Peavey 6505+ FET EMULATION

[mensur]

Can guys on the forum help me with creating of this super hi-gain preamp*(pedal)?I have the schematic of the 6505+, and i wanted to share it with you.I have drawn the LEAD channel, and i need some help. How to bias FET's on this circuit (what are the values of the trim pots)?Must I change values of the source(cathode) resistors?Are the resonance and presence controls ok, since i bypassed phase inverter?I will draw the PCB right away when you guys give me some answers.I will appreciate any help.
Here is the original schematic of the 6505+:

Here is my schematic:

[bancika]

That looks like 5150II amp, it says in the corner

[mensur]

6505+ is the relaunsh of 5150 Mark II head, 6505 is the relaunch of the 5150 head.I have the schematics from the Peavey, the circuits are the same.

http://www.wolfgangregistry.com/5150-info/peavey-6505-preview.html

[Derringer]

man ... that looks like QUITE the high parts count !


as far as trimpots, I'd imagine that you just need to run a 100k trimmer between your V+ to the drain of each FET. Mid bias is half of your V+. So if you're using a 9V battery, start by biasing the drain of each FET to 4.5V.

Once you get it up and running, you may want to then fiddle with the bias voltages, bring some up, bring some down, etc. to see if you can better approximate the 5150II sound.

[mensur]

Thanks,

Do you think that 100K trim will handle 24V?, and what about source(cathode) resistors?

[Derringer]

I "think" that you'll be ok using 1/4watt trimmers but 1/2watt would give you some more room for error. Also, since you're going to be stepping down 24v to ~12v, you will probably need something larger than a 100k trimmer. Hopefully someone else with more knowledge will chime in here and confirm. I don't know the equations necessary to find out exactly.


But, if it were me, I'd breadboard the first FET stage of your design and use a 1/2 watt 1M potentiometer between V+ and the drain and adjust it for 12V. Run a guitar or some other sound source through the stage and run the output into an amp so that you can hear that everything is working ok. If you can get 12V on the drain and you can make good sound, then shut everything down and measure the resistance across the 1M pot. That'll give you a good idea of the size trimmer you'll need.


And, if you can't find the right trimmers above 100k, well then just run another 1/2 or 1/4 watt resistor in series with the 100k trimmer to get the resistance up to an adjustable area.


and concerning the cathode resistors and caps, 1/4 watt resistors should be ok and make sure your caps can handle your V+ voltage .... just copy the cathode resistor and bypass cap values from the 5150II schem


Quick question ... have you built anything like this before? This project looks awfully complicated what with the high parts count.

[mensur]

I will be using the 200K trims, but i am worried about some values of source resistors in my schematic(R4-82K, R13-82K, R16-39K, R28-22K, R40, 1M, R44-33K).Do I need to lower them 10 times(82K to 8.2K)?If the source resistance is larger than drain resistance, there will be no gain.
I successfully built randall rg100es amp, boss ds-1, JCM800 valve preamp... I wanted to build some hi gain preamp sing FET's.

[drewl]

YIKES!
I hate working on the tube versions, I can't imagine trying to build a FET clone.

[Ed G.]

Before you get caught up making a pcb, I'd breadboard it first and see how it sounds.
I don't think that simply transferring a pcb from tube to jfet automatically gives you an approximation of the sound of the original.
I personally find to get a 'tight' distortion sound, some filtering and tone shaping has to be done. Higher voltages help, because jfets can't handle the voltage swing as well as a properly-running tube can. So you get distortion that comes on too quickly, sounding compressed, with muddy bottom end.
But on the breadboard, you can try substituting different things. First off, I find the input resistor is too high. It sounds good for tube, but I like it to be 1k or less to sound more 'open' and less muffled and muddy.

[Derringer]

I will be using the 200K trims, but i am worried about some values of source resistors in my schematic(R4-82K, R13-82K, R16-39K, R28-22K, R40, 1M, R44-33K).Do I need to lower them 10 times(82K to 8.2K)?If the source resistance is larger than drain resistance, there will be no gain.
I successfully built randall rg100es amp, boss ds-1, JCM800 valve preamp... I wanted to build some hi gain preamp sing FET's.

well then you're definitely more experienced than I

Is that a rule( source R greater than drain R = no gain) that just applies to triodes or FETs as well? And when you say no gain, do you mean literally no signal would come out? unity gain? something less than unity?

Maybe take a look at exactly what those non-typically biased triodes are doing. I'm curious, what are they doing to the signal with those large cathode resistors? Where exactly are the triodes being biased at? Cold stages?

FETs obviously work differently than tube triodes so maybe you have to look more at where and how each triode is biased and then try to approximate that with the FETs.


You might have to look at some graphs. Keep us updated. I'm definitely curious.

[DougH]

If this is some sort of 5150 derivative, start with a Dr. Boogey and mod if necessary. That will save you a lot of time and trouble.

Boogie Recto's, 5150's, and SLO's are all related.

[guitarman89]

I have drawn an engle fireball pre amp section with fet if you want (it is in my room's dark hole, so i've to look for it) i can post it!

[jayp5150]

If this is some sort of 5150 derivative, start with a Dr. Boogey and mod if necessary. That will save you a lot of time and trouble.

Boogie Recto's, 5150's, and SLO's are all related.

I almost mentioned that, and then I lost interest before I made it to the "post" button lol.

Someone has a SLO100 fet emu, I think, in the gallery. It might be a good starting point, but make sure that whoever needs it gets credit... (and I don't mean me, obviously).

[deaconque]

If you search for "freya", you'll find SoulSonic's SLO100 fet emu.

[km-r]

i kinda tried taking up the 6505 preamp once... i requested a schem from peavey...
IMHO, i think the parts count will make this circuit very troublesome... you even have to wrestle with the four gain stages of the DB, so, what would it be like with the 6505 with 6 12ax7's???

playing through a 6505 once, i really think all that preamp gain is not needed.

[dellamorte]

a little ot but just wondering if you could share more about the rg100es ? how did it sound? schematics?

[mensur]

I was just thinking, can this be done like the Dr.Boogey project, replace tubes with FET's and lower the source resistors in factor of 10?

[mensur]

What do you want to know about randall?

[snap]

Peaveys R17 and R13 look like 1k82 when I look at magnified other .pdf reproductions of a 5150 schematic.
For an 82k I`d rather use a 250V rated bypass cap, and still wouldn`t expect any signal coming out of the plate 

[mensur]

What do you mean?