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Post subject: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 11:48 am
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I have a '79 Super Reverb that I'm fixing up. New filter caps and power tube sockets.

I'd like to set the bias when I install the new tubes but it's unclear if I can do this by using my voltmeter or I need a tool that I plug in between the power tubes and chassis in order to read current.

I'd rather do it with just a voltage reading so I don't have to buy an expensive tool.

Thanks!


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 11:59 am
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Like any tube amp , you have to put an 1 ohms resistor on each tube's katode to ground.

With your volmeter on DC milivolts you'll have to read hown many volts at this resistor. This give you the current on each tube . On 1 ohms resistor voltage DC and current is the same .

If you read 0.035 volts or 35 milli. Volts this is same as 35 milli amp . You just have to adjust bias to have less than 60% on maximum power on each tube.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 12:30 pm
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Thanks for the reply! The cathode is pin 8 correct? Pin 8 of the socket is soldered to ground already. So would I need to un-solder that, and put the 1 ohm resistor between the socket pin and ground. or build a rig to go between the tube and the socket that is soldered to ground?


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 12:41 pm
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Just buy a decent bias probe. I assume you already have a DVM. You will not have to do anything to the amp and you can safely measure both cathode current AND plate voltage (which must be measured anyway in order to properly set the amp bias):

http://www.amp-head.com/product_info.ph ... ucts_id=70

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 12:56 pm
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russpatterson wrote:
Thanks for the reply! The cathode is pin 8 correct? Pin 8 of the socket is soldered to ground already. So would I need to un-solder that, and put the 1 ohm resistor between the socket pin and ground. or build a rig to go between the tube and the socket that is soldered to ground?


1- Yes , you have to unsolder the pin 8 to ground and put the 1 ohm resistor between pin 8 to ground. An 1/2 watt resistor is good . You live these resistor there for futur bias . This do not change anything in the amp working.

2- Like bluesky636 write you must also monitoring plate voltage pin 3 ( more than 400 DC volts, this is a lethal voltage ) .

Buying a Bias probe is very usefull if you do some bias a year. And the use is very safe.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 1:53 pm
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stratele52 wrote:
russpatterson wrote:
Thanks for the reply! The cathode is pin 8 correct? Pin 8 of the socket is soldered to ground already. So would I need to un-solder that, and put the 1 ohm resistor between the socket pin and ground. or build a rig to go between the tube and the socket that is soldered to ground?


1- Yes , you have to unsolder the pin 8 to ground and put the 1 ohm resistor between pin 8 to ground. An 1/2 watt resistor is good . You live these resistor there for futur bias . This do not change anything in the amp working.

2- Like bluesky636 write you must also monitoring plate voltage pin 3 ( more than 400 DC volts, this is a lethal voltage ) .

Buying a Bias probe is very usefull if you do some bias a year. And the use is very safe.


Although it is easy enough to add the one ohm resistor (you would have to add one for each cathode to measure each tube individually, or one for both cathodes and divide the resultant voltage in half) and can measure the plate voltage on pin 3 of each tube, one slip of the voltmeter probe can cause possible (fatal) damage to the amp and to you. I know from direct, personal experience. :oops:

A bias probe is a cheap investment in both your and the amp's safety. I won't bias an amp without one.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 4:07 pm
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bluesky636 wrote:
Just buy a decent bias probe. I assume you already have a DVM. You will not have to do anything to the amp and you can safely measure both cathode current AND plate voltage (which must be measured anyway in order to properly set the amp bias):

http://www.amp-head.com/product_info.ph ... ucts_id=70


+1

Money well spent for the ease of use and peace of mind it provides.

Gnome saing, mon?

8)

Arjay

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 6:42 pm
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Thanks all. Good advice. I'll order one of those rigs.

Can you point me to a good article on what should be looking for for the current and voltage when I do the biasing?


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 7:38 pm
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Russ, I like the dual or quad adapter Bias Probes. You can see how one tube effects the other. Can easily see which tube is more sensitive to bias voltage than the others. This is a nice way to match the tubes. If you have several 6L6GC or whatever output tubes to choose from. :D

Vibrolux reverb with home-built dual Bias Probe:

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 16, 2012 10:07 pm
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russpatterson wrote:
Thanks all. Good advice. I'll order one of those rigs.

Can you point me to a good article on what should be looking for for the current and voltage when I do the biasing?


1. Hook up bias probe and DVM per bias probe instructions.
2. Turn on main power.
3. After a few minutes, take amp out of standby mode.
4. Let amp warm up for about 15 minutes.
5. Measure cathode current of each tube.
6. Measure plate voltage of each tube.
7. Multiply cathode current (in milliamps) by plate voltage (in volts) for each tube. Product equals plate dissipation (in watts) at idle.
8. Adjust and measure cathode current and remeasure plate voltage (plate voltage will change as cathode current changes).
9. Set cathode current so that the product of cathode current and plate voltage is between 50% and 70% of the maximum plate dissipation of the tube (max for a 6L6GC is 30 watts, so you want to see anywhere between 15 and 21 watts as the product of your measurements) AND sounds good to you. After each adjustment, let amp stabilize for about 15 minutes. Final cathode current and plate voltage measurements are not important as long as the product of the two is within the 50% to 70% range of maximum plate dissipation.

Note that for bias measurements it is actually the product of PLATE current and plate voltage that you are calculating, but cathode current is close enough for government work and gives you a little safety margin.

You are done. Go have a beer and play your guitar. :D

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 8:54 am
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Thanks!


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 10:14 am
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Keep in mind, the ultra-linear SR has a fixed-value bias with a bias-balance control.

HTH

Arjay

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 11:10 am
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Retroverbial wrote:
Keep in mind, the ultra-linear SR has a fixed-value bias with a bias-balance control.

HTH

Arjay


That would have been good to know at the very beginning of this thread. Too bad the OP did not specify which SR (by circuit number) he has. Probably invalidates all that I wrote.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 11:41 am
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How would I determine the circuit number? The serial number is A957289. I found a web page that indicated that would be '79.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Sep 17, 2012 12:12 pm
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FMI/CBS abandoned circuit-revision ID numbers per se in the early '70s. The SR in question is the 70-watt ultra-linear variant which was developed and fielded in early 1978. These amps can be categorized by indentical Twin Reverb style tone stacks in both channels and a solid-state rectifier in the power supply. This UL variant shares its chassis architecture with both the Pro Reverb and Bandmaster Reverb from the same era, save for the SR's 2-ohm OT.

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