Fender Champion 600 reissue power supply resistors

[Mark]

I have a Champ 600 and have modded it to get it to sound better, these mods being a Weber Signature 6"speaker (it took forever to break in), new Mercury transformers and a power supply choke (which are part of their kit.) I removed the tone control circuit and used Mallory 150's which I thought sounded better than the stock caps.

Largely, I thought the accompanying mods to the transformers weren't up to scratch, the amp sounded great after the mods through a Celestion Blue with the amp cranked but I rarely do this. I wanted the amp to also have a good clean tone too.

The amp uses a 1K choke resistor and a 4.7K preamp supply resistor (replacing the stock 10K). I strongly suspect the 1K choke resistor NEEDS to be a 10K resistor to stop killing the screen grid of the output valve when the amp is cranked (it killed two TAD 6V6's) but for some reason the stock Chinese valve can take the abuse.

Again, I strongly suspect the 4.7K power supply resistor is making the preamp sound brighter with the increased voltage.

Any thoughts on this one?

Thanks.

[Stevem]

What is your output tube plate voltage and current reading at idle and then also both at max wattage?
Any of these Champs could use a dedicated 150 screen resistor if you ask me to ease the load !

To judge the amp for what can be done for better clean tones you need to look at both the preamp output wave forms and the output wave form on a scope if you have such?

Strong output tubes ( other then ones made in China) will die way faster then a weak tube if you have them idling at more then 90 percent

[Mark]

Thanks for your reply Steve. The link has the stock circuit, the power supply is the same including the 470 ohm screen grid resistor. The output transformer has been replaced with a bigger unit.

The amp works okay if the amp is operated clean, however the valves are toast when then amp is cranked.

The rail voltage 366VDC and the dissipated wattage is 12 watts. Yes I have a CRO and signal generator.

https://www.thetubestore.com/lib/thetub ... ematic.pdf

[Stevem]

With 366 volts on the plate of the output if you had a 1 ohm 1% 2 watt bias check resistor to ground off of the tail end of that 470 you should not read more then .034 vdc as this reading due to the 1 ohm resistor translates directly into idle current.

You should also not want anything more then 15 volts of audio drive signal hitting pin 5 of the output tube.

[Stevem]

With 366 volts on the plate of the output if you had a 1 ohm 1% 2 watt bias check resistor to ground off of the tail end of that 470 you should not read more then .034 vdc as this reading due to the 1 ohm resistor translates directly into idle current.

You should also not want anything more then 15 volts of audio drive signal hitting pin 5 of the output tube.

You should test out when the driver tube section starts to clip its output wave form and try to set the amp up so that both the driver and the output tube start to clip at near the same time.

This takes some playing around as the output tube will want to clip first which is fine within reason!

Since that build runs power supply diodes and not a tube I would step up C6 to a 47uf @ 450 volt cap which will help out with the amp staying clean longer when driven hard.

[Mark]

I should point out the amp has a Pi filter after the diodes and before output transformer primary. This keeps the amp pretty quiet. The choke is a Mercury Magnetics one, and the caps are both 22uF. I don't see an issue raising both caps to 47uF.

[Stevem]

Well the schematic you put up that I was going by does not show a choke so how was I to know?

[Mark]

Sorry about the omission. Mind you increasing the main filter caps is still a good idea.

[Mark]

I changed the circuit so it is a 5F1 Champ. The transformer set are the Mercury set which contains a choke to knock out the mains hum, a large output transformer and a Weber Signature 6" speaker.

I was pleased that the amp sounded thicker (with chime from the 6" speaker), and it sounded good when it was distorted which it hadn't before. I doubt the amp with sound as good through a Celestion speaker at volume but it sounds good as it is.