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Old 22nd January 2015, 05:51 AM   #1
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Default New to amps - Long-tail pair and heater hum

Sorry for a longish first pose here, but it's a complicated question, for me anyway. I'm new to building (good sounding) amps, but not to tubes. I've repaired antique radios for years, sometimes replacing old tubes with new and making adjustments. But a SE pentode at 10% THD was about as far as the amp end went.

Anyway, just for the heck of it I decided to build an amp to plug an MP3 player into. A couple watts, enough to drive a small speaker. 2 12T10 compactrons, the dual-control pentodes (G1 and G3 connected together) as a long-tail pair, the beam side as a push-pull pair. A output transformer pulled from an old Conn organ I got for free and there you go. The design is strictly by the book, copied from schematics I found on the web and tweaked for the tube's characteristics. I can post a schematic if anyone cares. It works OK, 2 watts at 0.5% THD (better if I add NFB, but good enough without it) and easily driven to 2 watts with an IPod at 3/4 volume.

The problem is that there is a lot of heater hum. Heater for sure - 60Hz and running the heaters on an external DC power supply eliminates it. I know how to route heater connections, everything is twisted tight and lead well away from the rest of the wiring. The best I could find on the problem was at The Valve Wizard, which explains a bit about this circuit. The elevated cathode voltage on the long-tail pair seems part of the problem.

I tried elevating the voltage on the heater circuit as described, and it sort of works in a voodoo way. The cathodes on the long tail pair are at about 70V, at around 45 volts on the heater circuit the hum is minimized. But the actual hum waveform (on a scope) varies wildly with the voltage I use, it's not a simple "get's worse or get's better" kind of thing.

Which basically means I don't know how it works and I don't know what I'm doing, and so I don't trust it. Anyone out there know how I should approach this? If I could get rid of the hum it would be a nice toy, and the 12T10's light up in an impressive way making a cool conversation piece if nothing else
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Old 22nd January 2015, 06:45 AM   #2
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Put a CCS in the tail. No more common mode noise, and less distortion.
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Old 22nd January 2015, 09:08 AM   #3
SY is offline SY  United States
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How do you have the heater referenced to AC ground? Can you do a quick sketch of that?
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Old 22nd January 2015, 11:16 AM   #4
DF96 is offline DF96  England
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A hum waveform varying wildly with heater reference DC voltage could mean more than one mode of entry, and probably one of them being heater-cathode leakage.

A well balanced LTP and PP output stage should cancel out much of the hum if it is in phase on both sides, but will happily amplify it if it is antiphase. Try reversing the heater supply to one valve.
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Old 22nd January 2015, 10:01 PM   #5
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One mistake I made with a valve amplifier was to mount the mains transformer too close to the valves. The valves were picking up stray magnetic fields.

One strange thing I did find was using an 12ax7 I was getting hum.
If I used the 12ax7 as consecutive stages I got zero hum but using only one stage I got a lot of hum.
For some reason the consecutive stages were cancelling out hum.
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Last edited by nigelwright7557; 22nd January 2015 at 10:03 PM.
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Old 23rd January 2015, 04:10 PM   #6
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All,
Thanks for the tips. Re SY's point, there was an issue with the grounding that I introduced when I rewired for 12 volt heaters. The original design used 24BF11 tubes, with a 24V center-tapped transformer (center tap grounded). I just moved one wire to the tap for 12 volts, so the heaters were grounded at one end, not the "center". Using a couple resistors to make a fake center and grounding it helped a bit.
Re "DF96" - this was a bigger issue and rather than waste words on the details I need to draw up some schematics, which will take a bit. Suffice it to say that a modification I made to allow negative feedback on the "follower" side of the long tail pair was introducing hum. When I just grounded the grid with a capacitor a large part of the hum went away.
It's now down to where the hum sounds barely louder than the hiss.
More work to do, and probably more questions, but let me get some diagrams up here first.
Thanks!
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Old 25th January 2015, 04:33 AM   #7
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Default Schematic

Here is the schematic. The original "NFB input" network used a cap that was 10X smaller and resistors 10X larger. Redesigning it as shown had the biggest impact on the hum. Not sure entirely why, but I can see how lowering the impedance from gate to ground could do thus.

The rest of the design is pretty conventional, except that I use a 110v -> 24V(CT) transformer connected to a "backwards" 240/480V -> 24V transformer to get the B supply. You can get 24V control transformers of various voltage and power ratings dirt cheap on EBay, especially when compared to "real" tube power supply transformers.
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Old 25th January 2015, 08:49 AM   #8
SY is offline SY  United States
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Can you show how you have the heaters wired?

The suppressor grid connection to the control grid- can you elaborate on that a bit?

And finally, you might consider wiring the screens together and using a single RC so that any noise there is guaranteed to be common mode. Minor point, but it saves two parts and potentially could increase noise rejection.
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Old 26th January 2015, 03:18 AM   #9
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The 'T10 compactron is a combination of a beam pentode for audio and a "dual control" pentode intended (according to the spec) as an FM detector. I'm not familiar with this use. The "ordinary" control grid G1 has a tranconductance of 1000 uMho. The "suppressor" grid G3 is built as a less sensitive control grid with a trans of 400 uMho. All I can say is I tried it connected to the cathode and connected to G1 and found that the tube was more linear and showed higher gain then they were connected together. It assume it still works as a suppressor too.

I have a lot of compactrons (I surf eBay for bargains when I get bored) and most of them are special purpose tubes for TVs. You just have to take what you get and mess around with them sometimes. I've used them for several radio circuits. This is the first time as an amp and I'm basically guessing and measuring the results.

I'll try wiring the screens together tonight and see what it does. I'm also looking into making a CCS per MrCurwen's suggestion. I've found a tube that might work (trying to be a purist here, I probably have some HV transistors that would work too.)
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Old 26th January 2015, 01:56 PM   #10
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In many cases simply grounding one end of the heater circuit may eliminate hum. If you elevate the heater circuit, you still need to AC ground one end by a suitable electrolytic capacitor.
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