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WRyan 21st March 2009 02:02 AM

Guitar Amp Advice
Hello Everyone,

I built a 5F1 champ clone and it sounds good but I think it could sound better. I am looking for ways in which to tweak the circuit in order to get better distortion sound.

The main problem I have with the current sound is that the distortion does not seem that pleasing or smooth… the clean is beautiful, but when pushed, it seems to give a kind of harsh distortion sound, most noticeable with chords. Soloing sounds pretty darn good, but even so, it lacks the dynamics, crunch and definition I imagine it could have.

Since this is my first build, I am wondering if I am doing something wrong, like running the tube at a bad operating point? I assume this is possible. I wouldn’t know how to change it. I just copied the vintage 5F1 circuit and used carbon film resistors, Zoso coupling caps, Sprauge cathode bypass caps, Sprauge Atom filter caps and Hammond transformers. This all goes to a Jensen 10” alnico reissue speaker. I tested with a Les Paul and a Stratocaster.

I notice my plate voltage on the 6V6 is 380 volts. This seems high to me. Is it possible that this is contributing to a harsh sound? I think this voltage is higher than what the actual vintage amps were used to seeing back in the days in which they were made. I know that some people use Variacs to lower voltage… evidently this gives a different, often described as more desirable tone. But this would also lower filament voltage.

I am wondering if there is an easy way for me to lower the plate voltage (and screen too?) in order to simulate that Variac lowered “brown” tone that many speak of. Can I just merely insert a resistor in series with the plate supply?... and screen supply? If so, I am kind of confused about how to go about calculating the value needed to drop say 20 volts at the plate? Would it even make a difference?

I have also considered that the Jensen speaker might be the culprit. Unfortunately, it’s the only one I have right now. I am thinking Celestion or Weber might be better for distortion?

Also, there is barely 20 hours worth of playing time so far – do I need to wait for caps and speaker to settle in?

Any help or advice in regards to any of this would be greatly appreciated.

Thanks !!

mjf 21st March 2009 09:38 AM

can you show a schematic (with voltages drawn in) of the amp you have built?

Geek 21st March 2009 10:21 AM

Hi mjf,


Hi WRyan,

The 5F1 has got to be the ultimate in simplicity... there's just too little to go wrong with the circuit. Assuming you followed the schematic exactly...

1) What is your speaker?
2) What brand tubes are you using? Some are naturally crunchy.
3) Is this from a kit, ready made or full DIY?

380V is a wee high, but it's what I run my 18W designs at and they live.


Robert McLean 21st March 2009 01:50 PM

Just a question , you may already have checked this, but do you have access to another 5F1 to compare the sound to, or have you heard one played recently ? Maybe what you are hearing is just the way that amp performs ...

WRyan 21st March 2009 05:06 PM

2 Attachment(s)
Thanks for the responses!

The speaker is a brand new Jensen 10" 25 watt alnico model P10R Vintage Series.

Tubes are:
1 12AX7A -C5 Ruby Tube
1 JJ Electronic 6V6 Spiral Filiment
1 5Y3GT Sovtek

This is a full DIY, not a kit.

I do not have another 5F1 to compare nor have I ever played one in real life. The closest I've had was a blackface Vibro Champ which I think I remember also being a bit harsh when cranked. It is very possible that I might be expecting this amp to do something it is not supposed to.

The only reason I feel like it ought to be different is because most of the time when I read descriptions of the characteristics of this amp, it is "creamy", "smooth", "lots of 2nd order harmonics"... and this seems to be true at lower volume levels. It is very sweet and has lots of chime, however, the harshness I hear when cranked seems to indicate more 3rd and 5th harmonics (I am definitely guessing on this), which I understand is what contributes to "harshness".

If I can figure out how to calculate and interpret the load line (I am still learning this), than I will be able to see if I am running it in a linear region or if I am way off. But then again, I don't know enough yet to know how changing these operating points affect the actual sound and feel.

