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Valve senior Mods.

#1 User is offline   Gooch Audio 

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Posted 11 September 2014 - 08:28 PM

Hi guys I'm new so I'm asking for help if anybody can point me in the direction of
links to mods for the valve senior combo or any suggestions or advice for the amp.
Any help will be greatly appreciated. Cheers
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#2 User is offline   Gooch Audio 

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Posted 12 October 2014 - 06:33 PM

Anyone?
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#3 User is offline   smg_luthier 

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Posted 15 November 2014 - 09:14 PM

Hey Gooch, Just came across your post, and so glad I did. I have a "recipe" that I've been dying to share. These amps can easily be transformed into little boutique monsters that will urinate all over just about any other amps in the 15-20 Watt range. T here is not really one single mod that will bring the amp to life, but a combination of upgrades and "tweaks". That being said, I am about to write a novel, so please be patient. I can assure you that it will be worth you time, and hopefully mine, too.
IMPORTANT DISCLAIMERS AND WARNINGS!!!!!!:
1)Guitar amps and their components use voltages that can and will harm, or even kill you. Be sure that when working on your amp, the power supply is unplugged, and the filter capacitors are fully drained. If you don't know how to drain the filter caps, you can Google it, or shoot me a PM.
2)Although I am a some-what skilled luthier, when it comes to amps, I am really just a tweaker and a parts changer. I really have no idea what I'm doing, but what I'm doing seems to work.
3)If and when I state a opinion, it is simply that; my opinion. My opinions are based on my personal experience. I own two of these amps, so it make it easy to A/B different components, and I have, on several occasions. If anyone has a different opinion, please do not be a "hater" (as the kids like to say on the World Wide Web), but offer an alternative suggestion.
4)MOST IMPORTANTLY, SEE #1!!!!
O.K., let's get started.
Tubes make a HUGE difference. I have found that the Mullard and Tung Sol reissues sound far superior to any other tubes. I am using a pair of Tung Sol 6V6s for the power tubes, a Mullard 12AX7 for the pre-amp tube, a Tung Sol 12AT7 for the reverb (this is important, I'll explain more later), and I like vintage (even if they're used) 12AT7s as phase inverters. They just seem to "feel" better. Using a 12AT7 as a PI will compromise gain, but in return, your amp will have much more "touch sensitivity"' or "dynamic response". And yes, a new 12AT7 will do just fine.
I am running two 12AT7s, which draw considerably higher plate current than 12AX7s, but this has not been an issue for the power transformer(s), and I've been running both of mine very hard, for some time now.
Simply by upgrading the tubes, I can assure you that you will notice a great improvement in tone, but there is so much more that can bee done,
Let's proceed with some interior mods/upgrades.
You can find a schematic of the amp here: http://forum.gibson....se/page__st__80 . You should keep a copy of this. I will be referring to it often. Remove the chassis from the cabinet, and remove the board from the chassis. Take notes, label & tag everything, and even take pics. This will help greatly when putting everything back together. If you need more help/advise with this, PM me.
First, you should bump up all of the filter caps. This will smooth the AC hum, and keep your amp whisper quiet, even when "dimed". Change C32 to 100uf 450V, and change C33, C34, C36 and C37 to 47uf 450V.
Next, all of the poly caps should be upgraded. I like the M150 series caps, which are currently produced by Cornell Dubilier. Orange Drops are also very good. (Funny, in a guitar, ODs are always my first choice, but in an amp, M150s are my first choice. I've never tried it the other way. If I try something once, and it works, i stick with it.) So, that being said, you should upgrade the following capacitors (with the same values):
C35/C38, C1/C2, C3/C4, C5/C10(this one is very important,largely responsible for the "voice" of the amp), C40/41, C13/14 (those are part of the reverb circuit, not hugely important for tone, but why not, while you're in there...) , C30/31, C21/22, C13/14, C15, C17/18,and C19/20. Not very expensive, or labor intensive, and wow, what a difference.
