Fender Guitar Amp Hum (yep, again)
Question:
Allright, I’m sure you’ve heard this one before, but here it goes: I’ve got a Fender Bassman 135. I recently changed the filter capacitors with Sprague Atoms and Illinois Capacitors. For the 220 V capacitors, Matt Griblin of Griblin Engineering (where i got the caps from) didn’t have any 220’s but sent some 100’s because supposedly, they were the caps that the Fender was supposed to use, or were the ones used on the earlier blackface designs. After I did this, the amp sounded super, no more flabby bass, but there was a noticeable loud hum at all volumes. He gave me directions on biasing the amp. I set the voltage at pin 5 around 46 volts, which is supposed to be a good range for the tubes. When I played around with the bias, the hum went down, but when I checked it with the vom, it was out of the acceptable range. I tried adjusting the hum balance, but it only affected higher frequency hum, which was minimal. This hum is centered around a low "b" whichever frequency that is. Any ideas? I’m not a technical wizard, but pretty clean with a soldering iron and I can read a schematic. I don’t know shit about plates and grids, but I know about the main components of a tube amp. check out my homepage and guitar links at www.cajunnet.com/~epguidry
Response:
>Allright, I’m sure you’ve heard this one before, but here it goes: >I’ve got a Fender Bassman 135. I recently changed the filter >capacitors with Sprague Atoms and Illinois Capacitors. For the 220 V >capacitors, Matt Griblin of Griblin Engineering (where i got the caps >from) didn’t have any 220’s but sent some 100’s because supposedly, >they were the caps that the Fender was supposed to use, or were the >ones used on the earlier blackface designs. After I did this, the amp >sounded super, no more flabby bass, but there was a noticeable loud >hum at all volumes.
There’s a problem here – I think you’re referring to the 220 uf (microfarad) caps, which are rated at 285 volts (on the Fender schematic.) These are used in series to obtain a capacitance of 100 uf. If you just drop in two 100uf caps, with no other changes, you’ll have an effective capacitance of 50 uf, which is too low. If the hum appeared after changing only the caps, that’s the problem. If you can’t get the correct caps, the circuit could be re-wired to run the two 100uf caps in parallel, for a total capacitance of 200uf, which should work fine. These caps should be rated at 550 volts or more. >He gave me directions on biasing the amp. I set the voltage at pin 5 >around 46 volts, which is supposed to be a good range for the tubes. >When I played around with the bias, the hum went down, but when I >checked it with the vom, it was out of the acceptable range. I tried >adjusting the hum balance, but it only affected higher frequency hum, >which was minimal. This hum is centered around a low "b" whichever >frequency that is. >Any ideas? I’m not a technical wizard, but pretty clean with a >soldering iron and I can read a schematic. I don’t know shit about >plates and grids, but I know about the main components of a tube amp.
You don’t need to be a wizard, but you do need to be very careful working on the inside of a tube amp. RF >check out my homepage and guitar links at www.cajunnet.com/~epguidry
Robert Fries 415-988-9475
Response:
>>I’ve got a Fender Bassman 135. I recently changed the filter >capacitors with Sprague Atoms and Illinois Capacitors. For the 220 V >capacitors, Matt Griblin of Griblin Engineering (where i got the caps >from) didn’t have any 220’s but sent some 100’s because supposedly, >they were the caps that the Fender was supposed to use, or were the >ones used on the earlier blackface designs. After I did this, the amp >sounded super, no more flabby bass, but there was a noticeable loud >hum at all volumes. >There’s a problem here – I think you’re referring to the 220 uf (microfarad) >caps, which are rated at 285 volts (on the Fender schematic.) These are used >in series to obtain a capacitance of 100 uf. If you just drop in two 100uf >caps, with no other changes, you’ll have an effective capacitance of 50 uf, >which is too low.
Actually, I took a good look at that schematic not too long ago and I’m not sure that they are hooked in series in the usual sense. The power supply is a rather different design than what is typically seen. It’s a bridge diode rectifier cicuit and the transformer has a center tap.
Response:
>I’ve got a Fender Bassman 135. I recently changed the filter >capacitors with Sprague Atoms and Illinois Capacitors. For the 220 V >capacitors, Matt Griblin of Griblin Engineering (where i got the caps >from) didn’t have any 220’s but sent some 100’s because supposedly, >they were the caps that the Fender was supposed to use, or were the >ones used on the earlier blackface designs. After I did this, the amp >sounded super, no more flabby bass, but there was a noticeable loud >hum at all volumes.
