Channel Switching JCM800 Design? + BIAS pot question

[ViperDoc]

I'm about to jump into a 50 watt JCM800/2204 kit build. If I wanted to switch between clean and dirty, how simply could I mod that?

Here's the diagram:



Schematic:



I also have other questions about PPI Master volumes and effects loops, but perhaps later.

Thanks!

[pdf64]

Do you really mean 'clean' and 'dirty', or between 'a bit dirty' and 'more dirt'? As the former would be somewhat trickier in regard of switching complexity and issues involved in implemention. And the latter, perhaps just do what players have done for decades and use a Tube Screamer for the 'more dirt' option.

Note that the 1N5408, being 1kV rated, is not suitable for this application. Rather disappointing that Mojo are specifying it.

A decent clean option in a non switching design would be useful IMO; anyone have insight into what Alan Philips does here? https://www.carolannamps.com/78-50h-master-head.html#

[martin manning]

I'm about to jump into a 50 watt JCM800/2204 kit build. If I wanted to switch between clean and dirty, how simply could I mod that?

This amp already has high and low inputs where the high input places the first two gain stages in series, and the low input skips the first one. I built a version of this circuit without the low input, but with an added switch that places a 100k across the 470k to ground in front of the third gain stage, which cleans it up quite a bit.

Pete is right about the rectifier spec. I believe you will be well over 450V with that PT, so two 1kV diodes in series on each leg is a good idea.

[pompeiisneaks]

According to the datasheet I'm looking at, the 1n5408 is designed at 1kv 3A... which is a lot more than the > 400V 2A this one's going to sit around no?

*and that's at half each side so the diode only sees 1/2 the voltage and half the current of the transformer? The transformer says it will see 360 V 1A per side?

Or am I missing something else? Sorry the reason I ask is I'm always trying to understand more.

What is the minimum spec you'd need then for this type of setup?

Edit: changed first voltage
~Phil

[pdf64]

According to the datasheet I'm looking at, the 1n5408 is designed at 1kv 3A... which is a lot more than the > 400V 2A this one's going to sit around no?...

That type of rectification (2 phase cap input) puts the full winding voltage on to the diodes. The relevent voltage here is the peak, not RMS.
That Mojo PT is listed as 360-0-360- @75mA, so the diodes will need a rating somewhat higher than 720 x 1.414 = 1018. Then need to factor in the mains voltage being at its upper limit (eg x~1.1), and the winding being unloaded due to a blown HT fuse or cold / dead / no tubes in circuit (eg x~1.05). And finally the winding voltage being at its upper tolerance (especially as Mojo PTs tend to have secondary voltage that are somewhat in excess of their published values), so another x~1.02.
And we get minimum nececcary Vrrm of 1200V.
Bad Mojo

See http://www.valvewizard.co.uk/bridge.html

[pompeiisneaks]

According to the datasheet I'm looking at, the 1n5408 is designed at 1kv 3A... which is a lot more than the > 400V 2A this one's going to sit around no?...

That type of rectification (2 phase cap input) puts the full winding voltage on to the diodes. The relevent voltage here is the peak, not RMS.
That Mojo PT is listed as 360-0-360- @75mA, so the diodes will need a rating somewhat higher than 720 x 1.414 = 1018. Then need to factor in the mains voltage being at its upper limit (eg x~1.1), and the winding being unloaded (eg x~1.05). And finally the winding voltage being at its upper tolerance (especially as Mojo PTs tend to have secondary voltage that are somewhat in excess of their published values), so another x~1.02.
And we get minimum nececcary Vrrm of 1200V.
Bad Mojo

See http://www.valvewizard.co.uk/bridge.html

Thanks! that was exactly what I needed. That makes total sense. I was incorrectly understanding how the transformer ratings were interpreted. so it doesn't have 360 on one side and -360 on the other that flip/flops, but its' the sum of the two to get the 'voltage' pre rectification. Doesn't the more commonly older type 1n4007 get used often in this type of setup, but it's only 1kv too, so it's under specced? So 1N4008 is right one for this use too?

Cheers!

~Phil

[ViperDoc]

Do you really mean 'clean' and 'dirty', or between 'a bit dirty' and 'more dirt'? As the former would be somewhat trickier in regard of switching complexity and issues involved in implemention. And the latter, perhaps just do what players have done for decades and use a Tube Screamer for the 'more dirt' option.

