Add modern features to vintage circuits?

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, Nov 10, 2019.

  1. NSB_Chris

    NSB_Chris Tele-Meister

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    I am planning to build a simple champ amp for a family member that is just starting to play guitar. I have a Champ AA764 circuit planned with the vibrato removed. I am not altering the main signal path circuit but have been contemplating adding some modern elements to the power supply etc. if they make sense.

    Some of these are exemplified in Rob’s VHT page:

    1: Add thermistor and 0.1uF capacitor (C53) to PT primary side. I see this on modern Fender circuits as well as others. Seems the VHT also put a 0.1uF capacitor (C48) on the main PT secondary. Do you guys bother to add this to your builds?

    Screen Shot 2019-11-08 at 6.37.54 AM.png

    2: It seems the VHT adds 0.1uF caps (C42, C44 etc.) at all of the B+ filters. I don’t see this on other circuits.
    Screen Shot 2019-11-08 at 6.21.18 AM.png

    3: At the input jacks, they have capacitors going from the tip and sleeve connections to, I am guessing, a local chassis grounding location. So they have a 100pF caps (C2) from the tip connection to chassis and a a .01uF cap (C1) from jack sleeve connection to the chassis. C1 is essentially adding a capacitor between the preamp star ground and the local chassis grounding point. I am sure C2 leaks very high frequency noise from the input signal to ground, but I am not sure exactly what C1 is accomplishing.
    Screen Shot 2019-11-08 at 6.15.50 AM.png


    I don’t see these features being added to a lot of builds on this forum. For manufacturers like VHT, I would expect they would not be motivated to add unnecessary components as it adds cost. They must feel that these features are required or are important for reducing noise.

    Thanks for any input you might have!
     
  2. RottenTheCat

    RottenTheCat Tele-Holic

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    Totally un-needed.

    If you want to do something nice fit a 10in speaker to that cabinet & an aftermarket transformer for 8 ohms. The eminence 10 inch alnico does great in that application by the way.

    Better yet build a 5f1, in a 5f3 cabinet with a 12-inch speaker. That makes a rippin little amplifier.
     
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  3. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Upside-down triangle symbol is signal ground. 3-pronged rake symbol is chassis ground. In vintage amps these are often the same, but they don't have to be. A capacitor sending high frequencies directly to the chassis is something I have seen recommended by https://www.aikenamps.com/index.php/tech-info to deal with any persistent RF noise problems one might get in an instrument input or reverb return for example.
     
  4. NSB_Chris

    NSB_Chris Tele-Meister

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    Thanks.
    My first amplifier build was a Mojotone Vibro Champ head kit. Sounds really good with several 1x12 cabs I have. The OT will definitely have an 8 ohm tap. I have been debating a 8" or 10" speaker as my niece will be playing at bedroom levels and surely won't be trying to get a lot of distortion out of the amp.

    I don't think my niece would use the vibrato, so I am just going to build it super simple with simple controls. I plan on a very simple pedal board that she can put on the front to get some reverb and maybe chorus. I had planned on putting a pedal power jack on the front face so that the simple pedal board plugs right in. I saw that Rob did that on his VHT, so that gave me resolve.

    Thanks. Will leave this capacitor off but out of curiosity I will look into it further. Still not fully clear on where they are putting that capacitor.
     
  5. RottenTheCat

    RottenTheCat Tele-Holic

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    Well I was gonna post a picture of a 1x12 5F1. I had Mojo do it up in black tolex, came out really nice. Cant remember the speaker I used for that one. It was a long time ago, so it was probably the Emi 12ALK (discontinued). I converted quite a few SF Champs for customers. You can "just" fit a 10 in there, and the baffle will have a "flat" on top and bottom of the cutout. On a Champ with removable baffle, you can do the whole opening. In fact... I converted some of those to 2x6v6 (using solid state rectifier and the 5y3 "hole" for the 2nd 6V6 tube). Could barely... just barely get a Deluxe reverb transformer set in there, so you had a 22 watt little killer Champ that would blow folks away. One guy was a professional player who used to carry that amp and his axe on the trains up in northeast, when he went to gigs. Nice little amp, not too heavy.
     
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  6. King Fan

    King Fan Friend of Leo's

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    FWIW, the "Champ AA764" is a Vibro-Champ AA764 with the vibrato already removed. :)

    champ_aa764_layout copy.png

    If you're building it for your niece who's just starting out, the only modern features it really needs are the kind we all do now for *safety* and noise reduction: Double-strand heaters, lose the death cap, modern (separate) household safety ground, modern AC wiring, maybe even switch both the hot and neutral.

    What else, let's see, upgrade to 25uF-50V bypass caps, upgrade the dropping resistors to 2 or 3W, and the bias resistor to 3w or more.

    This is a simple classic 'student amp' that has also been in a lot of famous concerts and recordings. Final thought; an 8-ohm OT with a 10" speaker is nice, but for her ears and her family's sake, keep it an inefficient speaker. These little guys get *loud*...
     
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  7. muchxs

    muchxs Doctor of Teleocity

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    I built exactly one 5F1 in a "5E3" cabinet. Guy who has it says he uses it as a chair for showgirls which is a nice word for strippers. He's in Nevada. I hear there are a few showgirls in every large town out there.

    :D :D :D

    That particular amp has a 1961 12" Oxford I forget which model but I got two of 'em super cheap out of an old console phonograph. A tweed amp through a vintage Oxford is a lesson in why those old Oxfords are legendary.

    A 5F1 in a 5E3 just doesn't look right. That little chassis looks lost in that big cabinet.

