Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 25, 2014, 02:24:33 pm
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Mr. Merlin in one of his books show an intriguing way to mod a Marshall Master Volume (Preamp)
the mod implement some auxiliary functions using a pair of switching input jack and a DPST switch
the circuit operation is quite clear ................., quite ........ exactly ...................
The explanation the author gives are clear, but one point remains obscure
(http://i.imgur.com/FaTXqOU.jpg)
the obsure (to me) point happens when S1 is closed and the anodes paralleled
when the HI input is used only V1a receives an input signal, V1b input results grounded and also V1a output is grounded via R1
and ............. I'm not able to follow the signal from input to output point :w2: :w2: :dontknow: :w2: :w2:
My brain try to tell me that there is something that has to do with the cathodes of V1a & V1b but my mind refuses
to give to this thing a sense
Please can someone explain what is happening ?
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Great confusion (to me) is also the kind of input jack used at the HI input, is it something like this N*J4** ?
(http://i.imgur.com/4K55zYw.jpg)
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May Thanks
K
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It seems to me that when the Hi Input Jack is used AND S1 is Closed:
* The Hi jack is internally double switched. Insertion of the phono plug disconnects the Hi jack from both of its ground connections: at the V1a input & at the bottom of S1.
* Signal enters V1a's grid; then exits V1a's anode.
* From there most of the signal passes up and around the R1 RC circuit. But this RC circuit does attenuate signal a bit - some signal passes through this circuit to ground, through the bottom of S1. But no signal reaches the input of V1b, even though there is a physical connection, because of that short to ground at the bottom of S1.
* The signal continues on to C-0 through the top of S1. The 2 plate resistors lift signal from bleeding to ground through the +HT circuit. So the main signal passes through the top of S1, then downstream; then through C-0 (at the top of V1b). In this way, the output of V1a bypasses V1b.
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I'll take a stab:
You can use the N*J6** as the HI input jack. "R" and "RN" are not used, but "SN" is used. This is the arrow that points to the top of the sleeve in Merlin's diag. When a mono plug is inserted into "HI" that arrow, the switch portion "SN" opens. If "R" and "RN" are present, they are unused.
HI used + LO unused + S1 open:
V1a signal-> to the LO jack via voltage divider R1 & 1M (attenuated to about 66%) -> V1b->Co.
V1a and Vb are cascaded.
HI unused + LO used + S1 open:
signal is applied to V1a and V1b in parallel, but V1a output signal is unused (i.e. doesn't reach Co).
HI unused + LO used + S1 closed:
signal is applied to V1a and V1b in parallel. the bottom half of S1 provides a redundant ground connection to the right side of R1 (HI's SN is still grounded), now the parallel plates and signal have path to ground via the 4.7n + R1 and whatever tone shaping that might apply (470k is high enough that it probably doesn't result in a much of a tone change. if R1=47k then it'd be a "low" tone type switch, but you'd need to lower V1b's Grid leak to 100K instead of 1M to maintain that R1&1M voltage divider network.
HI used + LO unused + S1 closed:
V1a signal direct to Co. via top of S1. Input to V1b is grounded via bottom half of S1.
(take S1 out of the picture) using the switch jacks in this fashion was a trick mesa boogie did early on in their first amps. I don't know if any did it in a guitar amp before then.
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Merlin shows a suggested layout in Fig 4-17. It can use standard jacks, by incorporating the "top" Hi jack switch into S1, as "explained" on p. 112 of "Designing Tube Preamps for Guitar and Bass". As presented it sure is a brain teaser, and could probably be more clear. :think1:
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If you go in the HI jack, and switch to Parallel, you get no output.
I think.
It perhaps should be rigged so the Parallel switch is ignored for HI input. But that probably needs a relay.
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I'm still in confusion :dontknow:
Here is what Mr. Merlin say about that circuit
K
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Maybe this pic will help,,,, or add more confusion :laugh:
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Ciao Steve
few seconds ago I've read the replay of an italian guy who is very expert about audio circuits and ......
Many Thanks your shematic shows in a very simple way to be understand the thing this guy explained me in words,
see a visible explanation is at the tops that I could hope
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Ahem, can I suggest to you to add this explanation to your ScapBook ?
Ciao
Franco
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Maybe this pic will help,,,, A picture is worth a thousand words!