resistors and then measure the voltage on the resistor. This might tell you where the short is occuring too. When you say tubes pulled does that mean all the tubes  or just the power tubes.

I pulled out all the tubes.  My rectifier tube was arcing and I thought the problem was the bias circuit.  Before I destroy any other tubes, I gotta find this short.  I will try your suggestion and measure the voltage at the resistor after disconnecting the bias lead.  Thanks!

I got about 320V and the resistor heated up significantly again.  I measured on the right side of the resistor as seen in the photo.  I also disconnected the bias lead to the 220k grid leak resistors as you suggested. 

[tommyjr98@gmail.com]

How about some resistance readings before you turn that thing on again.

You mean resistance of the transformer? I got 49.4 ohms measured at both plate pins on the output tube sockets. 

[tubeswell]

I have not changed any of the PT wiring since it was in a working state.  But to answer you question, I do not know whether it is the high tension winding or lower voltage bias supply winding.

So it was previously working? And you replaced the bias supply components and now its not working? (If this is the case, then you need to look carefully at your soldering. Look for unintentional shorts/resistor/cap/diode leads touching other parts of the circuit/ground, or excessive solder blobs touching other parts/the chassis)

[2deaf]

I want the resistance to ground from the junction of the 470R resistor and the diode.

[tommyjr98@gmail.com]

I want the resistance to ground from the junction of the 470R resistor and the diode.

I got .536k ohms

[tubeswell]

I want the resistance to ground from the junction of the 470R resistor and the diode.

I got .536k ohms

Set your meter to Ohms and check again

[2deaf]

I got .536k ohms

Sounds about right for the 470R resistor in series with the transformer winding.

[mresistor]

disconnect the lead going to the trem circuit

[tommyjr98@gmail.com]

I have not changed any of the PT wiring since it was in a working state.  But to answer you question, I do not know whether it is the high tension winding or lower voltage bias supply winding.

So it was previously working? And you replaced the bias supply components and now its not working? (If this is the case, then you need to look carefully at your soldering. Look for unintentional shorts/resistor/cap/diode leads touching other parts of the circuit/ground, or excessive solder blobs touching other parts/the chassis)

It was not working before I replaced the components.  That was my first attempt to fix the problem which was red plating in the power tubes after closing the standby switch.  No redplating with only the power switch closed.  The excessive solder blobs caused the problem in the first place I believe.  I was reflowing solder joints to eliminate crackling noise.  I don't have a scope so I just reflowed solder joints.  What happened I believe is a blob on the underside of an eyelet went through the coating of a wire under the board. Total amateur mistake on my part.  The result was an arcing rectifier tube and blown fuses.  I found the wire underneath and eliminated what I believed to be the short.  Replaced the rectifier and no more arcing or fuse blowing but I still got tube red plating. Then I changed out the bias parts and now that resistor is smoking.  I probably should have mentioned the whole story in the first place.  Sorry for any confusion!

[tommyjr98@gmail.com]

I want the resistance to ground from the junction of the 470R resistor and the diode.

I got .536k ohms

The meter was set to auto range.  I manually tried ohms but got OL.


Set your meter to Ohms and check again

[tommyjr98@gmail.com]

disconnect the lead going to the trem circuit

...and see if that stops the resistor from overheating?

[mresistor]

yes  but after reading your last post ..  it could be a short in the areas your reflowed.     I always use  solder wick and plenty of paste flux to suck up all the old solder before I resolder the joint..I dip my wick into the flux to make sure it has some on it ..  because of exactly what you stated  ..  solder blobs underneath the board.     can you lift  the board to see if there is any remaining solder that is making contact with the chassis?   and also lift the lead to the trem circuit.   to totally isolate the bias circuit .. 

[tommyjr98@gmail.com]

yes  but after reading your last post ..  it could be a short in the areas your reflowed.     I always use  solder wick and plenty of paste flux to suck up all the old solder before I resolder the joint..I dip my wick into the flux to make sure it has some on it ..  because of exactly what you stated  ..  solder blobs underneath the board.     can you lift  the board to see if there is any remaining solder that is making contact with the chassis?   and also lift the lead to the trem circuit.   to totally isolate the bias circuit ..

I did as you suggested.  The lead to the tremolo was lifted as well as the lead from the bias to the 220k grid resistors.  The bias resistor still heats up and now the bias cap just blew up.  Hmmmm.....

[2deaf]

The bias resistor still heats up and now the bias cap just blew up.  Hmmmm.....

Hmmmm, over voltage and reverse voltage will do that.  In both cases the leakage current takes a dramatic rise before the vent blows out.  Significant reverse voltage usually pops them rather quickly whereas over voltage usually smolders for awhile first.

320V (AC?) and the resistance reading suggest that the bias circuit is connected to the HT portion of the winding.  If so, the 470R resistor is way too small.   

[tubeswell]

320V (AC?) and the resistance reading suggest that the bias circuit is connected to the HT portion of the winding.  If so, the 470R resistor is way too small.

ditto. For bias supply sources from High Tension windings, you want a resistor more in the order of 100k (not 470R) - as per the Fender BF Princeton reverb schematic

[DummyLoad]

draw a schematic of what you actually have. not a post "something it looks like", that would help us help you immensely. you don't need to reverse engineer the whole amp, just the bias circuit from transformer connection (which winding - e.g., bias tap, HV, etc.) to the the output tubes.


