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Post subject: Need Help From You Vintage Amp Guys
Posted: Wed Sep 05, 2012 10:22 pm
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Rock Star
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Hey, Guys.

I know most of you stick to the vintage amp forum and don't venture much to the modern amp side of the house. But I am troubleshooting a bias problem with my Blues Deluxe Reissue that has both Shimmilou and I at our wits end. Please take a look here:

viewtopic.php?f=13&t=74557

The thread started out as a power tube shorting out and taking out a screen grid resistor and has morphed into an investigation into why I cannot bias my amp any higher than a little over 50% max plate dissipation. I appear to have a problem with low power supply voltage, but I have measured everything that Shimmilou and I can think of and cannot figure out what is going on. We are both banging our heads into the wall and I am at the point of delaring "it is what it is". :lol:

The thread is about 4 pages long right now with lots of measurement detail (I know you guys love that), so please come on over and join the fun. Whoever can figure out what is going on will win the grand prize of my undying gratitude. So please, "come on down" to

viewtopic.php?f=13&t=74557

See you there. :wink:

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 1:57 am
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Blue, I've had similar issues with some Peavey amps and modern PCB hi-fi amps. On three out of five--- I had good bias voltage. Good anode plate voltage... BUT hinky contact from the cathode (pin 8, of a 6L6GC) to ground. The chassis material doesn't seem as conducive to a good solder like the good old steel plates Leo used,

What I did what construct a copper rod (like 16-18 gauge) raised above the chassis. Solder all output cathodes to this rod. Then bolt the copper rod to one center point on the chassis. Then, epoxy it to prevent loosening.

Show you some photos, tomorrow--- when I get off of working in Paradise.

First, be sure all 6L6GC are operating in spec. I bias them in a KNOWN good test bed. Like a well working SF Bandrmaster. If they passed the muster, may want to consider this raised star-point.

Good luck!


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 5:46 am
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BMW,

Interesting thoughts, but ....

BMW2002Ti wrote:
Blue, I've had similar issues with some Peavey amps and modern PCB hi-fi amps. On three out of five--- I had good bias voltage. Good anode plate voltage... BUT hinky contact from the cathode (pin 8, of a 6L6GC) to ground. The chassis material doesn't seem as conducive to a good solder like the good old steel plates Leo used.


The cathodes of the power tubes in the BDRI are not connected directly to ground. A 1 ohm, 5% tolerance resistor (in parallel with a protection diode) is connected between the two cathodes and ground for use as a bias measurement test point. I have already verified that this path is solid but will go back and check it again to be sure. I have considered removing the 1 ohm resistor and diode and just grounding the cathodes since I use a bias probe with precision 1% tolerance 1 ohm resistors. I haven't done so since that would then require any future owner to have a bias probe. However, I could just simply jumper the resistor and diode with an easily removable wire from the cathodes to ground. I think I will try that tonight to see if it makes an difference.

BMW2002Ti wrote:
What I did what construct a copper rod (like 16-18 gauge) raised above the chassis. Solder all output cathodes to this rod. Then bolt the copper rod to one center point on the chassis. Then, epoxy it to prevent loosening.!/quote]

No place to install such a rod in the BDRI unless the chassis is completely gutted and rebuilt. That is not something I plan on undertaking. Based on the fact that the amp is dead quiet (no hum, hiss, or other noise), I don't really think it is a grounding problem. See my comment above about the cathodes of the power tubes.

BMW2002Ti wrote:
Show you some photos, tomorrow--- when I get off of working in Paradise.


I would definitely be interested in seeing these. Might give me some other ideas.

BMW2002Ti wrote:
First, be sure all 6L6GC are operating in spec. I bias them in a KNOWN good test bed. Like a well working SF Bandrmaster. If they passed the muster, may want to consider this raised star-point.


Well, I could check them in my 5F6A clone (I am able to bias that amp at 70% or hotter), but this behavior has been exhibited since the amp was new and running the stock GT 6L6Rs (Sovtek 5881 WXTs), the first set of JJ 6L6GCs I ran in the amp (the ones where a tube shorted), and the replacement set of JJ 6L6GCs. I don't think it is a tube issue.

BMW2002Ti wrote:
Good luck!


Thanks. Please read through the other thread for reference. I have performed multiple tests and measurements to try and get to the bottom of this with no luck. As I have noted in the other thread, the amp's performance is excellent: dead quiet with no hum or hiss and excellent tone. This issue has bugged me almost since day one and I would like to solve it without a major rebuild of the amp.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 8:40 am
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" A 1 ohm, 5% tolerance resistor. "

Blue, I'd go for a 1 or 10-ohm, 1% tolerance resistor. !-watt, heat resistant. I like the Mills WW in this crtical postion. Both for accuracy and stability in tolerance of er time & heat. If you use a 10-ohm resistor, just divide the mV you measure by 10.

http://www.partsconnexion.com/resistors_mills_mra5.html

Part Number: MILLS-64257

CC and MO resistors drift around, like any other. Affecting your bias readings, as much as drifting current flow in the output tube.

I star-ground them to a single point. Secure with a raised copper buss--- if you can. You'll get a much more consistent bias reading, IMHO.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 2:00 pm
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BMW,

Please read the thread. The problem is low power supply voltage, not drifting bias.

I don't need to change the 1 ohm resistor. I am going to bypass it. I use a bias probe that measures cathode current and plate voltage using 1 ohm, 1% resistors.

