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Using Ampeg SVT-CL with some power tubes removed

#24
Wasnex said:
IMHO Getting a V4 is a better option.

Normally the pull-the-tube trick is done with amps where you can pull half the tubes and double the expected load.

I am not smart enough to figure out the expected load from pulling 4 of 6 tubes--My guess is the expected load with only two 6550s in the amp is 6 ohms and 12 ohms, depending upon which output tap you use.

Since the amp is supposed to be safe with an impedance mismatch, treating the 2 ohm output as 4 ohms and treating the 4 ohm output as 8 ohms should be in the ball park. Running an impedance mismatch will be harder on the tubes and harder on the amp. The OP does this at his own risk.

The plate voltage will go up a bit with less tubes pulling current, and the rear panel bias lights will not function properly with only two tubes in the circuit. Quiescent current will need to be measured internally and should be calculated off the new plate voltage. I believe the factory target is 72mA for all three tubes, so the target for each of the remaining tubes will be a bit lower than 24mA after you account for the increase in plate voltage. Poking around in this amp should only be done by people who understand electronics and how to safely work around high voltage. This amp has enough juice to kill you.

IMHO the SVT is not really the best amp for a super saturated distortion anyway. I only like my vintage SVTs pushed to the point where they get grindy. IMHO they get a bit farty when pushed too hard. The CL should be able to get a bit nastier because it has a higher gain preamp. If the preamp is not developing sufficient drive, consider installing some fresh, high gain 12AX7s and/or hitting the front end with a clean boost or some sort of OD pedal with a hot output.
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please note "amp has enough juice to kill you". My humble advice, if you still wish to do as stated, consult/turn over to a professional.
 
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#25
cheechi said:
being very general, how long would you expect a sextet of modern 6550's to last in a vintage SVT?
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I couldn't say. I know some users claim they are consistently burning through tubes in just 3-4 months of hard usage, while others say they get a couple of years of service with regular usage. All I can say is if you thrash the amp, chances are the tubes will wear out faster, but the truth is a tube can fail at pretty much any time.
 
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#27
If your gain is maxed and it still isn't dirty enough, then you must be after some pure disgusting filth of a tone. And/or perhaps you have an extremely light touch on the bass. Just get the right dirt or "clean" boost pedal so that you hit the amp differently. That's a good amp for it. An outboard e.q. would help a ton with fine tuning the dirty sound as well.
 
#29
A vintage traynor YBA3 with a master, is loud as hell and gets plenty of grit at any volume. I sold my two
svt's when I got the traynor. I don't regret selling them, they were always f'ing breaking down. Ampeg service is primitive and they are downright rude to players. The Traynor never needs any work, it always works, doesn't eat tubes, built like a tank, sounds awesome.
 
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#31
Don't mess with the amp. That is a very inexperienced option. Get a pedal you can overdrive with and be done. One of the best ones on the market is:
Darkglass Alpha Omega Bass Distortion and Overdrive
Pricey, but a very professional unit, with a very good reputation.
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#33
cheechi said:
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this might help you also?
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IMHO the JJ ECC83S is actually a fairly low gain tube, but it seems to break up fairly early in some circuits. Tung Sol and Chinese 12AX7s tend to be fairly high gain and a bit more punchy. The Tung Sol, in particular, doesn't break over into distortion very easily in my experience. However, a good strategy might be to put a Tung Sol early in the amp (V1) so you get a lot of gain up front. Then roll a JJ ECC83S through the preamp to find where it produces the best drive.

Course you never know what's going to work in an amp...maybe replacing all the 12AX7s with JJ ECC83S will be perfect. So far, I have thought this tube sounded horrible in every bass amp I have rolled it through. Sounds nice in V2 of a couple of guitar amps though. Keep in mind what I think sounds awesome may sound awful to you.
 
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#35
BassmanPaul said:
Another thought is have you checked that the pre-amp tubes installed are actually what are supposed to be in there? If an earlier owner had installed say a 12AU7 in place of a 12AX7 to get more clean volume you’d be experiencing something like you describe.
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I bought the head new and it's currently stock.

five7 said:
What bass are you using.
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Rickenbacker 4003 with a medium heavy right hand.

I have a few pedals and generally don't like using them. I will explore other options. Thanks to all for your expertise and taking the time to reply!

--jack
 

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