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vintage ampeg svt midrange problem?

D

DrowningBrian

May 21, 2009
178
0
41
Atlanta, GA
#1
i love mine but i just got spoiled by an orange ad200b. it seems like the mids on the old svts are insane and boxy sounding, maybe its just mine. i just did a cap job and replaced the power tubes with JJ 6550s. its an MTI era svt. maybe there is something up with the amp, i dont know. no weird sounds. i play either a jazz bass or p bass fueling an ampeg 8x10. i played a blueline svt and it sounded perfect. idk. im just gonna shut up and play it but im just curious. killer rig!

thanks,
brian
 
#3
ive been thinking that too. there should be no reason why my gk800rb sounds better. my tech says these are built just as well as any other vintage svt. just not a pretty.lol. its almost like the mids are funky, it looks like the rocker switch for the mid was replaced at one point, but it works.
 
#8
The preamp tubes can make a difference so check that first. Scrubbing the tube pins and sockets with deoxit to remove oxidation can help.

As was mentioned, check the solder joints. I've seen joints were the solder looked good but the wires were loose, they could be pulled out easily. So they need to be checked carefully.

It is possible that there are components in the preamp that have drifted out of spec. It would be good if your tech checked for that. Especially look at the ones in the tone stage.
 
#10
The MTI SVT started using 12DW7 preamp tubes as did the earlier SVT's. Later they switched to 12AX7's along with the requisite circuit changes. A 12DW7 has one 12AX7 gain stage and one 12AU7 stage. The switch was made due to 12DW7's becoming scarce. (There are current production 12DW7's available today.)

Is is possible your SVT has the wrong preamp tube in the midrange circuit? The tone and effect of the midrange circuit would be affected by the use of a 12DW7 in a 12AX7 circuit and vice versa.

Many SVT's designed to use 12DW7's were modified to use 12AX7's and these conversions were fairly common due to scarcity of 12DW7's a few decades ago. Now, years down the line, have any modified amps not been identified as such? I would suspect quite a few.
 
#11
Since a 12DW7 is a 12AX7 stage and a 12AU7 stage, subbing a 12AX7 would create some problems. The 12AX7 stage is far higher gain than a 12AU7 stage, and is also easier to distort (higher sensitivity.) By contrast, a 12AU7 stage is much lower gain, higher current, and is much harder to distort.

If a 12DW7 SVT had 12AX7's put into it, there would be tonal changes, and distortion problems, unless the 12AU7 stages were modified to match 12AX7 gain characteristics. If a 12DW7 was used in a 12AX7 SVT, I would surmise the result would not be distortion, but rather lower gain and perhaps lackluster tone in the preamp.

There are also impedance differences, besides the gain differences.
 
#15
Steve Dallman said:
The MTI SVT started using 12DW7 preamp tubes as did the earlier SVT's. Later they switched to 12AX7's along with the requisite circuit changes. A 12DW7 has one 12AX7 gain stage and one 12AU7 stage. The switch was made due to 12DW7's becoming scarce. (There are current production 12DW7's available today.)

Is is possible your SVT has the wrong preamp tube in the midrange circuit? The tone and effect of the midrange circuit would be affected by the use of a 12DW7 in a 12AX7 circuit and vice versa.

Many SVT's designed to use 12DW7's were modified to use 12AX7's and these conversions were fairly common due to scarcity of 12DW7's a few decades ago. Now, years down the line, have any modified amps not been identified as such? I would suspect quite a few.
Click to expand...

thanks steve! good info. it originally had the 12ax7 in the preamp but the tech put it back to 70's specs with the 12dw7s and a few resistor changes.
 
#17
Steve Dallman said:
The second channel does not have the midrange circuit, just the first channel. The baxandall bass and treble is a separate circuit from the midrange circuit and is identical in both channels.
Click to expand...
So they say, but it's not in my 69. The second channel in mine is more like a guitar channel with reduced lows. I can get the same sounds out of each, but they don't behave the same way.
 
#20
I've worked on two different MTI era SVT's that had a wiring problem on the midrange of channel one. The midrange frequency switch wiring.

Have it checked out closely by a tech.

Both of these amps seemed to have been built wrong from the factory because the solder joints looked to have been original.

The difference was huge when this was fixed!
 

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