NAD Skunkworks SVT content!

[Wasnex]

Sometimes leaving a properly working amp alone rather than "fixing a problem that doesn't exist" is a better way to approach things.

What was the amp shipped with from the factory originally?

Reports suggest SVTs sound about the same regardless of whether they are set up with 12AX7 or 12DW7. My intent was to equip the OP with enough info to know how the circuit is configured, not recommend a change. The idea is to make sure the tubes match the circuit configuration.

My 69 hits max clean volume on channel 1 when the gain is turned up to about 10:30. My Skunk hits max clean somewhere between 11-11:30 so it not a lot of difference. My 69 does sound a bit more compressed though.

The model of 12AX7 could have some impact as well. For example, in my experience Svetlana 12AX7 and JJ ECC83S tend to be noticeably lower gain than a current production Tung Sol 12AX7 in every amp I have tried them in.

I have never heard of TNT, but it's probably just a Shuguang 12AX7A. 12AX7A New Chinese reissue Branded TNT | New Tube Company If this is accurate, I wouldn't expect it to be a low gain tube.

 

[agedhorse]

So, going back to my original question and suggestion, did the amp ship with 12AX7 or 12DY7 (which is a 12AX7/12AY7 in the same package)? Install whatever the amp shipped with and that's the benchmark.

The 12AX7 mod is for folks unable to source 12DY7's because at one time there was fear that they would become unobtanium.

 

[Wasnex]

Here's something I totally forgot about...only the 12dw7's were swapped out to 12ax7's. The 6c4 cathode follower and the 12bh7 driver tubes remained. And according to an ad on Reverb for a used one, they used US made Philips Sylvanias. But in the Ampeg book, they talk about buying all the GE 6550's they could find at the time because the tube plants were all closing down and they didn't know what was going to happen. Anyway, this thread from a few years back talks about the tubes it uses and their functions:

SVT-HD Tube Functions?

Now if SLM's claim that the Skunkworks SVT-HD's was the exact same signal path otherwise, I suppose converting it to use 12dw7's could fix the gain being lower than the originals. However, I think an easier fix would be just to turn the volume knob up Unless you're looking to crank it all the way up into fuzzland and it's not hitting the same level of distortion as an original or VR, I wouldn't sweat it even a little.

My skunk still has the original Philips/Sylvania 6550s in it.

Everything I know and have read suggests the circuit boards are authentic recreations of the vintage SVT circuit boards. One variance I noted when I recapped my skunk is the balance/bleeder resistors in the power supply where actually soldered directly to the capacitors. On my 69, these were placed on the Power PC circuit board.

I don't think the tube type (12DW7 vs 12AX7) is what impacts the gain. I am not certain, but I think different resistors were used to vary the working voltage sent to the preamp. Various schematics show a different resistor value between Nodes A and B, and the preamp is fed from Node B. There may be other reasons the gain is higher in some vintage of amps as well.

 

[Wasnex]

So, going back to my original question and suggestion, did the amp ship with 12AX7 or 12DY7 (which is a 12AX7/12AY7 in the same package)? Install whatever the amp shipped with and that's the benchmark.

The 12AX7 mod is for folks unable to source 12DY7's because at one time there was fear that they would become unobtanium.

I am pretty sure it was originally setup for 12AX7 and that is what it appears to be running currently. If the gain is unusually low, at some point it will be worth checking the values of R6, 7, and 25 to confirm the amp has not been retrofitted to run 12DW7. It's a 32 year old amp, so there may be a few surprises lurking inside.

 

[JimmyM]

So, going back to my original question and suggestion, did the amp ship with 12AX7 or 12DY7 (which is a 12AX7/12AY7 in the same package)? Install whatever the amp shipped with and that's the benchmark.

The 12AX7 mod is for folks unable to source 12DY7's because at one time there was fear that they would become unobtanium.

Sorry if I wasn't clearer before. It was made with 12ax7's. I just thought it was odd that they pulled the 12dw7's and kept the 6c4 and 12bh7's, which are half a 12au7 and 12ax7 respectively. And I completely agree with leaving it alone. If it ain't broke, don't fix it.

I am pretty sure it was originally setup for 12AX7 and that is what it appears to be running currently. If the gain is unusually low, at some point it will be worth checking the values of R6, 7, and 25 to confirm the amp has not been retrofitted to run 12DW7. It's a 32 year old amp, so there may be a few surprises lurking inside.

