Bassman Question

[autopilot]

my question is basically general in nature, but may lead to more.

i have a bassman that says AB165. It is silver face with black lines on the faceplate, which I am under the understanding that it may be "blackface" electronics.

upon inspection, the amp appears to be all original electronics, untampered, and clean. all solder points appear original. if you need pics, i can get some together. none of the caps appear leaky. i don't know enough about amp electronics. i got this amp for $150 a couple years ago and never had any issue previously. this is just to give you a frame of reference regarding the amp in question...

now, the problem i am experiencing is new, but i am also using the amp in a new way than i have previously.
i understand the amp is 2 ohm speaker output at 50w.
in the past, i always just used the amp as a guitar amp, for cleaner grit to blend with other guitar tracks.
but recently i started jamming on bass with a band and picked up an Ampeg 410HLF cab thinking i could use the bassman to drive it until i decide to possibly get a more powerful tube head in the future. the ampeg is a 4 ohm cab.

now, for the problem...
when i plug my bass in, a peavey rudy sarzo model, i can only turn my instrument volume up to about 2 or 3 (of 10).
the gain on the bassman can go all the way up as long as my instrument doesn't go any further up.
otherwise, if i turn my instrument up any louder, the bassman begins to behave rather unhappily.
it starts to make a loud crackle pop type of sound. it is a bit unsettling.
this happens whether plugged into the "bass" or "normal" inputs.
i had never had this problem using it as a guitar amp, so my immediate thought was that perhaps it has something to do with the bassman's input circuitry in general that limits the amount of input gain it can accept without overloading on bass frequencies, but it seemed odd to me that i can only turn up slightly (a bass into a bassman)!

also, this is the only time i get the loud crackle pop.
in any other circumstance the amp is quiet, and performs without a hitch.

so, is the "problem" the inherent nature of a bassman?
or do i have another issue to work out?
any suggestions are welcome!
and hopefully this isn't a dumb question. i have very little electronics experience.

[corduroyew]

1st off, your bassman has a 4ohm output not 2 ohm.

The crackle problem might be the speakers as much as your amp. The bassman is not a good bass amp. It' bass tone is a bit flabby and it's too much for a lot of speakers. I had a problem with them when I was using Jensen P12's. When I changed the speakers to C12's it sorted everything out for a 6 string guitar but I still couldn't use it for 7 string. When I started using a full range eminence legend (don't remember the exact model) speaker it reduced the crackle even more but to realy get rid of it I had to change some cap values to tighten up the bass end. I have never tried my amp with a real bass but I would think that it would have the same problems I was having, but to a greater degree.

Then again, I don't know much about this stuff either so I could be way off base.

[autopilot]

Thanks for clearing me up regarding the ohm output. That would mean the head does match the cab in that respect.

The cab is an Ampeg SVT-410HLF with a freq response down to 48Hz. It's one of the best cabs made by Ampeg in terms of quality and construction. So I am less likely to question the speaker's ability to reproduce low freq.

[Chuck H]

The bass you are using has an active pickup system in it. That could be creating more low end than the preamp can handle and still remain stable. Stability problems with tube gear and preamped instruments happen all the time. There is alot more voltage swing and bandwidth. That is not to say that a regular passive bass guitar wouldn't do it. But it probably wouldn't be as bad. No one at Fender ever imagined that someone would try to to send a bass signal with a +10dB boost at 40hz into that input. The crackle you hear is most likely a brief oscillation. You could probably modify the amp to accept that kind of input. But you would have to sacrifice gain in the preamp and most likely render it useless for guitar.

Chuck

[autopilot]

Good catch, but the bass is active or passive and has a switch. I don't have a battery loaded into the bass for the active electronics. I never had problems using the amp with guitars so it could be the bass frequencies are just too much for it.

I don't have an interest in modifying the amp that drastically as I do want to use it with guitar. At most, I would want to replace the capacitors and add a ground cord for starters. I notice a lot of people indicate there are inexpensive mods that really make it better, but I'm not sure how much of that I would get into.

Probably the next step is going to be replacing some capacitors. What's a good source for caps for these?

Chuck, where are you located? I am in Seattle.

[autopilot]

OK, new question regarding capacitors

I took the bassman apart tonight and removed the cap pan.
I now notice some of the old mallory caps are bulging slightly.
There are also the other caps in the audio section.

I have taken note of all the values of caps/looked at a schematic & diagram.
I am a novice but can solder well and understand some have polarity.
I have seen that most do not recommend replacing the blue caps so I will skip over those.

On the 450v/8uf, I notice mine has a 16uf. Which value should I use?
On the brown turd caps, are these also electrolytic? I really don't know what the difference is.
The orange ones may be replacements already...are they electrolytic?
Regarding the tri-caps (that's what i'm calling 'em)...can I simply use 2 polarized 25v/25uf electrolytic caps?
Is the white cap also electrolytic?
Then, what about the round brown discs? Do those need to be replaced?

Sorry if these seem like common sense questions, I just don't want to screw it up, but I like the opportunity to learn a bit.

I also read something about changing resistors under the cap pan? What is the purpose of doing that?

Here are pics of the amp
http://www.autopilotrecords.com/imag...sman%20001.jpg
http://www.autopilotrecords.com/imag...sman%20002.jpg
http://www.autopilotrecords.com/imag...sman%20003.jpg

thanks!

[corduroyew]

"On the 450v/8uf, I notice mine has a 16uf. Which value should I use?"

