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Old 26th September 2007, 11:46 PM   #1
PRNDL is offline PRNDL  United States
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Default Octal Champ works - pls check design

I've finished the Octal Champ and it sounds pretty good. It's a little thick, which I think is from the 6SL7 preamp tubes. It begins to distort at about 3, which is great for guitar.

The voltages are:
A = 351, B = 279

Preamp 1: anode = 159, cathode = 1.8
Preamp 2: anode = 134, cathode = 1.76
Output: anode = 338, cathode = 20

20/390 is about 50 mA (if my math is right), which is a bit high for the 6F6.

I could swap the 6F6 with a 6V6, since they have similar pins.

Any comments/suggestions/recommendation are greatly appreciated!
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Old 27th September 2007, 09:41 PM   #2
kevinkr is offline kevinkr  United States
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I'm not intimately familiar with the original champ design, but one area I would experiment with would be powering the 6SL7 pre-amp and driver circuitry separately from the screen grid of the output tube. Variations in screen current are going to modulate the plate supply to the first couple of stages and could result in instability when the amplifier is pushed hard.

Just something to think about, and it might not even be a problem. I would try a separate rc filter to feed the pre-amp circuitry fed from point A. (The stock configuration might even be a virtue - how about a switch to select either?)

One other thought might be to substitute the standard two pot fender tone stack - a lot more flexible. To calculate the ideal values use the Duncan tone stack design software or lift the values from another Fender amp you like. (Marshall too.)

You can increase the cathode bias resistor slightly and reduce the cathode current if you use 6F6.

Other than that it looks pretty good. I would try both the 6F6 and the 6V6 and decide which you like better. Even early 6L6G/6L6GB might be worth experimenting with in this amplifier.

How about some pictures.
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Old 27th September 2007, 11:03 PM   #3
Trout is offline Trout  United States
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Typically the Cathode resistor in the Champ is around 470-500 ohm and bypassed with a 25uf/50V electrolytic.

@ 390 ohm I can understand the higher current draw even with a 6F6.

I would think about bumping that up a bit for tube life.


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Old 28th September 2007, 01:52 PM   #4
gerryc is offline gerryc  United States
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Default Re: Octal Champ works - pls check design

Quote:
Originally posted by PRNDL
I've finished the Octal Champ and it sounds pretty good. It's a little thick, which I think is from the 6SL7 preamp tubes. It begins to distort at about 3, which is great for guitar.

The voltages are:
A = 351, B = 279

Preamp 1: anode = 159, cathode = 1.8
Preamp 2: anode = 134, cathode = 1.76
Output: anode = 338, cathode = 20

20/390 is about 50 mA (if my math is right), which is a bit high for the 6F6.

I could swap the 6F6 with a 6V6, since they have similar pins.

Any comments/suggestions/recommendation are greatly appreciated!
Looks like you finally got your amp up and running! What do you mean by "thick"? Is it too bassy, or too much in the lower midrange? It could also possibly be related to the distortion level in the output stage since there is no feedback in your circuit. Two things you could try: 1) reducing the coupling cap C2 to 0.022 uf to reduce the bass response, 2) trying some modest feedback like Fender put into the later Champs, like the 5F1 version here:

http://www.kbapps.com/audio/schemati.../champ5f1.html

You'll just have to ground one side of the OT secondary and add a resistor from the ungrounded side to the top of your R11. Bigger feedback resistor means less feedback, a smaller one means more. Don't be afraid to experiment. Just remember that unless you have info on the OT to know the relative primary to secondary polarity, you might have to swap the leads if the amp starts to oscillate or howl (positive feedback). You can always put a switch to turn the feedback on or off. Other amps have this as well.
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Old 28th September 2007, 02:27 PM   #5
PRNDL is offline PRNDL  United States
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Thanks everyone!

I've made some upgrades, including a second input with a 10K grid resistor for hi gain.

I also swapped out the .068 yellow Pyramid cap for a .047 white cap that looks similar to the white Goodall .022 without the name on it. The Pyramid came in the amp. The others were left over from another old amp.

The change improved the tone. It's smoother and less "thick", which was the bass. A .022F would most likely sound best.

