Hi All,
I own a Fender Pro Junior which I have owned for about 5 years. Shortly after purchase, I renewed all the electrolytic capacitors and replaced all the signal resistors with metal film. I also redressed the wiring routing etc. It's PCB shows 2005 so would think it is a series 2. It has always been excellent and has always been without hum and noise (no i/p, no volume) except for a very small hiss if putting an ear to the speaker.
Recently I had the V1 go very microphonic so replaced it with the same make and type (Philips NOS 5751) as I originally bought them as a pair. I was very surprised to find that the replacement valve produced quite a noticable 50Hz hum when fitted. I then visited my valve box and tried 4 different makes of 12AX7's. All but one gave quite a noticable hum at different levels. The one that didn't was a Groove Tube 12AX7 which was virtually silent. Not to be put off I then purchased 2 x JJ 5751 Gold Pin valves from a very reputable dealer as the valves were supposed to have quite a good review. Hoping for great things I tried both in the Amplifier. You can imagine my disappointment when they both had considerable hum, with one a lot worse than the other. I have now re-fitted the Groove Tube 12AX7 and now everything is good.
Question: Are all valves we buy now so unpredictable in their performance? I am now at a loss, as I could buy some more Groove Tube 12AX7's and find they are hummy too.
Roger
I own a Fender Pro Junior which I have owned for about 5 years. Shortly after purchase, I renewed all the electrolytic capacitors and replaced all the signal resistors with metal film. I also redressed the wiring routing etc. It's PCB shows 2005 so would think it is a series 2. It has always been excellent and has always been without hum and noise (no i/p, no volume) except for a very small hiss if putting an ear to the speaker.
Recently I had the V1 go very microphonic so replaced it with the same make and type (Philips NOS 5751) as I originally bought them as a pair. I was very surprised to find that the replacement valve produced quite a noticable 50Hz hum when fitted. I then visited my valve box and tried 4 different makes of 12AX7's. All but one gave quite a noticable hum at different levels. The one that didn't was a Groove Tube 12AX7 which was virtually silent. Not to be put off I then purchased 2 x JJ 5751 Gold Pin valves from a very reputable dealer as the valves were supposed to have quite a good review. Hoping for great things I tried both in the Amplifier. You can imagine my disappointment when they both had considerable hum, with one a lot worse than the other. I have now re-fitted the Groove Tube 12AX7 and now everything is good.
Question: Are all valves we buy now so unpredictable in their performance? I am now at a loss, as I could buy some more Groove Tube 12AX7's and find they are hummy too.
Roger
Valves are not so much unpredictable as solder joints. Better check the work you did in the past.
Absolutely no problem with solder joints. As a retired electronics engineer I've already made doubly sure of that. Each valve I have tried gives a predictable but different hum. The Groove Tube one is almost silent.
I'm really interested in this. The last few months I have got a uTracer 3+ valve curve tracer. If you are happy to post your tube l'd measure the traces and return them to you. Would be interesting to see if there are any clues hidden in the traces. PM me if interested.
OK. It might come from the heather/cathode isolation gone bad as you describe difference in hum level for each valve. There might be an issue with the heater potential at which the valves are operated. Are you able to post a schematic?
Yep,sounds like heater cathode short though it's weird you have several valves with hum.
Therefore you either have really bad luck and several knackered valves all with the same fault - unlikely.
Or there is something about the amp or circuit as implemented that's at fault, more likely.
So,the valves need testing for heater cathode shorts and the circuit/amp needs a shufty.
Andy.
Disco, boyfarrell, Diabolical Artificer,
Thanks for all your valued replies.
boyfarrell: All the valves I tried are now back in my valve box and as I didn't identify each valve as to the level of hum each one produced, I would have to remove the back panel and go through the exercise of trying them all in the amplifier again. I'll give it some thought.
I thought about the heater/cathode connection myself and thought that it could be a possibility with my 2nd hand valves. However I would have expected the new valves not to show this problem. I agree, that the 50Hz hum can only come from the heater/cathode area.
I do not know how to insert a PDF schematic into the post, but it is available on the web. There are two available. It is the later one (I think Rev E or F)
Thanks for all your valued replies.
boyfarrell: All the valves I tried are now back in my valve box and as I didn't identify each valve as to the level of hum each one produced, I would have to remove the back panel and go through the exercise of trying them all in the amplifier again. I'll give it some thought.
