Bad Output Transformer

[morta]

Hello all, I have a Bogen CHB35A PA amp (7868 tubes) converted for guitar with a bad output transformer. One side of the primary reads 130 ohms and the other is open. I've heard these OT's are tough to find. Any ideas? Thanks for your help.

[TigerG]

Have you checked the primary leads to make sure they haven't come detached? It's possible that the break is close enough to the outside that you can fix it. If not, if the primary is indeed open, you'll probably have to replace the OT or get it rewound. If you can't find that exact OT, there's bound to be something available that'll work as a replacement. If you need more info, let me know; I have some info on a Bogen OT at home (I'm at work right now).

[morta]

TigerG,
Thanks for the response. Actually, I did peel back the outer wrap of the OT last night and metered across the connections of the hook-up wire and the transformed windings. No luck, they were good. I will try to find a bogen parts machine for cheap that has a good output xfmr. My greatest fear is that I replace the xfmr and it pops again, from whatever the original cause was - I have new 7868 output tubes and also I'm going to meter out the components in the output stage to be sure. Thanks Again. Al

[Mars Amp Repair]

Hello,
I'd have to think that the Bogen as most older tube amps have nothing connected to the primary leads but the power tube plates. If there are any other components connected to the primary wires (ignoring the center-tap of course)...my guess is you might encounter a snubber capacitor...you can check those, of course.
If a power tube were drawing wayyyy too much current, you'd of course see the plates glowing red...hopefully you would merely monitor the bias current while firing it up...good luck on finding an OT. glen

[morta]

Haven't found a direct replacement for my bad Bogen CHB-35A output transformer yet, howeer, I may be able to get one from a CHB-100A. These amps both use 7868 tubes, however, the 100A uses 4 of them and the output transformer is somewhat biger in size. My question is this: Can I use the output transformer from the CHB-100A in the CHB-35A, or would there be diferent impedance given the parallel 7868s in the 100A. Thanks in advance for your help, Al

[TigerG]

If you use the OT from the 100A with only two output tubes rather than four, you'll need to double the impedance specified for each output tap. For example, if you have an 8 ohm load, hook it up to the 4-ohm output. Fortunately, the Bogen has all those output taps, so properly matching the impedances is easy.

[Chuck H]

FWIW you could use an actual guitar amp transformer now if you want to. You could even switch to a more common power tube too. Unless you were absolutely getting the tone you want from the original setup. Then you'll need to find that exact transformer. But if your thinking of going with a 100A transformer then your going to have to retrofit something anyhow.

Other than that I'm sure Hammond makes a transformer that will work well with the 7868 tubes. A Hammond 1620 has a 6600 primary (probably the same as your stock unit had) and is good for 158ma per side. It's rated at 20 watts, but you can ignore that. A tech at Hammond once told me that to use their transformers in guitar amps you should half the wattage rating. This is most likely due to guitar amps reproducing a narrower freq. band, no need to reproduce 20hz to 20khz (guitar amp is more like 55hz to 5.5khz) and the the fact that Hammond transformers are very conservatively rated. This tech told me that he didn't believe you could blow up one of their trannys if you went with a 25% rating in a guitar amp. And I can attest that they sound good too.

Chuck

[Enzo]

I agree, you certainly do not need all the CV taps unless you are planning on a 70v speaker distribution system. A plain guitar amp transformer sounds like a good idea.

[morta]

Hey Guys,

Thanks for all the feedback on the Bogen CHB-35A. I'm up for changing to different OT and getting away from the 7868's if necessary. If anyone has any recommendations I would really appreciate hearing them. I will definately do somthing to get this amp back on line. Thanks for the help. Al

[J Martin]

If that amp came into my shop, I would use the H 1620. The most painless solution as it wouldn't require any further tweaking of the amp circuitry. Besides, 7868's sound pretty good. I have an MX30A, a later version here right now for quitar amp conversion.

