So I have this 45wpc KT88 PP stereo power amp and lately there is hum in the right channel that sounds like 60hz. I swapped tubes and that didn't help, I checked all the power supply capacitors and they all seem fine. The builder used one power transformer for both channels but uses two 5AR4's (one for each channel) and looks to be wired as half wave rectifiers with the plates in parallel, from the power transformer one high voltage secondary goes to both plates on one 5AR4 and the other HV secondary goes to the other 5AR4's two plates, the center tap goes to ground.
The only thing that I can see that is strange is that the builder had placed .022uf caps in series from the heater center tap to ground. What is the purpose of these?
Thoughts on what to tackle first would be very helpful. Maybe I will replace the two .022uf caps on the heater supply or just remove them if they aren't that beneficial. Or for $60 I could just replace all the filter caps. All this will be cheaper than having it shipped to the builder.
The only thing that I can see that is strange is that the builder had placed .022uf caps in series from the heater center tap to ground. What is the purpose of these?
Thoughts on what to tackle first would be very helpful. Maybe I will replace the two .022uf caps on the heater supply or just remove them if they aren't that beneficial. Or for $60 I could just replace all the filter caps. All this will be cheaper than having it shipped to the builder.
Can you post a schematic of the amp? Would help a lot to see what the design looks like.
For instance, your description of the 5AR4 connections has me wondering if your power transformer has two separate HV secondaries or a single secondary with the 5AR4 plates in each tube wired in parallel.
The schematic would make this clear, and also might have other aspects of the design for others to look at which might shed some light on your problem.
As for the 0.022uF bypass on the heater CT, that sounds like high-frequency bypassing for the heater. The heaters do not have a ground connection (I presume) and thus float relative to the circuit ground. The bypass on the CT takes higher frequency signals to ground, but is not much of a bypass at 60Hz (nor should it be!).
If the amp played fine, and nothing has changed in your setup (new speakers, new source, changed cables, plugged it into the wall-power different, etc) before you started hearing the hum, it's likely a component has gone bad - likely either a bypass cap or a tube.
Troubleshooting by remote is challenging, but at least with a road map of the circuit folks here can start suggesting ideas to you.
Good luck
Sam
For instance, your description of the 5AR4 connections has me wondering if your power transformer has two separate HV secondaries or a single secondary with the 5AR4 plates in each tube wired in parallel.
The schematic would make this clear, and also might have other aspects of the design for others to look at which might shed some light on your problem.
As for the 0.022uF bypass on the heater CT, that sounds like high-frequency bypassing for the heater. The heaters do not have a ground connection (I presume) and thus float relative to the circuit ground. The bypass on the CT takes higher frequency signals to ground, but is not much of a bypass at 60Hz (nor should it be!).
If the amp played fine, and nothing has changed in your setup (new speakers, new source, changed cables, plugged it into the wall-power different, etc) before you started hearing the hum, it's likely a component has gone bad - likely either a bypass cap or a tube.
Troubleshooting by remote is challenging, but at least with a road map of the circuit folks here can start suggesting ideas to you.
Good luck
Sam
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Hello thanks for the resoponses,
I unfortunately do not have a schematic for the amp but it is basically pretty simple, tomorrow I can draw one up and post it.
The power transformer only has a single HV secondary, each tap feeding seperate 5AR4's (one for each channel) with the plates in parallel.
Nothing has changed in the system whatsoever, it goes Mcintosh C2200--->custom power amp--->Klipsch Cornwalls. One night changing CD's I noticed the hum so I changed tubes, switched around speaker wires etc.. The only tube I didn't swap was the Mullard GZ34's because I didn't think rectifiers tubes were a source of noise.
I unfortunately do not have a schematic for the amp but it is basically pretty simple, tomorrow I can draw one up and post it.
The power transformer only has a single HV secondary, each tap feeding seperate 5AR4's (one for each channel) with the plates in parallel.
