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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Oct 04, 2012 11:07 am
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Thanks for the info. I measured resistance across from the 4th lug on the pot (the one opposite the 3 lugs).

The resistance from the 4th lug to both, the top and bottom lugs reads just under 5K and changes slightly (less than 1K) when I rotate the pot.

The resistance from the 4th lug and the center lug is 5K at both extremes of pot rotation and goes to near 0 ohms at the center point of the pot rotation.

So it seems like disconnecting the bias feed and the 2.2M from that resistance will change the circuit. Leaving it disconnected is the way to go for this bias mod?


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Oct 04, 2012 12:10 pm
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russpatterson wrote:
Thanks for the info. I measured resistance across from the 4th lug on the pot (the one opposite the 3 lugs).

The resistance from the 4th lug to both, the top and bottom lugs reads just under 5K and changes slightly (less than 1K) when I rotate the pot.

The resistance from the 4th lug and the center lug is 5K at both extremes of pot rotation and goes to near 0 ohms at the center point of the pot rotation.

So it seems like disconnecting the bias feed and the 2.2M from that resistance will change the circuit. Leaving it disconnected is the way to go for this bias mod?


The purpose of that lug is to feed the -58 VDC bias voltage to the balance pot. It has nothing to do with the 2.2M ohm resistor which is also connected to the -58 VDC source. When the wiper of the bias balance pot is at the center point, the -58 VDC is applied equally to both output legs of the pot. Disconnecting that lug is only disconnecting the pot from the bias source which is being reconnected to the pot through another lug. The 2.2M ohm resistor remains connected to the -58 VDC source.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Oct 07, 2012 5:59 pm
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I made the mod to convert the bias balance to a bias adjust pot. I noticed that one poster recommended 50-70% max idle watts for the 6L6's and another poster recommended not over 60%. So I set it to 57% idle watts.

The tone is markedly better than with it stock. It sounds awesome! :D It's deeper, richer, fuller, not as thin sounding.

Thanks for all the help! That tube test rig from Amphead is a pretty sweet, thanks for the tip on that.

Now to put it back together and start on the 50's Fender Deluxe that stopped working a few years ago.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Oct 07, 2012 6:16 pm
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russpatterson wrote:
I made the mod to convert the bias balance to a bias adjust pot. I noticed that one poster recommended 50-70% max idle watts for the 6L6's and another poster recommended not over 60%. So I set it to 57% idle watts.

The tone is markedly better than with it stock. It sounds awesome! :D It's deeper, richer, fuller, not as thin sounding.

Thanks for all the help! That tube test rig from Amphead is a pretty sweet, thanks for the tip on that.

Now to put it back together and start on the 50's Fender Deluxe that stopped working a few years ago.


Glad to hear it. :D :D :D

Without going back and reading the entire thread, I don't know who said not to go over 60%. The commonly quoted range for bias adjustment is 50% to 70% of maximum plate dissipation. You can read about it here:

http://www.aikenamps.com/WhatIsBiasing.htm

http://www.aikenamps.com/Why70percent.html

I run my 5F6A clone with JJ 6L6GCs right at 70% and my BDRI (also with JJ 6L6GCs) at about 67% (I could go to 70% but the plate voltage drops more than I want. Long story.).

You might want to try bumping the bias up closer to 70% and see if it sounds any different. If not, put it back to where you had it. The tubes will last a little bit longer.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Oct 08, 2012 1:35 am
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You coud bias 50 to 70 % but with my experience Fender amp don't play better if more than 60 % and if you use the amp at high volume it is better to bias colder than hot.

Colder bias will save tubes

Like Bluesky write try many bias a keep the one you like. Most important is you know how to bias .


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Oct 08, 2012 5:10 am
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Stratele, all depends on the tube, too. I've seen many amps with 1950-60's made RCA 6L6GC pushed to near Class "A" & those tubes seem to last. I'm not sure why some ppl like the tonal results of such "hot" amps, as they lose a lot of the Fender tone.

But, if you took a cheaply made Russian "6L6GC/7581A" (like in link) and ran them in the same amp biased so hot --- the tubes may not make it to the end of the day.

The real rock of tubes, are those NOS 6V6GT's. How many old Champs and VC have you seen with OEM 6V6GT still in them? Pushing Class "A" (really more like hot AB1) and "10" all day for years and years.

http://www.vacuumtubesinc.com/VacuumTub ... 6L6GC.aspx


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Oct 08, 2012 7:57 am
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BMW2002Ti wrote:
Stratele, all depends on the tube, too. I've seen many amps with 1950-60's made RCA 6L6GC pushed to near Class "A" & those tubes seem to last. I'm not sure why some ppl like the tonal results of such "hot" amps, as they lose a lot of the Fender tone.

But, if you took a cheaply made Russian "6L6GC/7581A" (like in link) and ran them in the same amp biased so hot --- the tubes may not make it to the end of the day.

The real rock of tubes, are those NOS 6V6GT's. How many old Champs and VC have you seen with OEM 6V6GT still in them? Pushing Class "A" (really more like hot AB1) and "10" all day for years and years.

http://www.vacuumtubesinc.com/VacuumTub ... 6L6GC.aspx


Concur.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Oct 08, 2012 11:30 am
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BMW2002Ti

I quite agree with you that the tubes manufactured in the 1950s and 1960s were highly resistant to the other side and some recent tubes are the opposite.

I wanted to respond in general bearing in mind the recent quality tubes avalable today and my personal experience.

In any case, I would not advise anyone to push beyond the limits of the good old tubes of the 50 or 60. They are so rare and expensive.

In addition they are rarely used today and could misinform the forum members who read me.


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