Please look over my shoulder on this mod (Completed)

[Platefire]

Got this revere reel to reel amp I built out of spare parts back in 2007 that if I recall right? Geezer build his version too? Someone did and I think it was Geezer. To brief you on what it is, the amp is generally a 5D3 Deluxe but because the amp had a 6SC7 I used that for the PI like a 5C3--so we ended up calling it the 5DC3.

Well the amp sounds good but believe it or not has a little too much clean headroom and is a little stiff--so I was thinking of adding a MV and also using an un-used 12AX7 pre stage. So the plan was to install the a pre PI MV and use a DPDT switch to switch the extra preamp stage in an out so as to preserve the present amp operation. Attached is the original schematic and the schematic with mod. Please look over my shoulder and see it this mod is workable. I can't think of any reason it shouldn't work but I've never done this type of switch-a-rue on a gains stage before either--so let me know what you see. Might be a better way to use the gain stage to add in a little more flex?! Thanks! Plate

[LooseChange]

I think your plan will work. The original does not have much gain.
To warm it up I would try this...
Reduce the power supply filtering and parallel the preamp tube.

[DummyLoad]

cleaned up the schem - where there is a DOT there is a junction. i think this is what you're trying to achieve? hopefully geezy will chime in.

the resistor i added that is marked "R New ???" probably should be a 10K 1/2W the preceding 10K 3W can be a 1/2W as well...

--DL

[bluesbear]

I had a Blues Pearl version of a 5C3 and built another from scratch. I'm having a hard time getting my head around it not having enough gain, though. Mine had lots! I love 6SC7's; they're my favorite preamp tube. I wish someone would make a glass version again. I was lucky enough to find 20 Tung Sols for $4 each a few years ago. Sadly, I only have 4 left now. I'm saving them for something special.
I can't look at the sch file on this computer but what you're describing sounds okay. Ought to be sweet!
Dave

[tubenit]

Platefire,

I would suggest changing the input resistor for 33k (for more gain)  &  I would add a 10k grid resistor on the gain stage.

with respect, Tubenit

[VMS]

Hi, I'm PI-curious. 

If you take the PI from 5C3 then don't you also take this part from 5C3.

[Platefire]

Hi, been gone all day and just got back looking over all the responses, Thanks! To answer a few questions/comments buy bluesbear and VMS---this is not a pure 5C3. It has a 12AX7 preamp like a 5D3. The PI 6SC7 has the cathode resistor and bypass cap of a 5C3.   OK stop there!

When I was building this amp in 2007, I did a little reading up on the 5C3 and 5D3 circuits. What I recall about that research was the 5C3 had a problem with a little too much distortion in relation to the PI output being unbalanced and with 5D3 circuit on the output of the PI was changed by fender to properly balance the output to cut down on distortion. Rather I got that right or not, don't know? that's what I remember---So based on my research at that time, I applied that same 5D3 self balancing circuit on the end of my 6SC7 PI, even though the 5D3 uses a 12AX7 PI, because as I understood it the circuit correction corrected the distortion problem and provided more clean headroom. So if that's an improper mixture of two circuits, you tell me?

It's got the clean head room but it's a little cold on reponse and when pushed the distortion overtones are kind of fizzy. So the clean tones are pretty good and go pretty loud for an old deluxe, the touch response could be better and breakup in not too harmonically pleasing. So I hope that kind of explained how I ended up with what I got. So to generally break it down, it's a 5D3 with a 5C3 PI (6SC7 instead of 12AX7) and of course 6SC7 has a shared cathoded and reduced gain compared to a 12AX7. So I'm hoping these changes will put a little more life into it and make it more harmonically pleasing.  

So thanks for all the suggestions and I will be studying those schematics and checking my parts inventory to see if I have everything I need. I will also need to tear the amp down and find real estate to mount these extra parts. Platefire  

BTW-Tubenit-I'm not understanding your DPDT wiring? I'm sure it's right but still not understanding it yet!