Nothing has changed in the system whatsoever, it goes Mcintosh C2200--->custom power amp--->Klipsch Cornwalls. One night changing CD's I noticed the hum so I changed tubes, switched around speaker wires etc.. The only tube I didn't swap was the Mullard GZ34's because I didn't think rectifiers tubes were a source of noise.
Heaters should not float; they should always have a DC reference (the simplest is just ground). Some people don't understand this and think that a capacitor to ground is sufficient. If they don't know what voltage their heaters are floating at, how can they know that they are within the heater-cathode voltage spec? Although they don't realise it, they are relying on leakage currents or insulation breakdown to set the heater potential.
Go back to the front of the amp. Sometimes hum is picked up on the RCA cables crossing too close to a transformer from another piece o' gear, or flourescent light. The cable connector could be just a little dirty so the shielding doesn't do its job 100%. RCA connections and cables are highly susceptible to external 60Hz sources. Try swapping the input lines to see if the hum moves, if so it's external for sure.
The CT caps could be blocking a DC voltage component that was purposely designed to be on the heaters. If that is the case, then you can't just ground those CT's or you'll create a short. You need to see the schematic or do a DC voltage check to see if there is much DC on the heaters, first. The Dynaco ST-70 uses a heater CT cap too, so don't assume it's just a redundant component.
I seen some builders reference the one side of the heaters to the cathode and install a .022 cap on the heater ct.
and Ive seen the heaters referenced through two 100 ohm resistors voltage divided referenced to output cathode that is bypass with a 220 uf cap and a .0022 on the heater tap.
in both cases, the heaters are dc referenced so I wouldn't recommend grounding the center tap.
maybe replacement of these caps.
on another note, I had to switch my power outlet on my studio wall because no matter what I plugged into it, it hums.
and Ive seen the heaters referenced through two 100 ohm resistors voltage divided referenced to output cathode that is bypass with a 220 uf cap and a .0022 on the heater tap.
in both cases, the heaters are dc referenced so I wouldn't recommend grounding the center tap.
maybe replacement of these caps.
on another note, I had to switch my power outlet on my studio wall because no matter what I plugged into it, it hums.
Ok do I have more timet to look at the amp. I can't find any voltage divider or anything else the heater supply is connected to. It can't be a wall outlet ground loop because it would be in both channels I presume. The channels have seperate heater supplies and they seem to be left floating.
I haven't drawn a schematic but a block diagram looks like this;
First gain stage is a 12AX7 with both triodes in parallel, directly coupled to a 12AU7 LTP phase inverter into the KT88's. The KT88's are fixed bias.
I will set the amp up with dummy loads and take voltage readings and or start paralleling a known good cap with filter caps to see if the noise goes away, for the latter I will have to hook them up to speakers for I do not have a oscilloscope.
I haven't drawn a schematic but a block diagram looks like this;
First gain stage is a 12AX7 with both triodes in parallel, directly coupled to a 12AU7 LTP phase inverter into the KT88's. The KT88's are fixed bias.
I will set the amp up with dummy loads and take voltage readings and or start paralleling a known good cap with filter caps to see if the noise goes away, for the latter I will have to hook them up to speakers for I do not have a oscilloscope.
Just tried a new 5AR4 in that channel and the same noise persists.
I plugged her in and have 6.6VAC on the heaters with zero DC volts when checked to ground. So heaters are left floating, maybe builder forget to create artificial center tap? But if this was a design flaw it would have been present from day one.
I plugged her in and have 6.6VAC on the heaters with zero DC volts when checked to ground. So heaters are left floating, maybe builder forget to create artificial center tap? But if this was a design flaw it would have been present from day one.
Does this amp have a built-in volume control or are you using a preamp with RCA cables?
Do you have any input shorting plugs you can use at the input? If not just pull the stereo cables from the source device and use a paperclip or something to short the signal in. Then tell us if the hum is still there, better or worse.
