Scott LK-150 blowing fuses after change from SR to diode HELP!!

lenos

lenos

New Member
I have been working on an LK-150 that came from a console pull along with an LC-21 preamp. The history is unknown but these are both in near perfect condition. The goal was to get both up and running with fresh power supplies, capacitors etc.

For the LK-150, after a little bit of a thrash with the power supply electrolytics, I bit the bullet, bought and installed Hayseed Hamfest electrolytics. (I should have done that in the first place) It’s wired up per the update from 1962, as it was when I got it, and I’ve been using the build instructions, the update assembly instructions from the November 62 service bulletin and the “HH Scott LK-150 Nov62 revision schematic” for guidance.

I wired up V1 and V2 for 6U8A and am using NOS tubes in there. Power tubes are new Tung Sols and the rectifier tubes are the originals. It came to me with one of the 7199s fried, 3 original US Tung Sols and a 6L6.

Once everything was in place I brought it up slowly on a variac with the original selenium rectifier, checking voltages as I went and everything looked fine, though my voltages were a bit different than the ones showed on the schematic, they generally seem to be within reason. The oddball was the voltage at the 1.5k 2W resistor after the selenium rectifier, which was -107 V as compared to -67 shown on the schematic.

I hooked it up to speakers using a digital music player as input. It sounded awesome. I adjusted the bias using the push buttons (after correcting an unsoldered connection on one side of the meter). I listened for an hour or so and decided that all it needed to be complete was to replace the selenium rectifier with a diode, figure out what resistor was needed in there, install it and be done.

I installed a 1N4007 diode with a 10 k pot, set for 1.5 k, in place of the original SR and 1.5 k resistor. I fired it up and while monitoring the voltage at the output of the pot and attempted to adjust the pot to get the voltage into the -70 ish range. When I close to -70 V the fuse blew. Strange. I tried again and was able to get to about -90v without problem and figured that was close enough. The value needed was around 6 k and since I had the value sitting around in a 2 W, soldered it in and figured I was good to go.

With the 6 k in place, the voltage was around -90 so I figured I just needed to adjust the bias. When I tried doing so the fuse blew again, and I now noticed a brief flash in one of the power tubes, just as the fuse was blowing. I kept trying different things but the fuse kept blowing. I finally decide that I should start over, so reinstalled the SR and 1.5 K resistor. With the bias adjust pots set at about their midpoints I switched on and again blew a fuse. I’m baffled, any suggestions? Thanks.
 
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The -107VDC reading at the 1.5K resistor should have stopped you right there. Having the amplifier bias properly with -107VDC at the resistor is also very odd. Some selenium rectifiers are marked opposite of silicon rectifiers, that is, the band does not indicate polarity the same way.
You did not mention replacing the coupling caps, are they still original?
 
ramjet said:
The -107VDC reading at the 1.5K resistor should have stopped you right there. Having the amplifier bias properly with -107VDC at the resistor is also very odd. Some selenium rectifiers are marked opposite of silicon rectifiers, that is, the band does not indicate polarity the same way.
You did not mention replacing the coupling caps, are they still original?
Click to expand...
Coupling caps were changed. Yeah the -107 was odd but still the bias could be adjusted and it all played just fine. I understand the SR marking thin, which is confusing. As it was, the SR was hooked up according to the original instructions.
 
If we're talking bias supply it should be negative, though -107 seems like an awful lot. Even accounting for less voltage drop with a Si diode, that seems like more than I'd expect.

That should not make it pop fuses though. About all an excessively negative bias supply will do is make the output tubes not conduct enough. Unless its just way in excess of the cap ratings, then you might have some trouble. I'd expect the cap to pop or ooze before a fuse blew though.

What are you getting for bias voltage at the output tube grid? Pull the rectifier tube to test.
 
Check the Peak inverse voltage on the diode you used. Use something with a PIV in excess of 1000 volts, and make damn sure it and the cap are installed in the correct orientation. Often the bias leg, is a tap off of the main b+ winding. Possibility of having >100v at that diode under extreme conditions.
 
