Bad Output Transformer

[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