[roger_s]

Yep. It's those AC heaters. I'm not happy with the build--the 35v 2000uf electrolytic cap's too large and the 250v 1uf polyester's almost as large. That forced me to locate the bridge rectifier and it's caps at the rear of the amp with twisted pairs running from the AC source on the PC board back there and another pair running to the existing heater leads. Lower voltage caps should let me put the whole thing near the tubes with the rectifier soldered to the PCB.

The bottom line, despite this kludge is it works! My 336 came with two 6SN7's one touted as quiet; it's the one that I first used and noticed the hum with. The second is either Chinese or Russian and was supposed to be a hum monster. That's the one that I used to test my DC heaters. Dead quiet. If there's hum I couldn't hear it over the ambient noise my HD-650's were letting in. I had my RWA iMod plugged in but without playing anything. So, how about some tunes? They're great as well. I didn't spend a lot of time listening to the 336 before modding it but the SQ certainly hasn't been diminished by the heater mod. I'm a happy camper.

I'll post some pics after I've got a cleaner install.

===============================================

I just picked up a used DV 336 and it's got the oft-posted hum. Listening to it I thought it sounded like AC heater hum. I turned it off and the hum vanished--not a perfect test but it does take a while for the caps in the HV section to drain.

So I pulled of the bottom plate, powered it back up and measured the voltage on the heaters. Sure enough, it's AC and the leads aren't twisted. No wonder it hums. I see a couple of ways to kill the hum that I've not been able to find posted on Head-fi:

Put some snap-on toroids over the heater leads to soak up some of the hum or whip up a DC circuit with a SS rectifier, a 2000 uf cap and a 1uf bypass cap.

I suspect the toroids won't be the best solution since there's no way to wrap the leads. The DC circuit will be a pretty tight fit an might introduce noise of its own given its proximity to everything else.

So, despite my failed searches, has anyone tried either and how'd they work out? Or, do they sound like reasonable solutions?

Thanks.

Roger

 

[Dominat0r]

Im not sure if this applies, however look up the Fitz mod on the DV336i.

Also, have you tried another tube? I had a DV336i for a year...had a slew of tubes, however 1 tube did give a slight hum after warming up.

 

[DennyL]

I had a DarkVoice 336 in which I tried many different valves. Some valves just hummed, some didn't hum, and some hummed for the first 30 or so hours of use. Your ideas may well be right, but you hum could also be coming from a valve.

 

[manfred77]

I have a DV 336SE, it is almost new, maybe 100 hours use. With several different 6sn7 tubes, it has a hum, but with others it doesn't. All the tubes test well into the good range. Would the mod mentioned in the original post apply to the 336SE as well?

 

[nikongod]

Please observe good safety procedures. Lethal voltages can hide in all sorts of interesting and unexpected places on a tube amp. Death is final.
Quote:

Originally Posted by manfred77 /img/forum/go_quote.gif I have a DV 336SE, it is almost new, maybe 100 hours use. With several different 6sn7 tubes, it has a hum, but with others it doesn't. All the tubes test well into the good range. Would the mod mentioned in the original post apply to the 336SE as well?

If the hum is HIGHLY conditional on the tubes used (and some will play silently) it is more likely from tube micro-phonics than anything, even with "untwisted" heater wires.

yes, switching to DC heaters will cure most of it. I would sort out whether its a microphonics issue, a "leaking noise" (from untwisted wiring which should be twisted, and possibly shielded) issue, or an issue "uncurable" by other methods. Many high-bill tube amps come with DC supplies because the MFR dosnt want to deal with people with microphonic tubes complaining.

As a benefit to an AC heater, you dont have ANY extra switching noise in that line. Although it is smaller, the noise of clicky diodes, and RF generated by a current spike at the top of the AC cycle as it charges the cap can come out of the amp... One should be careful when converting an AC heater to DC to insure that the instantaneous current draw on the transformer dosnt get too high. Use a power resistor between the rectifier and the cap. As with anything in this hobby, it is a compromise.

 

[silverrain]

My 336SE has no hum.

EDIT: My next post makes this a moot point, now.

 

[silverrain]

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif Please observe good safety procedures. Lethal voltages can hide in all sorts of interesting and unexpected places on a tube amp. Death is final. .

Reading this, I think I will sell my tube amp and avoid ever owning one again, if they are this dangerous.

 

[nikongod]

unless you want to go poking around in the enclosure you are exposed to very little risk. SS amps dont eliminate this risk, and when they fail (SS amps) can often be MORE dangerous than a tube amp.
Quote:

Originally Posted by silverrain /img/forum/go_quote.gif Reading this, I think I will sell my tube amp and avoid ever owning one again, if they are this dangerous.

There are SS headphone amps that run on voltages as high as 60V (rail-to-rail) which is easily lethal of you catch it across your chest. In speaker-vile, there are SS amps that are pushing over 100V.

