Merlin VVR for Big amps Trouble on implementing it on a 2204 Amp

[kagliostro]

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



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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

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



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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

[stratele52]

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]


Keep  simple like Vintage amp . Remove this mod please.

[kagliostro]

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

[EL34]

K,
that is a nice schematic of the 2204
Do we have that one in the library?

[kagliostro]

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.........

[EL34]

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?

[kagliostro]

Now I've sent it to you 


Franco

[EL34]

Thanks, I uploaded it

[kagliostro]

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

[EL34]

hold on, I will go check
It may have not gone into the Marshall folder
------------------------------------------------------------
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

[VMS]

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/raw-bias-supplies

I think it has something to do with the original bias supply being high impedance one.

-V

[kagliostro]

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