WAS THIS A GOOD DEAL ??

zoomduck · Post by **zoomduck** » Tue Mar 27, 2007 9:45 am

I just bought an all original 1974 Ampeg B25B head . It needs only a volume pot for channel #2 . Got it for $375 . This amp has only 16 ohm outputs so I ordered a Jensen MOD 15" . I want to use this for recording so I do not need a high powered speaker . What happens if I connect an 8 ohm cab ? Looking forward to hearing my toaster 4004 Cii through it .

thx1955 · Post by **thx1955** » Tue Mar 27, 2007 10:00 am

Hi Barry,
I've also got a B25, but a little older, I've used this amp with an 8ohm 4x12 for coming on 8 years now with no problems.

I also use the twin channels to run my 4003 in Ric-O-Sound with great results.

zoomduck · Post by **zoomduck** » Tue Mar 27, 2007 10:59 am

Thanks Jim . I keep reading about the OHMs.....Some say never play into anything but 16 ohms....some say plug into the ext output for an 8 ohm connection with a cord plugged into the main output but not connected to a cab . Think I'll just wait for my 16 ohm speaker to be safe . Think I could replace the volume pot myself ?

thx1955 · Post by **thx1955** » Tue Mar 27, 2007 11:55 am

Either way will work, for recording you'll have plenty of power even at 16 Ohms.

I've got a B25 Schematic, as well as a great article on the flexibility of the B25 design and the various tubes you can use. Drop me an email if you'd like a copy.

johnallg · Post by **johnallg** » Tue Mar 27, 2007 12:51 pm

Barry, if you can handle a soldering iron, just pull the old pot loose (of course NO POWER PLUGGED IN) and mount the new pot. Then unsolder one wire at a time and solder to the new pot's same post. Should take 15 minutes if you are being really slow and meticulous.

edski · Post by **edski** » Tue Mar 27, 2007 1:36 pm

If the amp is designed to run at 16 ohms and you run it at 8 the amp will output more power than the specs say. It will also run hotter. Some amps that's great. Some amps that's bad. My guess with older vintage equipment like that is that they are spec'ed a little conservatively. Jim seems to indicate he's had no issues running it at 8...

I would guess (and I may be totally wrong) that tube amps are more forgiving with this issue. I never had an issue running my guitar amp down to 5.3 ohms, and tube amps are designed to get pretty hot.

thx1955 · Post by **thx1955** » Tue Mar 27, 2007 1:52 pm

Ed,
Tube amps are indeed a lot more forgiving.

I've had no issues at all with heat, or shortened tube life, there again I'm not pushing my B-25 anywhere close to it's max.

The B-25 chassis and cabinet has lots of room for good airflow, and the whole thing is being driven by a pair of 7027's, with the transformers are at either end, again allowing for good heat dissipation.

doctorwho · Post by **doctorwho** » Tue Mar 27, 2007 2:07 pm

I would recommend that this question be posted in the Tube Tech Forum section here:

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zoomduck · Post by **zoomduck** » Tue Mar 27, 2007 3:51 pm

OK . posted over there .

zoomduck · Post by **zoomduck** » Tue Mar 27, 2007 4:23 pm

John...the pot is soldered to a circuit board ?

johnallg · Post by **johnallg** » Tue Mar 27, 2007 4:41 pm

Being soldered to a board makes the skill level go up a tad - you have to have a solder sucker or the wicking braid. But putting the new pot in correctly is also simplified as it only goes in one way! On removal, you just get all the solder out of the holes the pot leads are in then pull the pot out - insert the new pot and resolder and thats it.

johnallg · Post by **johnallg** » Tue Mar 27, 2007 4:44 pm

And as for lower impedance speakers used with a tube amp, that lower impedance is reflected to the primary of the output transformer and the tube operating point is shifted causing them to run out of the design point, hotter. WIth the old tubes, it probably wouldn't matter very much if not pushed hard nor for long hard times, but with today's tubes not being as well made, well, I wouldn't.

charlyg · Post by **charlyg** » Tue Mar 27, 2007 5:39 pm

John, out of curiosity, where did you learn your electronics? My Dad and I were hams(still are) when I was in High School, and then the Navy taught me for two years. The reason my curiosity is piqued, you answer pretty much what I am thinking when it comes to this stuff. I doubt if I could explain it as well. Tip o' the hat!

elysrand · Post by **elysrand** » Tue Mar 27, 2007 5:50 pm

73s, OM! Tested novice in 1963, General in 1966, did Extra-Class (skipping Advanced) in 1968, didn't do squat to log anything after everything went solid state in 1975 or so

Still have my 2M and 6M Benton Harbor lunchboxes, Collins S-line, Yaesu hybrid, Drake TR$-C 80-10 though.

OOPS, not Rick-related, off-topic, sorry...

elysrand · Post by **elysrand** » Tue Mar 27, 2007 5:51 pm

73s, OM! Tested novice in 1963, General in 1966, did Extra-Class (skipping Advanced) in 1968, didn't do squat to log anything after everything went solid state in 1975 or so

Still have my 2M and 6M Benton Harbor lunchboxes, Collins S-line, Yaesu hybrid, Drake TR4C 80-10 though.

OOPS, not Rick-related, off-topic, sorry...