question about fender champ amp.

[fossilshark]

Ive been designing an amp I am going to start building very soon. Im combining a high gain Marshall preamp with the power amplifier of the fender champ amp. I drew up the final circuit combining the two and I noticed something was a little off with the output transformer. why is it set up like that it looks like one side of the primary is on tube output and the other side is on v+. also, is 450v too much? ive seen some schematics where that was used and it looked about right. please let me know if theres any other problems i missed with this circuit. im a bit of a beginner to tubes (im used to jfets). cheers!

[pompeiisneaks]

It depends on the tubes themselves.  Some manufacturers give them a big more power ratings than others, but the one I'm seeing has about 315 as the max for a 6V6.  Also you'd want to make sure whatever preamp tubes you're using can handle the voltages you expect at those points too.  The schematic you show has a 12AX7 as the first tube, but at my initial glance I don't see what the others are?

I would look at the datasheet for the exact 6V6's you expect to use to be sure, but common operating ranges seem to be between 180 and 315 v for the 6V6.

[pompeiisneaks]

Oh I also noticed that none of your preamp tubes have a cathode bypass cap.  That's common in drive stages but otherwise not normal for most cases.  You should look at potentially adding something in the 4 gain stages that is more clean, just to help overall.  I see 4 stages in that amp and it could get pretty heavy otherwise and drive too hard for pretty ugly preamp distortion.  That being said, it also can't hurt to experiment with it and see what you get.  There also seems to be a missing coupling cap between U3 and U4 which will let DC bleed into that stage which I'm pretty sure you don't want.  Also in tube schematics usually tubes (valves) are indicated with V# not U# but I don't know how important that is (i.e. V1-V5 instead of U1-U5 in your schematic). 

I'm looking it over in general as well, just to see if I see anything else.

~Phil

[pompeiisneaks]

Noted a few more things:  The cathode bypass cap on the output tube is .25u which is quite small for a bypass cap, the champ schematic you link has 25uF not .25, and that's a lot more in the normal range.

Youv'e got the filter caps at 450u which is REALLY high, normally most amps will have maybe 50 or 100uF for big voltages, but for smaller maybe something akin to 16 or so.  The champ you show uses 16 and then 8 uF caps at 450V and then a third at 8 again.  This is at the A, B and C points of the power rail.  I also see you have a cap, a resistor and a cap again, at 50uF later in the chain, usually you only need a filter cap at each stage taht provides more ripple control.  You may want to stick to the 16,8,8 for each of the stages instead.

~Phil

[PRR]

Why do you start with 450V, then drop through a _10K_ resistor?

The 6V6 with usual Champ loading acts like 5K-10K DC load. So your 450V will drop to 225V or 150V!

Ah, that 10K is the Screen Dropper. The Plate feed should *not* be dropped with more than a few hundred ohms.

The big Marshalls you copied do start from 450V, but with power stages 10 times more powerful. While they may put 400V-350V on the low level stages, they will probably work fine with less.

Use standard Champ Replacement PT and OT. The voltage you get will be correct for guitar 6V6 directly. Use about 10K to drop and filter that for small stages. Later Champs used smaller screen droppers.

I'm unsure about putting guitar input to U1 plate node. If you found that on a Marshall, OK.
_____________

Marshall "generally" used small or no cathode caps. Fender wanted full and lush, Marshall defined the early Who scream. So that is a reasonable choice. And fully open to tweaking later.

[pompeiisneaks]

Marshall "generally" used small or no cathode caps. Fender wanted full and lush, Marshall defined the early Who scream. So that is a reasonable choice. And fully open to tweaking later.

Ahh outstanding to learn, I don't yet know a ton about marshalls.  I do recall in some amp schematic's I'd seen the drive tube usually left that out to give it more bite, but that makes sense that the marshalls have that really nice crunch. 

~Phil

[fossilshark]

so theres no problems with the output transformers in my full schematic? also the triodes are half of 12ax7s and the pentode is a 6v6gt. my CAD program was designed for solid state haha

[drgonzonm]

so theres no problems with the output transformers in my full schematic? also the triodes are half of 12ax7s and the pentode is a 6v6gt. my CAD program was designed for solid state haha

If you can print the various tube components from jschem as pdfs then import into your cad program.

I have a question regarding the amp design.  I noticed that U4(v2b), is a cathode follower which feeds the FMV tone stack.  Will there be enough signal from the tone stack to drive the 6V6? 

[mresistor]

  Will there be enough signal from the tone stack to drive the 6V6?

It might work if he uses a 12AU7 .  But his preamp resembles a Marshall 2204 circuit which uses a cathode follower as the last stage in the preamp, but that drives a PI and then the output tubes. So I would also like to know if this arrangement is going to work as drawn up feeding the input to a 6V6 off the tone stack. There is no degenerative feedback, so maybe. If a 12AX7 doesn't do it then a 12AU7 might.

[HotBluePlates]

2 or 3 gain stages plus a cathode follower should be enough to smack a 6V6 silly (2 gain stages and a tone stack are plenty to push a 6V6 to distortion in a blackface Champ).

However, it might be worth investigating the circuit back-to-front to verify nothing restricts the circuit from driving the output tube when volume controls are set sufficiently-high.

[mresistor]

Thanks HPB,  maybe I should have said it might work "better" to use an AU .  But  the Champ uses that signal off the plate of the final preamp stage, not off the cathode. The signal off the cathode is going to be much smaller right?

[HotBluePlates]

...  But  the Champ uses that signal off the plate of the final preamp stage, not off the cathode. The signal off the cathode is going to be much smaller right?

Gain of a 12AX7 cathode follower will be something like 0.98 times whatever is on V3's plate. If there is a 20v at V3's plate, the cathode output of V4 is ~19.6v. So, hardly enough to make a difference.

And the tone stack will be driven by a cathode instead of a plate, so there will be less loss in the tone circuit.

And there's an extra gain stage between the input jack and the other 2 plate-loaded gain stages.

So overall, this will be much higher gain before the impact of the interstage voltage dividers is accounted for.

so theres no problems with the output transformers in my full schematic? ...

The power supply rail is drawn incorrectly, but the OT is not necessarily drawn wrong.

Original Champ might have had the B+ feed into the OT and the screen attached to the same point. That's a non-issue unless you feel like adding an extra stage of filtering.

[PRR]

> Will there be enough signal from the tone stack to drive the 6V6?

A good question.

It is not just "gain", gain is cheap.

The 6V6 needs 15V-25V of signal to make all the 5 Watts you paid for. If the tone-stack has 10:1 loss, the tone driver must output 150V-250V of signal, which is unlikely to be pretty. Even at a "most knobs mostly up" loss of 4:1 we need 60V-100V of signal. Triode driver with 300V supply is straining to make 60V signal.

The old guys would never do that. It's like a 400HP engine, but a pedal that only goes half-way, never gives all the HP you paid for.

This plan IS widely used on new-age guitar amps. They mostly break-up in the tone driver, the 6V6 hardly gets into its overload action. It means you can dial a "same" sound at 3 Watts or at 0.3 Watts. This may be more familiar to silicon-amp players where all "distortion" is in small stages and a master volume dials the acoustic level. (But even these normally let you ask for "100W" out of a 50W final.)

[fossilshark]

if i remember correctly i did base my design off of the marshall 2204 preamp. the tone stack was included in the preamp schematic. if it does not work then ill hit the output of the preamp with an oscilloscope to make sure its putting out the necessary power to drive the 6v6.