Fuzz/Phase shifting effect (muskito flying by) in the mids

raiken · Post by **raiken** » Sun Jul 27, 2008 1:50 pm

If you connect the zener clipper circuit between the bias supply and the output tube control grid (i.e. across the bias feed resistor), you want a zener voltage nearly equal to the bias voltage, because it will clip at a level equal to a zener voltage drop below the bias supply voltage, which will give you a symmetrically-clipped waveform, because the top of the waveform is clamped to ground.

If you instead connect the zener clipper circuit between the grid and gound, you want a zener voltage double the bias voltage, because it will clip at a level equal to a zener voltage drop below ground, again giving you a symmetrically-clipped waveform.

Randall Aiken

raiken · Post by **raiken** » Sun Jul 27, 2008 1:51 pm

Froumy wrote:Ohh yeah. Randall, seems like you have/had a book in the works. Is it done? And where/when can I buy it?

It's been in the works for years - I'll finish it one of these days.

Randall Aiken

raiken · Post by **raiken** » Sun Jul 27, 2008 1:56 pm

Ricky Lee wrote:Any problem using the Zeners when the 220k bias resistors have been removed for the PPIMV? Also, with the dual voltage tranny, where plates defer 400v/500v and naturally bias -35/-43 respectfully, would using a Zener value for the low voltage side (400v) be the most appropriate?

If you have a PPIMV, you are not driving the output tubes into clipping, so the bias shift won't occur, and you shouldn't have mosquitoes flying around. As you adjust the PPIMV down, the peak-to-peak signal level will decrease below the zener voltage, so the negative peaks of the waveform won't get clipped by the zener, but then, the positive peaks aren't getting clamped either.

Randall Aiken

Ricky Lee · Post by **Ricky Lee** » Sun Jul 27, 2008 2:19 pm

So with the PPIMV the Zeners are not needed?

raiken · Post by **raiken** » Sun Jul 27, 2008 2:20 pm

Ricky Lee wrote:So with the PPIMV the Zeners are not needed?

They are if you turn the master all the way up, or to the point where the output tubes start clipping, but at "sane" volume levels you won't hear the buzz of crossover distortion...it won't hurt to put them in, because they are essentially out of circuit when you turn the master down.

RA

Ricky Lee · Post by **Ricky Lee** » Sun Jul 27, 2008 2:27 pm

I was going to mention that with the MV essentially not affecting the amp when dimed so the Zeners would help in that case, which I may try since I occasionally have no need for the MV when running pedals thru the front door.

5150loveeddie · Post by **5150loveeddie** » Mon Jul 28, 2008 4:02 am

raiken wrote:[
You can make a unidirectional clipper with a zener diode in series with a regular diode, with the zener's cathode and regular diode's cathode connected together, and this assembly connected across the bias feed resistors, with the zener's anode connected to the coupling cap/output tube grid stopper connection and the regular diode's anode connected to the other end of the bias feed resistor that goes to the bias supply, so you only clip negative-going signals.

Randall Aiken

Sorry to be a pain in the butt but I'm not 100% sure I get this configuration right. the assembly connects across the two 220k's (in a Marshall 1959/87.. type circuit here), so where is across thos two resistors?? And the "other end of the bias feed resistor".... where is that? Where the two 220k's meets? I'm a french speaking person trying to understand new expressions or different English terminalogy!!

Thx for the clearing up!

Ricky Lee · Post by **Ricky Lee** » Mon Jul 28, 2008 9:12 am

Both are piggybacked across the 220k bias/splitter resistors, they come together at the bias feed, the ends (Zener) connect where the green/orange wires join the phase inverter coupling caps either .1/.022 depends on which you have, this junction joins them to the swampers either 1k5 or 5k6 on pins 5 of power tubes.

raiken · Post by **raiken** » Mon Jul 28, 2008 11:07 am

5150loveeddie wrote:
raiken wrote:[
You can make a unidirectional clipper with a zener diode in series with a regular diode, with the zener's cathode and regular diode's cathode connected together, and this assembly connected across the bias feed resistors, with the zener's anode connected to the coupling cap/output tube grid stopper connection and the regular diode's anode connected to the other end of the bias feed resistor that goes to the bias supply, so you only clip negative-going signals.

Randall Aiken
Sorry to be a pain in the butt but I'm not 100% sure I get this configuration right. the assembly connects across the two 220k's (in a Marshall 1959/87.. type circuit here), so where is across thos two resistors?? And the "other end of the bias feed resistor".... where is that? Where the two 220k's meets? I'm a french speaking person trying to understand new expressions or different English terminalogy!!

