Low Level Hum in My Fender Tube Amp

Discussion in 'Tube Audio' started by tube-a-lou, Jan 7, 2018.

  1. solderjunkie

    solderjunkie AK Subscriber Subscriber

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    Good catch on the screen supply. Do you notice any difference in operation?
     
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  2. tube-a-lou

    tube-a-lou Super Member

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    I still have the Gibson GA40T and now the Deluxe, I think the deluxe really sounds different with the
    6SC7 tubes in it, it just sounds fat and warm.
     
  3. tube-a-lou

    tube-a-lou Super Member

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    It sounds a bit cleaner and thicker tone I think so far. it still has a slight hum but it might
    be the way the amp and 6SC7 tubes work. I was thinking of modding it to take 12AY7's
    like the later Deluxe's but I'll leave it for now.
     
    Last edited: Jan 12, 2018
  4. tube-a-lou

    tube-a-lou Super Member

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    Here's my next question, I do want to try a few of the Jupiter caps in the amp they suppose to
    be remakes of the original Astrons caps made back in the day. So when I received the amp
    by the first position they have two .1uf 200volt caps, should I put the same voltage back in there
    or go to 600volts?
     
  5. knockbill

    knockbill Addicted Member

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    They are input caps,,, schem calls for 200V,,, should be fine, unless you only have 600V in stock!
     
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  6. tube-a-lou

    tube-a-lou Super Member

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    No I have to order them, so I'll have to get them.
     
  7. solderjunkie

    solderjunkie AK Subscriber Subscriber

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    If you want to return the amp to stock values you might consider changing those .1 signal caps back to .05 which I believe only come in 600v in Jupiters. Those are really nice caps (I've put them in both guitars and amps) and look the part, but they ain't cheap. They are 8x the price of say a Mallory 150. Do they sound 8x better? Probably not, but if I were recapping a vintage tweed I'd definitely consider them from looks alone. Too bad they don't have vintage style electrolytics as well..
     
  8. Alan0354

    Alan0354 Super Member

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    I am going to say something OP must not like to hear. I had been a guitarist for over 10 years, performing in concerts, gigging, on TV and radio when I was young. I successfully designed quite a few guitar amplifiers with power scaling, channel switching high gain amplifier. I know how a good amp should be and how to get the sound.

    That said, components selection is way way way way over rated. You change the tone circuit by just a little bit, you change the character of the amp. The circuit of the famous Fender 59 Bassman and the Marshall Plexi are almost the same, just the value changed in the tone circuit. Fender use 0.047uF and 0.1uF for the two tone cap, Marshall use two 0.022uF. That's the main thing that is different. They are completely different animal all together. I put a lot of effort experimenting the equalization and the tone circuit to say this. In my design, I actually add an extra equalization stage I can switch in and out with a switch. You change one value, you get a different amp!!!

    Back to choosing capacitor. I can tell you. I made it a point to use ALL ceramic cheapest cap from ebay for the whole amp except the 0.1uF.....only because I cannot get 0.1uF 600V ceramic cap that easily and I have those old film caps from the old Fenders I took out before. I made it a point to PROOF caps does not matter and I did. I have two fantastic sounding amp to show for.

    I made it a point not to use carbon comp resistor, everything is metal film resistor......from ebay China express.

    Then I bought ALL tubes from ebay, the cheapest Shinghan tubes. No matching, just put them in. Then adjust the balance.

    I use the cheap Classic Tone Bandmaster output transformer on both of my designs. I actually experimented changing to a Weber Marshall 50W Plexi replacement output transformer. It is completely different dimensions from the Classic Tone. I don't notice much difference. ( only reason I can say no noticeable difference because I did not have it side by side).

    Watch out, there might be psychosomatic effect in this. But I can tell you if there is noticeable difference, I would have notice.


    The other thing that can change the sound is the wiring and layout. Follow the original component placement of the amp.

    Fix the hum problem first, one at a time. Follow the original circuit and layout. If still hum, check the balance of the output tube. You are going around in circles. This should be a very easy problem to fix.
     
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  9. Alan0354

    Alan0354 Super Member

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    One more thing, if you really want to get into this, go to this forum:http://music-electronics-forum.com/activity.php?

    This is a forum for serious guitar amp people. There are knowledgeable people there that can give you insight what to do. I used to be on that forum until I lost interest in guitar amps. Go talk to those people, I am only one person's opinion. But I can tell you the time I was there, we don't talk about what cap or resistor or transformer to use. We all agree on it's the design.

