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Post subject: Hybrid fixed and cathode biased power tubes
Posted: Fri Jan 21, 2011 11:24 pm
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Anyone here try adding cathode bias to a fixed bias Fender amp? In order to try and "brown" the tone. And maybe get a bit of earlier and slower onset of overdrive?

I was thinking of adding a cathode resistor and bypass cap to a Princeton (yeah, still mulling over "tweeding" the amp to Tremolux (5E9-A), but keeping the original bias supply powered vibrato/tremolo circuit of the Princeton (AA-964).

This mod is described by O'Connor, but I was wondering if both cathode and fixed bias circuits could be active at the same time. And not use a switch, as O'Connor seems to described. Is this just a pipe dream?

Thanks, for any reply.


O'Connor's description:

http://www.londonpower.com/cath-fix.htm



Tremolux (5E9-A):

http://www.ampwares.com/schematics/tremolux_5e9a.pdf


Princeton (AA-964):

http://www.ampwares.com/schematics/princeton_aa964.pdf


Last edited by BMW2002Ti on Sat Jan 22, 2011 12:10 pm, edited 1 time in total.

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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Sat Jan 22, 2011 7:10 am
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Honestly I don't see why you would want to "brown" the tone of a Princeton.
I own a '62 Brown Princeton and it's not far from the tone of a tweed,I can't say it's better than what you have ....but you may prefer it,I just think there's plenty more amps out there to get the tone than modding that one.
It's your amp and money though,I don't know about the technical mods,someone else will have to figure if that would work.


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Sat Jan 22, 2011 12:09 pm
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Just asked to see if anyone tried this on any Fender amp. I have several stock BF Princeton Reverbs. And a basket case SF Princeton that I was thinking about experimenting with this idea.

Anyhow, thanks for your thoughts.


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Mon Jan 24, 2011 5:08 pm
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I never do that but I fix and built tubes amp ( not from kit) . This is to say that it work. It's easy to understand for me.
You have to disconect your negative bias circuit and put a Kathode resistor on power tube. Some modern tube amps have those switch at the rear to have those two bias mod


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Mon Jan 24, 2011 6:06 pm
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Check out the Twin Reverb AA568. That was a combination of both bias methods, used at the same time. Art

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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Mon Jan 24, 2011 6:13 pm
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Problem is, the vibrato of a AA964 Prninceton is fed via the bias power supply onto the power tubes. So to keep the vibrato and not have to rewire the circuit, I'd need to run a hybrid for both fixed and cathode biased power section.

Blencowe, in his new book describes this fix around and states it's works pretty good with the hybrid. See the links:

I'm going to try this mod, running a 500-800 ohm shared cathode resistor from the 6V6GT's. I'll try no bypass cap, then go 25mfd/50VDC, to 50/50, to 100/100 and listen to the results.

I do have one other question. O'Connor in explaining this hybrid drew an example schemo. He included a 1-ohm, 1/2 watt resistor tied to each leg of the 6V6GT cathode, prior to shared 500-ohm (if 6L6GC) or 800-ohm, 10-watt shared resistor. Does anyone know the function of these 1/2-watt resistors?

Thanks, for your comments!


http://www.londonpower.com/GRAPHICS/pwr-fig1.gif

O'Connor's article:

http://www.londonpower.com/cath-fix.htm



Princeton (AA964):

http://www.schematics.ca/modules/PDdown ... tle=Fender Princeton schematic aa964



Valve Wizard (Blencowe):

http://www.freewebs.com/valvewizard1/pp.html


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 2:43 am
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The 1 ohms resistor is only use to mesure bias at each tube. This is an easy method to know the bias of a tube with volmeter each side of this 1 ohms.

Ohms law ; 0n 1 ohm resistor , current ( MilliAmp) and voltage are the same. With voltmeter if you read 30 millivolts, your tube draw 30 milliamp.
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Vibrato, good question, I don't know if you can keep it working? I have to look at the schematic.

