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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 30, 2012 1:47 am
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bluesky636 wrote:

If you are happy with the sound, I wouldn't worry about installing a bias pot. Your call.



Yes if you are sure that the bias are good / safe.

Bias too hot ,you could damaged your tubes and shorten their live .
Bias too cold ; long tube live but sound is thin, lack of punch.

Better to spend some money and see a good tech for a proper bias and a balance pot installation if you are not sure you can do it.

You have a good amp , not the best of Fender but a well made and tough amp with good sound.
Be sure keep in good working order and it will last all of your life. Not like most of the tube amp today.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 30, 2012 4:18 pm
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Thanks again for all the info everyone.

Sorry for the pin 6, that's just how it's wired up. A 470 Ohm resistor is soldered across pin 4 and pin6. They just use pin 6's socket lug as a soldering node. I measured 490V on pin 6, then watched it drain to below 1V. What voltage should I check to make sure it's safe?

I built a filter cap draining rig with a 1K resistor and alligator clips but it seems that it drains quickly on it's own anyway, without using the cap draining rig.

For the additional biasing circuit. So just so I'm clear the 1.2K and 2.2K that you talk about are the ones in this picture?
Image

I would reduce the amount of resistance on one or both of those to increase the voltage to the tubes, (less negative)? What is the relationship between the two resistors?

I've done a lot of DC power stuff but never much with AC power circuits until now with this amp. Bear with me on the beginner questions.

It seems that the balancing pot has some use and that it would be better to preserve it and add a true biasing pot. If you buy 'matched' tubes does the differences make having that bias balancing pot not useful?

The tones way better now on the amp now since I put in new filter caps and replaced the worn out tube sockets. However I've got the bug and I want to mess with it to get a great tone without having to play it so loud. This amp with both volumes on 6's is where I used to like to play it at gigs for the good tone, but that's too loud for the living room.

Here's a link to the pdf from schematic heaven if you want to zoom in on it and still have it legible.
http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/cbs_70w_mstrvol_pullsw_super-pro-bmstr_rev.pdf


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 30, 2012 9:11 pm
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russpatterson wrote:
It seems that the balancing pot has some use and that it would be better to preserve it and add a true biasing pot. If you buy 'matched' tubes does the differences make having that bias balancing pot not useful?


The sole purpose of the balance pot, as I understand it, was so that you DID NOT have to have matched tubes. If a tube went bad, you could replace just the the one and use the balance pot so that they would be biased more or less equally. Today I think you would be better off modifying the balance pot to a bias pot and buying matched pairs. I would leave the rest of the bias circuit alone. Looking at the schematic, I think the mods in the book referenced earlier would work.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Mon Oct 01, 2012 7:03 am
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" The sole purpose of the balance pot, as I understand it, was so that you DID NOT have to have matched tubes, "

Bluesky, that is one benefit. The other is if you get the two push-pull partnered 6L6GC conducting as close to each other as possible, their resultant simple sine waves would be near mirror images. The theory being, that spurious non-symetric noise riding above the sine wave (like pink or white noise) would be better elminated.

That was the theory. But, we all know in practice that output tube change in character, independently from each other with use. So that "hum" pot advantage is a fleeting one.

The whole thing was brought over from the single-ended triode ppl that use the hum pot to balance the HEATER stage (=cathode) of their big receiver/transmitter tubes. Where the high current AC heater taps are being applied to each cathode.

Note the hum/balance pot on the tail-end of this 211 SET amp?

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 1:11 pm
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Thanks again for the information. I'm convinced I don't need to preserve the balancing pot; and should use it for a bias pot. However the wiring in my '79 Super Reverb is somewhat different than what's in the bias pot mod diagrams posted above:

-My balance pot has four (4) lugs on it instead of the usual three (3) shown in the diagram.

-All the resistor values are different on my amp, and the board is laid out a little differently as well.

-I could not find the diode that's shown in the diagrams for the 'Bias Board'. I don't think I have a separate bias board, but did trace a wire to a 70uF filter cap so I'm assuming it's the same 'bias filter cap' from the diagrams. There are two 70uF caps in the filter cap tank, one is connected to that pin on the balance pot (see diagram below).

Here's a link to the tube bias mod diagrams: http://www.el34world.com/charts/bias_conversions.htm

Here's how the '79 Super is wired now: https://docs.google.com/drawings/d/1lYtmd8RNe4xky8tdrXxnVL-y0L-s0PSfcyAa63D5cdA/edit

Here's how I'm proposing to wire it for the tube bias mod: https://docs.google.com/drawings/d/1uTigATRQuY31Q8VNqBddfpJ6w7gEEYeJkMpbsdH7joQ/edit

I think this is right but I'd appreciate a second opinion before I go experimenting on my amp's wiring. Thanks!


Last edited by russpatterson on Wed Oct 03, 2012 1:29 pm, edited 1 time in total.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 1:28 pm
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russpatterson wrote:


Here's how I'm proposing to wire it for the tube bias mod: https://docs.google.com/drawings/d/1uTigATRQuY31Q8VNqBddfpJ6w7gEEYeJkMpbsdH7joQ/edit

I think this is right but I'd appreciate a second opinion before I go experimenting on my amp's wiring. Thanks!


No this won't work for a Super Reverb; Where do you connect the 33 K ( or 3.3K ) going to photo resistor ( tremolo photocell ) ?

