Fender Bassman with 6550 Tubes??

Pete Knapton · Post by **Pete Knapton** » 24 Sep 2012 5:09 pm

Hi all, I've got a silver face Fender Bassman 100 with a JBL K130 15" speaker. The amps been serviced and it needs a complete set of tubes. I play a '57 Fender 6 string.
Does anyone use a Bassman with 6550 tubes (instead of 6L6) for steel guitar? . Cheers

Tim Marcus · Post by **Tim Marcus** » 24 Sep 2012 7:21 pm

Might not be a great idea for that amp as is. You might be able to convert it to run 6550's but they require much higher plate voltage than 6L6 and also the bassman 100 is likely wired as untralinear output with no screen supplies

Stephen Cowell · Post by **Stephen Cowell** » 24 Sep 2012 11:41 pm

You might be able to convert it to two 6550's... the question would be why? It will make more power with four 6L6's than two 6550's... or *four* 6550's. The amp was designed for that operating point... the output transformer was chosen for four 6L6's at that particular voltage, 6550's have a different impedance characteristic from 6L6. Also, be aware that Fender amps use tie points on the sockets that normally are tube elements for other tubes like EL34/6550... you definitely should not just plug them in unless you like the smell of magic smoke.

The 100W is not ultra-linear... the four-tube Fenders that are ultra-linear are the 135watt series. You get about 8watts per screen when you connect them through the OT taps.

Pete Knapton · Post by **Pete Knapton** » 25 Sep 2012 2:38 am

Thanks Tim, Yes I'm aware that the voltage will be different. I have a clever and experienced amp tech who can sort that. I was interested to know how the tonal qualities would suit a steel guitar. The (bass) reviews I've read suggest that 6550s will give a 'glassy and liquid' sound, with a very clean range from lows to highs.
Trying to get some feedback from someone using this setup with steel guitar. Cheers

Cartwright Thompson · Post by **Cartwright Thompson** » 25 Sep 2012 4:41 am

I run a pair of G.E. 6550's in a blackface twin with a single 8ohm D-130. They sound great, I like them more than 6L6's. I don't notice a loss of power ( maybe 65 watts vs. about 85 with 6L6's) and the tone seems "bigger". It's been a long time since I put them in but I think the only mods to the amp were bigger screen resistors and a bias adjustment.
A twin may be different than a Bassman 100 though.

Michael Brebes · Post by **Michael Brebes** » 25 Sep 2012 7:00 am

If you want more of that 6550 sound, you might want to try the Winged C 6L6 tubes. I think they sound a little cleaner and punchier than standard 6L6 tubes.

Brett Lanier · Post by **Brett Lanier** » 25 Sep 2012 8:26 am

I also run two 6550's in my twin reverb. Mine is a '77 so I guess the power section of my amp is probably very close to your bassman 100, if not the same.

Bigger screen resistors are a must, and with my amp, my tech brought the whole bias range down because it was still a little high with the bias pot turned all the way down. Also, with 2 power tubes, it's an 8 ohm load, so I use a single 8 ohm JBL K130.

In my opinion, switching to 6550's made the amp a whole lot better for pedal steel. Maybe you've noticed that with a twin(& probably a bassman100), if you turn the bass up past three or four, you don't get more bass, just flub. I still keep my bass knob at three or four, but the low notes sound much deeper and have more of a singing quality.

Having said that, you probably should only go for it if your tech has confidence in the mod and if he lives close by because you may run into a hitch here and there while the amp gets used to the change.

Tim Marcus · Post by **Tim Marcus** » 25 Sep 2012 9:45 am

you do not need to get 6550's into a Twin to fix the flubber problem.

You need to address the power supply, the eq and the size of the speaker first. 6L6s are not to blame for lack of low end - many bass amps have 4 x 6L6's

In fact, the greatest recording bass amp of all time - the Ampeg B15 has 2x6L6 on the output and there certainly is not any flub there.

A Twin may not have enough low end for several reasons:

Phase inverter has silverface values, which are too tight

EQ capacitors have drifted to not let bass through

Coupling caps have not been replaced in 35 years and the amp has no snap

Power supply has become soft because the filter caps have drifted

Brett Lanier · Post by **Brett Lanier** » 25 Sep 2012 11:32 am

Tim, I respect your opinion on this, and I'd agree that all the things you've mentioned are most important before switching to a different type of power tube. But... I wouldn't make a comparison if everything else about the amp wasn't constant; including the 15" JBL, caps, etc.

Tim Marcus · Post by **Tim Marcus** » 25 Sep 2012 11:53 am

true

but its a whole lot easier to get the amp up to snuff with new caps and a good power supply than it is to convert to 6550's

if it is a vintage amp, chances are that the coupling caps and power supply caps are doing a lot more damage to the tone than the output tubes, which will only amplify the signal they see

Donny Hinson · Post by **Donny Hinson** » 25 Sep 2012 5:34 pm

When it comes to modding tube amps, be wary of the fact that 98% of the "tube amp experts" out there know zippo about what a pedal steel should sound like, and what characteristics steel players like. With the exception of the few techs you'll find here on the Forum, most of those guys re-design amps for rockers and tele-twangers, and their tastes lean far more towards the "overdrive-crunch-distortion" arena.

"Glassy and silky" at low volumes usually means "distorts like hell" when pushed.

Cartwright Thompson · Post by **Cartwright Thompson** » 26 Sep 2012 2:15 am

Tim Marcus wrote:true

but its a whole lot easier to get the amp up to snuff with new caps and a good power supply than it is to convert to 6550's

if it is a vintage amp, chances are that the coupling caps and power supply caps are doing a lot more damage to the tone than the output tubes, which will only amplify the signal they see

You have to do all of these things anyway, then you can try different power tubes.
"Converting" isn't really the right word for it in the case of a twin.
I go back and forth between the two types. If I want to use a pair of 12's, I go with 4 6L6's. If the amp is set- up correctly, you simply re-bias it and you're ready to go.

Pete Knapton · Post by **Pete Knapton** » 26 Sep 2012 2:23 am

Thanks everyone for the very helpful replies. I'm glad other people use the JBL K130 speaker as I was a little unsure about it. I'm off to the amp tech tomorrow with a matched set of GE 6550s (a friend deals in vintage tubes) and a stack of info off the net for setting up the amp. Hopefully I'll end up with a nice steel guitar amp. Will keep you posted of the result.

Pete Knapton · Post by **Pete Knapton** » 13 Oct 2012 12:36 am

I've had a good play with the 6550s and the amp is still in tact. I like them, tho not as punchy as the 6L6, but nice for steel. Thanks everyone for your replies. Cheers Pete