Jose Plexi mods

[Spencerleehorton]

As far as I’ve read cathode bypass capacitor adds gain as one of its functions, so I can tweak these if needed.

 

[john12ax7]

For 3 stages you can start with the jcm800 for a known good sounding circuit and adjust from there.

 

[Spencerleehorton]

For 3 stages you can start with the jcm800 for a known good sounding circuit and adjust from there.

im already at that point as v1a is going into v1b into v2a, what im proposing is putting in another gain stage before them.
i will be able to switch with relays from plexi to jcm800 and add another gain stage to the jcm800 via a DPDT switch.

 

[john12ax7]

In that case slo100 is a good reference.

 

[dai h.]

here are some posts from Chris Barrow ("cbarrow") on Music Electronics Forum that look like very worthwhile reading:

https://music-electronics-forum.com...-amps/theory-design/6603-biasing-preamp-tubes

http://music-electronics-forum.com/showthread.php?t=7533

http://music-electronics-forum.com/showthread.php?t=12642

plus an "Amp Smoothing Options" thread on Hoffman Amps BBS:

https://el34world.com/Forum/index.php?topic=8579.0

Maybe check out the Headfirst Amps guy's google drive for ideas (schematics and mods--check out JEL (Jake E. Lee)--it's a somewhat tweaked 2204 circuit which seems to show some low frequency tweaking (220k R added across the 1MA pot (helping to reduce "mud"(LF intermodulation)?)) :

https://drive.google.com/drive/folders/1mgmE02iNQrU2n5xnbvnnDbonBQ0J_tNG

Might also want to look at construction details for amps using the same basic layout with (what I gather are) Jose-type mods (Friedman, Dan Gower) :

 

[Spencerleehorton]

I think I need to fully understand cathode bypass capacitor cap values and yes the SLO100 is a great schematic for doing what I want.
Thank you guys.

 

[john12ax7]

Randall Aiken is a good resource for the tech side of things.

http://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers

 

[Spencerleehorton]

Wow, that’s going to take some time to digest!!

 

[ruffrecords]

Randall Aiken is a good resource for the tech side of things.

http://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers

That is a pretty good summary.

Cheers

Ian

 

[dai h.]

personally, I look at it like this:



(seemingly more intuitive if drawn like this?) bigger cathode bypass (along with the .02uF coupling cap (which taps in to the output signal path (BLUE) to take the signal to the next stage) = more lows and 1MA as a load (higher = more lows passing and vice a versa, then to a lesser extent (I think) the 16uF decouping/bypass cap)

something that looks fun to play with ((click "launch me" to start the app)):

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

 

[Spencerleehorton]

Here it is with the extra gain stage at start.
Put 1uf caps on the 2 x 5.6k on the cathode, switches them in parallel.

 

[CJ]

Here is a lab experiment dealing with cathode bias and bypass capacitors>

 

[Spencerleehorton]

building the 5v relay psu and foot switch pedal, i will put in two masters, one for plexi channel and one for the OD channel.
i will use two relays so i can switch out each channel.
i will also put in the dual bias

 

[Spencerleehorton]

Did a bit of a rebuild and now have channel switching and dual master, plexi ch1 single volume, JCM800 ch2 and I’ve put in an extra gain stage after the second gain stage with its own gain knob, as the gain stage is now before the cathode follower and last in line as apposed to being first it does sound slightly different, a bit darker so will review some of the filtering as I thought it sounded better before.
Dual bias next stop.

 

[Alva Goldbook]

Hi all,

Armed now with a great source of knowledge and working my way through this book:

https://www.valvewizard.co.uk/Book1.html

I’m going to attempt to mod my other plexi 1959 50W I’ve built to try out some mods and see how it sounds.

I’ve added another first gain stage into the plexi and having two input jacks, one for standard plexi and the other for the extra gain stage.
The other jack sockets will be replaced with 1M gain pot and 1M master volume, as I’m going to remove the PPIMV pot and put back the two 220k resistors.

I’ve added the gain stage and so far the plexi sounds great at half gain but I’ve not changed the PPIMV to mst vol between tone stack and PI.
When I turn up extra gain stage past half way I get squeals and wah wah overloads the signal?
Could this be the PPIMV?
I’ll add the mst vol tomorrow and see if it improves.

Will also try mst vol pre tone stack to see how it sounds.

you know, cathode followers are cool, but they do offer just a single gain stage. Yet they use two triodes. Two triodes individually provide a lot more gain. You can also make a cathode follower clip more by reducing the tail resistor. So just like a cathodyne, where the plate and tail are the same resistor value, if you reduce the tail value the gain stage will clip more. Which you may or may not want going into the tone stack. Also, A plexi has a parallel front end. One side bright, one side dark. But there's no reason the bright side can't cascade into the dark side and get two series gain stages.

 

[Spencerleehorton]

I’m actually starting to turn it into a SLO50, lots of options with parallel the clean and OD channels, trying different things and trying to find what I like

 

[Spencerleehorton]

Starting to tidy everything up and my soldering iron breaks down!!!
Ordered a new Weller one which I hope will be delivered in the next couple of days!!!
Made my own eyelet pcb for the relays.
Waiting on some better screened cable and will make a start!!

 

[Spencerleehorton]

Here is the dual bias web!!

 

[Spencerleehorton]

Finishing the tidy up tomorrow, going to be somewhere between a Plexi/JCM800 and a SLO-50.

 

[Alva Goldbook]

I think I need to fully understand cathode bypass capacitor cap values and yes the SLO100 is a great schematic for doing what I want.
Thank you guys.

Cathode bypass caps do two things. 1) regardless of value increases gain. Like close to doubles it. And 2) changes what frequencies are boosted.

Despite seeing some plexi values that are high like 330uF, the reality is that for typical 12ax7 cathode bias resistors, there’s no point in increasing it more than say 25uF. That amplifies below the 82Hz frequency of the low E string. Changing caps to 10uF or 4u7 won’t have a big impact on tone.

But then you have very sudden and dramatic changes in tone from 2u2 to 470nF. The lower the value, the brighter the response, or more accurately, the less bass frequencies are amplified.

Film caps are used below 1uF, but there is no need to use a massive 600v film cap here. The bias voltage is less than 10v, so any cheaper small voltage film cap will do. 100v are more common.

Here is a handy calculator for finding the bypass cap value you want.

Remember, you can also shave off bass response by changing the coupling cap
value as well.

https://ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/