I'd bet money only Bob and maybe one other person in the world modded theirs to use 12dw7's

 

[agedhorse]

Then leave the 12AX7's in it and verify that it hasn't been screwed with.

 

[beans-on-toast]

If it ain't broke, don't fix it.

My neighbor's alley cat wasn't broke but they fixed it. They gave it a cat job. He wasn't the same afterwards. Less agressive.

 

[Wasnex]

Sorry if I wasn't clearer before. It was made with 12ax7's. I just thought it was odd that they pulled the 12dw7's and kept the 6c4 and 12bh7's, which are half a 12au7 and 12ax7 respectively. And I completely agree with leaving it alone. If it ain't broke, don't fix it.


I'd bet money only Bob and maybe one other person in the world modded theirs to use 12dw7's

I checked what was running in my amp's channel 1, but didn't write it down. I suspect it's 12AX7. I don't see any reason to change the circuit to run 12DW7. The amp sounds very good and is bit more punchy than my 69. I like having the variety.

Changing out the 6C4 is pretty invasive because it uses a different tube socket. I don't think anyone is still making 6C4 but they are not hard to find. I have several different brands. Supposedly this tube type is very prone to being microphonic.

I think you could mod the amp the run 12AU7 in place of the 12BH7, but I don't know why you would? There are a few different 12BH7s in production, and I think the JJ ECC99 is also fairly close.

One of my big Ashdowns has the driver slots setup to run JJ ECC802, which is their high grade 12AU7, or JJ ECC99.

 

[JimmyM]

I checked what was running in my amp's channel 1, but didn't write it down. I suspect it's 12AX7. I don't see any reason to change the circuit to run 12DW7. The amp sound very good and is bit more punchy than my 69. I like having the variety.

Changing out the 6C4 is pretty invasive because it uses a different tube socket. I don't think anyone is still making 6C4 but they are not hard to find. I have several different brands. Supposedly this tube type is very prone to being microphonic.

It's the truth. I once bought 4 of them in hopes of getting one to replace the two microphonic ones I already had, and they were all equally microphonic, and while I think Beans found some that were better than most, that was the only time I heard of someone getting some that weren't too microphonic. But the amp sounds just fine with them.

I think you could mod the amp the run 12AU7 in place of the 12BH7, but I don't know why you would? There are a few different 12BH7s in production, and I think the JJ ECC99 is also fairly close.

One of my big Ashdowns has the driver slots setup to run JJ ECC802, which is their high grade 12AU7, or JJ ECC99.

I've had problems with the 12bh7, too. I heard it was something about those tall single triode tubes not having very good support for the parts inside but I could be wrong.

 

[Wasnex]

I've had problems with the 12bh7, too. I heard it was something about those tall single triode tubes not having very good support for the parts inside but I could be wrong.

FYI 12BH7 is a dual triode, but they are tall though. I have never read anything about them being a problem, but that doesn't mean they are not.

 

[JimmyM]

FYI 12BH7 is a dual triode, but they are tall though. I have never read anything about them being a problem, but that doesn't mean they are not.

Mi mistako. Always thought it was half a 12au7, but I guess it's just similar.

 

[Wasnex]

Mi mistako. Always thought it was half a 12au7, but I guess it's just similar.

That's the 6C4.

12BH7 is similar to 12AU7, but it's a stronger tube. ECC99 is also in the same ballpark. In some circuits you might get away with swapping all three, but I don't recommend it unless the manufacturer says it is okay.

If you compare the driver circuits for the vintage SVT and CL, I believe the first section is exactly the same, but the second section has been reworked. To clarify, the vintage amps use 12BH7 in the drivers and the CL uses 12AU7. So the first section is fine with either tube, but the second section was revised.

 

[greengoblin]

Nice! I almost pulled the trigger on that head a few times as I really want a Skunkworks, but dont really need it. Also, I sent the seller a question and never heard back so I was a bit skeptical. Glad it went to a fellow TB'er..

Get some internal pics if you have time and don't mind. Curious if it has personal writing inside like some of the others (always think that is cool for some reason).

 

[Ampeg bass]

New to this site.
I have #116 and am looking for some info as well.
Also the 6c4 in my amp is really microphonic so I am going to replace it.
Also if anyone has pictures of the inside of your amp I'm curious to see them especially what caps are in it and if they are original.

 

[Ampeg bass]

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