Stone SGT60 drawing way too much current(blowing fuses)

[Stormrider]

Ok, I recently acquired a Stone 1x12 SGT 60 combo. Whenever any power tubes are inserted (it has 4x EL84s and 3x12AX7s) it lights my current limiter up like crazy. The tubes aren't glowing or anything. I tried new tubes, and only putting in a pair at a time, same result. I thought it might be a biasing problem, but after examining the boards, there's no bias pot! So at this point, I'm assuming that it's cathode biased, and perhaps there's a problem with one or more cathode resistors?

I tried searching google for a schematic, and turned up nothing on this amp at all. Not a single thing. Anyone have any ideas on this?

[52 Bill]

Whenever any power tubes are inserted (it has 4x EL84s and 3x12AX7s) it lights my current limiter up like crazy.

So you're saying that it doesn't blow fuses when the output tubes are pulled?

If the current draw only gets big when the output tubes are in the amp, then the problem lies in somthing that the tubes complete the circuit to. Check the cathode resistors and caps, the output transformer and the tube sockets and wiring as well.

[Stormrider]

So you're saying that it doesn't blow fuses when the output tubes are pulled?

If the current draw only gets big when the output tubes are in the amp, then the problem lies in somthing that the tubes complete the circuit to. Check the cathode resistors and caps, the output transformer and the tube sockets and wiring as well.

That's what I was thinking. I'm thinking of just shotgunning the entire output board, since it's kind of a pain to get out. Wiring and sockets seem to be good, but I'm going to double check again.

On a side note, does anyone know anything about this amp, or the company (Stone)? The only thing I've found on the internet about them is a webpage selling a small 10w solid state combo. I've never seen one before I got my hands on this one.

[Enzo]

DOn;t assume. If they are cathode biased, there will be a cathode resistor between the cathode pins and ground. SImple enough to use an ohm meter and find out. You can even do this from out side. Just find the cathode pin on the top side of the tube socket. If you measure 50 or 100 or 150 ohms or something, then you are right. if it is grounded, then it has fixed bias and there should be negative bias voltage on the grid pins if the sockets. if that voltage is absent, then the poor tubes will indeed conduct as hard as they can.

And while you are at it, check the plate and screen voltages.


Now let's assume it has cathode bias. "Problems with the cathode resistors?" Of the resistor is open, the tube won;t conduct current at all. NO fuses blowing. If the value has drifted - and that is almost always upward - the tube will draw less current. A shorted cathode resistor seems just too unlikely to consider. But simple to check.

The only other matter comes to mind is if pushing the tube into the socket causes a socket pin to shift position enough to short to a neighbor pin.

[Stormrider]

Ok, I'll check for the cathode resistor in the morning, as well as get the voltages. I don't think the sockets are causing it, since I also put the tubes in a pair at a time, to see if it was one of the tubes causing it. I'll go back and check by putting a tube in one socket, and moving it to the others, and see if there's two shorted sockets.

[Enzo]

You are probably right, but never hurts to check. I just mentioned it because too many people focus on "bad part" and don't really think about other ways a problem can arise.

Just eyeball the sockets from underneath, you should be able to spot if there are two pins about touching each other.

Bill's insight is the key to this - the tubes must be completing a circuit path to something.

Here is one not on the schematics. MAny EL84s have an internal connection to pin 1, but it can also have an internal connection to pins 6 or 8. But not all do. When the amp was designed and built, it is possible the tubes they were using did not, and the ones you have now do. In 6L6 amps it is common practice to wire a screen resistor from pin 4 - the screen - to pin 6. Pin 6 is not used on the tube, so it is a handy solder terminal. I don;t normally see that in EL84 amps, but I don;t know this amp company. Look at your EL84 sockets and see if pins 1,6, or 8 were used for wiring in any way. Plugging a tube into the socket would then complete the circuit to whatever is soldered there. You follow?

[Stormrider]

You are probably right, but never hurts to check. I just mentioned it because too many people focus on "bad part" and don't really think about other ways a problem can arise.

