Modding Valco/Wards GIM 9151A

Discussion in 'Amp Tech Center' started by ArcticWhite, Dec 10, 2016.

  1. ArcticWhite

    ArcticWhite Tele-Meister

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    I've been thinking about modifying my new Valco (circa 1965ish), and wanted to bounce a couple ideas around here first.

    See attached schematic for details, but the basic layout of this amp is this: two 12ax7 driven input channels, feeding a quad of 6L6 power tubes. The weird part is there are two output transformers which fed the original 2 by 12 cabinet speakers in a stereo arrangement.

    This has been modified already. The two output transfomers now feed a mono output jack and a traditional mono 2x12 cabinet.

    The amp produces about 75 watts of power, so it is very loud and has tons of Headroom.

    Mod ideas:
    Since there are 4 inputs feeding two channels (which are almost identical), there seems to be an opportunity to modify three of the existing inputs - so that the amp has four different personalities available.

    My first impulse is to mimic the 5e3 circuit (Tweed Fender Deluxe Reverb), and simply add 1 Meg resistors to one input each for channels 1 and 2. That should give me a bright channel and a normal one, each with a high and low input. Same as the 5e3.

    The only difference between the two channels that I can see, is that one has a small 500 pf capacitor that the other does not(off the cathode of V3a). It appears to be a brightness capacitor - but I could be wrong about that. At any rate, the two channels sound identical, so I don't think it is doing much. Perhaps I should try larger or smaller value cap in it's place. Or perhaps add one in different position? - the 5e3 uses one right in the tone control.

    But what I really would like is to add additional gain to one of the two preamp channels. I'd like to make one channel crunch earlier and harder than the other.

    I noticed in the schematic that the V3b half of one12ax7 is not used.

    Would it be possible to use V3b to add another preamp stage on the tail end of the channel 1 preamp circuit - before it dives into the rest of the pre amp circuit? Just before V1b?

    I think I can handle the easy option of copying the 5e3 input mods, but designing the new gain stage is beyond me. What do you think of this idea? Is it possible to make one channel extra crunchy? Would using V3b cause my house to burn down?

    The 9151A schematic is at the bottom as a pdf..
    20161206_164711.jpg 20161206_164746.jpg
     

    Attached Files:

    Last edited: Dec 10, 2016
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  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    All 4 inputs are wired like Fender low jacks. They have 47k of impedance and the two 47k grid stoppers form a voltage divider to cut the guitar signal in half. The 47k grid leak resistor on both channels' first gain stage also act as a voltage divider to cut even more guitar signal.

    You'll need more than the 1M resistors to convert the two inputs into the Fender standard Hi-Low configuration but doing so and changing the first gain stage grid leak resistors to 1M should really transform this amp.

    Channel 1's C15 is a cathode resistor bypass cap which increases gain but 470k resistors R14 and R15 form a voltage divider that cuts Channel 1's output signal in half. This amp really likes to cut gain so it should be a clean monster. Changing the R14 grid leak resistor to a standard 1M will boost gain.

    You can use the unused V3B triode as a second preamp gain stage.

    A decent amp tech can get this done pretty easily.
     
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  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You can wire the input jacks like the 5E3 standard. Don't miss the little jumper on the jacks that connect the ground and switch terminals. Click on the image for the full size.

    [​IMG]
     
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  4. BobbyZ

    BobbyZ Doctor of Teleocity

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    Got the GIM 9991 or is it 9111? Looks like that one with a pair of 6L6s, three inputs and I think two 12ax7s.
    Probably the first amp I modded, used the unused triode and added a gain stage that engages by plugging into one of the inputs. Suprising enough it worked ! About all I remember is the gain stage plan was something Ken Fisher wrote up.
    What it really did was add alot more volume to the amp so you can overdrive it. Stock it's pretty low volume.
    I'll have to dig it out sometime and mess with it.
     
  5. ArcticWhite

    ArcticWhite Tele-Meister

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    Hi Rob,
    Thanks for all the ideas - Question: this amp doesn't have switched input jacks like the 5e3. In order to do this mod, would I need to swap them out? Or would I simply put a 1 meg resistor to ground in front of 47k resistor on some of the inputs?
    Thanks!
     
    Last edited: Dec 12, 2016
  6. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Because of the way your inputs are wired you will need to add the two switched jacks. Just adding the 1M resistors won't change anything.
     
  7. Silverface

    Silverface Poster Extraordinaire Ad Free Member

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    Honestly - I think you're in over your head. I do not suggest doing mods unless you understand the function of the existing parts, and clearly understand both your target sound and what it will take to accomplish it. "Adding a gain stage" because you want "more gain like a XXXX" is a really bad idea - the theory of how gain is created (it's not just adding a tube and a few parts), current draw on the existing power transformer and its capabilities, lead dress/parts placement to avoid increasing noise, knowing how to calculate gain - and understanding what you sacrifice in clean tone when you add gain stages - are all critical pieces of knowledge essential to doing mods of that type.

    Even changing parts at the input jacks is foolish if you don't understand the function of the existing parts.

    Seriously, you can accomplish much more without problems by changing tubes, bias settings and speakers than by haphazardly making modifications you don't fully understand the effect of.
     
  8. ArcticWhite

    ArcticWhite Tele-Meister

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    Gee. Thanks.

    But I'm probably not as stupid as you think. First off, I'm here asking for help, not going off and just modding things. I know my limitations; that's why I'm asking.