Nice amp. Would not be easy to implement that type of ground in the BDRI:

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 5:41 pm
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Can anybody help?

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 5:45 pm
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Sorry you're having so much trouble Bill, wish I had a solution for you, but this is not my realm of expertise. I really hope you get it solved on your own, and don't have to break down and bring it in for repairs.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 5:53 pm
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Arnold Layne wrote:
Sorry you're having so much trouble Bill, wish I had a solution for you, but this is not my realm of expertise. I really hope you get it solved on your own, and don't have to break down and bring it in for repairs.


I have no intention of letting anyone else touch it. The thing is, it works fine and sounds great. It just will not bias to where I want it becasue of low DC voltages. But I cannot find any reason for them to be low.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 6:20 pm
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Blue, Sorry about the mix-up. NOW... -41VDC reading from a point that should measure -51VDC is TOO MUCH voltage. Or too high idle bias. The more negative the votlage of the bias supply, the colder the bias point.

" I then reflowed the solder on the pins where they attach to the circuit board and measured the resistance between the solder point and the metal pin connectors inside the spocket. All measured zero resistance.

I also tried to tighten the connectors inside the socket. These are different from the ones in the sockets of my 5F6A. The BDRI connectors are like little two prong clamps as opposed to the rolled connector in the 5F6A socket. However, even after attempting to tighten the connectors with a pointed metal tool, I still cannot read any voltage at the Fender bias test point with only V5 installed.

I have verified the crcuit path between the tube socket and the Bias test point but cannot get a voltage reading. I am 99.999% sure that the problem is the inside connector for pin 5 or pin 8. How do I tighten these? What am I missing? What do I do next? "


You may need to try new sockets. But first, have you tried those 6L6GC (Or 5881) in a known, good amp? See if they hold a tight idle point? [no input, speakers connected, zero volume(s)] I have had kinky brand new tubes, before.

is the OPT's secondary is well grounded? Are the points from the global feedback ok? You can bypass this GNFB circuit, while hunting down the culprit.

If all seems ok... I'd breadboard the amp's questionable wiring. Use jumpers with good alligator clips. And bypass points, to see if the amp start behaving.

Good luck! HTH!


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 6:32 pm
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:shock: You guys have been playing with Guitar amps for many moons huh.... 8) ....Biasing an amp is what i talk to my Amp guy about.. :oops: ...He is about to fit a Bias Switch to my BD, Seeing as it didn't have one standard making him able to Bias it correctly to the JJ millitry spec matched tubes he swares by....Sorry for the Hijack... :roll: ... but does he sound like he knows his stuff??

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 6:37 pm
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BMW,

I'm sorry. I should have been more specific. The problem you are referring to in blue was solved by the end of page 1. V5 had shorted internally and had taken out the screen grid resistor. That has all been fixed.

The problem I am dealing with now is the inability to bias the amp hotter than about 55% max plate dissipation. That problem begins at the bottom of page 2 and is time stamped 01 Sep 2012 23:14. Also, as I stated earlier, this problem occurs no matter which set of power tubes I have installed. Three sets of tubes exhibit the same behavior. It is not a tube problem. It is a low power supply voltage problem that I cannot find the cause of. I have documented all of the measurements and changes I have made in the amp on pages 2 through 5.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 6:38 pm
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ozrv wrote:
:shock: You guys have been playing with Guitar amps for many moons huh.... 8) ....Biasing an amp is what i talk to my Amp guy about.. :oops: ...He is about to fit a Bias Switch to my BD, Seeing as it didn't have one standard making him able to Bias it correctly to the JJ millitry spec matched tubes he swares by....Sorry for the Hijack... :roll: ... but does he sound like he knows his stuff??


Please start your own thread. I am working very hard to try and get this problem solved. Thanks.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 7:33 pm
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Blue,

Have you tried a UF4006 in place of the current diode of the bias supply? And you must play withthe dropping resistors to get -51VDC at the reading point. Or the range of the pot will not be great enough to bias the tube (if you have a bias pot).

If you have -51VDC at the point. And a good working pot. The only other cause of hinky, unstable bias currents: Are: 1.) bad output tubes, 2.) bad socket. 3.) bad connection from cathode to ground. 4.) maybe bad OPT secondary to ground. 5.) Funky global feedback network. (just disconnect this circuit off the secondary to ruled it-out).

Remove the phase inverter when biasing, to rule-out issues upstream.

If you can, jump the line going from the bias power supply to the tubes. The issue maybe in the line setup.

Good luck!


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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Thu Sep 06, 2012 10:58 pm
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Guys: PLEASE. Read the thread. I do not believe the problem is in the bias circuit. I have checked all those components and they are all good. The bias circuit in my BDRI is not what is shown in the BDRI schematic. It is the same configuration as the Hot Rod Deluxe III. I have documented all of this in the other thread. The problem is that all of the power supply voltages are dropping to levels below their spec'd value. At the factory bias setting of 30 mA per tube, the plate voltage and B+ are 20 volts lower than what it is supposed to be. At 40 mA per tube, the plate voltage drops to around 405 VDC. Even if I could get the cathode current higher, the plate voltage would drop even more. I have documented all of the component and voltage measurements that I have taken in the other thread. If my thinking is wrong, please explain why. Thanks.

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Post subject: Re: Need Help From You Vintage Amp Guys
Posted: Fri Sep 07, 2012 1:17 am
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Blue, think you have some kinda parasitic oscillation going on in the power section?

If you, try ferret beads around some suspect wires and see what happens.


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