Just eyeball the sockets from underneath, you should be able to spot if there are two pins about touching each other.

Bill's insight is the key to this - the tubes must be completing a circuit path to something.

Here is one not on the schematics. MAny EL84s have an internal connection to pin 1, but it can also have an internal connection to pins 6 or 8. But not all do. When the amp was designed and built, it is possible the tubes they were using did not, and the ones you have now do. In 6L6 amps it is common practice to wire a screen resistor from pin 4 - the screen - to pin 6. Pin 6 is not used on the tube, so it is a handy solder terminal. I don;t normally see that in EL84 amps, but I don;t know this amp company. Look at your EL84 sockets and see if pins 1,6, or 8 were used for wiring in any way. Plugging a tube into the socket would then complete the circuit to whatever is soldered there. You follow?

I knew about the problem with some older tubes, namely old Mullards that did some funky stuff due to the internal connections, but I'm using new Peavey tubes, and I did check the board, and 1, 6 and 8 are not connected to anything on any of the sockets. I did some measurements, I got 487.5 VDC on both the plate and screen, and 7.48 on the heater...that seems a bit high to me.

Also, it draws mucho current with just one tube in any socket, I went ahead and checked that. I'm thinking about clipping the leads to the OT next and see if that's causing the problem.

Also, it IS cathode biased. All 4 cathode pins read 48 ohms to ground.

[Enzo]

What DC voltage - if any - is present on the grid pins of the power tubes? With cathode bias, it should be zero.

With power tubes removed, B+ voltages in amps go sky high. You can;t right now, but once you can stick tubes in it, you may fiond it falls to a more reasonable value.

Does the amp have a standby switch? If so, does it have to be on for this, or does current max out even in standby?

DOn;t clip the transformer wires, unsolder them. Try disconnecting the center tap first. I bet the problem stops. If it doesn't matter which socket you stick a tube in, then I'd think it wouldn't matter which primary wire you disconnected on the OT.

Are you seeing 7.48VAC ACROSS the heaters? Or are the heaters sitting at 7.48VDC above ground?

What size bulb is in your limiter?

DO you have a scope? ANy chance the amp is oscillating hard when tubes are in it.

[Stormrider]

What DC voltage - if any - is present on the grid pins of the power tubes? With cathode bias, it should be zero.

With power tubes removed, B+ voltages in amps go sky high. You can;t right now, but once you can stick tubes in it, you may fiond it falls to a more reasonable value.

Does the amp have a standby switch? If so, does it have to be on for this, or does current max out even in standby?

DOn;t clip the transformer wires, unsolder them. Try disconnecting the center tap first. I bet the problem stops. If it doesn't matter which socket you stick a tube in, then I'd think it wouldn't matter which primary wire you disconnected on the OT.

Are you seeing 7.48VAC ACROSS the heaters? Or are the heaters sitting at 7.48VDC above ground?

What size bulb is in your limiter?

DO you have a scope? ANy chance the amp is oscillating hard when tubes are in it.

Haven't hooked up a scope yet, I'll take it to work one day this week and do it.

The 7.48 VAC is from heater to ground...forgot how they normally are wired, I'll recheck that later tonight.

I don't remember seeing ANY VDC on the grid pins, but I'll check again.

The problem only manifests when I turn the standby switch on, with just the power it runs fine.

Bulb in the limiter is a 100w bulb, amp is a 60w combo.

Once I get the other voltages, I'll repost. I might dig out my old scope and hook it up and see what I can get, but we have a much better setup at work.


EDIT

Ok, 7.4 VAC on the heater...isn't that really high? There's no DC voltage on the grid...but there is a small (.02 VAC) AC voltage there...hmmm

[Enzo]

While you are looking for any DC volts on the grtids, with power off, verify they all have a grid return resistor to ground - whatever it is, 220k, 330k, 470k, something. If they are there but the ultimate ground connection is missing, that would leave the tube grids unterminated, and they would try to conduct HARD. Hey, it's a long shot, but easy to check.

[Stormrider]

Grid resistors are good, 280k with a plus or minus ~1k