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Tubes in an Ampeg B25B...

Discussion in 'Amps and Cabs [BG]' started by fat jonny, Dec 20, 2001.


  1. fat jonny

    fat jonny

    Jul 8, 2000
    Columbus Ohio
    I just got an Ampeg B25B head and cabinet together for $400. It looks sweet and its in great shape and works fine. I was wondering if anyone knows what is the point of the 2 different channels on the amp? Which channel should i use or does it not matter? Also, would it be a good idea to change the tubes in the amp? What it make it any louder or sound better? which tubes would i get to replace them? Is there anything else i should know to use it right, I dont really know a lot about tube amps. Thanks!
     
  2. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Believe it or not, until the 1970s almost all amps had two channels. The thought was two players could plug in at the same time :eek:

    You can see photos of surf bands in the early 60s where the whole band is plugged into only one or two amps.

    On some dual channel amps, each channel sounds a little different, so try both and see what you think.
     
  3. On an older tube amp, especially if it hasn't had maintanence (sp?) in a while, it could be very good to take it to a tech and get new tubes, a rebias, and a cap job, if it's needed. New tubes, especially if the old ones are beat up, can really make a difference in sound and volume.
     
  4. Ampeg B25B? Where's Chris? :D :D

    You can use 2 channels as a boost as well - feed one into the other (not cascading gain, but paralleled channels). On some amps, such as a vintage Fender Bassman, you can balance the sound this way by running the bass channel with all bass and no treble, and the guitar/treble channel with the opposite (either channel is voiced differently).

    New tubes and a tech visit are a good idea, but sometimes the benefit of new tubes is hard to judge. I was running my Ampeg V4 for a while until I found that it was only running on one 7025 power tube (the rest were drawing current, but not working). Installed a new matched quad of 6L6's, and there really wasn't that much difference in tone/headroom (except the single 7025 would break up easier, but for me that was fine).
     
  5. How can an amp run on one tube?
    Most all old Ampegs are push/pull (requires 2 power tubes)

    Nil. you mean 7027A-, right?
    A 7025 is a mil spec 12AX7 preamp tube.

    You can direct replace 7027A tubes with 6L6GCs (the GC is imparitive)

    See fliptops.com
    or thetubestore.com

    The amp should be rebiased with any power tube replacements.

    A cap job will also bring the amp up sounding right.

    I think the B25B was a 60 watt
    (2-7027As) for power.
    You can find V-4, V-4B heads for about $250. $300.plus service costs.
     
  6. Someone call me? :D

    OK, the 2 channels actually sound a little different, #1 is kind of thinner sounding, and #2 is full bass. You'd think #2 would sound better, but I always end up going into #1 for some reason.

    You can find NOS 7027A's for not too much dough on eBay. They last forever.

    You need to have the filter caps replaced.

    The 7199 tube is pricey. You'll have to keep your eyes out for one of those as a spare. Don't get the Russian one, only USA NOS.

    The 5AR4 rectifier is kinda pricey too, but they also last forever once you have a good one. Yours is probably fine. Hell, all your tubes are probably fine. Just try and pick up some spares in case you ever have one go south.

    (2) 7027A's
    (1) 5AR4
    (1) 7199
    (2) 12AX7A's

    Really cool amp, by the way. I love mine.

    [​IMG]

    Chris
     
  7. Just to answer some of your specific questions I didn't hit above-

    Your tubes are probably fine. I wouldn't worry about replacing them unless it really sounds bad.

    Proper tube amp etiquette -
    Don't leave it anywhere you wouldn't want to sleep.
    It likes to be treated really nice.
    Let it ride in the front of the car.
    Don't throw it around.
    Set it down gently when carrying it, and don't bump it into anything.
    Don't let anyone else mess with it, or carry it for you - they won't care how they treat it.
    Turn on the AC power with the Standby switch OFF, wait about 20-30 seconds, then turn on the Standby. Leave it like that until you are done playing.
    Do not use the Standby switch to turn the amp off temporarily during a break. Once it's ON, just leave it full ON until you are done. Then turn it off by just killing AC power. After a few seconds, then flip Standby OFF so it will be OFF next time you Power the amp up.
    Don't leave it unattended while ON - you want to be able to kill power if something goes crappy on you.

