Champ clone

[syn]

Hi
      I'm about to build a champ clone, with vibrato added (5f1+the vibrato part of the AA764).Have a couple of questions:
Where do I connect the taper of the speed pot?
Would 4.7Meg for speed pot change much? I have som 4.7 meg pots laying around, and I guess 3meg pot will be difficult to source.
What  do I connect the "red spot" to?
How about regulated DC supply for the heater section?
Would like to make a pcb, but, many say that the "sound" is better when P2P for guitar amps? True?
Thank you

 

[peterc]

This was my first toob build in 1992, sounded absolutely gorgeous

Where do I connect the taper of the speed pot?

You only use the centre (wiper) & one of the outer terminals of the pot, select the one which makes the pot work correctly.

Would 4.7Meg for speed pot change much?

If you out a 10meg resistor in parallel, it will be approx 3 meg in total. The response might not be smooth, tho.

I never got to a DC supply on this project, only on mic pres. If you are careful with the heater routing & using twisted pair for the supply, it will be good.

P-P is definitely the way to go.......

 

[volker]

- Pot: Important for a nice travel from one end to the other is the reverse log taper. I guess you have a linear pot. The bigger value and the wrong taper and the fact that it is wired as a variable resistor in this cuircuit (that's why there aren't three terminals shown in the schematic) make it perfect for converting it to a reverse log pot using a parallel resistor. Look here how to do that.

- Red spot: The switch connects/disconnects that point to/from ground to switch the oscillator, i.e. the tremolo, on and off.

- DC heater: not necessary if you know how to wire the amp the right way. If you go for PCB with tube sockets on it, then yes. If you go for PCB/turret or eyelet board/P2P with tubes mounted to chassis, then not necessary. Never the less it's always a good way to avoid hum. But in an easy and low gain amp like that, you can definitely make it work flawlessly without it.

- PCB: The layout does have influence on the sound, doesn't matter if you build it P2P/eyelet or turret board/PCB. PCBs have their bad reputation from amp builders not knowing how to design it and old-fashioned guitarists talking about stuff they don't know anything about. If designed appropriately, a PCB amp will sound as perfect as any other wiring method. But there are many things to take into account, and if you haven't ever done it or built any tube amp at all, it is definitely not recommended. The Champ is really an easy circuit and makes a PCB not worth the hassle, especially for a one-off.


Volker

 

[abbey road d enfer]

- Pot: Important for a nice travel from one end to the other is the reverse log taper. I guess you have a linear pot. The bigger value and the wrong taper and the fact that it is wired as a variable resistor in this cuircuit (that's why there aren't three terminals shown in the schematic) make it perfect for converting it to a reverse log pot using a parallel resistor.

Sorry, volker; the fact that it is wired as a variable-resistor is precisely what makes it unsuitable for law-steering.

- PCB: The layout does have influence on the sound, doesn't matter if you build it P2P/eyelet or turret board/PCB. PCBs have their bad reputation from amp builders not knowing how to design it and old-fashioned guitarists talking about stuff they don't know anything about. If designed appropriately, a PCB amp will sound as perfect as any other wiring method. But there are many things to take into account, and if you haven't ever done it or built any tube amp at all, it is definitely not recommended.

+1

 

[PRR]

> How about regulated DC supply for the heater section?

For a CHAMP?? It is supposed to be cheap. Low hum tells you it is working.

> Would like to make a pcb,

That's the other thing. Flying balanced twisted AC heater leads can give very low hum; early Champs did not even balance or twist. If heater wires are on PCB, you can NOT properly route or twist them, and "must" turn to silly DC heat, well-filtered.

Build it just like shown in Fender's layout. Except use a 3-pin power cord per your country's electrical standards (as shown in your modified schematic) and twisted heaters.

Don't worry about trem speed pot value or taper. The nominal C-R-C-R-C-R resistance is 1meg. Ideally we'd vary all 3 resistors, but... instead we vary one resistor both ways from 1Meg. If you use different values you get different speeds. If you get too far out, it stops oscillating. If you use a different taper the "0" and "10" speeds are the same but the "5" speed is different than the original... this may turn out to be unimportant.

I guess the "ideal taper" would follow a third-root law, so any practical pot is a crude approximation. No matter, because your turn it to match the beat, not to any specific calibration. A linear pot is just a bit touchier at the fast rates. If you never play slow, try a 1Meg Lin. Or use an "audio" taper and let the knob work "backward".

 

[syn]

Thank you very much.

 

[volker]

- Pot: Important for a nice travel from one end to the other is the reverse log taper. I guess you have a linear pot. The bigger value and the wrong taper and the fact that it is wired as a variable resistor in this cuircuit (that's why there aren't three terminals shown in the schematic) make it perfect for converting it to a reverse log pot using a parallel resistor.

