Fender Champ preamplifier only

[Yazoo]

I am looking for a new project and started thinking about a preamp only version of the Fender Champ. I am still hesitant about trying to build a full version because I just don't have the knowledge or self-confidence to do this - properly biasing the power tube and so on. Just doing the preamp would be much simpler and I wouldn't have the expense of buying a full power transformer or output transformer. I could just use cheaper back to back transformers as I did in another tube project.

I already have a couple of power amps so the idea would be to use one of these with it. Do you think I would get that Champ sound if I did this. The tone section is in the preamp or that's the way my thinking goes.

[Rodgre]

I haven't checked schematics yet but I have a feeling that most Fender preamps (Champ, Deluxe, Vibroverb, Bassman, Twin, etc) are probably pretty similar. I would think you would be able to build any one of them similarly as long as you can get the correct voltages to the tube(s) from whatever power supply you're using. My question would be whether the output needs to be treated differently since you're sending it to an output jack (presumably to send to a power amp, but maybe you want to feed other pedals?) than the original circuit. Coupling caps? I'm not sure. Just thinking aloud. I might want to try a similar trick at some point to make a nice direct box preamp for bass in the studio.

Roger

[willienillie]

Which Champ are you talking about?  The 5F1 (tweed) has no tone control.  The preamp is pretty standard.  I don't think you'd get "that Champ sound" with just the preamp.

I say build the whole thing.  It's simpler than most pedals, though more expensive obviously.  The power tube is cathode biased, just like the preamp.  470r 5W resistor, 25µF/50V bypass cap, most 6V6s should bias fine.  Add a screen resistor.

[Steben]

There were different versions of the Champ. tweed, silverface, ....
But most of them (if not all) are a 12AX7 (two simple triode stages) followed by a power tube (6V6).
Some have decoupling caps at the cathode, some not.

[Steben]

I say build the whole thing.  It's simpler than most pedals, though more expensive obviously.

And more dangerous. Just a warning. It is not a no go, I've worked on tube amps. Just be careful.

It is more expensive in the direction of expensive renewal of broken parts as well...

[vigilante397]

Do you think I would get that Champ sound if I did this. The tone section is in the preamp or that's the way my thinking goes.

Just because the tone control lives in the preamp doesn't mean the inherent tone of the circuit comes from there. You may get something vaguely Fender-esque, but getting something that actually sounds like a Champ will depend a lot on what kind of power amp it's connected to.

That being said, why are you looking at the Champ in particular? Does the Champ have some magic to your ear that another amp doesn't have? I just figure if you're going through the effort to make a tube preamp, why settle for a Champ? I build a lot of tube preamp pedals (it's the only thing people even ask me for anymore) and I haven't had a single request for a Champ. Lots for the Alembic F-2B though, which is the Fender Bandmaster input section and which makes an excellent simple build for a very flexible preamp.

[Yazoo]

Thanks for the input everybody. I have already built the Alembic F2-B and it is pretty good. I am thinking about the Champ because of the stories about Eric Clapton and Larry Carlton. Carlton said Steely Dan insisted he use the exactly the same Champ on a previous record and paid for an airline seat for it. The Clapton story is that for 461 Ocean Boulevard, he used a Champ and a Pignose.

I've got time on my hands, still shut in because of Covid, and I thought it would be a good project at a reasonable cost. Too many of my past projects have run away from me in the past, mainly because I tend to get very enthusiastic and obsessed. This time I'm trying to show some self-control!

If I can get a Champ-like tone, that would be close enough.

Basically, if I can take the Champ schematic and cut off before the power amp stage and get a line out level, that would be great.

[Steben]

In all honesty... If you've already built tube amps (a preamp is an amp) I don't see any point in avoiding a power amp. The worst is electrocution and a power amp still draws less power than a short circuited heart stroke. Especially a SE 5W amp such as the Champ. Biasing the thing is easier than thinking of pentode preamps. It is as easy as ... a triode in fact.
The output transformer might scare you but it is just a device that needs the wires to be connected.

[PRR]

> version of the Fender Champ

https://el34world.com/charts/Schematics/files/Fender/Fender_Schematics.htm

There are a lot of "Champs". Significant differences among them.

champion_600_reissue
champ_12
champ_25_se
champ_5c1
champ_5e1
champ_5f1
champ_aa764
champ_cbs
champ_ii
champ_super
vibrochamp_ab764

_I_ suspect much of the "Champ tone" is limited power: you can beat the snot out of the 6V6 and still hear your tone. Also dinky speakers (which can be "BIG" on the recording). Some have wonky overload (5C1 input stage overloads "too" easy").

[Marcos - Munky]

I also say to go for the whole thing. Since you already have some experience with tubes and hi voltage power supply, the next step is to build a tube amp. And the Champ is one of the easiest tube amps to build.

