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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 10:06 am
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stratele52 wrote:
aclempoppi wrote:
No offense you guys. But I'd leave the 70 cap alone, the vib would suffer the change. The 33k would be my point of focus. JMO Art


Sorry but I just see a 3.3 K not 33K . Where is it ?


Hard to tell if it is 3.3K or 33K, but I believe we are talking about the resistor that hangs off the slider of the balance pot. I read it as 33K.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 10:37 am
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We don't talk about the same drawing. I was looking at drawing 12108. Ok now on the drawing on this forum , yes it look to be a 33 k .

Not it is not the resistor for the tube bias. Only for vibrato circuit.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 11:00 am
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stratele52 wrote:
We don't talk about the same drawing. I was looking at drawing 12108. Ok now on the drawing on this forum , yes it look to be a 33 k .

Not it is not the resistor for the tube bias. Only for vibrato circuit.


I agree. That's why I asked aclempoppi to explain his thinking on this. Waiting for a response.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 2:12 pm
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Sorry fellas I should have read the thread a little closer.

My 68 bandmaster is converted to bias adjustment but its much earlier.

It is a good reference book !

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 2:25 pm
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Your Bandmaster must be the later AC568, Chris.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 6:25 pm
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Yeah guys MY BAD!!! Where the heck are those reading glasses? Art

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Thu Sep 27, 2012 9:09 pm
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aclempoppi wrote:
Yeah guys MY BAD!!! Where the heck are those reading glasses? Art


Shame on you. :lol: :lol: :lol:

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sat Sep 29, 2012 5:00 pm
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I'm confused as to the agreed upon recommendation, resistor across the 70uF 100V cap or something to do with a 33K resistor somewhere?

I read -57VDC on both pin 5's without the 6L6's plugged in. Is the goal is to get that closer to -35VDC?

Question: if I let the voltage on pin 6 run down to below a volt, happens in less than a minute, am I to assume the filter caps are discharged and it's safe to work on the amp?

Thanks for the help.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sat Sep 29, 2012 6:21 pm
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I see the 33K resistor in question in the schematic. On the wiper of the balancing pot.

However it looks to be soldered to the back of the pot. Is that expected? I don't see it going to ground in the schematic. Is the back of the pot not ground?

The amp sounds pretty good with the JJ's as it is, even without the biasing pot installed to get the tubes to run hotter.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sat Sep 29, 2012 6:42 pm
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russpatterson wrote:
I see the 33K resistor in question in the schematic. On the wiper of the balancing pot.

However it looks to be soldered to the back of the pot. Is that expected? I don't see it going to ground in the schematic. Is the back of the pot not ground?

The amp sounds pretty good with the JJ's as it is, even without the biasing pot installed to get the tubes to run hotter.


Yes, the schematic shows the 33K resistor going to ground. The 100K attached to the trem speed control and one side of the opto-isolator also go to ground.

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

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sat Sep 29, 2012 7:06 pm
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russpatterson wrote:
I'm confused as to the agreed upon recommendation, resistor across the 70uF 100V cap or something to do with a 33K resistor somewhere?

I read -57VDC on both pin 5's without the 6L6's plugged in. Is the goal is to get that closer to -35VDC?

Question: if I let the voltage on pin 6 run down to below a volt, happens in less than a minute, am I to assume the filter caps are discharged and it's safe to work on the amp?


I'm not sure what playing with the 33K ohm resistor would do.

The bias voltage (-58 VDC) is acually set by the 2.2K ohm resistor that connects between the 1/2 wave rectifier and the input to the bias balance pot. I think it is that resistor that would have to be changed (made larger, I think) to increase the bias voltage (make it less negative). I'd like someone else to confirm that before making any recommendations.

Looking at the schematic, I think you could modify the bias balance pot to work as a bias pot as described in the Fender amp mod book we discussed earlier. I'd have to look at it some more. This might be useful:

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

Pin 6 of what? The power tube plate voltage is on pin 3 of each of the 6L6GCs. That is the voltage you really have to be careful of.

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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 30, 2012 1:04 am
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russpatterson wrote:

I read -57VDC on both pin 5's without the 6L6's plugged in. Is the goal is to get that closer to -35VDC?


NO .

The goal is your 6L6 should work at around 60% or less than their maximun power. -35 vdc was the voltage when the amp where desing WITH tubes avalaible at this time.

Today could be not the same.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 30, 2012 1:14 am
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russpatterson wrote:


Question: if I let the voltage on pin 6 run down to below a volt, happens in less than a minute, am I to assume the filter caps are discharged and it's safe to work on the amp?
Thanks for the help.


6L6 don't use pin 6 . Voltage are on pin 3.


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 30, 2012 1:23 am
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russpatterson wrote:
I see the 33K resistor in question in the schematic. On the wiper of the balancing pot.

However it looks to be soldered to the back of the pot. Is that expected? I don't see it going to ground in the schematic. Is the back of the pot not ground?
]


Yes

+1 bluesky636


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Post subject: Re: How to bias a 70's Super Reverb
Posted: Sun Sep 30, 2012 1:37 am
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russpatterson wrote:
I'm confused as to the agreed upon recommendation, resistor across the 70uF 100V cap or something to do with a 33K resistor somewhere?


Changing 33k resistor will change bias voltage, Yes you can work there to have your proper tube Bias.

Higher resistor will give you colder tube's bias by sending more negative voltage to 6L6. But you could not have more negative voltage than the 1.2 k and 2.2 k send to your bias circuit. That is why a "real " bias pot is usefull.

Same as lower resistor will give you hot tube's bias.

You can try many thing WITHOUT 6L6 in the amp and always be sure you have some negative voltage ( more then -25 ) at each 6L6 BEFORE put in the amp.


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