bassman 10 speaker load

[sniper]

okay, i got the amp and the home made case was in splinters. so i took it to the local tv repair guy who is an older gent that used to work on tv's when they had tubes. he checked it over, tried it out and pronounced it well, then gave me a demo of how it sounded with his instore stereo. while all the demo's tv's were running, all his test equipment and his puter which had a bigger screen than my dining table top.

the verdict was, no hum, no buzz and it worked.

now i found an old but nice bandmaster reverb cab it fit into and bought a couple of Celestion G12T-75 16 ohm speakers to make me an 8 ohm speaker cab. (you know what i am leading up to here...betcha huh!)

during the demo he showed me how the, we will call it the first speaker jack had to be plugged in order for the secondary jack (marked ext. speaker) to work.

the problem i have is the little line below the jacks says "8 ohms total". this is where the questions start! i plan on using the G12T-75's in parallel for a total load of 8 ohms into one jack. now...if i plug another speaker into the "ext speaker" jack, does the "first speaker" which evidently has a switch on it, center tap into the OT to halve the ohms impedence on both jacks for a total load of 8 ohms because they run in parallel when both jacks are used?  i am under the impression the chassis is relatively unmodified. the OT has been replaced at one time, but the numbers on the OT confirms it is of OEM type and matching part number.

in other words what ohm impedence do i have to have on the second speaker cabinet now that i just bought an 8 ohms JBL 15" speaker for the second cab build? DUH?   

second question: is a Faraday shield in the top of the amp head really needed? i think i can make one out of some aluminum screening and my stapler gun.

[Psycho Bass Guy]

if i plug another speaker into the "ext speaker" jack, does the "first speaker" which evidently has a switch on it, center tap into the OT to halve the ohms impedence on both jacks for a total load of 8 ohms because they run in parallel when both jacks are used?

No. The jack is NOT a switch for the OT different taps according to the schematic. There are only two leads at all off the OT. Either your OT is from another model Fender, or your repair guy is wrong about the switching jack.

in other words what ohm impedence do i have to have on the second speaker cabinet now that i just bought an 8 ohms JBL 15" speaker for the second cab build? DUH?   

Assuming it's really the Bassman 10 circuit, OT and all, if you want to run the amp at its rated output impedance, you can't add another cab.

second question: is a Faraday shield in the top of the amp head really needed? i think i can make one out of some aluminum screening and my stapler gun.

You said yourself that the amp had no hum. There's your answer.

[rahock]

For starters, Psycho Bass Guy knows a helluva lot more than than I do about this stuff , so put your faith in what he says.

My question is, why run another cabinet with the 15 JBL? That Bassman head is rated at 100-110 watts peak and about 50 watts RMS. I don't know how many watts those 2 12" Celestions are rated at, but I'm guessing that they are probably putting you at , near or maybe even over the power limits of that bassman head. What is the power rating on the Celestions and the JBL? I'm thinking your throwing too much speaker at your amp.
Rick

[sniper]

"No. The jack is NOT a switch for the OT different taps according to the schematic. There are only two leads at all off the OT. Either your OT is from another model Fender, or your repair guy is wrong about the switching jack."

i made an assumption about the center tap (but i am in the Ben Franklin stage when it comes to electronics) and cannot read a schematic. thanks. i know the jack is a switching jack of some sort because the second jack will not work unless the first jack is used. maybe a simple on/off type switch. BTW the master volume works!

"Assuming it's really the Bassman 10 circuit, OT and all, if you want to run the amp at its rated output impedance, you can't add another cab."

i believe it is a Bassman 10 curcuit. the trannies (OT = 021848  which is the 125A13A equiv and PT = 022798 which is the PT 125P5D equiv) check out parts number wise but i know it has been worked on. the stock curcuit is the diagnosis of Mike ... the repair guy.

thanks about the Faraday shield answer. it is a quiet little amp.
___________________________________________________________________________________________________________

the 12's are rated at 75 watts each, the JBL is rated at 200 watts @ 8 ohms. both are sensitive speakers at or above 97 db @ 1 watt.

i maybe killing it with too much speaker! hmmm...a rethink to a plan B on the speaker build might be in order!

so bottom line = 8 ohm cab then no other cab and 4 ohm cab then one other 4 ohm cab is okay?

thanks a bunch.



as recieved



as it is now. logo is bought, tube chart is bought, original B10 manuel is bought, and 70's grill cloth is ordered. the wood is actually 41 years old with new tolex and hardware (bought that way), original top bars. i don't mind the extra height as it gives me room to toss the plug cord in without whamming the tubes.