[DummyLoad]

OK, reading the above. if the original uses a divider for paraphase consisting of 1M and 25K, then the ratio is 40:1. if we use a 270K instead of the 220K calculate for lower resistor - 270K/40 is 6750 - use 6K8 resistor.

schematic
  - fixed PI - 270K was incorrectly wired.
  - fixed switching.
 

--DL

 

[tubenit]

I'm not understanding your DPDT wiring?

Trace the colored lines.   This is similar to Doug's hotswitch mod and how Dumble clones are done.

With respect, Tubenit

[bluesbear]

"So if that's an improper mixture of two circuits, you tell me?"

If it works, it has to be proper! During the transition from 5C3 to 5D3, Fender made some amps that had one 12AX7 and one 6SC7. They're rare but they do exist. Please keep experimenting. That's where new ideas come from. Those are both great amps. If you mix the attributes you like from each and make an amp you like better than either, you're officially an amp designer.
Dave

[DummyLoad]

...

 

[Platefire]

Tubenit, Thanks for the nice color coded schematic on the DPDT operation--that helps. I understand it better but still studying--there is a lot of different ways to hook up a DPDT-this is one of the more sophisticated ways;>)

DummyLoad---not sure where you getting your values for that circuit but I got mine right off a 5D3 attached. Platefire

BTW-on an added note, just got back from the hospital in Shreveport where my 9th grandchild was born--mother and babe are doing well--she's beautiful   

[DummyLoad]

DummyLoad---not sure where you getting your values for that circuit but I got mine right off a 5D3 attached. Platefire

i calculated the values based on the original 5C3 ratio of 1M and 25K.

it's a different animal altogether in the 5D3 - bias is diff and so is the ratio and what i don't understand is why they 270K is connected to the 220K/56K node and not grounded.  i'll deffer to experts...

so sorry for the confusion...i'll go away now. 

[Willabe]

BTW-on an added note, just got back from the hospital in Shreveport where my 9th grandchild was born--mother and babe are doing well--she's beautiful 

All right Platefire, good for you, good for you'r family!                

I wish you many, many happy hours and times with all your grand children!

Got any old enough to start to teach them how to play?       


       Brad       

[Platefire]

Willabe-My oldest grandson is graduating high school this year.

DummyLoad--you got it on me keeping track and using all the formulas to calculate proper loads. By the time I learn one formula, by the time I need it again it's gone--I have to be re-trained! So I wouldn't discourage you at all--a lot of this stuff is just over my head--so I was just copying what had already been done.  Applying that 5D3 PI circuit to the end of a 6SC7 as compared to a 12AX7--may need some tweaking? But I have to admit, I got pretty good results and clean sound is pretty outstanding which is my main sound---but I like my clean with a little bit of cool sounding grit on the top end of the volume spectrum as to achieve some nice clean with some compressed flex on the low end of the guitar volume and on the edge of breakup punch on the top end of the guitar volume control. That's why I become such a fan of master volumes--you can dial that in better for different size rooms--and for larger rooms you wouldn't even need the MV for that.

So I looking at basiclly keeping what I got as for as clean-but pushing it a bit. I'm sure the DPDT extra stage with give it a level of OD it never had at the flip of a switch. The MV will be available all the time with both levels of drive. Platefire 

[Platefire]

Tubenit

Just want to reinterate I appreciate the sch mod you did for me on this and I haven't forgot it and have it downloaded to my computer. I have just now figured out how I will open up some real estate on this amp to install these parts because space is tight and when I show the pixs you will be able to see what I mean. Thanks, Platefire

[Platefire]

Well I've located my real estate for my controls and got all the parts together but----Got main AC power cord within about 1" of master volume and DPDT. The amp is a converted Revere Reel to Reel case that I incorperated the R to R features to bury my controls in which made it almost impossible for add ons as here is a picture for you to see what I mean.

So I've got a recess control mounting plate that came off of a Hammond PR-40 Tone Cabinet that I am planning to mount the DPDT and 1 meg MV pot in and it will be sunk in the 1/4" plywood on top(see pix). Everything seems to be a go but I've never had to deal with 120V AC being this close to controls--so I'm wondering if this will cause interference. Thanks, Platefire

Note: The top 1/4" plywood panel is presently pulled back to show the approximate mounting position of
the recessed control panel will go. I haven't cut the hole yet for the recess panel until I get clear on this question. 

