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Post subject: Biasing an Original Blues Deluxe Post-Amp Tech Pot Install
Posted: Tue Jun 18, 2013 6:47 am
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Hi. The original Blues Deluxe does not have a bias pot so I asked a respected technician to fit one for me a few months ago. I think he just added a pot at resistor 87. Then he marked in black pen (dots) reading points at the power tube solder points. His instructions to me follow the pics:

Image

Image

His instructions:

- Amp needs to be on and warmed up
- Reading should be -50V (he insisted it was not mV and was definitely minus) not more than +/- 5V
- Attach the black probe to the ground plate
- Attach/touch the red probe to either of the two marked (black circle) solder points above the power tubes to get the reading
- Adjust the bias pot with a screwdriver as required
- Check the bias reading at both solder points to see how the tubes compare - they should be within 1V
- The amp is running with colder with a higher bias e.g. 55 which is better for tubes, a bias 45 would mean it's running hotter etc


The problem is I don't understand what he has actually done and what I am actually reading?

I have read statements such as these trying to find more information:

"My BDRI (using JJ 6L6GCs) is biased at 38 mA cathode current (approximately 36 mA plate current) at a plate voltage of 413 VDC (about 85 mV at the Fender test point)"

"I have mine biased at 60 mv at the test port"

"That corresponds to about 30 mA per tube cathode current and about 28 mA plate current. You need to know what the plate voltage is also to calculate the % of maximum plate dissipation. I run my JJ 6L6GCs at 40 mA cathode current/38 mA plate current at 410 VDC plate voltage. That corresponds to 53% maximum plate dissipation (which is still cold but sounds good) and measures as 88 mV at the Fender test point. You need to reread my post. 80 mV by itself means nothing. You need to measure the plate voltage also to calculate the % of maximum plate dissipation at idle"


My questions are:

- Why is he telling me -V, everyone else seems to discuss mV?
- What are cathode current, plate current, and plate voltage - and how are they measured individually?
- What is plate dissipation at idle and how is it calculated?
- How do these figures interact and what are ideal readings/percentages?

PS his instructions were as stated because I wrote them down. Any information appreciated. Thanks :)


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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Tue Jun 18, 2013 9:11 pm
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Hi sidepartings26,

Well, that's kind of a crap shoot as to exactly where the amp is biased. The voltage reading on either black dot is pin 5 of that output tube. This will be the negative voltage applied the tube's control grid. This negative voltage will control the idle current through the tube, but the current through the tube will be directly dependent on the tube rating. So, the same negative voltage applied to pin 5 can result in vastly different idle current for different tubes.

When we refer to mV or mA, we are referring to the actual current through the tube (Plate current or Cathode current), because it doesn't matter the negative voltage applied to pin 5, we need to know the tube current.

The only time that your instructions will be somewhat meaningful is if the tech biased your tubes to the proper range when he installed them, and if you always use these same exact tubes, with the same exact rating when replacing. And, if this is the case, then no adjustment will be necessary in the first place, which renders the instructions moot.

This scenario is the perfect definition of half-assed. He did what looks like a great job installing the bias pot, then left you in the dark about properly using the pot to bias your tubes to the correct range.

As you referenced in your post (looks like you cited an excellent source, bluesky636), you need to know the Plate voltage, and the current through the tubes to know exactly how your tubes are biased. For purposes of biasing many types of tubes, the negative voltage applied to the grid is meaningless and we rarely even reference it here on this forum.

Search the forum for posts about "Biasing", and if you don't find enough info to keep you reading for many days, come back here and ask any specific questions that you might have. I better stop for now, I just wanted to respond quickly in order to beat bluesky636 to your post. :lol: (inside joke)

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jun 19, 2013 3:55 am
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OK I get you. I had to review the post several times but I know what I need to research; I think.

Is the pot useless then given how non-critical you think the -V applied is, or, is it actually useful in that once I know how to figure out tube current and plate voltage I can control idle current with the pot?

In other words, do I at least after this install, have the tool I need to correctly bias the amplifier myself with various tubes/scenarios - once my knowledge catches up?

And, are these all measurements which can be made easily with a multimeter and basic equipment etc..