How are the socket contacts with the tubes? They may need tightening. Sounds like you could be losing contact with the signal grid and losing bias --- causing tubes to runaway. Blowing the fuse. :idea:


As for the bias voltage... check any dropping resistor prior to the bias supply rectifier diode. Could be out of range or damaged. There prolly is a resistor tied to ground somewhere downstream of that diode. Be sure it is okay and properly grounded to the chassis.
 
Thanks all for the responses.

The -107v was with the original selenium rectifier. I agree that -107 seems high but with the bias adjust pots I could get the meter centered, and as I mentioned the thing sounded great.

Also it sort a get-what-you-get situation because at the measurement point, just after the dropping resistor, there is not a lot in the circuit. The output from the transformer goes to the rectifier, then the dropping resistor (which is where I'm measuring) then the filter cap. Downstream there are two pots, one for each pair of power tubes, that are used to fine tune the bias. The dropping resistor is a 1.5K 2W and was pretty much dead on, resistance-wise.

I think it now may be a bad tube or sockets, since the only thing I did other than change the rectifier was remove and replace the tubes. I don't have a functioning tube tester but I will find one somewhere and get the tubes tested.

I'll check the bias voltage with the rectifier tubes removed and report back.

Again to recount.here is the timeline:
Replaced all electrolytics, paper caps etc, everything but mica and ceramic caps.
Brought it up on a variac, check voltages, everything seemed OK but the bias supply was high
Using the bias adjustment pots and the meter on the front of the unit I was able to adjust the bias to the proper level indicated on the meter.
Fired it up, everything was working great, at this point it is a working amp that just needs the selenium rectifier replaced with a diode
Replaced the selenium rectifier with a diode and the dropping resistor with a pot to determine what value I would need for the dropping resistor.
Brought up the voltage and started to adjust the pot to get the bias supply voltage down to what is should be.
When I got into the range of where it should be the fuse popped.
Replaced the fuse and tried again going slowly, seemed to work OK as long as I didn't try to get the voltage all the down to where it is supposed to be, The voltage was still a bit high, but everything seemed to be happy
Replaced the pot with a resistor of the correct value, fired it up and tried to adjust the bias using the adjust pots
Fuse blew and I notice a flash at one of the tubes.
Put the original dropping resistor and selenium rectifier back in, when I powered it back it the fuse blew.
I must have done some damage to something in the process of trying to adjust the bias an blowing fuses, flashing tubes, etc.

Oh, and when I removed the tubes I lost track of which one was flashing
.
Thanks again all.
 
What method are you using to bias your. Tubes?
The critical component is the idle current.
I usually install a resistor to measure across in the cathode circuit, sometimes i just use the resitor which is already there.
As the bias voltage gets higher or closer to zero, the current in the tube rapidly increases until it pulls enough to pop the fuse. You want to set it where the data sheet says to for that type of tube.
By all means, check the tubes and sockets, thats good advice, but also make sure the values you are adjusting it to are right.
 
Well after lots of futzing and some professional consulting it turned out that one on my rectifier tubes had gone teats up in the process of trying to figure it out. With th NFG rectifier, the power tubes were Red plating and causing the fuses to blow. Replacement of the rectifier tubes and completion of the diode installation and everything is now fine. Bias is dead on. This thing sounds great.

A curious thing is that before working on this project I had been working on an Eico HF 81 that had a really bad hum. I decided to put that aside and work on the Scotts, thinking it was going to be straightforward. When I got back to the Eico I noticed that one of the rectifier tubes didn’t look right. I replaced the rectifiers with some that I had on the shelf and the hum was gone. In both cases I started with new tubes. With the Scott I can see how my screwing around may have fried the rectifier tube, but the one in the Eico had no excuse. It had maybe two hours on it. This must be the year of the rectifier tube for me.
 
no special reason that losing a rectifier tube would cause the output tubes to do anything bad. It would make for low supply voltages and a ton of ripple since it effectively becomes half wave rectified though. You'd end up with very low current flow through the tubes, not high current.

High current flow could have taken out one of the rectifiers though, so I'd suggest pulling both and verifying the bias supply is working properly before putting them back in. If those are a 6550 at ~460v, I'm guessing the grids want to see somewhere in the -40 to -50v range.
 
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