[Platefire]

Bump-Wanting to get someone to comment on the 120VAC power cord being close to the MV pot and DPDT to switch in another gain stage I'm adding in on this modification. Low voltage grid AC within 1" of incoming 120 VAC power. Is a problem or a show stopper. See pixs in previous post. Platefire

[sluckey]

It's not a show stopper. I wouldn't do it. Surely there is a better place, maybe on the back near where the power cord enters the chassis.

However, tons of cheap consumer electronics had the power switch mounted on the back of the volume control. Think 5-tube super het table radios, B&W tvs, almost all those cheap entry level series filament guitar amps, and on and on. Even some high end hifi gear, although they usually had the ac power run in small flex EMT.

Just give it a try. It may be totally acceptable.

[Platefire]

Ok Thanks for looking and commenting Slucky

I a took lot closer look at that area and found by removing the abandoned pilot light socket right next to the pencel lead in the pix, I was able to bring the mounting plate forward a bit and gaining more space beween it and the power cord---so yeah I believe I will proceed.

Attached is a pix of the layout for the cutout to mount the mod controls plate. Platefire

[Platefire]

Well I did the mod today perzactly like tubenits schematic that he provided on a previous post. Based on preliminary testing, it is working great! Good thing is I still have the original clean sound plus a lot more extra spices! I really like what can be done with the extra gain stage kicked in. With the pre and master volume set just right you can go from clean to scream   with a flip of your guitar volume. Of course I'm still honey mooning this mod but initial testing is a blast!

Had to install a terminal strip to fly off the new plate resistor, coupeling cap, cathode resistor/bypass cap and grid resistor. I had plenty of room inside the old R to R case so I kind of let the leads fly around to have plenty of seperation between everything. Thats the good thing about that solid core wire, it's stiff enough that how ever you set it, it stays in place. It's kinda like the Royal Crown Hair Dressing, "Makes You Hair Stays Where you Puts it". The gut shot is kinda messy so beware--but it does work! Thanks for your help and support on this!   Platefire

[Platefire]

I real happy the way the mod works except for one thing---when I drive the pre-vol real hard and have the master vol cut way back it sucks out all my high end. The high end is fine in original mode with MV 100% where the pre-vol is the main volume and with OD stage cut off. I'm just wondering if I put a switchable bright cap on the MV would that help or maybe a brighter coupeling cap on the OD stage or both? Platefire

[Platefire]

Attached is revised schematic reflecting actual as-built circuit and voltages. Platefire

[tubenit]

Try a bright cap across the master volume.  Anything from 47p to 250p. I like 120p bright caps pretty well.

You can also try something like a 120p and 100k resistor across resistor and nonground terminal.

With respect, Tubenit

[LooseChange]

I like to add a series resistor on the center tab of the Master when going straaight into the PI.. Seems to keep the tone even as the pot is turned.

[Platefire]

Ok, Thanks! I know you guys know what your talking about and but I'm not sure I following completely.

Tubenit: Your refering for example a 120p and 100K (in parellel) resistor from input terminal to middle output terminal of MV?

LooseChange: your speaking of a resistor on the output of the MV going to the PI. Any paticular value to Use? Is this the same resistor arrangment that tubenit is refering too or is this different?

Sorry about all the questions but it's easy for me to get things screwed up  , har! Platefire

[Platefire]

The guitar I've been using on this amp lately is a Les Paul (not bright) and the amp is just about right for this guitar with the tone 100%. So in that case every other guitar I  have is brighter than the one I'm presently using--so I think I will leave well enough alone.

This mod really made this amp so much more versatile in that I can play it now at really low volume and dial in my fav tone and response which was impossible before. This is with the gain boost on, pre gain at 1 and MV at 1. When you crank up the volume with the gain boost engaged the distortion is really rich and robust with the LP--haven't got around to the strat and tele yet. Thanks again, Platefire