He is a lovely guy but I did the get the feeling he was keeping me in the dark for the good of his future revenue stream...

Thanks. :)


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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jun 19, 2013 4:37 am
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Oh no, the bias pot is exactly what you need to be able to easily adjust your bias for different output tubes. The bias pot does adjust the negative voltage at pin 5, just as your guy told you. It's just that knowing the voltage at pin 5 really means nothing, because it doesn't tell you the actual current through the output tubes.

Now, you need a multimeter to measure the Plate voltage at the tubes (pin 3) and some way to monitor the current through the tubes, such as a bias probe(s). Measure the Plate voltage, then while monitoring the current, you adjust the bias pot to get the desired current through the tubes.

Basically, the object is to bias the tubes to a desired range of idle wattage, which means the wattage at which the tubes are idling, when not playing. A typical range for idle is between 50% and 70% of the tubes' max Plate dissipation rating. In the case of most 6L6 tubes, the max rating is 30 watts (each tube), so the idle range will be between 15 watts and 21 watts per tube. The wattage is determined by multiplying the Plate voltage and the Plate current, and Cathode current can be substituted for Plate current. Most bias probes will measure Cathode current.

In the case of your amp, the Plate voltage should be about 430 volts with properly biased 6L6 output tubes. So, let's try to set the idle to 60% of the tubes' max rating, about 18 watts (30 x 0.6). Since watts equals current times voltage, we divide 18 watts by 430 volts to get approx 0.042 amps of current, or 42 mA. With bias probes in place, after measuring the Plate voltage, you would then try to adjust the bias pot until the bias probe reads 42 mA per tube. The Plate voltage will change slightly with bias pot adjustment, so measure Plate voltage and current, then recalculate to get the desired setting.

It is important to measure the Plate voltage because it can sometimes be different than what is expected. Most bias probes and multimeters will come with instructions for their use. A bias probe is pretty simple, consisting of a tube socket and some leads coming out of the socket. The probe is inserted between the tube and the amp, and most probes will connect to a multimeter and read DC mV which can be directly translated to DC mA.

There is a more complicated way to indirectly measure the current without using a bias probe, but involves more measuring and math, while the bias probe simplifies the process greatly.

There are many types of bias probes, and I use one that has it's own meter and digital display, the "Quadstage BiasPro MQ10" from A-Sharp Fretworks. Other forum members can recommend their preferred probes also. Any inexpensive multimeter that can measure from mV up to 600 VDC can be used.

http://www.asharpfretworks.com/5543.html

Almost forgot:
Terms like Plate, Cathode, grid, heater, are internal parts of a tube, each connected to certain pins on the tube base, and the base is the part of the tube that plugs into an amp's tube socket. You can search the internet for datasheets for many types of tubes, and these sheets will usually have a drawing of the tube's internal parts and the pin numbers to which these parts are connected. Check the schematic for your amp, and you can see the pin numbers for the tubes are marked there.

http://support.fender.com/schematics/guitar_amplifiers/Blues_Deluxe_schematic.pdf

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jun 19, 2013 6:37 am
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Or just get one of these :wink:
http://taweber.powweb.com/biasrite/br_page.htm

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jun 19, 2013 8:09 am
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Thanks Shimmilou, just popped back to edit my post to say that I now realise I need a probe, I was looking at the Eurotubes probes earlier... Anyhow I then read your absolutely brilliant answer - thanks so much for that. Your last note about the various terms referring to tube characteristics e.g. 'plate voltage' was very helpful as I imagined it would be something to do with the chassis ha ha!. I'll review this information for sure and go from here!


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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Thu Jun 20, 2013 8:34 am
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I have looked at a few bias probes now. The A-Sharp fretworks site mentions a separate 'companion plate voltage probe' - does the Q10 or MQ10 not measure plate voltage? I would be looking for something that measures plate voltage as well as cathode current...

The 'Amp-Head Dual MPD' looks pretty good - I noticed Bluesky636 mentioning it in my searches.

Something around $60-$80 is sort of my maximum and I don't mind using a multimeter.

Any suggestions?

