Q for the svt tube guru guys

[staindbass]

hi, i was looking online for info on the preamp tube signal path (which tubes are first in line as the signal travels) and found a schematic. it looks like V1 is first in line on channel one , as it is connected directly to the input jack. but on channel TWO it looks like it goes to the 12ax7 first, then travels over to channel one, going into the circuit after V1. this would give the two inputs totally different qualities ?, as the first tube is different (the first preamp tube has the most effect on sonics) . im looking for confirmation on this. maybe everyone knew but me ? its a linden nj blueline . looks like i can choose between 12dw4 on channel one, and 12ax7 on channel two. i figured all 4 inputs went through the same tubes. http://www.drtube.com/schematics/ampeg/svtpre-jp.gif

[beans-on-toast]

You are correct, the channels are different. The DW and AX tubes are similar but do have different tonal qualities. Channel 2 uses a 12AX7 tube which feeds into a tone stage. Channel 1 uses a 12DW7 and the tone stage has more tone shaping ability.

On your schematic, the channels are mixed together just before V5, the 6C4 tube.

[staindbass]

thanks! i never compared the two channels, i will now. im putting in a telefunken smoothplate 1950's i hope i hear a difference. johnny a.

[JimmyM]

ya johnny, the second channel is a bright channel. with eq you can make both channels sound about the same, but the second channel should be more rolled off on the lows and have more highs with the knobs at noon. i used to take a splitter and run both channels of my svt and blend them. sounds great, but i got lazy and now i just eq channel 1 to sound more like my blend.

always wanted to try a high dollar 12dw7 in my blue line. the closest i ever got was an amperex...but since i have such problems telling the difference between tubes, i never bothered with it.

so does yours still have the 6146b's in it or has it been converted?

[jimbilly]

Think of a 12AX7 (or similar) preamp tube is 2 halfs in one bottle (both relatively high gain), I understand that the 12DW7 is like half a 12AX7 and half a 12AU7 (half higher gain where needed and half lower gain for the other 'thing'). I believe you can substitute other similar tubes to for tone to taste, like 12AY7s and 12AV7s, and 12AU7s, although 12AU7s are often too low of gain when substituted in a higher gain spot.
Someone correct me if I'm wrong.

[staindbass]

with all the controls at noon and flat, channel 2 (12ax7) has clearer mids and highs. i tried to dial up the treble on channel 1 to match, but it just made more treble, still not as clear and transparent. channel 1 has a heavier darker bass sound that channel 2 can not match either. i changed the 12ax7 tele to a mullard then a 5751 but still pretty much the same outcome. i guess thats why people bridge the inputs on amps to use both channels at once. johnny a.

[staindbass]

hi jimmy ! yeah i got all the fancy tubes and brands, the differences are kind of slight, and i think those differences show more in hifi stereo systems. if you dont notice a difference, its not worth it. the paper tube chart in the amp says 6550 or 6146b / 8298a. i guess it can use all 3 ? im using modern gold lion 6550. the 12dw7 in the amp are the original gt. britain mullards, and measure higher than the 3 rca 12dw7 in my tube stash.

[JimmyM]

interesting findings. never really noticed the treble response in ch 1 compared to ch 2 other than there being more of it, but that may be because i'm not as big into the treble as you are. looks like i have a project this weekend.

the svt can use all 3 power tubes, but not as drop-in replacements. the sockets have to be rewired and a plate voltage wire with a cap is needed if you want to run 6146b's since they're transmitter tubes. but since it's already been converted (or maybe it came stock with them), i wouldn't lose sleep over it. an svt sounds like an svt sounds like an svt to me, bro.

[beans-on-toast]

The important thing to remember with any old amp is that although tone generalizations from one to the next do apply, they are unique.

Components such as resistors and capacitors have values that drift with time and this helps sculpt the sound. This could be part of what accounts for the differences in tone from one amp to the next.

When you take into account changes in component values and changes that come with tube substitution, the overall effect can be positive or even negative. A Telefunken might make a dramatic difference in one amp and only a subtle difference in another.

Sometime these dramatic difference come together in a way that makes an amp something very special.

[JimmyM]

as always david, you are the voice of reason and sanity in an unreasonable and insane world. i use a lot of rented svt's, and i find them remarkably consistent from amp to amp no matter when they were made (obviously not taking cl's into account here since they do have some noticeable differences), but i'm sure if i sat them down in the same cab and ab'd them, i would find the differences you speak of.

fortunately, the differences would likely be slight enough to where they were lost on me so maybe a more critical ear like johnny's or yours would see those slight differences as more apparent.

