Hopefully someone with some experience with this classic design can offer some ideas. But, I have my own questions and general thoughts (which you should take with a grain of salt because your problem may be specific to this design, which I've only read about in passing.)

The hum attentuates when you raise the volume? If the hum is actually decreasing in level when you do this, it seems like it could be a grounding issue. Does the level change when you just touch the knob?

Is it in both channels? I suppose it could be a transformer, or a tube, or tubes. Is that one tube that is taller properly seated? If the hum is only in one channel, I wonder if you might be able to swap tubes to see if there is a change or shift to a particular side.

Hi Don,

My suggestions :

1. Disconnect ALL the inputs to the Dynaco. Keep ONLY the power Cord and speakers connected. Turn on the Dynaco.

Do you hear a Hum ?

If not, your Dynaco, or its tubes are not the cause of the Hum.

( I suspect that there will be no Hum when the Dynaco is stand-alone )

2. If the Hum is present on the stand alone Dynaco, then its in the input tube(s) of the Dynaco, particularly if the Hum level increases if you turn up the Dynaco's volume level.

Your EL-34s are OK since the Hum is common to both channels.

3. If the Dynaco is Clean, the Hum is the result of a "Hum Loop" formed when the External Pre Amp is connected to the Dynaco, or the pre amp itself has hum ( I doubt the latter, though )

4. Check if the live and Neutral of the Pre Amp mains and the Dynaco mains are correct. I suspect 1 of these is connected the wrong way around.

5. If 4 does not get you results, Disconnect the Earth wire on the Dynaco's Mains lead. This will Ground the Dynaco, ONLY through the Pre amp.

I am not sure if this violates safety procedures, but I dont think it will, as long as the Dynaco is connected to the Pre....

This SHOULD CURE THE HUM ! ??

Do let us know.

Yeah, ground a 15A ckt through the shielding of an RCA interconenct, how much current capaity does the shielding carry in the event of a short in the other device. the shielding of an audio interconenct is not for carrying fault current.

It may be that you have a ground loop problem. The easiest way to find out if it is a ground loop problem is to use a cheater plug (adaptor for grounded plug to two prong socket)on the Stereo 70. It may be necessary to do some filing on the cheater plug to allow reversing it. If reversing the cheater plug results in less or no hum. Get the service shop to reverse the hot and neutral inside the Velleman (stereo 70). I used to own an original Dynaco Stereo 70. I sold it. It was one of the dumber things I've done.

I think you are going to have to experiment to see which sounds best. I lean toward using the preamp with the source and amps wide open.

Many CD players that offer a variable ouput can squash dynamics when employed. The same situation could happen with the amps and leave the sound kinda lifeless. This doesn't have to be the case, but just as passive line stages can take the boogie out of the sound, so could be the case here.

At least with the hum gone, you can give it the ear test.