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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 22, 2017 6:20 am
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Also, check the 220k-ohm grid-to ground resistor, on the 6V6GT. These often drift up in value (sometimes close to 1meg-ohm). The G2G resistor help set the idle bias. So, 500k-ohm reading or more, can increase the potential idle bias --- quite a bit. Drifting G2G also leads to unusual and inconsistent current draw, by the 6V6GT.

Some ppl go to 150 or 180k-ohm (lower the value, the lower the idle bias). Use a 1/2-watt carbon comp, if you have it. Metal oxide ones will alter the tone. I find they brighten the tone, quite a bit.

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Before. Position of 220k-ohm resistor red arrow. Note, how many VC and Champs use a ceramic disc cap for their 0.02mfd coupling cap between the second pre-amp stage and the 6V6GT? (green arrow). Not good for tone. :(

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After. Changed to a 220k-ohm, 1-watt, 1% precise Vishay-Roderstein carbon I had after fixing a CJ audio amp. 5% A-B CC resistor will work fine. A 180k-ohm, 1/2-watt resistor will drop the idle bias about 15-20% --- depending on each 6V6GT tube's specific current draw).

You can try a metal oxide, if you prefer a brighter tone. 5%, 1/2-watt MO will work just fine. And last twice as long as the AB (in other words 100 years, instead of just 50 years of constant use). :lol:

Changed the coupling cap to a 0.02mfd/600VDC Vintage Jupiter mylar cap. This combination makes for a nice vintage tone, with smoother OD than OEM.

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 22, 2017 7:44 pm
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Everything was in spec, so I finally wound up changing the bias resistor from 470 ohms to 680. Got the dissipation down to 15 watts and the plate to stop blushing. As to sound, it seems fine. I'll need to play it a little while to decide if that change matters to me. I typically don't play that loud, and don't really need the amp to break up.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 22, 2017 8:55 pm
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One other thing to try --- change the mfd value of the bypass cap across that, now, 680-ohm resistor. 50mfd/50VDC or even a 100mfd/100VDC cap. Gives a slighter stouter bass tone. Sooner OD.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 22, 2017 10:17 pm
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You could try raising the dropping resistor values as well to get a little less voltage on the 6v6. I had some success w/ that method on my Dlx, just need to monitor the resulting drops to the pre amp stage.
In the end I decided to purchase a "Brown Box" power manager instead of trying to build one. I highly recommend it for controlling input voltage & vintage operating conditions. Gives me piece of mind when I can glance down and see input voltage=115 and amperage draw=.5


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Tue May 23, 2017 7:31 am
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sfceric64 wrote:
You could try raising the dropping resistor values as well to get a little less voltage on the 6v6. I had some success w/ that method on my Dlx, just need to monitor the resulting drops to the pre amp stage.


Eric, there is a bit of a problem lowering anode plate voltage on Champs and VC amps. They are set to cathode-biased, single-ended, Class "A." So, if you lower the plate voltage, the tube will compensate by drawing more current through the cathode-tied resistor. Kind of an auto-bias, up to a point. It's why most 12A(_)7 tubes do not need to be re-biased. They are also set to cathode-biased, Class A, single-ended.

Now, in your Deluxe, changing the plate voltage works because the Deluxe has the 6V6GT tubes set to Class AB, push-pull, fixed bias. You set the idle bias by applying negative voltage to the grids.

Anyhoo... good components and clean layout --- esp, keeping hot resistors away from capacitors, as much as possible --- really helps these small amps. And the use of good 6V6GT tubes.

:)


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Tue May 23, 2017 8:23 pm
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Quote:
So, if you lower the plate voltage, the tube will compensate by drawing more current through the cathode-tied resistor
As w/ most mods, there's always a trade off to contend with; that's why in the end the Brown Box was the right answer for me.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Tue May 23, 2017 8:53 pm
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sfceric64 wrote:
Quote:
So, if you lower the plate voltage, the tube will compensate by drawing more current through the cathode-tied resistor
As w/ most mods, there's always a trade off to contend with; that's why in the end the Brown Box was the right answer for me.


