Bedrock 600 blowing fuses, B+ drops 100V with tubes in

[buffalono]

Hello All,

Need help troubleshooting a old Bedrock 600 amp that blows fuses on startup. I've checked the diodes and replaced the filter caps but it still seems to be drawing alot of current through a light bulb limiter. If I pull the power tubes the light bulb dims noticeably. With the power tubes out, (a quartet of EL 84s) the B+ is 425 volts, when they are in it drops to 295 volts. Also the max bias voltage is -14V, is this too low? I've checked the bias and the lowest I can get the EL84s to idle at is 12 watts. If I leave the power tubes in and pull the phase splitter tube the lightbulb also stays dim.

The voltages on the el84s are grid -14, plate 294 V, screen 292.

This amp is a old pc board amp so I don't want to get into wholesale part substitution because every time I look at it some component, trace or wire seems to break, therefore I'm asking the experts here for some advice on troubleshooting it.

I have some rough schematics I can post with the voltages I've measured on it. I also read positive voltages on the grids of v2 in the preamp which I think is set up as a cathode follower and in V3 which is a ltp.

V2 voltages are (pin 1) 296 V, (pin 2) 0V, (pin 3) 1V, (pin 6) 255V, (pin 7) 175V, (pin 8) 176V.

The voltages on the V3 phase splitter tube are (pin 1) 200 V, (pin 2) 14V, (pin 3) 24V, (pin 6) 231V, (pin 7) 15V, (pin 8) 24V.
The grid voltages are lower than the cathode voltage so I guess that is ok?

Thanks in advance for any advice!

[52 Bill]

...but it still seems to be drawing alot of current through a light bulb limiter.

The limiter will change all of the voltage readings depending upon the current draw of the amp. Don't try to figure out output bias, etc. with the limiter in place.

Have you tested the tubes? Try running the amp with one power tube at a time, watching the light intensity. Does one tube light up the lamp more than the others?

[R.G.]

Good advice.

One or more of your power tubes is pulling too much current. Swapping power tubes is the first step in debugging tube amps because tubes go bad with normal use much faster than anything else in there. There's a reason they're in sockets.

Another possibility is that one or more tubes has lost its bias setup and is acting like a vacuum rectifier, not an output tube.

[buffalono]

Thanks for the info about the limiter!

I've tried it with a single tube in each socket and the light bulb's increase is consistant. With one tube in the bulb is not as bright as with all four installed but it does get brighter the same amount in each socket. I have tried it with different tubes and get the same symptoms.

[g1]

Seems very odd that if you leave the power tubes in but pull the phase splitter the light goes dim. As dim as with the power tubes out? What happens to the voltage on the power tube plates when the phase splitter is pulled?

And here's a schematic, though not a great one, anybody have something better?

[buffalono]

Thanks for the schematic, I have that one too, wish it was better resolution. I also have some hand drawn schmatics from the same Bedrock forum site that a member kindly sent me.

The B+ is 445 volts with only the 2 preamp tubes installed. With the phase splitter pulled and the power tubes in it is 305 volts. With the power tubes out and the phase splitter in it is 426 volts. Finally I checked it with just a single power tube plus preamp and phase splitter. The 380 volt measurment didn't change as I rotated the single tube in each of the four power tube sockets. As I inserted each additional el84 the voltage would drop about 25 volts so that it would end up at the 295 volts with all four installed.

I also checked the 6 resistors between the grids and the bias voltage supply. They seemed ok, each tube had a 2.2 k resistor connected connected to thier respective grids and each pair of 2.2K resistors then fed into a 148K resistor that then let to the bias voltage supply.

I measured the power resistor that drops the screen voltage, it measured 326 ohms but was marked 270 ohms, I'm guessing that 20% difference is not a major problem.

I also measured the resistance from the screens el 84 pin 9 to the power supply diodes and found the same 326 ohm resistance.

What should the resistance measurement be from the power supply diodes to plates of the el 84s? About Zero ohms?

One thing I noticed were some resistors that were added to the back of the circuit board to each power tube socket when I had the the board removed from the chassis to replace the filter caps. They were hacked in by cutting the trace to the socket, I checked them for being shorted to ground and consistant resistance and they were ok. I don't remember which pin they were attached to or the resistance measurement but I could take it apart and check them again.

[52 Bill]

What should the resistance measurement be from the power supply diodes to plates of the el 84s? About Zero ohms?

The plates are connected to the B+ supply via the primary winding of the output transformer, so you should be reading the resistance of one half of the primary winding.

If all of the above voltage readings were taken with the limiter in place, I think it may be time for you to remove the limiter. Pull the tubes and see if the fuse blows when you power it up without the limiter. Then add the small tubes and re-power up. Then add the power tubes. Take voltage readings as you go along.

[buffalono]

I plugged the amp in without the limiter and got 630V B+ no tubes, with the preamp tubes in, it went down to 530V and with 2 of the 4 power tubes in, it stayed about the same. When I turned on the standby with a 3rd power tube, there was a full volume buzz that was so loud I was afraid I was going to blow the speaker so I shut it down. I had it plugged into a single 12" 100W speaker EV 12L and it was LOUD. I measured the bias voltage with 2 power tubes in and got - 26 Volts. With all the controls were set to 0, the volume of the buzz was the same. I have a new set of power tubes but I'm afraid to put them in.

[52 Bill]

When I turned on the standby with a 3rd power tube, there was a full volume buzz that was so loud I was afraid I was going to blow the speaker so I shut it down.

So there was no buzzing until you added a third power tube? What if you take out the third tube and install the fourth one instead?