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Scottkit trouble

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I have a Scottkit LK72 amp. I did the standard restoration stuff to it.
1, rebuild the -45v bias supply
2, replace all electrolytics
3, replace all coupling caps with orange drops
4, retube

Worked fine for about 6 months.
Then one of the EH brand output tubes started arcing inside. I replaced the pair with some old sovtek 7591s I had around just to test. It seemed OK.
Now I have trouble with arcing in the Tube Dr. brand GZ34 rectifier which blows the fuse. Replace fuse, works OK for awhile then rectifier arcing-blown fuse again.
So I check the B+ at the power supply. The power that feeds the center of the output xformers. It reads 435v, the schematic says 425v... should be OK right?
Then I read the GZ34 doesn't like too much filter cap right at the output. I have 30uf there with a 30ohm pwr resistor between the tube and the 1st filter cap. I should be OK there too.

I will keep this amp offline 'till I have some idea what is going on here.

Any suggestions??
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So I found the original GZ34 tube and plugged it in. No arcing and the B+ increased to 459 volts. I found out that the Tube amp doc.GZ34 rectifier tube that arcs is made in China. No surprise it's not up to the job.The bottles I thought were Sovtek are really JJ brand.

I now have a pair of JJs and a pair of Electro- Harmonix 7591s in this thing. I noticed the EH tubes are producing alot more heat than the JJs. Gotta wonder about the currents . There are no cathode resistors so I measured the DC resistance of the output transformers. I can figure out the individual tube currents that way. Each half of an output tranny is 100ohms +/- .
so I figured out the idling currents.

JJ pair: 24,24 ma...EH pair 41,43ma. A huge difference. For the record, the B+ is 459v @ the midpoints of the xformers, the -bias supply measures -44v
at the source. Now I really would appreciate some comments here cause I'm reeeeeaaaaly wondering now!

PS If I can find 2 of the original 7591s and if they work, i'll post the currents for those later.
The series SS diode tweak works. Except for PIV issues, the Sovtek 5AR4 is excellent. Adding the SS diodes shores the weak spot up. While it may not make a difference, I suggest the inherently quieter UF4007, instead of the 1N4007. Can't hurt, may help, and is only a few cents more per diode.

The series SS diode tweak may allow a bit more service to be squeezed out of well used OS 5AR4s. The SS diodes certainly don't create trouble when a "healthy" OS 5AR4 is installed.

The Octal tubes made by JJ and shipped to North America have proven to be very unreliable. Steer clear of JJ Octal based tubes, for now.


  • SS Series Diode Tweak.jpg
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For the record, the B+ is 459v @ the midpoints of the xformers,
the -bias supply measures -44v at the source.

According to the original Scott schematic, the nominal B+ voltage is 425VDC
(after the 30 Ohm resistor). The grid bias voltage should be -45VDC at the bias pot's wiper.

With fixed, non-adjustable grid bias, such plate current variation is to be expected.
The bias circuit could be modified to allow adjustable bias, while retaining the balance feature.
You could add a 1 Ohm resistor in each cathode to ground circuit for the current measurement,
which would be safer.
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the AC is 120v now not 110v like in 1961 so I'm not surprised at the B+ being a bit high.

In this amp, there are no balancing pots. The cathodes go straight to ground.

So: I will install the top shelf rectifiers with a new sovtek 5AR4

Install 1 ohm resistors on the cathodes of the 7591s. It looks like balancing pots might be hard to fit in. so maybe on those.

Ditch the JJ tubes and get another pair of the EH output tubes
[if the EH tubes are not too good either, please let me know now. After all 1 did fail by arcing already and the B+ is a bit higher now.]

Thanx for the help everyone.
The EH 7591 tubes need more negative bias voltage than old-stock 7591s.

I usually need to increase the negative bias voltage by about 2-3 volts, to get the EH tubes to bias in the same range as old-stock. But, once that's done, they work and sound fine. Higher B+ doesn't seem to faze the EH tubes, once you get the current in the right range.

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