how to replace preamp tubes ampeg svt classic

[Wasnex]

This is my personal opinion, but if you have a CL, you DO have a bad one. I build amps, fix amps, and examining the circuit it's a FrankenSVT compared to the original. I played one in a provided backline and it sounded like dogshht compared to the '69 I used to have. When the CL was developed, it was built around tubes in production. 6C4's? 12BH7's? 12DW7s? Unobtanium. A 6C4 is half of a 12AU7. A 12DW7 is half a 12AX7 and half a 12AU7. Do the arithmetic, the preamp would have to have 2 12AU7s and 1 1/2 12AX7s with the other half unused (ignoring the second channel) to use the same circuit. The CL uses 2 tubes. It uses 12AU7s as drivers instead of the beefier 12BH7s to push the 6550s. It's an SVT in name only. Dump it and get a used original from the 70's (the '69 needed a modification to keep from eating power tubes like candy) if you're looking for that Ampeg grit. There are pedals that do the SVT grind better than the CL does.

I think this is a bit overstated. Every CL I have played sounded good, and the CL can definitely can do some things better than a vintage-style SVT cannot. For the record I own a 69 and 87, and previously owned an MTI. Never owned a CL personally, but I am tempted to get a Heritage CL.

The 87 runs only 12AX7s in channel one. The MTI era factory mod for this involved swapping three resistors to rebias the circuit for 12AX7. I don't find that it sounds bad, or even different enough that most people would even notice. I have read comments for people who put their amps back to 12DW7 and they said the couldn't hardly tell the difference.


I concede that my 69 and 87 do sound and feel subtly different, but they both sound like SVTs. My understanding is the early SVTs made a bit more power with the original output tube type 6146, but 6550 can take more abuse. The 69 is a bit fuller and louder than my 87...but both get loud.

FYI, 6C4 tends to be very problematic. As you say, it's pretty similar to 1/2 of a 12AU7, but finding one that is not microphonic or hissy is next to impossible. The 6C4 in my 87 rattles obnoxiously if I crank the amp. I have left it alone as I don't generally play that loud. Also, I have a stash 10 or 11 brand new 6C4s in boxes...supposedly some of the best NOS Jan and Mullard versions made. Hopefully one or two of them are good enough, but they are still plentiful and cheap if I ever need more.

NOS 12DW7 is also still easy to find and fairly cheap. Not enough to supply production of a new amp line, but plenty of available tubes for vintage SVT users. Also JJ and EHX offer current production 7247, which is a different name for 12DW7. I have rolled various current production and NOS 7247 in my CTM 300. I couldn't here enough difference to warrant pulling the JJs out. Slight differences in gain, but pretty much no difference in tone.

The power amp in the CL is functionally pretty much the same as the vintage amp. It has some added bias sense and protection circuitry. Also the drivers have been changed to 12AU7...a few resistors were changed for this.

Pretty sure 12BH7 is totally overkill...the original designer was simply being conservative. If you don't believe me, then believe Jerrold Tiers who worked at Ampeg back in the day: https://www.talkbass.com/threads/driver-tubes-svt.638781/#post-8847352

 

[Stylee Mousse]

I liked my SVT-CL but it’s definitely not the grind machine. I ran it pretty clean and it sounded good to me. I can’t see why you couldn’t stick a gain stage in front of it. I used a full range modded rangemaster pedal for grit and it was very effective with that amp.

 

[agedhorse]

I think this is a bit overstated. Every CL I have played sounded good, and the CL can definitely can do some things better than a vintage-style SVT cannot. For the record I own a 69 and 87, and previously owned an MTI. Never owned a CL personally, but I am tempted to get a Heritage CL.

The 87 runs only 12AX7s in channel one. The MTI era factory mod for this involved swapping three resistors to rebias the circuit for 12AX7. I don't find that it sounds bad, or even different enough that most people would even notice. I have read comments for people who put their amps back to 12DW7 and they said the couldn't hardly tell the difference.


I concede that my 69 and 87 do sound and feel subtly different, but they both sound like SVTs. My understanding is the early SVTs made a bit more power with the original output tube type 6146, but 6550 can take more abuse. The 69 is a bit fuller and louder than my 87...but both get loud.

FYI, 6C4 tends to be very problematic. As you say, it's pretty similar to 1/2 of a 12AU7, but finding one that is not microphonic or hissy is next to impossible. The 6C4 in my 87 rattles obnoxiously if I crank the amp. I have left it alone as I don't generally play that loud. Also, I have a stash 10 or 11 brand new 6C4s in boxes...supposedly some of the best NOS Jan and Mullard versions made. Hopefully one or two of them are good enough, but they are still plentiful and cheap if I ever need more.

NOS 12DW7 is also still easy to find and fairly cheap. Not enough to supply production of a new amp line, but plenty of available tubes for vintage SVT users. Also JJ and EHX offer current production 7247, which is a different name for 12DW7. I have rolled various current production and NOS 7247 in my CTM 300. I couldn't here enough difference to warrant pulling the JJs out. Slight differences in gain, but pretty much no difference in tone.

