Bell & Howell 179

[lowell]

Hey all,
This amp is currently overhauled w/ a 3-prong cord, removal of V1 (opto tube), a 1/4" speaker jack and a standby switch. I'm a bit at a loss for understanding what's happening in this amp w/ the power supply. It seems there is NO power transformer and that the 120v wall supply goes through a 20ohm resistor to the tubes' heaters. There are 2 diodes but they're not wired as a half wave rectifier. Also there is V6 which is wired to a 3 coil inductor, what's this tube's purpose? Finally, the isolation transformer at the input I believe is there for protection being that there's no power transformer. Would anyone modify the input to the grid of V2 in any way? It's on pg 189-190 on this document.


http://www.film-tech.com/warehouse/manuals/AMPS.pdf

[lowell]

This thing is hummy as a mo-fo. When I pull V2 (1st preamp tube) the hum is gone. I'm thinking it's the filter caps, I've yet to do a cap job on this. We'll see.

[lowell]

wow this thing is driving me mad. I've tried to revert back to how the amp was yesterday but this hum is ridiculous. I'm now confused as to whether this is a new issue or if I caused it by removing some components I thought were unnecessary.

[lowell]

Can someone please explain why V2's heater is in series w/ the cathodes of the power tubes instead of being in line w/ all the other heaters?

[olddawg]

Although I can not open your schematic, it appears that this is a series filament design. You should not use it unless you have an isolation transformer. It can kill you. They are flat out dangerous. Series filament units should be avoided for conversions. You will not get much advice on a series filament amp because no one wants you to hurt yourself or be responsible for it in any way.

[lowell]

Yes I understand that. This amp has an isolation transformer. However I have 5 years experience working on amps, not that that's amazing or anything but it's enough seasoning to be confident in the way of the dangers of working w/ electricity.

That being said, I could use some help w/ this one. I'm not concerned w/ the heaters being wired in series, just curious why V2's heater is NOT. V2's heater is actually in series w/ the cathodes of the power tubes. That's quite odd to me.

[lowell]

How would YOU scope this?

Ok so I'm having difficulty scoping this amp. The common for the signal and some filter cap grounds are not actually connected to earth - thus the isolation transformer, so when I connect my scope ground to the common there is a spark... obviously because there is 100v on the common.

So I tried grounding the scope to the chassis (earth) and can see the signal quite good on the speaker jack and it is clear, but when I put the scope lead on the grid of 1 power tube there are 2 signals and they seem to be riding on different DC voltages... I'm assuming that it's because the scope is using the wrong reference as ground but not sure. I tried using the "ref" setting my scope lead and connecting the "ref" at the common but it still sparks. Any advice on scoping this amp would be helpful.

FYI I minimized the hum in realizing that common was not in fact connected to the 1st tubes isolated chassis, so in connecting that it's FAR less audible but still kinda sucky.

[olddawg]

Ok so I'm having difficulty scoping this amp. The common for the signal and some filter cap grounds are not actually connected to earth - thus the isolation transformer, so when I connect my scope ground to the common there is a spark... obviously because there is 100v on the common.

So I tried grounding the scope to the chassis (earth) and can see the signal quite good on the speaker jack and it is clear, but when I put the scope lead on the grid of 1 power tube there are 2 signals and they seem to be riding on different DC voltages... I'm assuming that it's because the scope is using the wrong reference as ground but not sure. I tried using the "ref" setting my scope lead and connecting the "ref" at the common but it still sparks. Any advice on scoping this amp would be helpful.

FYI I minimized the hum in realizing that common was not in fact connected to the 1st tubes isolated chassis, so in connecting that it's FAR less audible but still kinda sucky.

Use AC coupling mode on the scope channel?

[lowell]

that's what I had it on, but thanks. I'd think it's a matter of breaking the connection b/t the scopes earth and the amp's common, just don't know if that's an option.