Fender 600 champion reissue complete rebuild

[Mikante]

Ok the filter cap is now mounted.

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Meanwhile i m about to finish the cabinet, it is coming out nicely

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Here the speaker of choice

[martin manning]

I hope this sketch doesn't t look like I think it looks, messy :oops:

Still much harder to follow than a proper schematic, but at least it was obvious that you didn't have a cap in front of the choke.

I do not see a bypass capacitor across the 1k5, or a feedback resistor from the OT secondary (speaker jack) to the junction of the 1k5 and 47 ohm off the pin 8 cathode on the 12AX7 (second gain stage).

The Champ 600 has fully bypassed cathode resistors on both preamp stages (additional 6 dB gain for both), and reduced global negative feedback. I believe those features were included to make up for the insertion loss of the tone controls. If you intended to return to the 5F1 champ configuration, you need to add a 2k2 feedback resistor, delete the 47 ohm (ground the 1k5 directly), and delete the cathode bypass cap on the input stage.

I seems that what you are going for is basically a 5F1 Champ with a sold state rectifier, and an extra L-C filter in front of the plate node. Right now you have a fully bypassed cathode resistor on the input stage, so more drive available going into the second stage. You can use the volume control to dial that down, though.

[Mikante]

Hello,
I decided not to mount the second stage bypass capacitor.
The reason is that without the tone stack, this amp has a lot more gain and distorts much easier. I really like the sound without the fixed eq and i wanted to compensate for that.
There is no negative feedback right now because i didn t have the components. There is a 3 way rotary switch and i will put a 2,2k resistor which is the stock value, a 22k which is the old tweed value and no feedback at all so i can choose. I like the 22k the best but 2,2k works very good for a clean sound as well. I like it bypassed for solos or more heavy stuff.

[Mikante]

Today i have made a full test, tubes and speaker and guitar.
It works like a charm, I m so happy but I don t know how to post a video to let you hear how it sounds.
I m just a little low with the voltage, according to the schematic, first node is about 290v, should be 340.

[sluckey]

How much AC voltage do you measure ***BETWEEN*** the PT red leads at the rectifier diodes?

[martin manning]

I m just a little low with the voltage, according to the schematic, first node is about 290v, should be 340.

That is more than a little low, about 15%. I think making a complete voltage survey would be a good idea.

[Mark]

I have spoken a bit about my Champ 600. This is it.

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[Mikante]

Hello,
Did you convert the amp to a 5e1?
That output transformer is huge
Today i did some test.
I have 146v on both red wires from the pt.
I have 360 before and after the choke.
Then it goes down to 340 after the last dropping resistor.
The test was done without the speaker.
The amp sound very good but i can tell there is a lot more gain, breaks up pretty early and goes nuts at half volume. Lots of distortion even with the 2,2k resistor installed on the negative feedback and i didn t install the bypass cap on the second stage of the preamp tube.
The sound is a little stiff but the speaker is brand new.

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[Mark]

Did you convert the amp to a 5e1?
That output transformer is huge

More or less a 5E1, except the rail voltage is higher at 360VDC. The amp has a stack of bass in a 2X12” cab and sounds good with the 6” speaker. One of the Mercury mods was to have a 250pF cap from the grid of the 6V6 to the cathode. I removed the cap for normal brightness.

In a perfect world I would run with a 12” speaker. However, I’m through messing with it, for the moment.

The Gibson GA-5 resistor values are pretty cool with humbuckers.

Still want to try a pentode in lieu of the 12AX7.