Most people like the 16uF here. If I understand it correctly, the higher value gives you a more open sound.

"On the brown turd caps, are these also electrolytic?"

Nope. They are not electrolytic

"The orange ones may be replacements already...are they electrolytic?"

Nope, they are not electrolytic either.

"Regarding the tri-caps (that's what i'm calling 'em)...can I simply use 2 polarized 25v/25uf electrolytic caps?"

Yes

"Is the white cap also electrolytic?"
Should be. It's supposed to be a 25v/25uF cap there.

"Then, what about the round brown discs? Do those need to be replaced?"
Probably not but it wouldn't hurt. I'd probably do all the other work 1st and then wait and see if I still felt like I needed to change them.


"I also read something about changing resistors under the cap pan? What is the purpose of doing that?"

If I understand it correctly changing those resistors can help get rid of some buzzing so that would be a very good thing for you to do.

[autopilot]

thanks, are they polypropylene plastic film caps? i think i found other threads saying something about orange drops, which i think i have figured out are sprague 715 series orange drop caps. i can use those then on the brown & orange ones right?

what type of resistors are best?

[Dave Curtis, dB AudioTech]

I took the bassman apart tonight and removed the cap pan.
I now notice some of the old mallory caps are bulging slightly.
There are also the other caps in the audio section.

I have taken note of all the values of caps/looked at a schematic & diagram.
I am a novice but can solder well and understand some have polarity.
I have seen that most do not recommend replacing the blue caps so I will skip over those.

On the 450v/8uf, I notice mine has a 16uf. Which value should I use?
On the brown turd caps, are these also electrolytic? I really don't know what the difference is.

About 8uF A bit better bass response. That 16 is stock; you can go higher if you want.

The orange ones may be replacements already...are they electrolytic?
Regarding the tri-caps (that's what i'm calling 'em)...can I simply use 2 polarized 25v/25uf electrolytic caps?
Is the white cap also electrolytic?

You're right about the orange drops (but they're not 'lytics); are they the correct value? Yes you can use discreet 25uF/25V caps, but you'll find probably find only 22uF/35V caps; they're fine. Yes the white one is, too.

Then, what about the round brown discs? Do those need to be replaced?

Not if they're good.

Sorry if these seem like common sense questions, I just don't want to screw it up, but I like the opportunity to learn a bit.

I also read something about changing resistors under the cap pan? What is the purpose of doing that?

Reliability. What you want there is flameproof metal oxide resistors.

Here are pics of the amp
http://www.autopilotrecords.com/imag...sman%20001.jpg
http://www.autopilotrecords.com/imag...sman%20002.jpg
http://www.autopilotrecords.com/imag...sman%20003.jpg

thanks!

Cool. looks like a '68 (date codes on caps are late '67). Way overdue for a cap job, and will likely fix your problem. (power supply "complaining" about the bass) Don't forget the cap on the little bias board is + to ground. Have fun, and don't forget to post results.

-DC

[Alex R]

Standard electrolytic caps you find in amps are always cylindrical and have metal cans though they can go quite small. Usually the can looks different at each end - one end is a metal continuation of the case with a wire in the middle, the other insulated with a wire through the middle... sometimes two wires, with in that case no wire at the metal end. The wire connected to the can is the negative connection.

Small flat discs, generally brown, are ceramic disc caps.

Various types of poly caps are all kinds of shapes and colours (including cylindrical like orange drops are) but are not in metal cases. Usually they have been dipped in some insulating material, often shiny, though some have transparent cylindrical plastic containers.

Thought it might be worth stating the basics.

[autopilot]

resistor wattage?

hi,
can anyone tell me the wattage for the resistors under the cap pan?
i can't see it on the drawing, and just want to make sure they are 1/2W resistors.
thanks!
-Rich.

[Jag]

Schematic says 1W but I always use 2W flameproof in there.

[David Schwab]

You know, the amp might also be having a hard time driving that Ampeg cab. Some speakers need more power to drive them, the amp doesn't have it, so it breaks up.

I think the Bassman has become more popular with guitarist because it was underpowered as a bass amp, just as were many of the older Ampegs, like the V4B.

I couldn't imagine playing through a 50W bass amp, unless I was playing alone. Even my Mesa 400+ doesn't give enough headroom at times.

[Jag]

I agree with David, the Bassman is not really a very good bass amp despite the name.

You might be able to tighten up the bottom end using 2 220uf caps in place of the 70uf's that make up the first filter (that's generally the two on the left under the cap pan if you're looking at it from the back) but I think you'll probably still be disappointed unless your band is really quiet.

[Bruce / Mission Amps]

I agree with David, the Bassman is not really a very good bass amp despite the name.

You might be able to tighten up the bottom end using 2 220uf caps in place of the 70uf's that make up the first filter (that's generally the two on the left under the cap pan if you're looking at it from the back) but I think you'll probably still be disappointed unless your band is really quiet.

Me too.... stock, it is a horrible bass amp. Modified, it can be a very cool guitar amp.
Any $200-$300 transistor bass amp will blow it into the weeds.
I played with a bass player using a BF Bassman and an old C12N, 2x12 bottom and the guitar player was using a good sounding,
SF 15 watt Princeton Reverb.
The guitar player and I were always being told to come down a bit as the clean bass was getting distorted and covered up at that super loud level!!
HUH!? SUPER LOUD!??