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Post subject: 1969 SFSR 1069 circuity
Posted: Tue Jun 26, 2012 11:16 am
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I bought one one of theses used in 1985 for $300 I tried it a few time with my harmonica as I was in a band playing a lot, and did not like what it was producing so I put it in a case and left it there as I got less involved with playing in bands and stared to work more in sound. 3 years ago my now family bought me an electric guitar so I pulled out the SFSR and plugged in I was not sure if it was the amp that did not sound right or was it me being new at guitar playing so I packed it back up and said I have a lot to lean on the guitar before I try the SR again. I try to practice 2 hour a night if I can, so I have gotten better on the guitar and I unpacked the amp.
So I plugged my guitar in to the amp and this time I was sure the amp needed some help I started to follow this form and got some good advice by reading older post. Then I started checking thing the first thing I noticed it had 1 RCA 6L6 CG and 1 Scott 6L6 GB which have 2 very different watt ratings, so I ordered new tubes (Electro-Harmonix 6L6 Matched Power Tubes) and while I was at it I ordered new Filter Caps for under the dog house, and one for the bias circut.
Replaced the filter caps and the tubes plugged the guitar in and it sounded ok but I did not know what it should sound like. On 4 it was kind of loud at 6 it had some drive, but too loud to be in the same room with it, but I still was not so happy with the way it sounded.
More research and I find out about how a tube works, plate voltage, bias, so I get a bias probe it comes I find out 1 tube is set for 12ma the other about 37 ma I tweak the balance pot ( yes I do believe it is a balance pot ) so I I do this I notice that when one side goes down the other side goes Up and the middle is 26 to 27 ma and I can not get them to go to 32 ma or any other setting with out one of the tubes far away in value other then 26-27 ma. Also only one side will get over 36 ma the the other side will not go that high At the 26-27 ma bias the amp at 2 is very loud it now starts to get some drive around 4 but I still need to leave the room at 4 way loud.
Time the measure the plate voltage well I do that and I get 430 vdc I feel this is very low in value.
My question is what should I check next and is there a way to get the plat voltage higher than 430vdc if I put in a GZ34 in stead of the 5U4GB will that increase the plate voltage or is there possibly something wrong
sorry if this got long


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Post subject: Re: 1969 SFSR 1069 circuity
Posted: Tue Jun 26, 2012 11:45 am
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Joined: Mon Apr 05, 2010 2:37 pm
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Location: Natural Bridge, Virginia
Here is the schematic to your amp:

http://ampwares.com/schematics/super_reverb_aa1069.pdf

yes, you have a balance pot, not a bias adjust pot.

You might want to check the two 100K resistors that are in the bias balance circuit to see if they have drifted. The diode rectifier for the bias circuit could be bad too, but I don't see and type name listed for it on the schematic.

Why did you find it necvessary to change the power supply caps?

According to the schematic the plate voltage on your amp is spec'd at 437 volts. if you are measureing 430, that is fine. No need to change the rectifier to a different type.

Your amp is rated at 40 watts. Its gonna get loud above 4. The only way to avoid that and still get power tube drive with the amp is to use an attenuator.

If you don't know what the amp is supposed to sound like, I'm not sure what to tell you.

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Post subject: Re: 1969 SFSR 1069 circuity
Posted: Tue Jun 26, 2012 12:04 pm
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Thanks for your suggestion I will check the resistors to see if they drifted, do i need to remove them from the circuit or can I just meter them in place, should I just try to find a new diode and replace it
I have seen the schematic the problem is most of the work I have ever done on the bench was the engineer would tell me to swap out this part for that one and never did read them now as I look at them, it like a foreign language.
The power caps were Malory tan ones that were as old as the amp itself, I felt it was time.
As far as (If you don't know what the amp is supposed to sound like, I'm not sure what to tell you.) I understand there is nothing anyone can tell me what it is suppose to sound like I would need to compare other ones to get an idea


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Post subject: Re: 1969 SFSR 1069 circuity
Posted: Tue Jun 26, 2012 12:52 pm
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Rock Star
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Joined: Mon Apr 05, 2010 2:37 pm
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Location: Natural Bridge, Virginia
I would check the resistors in circuit first. Carbon comp resistors have a +/- 20% tolerance so they should measure between 120K and 80K. If greater than that, pull them out to check them and probably replace them. Might want to check the balance pot and teh other resistors associated with the bias circuit to be sure. See if there is any label on the diode.

You need to learn to read a schematic before you do much more with the amp.

Good choice on replacing the power supply caps. I reread your post. I had missed that you bought the amp used.

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Post subject: Re: 1969 SFSR 1069 circuity
Posted: Tue Jun 26, 2012 1:04 pm
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Your right about leaning to read the schematic I saw the 437 I also saw the 470 number and I assumed that was the plate voltage that is why I thought I had low voltage I will check my resistors tonight and see what i get Thanks for the reply


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Post subject: Re: 1969 SFSR 1069 circuity
Posted: Tue Jun 26, 2012 2:22 pm
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Joined: Mon Apr 05, 2010 2:37 pm
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Location: Natural Bridge, Virginia
470 is the value of the screen resistor. :wink:

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