V3 - An Important Preamp Stage

All about tube choices for the JVM series. See workbench for biasing and tube problems you may have.

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ach91
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Re: V3 - An Important Preamp Stage

Post by ach91 » Sun Feb 19, 2012 11:59 pm

Its not that the spiral filaments cause a problem its just that the New Sensor tubes are not constructed well enough to take the voltages of the cathode follower in a Marshall. If I remember correctly the stock tubes have spiral wound filaments but they arent New Sensor tubes.

As long as you dont have a New Sensor tube you should be fine. New Sensors will almost always fail where as other brands just have the typical tube chance that you might get a dud. NOS should be fine. Back then tubes were made to a standard.. The only decent hope these days for a good tube is Svetlana =C= or TAD tested and selected tubes.
Here's whats under the hood. I have some personal tweaks to these mods but here's generally what I have going on my amps per Andy's request.

JVM 410H:
-2203 modded OD1 and OD2 (see Casey's mods)
-Plexi modded Crunch channel (see Casey's mods)
-2203 negative feedback mod (see Casey's mods)
-6550 power section
-4H choke (plan on switching to 3H MM choke eventually)
-100uF caps on the screens (see Casey's mods)

Haze 40 C:
-Hotrodded JCM 800 mod (see "Convert Haze 40 into JCM 800" thread in Other Amps section)

DSL 100:
-No mods yet.

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spikei
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Re: V3 - An Important Preamp Stage

Post by spikei » Sat Feb 25, 2012 1:44 pm

Thanks for the advise, so am i right in thinking i should keep CP tubes like EH, Mullard, svetlana, sovtek and Tungsol away from V3 then, are there any others to keep away from ?
JVM 215c
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JVM 215c
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ach91
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Re: V3 - An Important Preamp Stage

Post by ach91 » Wed Feb 29, 2012 10:17 pm

Just New Sensor stuff really
Here's whats under the hood. I have some personal tweaks to these mods but here's generally what I have going on my amps per Andy's request.

JVM 410H:
-2203 modded OD1 and OD2 (see Casey's mods)
-Plexi modded Crunch channel (see Casey's mods)
-2203 negative feedback mod (see Casey's mods)
-6550 power section
-4H choke (plan on switching to 3H MM choke eventually)
-100uF caps on the screens (see Casey's mods)

Haze 40 C:
-Hotrodded JCM 800 mod (see "Convert Haze 40 into JCM 800" thread in Other Amps section)

DSL 100:
-No mods yet.

Russ
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Re: V3 - An Important Preamp Stage

Post by Russ » Wed Jan 16, 2013 10:38 pm

big dooley wrote:actually, a tube like the 12AT7 or the 12AU7 would work better as the send buffer and recovery of the effects loop... wilder still believes that a 12AX7 is best for the cathode follower... he prefers the 12AT7 for the FX-loop, but a 12AU7 will do as well

the problem is, that V3a is the cathode follower and V3b the FX-loop send buffer... so you would need reversed 12DW7 or JJ ECC823 to have the 12AX7 triode in V3a and the 12AU7-triode in V3b... then you would go by with a regular 12AU7/ECC82 in V4, although i don't know for sure, since V4b is the reverb mixer... if the reverb needs a 12AX7, you would have to put a 12DW7 in there again, but this time the regular one (JJ ECC832)...

i can understand that a 12AU7 in the PI would be horrible... you aren't driving the powertubes that way, like a 12AX7 does... for lower gain in the PI a 5751 or 12AT7 would be a better choice
I uploaded an image, hope I did it right. I want to be sure on where to stick this 12dw7 and reversed 12dw7 i bought. If v3a is the cathode follower, and v3b is fx loop send buffer, I'd want gain factor of 100 for side a and 20 for side b. According to this post I'd go with the reversed 12dw7 for v3. What it shows in the image is system I with pin 6 7 8 has gain at 100, where system II with pin 1 2 3 at 17, or 20. What i want to know is, as the signal goes through, does it hit system I first then system II? Basically v3a uses system I and v3b uses system II. If this is the case I'd need a 12dw7 for v3 instead of the reversed. Unless the signal hits system II before system I, then you'd be right. I've read alot of different info on the internet and wondering which tube is what? I've read both ways, driving me crazy. Not trying to say you or anyone's wrong, just want to be sure. I'd trust you guys more than some other forum.
Attachments
IMAG0224.jpg
12DW7 Data Sheet

Ned
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Re: V3 - An Important Preamp Stage

Post by Ned » Tue Sep 26, 2017 5:48 pm

Nearly 5 years later let's try again. I'm confused as well. The JJ site says 12AX7 is the first stage. I would assume V3A uses the first stage of the tube and V3B for the second stage. Where am I going wrong? Seems like you wouldn't want it reversed.

namklak
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Re: V3 - An Important Preamp Stage

Post by namklak » Tue Sep 26, 2017 7:13 pm

In the jvm, both V3a and V3b are configured as unity gain buffers, what are you trying to achieve?