This is what got me thinking about lowering the plate and screen voltage just to hear what it sounded like.

I drew in the voltages and posted. I only have a needle type reader, not digital, so the readings are not 100% accurate but they should be close. I really need to get myself a digital meter!

Again, any and all help is greatly appreciated.

Thanks so much guys!!

mashaffer 21st March 2009 05:35 PM

I suppose you could experiment with different values of bias resistor on the second triode which should change the distortion characteristics when it is overdriven. Experimenting with cathode bypass caps might also be interesting.

Just a thought.

VictoriaGuy 21st March 2009 06:04 PM

Great project!
As has been mentioned already, I wouldn't get too worried about load lines and much technical detail. You should be OK with the voltages you have, or you could try changing the dropping resistor(s) to get the 340/295/150 as per the original specs. However, these were +/- quite a few %, so you are definitely 'in the ballpark' where you are.
Quality of components- ie audiophile stuff- is generally wasted in a guitar amp, IMO. Just as long as you have fresh caps that aren't leaky and decent resistors, it should sound good.
Just double-check your wiring to make sure you have it right- sometimes guitar amps will perform with wiring errors or wrong components.
1) Speaker- this can make a big difference in the overdriven sound, but the Jensen should work OK. If you can rig up something with some cable adapters to just play music through your amp (from a tuner/walkman/ipod, etc) you can just put it in a closet somewhere (don't block the back or it will overheat) or lay it face down on a blanket and let it play for a few days. That will get rid of questions about 'break-in'.
2) Adjusting relative position of guitar and amp controls will affect the tone/breakup, as you know.
3) Tubes- You might be happier with a different preamp tube with lower gain- 12AU7,12AY7, etc will give different tone and 'breakup'.Good discussion of preamp tubes in a more complex amp will give you some ideas about possibilities for the V1 position in your amp.


jjman 21st March 2009 06:19 PM

1) Change the rectifier to any NOS and don't use Sovtek. Their 5y3's are not made to correct specs and provide voltage that is high.

2) Add a bypass cap to the 1st stage's cathode, as used on the 5E1 (not 5F1.) I use a 1u to favor the high end since my homebrew was a little dark. Use a 25u to keep the same "EQ" while increasing crunch and compression potential.

3) Increase the cathode resistor on the 6v6. Even with stock voltages, the design idles the 6v6 way too hot. Although the JJs are known to stand up to this abuse. I think I'm using 740ohm.

4) Disconnect the NFB resistor if you still want more crunch. I have mine switched but always leave it off.

Here's mine cranked at various guitar volume settings. It's my favorite amp as far as its tone:

Geek 21st March 2009 10:09 PM

Looks like you're in good hands here :D

You can also try "rolling" V1 for tone... different brands of 12AX7 as well as 5751 and 12AT7 for a little less gain and a different tone.

The OPT can also have an effect on performance.... 8K load is fair smooth and the de-facto standard for 6V6. Or, you could use as low as 6.6K for hard drivin' rock and roll crunch.

Nice speaker choice, BTW :)


Miles Prower 21st March 2009 10:45 PM


Originally posted by WRyan
The only reason I feel like it ought to be different is because most of the time when I read descriptions of the characteristics of this amp, it is "creamy", "smooth", "lots of 2nd order harmonics"... and this seems to be true at lower volume levels. It is very sweet and has lots of chime, however, the harshness I hear when cranked seems to indicate more 3rd and 5th harmonics (I am definitely guessing on this), which I understand is what contributes to "harshness".
This is exactly the type of behaviour I'd expect from that design. It includes gNFB (and it looks like a lot of it: B= 0.064 here) and that always makes clipping behaviour that much worse, and more like the clip behaviour of a solid state amp.

First order of business: make sure that feedback really is negative feedback, if you haven't done so already.

Next, you might want to try tailoring the gNFB to your preferences. My guess is that the way you built it is exactly how it was meant to sound (or very close to it, depending on VT differences).

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