Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jeff on April 24, 2011, 07:27:04 pm
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Could someone please explain what's going on with the first tube in this schematic? It looks to me like they are making a resistive divider out of the plate resistor. Why do this? What are the benefits of doing this over using a resistive divider after the coupling cap?
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It's a voltage divider, just another way to skin a cat, personal preference.
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The Weber circuit simply follows the original Fender one. Leo probably figured he could save one resistor by doing it that way. :icon_biggrin:
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http://www.blueguitar.org/new/articles/blue_gtr/amps/bp010629.pdf
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Ryan - thanks for that link. Splitting the cathode follower's cathode resistor is a neat idea. Just this morning, I read about using that approach to attenuate the "send" signal for an effects loop (Merlin's preamp book).
Jeff - in addition to saving a resistor, Leo may have been reducing noise too.
10K/100K => about 9%
To get the same signal attenuation "on top" of the Dwell pot, you would have to use a 2.5 meg resistor. Carbon comp in Leo's day. NOISY.
Just a guess...
Chip
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I suspect something esle is going on. Only because you're not saving a resistor. Either you put an extra resistor in series with the plate, or in series before the volume pot. And if it was in an attempt to aviod using a 2.5M resistor you could use a smaller pot. instead or 2.5M, 250K pot you could use a 250K, 25K pot. 25K were available to fender, they use them for mid pots. Besides we use 3M3's in Fender reverb channels. Maybe it does have to do with the responce of the cap but I would think a different value cap could be selected to make up for this. The difference would be in the next stages grid to ground but I thought in another post the input impedance of the tube is basically infinate and doesn't matter.
Maybe I'm over thinking this again but when I come across something unusual and don't understand I'm curious to know why it was done that way.
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Yes, you're overthinking.
Splitting the plate load to reduce signal to the next stage is ancient technology. Nothing unusual about it. In fact, way back when pots were tougher, it was not uncommon to see a pot used for a plate load, the ultimate adjustable plate load voltage divider.
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I need to tame the OD output in my TOS, its level is WAY higher than the clean channel. Looks like this is a great way to do it. I'm assuming I could apply it to the 5879 pentode stage? And where exactly would the snubbing cap go?