Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on May 15, 2014, 03:29:56 am
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A friend is encountering difficulties on implementing the Merlin's VVR with automatic bias regulation on a 2204 Amp
the circuit is attached here, the 220pf capacitor in red on the schematic is a correction that Merlin applied as to prevent oscillations
the 2204 schematic with indications about VVR connection is here
(http://s30.postimg.org/680m4ittt/schema_jcm_vvr.jpg)
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the problem encountered is that if the VVR bias circuit section is connected to the amp (see on the 2204 schematic the point labeled in green as "1 bias grezzo") at the bias section output of the VVR circuit there are only few mV
at AX84 there was a tread dedicated to this VVR and other guy had same problem
http://ax84.com/drums/bbs/dm.php?thread=480811 (http://ax84.com/drums/bbs/dm.php?thread=480811)
The solution of the problem was to change the original bias circuit of the amplifier that is being modified
adopting a full wave rectify (adding a second diode on the bias circuit) and lowering the drop resistor that in the 2204 bias circuit is 120k
something like this
(https://www.circuitlab.com/circuit/4432j5/screenshot/540x405/)
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in the attached pdf file I put an abridgement of the AX84 tread
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So the question is not about how to solve the problem, but how this happen ?
I'm not so skilled to understand by myself why is necessary to modify the existing bias circuit as to permit that the VVR bias circuit section do his job
I will be grateful with who can clarify this, for me, complicated thing
Many Thanks
K
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A friend is encountering difficulties on implementing the Merlin's VVR with automatic bias regulation on a 2204 Amp
Many Thanks
[size=78%]Why build such a complicated circuit to replace a bias that works well ?[/size] :BangHead:
Keep simple like Vintage amp . Remove this mod please.
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Keep simple like Vintage amp . Remove this mod please.
Err, NO, that VVR isn't an unuseful complication
it offers the possibility to implement a VVR on fixed bias amp giving an automatic regulation of the bias voltage in accordance with B+ voltage drop
if you want to enjoy your 2204 cranked at a low volume level, this, for me, is the best solution
I don't like attenuators and if you use often your amp at home, using a VVR increases the power tubes life respect the use of an attenuator
Ciao
K
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K,
that is a nice schematic of the 2204
Do we have that one in the library?
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Do we have that one in the library?
Err, I don't know, I'll give a look and if there isn't I'll send you a version without those green writes
Ciao
Franco
p.s.: I verified that it is missing, I've sent it at info2.........
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No problem, I was just asking because you send me so many nice schematics
I thought maybe you would already know if you had sent me this one?
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Now I've sent it to you :icon_biggrin:
Franco
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Thanks, I uploaded it
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Doug
Seems you uploaded it in a wrong way
it isn't between the Marshall schematics
the added schematic has a dedicaded button on the pricipal page
and I think you don't want that
Ciao
Franco
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hold on, I will go check
It may have not gone into the Marshall folder
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yes, that is what happened, it did not load into the marshall folder
I fixed it and it should be good now
Thanks for the heads up
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So the question is not about how to solve the problem, but how this happen ?
I'm not so skilled to understand by myself why is necessary to modify the existing bias circuit as to permit that the VVR bias circuit section do his job
I will be grateful with who can clarify this, for me, complicated thing
Many Thanks
K
Hi K.
Maybe these links will help you:
http://www.londonpower.com/bias-supply (http://www.londonpower.com/bias-supply)
http://www.londonpower.com/raw-bias-supplies (http://www.londonpower.com/raw-bias-supplies)
I think it has something to do with the original bias supply being high impedance one.
-V
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Many Thanks VMS
I remembered the London Power Raw Bias supply, but I didn't remembered exactly the purpose of that circuit
reading the explanations you linked was useful to remember and put focus attention on certain aspects about this thing
Thanks
K