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PostPosted: Sat Nov 06, 2021 10:02 pm 
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Hi All,

I think this is a quick question. I'm building the Champlifier (with the 6L6) and I'm looking around at tips on the Internet on making this amp as quiet as possible (like everyone else). I looked a Mike Marsh's site and he has a tip to run 2)100 ohm resistors to the ends of the pilot light tangs and run the other ends to ground. (Virtual center tap) I've seen this setup in other Champ kits. According to Mike this isn't necessary if the transformer has a filament supply center tap. The question is does the ClassicTone transformer have the filament supply center tap? Is there a way to tell in case I run into this scenario again. On the schematic or by looking at the transformer? Also, 1 or 2 amp slo blow fuse? Still learning.

Thanks,
Tubeamp


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PostPosted: Sun Nov 07, 2021 6:07 am 
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All the Champs I’ve built have had center taps for the filaments. On the other hand I’ve built several AX84.com “High-Octane” amps that use the virtual center tap for the filaments. I have never been completely happy with this method of lowering the amp’s noise level. This led me to search for a better solution. That solution was to elevate the the heater voltage using a voltage divider circuit. Below is the schematic for the circuit I found that I have tweaked the values of for the HO amp. To get rid of the noise I replaced the components with the following values. The 221k resistor was replaced with a 270k, the 56.2k with a 27k, the two 100Ω resistors were replaced with a 250Ω / 5W potentiometer and the capacitor was replaced with a 47uf. I placed the voltage divider across the 4th (C4) filter cap (200V in the H.O.). The 270k connects to B+ and the cap and 27k resistor are connected to ground. The wiper of the pot gets connected to the junction between the two resistors. In my H.O. build this elevated the ground to 18.5V. With the pot dialed to 125Ω on each side of the wiper the noise level was reduced significantly. The difference between having the voltage divider and removing it is night and day. The noise level was lowered so much that no dialing in of the pot was needed. In fact, you could probably get away with keeping the two 100Ω resistors and grounding them thru the voltage divider.

I use 2A Slo-Blo fuses in my Champ builds.

H.O. Schematic
https://ax84.com/archive/ax84.com/stati ... 101004.pdf

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PostPosted: Sun Nov 07, 2021 8:41 am 
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Curious to know what value resistor you’re using on the 6L6’s cathode. I’ve built a series of 6V6 based Champs recently and have found that the 470Ω value is way too small. A 470Ω resistor results in a plate dissipation of about 18 watts to 19 watts when it should only be 12W to 14W depending on the tube used. I’ve found that using a 600Ω resistor instead works much better.

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PostPosted: Sun Nov 07, 2021 4:14 pm 
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HI Stephen,

Thanks for the info! I've been pretty much going with Keith's instructions ( so I was going to go with the 470 ohm.) I haven't assembled anything from the kit yet, just checking around before I get into it. My understanding was I could use a 6V6 or a 6L6. (basically I was looking for a louder clean sound in a small package with the 6L6) Now that you have brought this to my attention I'll start looking at specific tubes (6L6 style) and their plate dissipation specs. I hadn't seen the voltage divider circuit before used in this manner and I will give it a try. Waiting on the speaker and cabinet so I have research time. I was also going to change the .022 caps to .01 to cut down on "flubby bass". I've built the Tweed Royal and I like to run it in Champ mode (clean) with my Tele. If I crank it too much it tends to be bass heavy. I was hoping to get a similar sound a little louder in a smaller package for practices and gigs where I'm mic'ing things.

Thanks! :D


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PostPosted: Sun Nov 07, 2021 9:26 pm 
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tubeamp wrote:
...The question is does the ClassicTone transformer have the filament supply center tap? Is there a way to tell in case I run into this scenario again. On the schematic or by looking at the transformer?

Yes, the ClassicTone power transformer has a center tap on the 6.3V filament heater secondary. Refer to the schematic here: http://byocelectronics.com/champlifierschematic.pdf Assuming that the wire color scheme stays the same, it's a green/yellow wire. ClassicTone went out of business a year ago (see THIS THREAD), so I don't know if that will change when Keith sources a replacement.

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PostPosted: Sun Nov 07, 2021 10:13 pm 
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Thank you duhvoodooman! Trying to keep learning.


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PostPosted: Mon Nov 22, 2021 1:18 pm 
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Stephen wrote:
Curious to know what value resistor you’re using on the 6L6’s cathode. I’ve built a series of 6V6 based Champs recently and have found that the 470Ω value is way too small. A 470Ω resistor results in a plate dissipation of about 18 watts to 19 watts when it should only be 12W to 14W depending on the tube used. I’ve found that using a 600Ω resistor instead works much better.


Hi Stephen,

Sorry it so long to respond. I'm learning and needed to do a little research. I've built the Tweed Royal and I just took voltage readings and wanted to run that by you. So I've built the amp based on the directions. I had a little extra noise so I followed Morgan's alternate grounding scheme (and adding additional shielding) otherwise it was built to spec. With the amp in Champ mode (SS) and both the tone and cut knobs off and volumes at 0, I see a plate voltage of 310VDC using JJ6V6-S tube(s). Based on Rob Robinette's calculator Cathode Bias (I'm pretty sure the Champ is cathode biased.) This amp is using 470 ohm 5 watt resistor.

