Clone 1974x to SLP ( RR ) plexi

[Plig88]

Hello guys , before 11 years I make my very very first tube amp , is clone of Marshall 1974x ,, before 3 years I turn the normal channel into cascade mod , but after one year to use , I bored and I reset to factory circuit .. so now I have a idea , I keep the tremolo channel and I change the normal channel to 1959 SLP PLEXI , basically Randy Rhodes mod ,, I found the schematic of SLP and the schematic of RR PLEXI and it’s similar ,. Just the first input is the classic SLP and the second input is the cascade mod RR .. my question is , can add presence on my 18 watt just like the real plexi ? I send you the original circuits and the circuits I make it . anyone an opinion ,

[Bieworm]

You could add presence to the circuit, but that would involve a negative feedback loop. That one will kill the nice tone you have when in tremolo channel mode.
Theoretically you could add a lot of relays switching to add the presence and NFB loop in plexi mode and remove it entirely when in 18W mode. It’s doable, but not as easy as it sounds.
Imho for a full plexi experience you’d need to add another tube, but that will depend on the properties of the power transformer. If you’re up to the challenge, go ahead, but be aware of the level of difficulty to pull this one off.

[Plig88]

First : Anyways I add 1 more tube for the preamp of plexi , maybe you mean to add onother tube for phase inverter , separated from the tremolo channel , that is you mean ?
And second : about the presence, I can put some relay ( something I think ) when put guitar cable into tremolo channel the presence stoped .

[Bieworm]

First : Anyways I add 1 more tube for the preamp of plexi , maybe you mean to add onother tube for phase inverter , separated from the tremolo channel , that is you mean ?
And second : about the presence, I can put some relay ( something I think ) when put guitar cable into tremolo channel the presence stoped .

Add an extra preamp tube, the PI is shared by both circuits. Look at the plexi circuit diagram and you’ll see there is a tube necessary in the preamp
For the presence switching you need to go to the drawing board and figure something out with a dpdt switch. It’s doable

[Plig88]

That’s I am saying , I add extra tube on last wiring diagram . Anyway , for the presence I make the same diagram like the plexi ?cut the ground from cathodes ( bottom ) of phase inverter and add 100k resistor from 4ohm tap ( output trans ) to cathodes ( bottom ) with 25k pot and parallel 4k7 to the ground ?

[Bieworm]

That’s I am saying , I add extra tube on last wiring diagram . Anyway , for the presence I make the same diagram like the plexi ?cut the ground from cathodes ( bottom ) of phase inverter and add 100k resistor from 4ohm tap ( output trans ) to cathodes ( bottom ) with 25k pot and parallel 4k7 to the ground ?

You’re oversimplifying it. You can’t use the NFB and presence values from an EL34 amp with an EL84 amp.

[Plig88]

I know that I can’t use the same values from l34 to el84 tubes , but I’m saying that’s the idea . I found a 18watt plexi schem , and the values for phase inverter are different from 1974x .. so I thinking to leave it without change the phase inverter and convert only the preamp section ,, and for presence I can use the ( this is I call fake presence ) . Just like the Mesa boogie dual rectifier preamp section some guy make it pedal call it dr boogie , and for the “presence” add a fake presence knob , check the circuit and tell me. The values are R17 = 2K2 c18= 33n Presence pot are 10k

[Plig88]

Hello ,, I make it ,, it’s works and the sound are soooo good ,,, but I have some issues. First of all . I’m not sure for the power transformer can take 1 more preamp tube ( 4 preamp tubes total + 2 EL84 ) , I see the specs for my power transformer and have 2x 290 volts and 120mA current , one issue is this and the other is , when I put the input on RR mode ( see the schematic ) and crank the 2 volumes all the way up , it’s like clogs the signal like little bit fuzz ,, something like this ,, but i forgot to say , the 2 volumes must be 1M both , and I can’t found here 1M ,, I found 2M2 stereo and I solder parallel and they give me 1M4 ,, if I put 1M exactly the sounds it’s better ? Thank you

[Plig88]

Everything’s is ok ,, I make a mistake on input Jack , and now everything works great , maybe I don’t put presence ,, it’s very good sound ,,,

[Bieworm]

If the transformer has enough heater current, like minimum 2.5A you're fine. 120mA is probably (just) enough for the HT. what are the voltages measured?