A special kind of hum - Snider amplifier

[kkatarn327]

On my bench I have a Snider boutique amplifier, the 234 Basic I believe, with EL84s. With those of you not familiar with these amps, as I was, it's a pretty basic AB amp; 3x12AX7 pre, 2xEL84 power, 5Y3 rectifier, master volume, treble & bass tone stack. It has some fancy boutique features on it, such as a half power switch, master volume toggle, and gain boost but relatively basic in design.

Anyways, I'm getting a low but very present hum out of it. It is both pre-amp and master volume dependent, and is also affected by the tone stack (i.e. the bass control full up will make the hum very bassy, vice versa). It is also inversely proportional to the signal, meaning that the louder (harder) the guitar is played the less hum there is. If you play hard enough, you can eradicate the hum entirely however only while you're playing; as soon as note decay starts the hum creeps back gradually.

I have eliminated the power/filter caps, power tubes, and anything power section-related as the source, being that the hum is so sensitive to the controls. I am 99% sure it isn't the preamp tubes as I have replaced all of them in multiple configurations, the only possibility being that my whole batch of tubes is bad. Being a boutique and relatively new amplifier, I doubt component deterioration or oxidation is the issue; regardless I thoroughly cleaned all jacks, pots and components. I also found no visible damage, faulty parts, or burn marks inside the amp.

This leaves my primary suspect to be the rectifier tube. First of all, the tubes are all brand new except the 5Y3, which is an old RCA tube. Secondly, I have never had an amp with a hum that had these or similar characteristics (particularly the proportionality to the guitar signal), and on the same token I rarely get amps in that still have rectifier tubes. Thirdly, I don't have any other rectifier tubes that I can swap to properly diagnose it so it is the last thing (that I can think of) I have not yet checked.

Despite this overwhelming evidence against the rectifier tube, I don't want to order a new one until I am more certain that the rectifier tube is in fact the problem, for many reasons (money being among the first). For those experienced tube amp players out there, is the hum that I have described in one way or another characteristic of an aging or failing rectifier tube?

EDIT: Also, forgot to mention that the hum is present whether the input is engaged or not.

[Billy R aka DynaFreak]

Rectifier tubes are not usually the cause of hum in tube amps.
Assuming the amp was properly designed and the layout is not contributing to the hum (i.e., the amp did not hum when new) then I would check preamp tubes, filament supplies and grounding, and power supply supply to the preamp. You have already isolated the problem to the stages before the tone control, and have checked the tubes. Is the heater supply AC or DC? A DC filament supply will have some filter caps associated with it that might be bad. An AC supply might have a center tap or "virtual center tap" ( a couple of low value resistors or a pot) that might be bad or have a bad connection. Lastly, the power supply for the preamp might have a bad cap. You can check for too much ripple on the power supply by setting your dmm on AC volts and probing the DC supply at the tube upstream of the plate resistor. You should not have more than a few mV of AC. anyway, thats a start, I am sure some of the experts can add more.

[kkatarn327]

Bingo! Going to test the 1000uF cap on the heater line I found that one of the legs had broken off. I shall replace this tomorrow and see if it solves the issue. Thanks for the tip!

[Chuck H]

If there's a 1000uf cap on the heater line I'd be interested in seeing a schematic of that circuit. Is it a DC heater supply?

The hum proportional to signal thing isn't uncommon. What happens is that the amp hums and is idling in class A so you have generous current and amplification in idle. Then when the amp is driven by a signal it is driven into AB1 almost instantly. So the hum decreases (either or both) because the power supply is sagging and actually reducing the amps output from it's idle state and/or the hum is decreased due to better AB cancellation while conducting signal than the amp has at idle. I have seen it before but it's never been a profound amount of hum and not in need of a repair.

I suppose the chances are that the broken 1000uf cap is responsible.

Other than that all I can tell you without seeing a schematic is that the hum being amplified in the preamp prior to the tone stack. Otherwise the tone stack and main volume control would not affect it. This doesn't mean that it's not a problem somewhere else in the amp. It only means it's being AMPLIFIED by the preamp prior to the tone stack.

[Enzo]

The first tube would be the one most sensitive to heater supply hum, so a broken heater cap is plausible.

The heaters can be DC and thus like a filter, but AC heaters can be elevated to DC and THAT would have a cap even though the heaters themselves were AC heated.

I hope your cap gets it. But if not, isolate the problem. First, is the hum 60Hz or 120Hz. 120Hz hum is from power supply ripple. 60Hz hum is from grounding or radiated fields or things like heater leakage.

[kkatarn327]

I was able to catch RadioShack before closing and nabbed a 1000uf cap. Replaced it and the amp is absolutely hum-free. Thanks for all the tips! I definitely learned a lot.