    This forum is like Diyaudio for hifi amps, they are just on guitar amps. Serious people are there.
     
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  10. tube-a-lou

    tube-a-lou Super Member

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    Thank you for the info I'll check it out, I did check my voltages tonight and my 6V6's are running
    a bit high at a tube dissapation at 13.7 watts on a Bias calculator that's online. Don't these tubes
    run at 12 watts to be there best.
     
  11. Alan0354

    Alan0354 Super Member

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    Here is the schematic for your amp, it's 5A3 that's for your year.

    Fender Deluxe 5A3 mark up.JPG

    I save the un-marked up schematic for your reference so you can download it.

    You see the two caps I marked TONE? You change those two, you can change the sound to your taste.

    You amp is cathode biased, it will bias to the design. Don't worry about the power dissipation, these kind of circuits has been the work horse for decades and proven to be reliable.

    Do you have hum BEFORE you did any modification? Or the hum appeared ONLY after you change everything? I it was quiet before, My bet is the matching of the two 6V6 tubes. You don't have a balance adjustment. If you still have the old tubes, put it back and see whether it still hum.

    I notice this old amp has the filament power by the transformer on one side, the other side is ground. They usually gives hum. So it's important for you to tell me whether you have hum before. If so, then the hum is normal. Of cause unless you want to modify it. Again I don't know your amp's worth since it's already been modified. It's up to you whether you want to dig into it more. We always do it the black face Fender did, we drive the filament with both sides of the transformer not touching ground, then we use 2 of the 100ohm resistor, one solder onto each side of the two filament wires, the other side of the two 100ohm resistor screw onto the chassis ground. That will get rid of the hum cause by the filament.



    EDIT:

    Don't worry about those 0.05uF 600V caps, those are just coupling caps, they do not change the sound of the amp, don't even touch them, don't change them.
     

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    Last edited: Jan 12, 2018
  12. tube-a-lou

    tube-a-lou Super Member

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    Hi thank you for the schematic but my amp is a 5B3 that's what I been using please check above.
    When I first received the amp the Hum was worse along with crackle's from the bad caps
    and resistor's
     
  13. solderjunkie

    solderjunkie AK Subscriber Subscriber

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    I concur with Alan0354 regarding capacitor types to an extent. I have used some of the cheapest mylar (chicklets) from Radio Shack as both tone and coupling caps as well as guitar tone caps and often preferred them over more "boutique" style caps. For a vintage amp, I suppose it depends upon the extent to which you wish to restore the old girl.

    I can't agree with this however as it is simply not true. Coupling cap values have a profound influence upon the sound of the amplifier.

    14 watts dissipation for 6V6's is still in the safe range and fairly normal, so I wouldn't worry about adjusting the bias resistor. Most of the Fenders I have had were biased in this range.
     
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  14. Alan0354

    Alan0354 Super Member

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    I meant don't have to change the capacitor, it's designed for that. It is really high pass filter. eg. if the grid resistor is 250K and cap is 0.05uF. The high pass is 12.7Hz. This is good for even bass guitar. If you low the value, you might cut the bass.

    As I said before, the two cap at the tone circuit are the key to the sound, those signal coupling caps are not important as long as you get the right frequency. That is if the 0.05uF cap gives you enough bass, changing to another brand is not going to make any difference. You want to change the sound, play with the two caps in the tone circuit. Look at the difference between the 5A3 and 5B3 circuit, only one cap changed as described in the next post.
     
    Last edited: Jan 12, 2018
  15. Alan0354

    Alan0354 Super Member

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    Basically the same circuit with slight change in values, my comment still applies.

    You can see the Bass tone cap is 0.01uF rather than 0.005uF in 5A3. Also, the input resistor is 50K vs 75K in 5A3.

    Fender Deluxe 5B3.JPG

    5B3 you have is a little more bassy in both ways. The higher the value the resistor at the input, the more highs you get from the guitar. The higher the value of the bass tone cap, the bassy it is.

    That's one thing I don't like the old tweet amps, to me, they are too muffled. I like the tone stack of the newer Black Face and the Silver Face. I would use the 0.005uF(0.0047uF standard value) instead of the 0.01uF. I don't like the tone stack on this one. only one knob to control the tone.

    I would use over 100K for the resistors. But this gets a little tricky to modify.
     
