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Post subject: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 2:17 pm
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I have just one question
inside the amp or the manual it said for the preamp tube 4x12AX7A and 1x12AT7
eurotube said is better to use ECC83S and tube depot is better to use the 12AX7A because is better for the tone and for the 12AT7 eurotube said is better to use the ECC81 and tube depot said is better to use the 12AT7 and for the power tube they say the same

what is better
thanks again and I would like to know if someone know what is the best bias for this amp


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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 3:00 pm
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I stick with the 12A's they sound s little smoother to me. As for the bias it should be on the back of the amp next to the two adjusters. One for bias, one for balance. Hope this helps.


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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 3:16 pm
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thank for the answer, but for the bias I mean for the power tube what is the range. I need to read on my volt meter


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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 6:33 pm
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simus35 wrote:
I have just one question
inside the amp or the manual it said for the preamp tube 4x12AX7A and 1x12AT7
eurotube said is better to use ECC83S and tube depot is better to use the 12AX7A because is better for the tone and for the 12AT7 eurotube said is better to use the ECC81 and tube depot said is better to use the 12AT7 and for the power tube they say the same

what is better
thanks again and I would like to know if someone know what is the best bias for this amp


ECC83S is the same as 12AX7. ECC81 is the same as 12AT7. ECC is the European designation, the other is the American designation. No difference functionally or electrically.

The power tubes should be biased between 50 and 70 percent of the maximum plate dissipation. You need to measure both cathode current and plate voltage. If the power tubes do not have a 1 ohm resistor installed in the cathode circuit, you either need to install one or use a bias probe that will allow you to measure both cathode current and plate voltage.

You will multiply cathode current and plate voltage together to come up with an idle power dissipation. You adjust cathode current and remeasure both until you achieve an idle dissipation of 50 to 70 percent of the maximum plate dissipation which for a 6L6GC is 30 watts. You can adjust the values within that range to what sounds best to you. I prefer my amps closer to 70%.

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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 6:49 pm
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thanks for the info bluesky636 that will help for other amp I have to Bias by the inside of the amp
but this one I found the info. because I just need to use my volt meter and plug in the back of the amp and turn the pot until I read 0.04 vdc and for the output balance to 0 vdc


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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 7:56 pm
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All you need is a volt meter. The correct reading should be 40 mv. This amp was made with 3 little probe holes in the back for a volt meter. You can't miss them. The center hole takes the red lead and the two outer holes (for the black lead) are for biasing and balancing. Right next to these 3 probe holes is the recommended factory bias setting. At least that's how it is on my Red Knob Showman and Red Knob Twins. Good Luck.


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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 8:08 pm
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Yes, I see that from the schematic. The bias pot sets the bias level for one of the two pairs of power tubes. The balance pot is then used to balance the other pair of power tubes with the first pair. Fender appears to have set the bias to an arbitrary value of 40 mA (40 mV measured across a 1 ohm resistor). Because that value is the sum of the two tube's cathode current, each tube is only drawing 20 mA. At a plate voltage of approximately 470 VDC (the value is hard to read on the schematic), each tube is dissipating only 9.5 watts at idle which is only 31% of the maximum plate dissipation (30 watts) for each tube. In my opinion, that is very cold and not the best setting for optimum performance. The only way to properly set the bias would be as I described.

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

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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 8:20 pm
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Here ya go. Scroll down to page 5:
http://ampwares.com/schematics/TheTwin_ ... Manual.pdf


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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 8:39 pm
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whipman wrote:


Uh, yeah. What I wrote is an explanation of what is going on when you set the bias per the instructions in the owner's manual. With the cathode current set to that value and the associated plate voltage, the amp is biased very cold. Fender does that with all their amps. Cold bias equals long tube life. It may not be the best bias for optimum performance.

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Post subject: Re: retube Fender dual showman red knob head
Posted: Mon Jan 21, 2013 10:12 pm
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Listen to bluesky636, he knows of what he speaks. The majority of Fender amps are biased very cold from the factory, and adjusting the bias can often be the best/easiest improvement in sound that you can make.

For this amp, 40 mA at each test point (TP 201 and TP 202, with TP 203 being ground reference for the negative meter lead) is way, way too cold. At approx 470 VDC on the Plate, I would shoot for at least double that, or approx 80 mA at each test point, which would give about 18 watts idle per tube, about 60%, considering that the Plate voltage will drop some when increasing the idle bias. Recheck the Plate voltage after setting the bias and adjust if necessary, per the instructions given by bluesky636. 8)

Fender makes up for it by biasing the BJr at over 100%. :shock: :lol:

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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 5:13 am
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thank for the info
but if I understand, I will need to buy 2 bias probes to read the correct setting?
because it will be my first time I will do that, and I don't want to send to the tech in my town because it will cost me over 100$ and even more some time.
thanks again and do I need to change something inside the amp to get the correct setting


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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 8:53 am
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No changes necessary. In this case you don't have to use bias probes, the amp has two test points used to measure the Cathode current, one TP for each side of output tube pairs, and you can check the Plate voltage at pin 3 of any output tube. Bias probes just make it safer (probes that also measure Plate voltage), easier and usually more accurate to check the bias, as the 1 ohm resistors that Fender uses are not precision as are used in most probes. It can be dangerous to measure Plate voltage at the tubes' pins because the pins are so close together, making it easy to slip and connect two pins together that shouldn't be ( :shock: ). In the case of an amp like this one, with four output tubes, using four bias probes will also check to see if the tubes are matched closely.

There are lots of good bias probes available, with or without the Plate voltage measuring capability. This is the set that I use, and you can add probes to measure Plate voltage, but I opt to check Plate voltage myself with a meter at safer spots inside the amp. "Quadstage BiasPro MQ10" with four probes. Many people get by just fine with two probes, they just have to shut down the amp, let the tubes cool and then switch the probes' positions in the case of four output tubes.

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

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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 9:12 am
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thanks shimmilou for the info and for Quadstage BiasPro MQ10 info too, I will buy one. is a good investment. and thanks again. by the way do you know a good web address I can find very good info to how to repair of fix tube amp, like how to change the cap and the famous dead cap too. because I think this is the next think I will need to make on my dual showman (25 years old)


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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 10:26 am
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One of the best places is this forum, just use the search function. Try the so-called "death cap" or "3 prong conversion" for starters and review the topics, many even have wiring diagrams and/or links to schematics. Lot's of repair topics here, and lot's of great advice.

Here is a drawing provided by one of our members. This drawing shows up in many threads about the rewiring from the 2-prong electrical plug to the 3-prong, and removal of the cap.

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Post subject: Re: retube Fender dual showman red knob head
Posted: Tue Jan 22, 2013 3:46 pm
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+1

Shimmilou finished up what I started. :wink:

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