    Yet somehow, in the recent past, I've managed to recap and blackface a 71 SFPR. I practice using a tube amped record player which I managed to convert for guitar - adding a Fender style tone stack, as well as a pot which somehow makes the thing go from clean to crunch at living room volume. Do I understand exactly how it works? No.
    Is it Punk Rock?
    **** yeah.

    So I'm here asking if anyone has any cool ideas for modding three inputs of a four input, dual channel amp that was made 50 years ago to sell in a department store. Your concern is noted, grandpaw. I'll be okay.
     
  9. ArcticWhite

    ArcticWhite Tele-Meister

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    Hey, I'm finally getting around to this project!

    A. I installed a pair of switched input jacks and associated 68k and 1 meg resistors on channel one, (as in the 5e3 design). Cool. It works.
    B. I changed the grid leak resistor (R14) to 1meg as Rob suggested.

    Overall, the tone is a little brighter, and more present. The amp is a little louder too. Progress! But there's still very little distortion available, even with the amp dimed, and guitar volume maxed.

    1. Is there anything else I can try to drive the output tubes a little harder? This amp is still a clean machine.

    2. I noticed that the caps on the power tube cathodes are mis-matched in the schematic. One uses a 20 mfd in the cap can, while the other uses a 35 mfd. Why might this be? Any reason to match them?

    3. This is originally a stereo amp design, with two separate output transformers feeding two tips of a stereo output jack. It has been converted to mono by combining both outputs to one mono jack.
    Q: Would it make any sense to delete one of the OTs and feed all the power tube output into one transformer?
     
    Last edited: Nov 3, 2018
  10. Wally

    Wally Telefied Ad Free Member

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    Arctic, I am not going to be much help here except to say that those two OT's do not a stereo amp make of this design. As the schematic shows, the design was made to function on one speaker load....with an extension speaker available. In order to be a stereo amp, there would be two separate preamps with two separate phase inverters and two separate and identical output sections. This amp does not fill those requirements. One has inverter feeds bot pairs and the two OT’s feed the same speaker load.
    I have no idea what they were attempting to achieve with the two OT’s and the two different cathode bias schemes there. Maybe vAlco bought a truckload of OT’s that were good for 2 x 6L6’s and decided to build a 4 x 6L6 amp without needing to order a sizable lot of larger OT’s? Have you made a voltage chart? What are the biasing numbers for each pair of 6av6’s. If you have cold biasing there, you will not get warm, Rich output ime.
     
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  11. Wally

    Wally Telefied Ad Free Member

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  12. Wally

    Wally Telefied Ad Free Member

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    A change to a smaller value cathode resistor on the input gain stages with a larger bypass cap will 8ncrease gain a bit.
     
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  13. ArcticWhite

    ArcticWhite Tele-Meister

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    Thanks Wally. Yeah I agree, it's a really weird scheme. It's not a true stereo system, but if you ran it into a mono cabinet, using a mono jack, one of the power tube pairs runs with no load. Yikes!
     
  14. ArcticWhite

    ArcticWhite Tele-Meister

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    Wally, can you tell me how to check the bias for the 6L6s? I've been messing about with amps for a while, but haven't figured out biasing yet. Is there a good thread if you don't have time?
    I will test voltages soon and report back.
     
  15. robrob

    robrob Poster Extraordinaire Ad Free Member

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  16. ArcticWhite

    ArcticWhite Tele-Meister

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    Thanks Rob!
    I knew you'd come through.
    Your stuff is the best. Direct, to the point, well-written, and easy to understand for people like me, to whom this stuff doesn't always come easy. Your page is probably the best in the world in its category.
    You're the Jimi Hendrix of tube amp explainers.
     
    Last edited: Nov 6, 2018
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  17. ArcticWhite

    ArcticWhite Tele-Meister

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    Questions about impedance

    This amp has a pair of output transformers. It originally had a pair of stereo output jacks, each connected to both output transformers. Output impedance was 8 ohms at each output (according to the schematic), but since these were stereo jacks - was that 8 ohms total, or 8 ohms per tip of the jack?
    See what I mean? Does this amp want to see a 4 ohm load, 8ohms, or 16?

    Gah! I can't figure it out .

    It has been converted to a more typical arrangement of two mono jacks, with the two tips bridged as shown. (See my sketch on the schematic, and the picture.)

    Is the output impedance still 8 ohms?
    Is this the correct/best way to connect the two output transformers?

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    Last edited: Nov 26, 2018
  18. ArcticWhite

    ArcticWhite Tele-Meister

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    Bump.
     
  19. ArcticWhite

    ArcticWhite Tele-Meister

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    I did all the stuff Rob suggested, and it really helped improve the amps tone.

    Still no crunch available, however.

    Take a look at R18 in the schematic. It's on the grid of the phase inverter. The 1 Meg value of this resistor seems kind of high to me. (Is R18 a grid stopper, or a grid resistor, or what?)

    Anyway, I bypassed R18 with a short jumper, and the amp opened up. Much louder, percussive, and with a little crunch.

    Any reason I shouldn't do this? Perhaps put in a smaller value resistor in its place (a 100k would make a voltage divider with R19 grid resistor).
    Or maybe place it after the grid resistor...

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  20. ArcticWhite

    ArcticWhite Tele-Meister

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    Bumperoo.

    Any idea what that 1 Meg resistor (R18) is for, why it's so big, and what should I do about it?
     
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