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Aclempoppy: I read about this AA568 that this combination is not great for a Fender amp.

Better to use one of both at a time, Or with switch, you choose class A or AB


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 2:49 am
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I look at he shematic ( london power/ Graphic ), the tremolo is disconnected when you are in Kathode bias ,

I think there no ( easy ) way to have this tremolo working whith this mod.


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 3:38 am
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Stratele, I know that a 1-ohm resistor is often used to measure voltage drop from cathode to ground on a fixed-biased amp. In order is measure idle bias. But, O'Connor has these resistors inline with the 10-watt shared cathode resistor when amp is being cathode-biased.

Won't these 1/2-watt resistors get fried when amp is in cathode-biased mode? And if not, why use a large 10-watt shared cathode resistor?

http://www.londonpower.com/GRAPHICS/pwr-fig1.gif

Article:

http://www.londonpower.com/cath-fix.htm

Thanks!


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 4:06 am
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Art,

Thanks, for the tip on the AC568 Bandmaster circuit. It has some kinda strange mods over usual cathode biasing. Independent cathode resistors (which would need to be matched) and a shared bypass cap ACROSS the two cathodes.

I never knew Fender ever made a hybrid biased amp.

http://www.ampwares.com/schematics/bandmaster_ac568.pdf

Another question (OT): Why does Fender place the filter choke in it PS AFTER the first set of filter cap stage. Is this basically to filter only the screen grids, gain stage, & PI and not the PT & main power tube's anode? Which I guess would require a much bigger choke (like 10-20H, rated at several hundred mA)?

Anyone ever try to use a large choke right after a solid-state. or tube rectifier? This is suppose to help reduce cold start cathode stripping of the tube rectifier and give it better transients (supposedly). I think most ppl I've talked to run a 100-220 nfd non-polar, high voltage cap from B+ to ground just before the choke to snub diode hash.

Much thanks, again!


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 4:12 am
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These 1 ohms is for mesuring bias and the 10 watts is for biasing the kathodes. Two differents use.

These 1 ohms can be use in any kind of amp bias

It look that you don't know much about tube amp. Is it safe that you do this mod yourself ? There are lethal voltage in tubes amps you know.


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 4:18 am
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Stratele, Are you telling me that 1/2-watt passes through these measuring resistors and 10-watts passes through the cathode resistor, when the amp is switched into cathode-biased mode? Look at the diagram, when amp is swtiched onto cathode-bias mode.

I understand how this works in fixed-biased, Class AB1 where one tube is "on" while the other is at "idle." But, in cathode-biased mode usually the amp is set to run close to Class "A" where both tubes are running hotter at idle.

Please, there is no need to insult me, if you can't answer the question.


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 4:32 am
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Sorry BMW I don't want to insult you but I give you the answer


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 4:41 am
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Stratele, the answer is simple. I had to look through some books to find it. One-ohm resistance is so low that very little voltage drop thus little heat is generated (thus you don't need a big wattage resistors). In fact, it can be an effective fuse if the power tube goes ballistic. And really starts sucking current.

The cathode tied resistor is much higher resistance and thus generated a greater voltage drop (and sets up the idle bias) and more heat. Thus the bigger wattage resistor is needed. Esp if the amp is set to Class "A" where both tubes are "on" a good percentage of the time.

The only cavaet to the one-ohm resistor is it must be precise (1%), if you are going to use it to measure volt drop and thus idle bias current. That's why some older amps use 10-ohm resistors, as the Volt-Ohm Meters were not as accurate at the time the amps were made.

If you had said that, I'd would have understood.

Thanks!


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Post subject: Re: Hybrid fixed and cathode biased power tubes
Posted: Tue Jan 25, 2011 4:56 am
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You are right BMW.

But about what you said :

".......That's why some older amps use 10-ohm resistors, as the Volt-Ohm Meters were not as accurate at the time the amps were made."

I never see that or heard about that


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