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100 W Silverface Twin Bias Adjustment Mod

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  • 100 W Silverface Twin Bias Adjustment Mod

    I am new to electronics, totally. I am a guitar player who took his twin reverb in for service, it was blowing fuses. I took it to my tech and it worked for a few hours then the same old symptoms came back. Low volume, funny smell and then a blown fuse. I did a lot of reading, and joined this forum for some expert support. I have learned about how the amp works, a little. I have done this: I put in new filter caps, replaced almost all of the coupling caps, replaced almost all of the small caps on the board, changed resistors to blackface values and improved the lead dress where I could. I am very pleased to say that now the amp seems to work very well. It has excellent tone, operates quietly but still the volume, though loud and pleasing to the ear is not loud enough to play in clubs, like my #1 amp a 66 SR. I have put the 1K resistors off the cathode pins of the 6L6R GT's that are new and added a 50K alpha audio taper pot to the hum balance to trim the current up and down at the cathodes. I have trimmed to 33mA range and balanced all four tubes between the hum and the new trim pot. My phase inverter voltages are:263 at pin 6, 278 at pin 1, and 131 at pin 3. My voltage readings at the power tubes are in the 455V range. I have -59 at the hum bias pot and 73 at the rectifier. I have changed the 80/75 caps to 100/100. Here is my question: I have a hair trigger on the new bias pot. I like being able to adjust up and down the cathode current but doubt it will stay there with all the moving around an amp gets. When I measure the resistance across the new bias pot I get 7.25K. I'd like to now bypass the new bias pot and put a fixed resistor back on the hum pot to lock in the voltage I have trimmed to. What value resistor would I use, though because the value I measure when the new pot is disconnected the resistance is more like 27K? The second question is where to look for more volume? I am using RCA preamp tubes as per the decal but my AX7's I think are AX7A's. I really love learning as much as I have over the last month and look forward to doing more of this new hobby by fixing up my 66 SR a little when this amp is bullet proof. Thanks to all, thanks for what I have learned here. Roy

  • #2
    Oh, the other thing I changed, it seemed the speakers were wired out of phase. The red dots on the Utahs were wired out of phase by the PO, I changed them back to the way it shows in the schematic, maybe the speakers were wired right, but labeled wrong? Now both red dots are wired to white. Thanks, Roy

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    • #3
      One potential issue you may have is the speakers. The Silverface amps came with a variety of oem speakers of different design and quality, and changing them can have a bigger impact than the black face mods.

      My 76 SFTR came with a pair of 16 ohm speakers that I believe were made by Eminence. The schematic and chassis were both labeled for two 8 ohm speakers for a load of 4 ohms. So, besides the speakers being pretty cheap, and the cones well degraded by time, elements, and abuse, they were also an impedance mismatch.

      Long story short, you may want to try different speakers. You may find a significant output gain, improved sound quality, and you can also tailor the sound to your desires. For example, a pair of Eminence red white a blues will give you a sensitivity of 102 db with a fairly American sound. The Cannabis Rex will tame the highs and give a smoother sound. Something like a Celestion Vintage 30 will give you a more modern British sound with more higher end bite. All would give a noticeable output boost over the stock speakers I had. A good first step may be to plug the amp into different external cabs to see how it sounds. You can also plug the speakers from the TR into another head or a combo with an external jack at the correct impedance. If you are using an amp you are familiar with, you will be able to get an idea of how the stock Utahs sound.

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      • #4
        I forgot to mention the polarity thing. It is real easy to see if they are in phase or not. Pull off the grill, or look in the back. You can dime the base controls, turn up the volume and push in and pull out your low E string. You should be able to generate enough slow movement to see if they are moving together or opposite. You can also use a 9v battery to see if they move in the same direction to DC.

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        • #5
          Thank you for your reply. I do have a 66 BFSR with original speakers and that amp is a killer. I'll try to hook it up tonight. I am really pleased with the sound I am getting from my TR after changing all the caps and resistors to BF specs. I put the 25K resistor back on the hum balance pot and disconnected my bias pot but now only 8.5mA at the cathode. Should I drop the resistor down some, say to 8K and try for a higher current? I'll keep my eye out for speakers, kinda low on cash these days so trying to make do with what I have. Thanks, Roy

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          • #6
            Dear mite-fine. My speakers are both pushing when 9 volts applied so I guess wiring is right. Thanks for the tip. I changed my resistor on the hum pot from 15K to 10K and now my idle current is right at 20mA. All 4 are within .04 of that, seems to be the best I can get. I am going to play it for a while now at that current, I will probably get good tube life there and try to tweak a little with preamp tubes to see if I can get a little more volume, I understand it would be ok to remove V1 if I don't plan to use the normal channel and maybe that will give the small boost I am looking for??? Thanks, Roy

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            • #7
              If you can turn it up enough to get breakup, anything you do to tweak the preamp section can only get you more gain and breakup, but not more volume or headroom. With a BF PI, I have to believe you can get decent breakup with the volume cranked. If it is still clean, then there is a problem to find.

              You mentioned the BFSR being louder which is what made me think speakers. The SR is 40 watt and the TR more like 80. That would make the TR 3 db louder all else equal, but if it has 96 db sensitive speakers and the SR 102, it will be 3 db quieter. Changing to a more sensitive speaker can be like multiplying the power by 4x or more!

              I hear you on the cash thing! I can't offer much help there.

              The

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              • #8
                I spent all day listening to sound bites and decided to go with the Texas Heats. The power rating is right for the amp and it gives nice sparkling highs, it should solve all my problems. The SR cabs speakers were much louder and had better tone, too, so somethings amiss with my speakers. Thanks again for all the support! Roy

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                • #9
                  I am glad you found the problem, sorry it was one of the more expensive things to replace. I think people often ignore speakers when modding amps because they are not cheap and also because they do not realize the impact speakers have on tone, both clean and distorted.

                  Something may have happened to the original speakers over the years, but they could have been just been a bargain find for one of the CBS accountants. Did you check impedance? Mine were 16 ohms which by itself cut the usable power in half. I was able to pull 2 power tubes and run 50 watts into the combined 8 ohm load with matched impedance for no noticeable change in output. Then when I replaced the speakers (for the first time) I went to 100 watts into 4 ohms plus the extra efficiency, tighter bass and other tone improvements and the overall change was quite dramatic. I hope you are as happy as I was.
                  Last edited by tboy; 12-07-2010, 07:29 PM. Reason: "uncorrected" spelling

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