[morta]

I See the Hammond 1620 at Parts Express and I think I'll order one. Thanks for your help. Al

[drewl]

A friend gave me one of those and I wish he hadn't......I was going to use the output tranny for a build, but it was dead....of course I found this out after it was installed.....I used the power trans on a really great sounding Marshall clone but it's humming like crazy...I'm sure it will die soon also....I just haven't gotten around to swapping it out yet....

[Bruce / Mission Amps]

Originally Posted by morta

I See the Hammond 1620 at Parts Express and I think I'll order one. Thanks for your help. Al

I have a couple extra Hammond 1620 OTs. Go see what partsexpress wants for one and let me know if you want one of mine.

[morta]

Bruce, Sorry already ordered by the time I read tis message. Thanks, A

[morta]

Guys, OK Thanks - I now have a Hammond 1620, but also a question on the hook-up to the Bogen CHB35A. In the Bogen schematic there is a connection from the 16 ohm secondary tap to V2B (is this feedbacK?) . Now what I'd like is to have the secondary wired up as an 8 Ohm output to my the speaker, but then how do I accomodate the tap off the 16 Ohm that feeds back to V2B? Thanks for your great help so far - getting close now.... Thanks,Al

[Enzo]

Wire the feedback to the 16 ohm tap and use the 8 for your speaker. The feedback signal voltage is present whether a speaker is also there or not. The NFB does not have to come from the tap with the speaker on it.

[Chuck H]

Enzo,

The Hammond OTs don't have "taps" per say. But rather a way to combine secondary leads to achieve the correct impedance. I didn't take much time trying to wrap my head around it, but I think it would be much easier to wire the OT for the desired ohmage, scrap the impedance switching, take the NFB from the mains jack and adjust the feedback resistor accordingly. But you should take a look see:

http://www.hammondmfg.com/1608.htm

Maybe use a 4 to 8 ohm switch, since that can be done with the comos available, for at least two options, take the feedback from the 8 ohm tap and half the feedback resistor?

Chuck

[Enzo]

Or just hook it up to whatever tap you wind up with and see how it sounds. It appears to have a 4 and an 8, so try the 8. Might sound fine as it is. Easy enough to tweak the FB resistor at a later time, if necessary.

[Satamax]

Hi everybody!

Morta, are you sure your multimeter is good enough to check the low resistance of your transformer? Does your transformer smell? Cos when i hear about dead tranies, it's always what i think about, resistance readings are a hint not a definate answer. I'd rather check this way.

http://www.radioremembered.org/outimp.htm

[morta]

Satamax,

I'm with you on that, however in the case of this particular bad Bogen output xfmr on the split primary, one side read 130 ohms and the other half read infinity with the multimeter so I didn't need to go any furher. The initial indication on the amp was was a very loud buzz and no sound output.

As for the new xfmr - I'm going to wire the secondary as the 8 ohm configuration, then tap off of it for the NFB and see what happens...

Thanks, Al

[morta]

Guys,

It's fixed. The Hammond 1620 is a good replacement for the OT on the Bogen CHB 35A. Secondary wired up as 8 Ohms and no NFB. Thanks everybody for all your help. Al

[Chuck H]

How's it sound compared to the original unit???

Chuck

[morta]

Sounds a tiny bit raspier when cranked up. Maybe that's because I didn't hook up the Neg FB this time. I think I actually might like it better.

[Chuck H]

Yeh, that rhasp is the same thing you get with a Marhall type amp with the "presence" cranked up. If the bottom end is tight enough without the NFB you can loose the rhasp by using a cap in series with a resistor of the stock value to feedback only high end. Use the smallest value that kills the uglies. Then you'll still be running no NFB on the low end and midrange.

Or you could get jiggy with it and build a custom feedback circuit to send a smaller amount of FB than the stock version, but still more top end NFB to tame the nasties.

Chuck

[morta]

Chuck,
Thanks for the info. I will take your advice and do some experimentation. Al