Bugera 1990 weirdness

[Prairie Dawg]

I've got a Bugera 1990 in house for what was supposed to be a tube swap but it seems as if there's a significant voltage drop off between the two circuit boards. Going off the power transformer I have 6.2v ac, the preamp tubes read 12v of dc across pins 4 and 5, but the power tube filaments read only 5.35v ac.

Is this the infamous board to board connector problem, or should I be looking elsewhere? Schematics for these are a lost cause, eh?

[Enzo]

Don't need a schematic. What voltage is across the heater winding at the transformer leads? What voltage is on the other side of the connector?

The heater winding is heavy yellow wires? Do they feed the board by way of a molex? Or do they plug onto posts? Seems to me they have ditched the molex years ago, and amps were coming from the factory already modified.

Seems to me there is a board for the power tubes? And a 7-8 pin ribbon over from the main board? explore the heater voltage on both ends of that journey.

[Prairie Dawg]

That's the plan. I know I don't need a schematic but I like having them handy. It saves time and effort-old hangover from the aircraft industry. They do use something like a molex to go from the preamp board to the power tube board and I figure it's in there.Bridging the filament leads from the power transformer I get 6.2v AC, and across 4 and 5 of the preamp tubes I get 12v DC which is sufficient although not great-they use DC on the preamp filament string. The only places where the power crosses over is the connector.

Well, that's tomorrow's debugging task. This guy's had problems with the amp off and on and I think this may be the source of his misery.

[Enzo]

The preamp tubes have their own DC supply for heaters, totally unrelated to the power tube heaters.

The "famous" molex problem was a connector on the end of the PT leads themselves. That was on early models like the 333.

DO indeed explore those board interconnections.

[Prairie Dawg]

To get a drop of a volt it's got to have some significant resistance somewhere. Should be an interesting journey.

[Prairie Dawg]

Here's what ended up happening.

I resoldered all the connections for pins 2 and 7 on the tube board. I also found that the problem was intermittent-sometimes I'd have voltage and sometimes I wouldn't, and it would jump around. I also measured the resistance across the interconnect cable and that was OK. I took the main board far enough out to get at the underside and resolder the spade connectors. Ultimately the cause of much of the problem was traced to a fuse holder that was made of metal that was pretty soft and didn't have much spring back-when the filament fuse was installed it spread the holder enough to make contact intermittent.

After all this I had a consistent 6.2v from the power transformer through the main board and out to the tube board to the filament connections. Putting the tubes in and turning off the heater in my shop the filament voltage stabilized at 5.99, which is usable but in my view insufficient to get best performance and life out of the 6L6GC power tubes.

I will be recommending that the owner consider installing a filament transformer to supply the filaments of the power tubes. The factory installed windings are not up to the job.

[Enzo]

5.99V is only 5% low. I would not consider it worth adding a transformer. I don't think the 6L6s will suffer at all.

[Jazz P Bass]

Isn't lower (within reason) better as far as heaters go?

[Prairie Dawg]

I wanna read a magazine by the glow and heat my house. It's cold here.

@Jazz, your gM will drop off pretty quick as your filament voltage goes below nominal. It's not as significant with a relatively fresh set of tubes but I noticed this when testing a bunch of 45s which are 2.5v filaments. The draw is substantial and it was loading my Hickok down to about 2 volts with the result that the tubes did not test nearly as good as they should have. Raising the line pot does not fix the problem because it is a global setting-it raises voltage throughout the tube tester.

Luckily enough my tester's got a 3.3v setting although I never have seen a 3.3 volt filament tube. Using that and monitoring filament voltage from an adjacent socket let me test them right about at 2.5v and the reject rate was significantly lower.

I guess the point is that I like the voltage to be spot on or maybe even a tad higher. I can deal with slightly shorter life if I can get design performance.

[Enzo]

Sure, running the heaters cooler should make them last longer. But really, in your career, how many tubes had burned out heaters? I can't think of many in my time. In my opinion, cathodes would boil themselves barren, plate and other elements will get mechanically loose causing noise or microphonics, elements will short together, long before most heaters give up the ghost. The heaters aren't what I worry about lasting.

I'd imagine that at 5% low on the heater, there'd be some tiny reduction in emission, shouldn't hurt the tube, and I doubt we'd hear it.