Note that the 1N5408, being 1kV rated, is not suitable for this application. Rather disappointing that Mojo are specifying it.

A decent clean option in a non switching design would be useful IMO; anyone have insight into what Alan Philips does here? https://www.carolannamps.com/78-50h-master-head.html#

Hi pdf64,

I don't need Blackface and Doom Metal 800 in the same amp, that's for sure. I would say less dirty that would clean up nicely with my guitar's volume knob would suffice for me. I do, however, like to tap into an amp's power tube distortion recipe for the higher gain stuff without slamming the amp's front end. I do love TS pedals, that's for sure. I thought I could somehow switch between two gain controls and keep the single tone stack in place. How would that differ tone wise from the low/high input switch?

Regarding the 1N5408 diodes, that's over my head. If they're under-spec'd, what would you recommend?

Thank you!

[martin manning]

I think adding 20% to the voltage is a little bit much. Adding +5% for unloaded voltage and +5% for line voltage variation seems more reasonable. I would measure the unloaded voltage and line voltage and see what it really is. Mojo spec sheets are always suspect, IMO. It's true that the upper end of guitar amp voltages will be putting right around 1kV across the rectifiers, but it is also common practice to use two 1N4007's in series.

[ViperDoc]

I'm about to jump into a 50 watt JCM800/2204 kit build. If I wanted to switch between clean and dirty, how simply could I mod that?

This amp already has high and low inputs where the high input places the first two gain stages in series, and the low input skips the first one. I built a version of this circuit without the low input, but with an added switch that places a 100k across the 470k to ground in front of the third gain stage, which cleans it up quite a bit.

Pete is right about the rectifier spec. I believe you will be well over 450V with that PT, so two 1kV diodes in series on each leg is a good idea.

Thanks, Martin! Any recommendations on what to sub in?

[ViperDoc]

I think adding 20% to the voltage is a little bit much. Adding +5% for unloaded voltage and +5% for line voltage variation seems more reasonable. I would measure the unloaded voltage and line voltage and see what it really is. Mojo spec sheets are always suspect, IMO. It's true that the upper end of guitar amp voltages will be putting right around 1kV across the rectifiers, but it is also common practice to use two 1N4007's in series.

I appreciate your help with finding the proper values. I see the 1N4007 diode--220KR--[standby]--1N5408 diode orientation. I am new to all of this, but it appears that the 1N4007 is in series with the 220KR and one 1N5408 when turned on, and the other 1N5408 is in parallel to the 50 uF cap on the other side of the winding. Is this right? If the 1N4007 and 1N5408 are under-spec'd, how would you "rectify that"? See what I did there?

Two 1N5408s in series on each leg and leave the single 1N4007 where it is?

Thanks.

[martin manning]

Yes, the 1N4007 after the 220k in the bias supply is fine. Just use two 1kV diodes in series in place of the single 1N5408's between the standby switches and point "A". They don't have to be 3A, 1A 1N4007's will do .

[ViperDoc]

Yes, the 1N4007 after the 220k in the bias supply is fine. Just use two 1kV diodes in series in place of the single 1N5408's between the standby switches and point "A". They don't have to be 3A, 1A 1N4007's will do .

Thanks. Since I already have them, is there anything wrong with placing an additional 1N4007 in front of each 1N5408? Or should they be the same?

[pdf64]

To even out the voltage across each, both must be the same type at least, preferably the same manufacturer and era, best to be from the same batch really.

[ViperDoc]

To even out the voltage across each, both must be the same type at least, preferably the same manufacturer and era, best to be from the same batch really.

Excellent, thank you, pdf64!

[ViperDoc]

I'm about to jump into a 50 watt JCM800/2204 kit build. If I wanted to switch between clean and dirty, how simply could I mod that?

This amp already has high and low inputs where the high input places the first two gain stages in series, and the low input skips the first one. I built a version of this circuit without the low input, but with an added switch that places a 100k across the 470k to ground in front of the third gain stage, which cleans it up quite a bit.

Pete is right about the rectifier spec. I believe you will be well over 450V with that PT, so two 1kV diodes in series on each leg is a good idea.

I see two 470K resistors on the schematic, I assume you were talking about the first one. When you say “across” the resistor, do you mean you switch in a parallel 100K at that point, or are you doing it at the grounded 470K to ground more of the signal? Or something else?

EDIT: I see now, you likely were describing the second one as you described. Ok.