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Old 5th June 2013, 02:35 AM   #1
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Default Heater Supply Center Tap

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.
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Old 5th June 2013, 04:50 AM   #2
DUG is offline DUG  Canada
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Don't start taking things out or replacing them just because you don't understand what they are for.

If it is not broken, don't fix it!

If you think the 0.022uF are the cause of the 60 (or 120) Hz hum, then swap them and see if the hum changes channel.

You have two channels...compare them.

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Old 5th June 2013, 05:49 AM   #3
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I don't understand what the capacitors are for that is why I am asking. I have never seen a capacitor in series to ground off the heater center tap, so if anyone can enlighten me thanks.
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Old 5th June 2013, 06:52 AM   #4
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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
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Last edited by rfengineer2013; 5th June 2013 at 07:00 AM. Reason: Transformer Confusion
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Old 5th June 2013, 07:14 AM   #5
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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.
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Old 5th June 2013, 09:21 AM   #6
DF96 is offline DF96  England
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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.
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Old 5th June 2013, 12:46 PM   #7
20to20 is offline 20to20  United States
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Quote:
Originally Posted by famousmockingbird View Post
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 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?
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.
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Old 5th June 2013, 04:09 PM   #8
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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.
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Old 5th June 2013, 06:45 PM   #9
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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.
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Old 5th June 2013, 06:57 PM   #10
DF96 is offline DF96  England
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Have you tried swapping the 5AR4? One may have a heater-cathode leak, and that might cause problems to the channel sharing that heater chain, especially if the heater chain is floating.
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