 

[n_maher]

Quote:

Originally Posted by silverrain /img/forum/go_quote.gif Reading this, I think I will sell my tube amp and avoid ever owning one again, if they are this dangerous.

Provided you're not monkeying around inside the amp you have little to fear. Most solid state amps (anything with an internal power supply, not a wallwart) have lethal voltages inside them as well in the form of the ~120VAC line voltage. It is true that you have to be very careful working on high-voltage tube amps though since they tend to be point to point built and therefore have a lot more exposed connections where high-voltage can lurk.

[edit]clearly ari and I are on the same wave length.

 

[roger_s]

Second that on the safety procedures. The 336's caps drain fairly fast--down to about 4v after a couple of hours but I still drop in a 1meg ohm drain resistor via alligator clips before doing anything. And only one hand in there at all times!

The 336's heater wires are not twisted and they're a pretty heavy gauge of solid core copper. I'd guess 20AWG, maybe 22. Stiff as all get out and cut exactly to length. If I was going to opt for twisted leads I'd snip the existing wires off about 3/4" above the tube sockets, strip the insulation and use what remains as binding posts for twisted pairs of stranded wire. There's not much room to work so I think re-purposing the existing wire would probably be a better option than trying to do a full-up replacement of them.

I opted not to go with a power resistor. Partly because I've not had problems with current draw in other AC to DC heater conversions and partly because it's already a very tight fit in there; I'm not sure a resistor could be installed safely.

In any event, the nice thing about this mod--assuming that the existing heater wires are not clipped--is that it is 100% reversible.

Roger

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif Please observe good safety procedures. Lethal voltages can hide in all sorts of interesting and unexpected places on a tube amp. Death is final. If the hum is HIGHLY conditional on the tubes used (and some will play silently) it is more likely from tube micro-phonics than anything, even with "untwisted" heater wires. yes, switching to DC heaters will cure most of it. I would sort out whether its a microphonics issue, a "leaking noise" (from untwisted wiring which should be twisted, and possibly shielded) issue, or an issue "uncurable" by other methods. Many high-bill tube amps come with DC supplies because the MFR dosnt want to deal with people with microphonic tubes complaining. As a benefit to an AC heater, you dont have ANY extra switching noise in that line. Although it is smaller, the noise of clicky diodes, and RF generated by a current spike at the top of the AC cycle as it charges the cap can come out of the amp... One should be careful when converting an AC heater to DC to insure that the instantaneous current draw on the transformer dosnt get too high. Use a power resistor between the rectifier and the cap. As with anything in this hobby, it is a compromise.

 

[Jo6Pak]

Quote:

Originally Posted by HighLife /img/forum/go_quote.gif Im not sure if this applies, however look up the Fitz mod on the DV336i.

I performed the Fitz Mod on my 336i. Two caps from Radio Shack and 5 minutes with the soldering iron. My hummers are now quiet.

 

[manfred77]

What size capacitors did you get from Radio Shack, I'm thinking of doing the Fitz Mod and I believe the size is 100uf/25v.

 

[Jo6Pak]

I used 220uf. I don't remember the voltage.

 

[roger_s]

After thinking about it for a while I decided a resistor in the circuit was a good idea given I've no idea what sort of transformer this thing's running. Prior to my DC mod the heater voltage was about 7.5DCV unloaded. Without the resistor my mod brought the voltage to about 8.7VDC. Now the electrolytic cap is tied to the positive side by a 1k ohm resistor. The voltage comes up much slower and tops out at about 6.46VDC. Much better and worth the hassle of making some interesting bends in the resistor's leads to keep the smoke and sparks in their places.

Thanks.

Roger

Quote:

Originally Posted by nikongod /img/forum/go_quote.gif Please observe good safety procedures. Lethal voltages can hide in all sorts of interesting and unexpected places on a tube amp. Death is final. If the hum is HIGHLY conditional on the tubes used (and some will play silently) it is more likely from tube micro-phonics than anything, even with "untwisted" heater wires. yes, switching to DC heaters will cure most of it. I would sort out whether its a microphonics issue, a "leaking noise" (from untwisted wiring which should be twisted, and possibly shielded) issue, or an issue "uncurable" by other methods. Many high-bill tube amps come with DC supplies because the MFR dosnt want to deal with people with microphonic tubes complaining. As a benefit to an AC heater, you dont have ANY extra switching noise in that line. Although it is smaller, the noise of clicky diodes, and RF generated by a current spike at the top of the AC cycle as it charges the cap can come out of the amp... One should be careful when converting an AC heater to DC to insure that the instantaneous current draw on the transformer dosnt get too high. Use a power resistor between the rectifier and the cap. As with anything in this hobby, it is a compromise.