Hammond AO-43 Conversion

[chris937]

Have you tried powering it up without the output tubes and seeing if it behaves the same way on the current limiter?

[Pdavis68]

No, that's a very good idea. That could rule out the power tubes as the cause. Should I leave the preamp tubes in? Or should I try it with all the tubes out? I've never turned an amp on without any tubes in it. Any special considerations or just do it?

[chris937]

I'd start with no tubes at all to rule out any possibilities. If that works, add the preamp tubes. If it still works, I'd be looking closely at those power tubes.

[Pdavis68]

Thanks. I'll try that tonight. I need to measure the resistance of the OPT secondary as well.

I've got some EL844s on the way anyway, so i won't be too bummed if these EL84s are bad. Though I'd prefer for them to be good as I'd like to use them in another amp.

[Pdavis68]

Thanks. I'll try that tonight. I need to measure the resistance of the OPT secondary as well.

I've got some EL844s on the way anyway, so i won't be too bummed if these EL84s are bad. Though I'd prefer for them to be good as I'd like to use them in another amp.

Well, life happened and I didn't get around to this. During the week is sometimes difficult and we've had a lot of busy weekends lately (I'm the kinda guy that can be perfectly content staying around the house for weeks on end, but my wife gets cabin fever if she's stuck at home for an entire day, so I don't usually get to be the troll I'd like to be.)

So I desoldered the OPT secondary and it's .9 ohms.

I pulled all the tubes, and my current limiter showed no light. Added the 12BH7 and 12AX7s and plugged in the reverb tank (I already had the speaker plugged in) and no light on my current limiter. Popped in the 6BQ5s, turned it on and the light came on, dimmed and a few seconds later came back on. So I'm thinking power tubes. Right?

So I've got some EL844s coming from AES and those should be here tomorrow, so hopefully that'll be the day of glory! We'll see.

I'm totally stoked to find out what this amp is going to sound like!

[cyclone]

I've converted a bunch of those to rock and roll amps, if you end up needing an OPT I've got five or six sitting around. PM me if you need one, trade for your wifes toll house cookies or old junque

[Pdavis68]

Thanks! I suspect the OPT is okay. I just need another set of power tubes to test with. I have a spare EL84 that I could stick in to replace one of the existing 6BQ5s, but they sure wouldn't be matched. I'll just wait 24 hours and try a matched set... I suspect all will be hunky dorey after that.

[Pdavis68]

Although, it occurs to me, should I do anything to make sure that there's not a problem that killed the tubes in the first place? I mean, this amp has been sitting in a garage or something for a number of years (mice ate away part of the original reverb filter cap and a lot of the external wiring. The internal wiring was oddly pristine), so the tubes are probably just bad from being in a bad environment all that time, but if there's something I should check before risking brand new tubes, please let me know.

[Jazz P Bass]

The #1 item that will kill an output tube is overdissipation.

[eschertron]

Although, it occurs to me, should I do anything to make sure that there's not a problem that killed the tubes in the first place? I mean, this amp has been sitting in a garage or something for a number of years (mice ate away part of the original reverb filter cap and a lot of the external wiring. The internal wiring was oddly pristine), so the tubes are probably just bad from being in a bad environment all that time, but if there's something I should check before risking brand new tubes, please let me know.

You don't want to play (or listen to!) the amp with just one output tube, but for testing you can put in one output tube, check the limiter, pull the tube and put in the other. Also swapping positions might reveal a socket-related issue. If you have 3 of the EL84 (or equiv) tubes and you find that one is bad and the other two good, then you'd be in business!

[olddawg]

So the tubes are probably just bad from being in a bad environment all that time, but if there's something I should check before risking brand new tubes, please let me know.

IMHO.. Tubes don't generally go bad from setting around. In fact people lust for old tubes. They are mechanical devices sealed in a vacuum. They can be damaged by physical abuse and the pins can corrode.

[Pdavis68]

Yeah, I guess even if they were in a garage, they'd be okay since they're presumably vacuum sealed.

eschertron, it wouldn't hurt to have mismatch tubes? I was thinking of maybe pulling them one at a time and replacing them with my EL84 to see if maybe just one was bad. I thought I had read somewhere that mismatched tubes could cause damage. I might have misread or misunderstood. Don't recall where I read it.

[Jazz P Bass]

Unless the tubes are way out of wack, a slight mismatch will not hurt the amp.

You may even like it.

[Pdavis68]

Oh well, at this point, it doesn't matter since my new set of matched tubes will have arrived by the time I get home. Getting excited. Going to be a long day...

[eschertron]

I still suggest that you try the limiter with each of the EL84/6BQ5s (one at a time) in one socket, then the other, to see if you can narrow the current draw to just one bad tube. if you have two bad tubes, but at least one good one that proves out the OT and associated circuit, then go for with the new tubes you have!