Do you have any input shorting plugs you can use at the input? If not just pull the stereo cables from the source device and use a paperclip or something to short the signal in. Then tell us if the hum is still there, better or worse.
I have the amp on my coffee table with the input jacks shorted, and there is no volume control on this amp.
The amp is only 7 years old. I didn't swap tubes from one side to the other but I tried other tubes I know that are good and the same noise is present.
Ok so I just used a known good 47uf 500v cap to parallel with the filter caps and bias supply caps and the noise is still present. I might try to ground that center tap and see what happens.
The amp is only 7 years old. I didn't swap tubes from one side to the other but I tried other tubes I know that are good and the same noise is present.
Ok so I just used a known good 47uf 500v cap to parallel with the filter caps and bias supply caps and the noise is still present. I might try to ground that center tap and see what happens.
Well I got side tracked and now I have to get ready for band rehearsal. I will look into it later tonight or tomorrow. If anyone can think of something I am overlooking that would be great, you know what they say two heads are better than one. What I forgot to do is run the amp with no phase inverter and see if the noise is present.
I was positive a cap went bad but after paralleling all the electrolytics on that side to no avail I am stumped. It's not like it's not listenable, you can barely notice the noise but I am anal and it will bother me. My Scott 299 is dead quiet through these speakers which is what I expect this custom amp to be (and it was).
I was positive a cap went bad but after paralleling all the electrolytics on that side to no avail I am stumped. It's not like it's not listenable, you can barely notice the noise but I am anal and it will bother me. My Scott 299 is dead quiet through these speakers which is what I expect this custom amp to be (and it was).
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Did you try moving around your speaker wires? You can pick up hum if one channel wire is too close to AC line/outlet. I'd be checking to be sure it's not environmental, I.E. wiring issue/light/light switches (dimmers) etc. Your speakers are fairly sensitive and can reproduce easily AC induced hum, maybe you moved something around recently and now the AC is too close to speaker wires... took the cover off the computer or plugged in the computer power supply charger to the same circuit.. .there's soooo many ways to get AC hum from what ya got laying around and you don't even think about it. I'd unplug everything else on the house circuit you are using for your system except the audio gear and be sure it's not anything in the room or on the circuit your audio is plugged into.
Cheers,
Bob
Cheers,
Bob
I'm with bobrown14 on this one. From what I've read, if I am reading it correctly, the OP has an amplifier that has been quiet for 7 years and now has a slight but noticeable hum in one channel.
The first question is " what has changed". If not in the setup, then in the environment.
Before digging "under the hood" and making circuit modifications or changes, it would make sense to pull the components apart - unplug everything from everything - and start plugging things back together one thing at a time.
Check the cable routings, make sure everything plugs into the same wall socket (or use a common power strip), untangle all the cables (if they're tangled ), etc.
OK, once you've done all that if the problem is still there then it's time to start digging into the amp.
DF makes a good point about the floating 6.3V heater - it is generally a good idea to bias the heaters using a voltage divider off the B+ (HT) voltage.
In older designs, and most commercial stuff sold during the Age of Tubes, the heaters were left floating, or RF-bypassed using a small capacitor (the 0.022uF cap in your circuit is probably there for this purpose).
In modern amps, especially with sensitive speakers, the 60Hz heater voltage can couple to the cathode of the tubes and come out the speakers as hum.
To make the bias circuit, all you need is two resistors and a bypass cap, 0.47uF or so, rated at >50V for a bias voltage to 30-40V. If you have a bigger electrolytic, it's OK to use it - the more the merrier - but you don't need excessive filtering on the bias network to achieve your purpose (the heater will still have 6.3VAC on it anyway...)
The heaters will draw no current from this bias network, so you can use a 150k-ish first resistor (off the B+), and calculate something in the 30k-ish range for the lower resistor (to ground) using Ohm's Law to get the desired bias voltage from the actual B+ voltage in your amp.