I spoke to my tech today actually and he said he thought he put in 1 Ohm resistors. Meaning, I could use the cathode resistor method. He didn't. I've looked into it, it's a lot to take in, for me at least, I read this article on it http://www.duncanamps.com/technical/lvbias.html which is pretty good. I'm handy soldering but if I was taking this route I'd like a picture at least of the job done to work with! (apparently leaving the 1 Ohm resistor in has little effect). That method measures cathode current right, and a separate solution is required to measure plate voltage - or is it just a case of a multimeter black to ground (chassis) and red to pin 3 (which I've read can be dangerous)?

Then again, the bias probe would be useful in various scenarios and I don't have a lot of time to research...

Any ideas?


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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Thu Jun 20, 2013 1:13 pm
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Yes, you need the additional Plate voltage probe if you want to measure Plate voltage with the MQ10. I just use a multimeter and measure Plate voltage at the OT terminals. It is always dangerous working on a live amp, there is high voltage present, and measuring at the tube socket connections can be risky because the meter lead can slip and short across two pins. But yes, measure the Plate voltage with the multimeter black to ground (chassis) and red to pin 3, but instead of pin 3 on the socket connection, it's better/safer to measure at the OT (output transformer) terminal next to it on the circuit board.

The one-ohm resistor is what is inside the tube socket of a bias probe, and is in series with the Cathode of the tube to measure current by measuring the mV across the resistor. It's important to use a precision resistor, such as 1% tolerance.

The main reasons that I like the MQ10 are that it is very well built, very accurate, and great customer support. The probes are detachable and I can use the 8-pin, 9-pin and 7591 probes with the same meter by simply unplugging one type and plugging in another type. If a probe fails, I can unplug it and get a new one without having to open it up and unsolder and re-solder a new probe. The capability of using up to 4 probes at the same time is a plus for those amps with 4 output tubes. Also, separate probes are listed for sale should I need one. Having the resistor in the socket is better/more accurate than the probes with the resistor at the other end of the probe leads. The built in meter to measure the current is nice. Of course I use my probes a lot for many different amps, your needs might be different.

The Weber looks nice also, or the Amp Head, or Eurotubes,........so many choices! :)

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jul 31, 2013 1:15 pm
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shimmilou wrote:
Basically, the object is to bias the tubes to a desired range of idle wattage, which means the wattage at which the tubes are idling, when not playing. A typical range for idle is between 50% and 70% of the tubes' max Plate dissipation rating. In the case of most 6L6 tubes, the max rating is 30 watts (each tube), so the idle range will be between 15 watts and 21 watts per tube. The wattage is determined by multiplying the Plate voltage and the Plate current, and Cathode current can be substituted for Plate current. Most bias probes will measure Cathode current.

In the case of your amp, the Plate voltage should be about 430 volts with properly biased 6L6 output tubes. So, let's try to set the idle to 60% of the tubes' max rating, about 18 watts (30 x 0.6). Since watts equals current times voltage, we divide 18 watts by 430 volts to get approx 0.042 amps of current, or 42 mA. With bias probes in place, after measuring the Plate voltage, you would then try to adjust the bias pot until the bias probe reads 42 mA per tube. The Plate voltage will change slightly with bias pot adjustment, so measure Plate voltage and current, then recalculate to get the desired setting.

It is important to measure the Plate voltage because it can sometimes be different than what is expected. Most bias probes and multimeters will come with instructions for their use. A bias probe is pretty simple, consisting of a tube socket and some leads coming out of the socket. The probe is inserted between the tube and the amp, and most probes will connect to a multimeter and read DC mV which can be directly translated to DC mA.


Using an Amp-Head dual bias I measured plate voltage at 441V. That little pot the tech installed let me take current up to 44mA but no further and not only has the plastic inside which allows turning pretty much disintegrated but the little blob stuff has come off. I've figured out how to drain the capacitors in the amp (straight from on to off with warm tubes and not using standby) and confirmed with the multimeter the voltage left as well as bravely touched the power section! The point being that I think the pot needs re-done and am happy to do it. I know 44mA is up there but a better job is needed; providing more headroom too.

Any suggestions as to a better pot and rating. And, a better way to install it e.g. drilling a hole in the chassis?