[Mikante]

Hello guys,
I have been playing the amp for a short while now and i can tell i am very pleased. The dynamic and response is incredible if i compare it with the stock version even after my modification.
The overall sound is better and the the handling of bass frequencies is perfect. This is also thanks to the 10 inch speaker against the pour little six inch.
I have added the tone control from the 5f2 design, i din t have a 1 mag pot so i have dropped in a 500k and looking to swap it soon.
I don t know if this is due to the different pot value but the control is usable just for a quarter of a turn and then it gets immediately shrill and looses the bass completely sounding more like a fixed position on a wha pedal.
But now the important part. There are some mid, mid-high frequencies summing up when the amp is turned up and start to distort. It is hard to explain but it is very unpleasant, and it is notable only when your playing by yourself or recording, you need to pay attention but when you notice it, you just can t ignore it anymore. This was happening even before i added the tone control and to be honest is the main reason why i added it in the first place.
How can i fine tune this? What would you look at? The bypass cathode? The coupling cap?
I have made a test with another speaker cabinet that i have built, it is made all of chip board panels and mdf buffle. I wanted to have a cab as sterile sounding as possible for testing, so i built it this way on purpose. Anyway, it din t change a thing, those horrible high mid frequencies were still there so i guess the spruce cabinet is out of the equation.
I would also like to measure the bias of the amp and i was going to use the method i read on the Robrobinette blog. This amp is a cathode biased. What i don t understand is, the method says to check from the center tap of the output transformer which i don t have on this amp.
I do have a virtual center tap after the last tube filaments, but that is the power transformer secondary. I am reading this wrong?

[martin manning]

I have added the tone control from the 5f2 design...

5F2 or 5F2-A? The -A has the 5E3 Tweed Deluxe type.

There are some mid, mid-high frequencies summing up when the amp is turned up and start to distort. It is hard to explain but it is very unpleasant, and it is notable only when your playing by yourself or recording, you need to pay attention but when you notice it, you just can t ignore it anymore. This was happening even before i added the tone control and to be honest is the main reason why i added it in the first place.
How can i fine tune this? What would you look at? The bypass cathode? The coupling cap?

Those are possibilities you can experiment with. By the way, what type of music are you playing?

I would also like to measure the bias of the amp and i was going to use the method i read on the Robrobinette blog. This amp is a cathode biased. What i don t understand is, the method says to check from the center tap of the output transformer which i don t have on this amp.

The equivalent for you is the B+ lead going to the output transformer, or the actual 6V6 anode (pin 3) voltage. You can use either of those for the anode voltage Va. The % dissipation is Va-k/Ia, or approximately (Va-Vk)*(Vk/Rk)/Pa max, which will be ~5% high.

[Mikante]

5f2, i just added i pot and two cap, i din t have to change anything and i really dig it. It is actually all i need as tone control from an amp.
The style i m playing? You almost never hear a clean sound coming out of my amp
Mostly rock, garage rock is my favorite but i actually vary a lot.
Going back the problem, It is a sound that isn t easy to describe, you notice it when you push the amp a little bit and it sums up with the distortion and becomes really annoying.
Sort of a “ish” sound, if that helps.
The amp sound great and i don t know where to begin looking for.
I should record the amp and check with the eq to find out the frequency band where this happens.

[martin manning]

5f2, i just added i pot and two cap...
Going back the problem, It is a sound that isn t easy to describe, you notice it when you push the amp a little bit and it sums up with the distortion and becomes really annoying.
Sort of a “ish” sound, if that helps.

So you have 5F2-A tine control. 5F2 is slightly different, with a 100k feeding the volume pot. Do you have the negative feedback resistor connecting to the junction of a 1k5 and 47 ohm at the cathode of the second 12AX7 stage? If so, a 22k feedback resistor will greatly reduce the NFB, to 1/10 of what the Champion 600 has with the 2k2 and the 47 ohm. Another thing you could try is connecting a cap across the 47 ohm, say 2-10u. It won't do much with the 22k FB resistor, but with the 2k2 you could accent the higher frequencies selectively.

The noise you are hearing could be a parasitic oscillation, which is usually caused by lead routing issues. Did that appear after you added the tone control? That is a common source. If you see a high frequency spike in your recording, that is probably what it is.

[Mikante]

Yeh i connected the 3 way rotary switch, i can choose between 2,2k and 22k or no feedback at all.
Just to be clear, it is not a noise, it is some frequency in the upper mid range summing up when you push the amp and becoming unpleasant.
This was present since day one of this amp.

[Mikante]

Is this tube running too hot?

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If you enlarge the picture you can really see a lot of blue glow.