The power amp in the CL is functionally pretty much the same as the vintage amp. It has some added bias sense and protection circuitry. Also the drivers have been changed to 12AU7...a few resistors were changed for this.

Pretty sure 12BH7 is totally overkill...the original designer was simply being conservative. If you don't believe me, then believe Jerrold Tiers who worked at Ampeg back in the day: https://www.talkbass.com/threads/driver-tubes-svt.638781/#post-8847352

Agreed, it’s ridiculously overstated.

The early ones have mostly been updated. They were not as reliable as the later versions, and the obsolete tube challenges are very real.

when I worked under Bill Hughes, I asked him about the decision of using the 6146 output tube, he said that it was “state of the art” at the time but had he had the chance to revisit that decision, he would have used the 6550 from the beginning. This is backed up by his use of 6550’s in his revisions as well as his later bass amp designs under Fender.

 

[Short4since64]

I think this is a bit overstated. Every CL I have played sounded good, and the CL can definitely can do some things better than a vintage-style SVT cannot. For the record I own a 69 and 87, and previously owned an MTI. Never owned a CL personally, but I am tempted to get a Heritage CL.

The 87 runs only 12AX7s in channel one. The MTI era factory mod for this involved swapping three resistors to rebias the circuit for 12AX7. I don't find that it sounds bad, or even different enough that most people would even notice. I have read comments for people who put their amps back to 12DW7 and they said the couldn't hardly tell the difference.


I concede that my 69 and 87 do sound and feel subtly different, but they both sound like SVTs. My understanding is the early SVTs made a bit more power with the original output tube type 6146, but 6550 can take more abuse. The 69 is a bit fuller and louder than my 87...but both get loud.

FYI, 6C4 tends to be very problematic. As you say, it's pretty similar to 1/2 of a 12AU7, but finding one that is not microphonic or hissy is next to impossible. The 6C4 in my 87 rattles obnoxiously if I crank the amp. I have left it alone as I don't generally play that loud. Also, I have a stash 10 or 11 brand new 6C4s in boxes...supposedly some of the best NOS Jan and Mullard versions made. Hopefully one or two of them are good enough, but they are still plentiful and cheap if I ever need more.

NOS 12DW7 is also still easy to find and fairly cheap. Not enough to supply production of a new amp line, but plenty of available tubes for vintage SVT users. Also JJ and EHX offer current production 7247, which is a different name for 12DW7. I have rolled various current production and NOS 7247 in my CTM 300. I couldn't here enough difference to warrant pulling the JJs out. Slight differences in gain, but pretty much no difference in tone.

The power amp in the CL is functionally pretty much the same as the vintage amp. It has some added bias sense and protection circuitry. Also the drivers have been changed to 12AU7...a few resistors were changed for this.

Pretty sure 12BH7 is totally overkill...the original designer was simply being conservative. If you don't believe me, then believe Jerrold Tiers who worked at Ampeg back in the day: https://www.talkbass.com/threads/driver-tubes-svt.638781/#post-8847352

I've had a 69 (mod applied) and a 74. Both used the 12DW7s as direct coupled cathode followers. In my opinion, that circuit topology and the harmonic signature it carries, as well as the AB2 output stage is the heart and soul of the SVT. The CLs don't have either.

 

[Short4since64]

Agreed, it’s ridiculously overstated.

The early ones have mostly been updated. They were not as reliable as the later versions, and the obsolete tube challenges are very real.

when I worked under Bill Hughes, I asked him about the decision of using the 6146 output tube, he said that it was “state of the art” at the time but had he had the chance to revisit that decision, he would have used the 6550 from the beginning. This is backed up by his use of 6550’s in his revisions as well as his later bass amp designs under Fender.

I liked my SVT-CL but it’s definitely not the grind machine. I ran it pretty clean and it sounded good to me. I can’t see why you couldn’t stick a gain stage in front of it. I used a full range modded rangemaster pedal for grit and it was very effective with that amp.

I liked my SVT-CL but it’s definitely not the grind machine. I ran it pretty clean and it sounded good to me. I can’t see why you couldn’t stick a gain stage in front of it. I used a full range modded rangemaster pedal for grit and it was very effective with that amp.

The whole point of the continued love for the SVT is the grind, and at the time, the power, as solid state amps were not yet especially reliable in 69. They now are, and extremely lightweight. By the time you buy a CL, the chiropractor visits for your sprung back, and the externals to create a simulation of the Ampeg growl, you could have bought a TT800 and been done with it. No power tubes, no biasing, a good warranty and no bull, plus a shhtload more power. If I wasn't using a spare power amp channel in the Heavy!! PA rack, which has to go to the gig regardless, to power my rig, I would be doing just that

 

[Stylee Mousse]

I used mine in the 90’s and not for its typical thing. I used a JBL loaded 215 rather than a fridge and liked things balanced and tight.

never vibed with the cool ss of that era, SWR, Eden etc. and yep that thing was heavy as all day but I was young and strong so it was no problem for me at all.