Next, Let's address the reverb circuit. This seems to be everyone's big issue with this amp (including mine). This one seems to be a "no brainer", and might I reidioterate that I don't know WTF I'm doing. The problem is that the tube is running too hot for the reverb tank. This can easily be tamed by changing the plate resistor (R28) to 200K(1/2 Watt) and the cathode resistor to 2.2K(1/2 Watt). And as I stated earlier, change the reverb driver tube to a 12AT7. This will transform the reverb from "Out of control, Useless, Over the top" to just about the best tube driven reverb circuit you've ever heard.
An other thing that can be improved upon is the tone stack. Change C7(C6) to a 470pF, and R5 to a 47K Ohm. I have already mentioned above upgrading C1/C2 & C3/C4. These simple changes/upgrades will make your tone controls much more responsive.
Next on the agenda would be the pre-amp, and gain. Again, I must state that I STRONGLY recommend a Mullard 12AX7 for the pre-amp tube. Before I address the tube, I would like to look at resistor #3. The factory value is 68K Ohms. I think this is a little high. Most people use a 10K in that application, but I've found that to be just a little too hot (and my intention was to build a really dirty amp). I am using a 33K in both of my Valve Srs.(BT Dubs, I mostly use Les Pauls, with H-Buckers or P90s). As I'm running them right now, I think I might like just a little bit more clean headroom, but I am running the preamp tube a little hot, and I can make a huge difference by simply rolling back my volume knob on my guitar (L.P.s are great for that. Neck volume at 7, bridge volume at 10, the toggle switch becomes your clean/dirty switch). You can experiment with different values for that resistor, but i think 22K or 33K is where it's at. So, on to the tube
In case you didn't know, a 12AX7 is a twin triode, and the tube can be thought of as two separate valves. The first triode is the first gain stage, and the second triode, the second... (duh). I am currently using 220K Ohm resistors for the plate loads (although I think 200K would be optimal). Change the plate resistors (R2 and R7)to 200K (or 220K) Ohms. Along with that, you should change the cathode resistors (R4 & R6). A value of 2.2K is optimal, but you can raise the value for less gain. I am running 2.2K for both, but you might try a 2.2K for R2, while for R7 increasing the value to 2.7K, 3.3K, 4.7K, or even as high as 10K, for more clean headroom. If you're new to this, and it seems a bit confusing, just change the plate resistors (R2 & R7) to 200K, and change R4 to 2.2K, and R6 to 2.7. You will not be disappointed.
After all of these issues are addressed, you can, if you feel the need, tweak the "Voicing" of the amp. This is not nearly as difficult or expensive as you might think. This would simply involve changing a few capacitors. I like to use audio grade electrolytic caps when possible. C25 is a 22uF 50V electrolytic. Increasing the value will give you more bass response. Some people like to use as high as 1000uf for the power amp (C25). This can be very "boomy" sounding, but the factory Valve Sr. cab is very bright (I have A/B tested my head & combo through the same cab, and run the head through the combo cab.) I think I have a 1000uf in C25 for the combo, but a 470uf or a 220uf will offer more bass response than the stock 22uf. For C8 (the first cathode coupling cap), I'm using a 220uf, for more bass response.
I could go on, but really, if you implement most of these mods/upgrades, you will have a killer sleeper of an amp. I could talk about output trannies (save your money, no audible difference) and speakers(shoot me a PM, I'll let you know what I use), but what I've laid out should really get you there.
Let me know if this helps you, and if anyone else has anything to offer, I would be very interested myself. Thanks.
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#4 User is offline   Gooch Audio 

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Posted 14 December 2016 - 03:40 PM

Hi,
Sorry for my late reply.
Thank you for posting all of this.
I am finally going to start working on this amp soon.
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#5 User is offline   Dennis G 

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Posted 17 December 2016 - 04:20 PM

smg_Luthier,

Thanks very much for your input. I have a Valve Sr combo (way out of warranty) that the head sounds like the opening 10 minutes of "Saving Pvt. Ryan" when I turn it on, and I've been going to take it to my favorite luthier (who also does amp repair) for a going over. I'll also give him your list and ask for his opinion as well (no offense intended).

As for your warnings, I know two things about amps. The first is that if you don't know what you're doing, they can kill you. The second is that I don't know what I'm doing LOL.
Dennis
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