The Fender Super Bassman 300 thread

[FronTowardEnemy]

Got my amp back from the tech and managed to get a rundown on things.

First off, clarification on my bias circuit. I misunderstood what he was saying. Mine actually had a problem and needed replacing, Fender supplied a new one free because it is a "known issue". Not like I said previously that it was junked and replaced without reason. My bad

He ran me through the theory of replacing SB tubes in pairs and this is how I understand it ...

The tubes operate in two groups of three, 1A 2A and 3A on one side and 1B 2B and 3B on the other side. The two sides need to match one another to operate properly, the thing that needs to match is called mutual inductance. The amp should leave the factory with all six power tubes close matched, but over time they will drift out of spec. The auto-bias circuit compensates for the drift but only up to a certain point, when one or more tubes drift too far off spec it can no longer compensate. This is when you get orange and red lights showing.

Mutual inductance is the thing that is measured when sellers grade tubes. So when they refer to a pair being of "the same grade" they are saying they are matched for mutual inductance.

If you need to replace one tube because it's drifted too far off spec or it just goes bad, you need to replace it's partner as well. Both new tubes need to be matched for mutual inductance in order to keep the TOTAL of the two A and B sides matching.

This is also the reason for the tubes being fused in pairs. Eg if 2A suddenly has a problem ... the fuse for the 2A and 2B pair will pop, switching off the pair. That way the total mutual inductance for both sides stays matched and the amp can continue to operate.

I've done my best to translate what I was told and it's giving me a headache , I'm a looooong way from being clued up on this stuff myself.

Thank you. This makes complete sense.

 

[rodl2005]

Got my amp back from the tech and managed to get a rundown on things.

First off, clarification on my bias circuit. I misunderstood what he was saying. Mine actually had a problem and needed replacing, Fender supplied a new one free because it is a "known issue". Not like I said previously that it was junked and replaced without reason. My bad

He ran me through the theory of replacing SB tubes in pairs and this is how I understand it ...

The tubes operate in two groups of three, 1A 2A and 3A on one side and 1B 2B and 3B on the other side. The two sides need to match one another to operate properly, the thing that needs to match is called mutual inductance. The amp should leave the factory with all six power tubes close matched, but over time they will drift out of spec. The auto-bias circuit compensates for the drift but only up to a certain point, when one or more tubes drift too far off spec it can no longer compensate. This is when you get orange and red lights showing.

Mutual inductance is the thing that is measured when sellers grade tubes. So when they refer to a pair being of "the same grade" they are saying they are matched for mutual inductance.

If you need to replace one tube because it's drifted too far off spec or it just goes bad, you need to replace it's partner as well. Both new tubes need to be matched for mutual inductance in order to keep the TOTAL of the two A and B sides matching.

This is also the reason for the tubes being fused in pairs. Eg if 2A suddenly has a problem ... the fuse for the 2A and 2B pair will pop, switching off the pair. That way the total mutual inductance for both sides stays matched and the amp can continue to operate.

I've done my best to translate what I was told and it's giving me a headache , I'm a looooong way from being clued up on this stuff myself.

Thanx for the post.
So if TUBE 1A goes futz, ya replace 1A AND 2A. that right?

Edit, sorry, you'd replace 1A AND 1B.
yes?

 

[dbbltime]

Thanx for the post.
So if TUBE 1A goes futz, ya replace 1A AND 2A. that right?[/QUOTE
I read it to mean replace 1A, and 1B.

 

[G-Dog]

Thanx for the post.
So if TUBE 1A goes futz, ya replace 1A AND 2A. that right?

I thought he said the A & B are a pair! So, if 1A goes bad, replace it and 1B.

 

[G-Dog]

. . . . . ...
The tubes operate in two groups of three, 1A 2A and 3A on one side and 1B 2B and 3B on the other side. The two sides need to match one another to operate properly, . . . . .
If you need to replace one tube because it's drifted too far off spec or it just goes bad, you need to replace it's partner as well. Both new tubes need to be matched for mutual inductance in order to keep the TOTAL of the two A and B sides matching.

This is also the reason for the tubes being fused in pairs. Eg if 2A suddenly has a problem ... the fuse for the 2A and 2B pair will pop, switching off the pair. That way the total mutual inductance for both sides stays matched and the amp can continue to operate. . . .

 

[rodl2005]

I thought he said the A & B are a pair! So, if 1A goes bad, replace it and 1B.

Sorry, you're right, and that's what I meant. Rushing about at work....
I'll edit above. Etc etc

Replace 1a and 1b if 1a goes kaput

 

[Tim1]

Jeez I love my Super Bassman but hate autobias, even though mine has never had a problem. It is just another thing that I don't want to worry or obsess about. I never worried about tubes in the seventies and eighties and never had a problem. I would rather not know and these days if my amp stops I have a lightweight class D backup in the bag. Again, I am yet to need the backup.

 

[flatwound62p]

Sorry, you're right, and that's what I meant. Rushing about at work....
I'll edit above. Etc etc

Replace 1a and 1b if 1a goes kaput

That's the one Took him 15 minutes of repeating himself with the schematic up on the big screen to get it through my thick head.

 

[rodl2005]

Mine too!

TBH tho, I just treat the auto bias thingy as a bonus..... even if it shows an Orange light, I'd keep playing (this gig) if it sounded ok.
Then check it out properly later.