Sorry, volker; the fact that it is wired as a variable-resistor is precisely what makes it unsuitable for law-steering.

Well, actually not. If you look at the last diagram from my link above, you will see that it works quite well.

 

[PRR]

> If you look at the last diagram from my link above

Text above that image: "there is no way to get a simulation of a log taper pot in the two terminal connection. For that we have to buy real audio taper pots."

 

[volker]

Yes, that's right. But typically you would want a 3M reverse audio/log pot for that Fender circuit (= 3M-RA in the schematic).
And that's what the last diagram describes.

Or if you want to buy it, that is the one built by CTS, that is usually available at the tube guitar amp shops.

 

[PRR]

The direction of the taper does not matter for this trick. (The linear pot is symmetric.)

2-terminal is different from 3-term potentiometer.

The intended total R value is:
"0" = 3.1Meg
"5" = ~~600K
"10"= 100K

Mid-turn is geometric mean.

The (reverse) audio taper will do this, in 2-term connection with 100K to set the "10" limit.

Shunting a fixed passive resistor will NOT do it.

At full-up the parallel value is Rp||Rt which is always less than Rt. So to use a 5:1 ratio Rp:Rt (to get the highest curve), and 3Meg max, we need a 18M linear pot and a 3.6meg resistor. At 50% or "5" we have 9Meg||3.6Meg or 2.6Meg. Not 0.6Meg as we wanted. Not really different than 3meg.

At full-down the value is of course the 100K stopper. As we move the wiper along the track, total resistance increases. To get from 100K to 200K total (a small significant change) we need 100K of Rp||Rt, or 112K of Rp. 112K of a 18Meg linear pot is 0.6% of rotation. On a "0-10" scale, "0.06".

Also the 600K nominal "mid-speed" is at 580K of pot (||3.6Meg) or 3% rotation, not 50%.

> CTS, that is usually available at the tube guitar amp shops

Yes, that works (of course). I wish there were an alternative. If you can show a way to get the 3M-600K-100K above from a linear pot, it would be very exciting.

 

[pietro_moog]

did you see the champ ultra at ceriatone?

i'll be after this project as soon as i can work wood in my garage (spring,now i'm freezing)

 

[syn]

did you see the champ ultra at ceriatone?

Yeah, It looks nice.

I have some more questions. How about metal film resistors? I'll be using carbon comp. for the plate resistors but was thinking to use MF for the rest (PSU wirewound or metal oxide)? Also any point in using output transformer for the princeton or deluxe? I'll be going for the 4x10 cabinet, probably.

 

[Soeren_DK]

did you see the champ ultra at ceriatone?

i'll be after this project as soon as i can work wood in my garage (spring,now i'm freezing)

Its really a great project. I have one in my living room, love it.
http://www.ceriatone.com/images/layoutPic/fenderLayout/Champ-Ultra-Layout.jpg

Cheers
Soren

 

[pietro_moog]

yeah, i think metal film is fine for PSU and every thing that doesn't receive the audio signal.
Fender Champ '57 (new fender quality champs) use carbon film.
i think i'll build mine with carbon comp for the signal part and MF for everything else.
what capacitors are you going to use?

please, post pictures of the work!

 

[syn]

Mallory 150 or Orange 715, I'll order both and see. I'll post the pictures once I'm done, month or two

Anyone any ideas on  princeton or deluxe OT for the champ output? Good? Bad?

 

[AnalogPackrat]

Anyone any ideas on  princeton or deluxe OT for the champ output? Good? Bad?

Bad.

Champ is single ended-->DC through the output primary-->gapped core.
Princeton and Deluxe are push-pull class AB-->no (very little) DC through the output primary-->interleaved and ungapped core.

These are not interchangeable.  You should be able to find a larger core SE output transformer for a single 6V6, but I wouldn't bother.  I own two SF VibroChamps.  They sound great with their dinky output iron.  Plug one into a bigger, more efficient cab and be amazed how good a little 5W "student" amp can sound.

A P

 

[CJ]

also, note that the champ is one of the few amps with a 4 ohm speaker,

and therefore, a 4 ohm secondary on the output trans.

 

[syn]

Aren't earlier Princetons SE also (5b2-5f2a)? I didn't look at the Deluxe schemo, just thought it's scaled up Champ- but it is not, sorry. However I'll definitely go for the Champ OT.
Thank you very much

 

[AnalogPackrat]

Yeah, the 4ohm speaker is another thing to think about.  Looking at the 5F2 champ and 5f2 princeton the only difference is the princeton gets a choke PSU (maybe a bit higher B+ with lower buzz) and a tone control.  Both had the same dinky 8" speaker (some champs had 6") and I'd bet they used the same output iron.

 

[syn]

Excellent. Any suggestions for the cabinet-speakers? I was thinking of something along 4x10" lines...
Thanks