Thx for the clearing up!

I've attached a schematic showing the connection - remember, you'll have to pick a zener that is close to your bias voltage, the one on the schematic may not be suitable. Also, the 1N4001 diode shown is probably not the best choice, as it is rated only for 50V. You should probably use a 1N4003 - 1N4007 or something like that, just to be on the safe side. Even a lowly 1N914 small-signal diode will probably work fine, as they are rated for 100V.

Randall Aiken

Froumy · Post by **Froumy** » Mon Jul 28, 2008 11:56 am

Randall, You are the MAN!

Thanks again for all of your help, and the electronics lessons!!! Can't wait for the book. Best of luck with it! I wouldn't complain if you explained the whole emitter follower/Mosfet source follower attenuator theory in there. (without stepping on anyone's feet) Thanks again!

Ohh yeah. Before I hop back in there.. Having a difference between phases of 20+ volts peak-peak(when dimed) from the PI section is too much, right? Guessing a tube or something simple.

Bob

raiken · Post by **raiken** » Mon Jul 28, 2008 12:06 pm

Froumy wrote:Randall, You are the MAN! Thanks again for all of your help, and the electronics lessons!!! Can't wait for the book. Best of luck with it! I wouldn't complain if you explained the whole emitter follower/Mosfet source follower attenuator theory in there. (without stepping on anyone's feet) Thanks again!

Ohh yeah. Before I hop back in there.. Having a difference between phases of 20+ volts peak-peak(when dimed) from the PI section is too much, right? Guessing a tube or something simple.

Bob

No, that is normal - you will get a larger output swing from one phase to the other when you hit clipping - that is a characteristic of a long-tail pair phase inverter. The "input" side, which is functioning as a common-cathode stage, will have a larger clipped output than the "other" side, which is effectively functioning as a common-grid stage - remember, it is amplifying and inverting the "clipped" wave from the other section.

Having said that, the relative matching of the two sections, while unimportant when the PI is not clipped (due to the local and global feedback that tends to stabilize the gains), will affect the drive when clipping, so you may see a difference with different tubes.

Randall Aiken

5150loveeddie · Post by **5150loveeddie** » Mon Jul 28, 2008 3:10 pm

raiken wrote:
5150loveeddie wrote:
raiken wrote:[
You can make a unidirectional clipper with a zener diode in series with a regular diode, with the zener's cathode and regular diode's cathode connected together, and this assembly connected across the bias feed resistors, with the zener's anode connected to the coupling cap/output tube grid stopper connection and the regular diode's anode connected to the other end of the bias feed resistor that goes to the bias supply, so you only clip negative-going signals.

Randall Aiken
Sorry to be a pain in the butt but I'm not 100% sure I get this configuration right. the assembly connects across the two 220k's (in a Marshall 1959/87.. type circuit here), so where is across thos two resistors?? And the "other end of the bias feed resistor".... where is that? Where the two 220k's meets? I'm a french speaking person trying to understand new expressions or different English terminalogy!!

Thx for the clearing up!
I've attached a schematic showing the connection - remember, you'll have to pick a zener that is close to your bias voltage, the one on the schematic may not be suitable. Also, the 1N4001 diode shown is probably not the best choice, as it is rated only for 50V. You should probably use a 1N4003 - 1N4007 or something like that, just to be on the safe side. Even a lowly 1N914 small-signal diode will probably work fine, as they are rated for 100V.

Randall Aiken

Ah I see it better now, thank you so much Randall for taking the time for all this, this if way cool!!

So there is two assemblies one for each side, that is what I wasn't sure about duh!!

I'll get the stuff toghter and start testing!!!

5150loveeddie · Post by **5150loveeddie** » Mon Jul 28, 2008 4:40 pm

Ricky Lee wrote:Both are piggybacked across the 220k bias/splitter resistors, they come together at the bias feed, the ends (Zener) connect where the green/orange wires join the phase inverter coupling caps either .1/.022 depends on which you have, this junction joins them to the swampers either 1k5 or 5k6 on pins 5 of power tubes.

Thx RL

Billy Batz · Post by **Billy Batz** » Mon Jul 28, 2008 5:40 pm

Wow this thread has progressed. Keep this one bookmarked

Ricky Lee · Post by **Ricky Lee** » Mon Jul 28, 2008 7:15 pm

I ordered the Zeners today as well as the 1N914 signal diodes so will soon see how this works. Just out of curiosity I want to try different values with the Zeners.

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Fuzz/Phase shifting effect (muskito flying by) in the mids

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