I don’t have the schematic in front of me, but I’m going to guess you meant 220uF and 100uF caps. I don’t know why this individual would have reccomended dropping the filter cap value this substantially. Something within +/- 10% would be fine, but to cut it by more than 50% isn’t a good thing. Another possibilty you don’t mention is the bias supply filter cap. Aging on this one can definately result in hum. The other possibilty that comes to mind is that you didn’t get a ground right after you finished the cap change. Mark The truth as I perceive it to be. Your perception will be different.
Response:
Doesn’t this amp have a balance bias control instead of a true bias setting pot? Unless it has been modified from stock, I believe this is the case. I don’t have a schematic, but know that most silverface amps have bias balance controls to match the power tubes. GUYS, AM I ON TARGET? If this is a balance bias amp, you should turn it on and then turn the bias control until the hum is minimal. This balances the power tubes, and thus cancels hum. Don’t do this until others comment on my suggestion, as I am just starting this type of work, and although I have read a lot, am admittedly far from expert. Best regards, Paul
Response:
: Allright, I’m sure you’ve heard this one before, but here it goes: : I’ve got a Fender Bassman 135. I recently changed the filter : capacitors with Sprague Atoms and Illinois Capacitors. For the 220 V : capacitors, Matt Griblin of Griblin Engineering (where i got the caps : from) didn’t have any 220’s but sent some 100’s because supposedly, : they were the caps that the Fender was supposed to use, or were the : ones used on the earlier blackface designs. After I did this, the amp : sounded super, no more flabby bass, but there was a noticeable loud : hum at all volumes. It is significant that this started after the caps were replaced. There is something wrong with the way the caps were installed. Getting the wiring to the first filter after the rectifiers wrong can cause hum. It is possible that one or more of the new caps are leaking. I’m not familiar with the 220V caps you say are now 100V, but I would hesitate EVER to lower the voltage of a filter cap unless I knew the circuit backwards and forwards and was certain it would never exceed the lower rating. Since 220V looks a little odd for a voltage rating, is that 220 uF instead? : check out my homepage and guitar links at www.cajunnet.com/~epguidry
Response:
– Hide quoted text — Show quoted text -> Doesn’t this amp have a balance bias control instead of a true bias > setting pot? Unless it has been modified from stock, I believe this is > the case. I don’t have a schematic, but know that most silverface amps > have bias balance controls to match the power tubes. > GUYS, AM I ON TARGET? > If this is a balance bias amp, you should turn it on and then turn the > bias control until the hum is minimal. This balances the power tubes, > and thus cancels hum. > Don’t do this until others comment on my suggestion, as I am just > starting this type of work, and although I have read a lot, am admittedly > far from expert. > Best regards, > Paul
I have two suggestions, hope this can help. I never worked on a Bassman 135, but looking at the schematics found in GT Tube Amp book, it looks similar to the high power Twin with the ultralinear output. In this configuration, the screen grids are tapped on a specially designed output transformer. If this is the case, then the bias balance pot adjustment is critical to minimise hum. Don’t use a meter, just listen and adjust for minimum hum. I once worked on a similar Twin that had another hum trouble. There was a hum balance pot, that did not look original, and also the filament winding had a grounded center tap. It worked fine, until the owner turned that pot all the way to one end, shorting half the filament winding and blowing the fuse. With a new fuse, the amp worked but the pot was left with one end burned open, resulting in being unable to balance hum properly. I think I replaced the pot and cut the center tap wire on the the filament winding, or did I remove the pot and leave the center tap on, I can’t remember. Claude
Response:
: >>I’ve got a Fender Bassman 135. I recently changed the filter : >>capacitors with Sprague Atoms and Illinois Capacitors. For the 220 V : >>capacitors, Matt Griblin of Griblin Engineering (where i got the caps : >>from) didn’t have any 220’s but sent some 100’s because supposedly, : >>they were the caps that the Fender was supposed to use, or were the : >>ones used on the earlier blackface designs. The caps should be 220 UF, 285V working voltage. They are in series for an effective capacitance of 110Uf. If you reduce this, you could increase hum. In general, I would not replace them unless they fail, since exact replacements are hard to find. That was the problem your supplier had, but I think he gave you a bum steer on replacements. Since this power supply has 520V B+ with the standby switch open, you must have at least a 285V rating on each cap (260 plus 10% for tolerance). That is the Fender spec. My amp measures higher than 520V, probably due to normal production variations in the power transformer windings, so you can see the need for conservative ratings. For new parts, I would want 300V ratings, min, and capacitance at least as high as before. Back to the hum issue. : >>sounded super, no more flabby bass, but there was a noticeable loud : >>hum at all volumes. I had this problem, and found the tubes were out of match. Looking further, one of the screen grid resistors had shorted, and weakened the tube (or vice versa?). I replaced it with an identical NOS 6L6 and solved the problem to a degree. I also upped the wattage rating on the resistors to 5W, from 2W. Note that while the tube tested good, it contributed to the problem by being so far out of match with the other tube. It needed a bias of -50V for the same current the other tubes passed at -58V, which is the nominal bias for this amp. But I believe the fault is the lack of true bias adjustments in this and most other Fender amps. The bias is fixed by resistors which are +-20% tolerance, and can be way off. The pot provided can only drop the bias to one side or the other, so can bring them in rough balance, at whatever bias the 20% resistors program. So, being a technoid, I added a bias adjust pot and precision resistors in the divider to set the bias where Fender meant it to be. Then I and balanced the hum out with the existing pot. Dead quiet in the output stage.