Bypass the lower resistor to power-ground using the bypass cap.
Connect the bias voltage to the 6.3V center tap or tack it to one side of the heater at a tube socket - whichever is easier. Usually, it's easiest to do this back in the power supply - whatever you do, be sure to use the B+ and not the plate of one of your tubes to tap the high voltage...
In this type of circuit, the heaters do NOT return to system ground through a DC connection - so in effect the heaters now "float" at a controlled voltage but still get all of their current from the 6.3VAC transformer.
Finally, be sure to check the 0.022uF cap that is presently on your 6.3VCT - make sure the voltage rating is higher than your bias voltage. They come in many flavors, but the rating should be not less than your bias voltage (if it's rated higher, that's OK) for this. If it is not rated high enough, remove it. Otherwise, leave it alone (it won't hurt anything as long as the voltage rating is higher than your bias voltage)
As always, this comes with the standard disclaimer that if you feel uncomfortable or unsafe working with high-voltage equipment - please ask the Ham next door for help! These amplifiers can give a nasty shock at best, and can be lethal at worst, to inexperienced people. We were all inexperienced at one time, and survived (some would question that...) by learning from mentors or other interested people - so if you start fooling around inside the amplifier you've been warned
Good luck
Sam
The first question is " what has changed". If not in the setup, then in the environment.
Before digging "under the hood" and making circuit modifications or changes, it would make sense to pull the components apart - unplug everything from everything - and start plugging things back together one thing at a time.
Check the cable routings, make sure everything plugs into the same wall socket (or use a common power strip), untangle all the cables (if they're tangled
), etc.OK, once you've done all that if the problem is still there then it's time to start digging into the amp.
DF makes a good point about the floating 6.3V heater - it is generally a good idea to bias the heaters using a voltage divider off the B+ (HT) voltage.
In older designs, and most commercial stuff sold during the Age of Tubes, the heaters were left floating, or RF-bypassed using a small capacitor (the 0.022uF cap in your circuit is probably there for this purpose).
In modern amps, especially with sensitive speakers, the 60Hz heater voltage can couple to the cathode of the tubes and come out the speakers as hum.
To make the bias circuit, all you need is two resistors and a bypass cap, 0.47uF or so, rated at >50V for a bias voltage to 30-40V. If you have a bigger electrolytic, it's OK to use it - the more the merrier - but you don't need excessive filtering on the bias network to achieve your purpose (the heater will still have 6.3VAC on it anyway...)
The heaters will draw no current from this bias network, so you can use a 150k-ish first resistor (off the B+), and calculate something in the 30k-ish range for the lower resistor (to ground) using Ohm's Law to get the desired bias voltage from the actual B+ voltage in your amp.
Bypass the lower resistor to power-ground using the bypass cap.
Connect the bias voltage to the 6.3V center tap or tack it to one side of the heater at a tube socket - whichever is easier. Usually, it's easiest to do this back in the power supply - whatever you do, be sure to use the B+ and not the plate of one of your tubes to tap the high voltage...
In this type of circuit, the heaters do NOT return to system ground through a DC connection - so in effect the heaters now "float" at a controlled voltage but still get all of their current from the 6.3VAC transformer.
Finally, be sure to check the 0.022uF cap that is presently on your 6.3VCT - make sure the voltage rating is higher than your bias voltage. They come in many flavors, but the rating should be not less than your bias voltage (if it's rated higher, that's OK) for this. If it is not rated high enough, remove it. Otherwise, leave it alone (it won't hurt anything as long as the voltage rating is higher than your bias voltage)
As always, this comes with the standard disclaimer that if you feel uncomfortable or unsafe working with high-voltage equipment - please ask the Ham next door for help! These amplifiers can give a nasty shock at best, and can be lethal at worst, to inexperienced people. We were all inexperienced at one time, and survived (some would question that...) by learning from mentors or other interested people - so if you start fooling around inside the amplifier you've been warned
Good luck
Sam
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