85% Cool = 38.4 DC milliamps 95% Average =42.9 100 % Max Safe Dissipation =45.2

I'm seeing 42.3 milliamps which falls in the average range or close enough. This shows a plate dissipation of 14 watts.
Are you measuring the actual plate dissipation in your Champ builds or using a calculator? Let me know if I'm calculating things correctly. If I'm doing this wrong let me know what I need to do to get the correct info.

I just got a text that my Champ cabinet is on the way so I'll be building the Champlifier starting tonight. Once I get this up and running I can give you more of an apples to apples comparison. I plan on trying it with both the JJ6V6-S tubes and the JJ6L6GC tubes and I can let you know how things are measuring and sounding.

Thank you!


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PostPosted: Tue Nov 23, 2021 5:40 am 
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I took measurements (plate & cathode voltages and actual cathode resistor value) and then confirmed my findings using a bias calculator. Did you subtract the cathode voltage from the plate voltage when you plugged in values for the bias calculator?

Here’s a video where the 470Ω cathode resistor had 52ma going thru it which is way too much (around the 3:35 point). He replaces the resistor with a 1kΩ that drops the current down to 34ma. A 600Ω resistor (598 actual ohms) worked best for me. This gave me a 90% dissipation.

https://www.youtube.com/watch?v=mjZjvDYN-Og&t=507s

Also take note of the voltage rating of your cathode bypass cap. I see many schematics that spec a 25V cap here. A higher voltage cap should be used here instead. This is also mentioned in the video.

What does your wall voltage measure at? Mine is typically 120 to 125 VAC.


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PostPosted: Wed Nov 24, 2021 12:18 am 
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I'm on the right track now. Thanks for the assistance! I'll measure all the voltages and make sure I subtract the cathode voltage. (I didn't do that which is why the numbers looked good in the calculator). I didn't open the amp and used a bias probe which didn't get me the cathode voltage. I'll also get an accurate value of the 470 ohm (10%) resistor.

The D-Lab video was really informative and I found another Champ based video from D-Lab, that explains testing the voltages and a little more about tracing the Champ circuit.

https://www.youtube.com/watch?v=GV_rxDdf4iM

I'm putting the Champlifier together now and I can test using my Tweed Royal (in Champ mode) to see how that looks while the Champlifier is under construction. It appears that I'll have to change the 470 ohm resistor and the 25uf (25v) cap in both amps. Then I'll be able to give you an answer to what resistor I'm going to use when I put in the JJ6L6GC in the Champlifier.

I tested my home wall voltage and I was getting 124.6VAC at the bench.

Thanks for all the info!!! I'm learning a lot and I'll let you know how things are going.

Happy Thanksgiving to All!!


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PostPosted: Wed Nov 24, 2021 10:44 pm 
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Got the info:

Plate voltage: 330-333
Cathode voltage: 20
Resistor actual value: 466 ohm.

So, I figured out a bunch of stuff. So if I figured this correctly 330 - 20 = 310. If I do the calc. manually 20 / 466 = .043
This what I got out of the calculator, but I was looking at the cathode biased specs. The Champ is fixed Class A, correct? That means I'm running in excess of 90% dissipation.

From the calculator:

Class A Fixed Bias 70% Cool = 31.6 DC milliamps 80% Average = 36.1 90% Max Safe Dissipation = 40.6

So I'm running hotter than I should be on the Tweed Royal in Champ Mode. How do I calculate/estimate what I should replace the 470 ohm 5 watt resistor with? The amp sounds perfect with vintage style single coils, but distorts a little earlier than I'd like with humbuckers. I just can't use the higher sensitivity inputs.

Ran thru the same process for the amp in push pull Deluxe mode.

Plate voltage: 298
Cathode voltage: 16
Resistor actual value: 466 ohm.
298 - 16 = 282. If I do the calc. manually 16 / 466 = .0343

Class AB Fixed Bias 50% Cool = 24.9 DC milliamps 60% Average = 29.9 70% Max Safe Dissipation = 34.9*

Let me know if I went thru this properly. Again the amp sounds good, maybe it could sound better. The way the amp is designed to be both a Champ style and a Deluxe style amp this may be close enough. The Champlifier I'm building now can be tuned more specifically because it is only running Class A with a 6L6GC.

Also, I appears the Tube Amp Depot Bias Scout doesn't measure plate voltage, but the actual plate to cathode voltage based on taking the raw measurements. I can go thru the Bias Scout build and figure it out.

Thanks!


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PostPosted: Thu Nov 25, 2021 8:55 am 
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My understanding is that the max. dissipation for cathode-biased Class A amps like the Champlifier is 100% of their output tube rating, and the 90% guideline is for fixed bias Class A amps. Your result works out to 95%, which seems very reasonable.

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PostPosted: Thu Nov 25, 2021 9:48 am 
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When will these be back in stock?


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PostPosted: Fri Nov 26, 2021 11:06 pm 
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Thank you duhvoodooman! On to the Champlifier with a 6L6GC and see how that sounds.(I'll be checking the plate dissipation on that one for sure) I'm hoping for a light, easy to carry, good sounding practice amp.


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