Help me with my design of tube amp with added gimmics

[Learner]

PSpice voodoo!????

Hi guys,
I am trying to build a tube amp with my own added gimmics, this is the result of the simulation.

I've found flaws from this simulator previously, such as running a DC voltage into a step up transformer at the primary but got step up DC at the secondary output. This has made me a bit weary about trusting the simulation result and I was wondering if you guys could check it out and advise me if this would actually work with a pentode? or even a tetrode as shown in the simulation?

I would probably wire the supressor straight to the cathode and screen as shown in the tetrode below....

**broken link removed**

 

[Borber]

Re: PSpice voodoo!????

I am not shure your gimmics are OK. Pentode first grid current is about +800µA because of positive Ug1-c voltage of about 1.5V. This is may be possible because of very low Ug2-c voltage. Output voltage and frequency characteristics are OK but harmonic distortion is about 2%.

 

[Learner]

Re: PSpice voodoo!????

Any advise on what I can do to minimise the harmonic distortion? Also, don't you think its bizzare to have the R6 there? I mean, would this circuit actually work in real life!????

 

[Borber]

Re: PSpice voodoo!????

Working with positive grid voltage is more than unusual. I am not shure I understand why you need current generators in cathode circuit for bias. If you know how much of gain you need you can play with anode resistor and negative feedback (unbypassed resistor) in cathode circuit. This will lower distortion.

 

[Learner]

Re: PSpice voodoo!????

Hi Borber,
I've got rid of the R6 but it doesnt seem to make much difference except have more gain.

What I am trying to do is to turn this into a VCA and use the op amp + terminal to control the current therefore the current generator at the cathode, at the moment it doesnt seem to be working very well due to the distortion....

Since I can't change the plate resistor because it is already set for max plate current but it seem to improve a bit more in the simulation when I lower the resistance, I 've played around with the Q1 & Q2 shunting resistor but it doesnt seem to help much. Any other thoughts?

Would this be as good as it will ever get for this circuit?

BTW, do you think this circuit will actually work as simulated!???

Thanks Borber!

**broken link removed**

 

[Borber]

Re: PSpice voodoo!????

Distortion is not caused by generators in cathode circuit but the tube nonlinearity is causing it. If you satisfied with lower gain (23dB max) insert resistor between cathode and C-E connection of Q1-Q2. Do not bypass this resistor with 500uF cap. Distortion will be about 1.3%.

 

[Learner]

Re: PSpice voodoo!????

Hi Borber,
If I place a resistor at the cathode that'll defeat the purpose of the Q1&Q2 current source, since the current will just bypass Q1&Q2 and go straight to the cathode via the resistor if the resistor value is low.....

 

[Borber]

Re: PSpice voodoo!????

I am not shure you understand properly. Do as follows:

 

[Learner]

Re: PSpice voodoo!????

Now I see what you mean, thanks Borber!

Do you think this circuit will actually work as predicted in PSipice in realife???

 

[Borber]

Re: PSpice voodoo!????

Basically it will work but not exactly as simulated.