Why they write the 2.2 M go to reverb ? It is tremolo.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 1:33 pm
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russpatterson wrote:

-My balance pot has four (4) lugs on it instead of the usual three (3) shown in the diagram.



I see 4 lugs on the diagram. What diagram are you looking ?


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 1:48 pm
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stratele52 wrote:

No this won't work for a Super Reverb; Where do you connect the 33 K ( or 3.3K ) going to photo resistor ( tremolo photocell ) ?

Why they write the 2.2 M go to reverb ? It is tremolo.


You are correct. My mistake. That's not a 15K soldered to the back of the pot that's the 33K resistor. Also from the 2.2M resistor it does go to the vibrato pedal input jack. I corrected the drawings on Google Docs.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 1:50 pm
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stratele52 wrote:
I see 4 lugs on the diagram. What diagram are you looking ?


The diagram that was posted earlier in this thread as a reference for how to use the balance pot as a bias pot. This link show's only 3 lugs on the pot.

http://www.el34world.com/charts/bias_conversions.htm


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 2:39 pm
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Here's how I'm proposing to wire it for the tube bias mod: https://docs.google.com/drawings/d/1uTi ... H7joQ/edit
-Russpaterson

Ok this mod should work .


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 3:06 pm
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Thanks. So 1/2 watt on the 220k resistors? I'll order those.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 3:09 pm
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russpatterson wrote:
Thanks. So 1/2 watt on the 220k resistors? I'll order those.


Yes 1/2 watt is enough.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Wed Oct 03, 2012 4:31 pm
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russpatterson wrote:
Thanks again for the information. I'm convinced I don't need to preserve the balancing pot; and should use it for a bias pot. However the wiring in my '79 Super Reverb is somewhat different than what's in the bias pot mod diagrams posted above:

-My balance pot has four (4) lugs on it instead of the usual three (3) shown in the diagram.

-All the resistor values are different on my amp, and the board is laid out a little differently as well.

-I could not find the diode that's shown in the diagrams for the 'Bias Board'. I don't think I have a separate bias board, but did trace a wire to a 70uF filter cap so I'm assuming it's the same 'bias filter cap' from the diagrams. There are two 70uF caps in the filter cap tank, one is connected to that pin on the balance pot (see diagram below).

Here's a link to the tube bias mod diagrams: http://www.el34world.com/charts/bias_conversions.htm

Here's how the '79 Super is wired now: https://docs.google.com/drawings/d/1lYtmd8RNe4xky8tdrXxnVL-y0L-s0PSfcyAa63D5cdA/edit

Here's how I'm proposing to wire it for the tube bias mod: https://docs.google.com/drawings/d/1uTigATRQuY31Q8VNqBddfpJ6w7gEEYeJkMpbsdH7joQ/edit

I think this is right but I'd appreciate a second opinion before I go experimenting on my amp's wiring. Thanks!


No, I do not think your proposed drawing is correct. You have the wrong pot lugs connected/disconnected. I don't have any way to draw and post the correct diagram easily, so you will have to try and follow what I say.

You need to look again at the following bias conversion site:

http://www.el34world.com/charts/bias_conversions.htm

Ignore the diagrams for the AB568 Vibrolux Reverb amp. Your balance pot is more like the AA1069 Bandmaster Reverb amp and AA270 Twin Reverb amp. Your conversion should look like the mods to the balance pot as shown in the four drawings at the bottom of the page. After the mod the fixed lug that is opposite the three lugs that you normally see on a pot is disconnected and not used. Your proposed drawing shows it in use and one of the regular pot lugs is disconnected instead. Keep in mind that your circuit is not exactly like those shown on the bias conversion page, so you must make sure that all connections for the tremolo circuit are accounted for.

You drawing needs to be corrected to show:

The bias supply voltage (-58 VDC) and the 2.2M ohm resistor should go to the bottom right lug on your drawing. The 33K ohm resistor should go between the top right lug of the pot and ground. The center lug of the pot is the adjustible bias output that goes to the connection of the 220K ohm resistors.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Oct 04, 2012 9:17 am
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Thanks for the reply. Here's a V2 drawing. Is this what you intended?

https://docs.google.com/drawings/d/1gcFNxnLbSU3piK7in7RV3rFqBBuMe1LEk9N869IJVyI/edit

1) What is that fourth lug, on the opposite side of the pot from the three, for?

2) Does it matter if the 33K is on the top or the bottom lug, just so long as the bias supply comes in from the opposite lug and the center lug goes to the 220K junction?


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Oct 04, 2012 9:35 am
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russpatterson wrote:
Thanks for the reply. Here's a V2 drawing. Is this what you intended?

https://docs.google.com/drawings/d/1gcFNxnLbSU3piK7in7RV3rFqBBuMe1LEk9N869IJVyI/edit

1) What is that fourth lug, on the opposite side of the pot from the three, for?

2) Does it matter if the 33K is on the top or the bottom lug, just so long as the bias supply comes in from the opposite lug and the center lug goes to the 220K junction?


Yes, that drawing will work fine.

I have never opened one of those type pots, but I am guessing that the 4th lug is a fixed tap into the pot.

Changing the lug that the 33K resistor is connected to will change the direction the pot needs to be turned to increase/decrease the bias voltage. Wire it up as shown in the diagram.

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