Fender Champ PT

[basschops1528]

Thought I'd just make a new topic. So for this 5F1 Champ project I need a little advice on which PT to use. I'm going with the 771 OT and using a 5Y3: I'm not sure whether or not to pair it with the 759 or 125P1B. The Mojo has a lower HT @ 290-0-290 while the 125P1B is @ 325-0-325. The 125P1B can supply 70mA @ 6-18 watts (Beefy!) while the 759 delivers only slightly more B+ current at 75mA @ 5 watts, but I believe the voltage is what I should be concerned about most if I want an early breakup for a Clapton/Blues sound. I have no idea what the voltage drop on this JJ 5Y3 is, however. Could someone maybe explain how voltage and current come into play here and help me decide which PT would be best for my needs?

[Paul1453]

I am under the impression that a lower B+ voltage leads to an earlier breakup of the output tube.
Gathered from some of the "Brown sound" threads and anecdotal evidence.   

Here's a snip from the 6V6 data sheet.

I don't know where people get this information for higher voltages on the 6V6.

You can see that at a mere 250V the 6V6 pulls more current and wants a lower plate load.
These are the operating parameters that I would expect an earlier breakup from.

Going to the highest listed 315V we get 1 more Watt of output, with more distortion, lower current usage, and a significantly higher plate load cited.

You seem to be getting hooked on this amp building stuff like some of the rest of us.
Maybe it's time for you to make a simple breadboard?

A simple 2 tube circuit is very easy to assemble and test different settings on a breadboard.
For these simple lower voltage lower current circuits you don't need any fancy bench power supply.
You might be able to get by with a cheap 6.3VAC heater transformer,
and then something like a 12VDC wall wart, and a cheap Chinese step up board like I have used.
I got my Step up boards for less than $8 delivered, and you probably have a 12VDC power transformer for some other little gadget laying around already.

I forgot to add the pic.

[labb]

The original Champ had 340 vdc out of the rectifier to the first cap. 150 vdc on the anodes of the preamp tube.

[mresistor]

The Mojo 759 does have a 3 amp rating on the 6.3 filaments, that's a plus, and the lower voltage might just be what you're looking for.  ;-)  the JJ 5Y3 has a voltage drop mirroring the NOS 5Y3's and its appx 60 volts.
I think I would go with the mojo PT and look at the Mercury OT's.

[Paul1453]

A simple little breadboard set-up allows you to try out a circuit.
You get to hear for yourself how things sound when you change voltage levels, bias resistors, caps, etc.

It can be simple and inexpensive to construct.
Here are a couple of items I recommend if you are interested in trying it out.

Terminal strips.  These cost less than $1 each delivered and make life simple.
I use two of these for the power supply.  1 for a ground bus, and 1 for the resistors.
You just screw your E-caps - leads to the ground bus.  Screw down your resistors and the E-caps + lead to the other strip and your done.
Want to change a voltage level?  Unscrew a resistor and pop a different value in there.  A 1 minute job.

Then you can use 2 more strips for your circuit components.
Want to change your cathode bias?  Another simple resistor swap.
You want to enhance or cut bass levels, screw a different value coupling cap in there.
These make it easy to experiment and really get a better understanding of your circuit.

Then a DC-DC boost board.  For less than $8 I had to give one of these a try.
I was pleasantly surprised, and just ordered some more.
With a 12VDC wall wart you can dial up nearly 400V for your B+.
This can give you an idea about which real PT you want to use in your build.
Or you can try these things out on the BB, and then just mount them in a housing like I did on this one.

These parts make experimenting with a circuit quick and easy.   

[basschops1528]

Thanks mresistor, I think I'm gonna go with the 759. Paul I'm gonna print out your last post and possibly start looking for BB parts. Thanks for the great insight!

[basschops1528]

I may have made a couple mistakes, correct me where needed. But this is what I did in Duncan's PS builder:

[sluckey]

R3 is should be much higher than 7K.

[basschops1528]

R3 would represent V1A/B plates, correct? I'm going to re-do the circuit just for the power tube.

[basschops1528]

Here's the update. I can't figure out how to apply all the appropriate loads to the circuit when fully built. It seems you can only apply a load at the last node, in this case, "C+", the preamp tube. On another note, when estimating current/power, do you take into account the plate resistance (from tube chart) and cathode resistance?

[Paul1453]

With PSU start using current taps.

Take the current values, idle and max, from the data sheets.

