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Post subject: Need some amp advice.
Posted: Sat Nov 12, 2011 5:42 pm
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I think I have a problem with my '66 BF Pro Reverb,a couple of days ago it blew a fuse when I tuned it from standby to on,I pulled the power tubes,installed a new fuse and turned it back on,no problem so I installed the tubes back in one at a time and it was fine,so I've played it several times since.
Today,same thing,fuse blows when the standby is flipped on....so I go through the same routine,no problems,but I decided to try a different set a 6L6GCs and rebias....I found the plate voltage to be around 495vdc,so I pulled the 5AR4 rectifier and put a new one in and still high voltage,the notes I have from the last bias measurements show that it was in the 430vdc range with 33/32.5 mA.
Is there a problem in the power supply?


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Post subject: Re: Need some amp advice.
Posted: Sat Nov 12, 2011 6:53 pm
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Rebelsoul, try measuring the outlet voltage. You maybe running much higher than usual.


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Post subject: Re: Need some amp advice.
Posted: Sat Nov 12, 2011 7:20 pm
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Wall outlet is 126.6 vac,which is higher,usually it's around 125volts.
I've just never seen that much on the plates in that amp....almost 500 vdc.


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Post subject: Re: Need some amp advice.
Posted: Sat Nov 12, 2011 7:32 pm
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What are your bias readings with this plate voltage of 495VDC? Hopefully, still in the mid 30mV (= mid-30mA per tube).

Are all the gain stage tubes plugged in? And all the effects sections working, as usual?


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Post subject: Re: Need some amp advice.
Posted: Sat Nov 12, 2011 10:51 pm
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Check the incoming voltage at pins 4 and 6 of your rectifier tube -- for an AA165 circuit you should see 340VAC at each pin. If it's significantly higher, I suspect a problem with the power tranny.

HTH

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Need some amp advice.
Posted: Sat Nov 12, 2011 11:56 pm
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I'll spend some time on it Sunday...we'll track it down...the bias read in the low 32mA but the pot wouldn't let me drop it lower,I played it for a short time,the tubes didn't cherry plate and it sounds great...I'll check the voltage like you said Arjay and see what I get.
It hasn't gotten a lot of playing time the last few months,only lately,so this might have just been something waiting to surface with some regular use,I don't know....we'll see.
Thanks guys.


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Post subject: Re: Need some amp advice.
Posted: Sun Nov 13, 2011 3:20 am
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32 ma is good for bias.

You could have a bad contact in the 6L6 's socket. Try to clean them and tightened them too.

When you remove your tube and put it back it clean ( little bit ) socket contacts, but not enough for working long time.
Check for solder on 6L6's sockets and the 470 ohms .


If one of your 6L6 is in and out ( with bad contact ) this cause fuse blow.


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Post subject: Re: Need some amp advice.
Posted: Sun Nov 13, 2011 11:23 am
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Retroverbial wrote:
Check the incoming voltage at pins 4 and 6 of your rectifier tube -- for an AA165 circuit you should see 340VAC at each pin. If it's significantly higher, I suspect a problem with the power tranny.

HTH

Arjay

This could be the source of the problem....on pin 4 there's 390.8 vac....pin 6, reads 388.7.
Much higher than the schematic shows....these readings were taken plugged into the wall...I failed to mention that both times when the fuses blew that it was plugged into a box that reduces line voltage by 6/12 vac,depending on what position the switch is in,which in this case was around 119-120vac.


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Post subject: Re: Need some amp advice.
Posted: Sun Nov 13, 2011 11:41 am
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Rebelsoul wrote:
Retroverbial wrote:
Check the incoming voltage at pins 4 and 6 of your rectifier tube -- for an AA165 circuit you should see 340VAC at each pin. If it's significantly higher, I suspect a problem with the power tranny.

HTH

Arjay

This could be the source of the problem....on pin 4 there's 390.8 vac....pin 6, reads 388.7.
Much higher than the schematic shows....these readings were taken plugged into the wall...I failed to mention that both times when the fuses blew that it was plugged into a box that reduces line voltage by 6/12 vac,depending on what position the switch is in,which in this case was around 119-120vac.