I thought about the heater/cathode connection myself and thought that it could be a possibility with my 2nd hand valves. However I would have expected the new valves not to show this problem. I agree, that the 50Hz hum can only come from the heater/cathode area.
I do not know how to insert a PDF schematic into the post, but it is available on the web. There are two available. It is the later one (I think Rev E or F)
12AX7 requiring 300mA heater current (6.3V), however 5751 expecting 350mA.
If the tube heaters are paralelled, the growth is 100mA.
Weak heater PSU can cause this syndrome.
If the tube heaters are paralelled, the growth is 100mA.
Weak heater PSU can cause this syndrome.
I agree with the analysis of Diabolical Artificer but it doesn't seem to explain explain why the 12AX7 Groove Tube doesn't hum.
When looking for a schematic I noticed that early versions don't have a cathode bypass capacitor in the first stage, which makes the circuitry a bit susceptible for hum. Later versions do have a cathode bypass capacitor in the first stage. The filament supply has a anti-hum provision (the 2 x 47 Ohm resistors to ground) but ideally this would be a wire-wound pot of about 100 Ohm with the wiper to ground so you can adjust for minimum hum.
I have no idea if the above explains anything about the actual hum problem. If the amplifier worked for a long time without any hum, than the design as it is must be good enough.
When looking for a schematic I noticed that early versions don't have a cathode bypass capacitor in the first stage, which makes the circuitry a bit susceptible for hum. Later versions do have a cathode bypass capacitor in the first stage. The filament supply has a anti-hum provision (the 2 x 47 Ohm resistors to ground) but ideally this would be a wire-wound pot of about 100 Ohm with the wiper to ground so you can adjust for minimum hum.
I have no idea if the above explains anything about the actual hum problem. If the amplifier worked for a long time without any hum, than the design as it is must be good enough.
I recall — very murky, but still… — that some brands of AX had lightly twisted filament wires in order to cancel hum, seeing as the AX7's tend to be deployed 'up front' in the most sensitive parts of amplifiers.
Again, murky old memories.
The idea was like any small signal compact valve, the parallel filament wires could be formed and then as a bundle, given a 2 or 3 turn 'twist' before insertion in the emitter cathode sleeve, serving to act as a fairly reliable induced EMF cancellation technique.
There are a lot of 'old timers' here, which might remember a whole lot more.
BUT, the point is, that if your amp has a particular sensitivity to this effect, you always could resolve it by converting to filtered DC heater power. Pretty simple, actually. Schottky bridge rectify the 6.3 VAC (or 12.6 VAC), stuff into a 1000 µF can, then an LDO regulator such as the TI LM2940N ... 1000 mA of excellent regulation. Silent as a mouse.
Just saying: if hum-from-leakage is a problem, then engineer a solution that renders your amp completely independent of valve superiority and/or shortcomings.
Right?
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
Again, murky old memories.
The idea was like any small signal compact valve, the parallel filament wires could be formed and then as a bundle, given a 2 or 3 turn 'twist' before insertion in the emitter cathode sleeve, serving to act as a fairly reliable induced EMF cancellation technique.
There are a lot of 'old timers' here, which might remember a whole lot more.
BUT, the point is, that if your amp has a particular sensitivity to this effect, you always could resolve it by converting to filtered DC heater power. Pretty simple, actually. Schottky bridge rectify the 6.3 VAC (or 12.6 VAC), stuff into a 1000 µF can, then an LDO regulator such as the TI LM2940N ... 1000 mA of excellent regulation. Silent as a mouse.
Just saying: if hum-from-leakage is a problem, then engineer a solution that renders your amp completely independent of valve superiority and/or shortcomings.
Right?
⋅-⋅-⋅ Just saying, ⋅-⋅-⋅
⋅-=≡ GoatGuy ✓ ≡=-⋅
euro21 and PFL200,
My heater supply is 6.8vac which I know is a little high. I have tried two other mains transformers in it (for a different reason) and this one gives the lowest reading on full load. The one I have fitted at the moment is a Blues Junior one as it has a 240 volt primary tap on it which the Pro Junior one doesn't. The heater circuit is also fitted with the two 47 ohm resistors with their centre connection to ground. Both resistors measure roughly the same.