Then set a delay for max current draw.

Something like this.

[pompeiisneaks]

Oh cool, I wanted to know how that worked.  Question on I1 and I2 you have two listed current values, why?  Is that an operating range or what?  The others only have one. 

~Phil

[Paul1453]

Simple circuits are best, IMO.

I keep being drawn back to that simple 6L6 shooter amp for just noodling around in the basement.

An EF86 feeding a 6L6 at 250V, with a tiny 6V6/6AQ5/EL84 SE OT.
A little SE OT with a 2.7K load at 8 ohms and about 6.5Watts output.
That circuit really sounds good to me.
Check out that thread if you are interested in more details.   

I know you could build that circuit with new parts for around $100.   

[mresistor]

Paul - is that green line the in-rush current/voltage spike?

[Paul1453]

Oh cool, I wanted to know how that worked.  Question on I1 and I2 you have two listed current values, why?  Is that an operating range or what?  The others only have one. 

~Phil

Those are the idle and max current values, and max current kicks in after the delay you set.

The preamp tubes don't change their current requirements much, so only 1 value for them.

Dummyload, clued me in to using those features.   

[Paul1453]

Paul - is that green line the in-rush current/voltage spike?

Yes, that lets you know if you are overstressing your e-caps.
I used 450V e-caps so it's cool, 400V e-caps might be a problem.

My first 2 e-caps are 450V, the 10uF are 400V.
I think you could get by with just the 1st e-cap being 450V.   

[PRR]

> how to apply all the appropriate loads

That's as pointless as applying all the loads to a bridge design. Get the "heavy" loads: the road-bed and a solid line of heavy trucks. Get an idea where you come out for stress and beam size. Add a little fudge to account for pedestrians, hand-rails, lamps.... no real accuracy is warranted for that.

If you have a Power amp, you have 1 or 2 or 4 plates sucking big current, screen(s) current may be non-negligible, and little-tube current which is small like a dog-pack on the Brooklyn Bridge. Round-up the Power-plate(s) current and design the supply for that.

In a "Champ", the 6V6 load should indeed be in the area of 7K. With G2 and two triodes, maybe 6.3K. And you never know "exactly" in advance. Just run the sim for 7K and 6K, see if either one gives horrible answers (really shouldn't).

Once you find the main B+ is, say, 356V (more or less), you can easily figure the small-tube B+ node(s) as resistor dividers. Figure most simple triode stages as 2X to 3X the plate resistor. The standard 12AX7 stage with 100K+1.5K will act like a 330K resistor.

[Paul1453]

Simple circuits are best, IMO.

I keep being drawn back to that simple 6L6 shooter amp for just noodling around in the basement.

An EF86 feeding a 6L6 at 250V, with a tiny 6V6/6AQ5/EL84 SE OT.
A little SE OT with a 2.7K load at 8 ohms and about 6.5Watts output.
That circuit really sounds good to me.
Check out that thread if you are interested in more details.   

I know you could build that circuit with new parts for around $100.   

basschops

This 6L6 shooter circuit might not be what you are looking for to get early breakup.
It really doesn't start to breakup until 3/4 or more volume setting.
The thing is, it responds to an aggressive pick attack with much more power than a 6V6.
If you put your favorite distortion pedal on it's input, it might be just what the Doctor ordered.   

I did experiment some with a simple 12AX7 feeding an EL34 at 250V that breaks up much earlier.
I used the same tiny SE OT on it.  The EL34 puts out around 9 Watts at 250V.
But the tiny OT didn't seem to mind, and didn't burn up or get hot.
I'm still experimenting with the EL34 circuit, considering adding another 12AX7 and a tone stack.
One problem with the EL34 is that it uses 1.5A for heater current, almost double that of the 6L6.
I used a 12VDC 2.5A PS to power the heaters and the boost board with no problem on each.
You use a dropping resistor to get 6.3VDC to power the heaters, and a parallel 12VDC connection to the boost board.
With the DC heaters the 6L6 circuit is absolutely silent with the volume maxed and no input.
Just bump a string and you will realize it is still on with the volume maxed.
I got my PS brick out of the trash at work.   

We all make mistakes sometimes, and I killed a used heavy metal iron core PT before. 
It is much easier to accept burning up an $8 board than a $68 PT if you do make a mistake.   

[basschops1528]

I think this may be what I'm looking for and I might be using the software correctly. What do you guys think?

[sluckey]

Much better. Still need a current tap at the C2 node for screen current.

