Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: john_t on May 18, 2010, 09:39:40 am

Title: output ohm question?
Post by: john_t on May 18, 2010, 09:39:40 am

I have a 022905 output transformer. I believe this has a 40m secondary . Will a 8 ohm speaker work ok with this ot for my GA-5

Title: Re: output ohm question?
Post by: HotBluePlates on May 18, 2010, 12:15:46 pm

Hoffman's Fender Transformer Info (http://www.el34world.com/Transformers/fentrans.htm)

Yep, 4 ohm secondary.

If you put an 8 ohm speaker on it, it will play but the primary impedance will be too high for maximum output. Since this is basically a Champ-size OT, maximum power may not be an issue to you. It will make sound just fine.

Title: Re: output ohm question?
Post by: john_t on May 18, 2010, 05:01:53 pm

Thanks for the help. I Have an extra 8 OHM laying around so it would save me from getting another speaker. I did try it briefly and cant imagine needing more out put. It is plenty loud now. I was not sure how it would work out in the long term.

Thanks HBP.

Title: Re: output ohm question?
Post by: Dave on May 18, 2010, 05:55:27 pm

Like HBP said, it will work. Now you are asking about the long term which is a different question.
If you drive your Toyota from Dallas to Cleveland in third gear, it will work, but what impact will it have in the long term?
Extended wear and tear but it will work.
Some people like to run their stuff at a 100% mismatch. I don't. I use all 5 of my gears too.

Dave

Title: Re: output ohm question?
Post by: john_t on May 18, 2010, 06:57:41 pm

Not that it matters, but which will I be driving harder The amp or the speaker.  If I were to ruin one I would rather take out the speaker. I don't mind getting another  speaker rated at 4 OHMs. I only ask because I have two extra 10s Iam not using. Only there 8 OHM.

Title: Re: output ohm question?
Post by: Dave on May 18, 2010, 09:17:39 pm

The speakers won't care. Its the amp that will be abused.
If you have two 8 ohm 10 inch speakers. Wire them in parallel. Voila 4 ohms.

Dave

Title: Re: output ohm question?
Post by: HotBluePlates on May 19, 2010, 04:14:07 am

The only possible harm due to a too-high primary impedance is excessive screen current. Depending on the specific aspects of any given case, this may or may not occur.

If you do not have screen grid resistors (usually 470 ohms in Fender amps) and are concerned about screen current, add them.

I don't believe you will have any significant issues in a Champ-size amp with the mismatch. A bigger, badder amp with hot-rod output tubes... the story might be different.