    Don't ever run the amp without the cabinet hooked up! Ampeg's have a shorting jack to hopefully prevent damage, but as a general rule, don't plug the amp into the wall until the cabinet is connected to the amp!

    Unplug the amp from the wall when you are not running it. This can prevent lightning run-ins. Lightning can fry an amp even if it's not ON.

    I think that's about it.

    Don't ever sell that amp.

    Chris
     
  8. I was glad to see this thread. A few years ago I helped out an acquaintance by buying his old bass gear (he was moving out of town and was broke). I got a couple of usable pieces and a dead, fairly beat up B-25 head and cab. He said the only thing wrong with the head was that the power switch was broken.

    When I asked down at Midtown Music (local vintage store) if they had any interest in it, they said the only things they were buying were flip tops and SVTs so I kind of forgot about it.

    It seems that maybe there is some interest in this amp. Is it worth spending the money to get it checked out by a tech? Chris, can you recommend anyone? BTW, it has a pretty cool sticker on it to remind you to hook up the cab.
    [​IMG]



    note: attachment is the SAME as the visible graphic
     
  9. Dude, I'm about an hour south of Atlanta on I-85. You still got the 2-15 cabinet too? I want one that's in better condition than mine pictured above (the eBay seller failed to mention it had been re-tolexed). You wanna trade it for any amps or anything? Bassman 135 head? Kay 720 40Watt tube combo?

    I'm sure Wizard Electronics, or whatever that amp repair place is over by Guitar Center, could service it. I can if need be, I build and repair my own tube bass amps on the side, it just takes me months to get around to working on stuff (2 kids keep me busy).

    Definitely let me know if you want to sell it cheap, or trade for something else. I love B25 stacks, and would dig another one, either the whole stack or just the speaker cabinet.:D

    Chris


     
  10. Thanks for the response and interest, Chris. Both the head and cab are beat up. I'm not even sure that the cab is the original one. I know the grill cloth is different.

    When things slow down a bit after the first I'll check it out and take the head in for a diagnostic. I'll drop you a line then.
     
  11. Duh! Umm, yep, 7027A's (i've been in 12ax7 land for a bit... :D )...
    My tech said that all 4 were drawing current (all were evenly lit) but only 1 was actually doing anything. I can't remember if I tried the V4 with only one of the tubes - I did try a single 7027A in a Bassman 50-like head once and it worked OK (great breakup).

    I thought Ampeg's were self-biasing?
     
  12. I would not run the (or any push/pull) amp if one tube is the only thing happening. There are a lot better ways to get dstortion, maybe a SansAmp BDDI, or an overdrive pedal.
    I would have the amp serviced, and retubed.

    Are you sure there are 4 x 7027As?. That would make it a 100+ watt amp.I thought the B25 was a 50 watt (2x 7027As)??

    I do not believe that they are self biasing, or ever were.
     
    Georod likes this.
  13. Yep, the B25B has (2) 7027A's for 55 Watts rated output. The V4B has (4) 7027A's for 100 Watts rated output.

    The B25B and V4B use fixed bias (cathodes of power tubes connected to ground, negative voltage applied to power tube grids to control idle current), but it is not adjustable. A potentiometer has to be added to give bias adjustment.

    Self biasing is the same thing as cathode-biasing, which means a resistor is put between the cathodes of the power tubes and ground, and the grids are at ground potential. The current flowing through the tube also flows through the cathode bias resistor and creates a voltage drop that controls idle current. That's self biasing to some extent, because plugging in a tube which is more conductive will cause a greater voltage to drop across the bias resistor, which tries to turn the tube off. So it's kind of like a little local feedback to keep the operating points stable. Self Bias wastes some power supply voltage, though, and results in a little less power.