What I'd like to do is a comparison with Mallory's vs. the Russian Paper in oil, and Teflon caps that I asked about in another post.

The amp has a huge choke, so I'm not so worried about separating the preamp from the power tube grid supply.

The original amp came with a 390 resistor for the output tube bias. I left it in, but thought I'd ask since 50mA is a little bit higher than the 35mA normally used in 6V6's. It's a 6F6 in a metal can, which may mean that it's heavy duty and can take it.

Today I plan on making a head for it.
Maybe tonight I'll post some photos and sound clips.

It's got that smooth tube tone and distorts nicely at about 5-7. It's not super high gain, but it does sound cool.
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Old 28th September 2007, 03:16 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by PRNDL
Thanks everyone!


The original amp came with a 390 resistor for the output tube bias. I left it in, but thought I'd ask since 50mA is a little bit higher than the 35mA normally used in 6V6's. It's a 6F6 in a metal can, which may mean that it's heavy duty and can take it.

The early metal versions quite often had lower dissipation ratings than later glass versions, and unfortunately you can't see the plate starting to glow if the dissipation is excessive - I would definitely drop the idle current a bit to be on the safe side.

I'm quite sure I understand your supply choke comment - are you saying that the pre-amp circuitry is not powered off of the same point in the supply as the output tube screen grid or something else?

Sounds like you are getting closer to where you want to be with this amp though..
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Old 28th September 2007, 03:32 PM   #7
PRNDL is offline PRNDL  United States
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>> 'm quite sure I understand your supply choke comment - are you saying that the pre-amp circuitry is not powered off of the same point in the supply as the output tube screen grid or something else?

You are correct. It's off the same point, which is not optimal. I based the design off of a Champ.

The 5E1 Champ has a choke, but doesn't connect anything to the grid bias.

The 5F1 Champ doesn't have a choke, but connects the output tube anode directly to the rectifier tube DC out.

I assume the 5F1 is more susceptible to DC ripple (hum), but is more reactive to power supply sag.

I could try removing the grid bias and see how it sounds. I believe that's triode connected, which should lower the output power and make it distort a little sooner, which might be nice.

Thanks again ... it's nice to bounce ideas off people that know what they're doing.
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Old 28th September 2007, 05:19 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by PRNDL
>> 'm quite sure I understand your supply choke comment - are you saying that the pre-amp circuitry is not powered off of the same point in the supply as the output tube screen grid or something else?

You are correct. It's off the same point, which is not optimal. I based the design off of a Champ.

The 5E1 Champ has a choke, but doesn't connect anything to the grid bias.

The 5F1 Champ doesn't have a choke, but connects the output tube anode directly to the rectifier tube DC out.

I assume the 5F1 is more susceptible to DC ripple (hum), but is more reactive to power supply sag.

I could try removing the grid bias and see how it sounds. I believe that's triode connected, which should lower the output power and make it distort a little sooner, which might be nice.

Thanks again ... it's nice to bounce ideas off people that know what they're doing.

Hi PRNDL,
I'm flattered, but my typing at least leaves something to be desired.. LOL I meant to say I didn't quite understand what you meant..
Triode connection should be interesting, but might not give you the crunch and clipping behavior needed for a typical guitar amplifier, but it might give rise to some other interesting and desirable sounds.. A switch to change modes might be good. (I've seen some custom amps with this feature so there is precedent.. )

The other thing you might want to try is powering the pre-amplifier circuitry from its own rc decoupling network so that screen current variations don't affect the supply voltage to the pre-amp and driver stages. Again a switch to select between the two possibilities might or might not give you something worth thinking about..
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Old 3rd October 2007, 12:25 AM   #9
PRNDL is offline PRNDL  United States
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Default Photos available

I've uploaded photos of the Octal Champ.

A chassis pic is on my web site
http://www.naturdoctor.com/Chapters/...ctalChamp.html

Click the image to open in full size.
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Old 3rd October 2007, 03:54 AM   #10
kevinkr is offline kevinkr  United States
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Hi PRNDL,
That's one cool looking head, I also checked out the chassis at the link you provided. It sounds like a lot of fun, now if I only knew how to play guitar...
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