Have you read this?
viewtopic.php?f=7&t=10213
JVM205h. Added 100K from R94/C43 to ground for V1b (OD Channel) gain reduction. Installed 1000pF in series with 47K, in parallel with R58 (82K), to reduce power amp squeal. V3 Valve Wizard diode protection mod. Active power soak for more EL34 goodness.
Guitars: Angus Young Sig SG w/ Pearly Gates Neck and Custom Custom Bridge, Les Paul Special (2 pups) w/ SD Stacked P90s. And occasionally a Warmoth Strat w/ Dimarzio True Velvet in N, M, SD P-Rails in B.
HD500x in 4CM for all pedals and Midi control of 205h (grounds lifted on two of the signal cables).

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Re: V3 - An Important Preamp Stage

Post by Ned » Wed Sep 27, 2017 12:39 am

Yes I have. Nice tip and am inclined to have that mod done. The only reversed 12DW7 I have seen is a JJ which looks like a no no in V3. This would be to improve the effects loop as discussed in another thread. It appears to me in that thread that the reversed one(823) has the 12AX7 first and then the 12AU7. In that picture from Russ, it does not show where it says "ECC832 is 12DW7 pre-amplifying double triode. It combines 12AX7 and 12AU7, where the 12AX7 is used as 1st stage and the 12AU7 as 2nd stage." This is from the JJ site which curiously does not list the 823 although I have seen them for sale from several vendors. Per your suggestion from one of my post's, I poked around and found the channel block diagram for the 410H which I assume would be the same for a HJS. I was thinking of experimenting with a 823 and/or 832 in V1, or V2, or both to try and get the levels and gain a bit closer. Just trying to order the right ones.
On a side note, I've been to countless concerts by many bands over many years. I'll never forget the one and only time a singer brought me to tears. It was Greg during It's Not My Cross To Bear. Glad to see you're keeping the boys alive.

Ned

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Re: V3 - An Important Preamp Stage

Post by namklak » Wed Sep 27, 2017 4:22 pm

Ned wrote:
Wed Sep 27, 2017 12:39 am
On a side note, I've been to countless concerts by many bands over many years. I'll never forget the one and only time a singer brought me to tears. It was Greg during It's Not My Cross To Bear. Glad to see you're keeping the boys alive.

Ned
Thanks, it's an honor and way too much fun. Following Gregg's death we did that song at every gig. On a side note, if I was just covering say Duane, I wouldn't need 4cm. But I have to cover Duane, Dickey, Warren, Derek, Dan Toller, and all of Jimmy Hall's guitarists too (which includes some hack named Jeff Beck). Thank you JVM and midi and 4cm.

So you said you are trying to maybe improve V3b, which drives the effects loop. That may be a reasonable endeavor - although if you have just one or a couple of active devices in the loop, it may not be necessary. If you do the Valve Wizard mod, you can put anything you want in V3.
JVM205h. Added 100K from R94/C43 to ground for V1b (OD Channel) gain reduction. Installed 1000pF in series with 47K, in parallel with R58 (82K), to reduce power amp squeal. V3 Valve Wizard diode protection mod. Active power soak for more EL34 goodness.
Guitars: Angus Young Sig SG w/ Pearly Gates Neck and Custom Custom Bridge, Les Paul Special (2 pups) w/ SD Stacked P90s. And occasionally a Warmoth Strat w/ Dimarzio True Velvet in N, M, SD P-Rails in B.
HD500x in 4CM for all pedals and Midi control of 205h (grounds lifted on two of the signal cables).

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Re: V3 - An Important Preamp Stage

Post by Hot 'Lanta Cat » Mon Dec 11, 2017 2:09 am

One thing I've learned about tubes & tone is that you just have to try it for yourself. I'm a NOS tube junkie and have found very little & profound changes is amp tone depending on the amp & the choice of tube.

Having a vintage 64 Fender Champ is [for me] a great way to hear the differences in 12ax7's because the amp's circuit has only one 12ax7 preamp tube. So just plug in the new tube and see what you think?
Same goes for the 6V6 & 5Y3 tubes that the Champ only uses one of each.

How does this relate to the JVM?
Well listen to a modern JJ then compare it with a 64 Blackburn Mullard and you won't believe the difference in sound quality.

If your thing is improving tone, use a clean amp to test preamp tubes. If you want to make changes to gain, you probably won't use the Champ but can use the amp intended for that.

I'm about to load my new JVM 215c combo with my 3 NOS Mullard ECC83 preamp tubes one at a time starting with V1, then after evaluating the changes from V1 move on to V2 and so on. I'm excited about the trial, but also know from experience just because the Mullards are expensive and work well in other applications, they may not work well in my JVM?

Tubes & speakers can make a huge difference.

Metal Heads may recommend this speaker or these tubes but a classic rock player may hate them?
Trust your ears not someone's YouTube review.

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