    Chris
     
  14. fat jonny

    fat jonny

    Jul 8, 2000
    Columbus Ohio
    I am loving the tone i'm getting from this amp but i have a couple more questions. When its on and the volume is up about 3/4 of the way, the 7199 tube is pretty dim and both of the 12ax7's are so dim you can barely even tell theyre lit up at all. Is this normal or are 12ax7's not supposed to glow like the other tubes? All the tubes say magnavox on them so I assume they're still the original ones. Also, is it safe to turn the amp up all the way or will that blow it? Is it safe for the speakers? It distorts quite a bit when i turn it up loud but its a good sounding distortion IMO. If I were to run my ric 4001 into a splitter box and send one of the signals to channel 1 and the other to channel 2, what would that do? Also, on the back of the amp there is an ext. amp jack and an ext. speaker jack; how would I go about using those? I know the cabinet with the B25B is a 16 ohm so if I wanted to use another cab, what should it be rated at? What is the purpose of having these? also, there is an AC outlet on the amp, what is that used for? Sorry about all these questions but you guys really seem to know what your talking about so I figured I would ask. Thanks a lot!
     
  15. Originally posted by fat jonny
    I am loving the tone i'm getting from this amp but i have a couple more questions. When its on and the volume is up about 3/4 of the way, the 7199 tube is pretty dim and both of the 12ax7's are so dim you can barely even tell theyre lit up at all. Is this normal or are 12ax7's not supposed to glow like the other tubes? All the tubes say magnavox on them so I assume they're still the original ones.

    The filament glow may be bright, may be dull. No big deal if it sounds fine. Some tubes' heaters glow brighter than others. Don't sweat it. Yeah, Magnavox bought Ampeg back in the 70's. My V4-B is a Magnavox era Ampeg. Those tubes are probably still good and probably will be for years.

    Also, is it safe to turn the amp up all the way or will that blow it? Is it safe for the speakers? It distorts quite a bit when i turn it up loud but its a good sounding distortion IMO.

    Only turn it up to the point where distortion sets in - probably about 11-12 o'clock on the volume control. If you want to go nuts, turn it up to 1 o'clock. You CAN turn it up all the way, but you stand a greater chance of blowing speakers or something. It's a cool sound, but you really shouldn't turn it up much past the point where distortion justs starts to be heard (like 11-12 o'clock). If you need more volume than that, you need a different amp. Run it at its sweet spot(like 11-12 o'clock), and you'll get the best sound and performance out of it without killing the amp or speakers.

    If I were to run my ric 4001 into a splitter box and send one of the signals to channel 1 and the other to channel 2, what would that do?

    You would have a volume-Bass-Treble control for each individual pickup. Pretty cool when I tried it with my 4001, but not cool enough to do all the time. If you have pedals or effects, you could put them on the bridge pickup signal and leave the neck pickup signal dry for a really good sound, though. I ran my 2 signals through 2 different amps once, and it was cool. The Bass heavy Neck pickup through my V4-B and a 1-18" cab, and the trebley bridge pickup through an Earth 100Watt tube head into a 2-15" cab. That was way too loud. :D

    Also, on the back of the amp there is an ext. amp jack and an ext. speaker jack; how would I go about using those? I know the cabinet with the B25B is a 16 ohm so if I wanted to use another cab, what should it be rated at? What is the purpose of having these? also, there is an AC outlet on the amp, what is that used for? Sorry about all these questions but you guys really seem to know what your talking about so I figured I would ask. Thanks a lot!

    External Amp gives you a place to pick up the signal after the B25B preamp and run it over into another amp. External Speaker will accept another 16 ohm speaker cabinet to be plugged in - 16 OHMS ONLY!!! It was made to let you run another B25B cabinet. That looks massive, 2 of the 2-15 bottoms sitting beside each other with a single B25B sitting on top of the pair! The B25B cannot handle an 8 ohm cab or 4 ohm cab at all in it's stock state. Only 1 or 2 16 ohm cabs. It can be modified slightly to allow a single 8 ohm cab to be plugged in alone. The AC outlet is a convenience outlet. It's there for convenience - plug in an effect, a small TV, whatever, up to the rated power output of the outlet which is marked next to it. If you want your amp to last forever, buy a small fan, plug it into that outlet, and point it so it is blowing onto the power tubes from behind. You don't have to, but if you were to do that, you'd probably never have to replace those tubes, or at least not for like 5-10 more years. :D Vacuum tubes of course need some heat to work, but the cooler you can keep them, the longer they last.

