fixed bias/bassman layout?

[drew_half_empty]

well, i've been having some popping & volume drops in my bassman (early 70's, master volume bassman 50) and i wanna check & see if its a preamp tube thats doin all that

i have my hotrod, which has some 12ax7's in it, so, would it be safe to take one out of the hot rod, put it in the bassman, fire it up, and see if it still does it, or would i have to get the bias section rebuilt & rebias the hot rod after i took the tube out & then put it back in?

also, does anyone know which preamp tubes on the bassman are for the guitar side? Theres like 4 of them altogether

 

[drew_half_empty]

Re: fixed bias/bassman layout?

bump

nevermind about part of that, it also does it when i'm plugged into the bass input

could it be the phase inverter? It's a NOS phillips 12ay7, the rest are groove tubes, so can't be that old

 

[Scott_F]

Re: fixed bias/bassman layout?

Any of the 12's can be put in one of those preamp slots.

Find one that works well from the newer amp, 12AT7, 12AX7, whatever and start to replace it one position at a time in the older amp.

Also, you can take a pencil and gently click on the side of the preamp tubes while they're hot and glowing to see if they clatter. If so, get rid of them. Not just a little clink, but a huge clatter. You'll know it if/when you hear it.

Does it pop and sputter while just sitting there or only when your'e playing?

 

[Curly]

Re: fixed bias/bassman layout?

well, i've been having some popping & volume drops in my bassman (early 70's, master volume bassman 50) and i wanna check & see if its a preamp tube thats doin all that

you can isolate the part of the circuit that's a problem by tapping on the preamp tubes (lightly!) with a chopstick, or a fingernail

i have my hotrod, which has some 12ax7's in it, so, would it be safe to take one out of the hot rod, put it in the bassman, fire it up, and see if it still does it, or would i have to get the bias section rebuilt & rebias the hot rod after i took the tube out & then put it back in?

biasing involves the power tubes, so subbing preamp tubes from your HRD has no affect on its bias

also, does anyone know which preamp tubes on the bassman are for the guitar side? Theres like 4 of them altogether

tube layout

as I've said before, I'd suggest getting a good book on tube amps, such as Gerald Weber's "A Hip Guide". This will be a good investment, and will likely pay for itself. He has a very good section on "how to make your amp sound right"
tube amps are not complicated, but they do require maintenance, so you should know something about what you're doing, if for no other reason than the fact that tube amps contain lethal voltages
you can also find plenty of free tech info on the web, at places like ampage, aiken amps, geofx, etc

 

[drew_half_empty]

Re: fixed bias/bassman layout?

well scott, it does it whether or not i have a guitar plugged in, it's most noticable when i turn standby off

also, it has spring metal shields, will it hurt to take them off while I'm checking the tubes? Assuming it's 'hot & running'

 

[drew_half_empty]

Re: fixed bias/bassman layout?

ahh, found it! When i push one of the power tubes ever so slightly to one side or another, the popping starts

it also appears to be sitting lower than the other one, but i can't push it up any farther, it's sitting in there kinda loosely

 

[Curly]

Re: fixed bias/bassman layout?

it's likely the pin inserts on the socket need to be retensioned

 

[drew_half_empty]

Re: fixed bias/bassman layout?

how do i go about doing that?

or is it better left to a tech

 

[Curly]

Re: fixed bias/bassman layout?

I'd have to say leave it to a tech, since some of the pins on the power tube sockets carry very high voltages

 

[Lewguitar]

Re: fixed bias/bassman layout?

I'd have to say leave it to a tech, since some of the pins on the power tube sockets carry very high voltages

Yes they do. You can get knocked on your butt even with the amp unplugged because the filter caps are connected to the power tube sockets and filter caps can store a charge for some time...even with the amp unplugged.

If you know how to discharge the filter caps safely, then you can clean and retension those pin sockets.

I don't know whether to tell you how to do it or not! I'd hate for someone to not do it right and then get hurt...

Lew

 

[drew_half_empty]

Re: fixed bias/bassman layout?

well it shouldn't cost too much for a tech to do it, right?

 

[Kent S.]

Re: fixed bias/bassman layout?

In all seriousness Drew, there are many wondeful books available about tube amps, and servicing, they cover many basic things like this, are inexpensive, fun and fact filled (cool history and stories often included)... and with step by step instructions about common problems, rountine maintenance, and trouble shooting ... Some even have common mods and alterations.
It would behoove any tube amp owner to pick one or two up.

 

[casblah]

Re: fixed bias/bassman layout?

If you're taking it to a tech anyways, have him put some new filter caps in it while its apart. The amp is prolly due for some after 30 years.