Champ based clone, high voltage

[John Fisher]

Hello! I recently built a simple Champ style amp out of things I had lying around.
The power tranny does not have a 5 volt winding for the rectifier tube so I made the rectifier solid state. The voltages are a little higher then needed. About 30 volts higher on the high voltage. The amp sounds very clean and I have to put in up to about 7 or 8 before it will start to overdrive even just a little. I measured the current on the 6V6 and it is about 42ma with about 360 volts into it. Judging by specs that is a little high. Should I just raise up the 470 ohm cathode resistor a little to bring down the current to about 35ma? Will this also make the amp a little warmer sounding? Should I also increase the 1.5k cathode resistors that are on the 12AX7 since there is a higher voltage there too?
Thanks!

[Chuck H]

Maybe try a 510 ohm resistor. 38mA to 40mA is probably your target. Your actually in the park for typical class A operation (which is often as not biased hotter than 100%). If the tube isn't red plating and the amp is working well I wouldn't sweat it. I've seen Vibro Champs with 425Vp that I biased right at 100% and worked fine. 360Vp is a walk in the woods. Also, keep in mind that if current increases the voltage drop across the cathode resistor will also increase. This does serve to regulate bias for this sort of amp. A little hotter than 100% will probably "sound" best if the amp is going to be clipping a lot. Vp is a concearn, of course. But 360Vp isn't at all scary for most modern 6V6's. I did blow up an RCA 6V6 in that Champ though! The EH I replaced it with is working fine after five years of light use.

[John Fisher]

Thanks Chuck! That very helpful! The amp is squeaky clean! Is there a way to give it a bit more of a smooth overdrive when cranked. I does sound good with pedals though.

[John Fisher]

maybe I'm looking for a little more natural tube compression

[EFK]

You can put a big resistor off the rectifier diodes if you want a bit more 'tube' feel, maybe a 15W or 20W at 100 ohms give or take a little. That will add a bit of sag and compression and will bring your voltages down a little also, probably not so useful in a true class A amp but it will at the lease lower your voltages slightly. What size resistor do you have between plate and screen nodes? You can increase that also, which will not only add compression but will also serve to help restrict the current so you may be able to actually lower the value of the cathode resistor, which MY ears (jmho) usually find to be a good thing in a very subtle way, although I'm not smart enough to explain why!!!!

[Chuck H]

you can always drive the power tube grid harder too. Just reduce any voltage division in the signal path. If it's a Champ clone it should OD pretty well. Can you post a schematic and voltages?

[Tom Phillips]

...The amp is squeaky clean! Is there a way to give it a bit more of a smooth overdrive when cranked. I does sound good with pedals though.

What pre-amp & tone stack circuit are you using. If you have a fancy tone stack you may not have the voltage drive to push the output stage sufficiently to attain the overdrive you want. The original Champ didn't have a tone control at all. Later models incorporated a Fender Treble & Bass control tone stack which attenuate the signal. However, the amp still just had the same two triode stages before the 6V6 so, overall, there was less signal available to push the 6V6 into overdrive. If you do have the full tone stack you can try disconnecting the ground at the bottom of the tone stack. That will disable the tone stack an increase the maximum signal available. This is an easy test to do. If you like it then you could add a "tone stack lift" switch.

Can you post a schematic of your build?

[Bruce / Mission Amps]

Right. Something else is wrong....squeaky clean?
A simple Champ implies a tweed 5F1 to me, Vol and two input jacks.
If so, at 360vdc on the power tube plate, dropped at the screen node and divided out through the B+ rail to the preamp, a cathode bypassed 12AX7 ought to be able to drive the 6V6 way past distortion.

[John Fisher]

Ok I'm back! I've been busy the last couple of days but I am finally here. The amp circuits is a Champ 5F1. The amp is very clean sounding but when I get up to about 7 or 8 it will start to distort. I am not so crazy about the distortion as it is a little bit gritty sounding. I want more of the Deluxe sounding distortion. As far as clean sounding it sounds good but I don't like the transition so much of when it starts to clip. The voltage as it comes out of the diodes is about 350 volts (depending on the wall voltage as it varies a bit here throughout the day.) The voltage after the the next 10k drop resistor is 317 volts. The voltage after the next 22Kdrop resistor is 272 volts. The voltage on the plate of the 6V6 is 340 volts. The voltage at the plate of the first stage of the 12AX7 is 179 volts and the voltage at the plate of the second preamp stage is 164 volts. Could it be that using diodes in the power supply is what gives it the sounds it has? Should I ditch the feedback resistor? Boy this is such a simple amp!

[Austin]

I doubt the ss rectifier is having much effect on the sound. Why not lift the feedback so you could easily put it back and see how that sounds?

[John Fisher]

I lifted the feedback resistor and it sounds much louder and better. I am ashamed to say that I discovered that the speaker was crappy. I tried others with much better and different results. Thanks so much for everyone's help. Even the simplest amp can be a learning experience.

[Chuck H]

Still... The speaker won't lower the gain of the amp. When you say that the amp starts to break up at 7 or 8 are we talking about a 0-12 volume control or a 0-10? If your using a single coil pickup with the guitar volume control all the way up you should have breakup starting at about two O'clock on the volume knob. Much lower with humbuckers and a little higher with single coils in parallel. This is with the feedback resistor in place. I mention this because removing the feedback resistor or trying a different speaker may not have fixed the initial propblem even if the amp sounds much better now. Much better to fix the initial problem and then tweak. If you cut your finger off and go to the hospital, they try to reattach your finger. They don't attempt to add a finger on the other side of your thumb and call it good.

[Tom Phillips]

...Much better to fix the initial problem and then tweak...

+1
A VERY important point.