Response:
>But I believe the fault is the lack of true bias adjustments in this >and most other Fender amps.
I disagree with the ‘most other Fender amps’ part. All 60’s Blackface and most Silverface amps have a pot which sets the bias voltage on both tubes, and has no balance adjustment. > The bias is fixed by resistors which are >+-20% tolerance, and can be way off. The pot provided can only drop >the bias to one side or the other, so can bring them in rough balance, >at whatever bias the 20% resistors program. So, being a technoid, >I added a bias adjust pot and precision resistors in the divider to >set the bias where Fender meant it to be. Then I and balanced the hum >out with the existing pot. Dead quiet in the output stage.
I’d just rewire it to the Blackface-style bias pot, and not worry too much about precision resistors. RF Robert Fries 415-988-9475
Response:
>>But I believe the fault is the lack of true bias adjustments in this >and most other Fender amps. >I disagree with the ‘most other Fender amps’ part. All 60’s Blackface >and most Silverface amps have a pot which sets the bias voltage on both tubes, >and has no balance adjustment.
Most? My ‘68 Dual Showman was a balance bias and that’s toward the beginning of the silverface run.
Response:
: >But I believe the fault is the lack of true bias adjustments in this : >and most other Fender amps. : I disagree with the ‘most other Fender amps’ part. All 60’s Blackface : and most Silverface amps have a pot which sets the bias voltage on both tubes, and has no balance adjustment. You are correct, there is a bias adjust. In every Fender I have seen it is a bias "balance", 10K ohm. This is not to be confuse with the hum balance pot on the filament supply. The bias pot is fed at a center tap with the output of the bias voltage divider, which is potentially inaccurate due to use of carbon comp resistors with wide tolerance and temperature drift. This is suitable for balancing the bias current, but not for setting its level. This is very effective for hum cancellation and tube balance at zero-signal. It is essentially "idiot-proof", since it does not allow a user to damage the amp by misadjustment. I think it is appropriate for a mass market of non-technical users. If you believe bias is critical, you may want it set to the nominal design value. Given the balance pot is already in the circuit, I chose to keep it, but to more accurately set the bias with a 20 turn trimpot inside the amp. I also added 1 ohm 1% metal film cathode resistors to aid in monitoring the current of each tube, and used 1% bias feed resistors. This is overkill, but I had plans to try different output tubes (EL34s, 6550s) so wanted adjustable bias. This is not the only way to get it, but it does lend itself to easy restoration to stock. I am trying several other things on this amp out of curiosity. Adding an effects out/in jack, more gain, tweaking the phase splitter, automatic B+ delay and filament soft-start are on the agenda. For my purposes, this amp is ideal. Realize this is not an expensive vintage amp, it is a silver face Bassman 70, lacking reverb. That hammers the resale value way down. I have an effects unit, so I can mimic the cheesy tremolo and reverb effects (and add others) that the more expensive "vintage" Fenders have built in. I don’t think I would be so cavalier hacking on a black face Twin Reverb.
Response:
>For my purposes, this amp is ideal. Realize this is not an expensive >vintage amp, it is a silver face Bassman 70, lacking reverb. >That hammers the resale value way down. I have an effects unit,
What do those things go for anyway? I’m thinking if an early ’70’s Bassman sells for about $225, one of those Bassman 70 or 135’s should be even less. Also, what’s Sovtek Mig 100 head worth used, a little torn tolex etc?
Response:
I was interested if you feel that getting a current reading at your 1ohm resistor is more acurate than taking a milliamp reading between the B-plus, and each tube’s plate? (shunting the output transformer)?
Response:
>I was interested if you feel that getting a current reading at your 1ohm >resistor is more acurate than taking a milliamp reading between the >B-plus, and each tube’s plate? (shunting the output transformer)?
They’re both equally accurate, depending on your skills and equipment. With the cathode resistors, you can get a very precise measurement of the cathode current, with the shunt method, you get a reading pretty close to the plate current. They won’t give the same readings, due (mainly) to the current in the screen grids. RF Robert Fries 415-988-9475
Response:
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