Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubeswell on October 05, 2014, 05:05:26 pm
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In the act of responding to a post on another forum I suggested tone stack ideas as an adaptation of Merlin's morph control.
Herewith are a couple of untested ideas in this regard.
Feel free to add, subtract or whatever
(Edit - I changed the attachment a couple of times at the start trying to get one of the ideas to stack up better)
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That's a very strange idea. I think you should post a link to "Merlin's morph control" to get us started.
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The morph control on a 5879 and the morph control for a 5879 driven from a 12AU7 CF are in the attachments
(I realise the screen is probably running a bit too hot in this circuit (Ig2 = 4.4mA, whereas total tube current is 2.4mA) - probably should have used 470k Rg2.
Anyhow using these circuit values I worked out the gain with the screen fully bypassed and the gain with the screen unbypassed (to get started):
The 5879 gain fully bypassed is 0.001A/V x 100,000R = 100.
For 5879 gain with unbypassed g2:
triode-strapped ra = 17k, 1a:1g2 = 5:1,
therefore internal screen resistance = 17k x (1+5) = 102k
therefore loss of gain due to impedance at screen grid = 1-[370k/(370k+102k)] = 0.216,
therefore gain with unbypassed screen = 0.001A/V x 100k x .216 = 21.6
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This thing is interesting
As far as I remember Merlin wrote something about a tone control (or tone cut) connected in a similar way
we usually don't see TS following a pentode probably because a TS is a load too high for a pentode
so the use of a CF following a pentode with the purpose to add a TS after it is a must to me
What I don't understand is why, if you put on the circuit after the pentode a CF, you don't connect the TS after the CF ?
There is a particular reason for that ?
Franco
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What I don't understand is why, if you put on the circuit after the pentode a CF, you don't connect the TS after the CF ?
There is a particular reason for that ?
Franco
Yes indeed by all means you could have a TS shaping the frequencies into the following stages, and 99 percent of geetar players would probably want some such tone control.
Nevertheless the ideas at the start of the thread are merely an elaborate (and probably somewhat fanciful) suggestion for shaping the screen feedback in triode mode (which will shape the frequencies at the CF output - when in triode mode), because it is a possibility with this type of circuit. No other reason for suggesting it than that.