 

[azwhofan]

Here's my auto- bias lights last Sunday.
View attachment 3212672

I wouldn't have bothered with em at all, but this discussion prompted me to check.
I was set up close to the rear wall, so I couldn't get me head in behind to see, so I stuck my phone in and took a pic.
All green, all GO, and it sounded superb too.
Thru the HG410 too

View attachment 3212675
This pic was the night before, but same set up.

Rod, when did you get the HG410 and how would you compare it to the NEO 115 and/or NEO 610??

 

[rodl2005]

Rod, when did you get the HG410 and how would you compare it to the NEO 115 and/or NEO 610??

I've not played the 610 Fender cab, that I can remember at least.
Compared with the 115 Bassman, it's "similar " in that they're both tight sounding, the Bassman 115s are almost a "sealed " sounding cab, altho its ported.
The 115 is, as you'd expect, rounder and a tad more "vintage" sounding.... to me, it sounds like a real old school 115. With the horn off anyway.
The HG410 is VERY smooth, especially up high. The tweeter is astonishing. The smoothest transition from woofers to horn I've experienced. It's the ONLY cab ever that I've left the horn ON, even when using overdrive.
I REALLY woulda been completely happy with the Fender 115s, but I got a new car recently and they don't quite fit in the back, without the rear seats being down etc etc etc (1st world problems, I know)
The HG410 allows me to have it in the rear of my car completely unseen.
It's also quite dependent on the rear wall, with its rear facing driver, not that this is a problem, but it means you can adjust the cab to get the very best outta it, by simply moving it a little, depending on the room etc.
Last week, I was at an outdoor yacht club, on the back of a truck situation... it sounded a little boomy, until I moved it perhaps 20° and it just shone!
The HG410 is still very new to me, and judging by the gigs I've done with it so far, it's pretty astounding. I was hoping for a ONE CAB SOLUTION, and so far, it's hitting the mark.
One 115 would've struggled .... probly suffered actually in this above situation, but this HG410 handled everything I threw at it, and seemed to be begging for more.
I'll still be keeping my 115s, or at least one, as they really do sound terrific
Whether my NV215 stays, will depend on just how much punishment the HG can take.
Cranking the Super Bassman into it OUTDOORS soon should tell me.

Edit. The HG410 also doesn't SEEM, so far at least, to get real "splanky" in the top end. No click, clank with round wound strings. Even with the tweeter on fully, it's not "glassy". Which, for me, is a great thing. There's still plenty of highs, just not that REAL toppy splank. COOL!

 

[shwashwa]

i have a quick question about the 100T but im sure it applies here also. if im plugged into input one, can i plug a cable into input 2 and jump to another amp?

 

[rodl2005]

i have a quick question about the 100T but im sure it applies here also. if im plugged into input one, can i plug a cable into input 2 and jump to another amp?

Don't think so. If I understand you correctly at least.
I sometimes plug my bass and my keyboard bass into each input, but it has to go to the same cab outta the rear speaker outs.

 

[Meghans Dad]

i have a quick question about the 100T but im sure it applies here also. if im plugged into input one, can i plug a cable into input 2 and jump to another amp?

I don't think so either; if you check out the circuit I think you find that you are doing the same thing as splitting your instrument signal with a y-connector cable. I don't know what happens in you bridge 2 pre-amps together but it sounds like more of a risk than I would be willing to take. I think it would be safer to get a proper ABY splitter box that isolates the signal.

 

[shwashwa]

I don't think so either; if you check out the circuit I think you find that you are doing the same thing as splitting your instrument signal with a y-connector cable. I don't know what happens in you bridge 2 pre-amps together but it sounds like more of a risk than I would be willing to take. I think it would be safer to get a proper ABY splitter box that isolates the signal.

interesting, what I'm proposing is quite common with vintage bassman amps, are you guys understanding what I'm asking?

 

[rodl2005]

They have 2 separate channels, with inputs for each. This SB has the 2 channels, but only one lotta inputs.... you can switch between the channels, but can't run each channel separately nor slave or chain em together.

 

[Tim1]

interesting, what I'm proposing is quite common with vintage bassman amps, are you guys understanding what I'm asking?

I know what you mean. Guitarists used to daisy chain multiple amps together in this way for big stages. It was quite common to see Marshall stacks with the old springy coiled cords linking one to the other. Looked cool. I assume that the inputs were parallel linked so effectively it was the same as a Y connector. However, if I read it correctly the two inputs in the SB300 are probably linked but one is padded -6db presumably for active basses, and if you plug into both then both assume -6db sensitivity. Not really what you want.

 

[shwashwa]

I know what you mean. Guitarists used to daisy chain multiple amps together in this way for big stages. It was quite common to see Marshall stacks with the old springy coiled cords linking one to the other. Looked cool. I assume that the inputs were parallel linked so effectively it was the same as a Y connector. However, if I read it correctly the two inputs in the SB300 are probably linked but one is padded -6db presumably for active basses, and if you plug into both then both assume -6db sensitivity. Not really what you want.

thanks, yes, looking to run 2 different amps, not chain the channels together

 

[rodl2005]

I see. Well, that's possible ain't it? Outta the power amp out, as long ss there's a load on the SB???

 

[Ryan L.]

Sold my Neo 410, and now have another Neo 115 on the way to pair with my SB300. Never thought I’d be a 15’s guy, but the SB300/Neo 15 combo just kills. I found I preferred the 115 to the 410, after many gigs swapping between the 2.