Tube preamp into SS power amp

[Dunner84]

Hello

By order of my wife, I was tasked with getting rid of some of my "junk". While I was going through things I found an old squier 185 pro that I was given. The amp is in mint condition minus some broken jacks. They are the fender 9 pin PCB type. I never got around to ordering any due to other projects and forgetting them on larger parts orders, so I never fixed the thing. The idea was to fix it and sell it or give it away. It's not a bad sounding thing, it's just not great.

I also found a pre amp pcb from London Power that I ordered a while back, and I thought why not rip all the icky pre-amp stuff out of the squier and replace it with this cool dumble inspired pre-amp kit.

I had a look through the squier schematic, and I think I know where to hack away and connect the pre-amp, but I thought I would get some fresh eyes one it. The pre-amp kit has its own filtered power supply for the tubes, so I am really just looking at plugging it directly to the power section of this SS amp.

Here is a copy of the schematic I have. I would appreciate any insight.

Cheers

[phatt]

Only You can answer the Question,,,     Is it worth the effort? 8|
Best of luck ,, cause you are going to need lotsa luck trying to separate the preamp from power amp because it's all on one PCB. xP
As it's in good nick I'd replace the jacks and sell it. :tu:

But if you must,, then build the Valve preamp first and jumper that into the poweramp return and see if it works.
I doubt it will be a plug and play as I'd guess there would be a lot of tweaking to get it to sound right.
Just throwing a Valve circuit in front of a transistor power stage does not automatically make for the mojo.

A mate had a similar amp and I tried and tried to get it to work the way he wanted,, no luck.
In the end I built him a separate preamp and bypassed the whole front panel.
Phil.

[Dunner84]

Thanks for the input. Maybe I am going about this the wrong way. To me the real value in this whole plan is the dumble pre-amp kit. I bought it a while back to check it out with the intention of building an all tube amp. With the exchange, shipping to Canada and the overall cost of transformers, I never got around to building the amp I wanted to build.
As for this squier amp, the real value is in the cabinet, speakers, chassis etc.. I couldn't sell it for the cost of those components. The jacks I need are 2 bucks a piece... Plus 20+ dollars shipping, so considering the resale value it hasn't been worthwhile to fix.
I have ran with tube/SS circuits in the passed with good results, and I thought that due to the shear power of this one it would have been a good candidate for the dumble pre-amp because that's where most of the sweetness is in those amps.

I'd hate to scrap a perfectly functional SS amp just for the hard bits.. but maybe thats what I should do.

If I build the pre-amp and run it through the effects return, does that effectively remove the squier preamp from the equation, or will it still load down the circuit?

Thanks again for the input!

[g1]

Yes, that will bypass the whole 185 preamp. 
Agree with Phil that this is your best option.  Then you still have the amp intact if you ever want to sell it, and you will have the valve preamp as a stand alone unit so you can try it with other amps or power amps.
  Those jacks should be available at any Fender dealer or Fender repair shop in Canada.   Part # is 0990913000

[J M Fahey]

I have been looking at the circuit: a beast of n amp, VERY loud.
Not sure about what speakers you will be using.
IF the original single 12", it´s good, loud ... and muddy.
It´s Eminence made, so far so good, but to have it stand all that power (100W clean into 8 ohms means almost 200W squarewave when run full throttle) they used *thick*  wire in a large bobbin, plus *thick*  cone paper plus *lots*  of adhesive, all that weight adds up and you lose sparkle bybthebtruckload.

Not bad in an SS amp because it softens the buzz, might be acceptable for you, let your ears decide.
Definitely not a twanger´s delight.

As to the amp, technically you should go straight to Power Amp input jack; but in this particular amp they actually turned it into a variable level effects loop , selected by S4, they add a "gain offset" pot, the works.
In my view, they complicate it needlessly.
I suggest you get *straight*  into the real power amp input: lift the top leg (as seen on the schematic, the real one on the PCB may point anywhere) of C44 , wire preamp output there.
When you lift one leg of C44 out of its PCB hole, vap body will float in the air supported by justb one leg, which will eventually break because of flexing or vibration, so glue its body to PCB with some hot glue or better some RTV.
Add 2 Leds across C62, one pointing up, the other pointing down, you can use the small 3mm ones if you wish, they will clip any excessive peaks the tube preamp might send to Q7 base.

Now to a physical problem: where will you mount your preamp potentiometers?
Front panel is already occupied by original ones, which are soldered to a *large*  board with everything in it, and you can´t just use a bandsaw to split it, so on second thought, I suggest you build your preamp on a separate case (your choice) and feed it to original power amp input.
You might even make a cool looking "mini head" ... no own power amp of course but designed to drive something else.
Care to share its schematic in case there´s something we didn´t foresee?