Mini Silverface Champ with One Tube Reverb?

Discussion in 'Shock Brother's DIY Amps' started by tvas22, Aug 23, 2019.

  1. tvas22

    tvas22 Tele-Meister

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    Hey,

    I was wondering if I could get some eyes on this to see if anything looks obviously wrong or wouldn't work with this? I would like to apologise for the scaling of everything and the messy lead dress. I'm still pretty new to this and particularly using DIYLC. Some things are missing (such as filament wires) but I will add that in. I am planning on using generic turret board again which is why everything is done top row to bottom row, so that adds to the messiness a little.

    I have a spare 370AX power transformer and Hammond 125A output transformer after I double ordered for a Bassman Micro build, and have been wanting to build something with them that's a little different to that.

    I love a silverface champ, so I was wondering if I could mash together RobRob's 12AU7 push-pull design with a silverface champ? I have also wanted to put a one-tube reverb circuit in a build too, so I thought why not chuck that in. I used juneaumike's version of tubenit's one tube reverb with a 12DW7, but with a 3M pot and 100k mixing resistor based on bitsleftover's experience.

    I would really appreciate any advice or pointers you could give me. Does anything look like it won't work? mini silverface reverb.png
     
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  2. sds1

    sds1 Tele-Afflicted

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    A schematic diagram would get you some critical eyes on the circuit itself.

    Layout looks pretty good assuming it matches the circuit you are building. I might suggest rotating the sockets so that pins 1/9 are facing the board, you might get better lead dress that way. But I'm sure it will work fine the way you have it, too.

    Also you'd have to explain that switch to me, I see it in part switches the V1A cathode bypass cap in and out but not sure I understand the other side of the switch that apparently puts that cathode on the speaker jack? If that's just a convenient grounding point for a 56k in parallel with 1.5k cathode resistor... I still don't get it. 56k || 1.5k is not much of a warming up of the bias, also I don't think it's ideal to be grounding the V1A cathode in 2 places nor on the speaker jack at all.
     
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  3. tvas22

    tvas22 Tele-Meister

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  4. sds1

    sds1 Tele-Afflicted

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    Yeah if you edit the properties of the socket you'll see an angle of rotation value in there.

    Nope that looks good, thanks for clarifying! Threw me for a loop but that's on me. :)
     
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  5. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I don't understand the node that is feeding the reverb transformer primary. I see a 4.7k resistor coming from what I guess is the reservoir (B+1) cap, but no filter cap for the reverb. I suppose you can put a resistance in series w. a transformer, but why not add a capacitor to the reverb driver voltage supply?

    Looks cool, looking forward to seeing more
     
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  6. sds1

    sds1 Tele-Afflicted

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    Good catch @Snfoilhat, I reckon you could just add a cap at that node, but why not take advantage of the cumulative filtering provided by nodes in series , especially given there's no choke here? Or, I suppose you could just power both v1 and V2 off of the b+1 node and delete the 4.7k. A third node seems like the right choice though as you otherwise risk interaction between gain stages. Would be a great exercise for you to calculate the expected voltage /current draw at each node in case you've not done so already. Designate it on your schematic. :cool:
     
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  7. tvas22

    tvas22 Tele-Meister

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    Thank you for your advice so far, really appreciated.

    I will stress that I am still a beginner, so you’ve given me a lot to read up on already.

    If I was to add a cap to the 4.7k, its ground should go to the input jack right?

    I do actually have a spare 155H choke that I could use if that would be a better alternative?
     
  8. sds1

    sds1 Tele-Afflicted

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    You're welcome, ask more questions if you need to. :)

    Yes, you would want to ground that capacitor on the preamp bus.

    The choke will provide better filtering (less ripple) and less of a voltage drop, possibly less perceived voltage sag... technically I think it leads to a better power supply but Champs don't have chokes so that is a departure from the essence of a Champ. But so is reverb, and so is the SE parallel output section so... this is your design now. :)

    The reverb circuit would get the node after the choke, then you can calculate an appropriate dropping resistor for what would be B+2 to power V1.

    It's worth noting that Rob's Champ Micro design has been revised to include extra filtering (3 filter caps instead of 2) so this possibly helps to validate the need/want for a choke.

    If you can get that schematic drawn up we can help you finalize the power supply and get the voltages dialed in to match whatever design you want to follow. For example, Rob's B+2 is at 250VDC.

    See what other folks say. :)
     
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  9. tvas22

    tvas22 Tele-Meister

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    Thank you again - I have started work on the schematic.
     
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  10. tvas22

    tvas22 Tele-Meister

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    OK, I had a go. This is the first time I've ever written up an amp schematic - I checked it over, hopefully there's not too many mistakes.
     

