Odd, low level hum, Dynaco Mk II

I've got a puzzling low-level hum in a Mk II power amp with stock Dynaco board. It's one that I want to recondition as a spare, for when one of my regular ones is out for tweaking.

The hum is at 120Hz. It is low enough so that on a typical bookshelf-monitor, it is below the level of typical phono hum when the volume on the preamp is set at normal listening level. However, on high-efficiency speakers it's audible at the listening chair. It occurs even when the input jack is shorted.

I first swapped the 6AN8 tube for one that was in one of my "quiet" amps. No change.

I then tried a set of EL34 output tubes from a "quiet" amp. No change.

I tried another 5U4 rectifier. No change.

I lifted the coupling capacitors from the triode section of the 6AN8 to the grids on the output tubes. No DC leakage on either one (the originals have been replaced with Sprague Orange Drops). So, now the only thing the grids on the EL34s were seeing was the bias voltage. No change, and this would, I think, eliminate the input board as a cause.

I swapped in a new silicon diode and filter capacitors into the bias circuit. No change.

The power supply has been beefed up considerably on the underchassis, with each section at about 150-200uF, even the first one, which is too large as per "conventional wisdom". Each section seems OK, no excessive leakage when checked with an ohm meter, and no improvement when more capacitance was jumped across them. I lifted the first section and tried 30uF, as per "conventional wisdom." The hum got WORSE.

I installed resistors, 100 ohms/1 watt in series with the screen grid leads from the output transformer, as a suggested stability measure. No change. I then used the same resistors to connect the amp in triode mode. No change. I put them back in series with the screen grid leads.

I installed separate bias controls for the two output tubes, since I was planning to do that anyway; and separate cathode resistors at the output tubes. Adjusted the EL34s for balance by meter AND tried to do so for minimum audible hum, but there was no improvement when adjusted optimally.

I reconfigured the grounding scheme to a star ground at the quad filter capacitor, which is no longer in use but is still mounted to the chassis. No change.

The amp already has a three-wire power cord. I lifted the ground. No change. I swapped the polarity of the cord. No change.

I can't think of anything else but an unbalance of some sort in the output transformer, which would obviate the entire project. Any more ideas out there?

I like this challenge. Can you take some good photos of the underside of the amp? Have you tried moving the filament CT ground from the capacitor to a ground point on the PC board?

Although its not the problem here I would pay attention to the maximum cap size recommended after the 5U4. You'll risk damaging your rectifier exceeding the recommendations. The amp shouldn't hum with the recommended caps in any case.

Dan

danf wrote:I like this challenge. Can you take some good photos of the underside of the amp? Have you tried moving the filament CT ground from the capacitor to a ground point on the PC board?

Although its not the problem here I would pay attention to the maximum cap size recommended after the 5U4. You'll risk damaging your rectifier exceeding the recommendations. The amp shouldn't hum with the recommended caps in any case.

Dan

That's one thing that's so puzzling. This amp has capacitance in gobs, about an order of magnitude higher than stock. So do my two "regular" Mk II, but they're absolutely silent!

I'll try moving the filament center tap ground again. I've tried it grounded to the terminal strip as stock, ungrounded, and grounded to the star ground at the four-section cap. No differences anywhere. Now I'll try grounding it to the PC-1 ground.

Since last time I've lifted each of the filter sections and clip-wired in a 200uF/450V capacitors left over from a prior project, with no resulting changes (I did lower the line voltage when I did this, so the 450V test capacitor would not be overvoltaged).

No luck on moving the CT ground on the filament supply to the PC-1 board, either.

I'll see if I can locate a comparable choke (OTHER than taking one out of my Stereo 70!). In the meantime I put a 47 ohm 100 watt power resistor into the circuit temporarily (no way it would FIT!) and there was no change.

The only clue I've come up with is:

> The power supply has been beefed up considerably on the
> underchassis, with each section at about 150-200uF, even the
> first one, which is too large as per "conventional wisdom".
> Each section seems OK, no excessive leakage when checked with
> an ohm meter, and no improvement when more capacitance was
> jumped across them. I lifted the first section and tried
> 30uF, as per "conventional wisdom." The hum got WORSE.

This suggests to me that there is excessive current draw someplace downstream from that first filter capacitor section. The 50 ohm resistor does look like it has run quite hot at some point in its life.

You might want to check your preamp, they can make hum noises too.

Or else, why not try increasing the negative feedback, the Mk II may be over-sensitive now.

Don't think so.

Note that I mentioned the hum is present even with the input jack shorted; i.e., no outside signal coming into the amplifier

AND that the hum is present with the coupling capacitors to the output tube grids disconnected, so the grids see no signal from the 6AN8 stages, only the bias voltage...which section I also essentially rebuilt with a new rectifier and new filter capacitors and separate bias adjustments for each output tube.

I suppose I should next try MEASURING the S/N to see how far off spec it really is.

Are you sure the output tubes are really well matched? If you have a mismatched pair you can set the cathode voltages equal, but that doesn't mean they become matched. A mismatched pair of output tubes can produce hum too.

Also, are the cathode resistors common just like in a stock model? Or have you wired them individually? You'd better wire them common as they can cancel each other's hum and distortion a little too.

A couple of detailed photos would be very helpful in further diagnosis.

Dan

Wieslaw Lipowski wrote:Are you sure the output tubes are really well matched? If you have a mismatched pair you can set the cathode voltages equal, but that doesn't mean they become matched. A mismatched pair of output tubes can produce hum too.

Also, are the cathode resistors common just like in a stock model? Or have you wired them individually? You'd better wire them common as they can cancel each other's hum and distortion a little too.

Tubes that I tried have been:
A new pair of JJ EL34
A new pair of "Edicron" (a reseller from Britain as per earlier thread) EL34
A used pair of Mullard EL34 from one of my other, quiet Mk II.

None of them did anything different with the hum; though the bias settings were all different.

The cathode resistors were separated at the same time I put in the separate bias controls, and unused pinouts on the side-chassis mount octal socket are used as measurement points.

As to the other question about photos: can't do right now, the digital SLR is on loan!

The next step is to try a choke similar to what's used in the Stereo 70 and Mark III. I just looked at the free assembly manuals on the tubes4hifi site, and at the specifications in the Dynaco catalog posted on this forum. The specification for hum/noise on the Stereo 70 and Mark III is -90dB. The same specification for the Mark II is -80dB. I'm not that confident that what I'm hearing is not close to that specification, which also says the newer Stereo 70 and Mark III are specified as being substantially quieter. Particularly in the case of the Mark III, the only reason I can see for that is the filter choke.

On the advice of another tech, I put all the ground connections together on three solder lugs firmly bolted to the chassis at the same place, adjacent to where the ground for the power transformer was originally connected to the ground lug of a terminal strip. So now instead of a star ground at the location of the quad cap, and instead of the factory layout where some grounds were far removed from the power transformer ground, now all of the grounds are together, near the terminal strip. The improvement is noticeable. Subtle, but definitely there, and the difference is audible on a Speakerlab K-Horn clone. I don't really know why this helped, but it did.

That's the first progress thus far. I've not been able to get a good measurement of S/N on the bench due to noise in the test setup (now that I've retired from active repair work, I'm on a makeshift bench in my garage). But I'm suspecting that what I've got now is getting close enough to the Mark II specification, so I'll call it that, for now.