My PCB has 2005 printed on it and I think it is the revision 'F' board. It is fitted with the 47uF cathode bypass capacitor on V1 and also the snubber capacitors on the bridge rectifier. The amplifier is working without hum at the moment and is fine, but only because it is fitted with the 12AX7 Groove Tube in V1.
My heater supply is 6.8vac which I know is a little high. I have tried two other mains transformers in it (for a different reason) and this one gives the lowest reading on full load. The one I have fitted at the moment is a Blues Junior one as it has a 240 volt primary tap on it which the Pro Junior one doesn't. The heater circuit is also fitted with the two 47 ohm resistors with their centre connection to ground. Both resistors measure roughly the same.
My PCB has 2005 printed on it and I think it is the revision 'F' board. It is fitted with the 47uF cathode bypass capacitor on V1 and also the snubber capacitors on the bridge rectifier. The amplifier is working without hum at the moment and is fine, but only because it is fitted with the 12AX7 Groove Tube in V1.
GoatGuy,
Interesting take on the heater construction of the AX7 valves. I hadn't heard of that before.
I have read quite a few articles on people changing their heater power from AC to DC to very good effect. It makes quite a lot of sense, especially on newbuilds. I don't really want to go to all that trouble though in a commercially made amplfier that is currently working fine.
It is a very popular Amplifier and Fender have sold many thousands of them over the years and still do. My original question really was about the inconsistancy of the hum/noise levels between the same make and type of valve.
Interesting take on the heater construction of the AX7 valves. I hadn't heard of that before.
I have read quite a few articles on people changing their heater power from AC to DC to very good effect. It makes quite a lot of sense, especially on newbuilds. I don't really want to go to all that trouble though in a commercially made amplfier that is currently working fine.
It is a very popular Amplifier and Fender have sold many thousands of them over the years and still do. My original question really was about the inconsistancy of the hum/noise levels between the same make and type of valve.
All tubes (2x12AX7, 2xEL84) heater (6.3V) are paralleled.
Try to measure heater voltage with 12AX7 and 5751 too.
Try to measure heater voltage with 12AX7 and 5751 too.
I suspect it's just variations on hum susceptibility among the different 12AX7's. Is it 60 cycle AC hum? What are the brands/types that are humming? 7025 and other types were made to deal with this problem with spiral wound heaters. I see in the schematic the heaters are referenced to ground, you could try raising the reference to 10 or 20 volts if you have that available, or convert just the first tube to DC heat.
Correction: 50 cycle hum in your case.
Correction: 50 cycle hum in your case.
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What goatguy and jgf said.
"Same model name" 12AX7 does not mean they are built exactly the same, and one brand to brand difference comes from heater construction as mentioned by them.
Didn´t keep exact statistics but if you ask me, after using/replacing many of them found that, surprisingly, "chinese" ones tend to hum less than others, although there was *one* Russian type, don´t remember the letter salad after the 12AX7 bit he he, was significantly more silent than others.
Most are sold under different distributor trade names, and your particular GT one might have been actually chinese or the good russian one.
I am talking modern production ones, I use them for servicing or new builds, not a believer in Mojo (and that´s an understatement) so never pay premium prices for "assumed" advantages.
Since you are happy with the current one, so be it, but if you have to replace it in the future, you have 3 tools:
* use DC
* elevate heaters
* replace fixed filament ground reference resistors by 100 ohm trimmer and search for minimum hum
"Same model name" 12AX7 does not mean they are built exactly the same, and one brand to brand difference comes from heater construction as mentioned by them.
Didn´t keep exact statistics but if you ask me, after using/replacing many of them found that, surprisingly, "chinese" ones tend to hum less than others, although there was *one* Russian type, don´t remember the letter salad after the 12AX7 bit he he, was significantly more silent than others.
Most are sold under different distributor trade names, and your particular GT one might have been actually chinese or the good russian one.
I am talking modern production ones, I use them for servicing or new builds, not a believer in Mojo (and that´s an understatement) so never pay premium prices for "assumed" advantages.
Since you are happy with the current one, so be it, but if you have to replace it in the future, you have 3 tools:
* use DC
* elevate heaters
* replace fixed filament ground reference resistors by 100 ohm trimmer and search for minimum hum
euro21: I will do that.
jgf: The hum is 50hz.
