Dim Bulb usage questions

[Fred G.]

Hi all,

I am repairing an old Harmony B525 amp. The original complaint from the owner was that it was "cutting in and out". Upon examining the amp. I originally determined it was simply a dirty switch (this amp was grungy as hell and had rust on the chassis, bug nests inside, and looked like it had spent several years in a barn).

After cleaning the switch, the amp seemed to be working fine. I suggested replacing the power cord from the rotten two-prong cord to a grounded 3-prong, which I did, as well as disconnecting the old "line reverse" switch.

Before powering up the amp (this time), I decided to use a dim bulb tester beforehand.

The tester has a 100W bulb.

When I fire up the amp, now, the bulb starts out bright, goes dim, and stays dim for a few seconds, and then it lights up brightly. The delay before the bulb lights up is longer than I have seen in prior use of this kind of test...

I have checked and double-checked my wiring on the power cord; everything looks fine. Checking out the switching for continuity in the wiring, etc., with the amp powered off checks out fine.

I'm assuming the bulb tester is behaving this way because of a possible short in a tube or filter cap, rather than the transformer, after the amp has fully heated up. I haven't had time to test more thoroughly, but I wanted to throw this out for feedback. I read somewhere that this type of dim-bulb test behavior can be normal in some amps? That's what has confused me.

Thanks,

Fred G.

[Fred G.]

Hi all,

I am repairing an old Harmony B525 amp. The original complaint from the owner was that it was "cutting in and out". Upon examining the amp. I originally determined it was simply a dirty switch (this amp was grungy as hell and had rust on the chassis, bug nests inside, and looked like it had spent several years in a barn).

After cleaning the switch, the amp seemed to be working fine. I suggested replacing the power cord from the rotten two-prong cord to a grounded 3-prong, which I did, as well as disconnecting the old "line reverse" switch.

Before powering up the amp (this time), I decided to use a dim bulb tester beforehand.

The tester has a 100W bulb.

When I fire up the amp, now, the bulb starts out bright, goes dim, and stays dim for a few seconds, and then it lights up brightly. The delay before the bulb lights up is longer than I have seen in prior use of this kind of test...

I have checked and double-checked my wiring on the power cord; everything looks fine. Checking out the switching for continuity in the wiring, etc., with the amp powered off checks out fine.

I'm assuming the bulb tester is behaving this way because of a possible short in a tube or filter cap, rather than the transformer, after the amp has fully heated up. I haven't had time to test more thoroughly, but I wanted to throw this out for feedback. I read somewhere that this type of dim-bulb test behavior can be normal in some amps? That's what has confused me.

Thanks,

Fred G.

To clarify - the bulb lights up brightly, goes dim, stays dim for several seconds, and then fully lights back up again. It's the seemingly longer than usual length of time that the bulb is dim after first lighting up fully before lighting up fully again is what I'm questioning.

FWIW - it has a solid-state rectifier and no standby switch.

[xtian]

Sounds normal. The bulb flashes brightly right on power up, because the reservoir caps fill quickly, with a big surge. Then the bulb dims--only the tube heaters are drawing current for the next few to 15 seconds while the cathodes heat up. Then finally the current draw will grow as the tubes begin to conduct.

[Fred G.]

Sounds normal. The bulb flashes brightly right on power up, because the reservoir caps fill quickly, with a big surge. Then the bulb dims--only the tube heaters are drawing current for the next few to 15 seconds while the cathodes heat up. Then finally the current draw will grow as the tubes begin to conduct.

Oh...I'm new to using a dim bulb tester - most of the reading I've done has suggested the bulb should light up brightly initially, and then go dim and stay that way? So that's not really the case, and it will get bright again once everything has powered up?

Not challenging your answer, at all, just trying to get my arms around this. I'm getting conflicting feedback, and it's really confusing me. I have read some sources that suggested this is normal behavior, and others say the opposite!

I may be getting confused by some of the cookbook instructions, though - this behavior occurs with all the tubes in the amp, I hadn't tested with tubes removed - perhaps this would explain the discrepancy, then?

Thanks for your patience and feedback!

[glebert]

The bulb will react differently on a solid state vs. a tube amp. Solid state should be bright, get dim, and stay that way. Tube amps will depend if you have a standby switch or not. I would agree with xtian that this sounds normal for this amp. Also, don't go looking for problems

[xtian]

In tube amps, we use a bulb limiter for first-time start-up testing with tubes REMOVED. We're looking for accidental/unintentional shorts that would destroy components if the bulb limiter were not used. The bulb should flash, then stay dim because no current is being used for the tubes.

[pdf64]

Is this a cathode bias amp? If so a high dissipation at idle is to be expected. I’d expect the bulb to get ‘bright-ish’ but not full brightness. The latter may indicate a fault.

[Fred G.]

In tube amps, we use a bulb limiter for first-time start-up testing with tubes REMOVED. We're looking for accidental/unintentional shorts that would destroy components if the bulb limiter were not used. The bulb should flash, then stay dim because no current is being used for the tubes.

Welp..."yes" to all.

The amp is indeed cathode-biased. When I retested it (again, with tubes in) it was lit up pretty well, but, no, it wasn't fully bright, at second take.

I just finally said the hell with it, plugged in a guitar, played it, and everything was fine - all the way up to fully maxed out. Nice 'n quiet, decent crunch at 9-10 on the dial. A sweet-sounding Jensen 15'' speaker. This amp has potential! A cap job, major cleanup, and new tubes would really make that thing shine. Weren't those pretty much Valcro/Supro amps under the Harmony brand?

I can get a little squeamish when I'm working on someone else's gear - it's for a local Mom 'n Pop music store that sells a lot of vintage gear - he scouts out those funky old amps and picks up quite a few of them - and I'm working on getting all their amp repair biz, so I'm extra nervous about f**king up something.

Thanks, everyone, for your input - it was valuable and appreciated!

Fred G.

[tdlunsfo]

In my experience using a dim bulb with tube amps is the bulb will start to glow again during tube conduction but never should be as bright as it is on powerup. This is all assuming your bulb wattage is appropriate and 100w should be plenty in this case. I'm not a Harmony expert but did recently read that a lot of them where made by sound projects who is the same company that made Lectrolab. I own a Lectrolab r700c. They were cheaply made but this amp has a 15" jensen and 2 vintage 7189 tubes. Sounds glorious. That being said, Sound Projects and Valco were just down the road from each other and it appears to be lost to time if they ever worked together or where simply rivals copying each others designs.