I don't think the problem is there.
For information, on vintage amp schematic , the voltage is for a wall outlet of 110 ac , today we have 120 ac and more.


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Post subject: Re: Need some amp advice.
Posted: Sun Nov 13, 2011 11:52 am
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Rebelsoul wrote:
Retroverbial wrote:
Check the incoming voltage at pins 4 and 6 of your rectifier tube -- for an AA165 circuit you should see 340VAC at each pin. If it's significantly higher, I suspect a problem with the power tranny.

HTH

Arjay

This could be the source of the problem....on pin 4 there's 390.8 vac....pin 6, reads 388.7.
Much higher than the schematic shows....these readings were taken plugged into the wall...I failed to mention that both times when the fuses blew that it was plugged into a box that reduces line voltage by 6/12 vac,depending on what position the switch is in,which in this case was around 119-120vac.


Indeed......fifty volts higher than spec.

I'm surprised it didn't fry your GZ34!

Iterating, I still suspect the PT.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Need some amp advice.
Posted: Sun Nov 13, 2011 11:58 am
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Rebelsoul,

May want to measure voltages off the power supply B+ near the two 70mfd/350 mfd caps. And the resistance of the two 220K-ohm, 1-watt equalizing resistance across those caps.

And check the choke TR2 for high resistance. This could shift more voltage to the power tubes.

And the actual value of the 1K-ohm 1-watt, 4700 ohm 1-watt dropping resistors that supply the gain stage tubes. I find it these dropping resistor drift too high... less voltage goes to the gain stage and there could be a shift to more voltage to the power tubes.

And how old are the filter caps in the power supply?

In other words, a thorough check of the power supply. Including the VAC measured off the secondary wires of the PT.


Good luck!


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


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Post subject: Re: Need some amp advice.
Posted: Sun Nov 13, 2011 6:24 pm
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Thanks guys...I have only owned this amp for about three years and the filter caps were replaced before I bought it,so I don't know how long ago that was...most everything else is original so a good thorough going over is in order,it's a great amp so I need to get it back into shape no matter what it needs...this has only just happened because I put some Winged C 6L6GCs in it a couple of months ago and the plate voltage was normal.


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Post subject: Re: Need some amp advice.
Posted: Sun Nov 13, 2011 7:18 pm
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" the bias read in the low 32mA but the pot wouldn't let me drop it lower. "


Ok, sounds like you may have a one or both tubes drawing too much current. First clean the sockets and pins. And you may need to tighten the socket tangs that hold the pins. Unplug amp & drain main filter caps first. I use a Dental probe to tighten the tangs.

Now, before you replace the power tubes, check the negative voltage off the bias supply. Should be around a STEADY (-51VDC) at the Bias Pot or the branch point of the two 220k-ohm grid-to-ground resistors (see schematic). Anything way off or drifting voltage could mean you need to work on that Bias Power supply section (new diode, pot, resistors, cap).

Good luck! Keep us informed.



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


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Post subject: Re: Need some amp advice.
Posted: Mon Nov 14, 2011 6:19 am
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I'll do that BMW,I'm interested in seeing what that reads,it seemed that the bias pot had very little adjustment from the bottom,I didn't look at it when I pulled the chassis.
I appreciate the advice from y'all...I know how to work safely inside the chassis,and know how to do some things but I'm still learning the more techinical things....replacing components is fairly simple when you've figured out the problem...that troubleshooting can get complicated. :wink:


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Post subject: Re: Need some amp advice.
Posted: Mon Nov 14, 2011 7:56 am
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Rebel, Good luck with all that. Usually the bias point of the tubes should be about half-way in the range of the bias pot. When you need to crank the thing full counterwise or full-clockwise to get the correct range---either there's something going on with the tube (drawing too much current-- full counter. Or dying tubes-- full clock).

Or the Bias Supply isn't supplying correct negative voltage with pot at midrange position. At least that is my experience.

Shorted power tubes or rectifier may alter the overall B+ voltage (power tube) or PIV (rectifier), as its being a hog sucking up mostly current. But, if you have the bias turned fully down, for the power tubes and they are current hogs--- the tubes may change the voltage draw to compensate. It's really the WATTAGE they want. And (watts) = (Voltage) times (current). So you get the idea.


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