Perhaps true, but we're dealing with a '72 Champ that was produced in the era of higher wall-socket voltages. There's something else at play here, methinks.

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Tue May 23, 2017 10:36 pm
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Retroverbial wrote:

Perhaps true, but we're dealing with a '72 Champ that was produced in the era of higher wall-socket voltages. There's something else at play here, methinks.

Arjay


Short of an exhaustive survey of input/output voltage ratios for a sampling of Champ power transformers, maybe we'll never know what 'normal' is. Plus all the variables involving the tubes, etc... I'm pleased with my solution of having re-biased the output tube.

It's also entirely possible my NOS tubes aren't as awesome as they should be. I'm shipping the 12AX7A back to the seller because it's so microphonic it actually rings when you tap it. It also howled when the volume was turned up... Makes me wonder about the quality of the other tubes I got from him.

Let's remember, we're talking about a Fender Champ. While I'm fond of it (having owned it for most of my life), you just can't be too precious about it.

I played the amp several hours this evening, and it works for me. It sounds pretty decent, considering I'm running a very tired old 7025 in it. Not the slightest bit of plate blushing on the 6V6 now, so I'm comfortable with that. I'm not playing at especially loud volume as I am protective of whats left of my hearing. I have pedals for distortion if I desire that, so I don't need to drive the amp to saturation. Mostly using it with a Trio Plus to play/practice against, so distortion isn't really desirable there.

Now I can concentrate on my next steps.


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Wed May 24, 2017 3:34 am
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Any quality 12AX7 with short plates and a spiral-wound heater element should deliver a S/N ratio similar to that of an OEM 7025. Ditto for a 5751 albeit with slightly reduced gain.

Arjay

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Wed May 24, 2017 7:43 am
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Retroverbial wrote:
Any quality 12AX7 with short plates and a spiral-wound heater element should deliver a S/N ratio similar to that of an OEM 7025. Ditto for a 5751 albeit with slightly reduced gain.

Arjay


Agreed! You really don't need a spiral-wound heater tube, like a mil spec 7025 or 12AX7A, with the Champ or VC amps. Any good, strong testing 12AX7 will work. Should be strong testing, as the amp only uses this one 12AX7 for the gain and tone control sections. There is a method to bypass the tone control --- and get more gain --- ala a 5F1 circuit).

Anyhow, I has some NOS 12AX7, if you are interested. PST me. I'll send a photo of some of my stock.

:)


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Mon May 29, 2017 12:09 am
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Finally found some Hungarian Tungsram ECC83/12AX7. These are nice tubes. Yellow label = 1980s. Yellow and Red label = 1960-70s. Should have number tag on the anode post.

these tubes are not talked about much, these days. Look for them. Usually NOS or near NOS are very strong testing. Big, powerful tone. Clean mids, good bass.

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Tue May 30, 2017 1:48 am
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BMW2002Ti wrote:
...if you lower the plate voltage, the tube will compensate by drawing more current through the cathode-tied resistor...


Not true. I can only refer you to a thing called "Ohm's Law". It's not just a good idea, it's the law that electrons must follow. If you lower the voltage, all else remaining the same, the current will also lower. :idea:

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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Tue May 30, 2017 7:52 am
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Not only that, but I think it's important to keep the output tube in Class A, or near it. As the Champ and VC are single-ended amps. Need near 100% conduction of entire sine wave, right?


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Post subject: Re: '72 Champ Voltage & Bias questions
Posted: Wed May 31, 2017 12:11 am
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Class A simply means that the tube is conducting (pulling current) at all times during every cycle (sine wave), not always at 100%, or else there would be no output, but the current is never zero. Idle will be close to, or above 100%, then the current will vary up and down to produce the output sine wave. But you are correct, class A is what gives these amps their unique tone.

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