St-70 Transformer vibrations

[jshull]

Hi all
Just finished my rebuild and all sounds great also did the revision b I have one question I seem to have a vibration coming from the transformer is this normal? I do not hear it but you can feel it if you touch the tubes or transformer. Thanks John

[TomMcNally]

It may be the choke vibrating, they are famous for that. Try tightening the screws, or shimming it with some cardboard underneath first.

[jshull]

Thanks I will check the choke to see. The only other problem I have is that when setting the pots I can not get a stable reading voltage jumps up and down very fast I have a load on the outputs I can get a stable reading if I use a analog simpson but if I use a fluke it jumps so fast I cannot read it is this norm or do I have a problem with the amp somewhere thanks john

[Shannon Parks]

Thanks I will check the choke to see. The only other problem I have is that when setting the pots I can not get a stable reading voltage jumps up and down very fast I have a load on the outputs I can get a stable reading if I use a analog simpson but if I use a fluke it jumps so fast I cannot read it is this norm or do I have a problem with the amp somewhere thanks john

Hi John,

What Fluke model is it? Turn it off auto-ranging and set it to DC if possible. Also try a different ground point. Shorting the inputs is another good idea if you haven't already.

Back to the transformer question, I will guess all power transformers will have some type of line vibration if you touch them - some worse than others. If you can't hear it rattling around in the chassis, I wouldn't worry about it.

Shannon

[jshull]

Hi Shannon
I have a fluke177 has a 600 mv range I Have been setting the wire wound dynaco pots and the pots on the board with the amplifier playing
Is it better to use the shorting plugs and 8ohm resisters to do this I have seen a post to use light bulbs?? on the outputs.
John

[erichayes]

Hi All,

A few notes:

Analog meters, by nature, have inertia in their movements and can generally resolve to only 3 significant figures by interpolation. In fact, the old VU meter has movement ballistics called out in its specifications.

Digital multimeters are designed to have no lag time (other than the gate rate) and read directly to 3 or 4 SF. Many have an "analog" bar display to make it easier to follow trends, as the least SF is usually doing a jig when measuring all but the most stable of voltages.

When measuring quiescent cathode current while setting bias, it is not only desirable but necessary to have no signal on the input. The jumping around of the meter reading is actually the signal modulating the DC voltage flowing through the cathode. A steady-state signal, such as an oscillator, can be connected to the input for purposes of monitoring cathode current vs power output, but the initial setting must be at zero signal.

A light bulb can indeed be used as a load. Use a #47 or 1847 for low level loading, such as bias setting. Use a #1156 back-up/turn signal bulb if you're planning to twist the amp's tail a bit.

[erichayes]

Hi All,

Just an afterthought . . .

If you are using any type of analog meter or gauge, be it the VOM you inherited from your dad, or the O2 and Ace gauges on your gas welding rig (or your banjo barometer for that matter), remember to tap the faceplate to jog the needle from its "stuck" point--there's always a "stuck" point--to get the most accurate reading. Sometimes you have to tap them more than once.