[beans-on-toast]

It helps when you have instruments to tell you what you are hearing. Sometimes they can help you to know what to listen for. A good set of ears, which I wish I had, is more valuable than any instrument. What your ears tell you should always trump a set of readings.

One thing about backline rentals, they have the benefit of regular maintenance and are usually relatively new. Some companies even strive to keep these amps up to original spec. They also tend to have similar Russian or Chinese tubes installed. These amps are going to sound fairly consistent as you said.

This is not the same as your average old SVT that has had little more that a cap job over the years that most players own. That's more of what I was thinking of.

[staindbass]

beans and toast, good point. not every experiment with this old stuff will lead to the same conclusion with the variables. i tried using both cnannels, bridging the second channel from the bright input on ch 1, to the normal input on channel 2. i looked on the schematic and its going thru a resistor on the bright channel 1 input (which is now being used as an output) before going into channel 2. i tested channel 2 alone, by going directly in and by bridging thru channel 1. the bridging still affects the clarity and sounds a shade clacky instead of lazer clear. i think the Y cord like jimmy used is the way to go. or bridge it inside at the input jacks. even so, i used channel 1 with the treble at zero, (since the ch2 has better treble) and channel 2 with the bass at zero (cuz ch1 has better bass) and if i blend it just right, it sounds better than either channel alone. the y cord will bring me one step further . i have a/b tested mine against new reissues, and its close, but the original is just a shade better in all aspects. again, a slight difference, most people may not notice. but now that i notice it i prefer the vintage.

[beans-on-toast]

The advantage of combining channels, bridging or using a Y cord, is that you now have two sets of tone controls to help you sculpt your sound.

When the channels are mixed together before going to power amp, the signals from each channel are combined, adding and subtracting from each other providing more tonal complexity.

[aborgman]

Originally Posted by beans-on-toast

You are correct, the channels are different. The DW and AX tubes are similar but do have different tonal qualities. Channel 2 uses a 12AX7 tube which feeds into a tone stage. Channel 1 uses a 12DW7 and the tone stage has more tone shaping ability.

On your schematic, the channels are mixed together just before V5, the 6C4 tube.

A 12DW7 is dual triode that is one half 12AX7 and one half 12AU7.

So the 12AX7 half sounds just like a 12AX7... and the 12AU7 section sounds like a 12AU7.

[beans-on-toast]

It's a neat tube in that half can be used for an audio stage and the other half for a phase inverter. Saves using two separate tubes in a simple amp.

I had a Silvertone amp in the early 60's that used only a 12DW7 and a power tube. It was quite nice.

[JimmyM]

Originally Posted by beans-on-toast

The advantage of combining channels, bridging or using a Y cord, is that you now have two sets of tone controls to help you sculpt your sound.

When the channels are mixed together before going to power amp, the signals from each channel are combined, adding and subtracting from each other providing more tonal complexity.

you know, you're right. that tone is more tonally complex than just one channel by itself. i'm going to go back to doing that...that is, if i could ever stop going for the micro vr when gig time comes. i admit it...i've been pussing out on the tubes and big cabs. hey, not like i'm in staind or anything...i hear that dude's loud

[stingraysvt]

So has anyone tried running a split signal into the second channel i.e. DRY CH1/WET CH 2?

Never tried myself just curious after reading these posts...

[K2000]

Dennis Kager told me that Channel 2 is the circuit from the B15 fliptop, retained in the SVT for the guys who liked that setup. He also said there's no advantage to jumping the channels... no additional output, and any additional EQ control is moot -- you're combining a more detailed (detailed? you know what I mean) EQ section (in Channel 1) with less EQ options found in Channel 2... no point in doing that. If you want lots of EQ control, just use Channel 1.

Maybe I'm mistaken, but that's how I remember the conversation, and I asked him those specific questions about Ch 1 and 2.

Just to be clear, I'm not saying that jumping channels never does anything for anybody - it definitely does in Fender-style amps. Mr. Kager told me the SVT circuits are different than a typical Fender 2 channel amp, and that channel jumping the SVT is pointless for my purposes (extra volume, extra EQ options)

If you guys hear things differently, or have other applications, I would not argue with that. Heck, I'm not arguing anything, just parroting what I was told.

[mongo2]

Originally Posted by beans-on-toast

A good set of ears, which I wish I had, is more valuable than any instrument. What your ears tell you should always trump a set of readings.

Just be cardful that your eyes don't tell your ears what to hear.

[nysbob]

I recently tried both channels on mine ('76 Magnavox) and found no real advantage to it. I much prefer the switchable mid slopes on channel 1 - to me that's the key to making an SVT sound right - and I don't need any further control than that.