High Voltage Solid State input stages (tube equivalent)

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bajaman
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Post by bajaman »

Okay - not strictly a ready to build project but something i have been working on recently with good results.
This is the first schematic and analysis in a series of designs I will upload and share, and where better to start than the input stage of the classic Marshall 1959 / 1987 brilliant channel.
The graph shows the responses of the BAJA solid state (green) and the Marshall vacuum tube (red) when driven with a 1 volt peak to peak sine wave input signal. The left vertical axis is in decibels.
The responses are almost identical as you will see, however when driven by a higher signal (6 volts p to p) the transistor stage clips symetrically whereas the tube stage does not clip on the bottom of the waveform until at least well over 200v p to p ! For most input stage designs though we are seldom going to input 1 volt p to p from a guitar pickup so the design functions very well as an equivalent high voltage input stage and consequently saves using a vacuum tube in this position.
I do hope you all enjoy this first installment :wink:
bajaman
BAJA solid state high voltage Marshall 1959 brilliant input stage 070414.gif
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Post by Philster »

Cool! Are you going to work through the tone stack and PI too? Look forward to seeing more,
Philly :)

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Chugs
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Post by Chugs »

That is really cool! :applause: Very interesting.

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Post by Duckman »

Great work, Baja!
Wich is the actual input impedance? Those Vr 15K resistors confuse me :oops:

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Post by bajaman »

Wich is the actual input impedance? Those Vr 15K resistors confuse me :oops:
They (15k resistors) are both effectively in parallel with each other (7.5k), assuming a low impedance power supply, so the total input impedance is 1M PLUS 7.5k at most frequencies - the impedance gradually rises below 10Hz due to the increasing reactance of the 100n input capacitor, but at frequencies that we are interested in hearing this capacitor is effectively transparent or zero ohms impedance :wink:
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bajaman
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Post by bajaman »

Here is the Fender input stage :wink:
BAJA solid state high voltage Fender Twin input stage 080414.gif
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Post by DrNomis »

Really interesting Bajaman, I'm assuming that to build these circuits you need semiconductor components with suitable voltage-ratings....... :thumbsup
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Post by atreidesheir »

Thanks Steve,
I need to use some of those 2sk117.
Lots of potential here.
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bajaman
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Post by bajaman »

You do not need 300v DC - any power supply rail voltage from 200v DC will work just as well - I have built these versions using 220v DC power supply rail and using standard polyester or MKT capacitors and low wattage metal film resistors with no problems. The 22uf electrolytic capacitors can be 16v types although 25v is perhaps a better choice. The 10uf electrolytic in the Jfet bias supply rail needs to be at least a 50v DC rating though. With the component values shown the 2SK117 biases up at 12v DC to 15v DC on it's gate, depending on HT supply voltage. However, it is important to use a high voltage output coupling capacitor because there is well over 100v DC on the collector of the BF422 gain stage :!:
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Post by bajaman »

incidentally - this is a very low noise circuit :thumbsup
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Post by bajaman »

and now - the VOX AC30 normal channel input stage.
Strictly speaking the Vox AC30 runs off a much lower supply rail - typically around 260v DC :wink:
I have kept all the simulations at 300v DC normalised with a 1v AC peak to peak signal to allow easier comparison gain wise etc.
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bajaman
BAJA solid state high voltage Vox AC30 normal channel input stage 080414.gif
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Post by bajaman »

#4
The VOX AC30 brilliant channel input stage
BAJA solid state high voltage Vox AC30 brilliant channel input stage 080414.gif
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Post by bajaman »

#5
The Soldano SLO input stage
BAJA solid state high voltage Soldano SLO input stage 080414.gif
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Post by Mbas974 »

Outstanding job here !
I love the preamp used as stomps !!

Did you ever consider put a diode on gate-source of first Fet like the AMT Legend series design to better simulate the tube ?
...using this software should be easy to see the delta :-)

Need construction tips for PCB design ; Is the BF422 is getting hot ?
Any suggested charge pump circuit to use ?

Many thanks in advance.

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Post by bajaman »

#6
The Dumble ODS high plate input stage
BAJA solid state high voltage Dumble ODS input stage 090414.gif
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Post by bajaman »

......and now for something a little different :wink:
The Marshall 1959 "plexi" phase inverter driver stage - I have left the presence control off for clarity and once again the circuit is normalised at 300v DC and signal input of 1v peak to peak.
I will show how to achieve a similar presence and resonance boost in a future post - stay tuned. :)
cheers
bajaman
BAJA solid state high voltage Marshall 1959 phase inverter drive stage 090414.gif
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Post by HENK »

Great!!! :horsey:
More Vox-ish,please?!

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Post by hgamal »

I would like to see some non linear response comparison between the two circuits. I will be very surprised if the two circuits present the same signal distortion!

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bajaman
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Post by bajaman »

I would like to see some non linear response comparison between the two circuits. I will be very surprised if the two circuits present the same signal distortion!
Of course they won't, BUT, they are not designed to be overdriven :wink: They are designed to replace input and phase splitter stages which are not required to be driven into distortion.
:!:
As CLEAN gain stages they all function to all intents and purposes exactly the same as their vacuum tube equivalents - you would be very hard pressed to hear any difference between these and their tube equivalents in Real World input and phase inverter output tube driver stages. It is the stages AFTER these that are driven into non-linear distortion in real amplifiers and this is where the vacuum tube excels at producing the BEST non-linear distortion in my opinion. The phase inverter design will accept at least 3 volts peak to peak before going into it's non-linear area of operation - the output tubes will be well and truly saturated in distortion LONG before this input level of signal is reached :wink:
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bajaman
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Post by bajaman »

......and now for something completely different :wink:
The Marshall 1959 "plexi" phase inverter driver stage including the presence control :wink: - once again the circuit is normalised at 300v DC and signal input of 1v peak to peak.
The first graph shows the comparitive responses of the tube and solid state stages at minimum presence and the bottom graph shows the responses at maximum presence.
As in previous graphs, the red line is the tube response and the green line is the solid state equivalent.
cheers
bajaman
BAJA solid state high voltage Marshall 1959 phase inverter drive stage 110414.gif
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