I do not have an immediate 10 or 20v to hand without installing one especially for the purpose and I am reluctant to start modifying the amplifier just because it looks like a few 'dodgy' valves. I would really prefer to know the reason for why this is happening rather than to arrange a get-around.
The valve types tried were one 12AX7 Groove Tubes (works fine), two 12AX7 Electro Harmonix, one 5751 NOS Philips, two brand new 5751 JJ Gold Pin.
The 7025 valve choice sounds interesting.
jgf: The hum is 50hz.
I do not have an immediate 10 or 20v to hand without installing one especially for the purpose and I am reluctant to start modifying the amplifier just because it looks like a few 'dodgy' valves. I would really prefer to know the reason for why this is happening rather than to arrange a get-around.
The valve types tried were one 12AX7 Groove Tubes (works fine), two 12AX7 Electro Harmonix, one 5751 NOS Philips, two brand new 5751 JJ Gold Pin.
The 7025 valve choice sounds interesting.
JMFahey: Thanks for your input. I am very aware that most valves nwoadays are 'badged' so we don't really know whose make we are getting. That is why I have my doubts that if I order another Groove Tube one as a backup it may be as hummy as the rest. Thanks for your tips.
I’ve had brand new New Sensor 5751’s that had the h-k hum and others that didn’t. And the hum only showed up in the concertina where the cathode was up at 80 volts. Same tubes with the cathode directly grounded are quiet. 80 volts *is* within rating, but you just can’t always get away with it.
I’ve also had a NOS 12SN7 whose h-k leakage caused far worse problems than a little hum. It caused an LTP phase splitter to *latch up* about half the time on power up send 60 Hz at full clip out one side of my push pull output stage, with the other side inactive. Not fatal, but a loud and obnoxious 60/120 Hz buzz at half power into the 18” speaker. Not enough to magnetize my OPT, fortunately. Again, same tube used in a directly grounded cathode preamp stage works fine.
I’ve also had a NOS 12SN7 whose h-k leakage caused far worse problems than a little hum. It caused an LTP phase splitter to *latch up* about half the time on power up send 60 Hz at full clip out one side of my push pull output stage, with the other side inactive. Not fatal, but a loud and obnoxious 60/120 Hz buzz at half power into the 18” speaker. Not enough to magnetize my OPT, fortunately. Again, same tube used in a directly grounded cathode preamp stage works fine.
euro21: I have just measured the heater voltage with two 12AX7's and two new 5751's. The voltage on all was the same at 6.96 volts. That was higher than normal so I checked the mains which is usually about 242 VAC. Today it is at 258 VAC.
By the way, the Groove Tube valve which is nice and quiet is a Sovtek 12AX7WC shown by the writing on the side of the valve under the logo.
wg_ski: Thanks for the info. I'm glad it's not just me thats had problems. That half power buzz through an 18" speaker must have sounded horrendous.
By the way, the Groove Tube valve which is nice and quiet is a Sovtek 12AX7WC shown by the writing on the side of the valve under the logo.
wg_ski: Thanks for the info. I'm glad it's not just me thats had problems. That half power buzz through an 18" speaker must have sounded horrendous.
Thanks for the feedback, all. As is unsurprising, Fahey summarized it well. Missing of course is the 4th option... inserting the magic valve that has nulled out the induced hum, as you already have found.
BTW, one of the 'nice' things about using DC is that it can usually be an external modification to the PCB in many cases. There are some nice LDO (low drop-out) regulators out there, and Schottky diodes are almost free. $10 pretty much would solve the issue, regardless of what tubes you swap in the future. Maybe it is the irredeemable experimenter-enthusiast in me, but i almost always tweak in favor of future ease-of-support. Yah, i know, a personal problem.
;-) GoatGuy
LM1085IT-12/NOPB as an example of a 3000 mA $2 buck 12 V single package reg.
BTW, one of the 'nice' things about using DC is that it can usually be an external modification to the PCB in many cases. There are some nice LDO (low drop-out) regulators out there, and Schottky diodes are almost free. $10 pretty much would solve the issue, regardless of what tubes you swap in the future. Maybe it is the irredeemable experimenter-enthusiast in me, but i almost always tweak in favor of future ease-of-support. Yah, i know, a personal problem.
;-) GoatGuy
LM1085IT-12/NOPB as an example of a 3000 mA $2 buck 12 V single package reg.