DynaKit ST-70 two of them

Back from the dead

Drawing 1.75 amps

I put 1" feet on each amp so I can get fingers underneath it to pick up

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Nice turn around in cosmetics. Surprised the chassis came up that nice honestly. I'd have expected more pitting and staining.
 
I revisited amp #1 and measured the same points using my multi-point test rig
The measurements were closer to normal across the board. I think this reflects the new can and precision resistors
I noticed one bias pot is rather stiff to move and one is very sensitive but they do set to 1.56
I swapped out the tube set from each amp and amp #1 is now dead quiet where it had a hum before with the original set of tubes. I suspect the 7199 tubes

Drawing just little over 1.5 amps
 
H-K leakage on the drivers, or bad balance on the outputs could both cause hum. Hard to tell on either unless you can test the 7199 and probably add individual bias test points to check the balance. It would yield different hum frequency though, H-K leakage would be 60 hz and balance issues would be 120.
 
Ahh Gosh Darn the Blankety Heck

Amp #2, after playing beautifully for a couple of hours now pegs the ammeter and blows fuses at B+

I pulled the rectifier tube and it still pegs the amp meter like never before
I have now exhausted my 3A slow blow fuse supply

Amp #2 was drawing 1.75 amps as opposed to amp #1 which drew 1.5 amps

Maybe getting that new PT was a smart move after all
 
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Ouch! :( First check for shorts between the power cord and the primary of the power transformer. Unplug the amp from the wall, turn on the power switch, disconnect the primary of the power transformer from the rest of the works, and use an ohmmeter to check for continuity at the prongs of the power cord, and at the terminals of the fuse holder. If both check open, see what reading you get at the primary of the power transformer. Did you replace the filter capacitor can in amp #2? Check and see if any of the sections have shorted to ground. Good luck!
-Adam
 
unplug the rest of the tubes. If the heater supply shorted somewhere, you'd have the same results. If its blowing fuses with no rectifier, its nothing in the HVDC circuits. Time for the old dim bulb tester.
 
Thanks guys,
Amp #2 is original except for the two 50uf caps and new 10k resistors I changed. Original can cap

I'll run the tests and report back

Amp #1, with new components is now in test with the PAS3
 
I had been monitoring bias and was in the actual process of checking bias when the fuse blew.

Okay, something I did. replace fuse and power up, powers on and when recitfier starts working the fuse blows.

Okay add ammeter and note ammeter pegs, too much to bear I shut it down

connecting the ohm meter to the power plugs I get 22 ohm reading
 
Sounds like shorted PS cap (maybe shorted rectifier tube)
If you run your tests through DBT it will likely save your fuse supply until you track it down
I see Gadget got to that before me:dunno:(DBT)
 
Where are you on this now?

Now you need two more so they can play doubles:thumbsup:

I was looking back through your thread and there did not seem to be any mention on the 'testing' of the tubes (?),.... maybe I missed that part (?). You might want to test and look to see what their output is, any shorts, gas and voltage. Yes, you'll need a decent tester for this and I didn't see one on your bench. Otherwise, you'll need some quality matched doubles. I purchased a matched quad but this is not necessary.

When I restored my ST-70 4 months ago, my EL-34s were not matched and the bias output was really funky. I couldn't get the bias voltages nailed down as they were constantly fluctuating. I also replaced the bias resistors after speaking with Dynakitparts. Once I installed the new bias resistors and replaced the tubes, my ST-70 was stable.
 
^^^ Oh, my bias resistors were out of tolerance by 10-15% too!,... you mentioned that you were adjusting bias when your fuse blew. Those pots are really touchy so tweak them ever so slightly.
 
Ouch! :( First check for shorts between the power cord and the primary of the power transformer. Unplug the amp from the wall, turn on the power switch, disconnect the primary of the power transformer from the rest of the works, and use an ohmmeter to check for continuity at the prongs of the power cord, and at the terminals of the fuse holder. If both check open, see what reading you get at the primary of the power transformer. Did you replace the filter capacitor can in amp #2? Check and see if any of the sections have shorted to ground. Good luck!
-Adam

disconnect the primary of the power transformer from the rest of the works - I'm not clear on what to do here

I ohm'ed the power cord and it does register resistance, like 8.5 ohm. It does not seem to be shorted
Using my ohm meter and capacitance measuring device I do not see any shorted sections of the can. I did not change the can cap

No visual burnt or 'sploded components



unplug the rest of the tubes. If the heater supply shorted somewhere, you'd have the same results. If its blowing fuses with no rectifier, its nothing in the HVDC circuits. Time for the old dim bulb tester.

soooo...unplug all the tubes, connect speakers, connect ammeter, connect dim bulb, power up and see if the fuse blows? Got it

I have one fuse left. I'll go get more 3A slow blow fuses

Thanks guys

I'll also read up on others who have had similar experiences

Its great to have a working spare amp to check against
 
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