    I built more of my ADD version. That stands for "All Donut Diet". It looks like a Champ that was raised on fat fried donuts.
     
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  8. muchxs

    muchxs Doctor of Teleocity

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  9. NSB_Chris

    NSB_Chris Tele-Meister

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    Good point and good advice. I am building the cabinet so I can do whatever within reason. Still have some time to decide on the speaker. Definitely have the safety and noise reduction things you mentioned in the plan. Already have the DPST switch for the main. The PT I am planning is the Classic Tone 40-18027 which has a center tap on the heater secondary and I may elevate that on the power tube cathode pin. The OT I am planning is the Classic Tone 40-18030.

    It seems nobody bothers to put an inrush thermister on the PT primary.
     
  10. NSB_Chris

    NSB_Chris Tele-Meister

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    @King Fan

    Thanks for pointing out that there is a Fender schematic for the Champ AA764 (no vibrato). Once you pointed it out I remembered having seen it posted somewhere. I often go to The Tube Store's archive because it is convenient, but they don't have it so I missed it. It looks like the basic circuit path of the Champ AA764 was traced from the Vibro Champ AA764 but left out the vibrato obviously, but also a few other things. I also found the CBS Champ schematic which is almost an identical circuit to the Champ AA764. Both the Champ and the CBS have what looks like a 2uF cathode bypass cap on the second gain stage instead of the 10uF shown in the Vibro Champ. The CBS and the Vibro Champ have a 330pF cap going from the grid to the cathode on the 6V6GT tube, but the Champ does not. Minor differences but I will honor the Champ for this one.
     
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  11. RottenTheCat

    RottenTheCat Tele-Holic

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    champ.jpg

    Found a picture of what I described. Mojo cabinet, they supply tweed type hardware, so a trip to Lowes gave me some stainless hardware, screws, beauty washers. Got tilt back legs too!
     
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  12. NSB_Chris

    NSB_Chris Tele-Meister

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    I was considering putting tilt back legs also. Cool that you added those. I cobbled an amp into an old Fender Twin cabinet I got on eBay. It has the tilt back legs and love it. I think I will go ahead and add those!
     
  13. King Fan

    King Fan Friend of Leo's

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    Good eye comparing the schematics. (Do I recall the cap on the 6V6 socket was a CBS addition maybe to suppress parasitic oscillations? -- which some say may have come from their sloppier lead dress?)

    It'd be fun to compare the V1b bypass cap at 10uF to the 2uF. My VC with the 10 is a nice blend of tweed-Champ raw + baby-PR nice. Hmm, does the trem circuit add some gain, so more bypass at V1b? Above my paygrade.

    What do you plan to do about B+? Seems like the BF version shows 360v and the SF version 420? Was there a PT change between BF and the 70s?

    Which brings up a big little deal. I'm not sure if Champs did it, but as I researched the *VC*, actual gut shots, even on the BFs, show that Fender went to 40/20/20 filtering early on (as eventually shown on the Bronco version of the schematic) by simply soldering a jumper between the first two cap can lugs. I *think* modern cap cans are 20uF x 4, and I know the 5Y3 (despite data sheet ratings for 30uF) can handle 40 all day long. A bit of free extra filtering is a nice deal -- might be extra nice on a "student" amp with a 10" if you wanted less hum or less flub.

    I know somebody who'd know for sure about all this, and that's @muchxs
     
  14. muchxs

    muchxs Doctor of Teleocity

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    Champ, Vibro Champ, Bronco, Princeton and Princeton Reverb all share the same PT within a particular era.

    Fender must have got a good deal on 40/20/20 or 40/20/20/20 Mallory cap cans. The first filter is often 40uf, no jumper required.

    CE Dist makes new Mallory style cap cans on the original Mallory tooling. If you want 40uf for your first node, you got it!

    That works well with a 5U4G rectifier. Just don't use a 5U4G in a pre- 1970 Champ, Vibro Champ etc. with the original 022772 PT.
     
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  15. NSB_Chris

    NSB_Chris Tele-Meister

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    I was going to put in separate filter caps to be able to split the grounds between preamp and power amp sections so I can up the value on one. I think I have an extra 47uF filter cap laying around. Is that too much?
     
  16. tubelectron

    tubelectron Tele-Holic

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    Forget all that supplementary stuff that is completely useless on a Champ !

    What I would recommend you instead :

    - a 10" speaker, as mentioned by @RottenTheCat, but it's nonetheless possible to find good sounding 8", like the ones sold by vintage47.

    - using a choke Hammond 194A (4H/50mA/175R) for filtering ALL the plate supply, along with a JJ 40+20+20+20/500V multi-cap - Like below (sample from one of my amp schems) :

    [​IMG]
    +1 = plate
    +2 = screen
    +3 = preamp

    - using a bigger output transformer than the original Champ size. The Hammond 125BSE is just fine for this (wired for 5000R primary / 8R secondary).

    - to make as short, direct and compact wiring as possible... ;)

    -tbln
     
  17. RottenTheCat

    RottenTheCat Tele-Holic

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    Re cap sizes....p/p can get by with less than s/e on output valve plate
     
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  18. King Fan

    King Fan Friend of Leo's

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    Split grounds? Cool -- another logical modern feature. :)

    I can see where 47uF makes sense; heck, how many companies even make a 40? You hear about folks running > 40 uF on a 5Y3... but you'll want solid backing when the data-sheet nazis come after you, and, more important, you don't want your niece's amp eating rectifiers. The easy way out if there's room would be to parallel 2x20 caps for your reservoir...
     
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