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  11. sds1

    sds1 Tele-Afflicted

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    Good job man! Haven't gone thru it completely (heading to gig in a second here) but I don't see that NFB mod drawn in?
     
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  12. sds1

    sds1 Tele-Afflicted

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    Also, if you're around @robrob ... how do you feel about that choke? It's a departure from the classic Champ power supply circuit but any downsides? He has it on-hand, any reason not to use it?
     
  13. tvas22

    tvas22 Tele-Meister

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    Ah, I forgot to add that in. I am intending to do it still, will add it in tomorrow.
     
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  14. tvas22

    tvas22 Tele-Meister

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    OK, I think this is everything
     

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  15. D'tar

    D'tar Friend of Leo's

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    Possibly... From Robs micro champ input


    upload_2019-8-28_9-59-35.png
     
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  16. D'tar

    D'tar Friend of Leo's

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    just for conversation sake. I would use v1a simply to amplify the guitar input signal and tone shape further into the circuit. The switchable bypass is not a bad idea however. For NFB, one might look to the sf champ itself and add a small value tail resistor and the 2k7 nfb as such @V1b cathode

    upload_2019-8-28_10-44-48.png
     
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  17. tvas22

    tvas22 Tele-Meister

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    Ah, that's my first two mistakes. I missed off the 1M resistor and meant to use a 68k grid stopper rather than 6.8k. Cheers. I will add in the switched input too.

    D'tar - where would you move the tone shaping to? If it goes after v1b then the tone stack will affect the reverb, or do you think I should put in another tube? I agree about the NFB resistor - I will change it to the 2k7 going to the v1b cathode and leave the switchable bypass just as on/off.
     
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  18. D'tar

    D'tar Friend of Leo's

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    appologies... I would leave the tonestack as is but move the nfb to v1b instead of v1a and use a 33k input grid stopper.
     
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  19. sds1

    sds1 Tele-Afflicted

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    To elaborate on @D'tar 's comment, 68k parallel grid stoppers are found on two input jack configurations -- in parallel they are effectively 34k. So for single input jack setups we use a 33k grid stopper.

    As a matter of convention, it's confusing (to me anyhow) to have your V1A cathode drawn like that. I realize you have wires crossing everywhere, to keep the drawing clean just use jumper labels as necessary, as you have for Vaa, Vbb, etc.

    Let's work your estimated power supply out real quick, it's some simple math and a quick peek at the tube datasheets.

    First, your diode rectifier creates a DC voltage that is a factor of the AC input and magic value 1.41.

    B+ = 240 * 1.41 = 338.4. You can designate 334VDC at the top of the first filter cap in your drawing.

    The choke will drop just a few volts, there's a way to calculate it but I'm just going to call it 5VDC. It is probably less I dunno. We're doing some rough-ish estimating.

    Vaa = B+ - 5VDC = 329VDC. You can designate this on your drawing at Vaa.

    The voltage drop from Vaa to Vbb is a factor of the dropping resistor and the total current draw downstream of Vbb. In this case it's the plate current drawn by V1A and V1B. Pull up the 12AX7 datasheet and you'll see a "typical" plate current (Ia) of 1.2mA per triode. This will vary with the operating point, if you wanted to get a more accurate number to work with you could draw load lines and figure it out. I think 1.2mA is a fine value for estimating, now we use Ohm's law to calculate a voltage drop for 2.4mA draw across 3.3k resistor:

    3300R * .0024A = 7.92V. Call it 8V, so you can designate the voltage at Vbb to be 321VDC.

    You can cheat if you want, there are Ohm's law calculators available online. I cheated:

    [​IMG]

    Notice the power dissipation across that resistor is a mere .01W. I'm pointing this out because a 1/2W resistor would be ample.

    Your Vbb is pretty damn close to where the Silverface Champ runs the preamp at (330VDC if I'm looking at the correct schematic).

    You could also work the problem a different way, if you wanted to match Rob's Champ Micro preamp voltage of 250VDC, you'd need to drop 84VDC to get there. Ohm's Law once again, this time solving for resistance:

    [​IMG]

    So a 35K dropping resistor would get you there. 33k or 39k would do just fine, if you play with the calculator you'll see that for such a small current draw it's hard to affect the voltage drop with small changes in resistance.

    Hope this helps!
     
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  20. tvas22

    tvas22 Tele-Meister

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    Thanks so much for your replies. That's all really useful information. I have had a go at implementing the changes you have all recommended on the schematic attached.

    I'm a little confused about Rob's Champ Micro preamp voltage, as on the page it lists 304v at B+2 for ~150v on v1's plates, while fender's 5f1 layout gives 250v at B+2 for 150v on the plates - will it just vary case by case?

    What will the difference be in running it at the higher voltage given by the silverface layout?

    Also, does it matter too much what voltage I'm running in to the output and reverb transformer?
     

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