--pete

[tommyjr98@gmail.com]

The bias resistor still heats up and now the bias cap just blew up.  Hmmmm.....

Hmmmm, over voltage and reverse voltage will do that.  In both cases the leakage current takes a dramatic rise before the vent blows out.  Significant reverse voltage usually pops them rather quickly whereas over voltage usually smolders for awhile first.

320V (AC?) and the resistance reading suggest that the bias circuit is connected to the HT portion of the winding.  If so, the 470R resistor is way too small.

You're right about the components.  In my haste, I replaced the original 47k with a 470 ohm resistor.  Doh!  I also replaced the original 33u100v cap with the stock AB763 value of 22u50v.  The original 33u100v cap measured low around 18u out of the circuit.  That may have been the cause of the red plating.  I'll have to procure another cap as I do already have the resistor.  When I get the parts put it, I'll update the post. 

Lesson learned.  I need to slow down.  Also, not having much experience with transformers and which windings are which, I foolishly assumed (the old saying is ringing in my head) that the transformer wiring was the same as an AB763.  I'll make a point to learn more about transformer wiring's.  Thank you for the assistance!

[tommyjr98@gmail.com]

draw a schematic of what you actually have. not a post "something it looks like", that would help us help you immensely. you don't need to reverse engineer the whole amp, just the bias circuit from transformer connection (which winding - e.g., bias tap, HV, etc.) to the the output tubes.


--pete

I will attempt to draw one on my lunch break to day.  You're right about about identifying what it actually is rather than what it looks like.  That would be much more helpful. 

[mresistor]

Some clear well lighted photos would be good to post too. A pic of the power supply end so we could see the whole circuit in that area.. also the preamp if you want to.

[tommyjr98@gmail.com]

draw a schematic of what you actually have. not a post "something it looks like", that would help us help you immensely. you don't need to reverse engineer the whole amp, just the bias circuit from transformer connection (which winding - e.g., bias tap, HV, etc.) to the the output tubes.


--pete

I will attempt to draw one on my lunch break to day.  You're right about about identifying what it actually is rather than what it looks like.  That would be much more helpful.

Attached is the schematic.  This is how it came when I purchased the amp used before I did anything to it.

[PRR]

> Some clear well lighted photos would be good

Also don't fat-crayon the drawing. The rectifier is a key detail. Your image has it covered-up. I didn't see it at first. When I asked "is that a rectifier under there?" I was not sure which way it was pointing.

As far as I can tell from incomplete information, the visible wiring is right, but wires run off in different directions?? and the solder is so blobby that there can be both bad (tarnish) joints and solder-drips.

You can keep buying new tubes, or calm-down and re-build this section carefully, checking as you go.

[tommyjr98@gmail.com]

> Some clear well lighted photos would be good

Also don't fat-crayon the drawing. The rectifier is a key detail. Your image has it covered-up. I didn't see it at first. When I asked "is that a rectifier under there?" I was not sure which way it was pointing.

As far as I can tell from incomplete information, the visible wiring is right, but wires run off in different directions?? and the solder is so blobby that there can be both bad (tarnish) joints and solder-drips.

You can keep buying new tubes, or calm-down and re-build this section carefully, checking as you go.

Here is the power supply section.

[tommyjr98@gmail.com]

Some clear well lighted photos would be good to post too. A pic of the power supply end so we could see the whole circuit in that area.. also the preamp if you want to.

Here is the Preamp section.  A few wires are lifted from testing. 

[sluckey]

Attached is the schematic.  This is how it came when I purchased the amp used before I did anything to it.

Well, make it look like that again.

[tommyjr98@gmail.com]

Attached is the schematic.  This is how it came when I purchased the amp used before I did anything to it.

Well, make it look like that again.

Agreed.

[mresistor]

interesting he's using a Vactrol optocoupler for the tremolo..

[tommyjr98@gmail.com]

interesting he's using a Vactrol optocoupler for the tremolo..

Now you have piqued my curiosity.  Please elaborate...

[mresistor]

Actually I can't elaborate or tell you much as I would also like to know how the circuit is setup to use the Vactrol optocoupler.    Most fender circuits use the roach  which is a neon bulb coupled to a light sensitive resistor,,  and I make my own trem "roach" using a superbright led and a light sensitive resistor but this amp is the first I've seen using a Vactrol optocouler.     

[Pietro]

Personally Tommy...my advice would be to call someone in locally to do some serious re-wiring.
I.m not the most neat worker in the world but this amp has been worked on by someone who should.nt have.

Maybe someone here is living in your area?

[labb]

This is a shot in the dark...Some one has the bias circuit really messed up. I think that the bias range resistor (your 47k)needs to be something like 220k. The bias pot needs to be about 50K. You are probably going to need about -40 volts for the bias voltage for the B+ the amp is running at... How about one of you Marshall circuit folks chiming in.