Blowing Up Meters

[RWood]

This one has me stumped.

Conn organ amp rebuild into a 6G6 Bassman circuit; you may have seen my thread about the dual rectifers. I am still running off of one rectifier because I got a little sidetracked by this interesting meter meltdown phenomenon.

Here's what happened - I finished the B+ circuit, five nodes, then built the bias circuit and got a good range of negative voltage for the power tube grids. I now tried it with power tubes in and although B+ was pretty high, 480VDC, I was able to dial down the bias to a conservative level.

The next night I tried it with preamp tubes and although I have what I suspect are some oscillation issues I am getting sound. Here's where it got interesting. With my GB meter (cheap, Lowes, overpriced at $19.95) I could measure all the B+ nodes just fine but when I touched it to the power tube plates, *POP* and the meter literally melted on the inside. Bad smell, deformed readout on the panel, the works.

Undaunted, I tried it next with my (cheaper, Harbor Freight, $2.99) meter I got the same *POP* and a glow from inside the meter, but it survived the momentary blast. Impressive, but what the hell? Just to make sure I wasn't losing my mind I tried it on all the B+s and got normal readings; it's only the 6L6 pin 3s that are causing this. Next I noticed that, when I tried to use that meter in series with the cathode to ground, it was not reading mA, so I must have fried that part of it. I made one more fatal volts attempt on the plates and the meter glowed inside and self destructed. Adios.

So...... what do you think is causing this? The only possibility I am coming up with are inaudible oscillations that are being amplified through the power tube and appearing on the plates...... but appearing as what? AC voltage? DC?

I have an old Philips solid state scope that I would like to learn how to use and to look at parts of the circuit, maybe I can use this as a test case and a reason to get into it. Naturally I don't want to fry it, too, so I imagine I would inject a signal, then compare it to whatever is on the power tube grids and work my way backwards towards the input.

Any advice appreciated, including any known online sources for a beginner using a scope on a guitar amp. I have searched but haven't found anything good.

Thanks,

RWood

[loudthud]

Was there any kind of load connected to the amp when the meters melted down? Are the speaker connections shorted when no load is connected?

Back in the days of the VTVM (Vacuum Tube Volt Meter) there was a 1 Megohm resistor inside the tip of the probe used for voltage measurements. Meters were said to have 11 megohm input resistance. This greatly reduced the tendancy of circuits to oscillate when making plate voltage measurements. I think what's happening is that when the probe is connected to the plate, it creates a large antenna that causes oscillation. You might try disconnecting the feedback around the output stage to prevent this. Those output tube plates will also destroy scope probes because of the high peak voltages that can appear there.

[RWood]

Yes, I had an 8Ω speaker connected through a non-shorting jack. The OT calls for a 4Ω load but that was what I had on hand.

Regarding feedback, I had gotten audible oscillations at first, with the NFB conenction made, so I disconnected the lead from the OT. I want to swap the two power tube plate leads to see if this takes care of my 'first level' of oscillations, but the meltdowns occurred when there was no NFB.

Should I build-in a 1MΩ resistor into the probe I use for voltage measurements? I certainly have a few unused probes!

[RWood]

You might try disconnecting the feedback around the output stage to prevent this.

Thanks Loudthud, you got me looking in the right place. The leg of the presence control cap was shorting against the back of the pot. No more oscillation, normal readings from the plates, and I am restocked with $2.99 meters! I also put in a bid on a VTVM, for posterity's sake.

About this Bassman 6G6 circuit, I do have two questions:

1. Since this is a one-channel (bass channel) amp, I omitted the 470k mixing resistor after the treble control, heading to the PI. Was I correct to do this?
2. The bass control is doing very little here - almost no effect. I am surprised since it has a gain stage almost all to itself. What can I tweak here to increase the amount of bass that can be dialed in?

Thanks!

RWood

[Steve Conner]

Ow! I feel your pain. I remember once working on an old ham radio transmitter, and the B+ voltage was enough to blow the guts out of my DMM. It wasn't oscillating like your build, but it ran off 750V and that was just too much for my cheap meter.

At times like these you want one of the old Simpson analog multimeters, or the British equivalent, the Avo Model 8. That would read over 2000V, and you could use it for things like checking the transformer on a microwave oven.