    Last edited: Jan 12, 2018
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  16. Bill Ferris

    Bill Ferris Well-Known Member

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    What I`ve found to be an often over looked but very important and simple thing to do on all Fender tube amps is to first tighten all input/output jack`s nut with a 1/2" nut driver first, then all controls, especially the ones with their low side tab soldered to the back case/cover of the control .
    Then look at all solder ground connections from the brass control back plate ground buss to the chassis.

    When these are addressed, often there is little to no hum/noise, all other things being in good order in these fairly high gain amps, and they sound more dynamic with all their grounds intact/tight..

    Good idea to clean the shorting tine on the input jacks with very, very fine sand paper strip that`s drawn though it pinched between the closed section, or use a relay burnishing file..

    A 9mm gun cleaning brush run in and out a few times in all 1/4 " jacks will let follow up Deoxit application do a better job afterwards.

    First thing I addressed on every one of the hundreds that crossed my bench/benches for well over 43 years while giving the amp`s internals a first time visual going over..
    These old amps have lived a very high vibratory, long and hard life, especially the combos..
     
    Last edited: Jan 13, 2018
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  17. tube-a-lou

    tube-a-lou Super Member

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    I see, well again when I received the amp it had 75K in the amp which were original so I replace those
    with the same type. also I think the .01/.005uf is the famous sagamo caps and in all my amps they don't
    hold up, the other one I know does it's one of those square caps (forgot there name now I just got up) with
    the dot's on it. I did put in a new Bias resistor in it last night a 300 ohm and I got it down to 13 watt's.

    I just checked the cap in mine and it's a .005uf cap not .01uf
     
    Last edited: Jan 13, 2018
  18. Alan0354

    Alan0354 Super Member

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    Sound like whoever worked on the amp, it modified back to 5A3. I would change the 300ohm back to 250ohm. Don't worry about the power dissipation, it works. People have been pushing the tubes pass the spec and they live a good long live. 250ohm gives you a bigger class A region.

    As I said, I don't believe in different caps as long as it's the same value. BUT, be careful, the value of those caps can VARY a lot even though they claim are the SAME value. Like a 0.05uF cap can measure from 0.04 to 0.06uF. If it is the tone cap, it WILL make enough difference for you to hear it and you might think one brand sounds different than the other. I did not measure and test, but I know for a fact that volume pot from different brand measure differently.

    In guitar, the volume pot really affect the sound of the guitar. the lower the value, the more highs ( sparkle) you loss. I don't remember the brand name, but people keep saying that one sounds better for fender Strat. It's 250K pot. But they consistently measure 220K or so. So this pot gives a little warmer sound for strat. If you use other brand, they might measure 270K, that's enough to make the difference that people can hear. Capacitor is the same, a lot spec 10% or 20%, that's enough to make an obvious difference in the sound.

    You can do experiment, measure the cap of different brand, put in the caps of exact same value, then listen. My bet is you won't hear the difference.

    Your 75K is the load for the guitar, changing the value will change the sound. The two tone caps will change the sound if you change values. Those 0.05uF caps will NOT change the sound as long as you have the SAME value. As I said, I use cheap caps, never have any complain.
     
  19. Alan0354

    Alan0354 Super Member

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    As I said, go to Music Electronic forum and ask on the caps and resistors and modification. Even though I designed guitar amps, I have limited experience as I did not spend my life on it. I am more into theory, different type of electronics. Guitar amp does not need theory, it's a lot of trial and error. Those people there are serious guitar amp people, some have a life time experience in guitar amps. That worth more than a long time engineer.
     
  20. Alan0354

    Alan0354 Super Member

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    Something unrelated to your amp. If you are old enough and use effect pedals back in the older days, you would notice if you string a few pedals, you guitar sounded very muddy even you switch out all the effect. I used to think it's the connection and connector or switch. Actually what happened was those pedals are made too cheap, they don't sue DPDT foots witch. They use DPST to basically choose either the input or the output of the pedal circuit. That is the circuit is ALWAYS loading the guitar. Those circuit has input impedance like 200K or so. So every pedal you add, you add a 200K resistor in parallel with your volume pot in the guitar. So the more pedal you string together, the guitar sound like crap sooner or later.

    The famous Cry Baby wahwah is a very good example.

    Now, the pedals mostly use "true bypass", that is they use DPDT to truly switching out the pedal circuit so it does not add any loading to the guitar. You can string a whole string of pedals and it won't affect the sound.

    People that don't know electronics think that's Voodoo, that certain switch or certain circuit won't affect the sound. Hell, I used to think that way too.
     
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