Adding Diode Clipping

[Amled87]

So, previous posts I've got alot of useful information and help building a few different JCM 800 style preamps (one is a 2204 preamp following the schematic, the other has some modifications including lowering cathode resistors all down to 820R, adjusting the voltage dividers to not over "overdrive" each following stage and adding "snubbers from plate to cathode to handle any high end oscillating issues I was having.

What I'm working on now is adding a few different diode arrangements however, I'm having issues on where I should put them. In the Jose style circuit, the master is moved pre-tone stack with the diodes connected to the master via 10k resistor and 22nF cap.

My master is post-tonestack and adding the diodes to the master causes the treble pot to directly interfere with the diodes because of the resistance from the treble pot.

I'd love some tips of where I should add the diodes (two 20v zeners) via switch if anyone cares to help!

I've posted the schematic of the 2204 I used for my build.

 

[abbey road d enfer]

I'd love some tips of where I should add the diodes (two 20v zeners) via switch if anyone cares to help!

There are very few places in this amp where the voltages approaches 20V peak, basically only the output stage, and it would be extremely dangerous to put those zeners there. It would also reduce considerably the output power.
As a rule, any clipping device should be put before a volume control.
In a 2204, it should be put around the second tube. You may experiment with different zener values.

 

[merlin]

Add an extra coupling cap and put the diodes right before the tone stack. Always better to have clipping before EQ.

 

[Amled87]

Add an extra coupling cap and put the diodes right before the tone stack. Always better to have clipping before EQ.

There are very few places in this amp where the voltages approaches 20V peak, basically only the output stage, and it would be extremely dangerous to put those zeners there. It would also reduce considerably the output power.
As a rule, any clipping device should be put before a volume control.
In a 2204, it should be put around the second tube. You may experiment with different zener values.

I mean be a little green here, but wouldn't placing a coupling cap in this position block the DC being fed to the DC coupled cathode follower? As Merlin suggested wouldn't it be easier to come of the 100k cathode into a coupling cap and then to the diodes pre-tonestack?

 

[abbey road d enfer]

I mean be a little green here, but wouldn't placing a coupling cap in this position block the DC being fed to the DC coupled cathode follower?

Look at the picture. The anode of the first tube is still connected to the grid of the second one. The 0.1uF cap is there to isolate DC from the diodes.

As Merlin suggested wouldn't it be easier to come of the 100k cathode into a coupling cap and then to the diodes pre-tonestack?

The cathode follower has a low output impedance. I don't like the idea of having the tube hitting a brickwall.

 

[Amled87]

Look at the picture. The anode of the first tube is still connected to the grid of the second one. The 0.1uF cap is there to isolate DC from the diodes.

The cathode follower has a low output impedance. I don't like the idea of having the tube hitting a brickwall.

Thank you for the explanation, I see it now. On my build, the 100k anode resistor is on the socket for V2a/V2b. I ran a wire from pin 1 (anode of the 100k/820R section) into a .1uf Coupling cap and then to the zeners. Seemed to work just fine.

 

[JohnRoberts]

Add an extra coupling cap and put the diodes right before the tone stack. Always better to have clipping before EQ.

For today's TMI about diode clipping and EQ... back last century I patented a diode clipper built into a Baxandall tone control (US05509080 Roberts).

Long story short in the fixed install business speaker networks are driven through step up (and step down) voltage transformers. Loud, LF bass requires more copper and iron, than modest levels at higher frequencies. BUT customers (who are always right) wanted to boost bass to make the crappy ceiling speakers sound better. Boosting the very LF with a shelving Baxandall tone control could cause output magnetics to saturate without some form of limiting. My strategy to let the customers have their bass boost without killing the amps, was to incorporate a simple diode clipper into only the bass half of a bass /treble tone control EQ. The diode clipper effectively clamped the bass output below output transformer saturation levels. Since the HF portion of the audio waveform was passed unmolested it sounded unexpectedly clean for a hard level clipper.

JR

 

[Amled87]

For today's TMI about diode clipping and EQ... back last century I patented a diode clipper built into a Baxandall tone control (US05509080 Roberts).

Long story short in the fixed install business speaker networks are driven through step up (and step down) voltage transformers. Loud, LF bass requires more copper and iron, than modest levels at higher frequencies. BUT customers (who are always right) wanted to boost bass to make the crappy ceiling speakers sound better. Boosting the very LF with a shelving Baxandall tone control could cause output magnetics to saturate without some form of limiting. My strategy to let the customers have their bass boost without killing the amps, was to incorporate a simple diode clipper into only the bass half of a bass /treble tone control EQ. The diode clipper effectively clamped the bass output below output transformer saturation levels. Since the HF portion of the audio waveform was passed unmolested it sounded unexpectedly clean for a hard level clipper.

JR

I actually enjoyed reading this and gave ideas for something else. Seems to never end lol. Just put in orders for the BoM for a pair of the Nyan 73' EQ and been looking over Ruffrecords notes and getting a list together for the mic preamp in his EZtubemixer.

In all honesty, I've tried the clippers both ways now with leaving the master after the EQ and also moving the master pre-eq like in the Jose style circuit. Used a rotary switch to change between several different combinations and no matter what the 800 style preamp just sounded better to me with a boost out front. I'm going into the effects return of another tube amp and or using a dedicated EL34 power amp.

 

[mr coffee]

You could just modify the 2204 to be a 2205 preamp. Sounds like what you are aiming for.