Edit:

"Looking at the schematic the bias resistor should be R87 (27k). Depending on the idle current and plate voltage, what value pot to use is subjective. First off, to install a bias pot....remove R87, jump a long wire from one hole of R87 to the wipe (center leg) and the right leg (if looking from the back of the pot with the lugs at top view) of the bias pot, yes you connect both legs to the same wire. This turns the pot into a variable resistor. The other hole gets a long wire soldered in, and goes to the unused leg of the bias pot (left leg) with a resistor in series to this leg of the pot. I would use a 10k linear pot (Fender has a great one for this) and a 15k - 22k 1/2 watt resistor. Depending on where you want the sweep to be in terms of voltage and current will help determine the appropriate resistor to run series (15k-22k 1/2watt to start with)"

This looks like a decent solution except the resistor is not specified. Currently it's 16k that the tech put in. What would be better?

Also the 10K pot is not what I have seen recommended elsewhere (e.g. by Shimmilou - 25k) but is it 10K in the Torres kit?

A pot would be something like this I suppose: http://www.ebay.co.uk/itm/10k-linear-bias-potentiometer-pot-CTS-fits-Fender-tube-amps-amplifier-/150743330377


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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jul 31, 2013 2:27 pm
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That Weber Bias Rite looks like a handy tool to have around for biasing my 5E3 when it gets here. Should be here in a few days or so now as it is en route as of yesterday. Is it safe to assume that all octal tubes like the 6V6 use the same socket? That is, could I use the same device (BR2, I assume) to bias my 2x EL34 amp as well?

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jul 31, 2013 5:25 pm
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sidepartings26 wrote:
shimmilou wrote:
Basically, the object is to bias the tubes to a desired range of idle wattage, which means the wattage at which the tubes are idling, when not playing. A typical range for idle is between 50% and 70% of the tubes' max Plate dissipation rating. In the case of most 6L6 tubes, the max rating is 30 watts (each tube), so the idle range will be between 15 watts and 21 watts per tube. The wattage is determined by multiplying the Plate voltage and the Plate current, and Cathode current can be substituted for Plate current. Most bias probes will measure Cathode current.

In the case of your amp, the Plate voltage should be about 430 volts with properly biased 6L6 output tubes. So, let's try to set the idle to 60% of the tubes' max rating, about 18 watts (30 x 0.6). Since watts equals current times voltage, we divide 18 watts by 430 volts to get approx 0.042 amps of current, or 42 mA. With bias probes in place, after measuring the Plate voltage, you would then try to adjust the bias pot until the bias probe reads 42 mA per tube. The Plate voltage will change slightly with bias pot adjustment, so measure Plate voltage and current, then recalculate to get the desired setting.

It is important to measure the Plate voltage because it can sometimes be different than what is expected. Most bias probes and multimeters will come with instructions for their use. A bias probe is pretty simple, consisting of a tube socket and some leads coming out of the socket. The probe is inserted between the tube and the amp, and most probes will connect to a multimeter and read DC mV which can be directly translated to DC mA.


Using an Amp-Head dual bias I measured plate voltage at 441V. That little pot the tech installed let me take current up to 44mA but no further and not only has the plastic inside which allows turning pretty much disintegrated but the little blob stuff has come off. I've figured out how to drain the capacitors in the amp (straight from on to off with warm tubes and not using standby) and confirmed with the multimeter the voltage left as well as bravely touched the power section! The point being that I think the pot needs re-done and am happy to do it. I know 44mA is up there but a better job is needed; providing more headroom too.

Any suggestions as to a better pot and rating. And, a better way to install it e.g. drilling a hole in the chassis?

Edit:

"Looking at the schematic the bias resistor should be R87 (27k). Depending on the idle current and plate voltage, what value pot to use is subjective. First off, to install a bias pot....remove R87, jump a long wire from one hole of R87 to the wipe (center leg) and the right leg (if looking from the back of the pot with the lugs at top view) of the bias pot, yes you connect both legs to the same wire. This turns the pot into a variable resistor. The other hole gets a long wire soldered in, and goes to the unused leg of the bias pot (left leg) with a resistor in series to this leg of the pot. I would use a 10k linear pot (Fender has a great one for this) and a 15k - 22k 1/2 watt resistor. Depending on where you want the sweep to be in terms of voltage and current will help determine the appropriate resistor to run series (15k-22k 1/2watt to start with)"

This looks like a decent solution except the resistor is not specified. Currently it's 16k that the tech put in. What would be better?