[Yazoo]

Thanks for the advice. I think I would go for the 5F1 Champ. I would still like to try just the preamp, mainly because I could keep the cost down. The Champ power transformer comes in at about £60 and the ouput transformer about £30, whereas a preamp would come in around £50 all in. I can always go the whole hog later if I want to.

So, looking at the 5F1 schematic, can I just cut off the power stage by leaving everything after the .02 coupling capacitor after the second half of the 12ax7?

I realise I won't be able to use the negative feedback from the output transformer, unless there is another way of doing this.

[vigilante397]

Leave the .02u cap on the 12AX7 plate, you're going to need that. What I would do is replace the 200k pull down resistor after said capacitor and replace it with a 250k pot, call that one "master" and you're good to go. Then you just leave out the 6V6 and the output transformer.

[Yazoo]

Great, many thanks.

[willienillie]

One thing to note about the 5F1, the original Fender schematic is missing a 25µF bypass cap across the first stage's cathode resistor.  It's an error on the schematic, the actual amps had the cap.  The circuit sounds quite bland without it.

[Yazoo]

OK, I've built it and it sounds good, but, as usual I've got carried away by the project. I realised there is a 12 volt version of the 6V6, the 12V6, which uses half the heater current of the 6V6 and so would be OK with the back-to-back transformers I am using, so I am going to add this to my build.

The 12V6 only seems to be available as NOS but isn't very expensive, though I've got hit by postage from the US. It still works out at about the same price as a new 6V6. My plate voltage will be a bit lower but I'm hoping this will be OK. The UK is not as well served by sellers for this kind of gear which is frustrating. I ended up ordering the output transformer from Banzai in Berlin. The only place I could find in the UK wanted £47 plus postage. It's actually cheaper from Germany - until full-on Brexit kicks in. 

[Steben]

Lol... it still needs an output transformer... so WHY did you not want to build a power amp? 

[davent]

OK, I've built it and it sounds good, but, as usual I've got carried away by the project. I realised there is a 12 volt version of the 6V6, the 12V6, which uses half the heater current of the 6V6 and so would be OK with the back-to-back transformers I am using, so I am going to add this to my build.

The 12V6 only seems to be available as NOS but isn't very expensive, though I've got hit by postage from the US. It still works out at about the same price as a new 6V6. My plate voltage will be a bit lower but I'm hoping this will be OK. The UK is not as well served by sellers for this kind of gear which is frustrating. I ended up ordering the output transformer from Banzai in Berlin. The only place I could find in the UK wanted £47 plus postage. It's actually cheaper from Germany - until full-on Brexit kicks in. 

Not sure how shipping/taxes from Mouser UK works but they have the Hammond 1760c OT for £36.41 and states free shipping for this item and 53 in stock. I have two of these transformer waiting to go into unstarted builds.

https://www.mouser.co.uk/ProductDetail/Hammond-Manufacturing/1760C?qs=itkJorDwoMJ03yKzOK2DCw==
dave

[anotherjim]

That's a very attractive price for an OT! Put a small amp project into pocket money territory.

[Yazoo]

I have used Mouser before and they are good. The problem is if I get charged import duties. The duties aren't very high but there is a standard £8 handling fee the darlings at HM Revenue and Customs automatically put on top!

As usual, I started this project with the idea of not spending too much. The preamp does sound good and isn't too expensive with the back-to-back transformers which cost about £10 all in. As a result though, I can only get around 300 volts and 12 volts, so it's not possible to use a 6V6, as well as the current draw, probably too much for them. I was also unsure about biasing the 6V6 as I have never done this before.

Once I came across the 12V6 and decided to try it, I read the articles on Rob Robinette's site which gave me the confidence to go ahead. I also found an LTSpice model of the 5F1 which was really useful for confirming some resistor wattage values.

I'm waiting for the parts to come now, so fingers crossed.

[davent]

I have used Mouser before and they are good. The problem is if I get charged import duties. The duties aren't very high but there is a standard £8 handling fee the darlings at HM Revenue and Customs automatically put on top!

As usual, I started this project with the idea of not spending too much. The preamp does sound good and isn't too expensive with the back-to-back transformers which cost about £10 all in. As a result though, I can only get around 300 volts and 12 volts, so it's not possible to use a 6V6, as well as the current draw, probably too much for them. I was also unsure about biasing the 6V6 as I have never done this before.

Once I came across the 12V6 and decided to try it, I read the articles on Rob Robinette's site which gave me the confidence to go ahead. I also found an LTSpice model of the 5F1 which was really useful for confirming some resistor wattage values.

I'm waiting for the parts to come now, so fingers crossed.

Yeah that's what i was wondering about, i'm in Canada, Mouser & Digikey are both dependable and you pay what you see, no surprise ransom charges that arrive with the package to be paid or forfeit the goods.

The tube hifi amp i built years ago called for 12V6's and 12SL7's but my transformer had an adequate 6V winding so built it with the more easily obtained and plentiful '6' versions of the tubes, still going strong.
dave