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Using Ampeg SVT-CL with some power tubes removed

JackTheRipper

JackTheRipper

Supporting Member
Mar 5, 2008
784
1,372
San Diego, CA
#1
I want to get more grit from my Ampeg SVT-CL. I run it through a 810e cab and even with the gain at max, I don't get as much grit as I want at a low enough volume. I've tried a couple of different preamp tubes, but still want more grit out of it. I'm thinking that I want to remove some of the power tubes to run it essentially as a 100-watt amp. From what I've read it can be done and safely. True?

I'm guessing that I would need to remove 4 of the power tubes. Which ones? I remember reading that the tubes are arranged in "triangles". I found the attached image here on TB and drew the triangles. I'm guessing that I would want to remove 2 tubes from each triangle and re bias.

Any advice appreciated,

--jack

AmpegTubes.png
 
#3
Wait. .... even with the "gain maxed"?????

It should be pretty angry with the gain maxed. If I turn my gain past noon it is super distorted.

Is your volume on your bass dimed? Seems to me you're sending a pretty weak signal to the input if you can't make it mad as he!! with the gain maxed out. It should be pure DIRT! (not necessarily a bad thing)
 
#5
two fingers said:
Wait. .... even with the "gain maxed"?????

It should be pretty angry with the gain maxed. If I turn my gain past noon it is super distorted.

Is your volume on your bass dimed? Seems to me you're sending a pretty weak signal to the input if you can't make it mad as he!! with the gain maxed out. It should be pure DIRT! (not necessarily a bad thing)
Click to expand...

Do you mean at low volumes? I've owned a couple SVT-CLs and I have never been able to get anything close to dirty at low volumes. Gain at 10, active or passive.
 
#8
two fingers said:
Wait. .... even with the "gain maxed"?????

It should be pretty angry with the gain maxed. If I turn my gain past noon it is super distorted.

Is your volume on your bass dimed? Seems to me you're sending a pretty weak signal to the input if you can't make it mad as he!! with the gain maxed out. It should be pure DIRT! (not necessarily a bad thing)
Click to expand...

Yes, volume on bass maxed and gain maxed. Master volume on amp is between 1 and 2. Not "super distorted" or even close.

--jack
 
#10
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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#11
BlacknWhite said:
Do you mean at low volumes? I've owned a couple SVT-CLs and I have never been able to get anything close to dirty at low volumes. Gain at 10, active or passive.
Click to expand...

With the gain dimed and the volume on my bass dimed, mine is angry as crap at minimum volume. In other words, if you can hear it at all, it's dirty.
 
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#15
When I had my SVT CL, I’d dime the gain, have the volume at maybe 2 or 3, and achieve maximum, punishing grind!

Are your preamp tubes glowing fully? Might need to replace them. I had an issue with one glowing dimmer than the other and promptly installed a new set with a noticeable change in overall loudness & tone.
 
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#17
I would not do this, the already high plate & screen voltages will creep upwards along with a healthy plate impedance mismatch (which stresses the tubes even harder at higher plate voltage).

If you don't have enough level off of your bass to overdrive the preamp, I would suggest trying a clean boost pedal before messing with output tubes.
 
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#18
agedhorse said:
I would not do this, the already high plate & screen voltages will creep upwards along with a healthy plate impedance mismatch (which stresses the tubes even harder at higher plate voltage).
Click to expand...

Me either. I have read several threads where people have a hard time keeping a set of modern 6550s alive for long in a pushed SVT. Seems silly to artificially create a situation that stresses the tubes even more.
 
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#19
agedhorse said:
I would not do this, the already high plate & screen voltages will creep upwards along with a healthy plate impedance mismatch (which stresses the tubes even harder at higher plate voltage).

If you don't have enough level off of your bass to overdrive the preamp, I would suggest trying a clean boost pedal before messing with output tubes.
Click to expand...

Great idea. I was thinking hotter pickups. But this is both simpler and reversible simply by taking the pedal out of the chain.
 

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