Also the 10K pot is not what I have seen recommended elsewhere (e.g. by Shimmilou - 25k) but is it 10K in the Torres kit?

A pot would be something like this I suppose: http://www.ebay.co.uk/itm/10k-linear-bias-potentiometer-pot-CTS-fits-Fender-tube-amps-amplifier-/150743330377


Although you have a USA made Blues Deluxe, you might want to read the section on bias circuit modifications in my BDRI mod thread:

viewtopic.php?f=13&t=80142

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Last edited by bluesky636 on Thu Aug 01, 2013 9:05 pm, edited 1 time in total.

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jul 31, 2013 5:30 pm
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BMW-KTM wrote:
That Weber Bias Rite looks like a handy tool to have around for biasing my 5E3 when it gets here. Should be here in a few days or so now as it is en route as of yesterday. Is it safe to assume that all octal tubes like the 6V6 use the same socket? That is, could I use the same device (BR2, I assume) to bias my 2x EL34 amp as well?


Yes, the octal tube probe will work with 6V6, 6L6, and EL34 type tubes. However, measuring and calculating power tube bias is different for a cathode biased amp (like a 5E3) than it is for a fixed bias amp (59 Bassman). See the calculator entitled "Calculate Plate Dissipation In Cathode Biased Output Stage Based On Voltage And Resistance Readings" on the Weber bias calculator page:

http://www.webervst.com/tubes1/calcbias.htm

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Wed Jul 31, 2013 6:29 pm
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sidepartings26,

As you've noted, the reason that your current is limited, is because of the values of the pot and resistor that were installed for you. In this case, I might go with a 25K pot and a 10K resistor to give you a broader current range. The reason to use the resistor connected to the bias pot, is to limit the current through the tubes with the pot turned to it's limit. If the resistor weren't there, you could adjust the bias to a point that the current through the tube was too high, possibly damaging the tube and/or amp. Of course, getting more current range through the tube means an increased chance of setting the current too high. Some people are cautious and might use component values that help prevent too much current, conversely someone else might just use a 50K pot with no resistor for their own personal amp, knowing to properly set the pot to an acceptable value.

With what you've learned, it sounds like you are confident in replacing the pot yourself if you decide to. Go for it, and let us know how things work out. Frankly though, I think that approx 19 watts of idle (44 mA @ 430 to 440 Plate volts) is more than enough for your 6L6, and a higher bias will NOT give you more clean headroom, just the opposite, it will lead to earlier breakup. A lower idle, of say 16 to 17 watts will give more headroom. Just use some more, and better, silicon to secure the pot and you're good to go.

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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Thu Aug 01, 2013 5:38 pm
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I picked up two resistors today. 10K and 12K. They are metal film 2 watt though and I was looking for 1/2 Watt - is that ok?

The specs are:

Image

The product page is:

http://www.maplin.co.uk/metal-film-2w-resistor-5729

On that basis, is a 25K linear pot still best to achieve 35-47mA type of range?

Thanks Shimmilou. I will post pictures when done and hopefully that way I can give back to this community which has been awsome!


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Post subject: Re: Biasing an Original Blues Deluxe Post-Amp Tech Pot Insta
Posted: Thu Aug 01, 2013 7:42 pm
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It is OK to use a higher wattage resistor, but a 2-watt will be physically much bigger than a 1/4 or 1/2-watt. The range of current is partly dependent on the rating of the tube also, and selecting the resistor and pot value is sometimes partly an educated guess based on the values that you have now and the range of current that you get now.

Basically you have shown that reducing that value of the bias resistor to 16K (the value of the resistor alone), by turning the pot all of the way down, did not give you your desired current through the tubes, so a lower value such as a 10K is necessary for what you want. A 25K pot is good because the full value of the pot plus the value of the resistor would be 35K, which is close to the value of the original bias resistor of 27K.

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