I have a strange amp.

[Arthur Dent]

I recently acquired an old Arc amp, made in Canada, for a pretty low price. It hums just enough to be annoying. I replaced all the electrolytics, and that helped a little bit, but not much.
The hum is coming from the preamp, as it gets a lot quieter when the volume is turned down.

See, the wierd thing about that amp is that it lacks a power transformer. The 120vac from the wall outlet is half-wave rectified and used directly as B+. I think the hum is coming from the filaments. The filaments of the three tubes, a 12ax7, a 35z5 diode, and a 50l6 power tube, are wired in series, straight to the mains. This means that there's about 40 volts on each tube's filament. The 50l6 has a 50 volt filament, and the 35z5 a 35 volt filament, so it shouldn't cause too much of a problem there. But the 12ax7 is getting more than three times its rated filament voltage. I would assume that is where the problem is.

To fix this, I have ordered a filament transformer from Mouser. I'm going to wire the 12ax7's heater correctly, and I can replace the rectifier with a solid state diode. I also did some searching on tube data to find a tube that would work with the 50L6. It looks to me like a 6L6 might work reasonably well in that circuit. It looks like the only difference between any tubes named with a number and then "L6" is the heater voltage (marked by the first number) and the voltage handling. Is that actually the case? I'm also wondering if an EL34 would work better. I'm looking for a pretty bright sound.

[Rob Mercure]

Hey Arthur,

Well, first off the 50L6 isn't a 50V 6L6 although you might be able to get a 6L6 to work with the right heater voltage. In fact only the original 12L6 is a direct 6L6 variant (and only the original 19W version) - all the other __L6s are lower power dissipation tubes (25, 35, 50) and these fall into two "families." And there should be a voltage dropping resistor in the series heater string to make up for the difference between the 97 volts of tube heaters and the 120 VAC power line.

But the real problem is that even if you add a heater tranny and change the tubes one side of the chassis is still connected to the power line and you can get the shit, or perhaps life, shocked out of you if the plug is inserted incorrectly.

What you actually need is an isolation transformer - a 1:1 ration tranny that will power the amp while isolating the chassis from direct power line connection. This is what you should order if you're ordering transformers - but you can also make one by using two identical "salvage" transformers hooking the "secondaries" to each other and driving one "primary" from the AC line and deriving an isolated 120 VAC from the other "primary" - one of the best uses for old monitor, etc. trannies.

These amps were cheaply made but shouldn't hum to an horribly objectionable level - but perhaps even now you've got the plug reversed and the chassis is hot to the surrounding house wiring - nice way to get some hum!

Hope this helps.

Rob

[Enzo]

Arthur, there are a whole lot of misconceptions gathered together in your post, and I fear for your safety.

First, this was when new an inexpensive amp, and it was a single ended one too. Single ended amps are more likely to be hummy, and especially with half wave power supplies. But they never really were quiet when running perfectly.

I have a service bulletin in my files from Fender about the CHamp. The service bulletin tells the service shops NOT to try to get all the hum out of them because it is normal for the amp. Not a defect in other words. Sure a good tech could make it a lot better by moding it all over, but they were not going to pay warranty claims for it. Your amp always had some hum.

Did you measure 36v across the heater of the 12AX7? It will be extremely bright - at least for a moment - if it actually has that. As ROb said, there should be a series resistor to drop the excess volts. I think you are assuming voltage that are not there.

Series VOLTAGES do not divide evenly. CURRENT is the same throughout a series circuit, but voltage drops are made by the individual resistances. SO if you figured three tubes, divide 3 into 120 volts, that is not even close to correct.

In series, the three tubes should drop more or less their rated voltage all by themselves, with the proper series resistor as stated.

This is what we call a hot chassis - the mains voltage is directly rectified and the chassis is ultimately connected to it. Good practice - to the extent we would call it that in this day and age - would not have either side of the mains connected directly to the chassis. The common of the power - hopefully the mains neutral - would be run by wires to points in the circuit. The preamp curcuit is grounded to the chassis, and there will be a reasonable large resistor from power common to chassis - like 47k or something. And likely a .1 cap in parallel with it for AC grounding.

The hot chassis is VERY dangerous and should really only be run through an isolation transformer.

Even if you put a heater transformer in for the 12AX7, and replaced the 35Z5 with a diode, how were you going to heat the 50L6? You would need 50v from somewhere.

If you were to install a transformer, then do it for real, just get a small power transformer with 6v for the 12AX7 and some power tube, and a high voltage winding. The 50L6 with be replaced with a 6V6 in the same socket with a heater rewire of course. A lot closer than a 6L6.

Of course if you do that, then you need to consider the existing caps. Are they rated for higher voltage?

The 50L6 is a low power tube and nothing like a 6L6, and it is rated only up to about 200v. The AC mains will rectify up to about 170VDC. That is pretty low plate voltage for a 6L6, EL34, or what have you.

Heater voltage problems are NOT the source of hum, assuming any excess over normal levels, but the thing is not probably very well shielded. And that hot chassis doesn't help.

[Arthur Dent]

Actually, the chassis of the amp is not directly attached to the mains. The mains are attached through a 270k resistor to the chassis, which is earth grounded. Most of the components in the amp are not actually chassis grounded, the power switch is used as a ground of sorts.

I measured 15 volts accross the heater on the 12ax7, still much too high. The B+ is about 130 volts, which could be a problem, as the 35z5 rectifier is only rated for 100.

The hum is certainly more than what's normal for an SE amp. It is true that the amp was not well shielded, the chassis was only closed on three sides. Also, there were two extra input jacks of the non-shorting variety. I shielded the thing on the three open sides with aluminum tape and removed all the wiring for the extra jacks. That made an insignificant difference.

[Rob Mercure]

Arthur,

Not to be too awfully flip but if you insert the plug where the "hot" power line lead connects to the 270K resistor and then you touch the chassis and something that is grounded I truly think that you are, to understate extremely, going to be "uncomfortable." And what happens if that resistor shorts, decreases in value, or the end that isn't connected to the line accidentally touches/is connected to ground?

Both Enzo and I are quite serious about the shock potential of these circuits. While they were "kosher" up to the 1960s I'm pretty sure that none have been manufactured since while many other tube circuits are in production - do you wonder why?

I don't know if you've ever gotten yourself across 120 VAC but when I was about 13 I made a "worm shocker" - a device to produce a plethora of bait by connecting a metal rod driven into the ground to the hot mains lead. I was being clever and using an old bow saw frame with a rubber grip that I was sure was well insulated. Stupid me! I had wet the lawn to better "annoy" the worms and when my wet tennis shoes made one side of the circuit and my hand hit a hole in the rubber I was "clamped" - the current locked my muscles and I could not release the saw frame that I had just pulled out of the ground. If I had not been off balance and my fall had not pulled the plug from the outlet I'd have died 40 years ago! This is something I'm pretty sure I'll remember until I do kick off!

Do not fool with this thing until you've got an isolation transformer. If you're a cheapskate like me use two identical salvaged trannies "secondary to secondary" to isolate the chassis but use a three wire cord and connect ground to the chassis. Hell, pay the shipping and I'll give you a couple of trannies - I hate the smell of burning meat!

Rob+

[Enzo]

My RCA book shows the 35Z5 rated for 700 volts.

However it is wired, the lack of a power transformer means there is a connection to the chassis, even if through a resistor. This increases shock potential - no pun intended - tremendously.

PLEASE be careful.