Power amp in

[Miyagi_83]

Hello, everyone.
I've got this old solid-state 2x12 guitar combo from East Germany called Vermona Regent 1040 (schematic attached should anyone need it). Generally, the amp is operational. It does need some care and attention, like a cap job among others, but it works. I'm not too big on the preamp, though, so I figured I'd leave the output section intact and work on the A NEW preamp.
So, my idea was to first build some kind of input buffer with voltage limiting. Having researched the net, I figured I'd try something like in the attached image. So, if anyone more skilled than me could take a look and tell me whether there are any design flaws there, I'd greatly appreciate that.

Thanks in advance,
M.

-EDITED for clarification

[g1]

What is Vr ?

[Miyagi_83]

Reference voltage.

[Miyagi_83]

What is Vr ?

Reference voltage.

I didn't mean to be short with you, g1. I just had something urgent to attend to. Hope I didn't come off as rude.

[phatt]

Adding any circuit is unlikely to improve a bad preamp design.
My advice is purchase/build a decent preamp circuit and bypass the whole front end of the Amp.
Phil.

[Miyagi_83]

Thanks for chiming in, Phil
I just realized that I didn't make my intention clear enough, though.
The circuit I posted is meant as an input into the power amp. I want to test different preamp ideas, but I figured it was good practice to buffer a solid-state amplifier's input from the outside world. I don't care for the onboard preamp there, so I'm not going to bother trying to improve that.
I suppose I need voltage limiting because some preamplifiers are able to put out a pretty hot signal.
I'd like to know if the schematic in my original post makes sense and whether or not it needs any refinements. I'm pretty good with tubes, not so much with solid-state so I might be asking a rookie question here. Thanks for your time
M.

[Tassieviking]

I would first check what sort of signal is going into C215 which goes into the last OP amp in the pre-amp circuit, or the signal between the pre-amp and the power-amp (C).
Then figure where the best place is to add an effect loop send / return.
Then just use a nice pre-amp pedal straight into the effects return.
To fix that amp you might have to replace both speakers and all the electronics to get a nice sounding amp, and what are you going to fill all those pot spaces with ? You would need to get a new front panel if you change the pre-amp.

You cannot view this attachment.

https://elektrotanya.com/vermona_regent_1040_guitar_audio_amplifier_mixer_sch.pdf/download.html

[Miyagi_83]

I would first check what sort of signal is going into C215 which goes into the last OP amp in the pre-amp circuit, or the signal between the pre-amp and the power-amp (C).

Thanks for the reply. I'm thinking about placing the fx loop return just before the power amp, right before R501 / C501. The schematic shows a voltage input of 775 mV, so I was going to limit any incoming signal to that, or just under, like with a pair of silicon diodes in anti-parallel. I'm wondering if the setup I drew by hand makes sense there.

To fix that amp you might have to replace both speakers and all the electronics to get a nice sounding amp, and what are you going to fill all those pot spaces with ? You would need to get a new front panel if you change the pre-amp.

The speakers are not too bad, honestly. I soldered a socket to them and play my amps through them at home. As far as the front panel is concerned, that's not a problem. As a DIY-er, I'll figure something out.

[phatt]

You just need a 100k pot in front of "C", the input to the Power stage.
If you can only get to halfway before it distorts use a 50k pot or add some series resistance before the 100k pot. Your ears will tell you if it way off.
Phil.

[Miyagi_83]

As simple as that? Cool
Thank you, sir

[Miyagi_83]

Ok, one more question. If I want to keep it simple, as Phil suggested, how do I install overvoltage protection, in case a tube preamp is plugged in? Like this (attached)?
BTW, is it more common to say 'tube' or 'valve' in Oz? 🤔

[Tassieviking]

What sort of preamp are you wanting to use on that amp ?
Are you looking for a clean tone or distorted, Fender, Marshall, Vox ?
If you want to build a different type of pedal then have a look at the C2CE range of PCB's.
They are all Tube driven at high voltage but run on a 9V-13V pedal supply, although your power supply has to be able to deliver plenty of power. 0.5A to 1A.
https://c2celectronics.com/
Nathan of C2CE & Sushi Box also sells ready made pedals if you don't feel safe dealing with high voltages in a pedal, his other shop Sushi Box is here :
https://www.sushiboxfx.com/

It would be easy to make your own PCB's for his designs but since he is a small company I just buy his PCB's to make sure he keeps the business going, and also because he has my respect for developing the PCB's for us all to build.

[Miyagi_83]

What sort of preamp are you wanting to use on that amp ?

Definitely something switchable. I like to have some versatility. Tone-wise, I'm not exactly sure what, yet. First, I need that power amp ready to rumble. Then I'll try different stuff to see what I like best. I have a couple of solid-state and tube preamps in mind. I'll certainly post about that.

If you want to build a different type of pedal then have a look at the C2CE range of PCB's.
They are all Tube driven at high voltage but run on a 9V-13V pedal supply, although your power supply has to be able to deliver plenty of power. 0.5A to 1A.
https://c2celectronics.com/
Nathan of C2CE & Sushi Box also sells ready made pedals if you don't feel safe dealing with high voltages in a pedal, his other shop Sushi Box is here :
https://www.sushiboxfx.com/

Thanks for the hint. I sure will look it up.

[g1]

Reference voltage.

The reason I asked is it is usually only required for op amps when single ended supply is used (along with DC blocking caps at input and output). 
You will see the other op amps in the Regent are not using it as the Regent has split (+/-) supply for the op amps so Vr is not needed.

[Miyagi_83]

The reason I asked is it is usually only required for op amps when single ended supply is used (along with DC blocking caps at input and output). 
You will see the other op amps in the Regent are not using it as the Regent has split (+/-) supply for the op amps so Vr is not needed.

All right. So, in my case, Vr should be 0V, correct?