Ampeg B25B Head

Small combos like Mercury and Jet, Jess Oliver's fliptop Portaflex, the Gemini, Echotwin, B25, V4, VT22 up to the 300 watt SVT

Re: Ampeg B25B Head

Postby Captain Cap on Thu Jun 23, 2016 3:10 pm

Hello everybody, first excuse my poor english (I come from the dark side : France,brrrrrrr)
I also have an Ampeg B25B Head with a 15" JBL speaker 8 Ohms in a cab (homemade by the owner before me). He told me,a long time ago to put a dummy jack in the ext.speaker (n°2) and never take it out.
I recently ask to a Vintage head technician and he told me that if I put the 8 Ohm cab in ext Speaker (n°2) it will automatically switch the head in 8 Ohm and I don't need to put a dummy jack (He saw the electronic schematic). So I have two different answers and don't know which one is good (maybe both?). And what happen if I put two speaker 8 Ohms on both speaker out?
Thanks for your help, because I didn't understood the old answers on that forum.
Best regards.
Captain Cap
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Re: Ampeg B25B Head

Postby Dean Jr. on Sat Jul 02, 2016 4:39 pm

Your original tech gave you proper advice. The main speaker jack wants to see a 16 ohm load when used by itself.
IF you want to use a second 16 ohm load, you plug it into the extension spkr jack and the circuit automatically taps the OT for the 8 ohm winding.....2 x 16 ohm loads in parallel = an 8 ohm load. Your original tech was telling you that in order to use a single 8 ohm load, you should plug that speaker load in to the main jack and use the dummy plug in the ext spkr jack to switch the circuit onto the 8 ohm tap.
Last edited by Dean Jr. on Thu Jul 21, 2016 3:18 pm, edited 1 time in total.
Dean Jr.
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Re: Ampeg B25B Head

Postby jaywalker on Thu Jul 21, 2016 2:26 am

Go ahead and call me crazy but I think it does not matter which jack you have the dummy in as long as an 8 ohm cab is plugged into the other jack. Look at the schematic (which is a photocopy from my amp, btw). I generally do put the dummy into the "ext. speaker" jack just out of habit. I recomend using a "pancake" style jack, terminals insulated, so it does not extend beyond the cabinet's edge (don't want it breaking off during transport do we?). I painted mine yellow because it's special.
After all this talk, I just looked at mine and I forgot to put the dummy in :dumass:

Please correct me if I'm wrong...won't be the first time.
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