[Pilgrim]

i don't mind the extra height as it gives me room to toss the plug cord in without whamming the tubes.

Instant flashback: I was just tucking the AC cord into my blackface Bassman head yesterday and noticed that doing so always pushes one of the 6L6 tubes a bit sideways. 

Shared experiences!

[Psycho Bass Guy]

i made an assumption about the center tap (but i am in the Ben Franklin stage when it comes to electronics) and cannot read a schematic. thanks. i know the jack is a switching jack of some sort because the second jack will not work unless the first jack is used. maybe a simple on/off type switch.

Fender used shorting jacks to protect the amp from no-load situations. Until you insert a plug into the primary output, the amp is dumping its output directly to ground. They're technically not a switching jack like in the original Ampeg SVT's extension jack, which selects between separate OT taps.

the 12's are rated at 75 watts each, the JBL is rated at 200 watts @ 8 ohms. both are sensitive speakers at or above 97 db @ 1 watt.

Keep in mind that 97 dB is at 1 kHz; at bass frequencies you'll be lucky to have 80 or 85 dB.  

so bottom line = 8 ohm cab then no other cab and 4 ohm cab then one other 4 ohm cab is okay?

Yes, HOWEVER you could go lower in impedance-eg more speakers, but you will reduce your output power and wear the output tubes faster. Since tubes are relatively low current devices, lower than rated loads aren't the danger to them that they are to transistors. In most cases, you'll lose any volume gained by the extra speakers to the decreased headroom of a lower load, but it depends on the speakers, the OT, the tube quality, their current and the amount of gain you use. IOW, you could try it and see without it really hurting anything. If you see your output tubes' plates start turning colors from overdisappation, then you'll know not to do it, but if your amp is biased fairly conservatively, it might be just fine.  I'd wager it won't be a good idea, though. Where your speakers are paralleled, they will have a greater voltage drop than a series or series/parallel combination.

[lowend1]

I'd just like to say that I'm glad PBG has started to frequent the Outpost. The amp section of this forum has had a tendency to lapse from time to time.
Regarding the switching jack on the SVT - is there an intrinsic problem with these? When I hook up a second cab, the main cab shuts off. I think I remember reading somewhere that this was relatively common on the older models...

[Psycho Bass Guy]

Regarding the switching jack on the SVT - is there an intrinsic problem with these? When I hook up a second cab, the main cab shuts off. I think I remember reading somewhere that this was relatively common on the older models...

It's either gone bad or is really dirty.  Give it a serious cleaning and see if that changes.

[rahock]

Yes indeed , it's good to have you around again Psycho Bass Guy
Rick

[Psycho Bass Guy]

I appreciate it. It's not that I've been boycotting the internet or anything, there just hasn't been much I've wanted to talk about.  I spent ten years helping Talkbass become what it is now, and the lowdown pieces of shit permanently banned me for telling too much truth to the detriment of some manufacturers and their endorsees who shill there. It tends to take the wind out of one's sails. 

[Highlander]

My '68 Marshall might appreciate that knowledge... 

[rahock]

I appreciate it. It's not that I've been boycotting the internet or anything, there just hasn't been much I've wanted to talk about.  I spent ten years helping Talkbass become what it is now, and the lowdown pieces of shit permanently banned me for telling too much truth to the detriment of some manufacturers and their endorsees who shill there. It tends to take the wind out of one's sails. 

Telling it like it is doesn't often lead you down the path to political correctness. I always had considerable difficulty trying to get down that freakin' path myself
Funk 'em.
Rick

[lowend1]

Telling it like it is doesn't often lead you down the path to political correctness. I always had considerable difficulty trying to get down that freakin' path myself
Funk 'em.
Rick

Me three.

[Dave W]

Psycho Bass Guy has told us what happened, and it's really too bad, but let's not pile on TB here.

No sponsors or their endorsers to tiptoe around here, thank goodness.

[rahock]

Yeah, I agree. Trash talking doen't take you down a very nice path either. 'Nuff said.
This place has been a refreshing break from all that......even the Fender haters are pretty polite
Rick