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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Dec 29, 2012 2:18 pm
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Shimmy, Now I remember why you don't need to add SGRs to old SE Champs and VC's...

The tap already is downstream through a 10k-ohm, 1 watt resistor in the power rail (R14). So, the screen will always have voltage below the anode plate. As long as this resistor is intact.

So the 470-ohm, 2 watt resistor is superfluous. IMHO.... :mrgreen:


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Dec 29, 2012 2:36 pm
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I have wondered about that myself, if there was any advantage to having SGRs when the screen supply voltage was already lower than the Plate voltage anyway. :?: For amps that have a choke instead of the dropping resistor for the screen supply, it makes sense to have SGRs. 470 ohm SGRs won't make much, if any, difference in sound. That reminds me of the 10 ohm SGRs in some early Carvin amps, some even had 22 ohm, that were later changed to 470 ohm, even though there was a 350 ohm dropping resistor for the screen supply in all three versions.

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Dec 29, 2012 2:48 pm
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Shimmy, 470-ohm = about 5 volt drop in a 450 VDC line. Marshalls usually run 1k-ohm to get a 10 volt drop, as the "cheaply" made Mullard EL34 (imagine, they were 50¢ each, in 1965) --- weren't as hardy as the venerable RCA 6L6GC.

There is no advantage dropping the screen voltage much more on a good 6V6GT. These tubes were even tougher than the 6L6GC.

Interesting, you brought up chokes. They do have a bit of DCR (low, if they are good ones). Most in PS rails I've used have about 100-200 ohms DCR. Ones, I've seen in the signal tube line ---- after the output tube tap (like Fenders) --- can have as much as 500 ohms (thinner wiring).

One thing that could help keep the 6V6GT happy, in Champs, VC, and this EC VC --- is a good grid stopper. A low inductance 1k-ohm, 1 watt resistor will brown the tone a bit. But, can also stop instabilities, like oscillations --- that prematurely ruin the output tube. I have added this resistor to many Champs and VCs. Esp those that will not be using NOS US made 6V6GT. Or those super tough JJ 6V6S tubes. Seems to add to output tube life.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Fri Aug 08, 2014 4:55 am
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Based on the schematic posted earlier in this thread, it looks like the "low power" switch on the this amp simply changes the output from 4 ohms to 8 ohms. Is that correct? In other forums, people were guessing this low power mode was something that dropped the amp's voltage, but I don't see that in the schematic (I was actually hoping it did so I could possibly use some of those 6F6 and 6K6 tubes I have lying around). Am I correct that it does not drop the voltage?


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Fri Aug 08, 2014 5:57 am
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Image

The High power, or up position bypasses the 4 and 8-ohm resistors. The Middle power or center position has the output going through the 4-ohm resistor before the speaker, dropping the output. The Low power, or bottom position has the output going through an 8-ohm resistor, before the speaker. Dropping the output twice as much as the 4-ohm resistor.

Yes, the impedance seen by the output tranny changes with each switch position. But, more importantly, with a 25-watt resistor the output is cut down via the resistor changing much of the output to heat --- before the speaker "sees" the output.


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Fri Aug 08, 2014 11:55 am
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Thanks for your help in interpreting it!


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Sep 27, 2014 5:48 am
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Thanks for that, clear as mud… having said that I might have a go at building one!


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Sep 27, 2014 9:18 am
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BMW2002Ti wrote:
...Yes, the impedance seen by the output tranny changes with each switch position...


More evidence that changing the output load impedance on a tube amp is OK. (i.e., different speaker impedance than spec won't blow up a tube amp) :wink:

Note that the load is always either 4 ohms or 8 ohms total, only varying how much of the signal goes to the speaker (full, 1/2, or 1/4).

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jul 13, 2019 1:28 pm
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So... Can I assume the 2nd 12ax7 is for vibrato and not tone? Right now have a rca black plate 6v6, and an RCA 5y3..in rectifier, and for now a tad 7025 for #1 and a basic 12ax7 in 2nd position, loving the tone, but does the 2nd pos preamp affect to nat all?
Thanks for your help
Lessdrop


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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jul 13, 2019 2:14 pm
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Lessdrop wrote:
So... Can I assume the 2nd 12ax7 is for vibrato and not tone?


Yes. V2A functions as an oscillator for the tremolo and V2B buffers the signal and provides the depth control.

Arjay

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Post subject: Re: EC Vibro-Champ problem and bias question
Posted: Sat Jul 13, 2019 3:05 pm
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Thanks for the quick reply! I'm always searching for the tone, tube swapping, till sometimes I get lucky and find it!!! Nice to learn what go where? What, me worry? @


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