Ad Widget

Collapse

Announcement

Collapse
No announcement yet.

JMI Vox AC10 add-on top boost?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • seven
    replied
    Originally posted by drewl View Post
    Nice, with the right transformers that thing could be way more than 10 watts.
    Thanks. Planning to use transformers spec'ed similarly to originals, I don't need more than 10 watts (e.g., home use instead of gigging). If I did gig with it, I'd probably just have it mic'ed.

    Originally posted by drewl View Post
    I'd set it up with relays for switching along with a manual control, but that's just my insanity.
    Am thinking about using a relay, actually. Just purchased a power supply board and relay board from Hoffman.

    https://el34world.com/projects/relay_switch.htm
    https://el34world.com/projects/hotswitch.htm

    Leave a comment:


  • drewl
    replied
    Nice, with the right transformers that thing could be way more than 10 watts.
    I'd set it up with relays for switching along with a manual control, but that's just my insanity.

    Leave a comment:


  • seven
    replied
    Thank you again Chuck!

    Leave a comment:


  • Chuck H
    replied
    Originally posted by seven View Post
    Thanks Chuck and Helmholtz for the explanation and guidance.

    How does this look?

    Click image for larger version Name:	1paNOrV.png Views:	0 Size:	112.6 KB ID:	1009429
    Nope. Like this. Notice that it was only the "pole" (and the actual grid of the tube) that didn't have a reference to 0VDC.

    Click image for larger version

Name:	ctsfq1.png
Views:	51
Size:	133.1 KB
ID:	1009434
    Attached Files

    Leave a comment:


  • seven
    replied
    Thanks Chuck and Helmholtz for the explanation and guidance.

    How does this look?

    Click image for larger version  Name:	1paNOrV.png Views:	0 Size:	112.6 KB ID:	1009429

    Leave a comment:


  • Chuck H
    replied
    Originally posted by seven View Post
    the original Vox Top Boost schematic doesn't show the 1M grid leak resistor for that triode...
    The grid leak for the stock circuit is the 500k volume pot feeding the Top Boost circuit grid. But this circuit isn't switched so there's no need for the extra 1M to ground. Signal loss will be small. As if you were using a 333k volume pot rather than a 500k. If it bothers you then you could use a 3.3M resistor instead. Making it as if you were using a 430k volume pot. Which I think would be within spec. of the original circuit.

    Leave a comment:


  • Chuck H
    replied
    Using a plain DPDT for switching at the amp itself will be fine. No need for a relay.

    The 0DCV reference resistors would be the same thing as Helmholtz mentioned as the "grid leak" for V2A. That resistor needs to be there to keep bias on that triode. Without it the tube loses bias briefly when the switch is thrown. It will certainly pop and though it might be ok otherwise, it might not. Best practice would be to include a 1M grid leak on V2A

    Leave a comment:


  • Helmholtz
    replied
    You're mixing up grid stopper and grid leak.
    A grid leak resistor wired from grid to ground is used to make sure the grid sits at 0 VDC.
    In the original AC 30 the V2-A grid is hard-wired to the vol. pot wiper, so no extra resistor required.
    But when switching the grid might be shortly left without a DC path to ground which could cause popping.

    Leave a comment:


  • seven
    replied
    Originally posted by Helmholtz View Post
    I just noticed: V2-A should have a dedicated 1M grid leak resistor.
    Hi Helmholtz, help me to understand... the original Vox Top Boost schematic doesn't show the 1M grid leak resistor for that triode... is the purpose of the grid leak resistor to reduce (eliminate?) RF? Does it cause a loss of gain?

    Leave a comment:


  • Helmholtz
    replied
    I just noticed: V2-A should have a dedicated 1M grid leak resistor.

    Leave a comment:


  • seven
    replied
    Originally posted by Chuck H View Post

    If you mean to switch it remotely from the amp, like a footswitch, a regular DPDT switch and flying a cable out of the amp won't do. It would be very noisy and lossy in the HF.
    Thank you Chuck for your feedback. I was planning on using a DPDT on a push-pull pot... I guess that won't quite work...

    Originally posted by Chuck H View Post
    You would need to do it with a relay and the footswitch would trigger the relay power.
    So, a relay. I presume I can power the relay off the filaments?

    Originally posted by Chuck H View Post
    You'll also need to add 0DCV reference resistors to the pole circuit on the switch as shown or it will likely make a pop when switched.
    Is the "switch" here the switch to trigger the relay or is it the relay switch itself?

    Lastly, are there any examples you could point me to that I could reference to see how this is all implemented?

    Thank you!
    Last edited by seven; 01-12-2025, 05:54 PM.

    Leave a comment:


  • Chuck H
    replied
    Originally posted by seven View Post
    Do you think I would be able to have it switchable during operation,..
    If you mean to switch it remotely from the amp, like a footswitch, a regular DPDT switch and flying a cable out of the amp won't do. It would be very noisy and lossy in the HF. You would need to do it with a relay and the footswitch would trigger the relay power. You'll also need to add 0DCV reference resistors to the pole circuit on the switch as shown or it will likely make a pop when switched.

    Leave a comment:


  • seven
    replied
    Originally posted by Helmholtz View Post

    So you already built it?
    If so, how do you like it?
    Haven't built it yet, just sharing what another forum member shared with me. What do you think of the schematic?

    Leave a comment:


  • Helmholtz
    replied
    Originally posted by seven View Post
    Leaving this here in case someone wants to try something similar...
    So you already built it?
    If so, how do you like it?

    Leave a comment:


  • seven
    replied
    Leaving this here in case someone wants to try something similar...

    Click image for larger version

Name:	yJRKeTh.png
Views:	64
Size:	72.2 KB
ID:	1009386

    Leave a comment:

Working...
X