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Old 26th September 2010, 10:01 PM   #1
Leb is offline Leb
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Default Changing bass amp output transformer

I own a 1980 Fender Studio Bass amp. It's a 200 watt tube amp and is designed to be used only with an 8 ohm load. It has only one speaker output, but I would like to have two 8 ohm outputs and be able to run it at 4 ohms. Is it possible to change the output transformer to allow me to do this? If so, do any other components need to be changed? Will a different OT change the sound? Any help would be appreciated.
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Old 26th September 2010, 11:11 PM   #2
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A different transformer would allow 8 and 4 ohms use.
Not sure if you would get one off the shelf.
You might have to get oen specially made.
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Old 27th September 2010, 12:30 AM   #3
TheGimp is offline TheGimp  United States
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Is that the one with 6 each 6L6GC tubes in the output?

If so yo will have a difficult time finding the correct transformer due to the high output and low primary impedance (probably 3K).

There was an output transformer on ebay that was made for CBS that was rated 160-240W 5K p-p with multiple secondaries, but I lost the link. Someone might chime in with it.
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Old 27th September 2010, 12:30 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

It will be fairly load tolerant. You can run 4 ohms off a 8 ohm tap.
Unlikely to get any more real power but most valve amps put
out very near the same power into 4 ohms and 8 ohms.
(Dynamically if not continuously).

A 200W output transformer is never going to be cheap, and not simply
not worth it as keeping it original is paramount to value. You dont say
what cabinets you want to run off it, and there is some choice here,
e.g. a 16 ohm "bright box" could be added to a basic 8 ohm cabinet.

rgds, sreten.
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Old 27th September 2010, 12:44 AM   #5
TheGimp is offline TheGimp  United States
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Aha, found it:

160/200 watt Tube Output Transformer for 6550 EL34 etc. - eBay (item 110268290224 end time Oct-24-10 12:19:54 PDT)
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Old 27th September 2010, 04:02 AM   #6
Leb is offline Leb
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Thanks for the posts, guys. Hey sreten, are you saying that I can run a 4 ohm load and not burn out my amp? It would be perfect if that were the case.
What is a bright box?
Yup, this amp has 6 6L6GC's.
Does the OT have any involvement in the sound of an amp, or would any OT do as long as the specs are correct?
Thanks Gimp for taking the time to look that up on Ebay.
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Old 27th September 2010, 04:21 AM   #7
TheGimp is offline TheGimp  United States
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If you run an amp designed for 8 ohms with a 4 ohm load you will get higher distortion, lower power out, and no other problems,

Unless the amp is on the verger of stability problems.

Normally this would be more of a problem with a 16 ohm speaker on a 8 ohm load, so I don't think you will have a problem.

Just watch the tubes for red plate. If you see it, shut down.
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Old 27th September 2010, 07:33 AM   #8
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The Studio Bass is a tough-built amp with a good transformer. The sound of the amp is defined by the quality of the OT - maybe the most important component. I would not buy a used OT from ebay (the big ones are often abused and can develop shorted sections).

You could get a new one with 4 & 8 Taps from a known-good trafo builder. I recently got a quote from Majestic in the UK for a 200W: GBP258 (~$458 US). That's a high quality part.

If the amp is really being used to play electric bass on stage, the full power of the amp will certainly be needed. Using 4-Ohm on the 8-ohm tap? The amp can probably take it, but the stress on the 6L6 tubes will be tough, and they will be short-lived.

Make sure there's a F [fast, flink] fuse in the HT if you try this, and be very sure the Main B+/HT electrolytic caps have been renewed. If they're the old 1980 originals, the increased current level from driving low impedances will heat them, and they may blow corrosive vapour all around the amp's internals. Treat it to a new set of Panasonic TSHA/HB electrolytics - the sound will improve along with the reliability.
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Old 27th September 2010, 10:28 PM   #9
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by Leb View Post
What is a bright box?

Hi,

Something like a 2x 8" or 2x10", using high efficiency drivers in series for
16ohm, with basically no bass bass to speak of. The point is to brighten
the sound, give more midrange projection, more midrange volume, and
extend the top end cutoff of the main bass cabinet somewhat higher.

IF EQ'd back to "normal" balance, you still get the extra dynamic range.

rgds, sreten.
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