[basschops1528]

Sluckey, is this the right current for the screens?

[sluckey]

I don't know if that 1mA value is correct, but that's the correct way to represent whatever the current value is.

[basschops1528]

i wasn't sure. PRR said the screens are pretty negligible so I defaulted 1mA. Would I use the screen voltage / screen resistor value?

[sluckey]

You sure are obsessing a lot about a little champ PT. Why not just buy one from Doug. I'm pretty sure it will work just fine.

[Paul1453]

i wasn't sure. PRR said the screens are pretty negligible so I defaulted 1mA. Would I use the screen voltage / screen resistor value?

This PSU simulator will get you in the ballpark, or identify a real problem.

I use it to give me a good starting point when modifying/adjusting PS voltage levels for my vintage parts.
I've always had to make minor adjustments to it when I've actually powered up my circuit and parts.
It has never suggested values that put my vintage parts in danger though.

It usually gives me very close results for the output tubes plates and screens when I use current taps and DS current values.
It's the other nodes that I end up tweaking in my build.   

[Paul1453]

i wasn't sure. PRR said the screens are pretty negligible so I defaulted 1mA. Would I use the screen voltage / screen resistor value?

Read PRR's statement a little closer.

Screen current may be negligible on a small tube SE build.
What does the DS show for max screen current for the 6V6 in Class A?

On bigger bottle PP, or Quad output 100W, builds screen current definitely needs to be factored.

You are already in the ballpark with what you have done.
Fine tuning can really only be done with real parts in actual operating conditions.   

[PRR]

> screens are pretty negligible

Read the tube data-sheet.

It may not have an exact-match to your conditions, but it will be a fair ballpark.

Do agree you can just buy the parts and use typical values and get a fine amp without brain strain.

[basschops1528]

Sluckey, I'm learning a lot along the way. Even a simple build like the Champ can teach me a lot.
PRR , I ended up going with the mojo759. At 290-0-290 I think itll be perfect.
Paul, thanks for the help with the software as well

[sluckey]

Sluckey, I'm learning a lot along the way. Even a simple build like the Champ can teach me a lot.

I was too focused on the "which PT for a champ" part and kinda forgot about the learning part. Sorry. That PS simulator is a good tool and the simple champ is a good amp to learn how to use it. You will get even more out of it once you have the amp built and can plug in some real world measured current values. Hopefully the sim will support your actual measured values.

[basschops1528]

Hello all, been a while!

Finally working on that little Fender Champ. BTW this guy Jose sells inexpensive (compared to the big guys online) unfinished cabinets from California on Ebay.

If anyone is interested I will get the name.

Anyway, I couldn't remember which thread it was in or who said it, but one of the mods suggested putting two diodes across the tube rectifier in case it failed short/open but

I didn't write down the configuration on my schematic. Does anyone have a drawing handy that I could reference? And will the diodes affect the slow ramp-up of the PS section

by essentially bypassing the tube recto?

[sluckey]

Maybe this...

[Paul1453]

With the diodes installed as Sluckey diagrammed the rectifier tube is in series with the diodes.

That means the HV will not pass through the rectifier tube until it warms up, just like normal.

Now if you made a jumper socket and installed that instead of the tube rectifier, you would have a SS rectifier.

A SS rectifier immediately passes the HV without the warm up time.

It is also more efficient at rectification.  Approximately 1.4x vs 1.2x the AC input supplied.

This boosts the HV by 20% above the tube rectifiers output.  It also eliminates the voltage dropped by the tube rectifier.

For example let's say you are getting +250Vdc from a 5Y3 tube rectifier.

With the protection diodes installed and a jumper socket inserted,

you will get +300Vdc from the SS rectifier and maybe +25Vdc more from eliminating the tube rectifiers voltage drop. 

So around +325Vdc instead of the +250Vdc from the tube.  Which is there immediately instead of ramping up as the tube warms up.

I don't think anyone mentioned using a jumper socket instead of the tube rectifier to boost the B+

This will change the bias of a cathode biased output circuit and could possibly push the output tube too hard.

So if you do decide to make a jumper socket, understand the implications of that and be careful with it!

[basschops1528]

Thanks Paul for the clarification. I realized the configuration the diodes were in this morning.

However I'm "really concerned" about the extra voltage drop of the diodes in series with the tube.

I think it will significantly affect the tone. 

[Paul1453]

approximately .7Vdc drop for a diode is nothing!