Fender Studio Bass "Head"

[Tim Skaggs]

I've got the amp section out of a Fender Studio Bass combo amp that I converted to a "head" since it sounds so much better through my EV 15" than it sounds through its own cabinet with the same speaker. The original combo cabinet is vary shallow, & it sounds like it.

I would like to use this amp with two 8 ohm cabinets, but it's an 8-ohm only amp. I'm fully aware of impedance matching & amp overheating, but I noticed a comment in the stickies that a tube amp is in more danger playing into a higher than rated impedance vs a lower than rated imp. I know a SS amp will die if running a lower than rated imp load. Are all tube amps more forgiving of lower than rated imp?

One thing I belive makes a difference is if the amp output is coupled to the output jack via transformer. I don't know if my amp is or not. I can't quite accept the info in the stickies & risk frying a nice old (circa 1980) Fender tube amp.

[johnk_10]

yes, your studio bass has an output transformer, as all tube amps do.

in my opinion, running it at 4 ohms will not hurt it at all. i've run alot of fender amps at half their rated impedance for years and have never had a problem doing it.

[Tim Skaggs]

Thanks for the info. I suspected my amp was a transformer coupled output, but wasn't sure. I had read of a few modern tube amps that were not.

I suppose the transformer primary provides a relatively consistent load to the amp if the load on the secondary is "close" to rated impedance?

Just trying not to fry.....

[Bluesbob]

Here's a URL for the schematic:

http://www.schematicheaven.com/fende...200w_schem.pdf

I love how they ask you to interpret the addition of the other power tubes.
There is one in the shop with a blown PT. Nobody wants to spring for a replacement. I dunno why not, it would make a great head, like you've done.
Fender tube amps are designed to tolerate a 100% impedance mis-match, so 4 or 16 ohms shouldn't be a problem IF everything else (caps, tubes, bias, etc.) is OK.

[Tim Skaggs]

Excellent information & thanks. I hadn't thought about searching for the schematic although I'm not suprised it's available. The old amp sounds absolutely fantastic through the single 15 when that 15 is in an adequate cabinet, but the amp is only rated at 200 watts, and with only six 6L6 tubes, I would expect it's only around 160 - 180 watts. In some venues I could use a bit more volume & that's why I want to use a second cab. Thanks again.

[BassmanPaul]

Originally Posted by Bluesbob

Here's a URL for the schematic:

http://www.schematicheaven.com/fende...200w_schem.pdf

I love how they ask you to interpret the addition of the other power tubes.
There is one in the shop with a blown PT. Nobody wants to spring for a replacement. I dunno why not, it would make a great head, like you've done.
Fender tube amps are designed to tolerate a 100% impedance mis-match, so 4 or 16 ohms shouldn't be a problem IF everything else (caps, tubes, bias, etc.) is OK.

Can I have it????

Paul

[BFISHER1970]

Played my old studio bass into 4 ohms for years without a hiccup

[johnk_10]

here's one that i had:
http://johnkvintageguitars.homestead...tudioBass.html

and here's what i got to replace it so i'd have an actual 4 ohm output tranny:

[BFISHER1970]

Very nice....did you change the tone stack at all?

[johnk_10]

nope, no need to. it sounds just like all three of the studio bass heads that i had.

[Bluesbob]

D 130's?

[johnk_10]

NOS-K140's.

[Bluesbob]

What? Huh?

[Tim Skaggs]

Are those two 15"s in a sealed cab? Is that a 4 ohm cab?

[johnk_10]

yep. its a stock, sealed Bassman 50 cab, only i installed two 8 ohm JBL K140's in it wired in parallel.