    Chris
     
  16. fat jonny

    fat jonny

    Jul 8, 2000
    Columbus Ohio
    Thanks a lot for your help Chris. What is your opinion of the V4B in comparison to the B25B? I was considering both but I decided on the B25B cuz I got a good deal on one locally (I dont know how in the hell you would ship that cab). I was thinking I would need the extra power of the V4B but Ive heard doubling your wattage doesnt do that much in terms of volume. Besides, the B25B is surprisingly loud for its rating. Maybe down the road I will get a V4B and an 8x10 and split the pickups on my ric like you were talking about. Damn i bet thatd be sweet. Thanks again for your help man!

    Jonny
     
  17. I got my V4B back in 1988 when I got into my first band in Hi School. I started out with a Fender Bassman 50 and single 15 cab. It wasn't loud enough. So I traded a Harmony guitar to a guy for a Fender Bandmaster Reverb Stack, since the cab was bigger than the 1x15 cab I had. Still not loud enough. (I was stupid back then and didn't understand all this amp crap - the Bandmaster only had 2-12's in that big old cabinet, and even though the head was bigger than the Bassman, it had the same power output-40Watts or so.)

    Then I found the V4B and bought it. Played it through the Bandmaster cabinet modified with 2-15's. Better, but still not loud enough since the Bandmaster cabinet was not designed for what I had done. (The band had 3 guitarists, each with 50 or 100 Watt Marshall half-stacks. Loud as crap.)

    Then our drummer said he had let a guy borrow this giant Swoop cab, and we could go get it back. It was a Distex 1-18" Sugar Scoop cabinet with a Pyle 18" woofer. Size of a small refrigerator, with the big swoop on the bottom and the 18 right in front on top. Really mean looking, like a cyclops.

    I was plenty loud. I mean loud as ****. That's when I realized everything had a lot to do with the cabinet, not just the amp. And with an efficient cab and 100 tube Watts, you could rattle the doors off a barn.

    So, you're better off with the B25 for practicing, and even for gigging if you can mic the cab and go through a PA. The B25B is a lot more useful for jamming. The sounds are similar to a V4B. The V4B is definitely a monster though. Really. :D If you ever luck into one, get it, but don't loose sleep over it. It's louder than the B25B, but not a whole lot louder.

    Chris
     
  18. Hey Chris, what's the mod to allow a 8 ohm cab? I checked the cab that came with my head and it's a single 15, looks maybe like a Fender cab, dunno. At any rate, I'm going to have the head checked out next week. I want to get the tech to look for the mod, so any info would be helpful.

    BTW, I called Midtown to see who they recommended for amp repair and they are hiring a tech to work out of the store. They've not been satisfied sending tube amps out for repair. I did call Wizard and they charge a flat $45 just to look at an amp.
     
  19. There are 2 taps off the output transformer - 16 ohms and 8 ohms. The amp uses a switching jack that has its ground isolated from chassis using fiber washers. And the first cab is plugged in using an attached cord.

    The way I modded mine, I got rid of the attached cord. I wired the 2 output taps to a SPDT switch, and the common of the switch goes to a shorting jack that does not have to be isolated from the chassis. I only have (1) output jack, which was the EXT SPKR jack, but that's cool with me. I don't need more. The switch is used to select "16" or "8". Simple as that.

    Another option could be to leave the attached cord disconnected, and if you plug into the EXT SPKR jack you would be tapping the "8 ohm" connection automatically. So if you have (1) 8 ohm cab, just plug it into the EXT SPKR jack with nothing connected to the amp's attached speaker cable.

    http://members.aol.com/portaflex/schems/b-25.gif
    [​IMG]

    Chris
     
  20. Thanks Chris. Sounds a like a cool mod that you did. I had found the schematic during my info search and it looked to my untrained eyes like the external jack was 8 ohms (the attached cable was obviously into 16 ohm). I didn't know about the switching part. My amp has (on the outside) the old xlr replaced by a 6" piece of speaker cable and a 1/4" female attached to the wooden frame.