I'm confused - I thought they only made these for a couple years but see some called B25 , some B25B , some with toggle switches and the standby on the front and some with rocker switches with the standby on the back?
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Ampeg B25 layout?
- Thread starter Voodoo Guitar
- Start date
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- ampeg
The B-25 was only made in 1969, but the B-25B lasted from 69 till Magnavox sold Ampeg to MTI in 1980. And then the owner of MTI shut down production on everything except the SVT and 810 and came out with all new crappy products.
Holy cow! 7199's are $70 at Tube Depot! Yeah, I'd do the 6U8A sub route, too.
If you shop around, there are some better prices, but prices are nuts.
$54 at tubesandmore.com, $49 or $60 at tctubes.com. At lot of justification to get an adapter.
Is there any advantage to the older two-wire-cord standby-on-the-back versions? I just bought a newer one off of fleaBay that was gone through 10 years ago and used intermittently since (owner bought a V4B shortly after the B25B was serviced). I've had an early (16 ohm only back-mounted-speakers) cab for a few years now...
I don't think there were many circuit revisions on the B25B line. The later 70s ones should be pretty similar to the early 70s. I need to dig through my schematics and see if I can find any differences noted, but most schematics you see were just the early ones so it would stand to reason there weren't many updates/changes.
I can't comment on the B25 but in the other ampeg amps the quality of the circuit boards varied. The earlier revisions were better than the later ones. They were cutting costs. It's a minor detail, they all work well.
I'm not sure the years but they started labeling the boards better at one point, probably about 73/74. My B25B (~72) and 1970 V4 both have the boards with the brown side under the components and no part labeling on the green side like this:
B25B
V4
My 72-73 V4 and SVT both have the components labeled on the main boards
I don't know if the numbering corresponded with a change in materials to lower quality, but it certainly helps with repair work.
any tips for removing those 7027 retainer clips?? I have a nice pair of 6550s i can't wait to try. Sounding pretty good with 6L6s right now.
I'm not sure the years but they started labeling the boards better at one point, probably about 73/74. My B25B (~72) and 1970 V4 both have the boards with the brown side under the components and no part labeling on the green side like this:
B25B
V4
My 72-73 V4 and SVT both have the components labeled on the main boards
I don't know if the numbering corresponded with a change in materials to lower quality, but it certainly helps with repair work.
I find that the better boards are thicker and the traces are less prone to damage from heat. My 69 and 71 boards are way better than the ones in my 74 V4B.
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Do you mean the bear claw type? They can be difficult to remove without damaging the area. I've used a rotary tool to grind off the head. Very messy, requires covering the amp with plastic with a slit cut open for the socket being worked on.
Yep thats them. I suppose I could clip them off, or just stick with 6L6s or maybe KT66s.
Also, just noticed something that i can't believe I missed... my 6l6GCs are showing 117ma on the bias probe. No redplating, and they sound pretty damn good (until that 7199 PI noise starts), but i know that's about twice as high as i should be getting. Looks like the 100k 2W resistor has drifted to almost twice it's value! Reading around 168k on my VM. Everything else in the bias circuit measures fine. Glad I caught this before leaving it on for any extended time.
I'm thinking the same thing to get a smidge more power - that and replacing the tube rectifier with solid state. I also want to do the C17(?) mod to add subsonic filtering (HPF), maybe do the coupling caps in the two preamp sections too - I'm a real fan of HPF.
Theoretically with a pair of 6550's properly biased you could run three 16 ohm cabs on the 8 ohm tap for ~80 or so watts.
Maybe I'll start a B25B mods thread
.
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I think my 7027A were reading about 28mA plate current at 560V plate voltage with the stock 56K bias resistor, would put them around 50% dissipation for a 6L6GC, a little colder for 7027A, but ampeg set these pretty cool.
edit: yup
Those 100K bias resistors should be matched. They specify 5% but 1% is even better.
I'll be checking and likely replacing them as well as the 2w 100k in the bias circuit. Almost thought my meter was broken when i saw that bias current reading.
Yes, the 100K 2W dropping resistor in the bias circuit should be changed, they are prone to failure. Use a 5W if you can get one to fit. Also check the dropping resistors in the power supply. Any of the parts that run hot. Also replace the cap in the bias circuit.better to be safe than sorry.
I did the bias cap as well as the 1k 2w dropping resistor... the 10k 5w was still spot on. Hopefully those 100ks will solve my bias issues, as the 56k & 47k all read fine.
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