Crate V18 conversion

[jeff]

Thought I start a post of my project to:
 1) Show what I'm doing(in case anyone's interested/wants to try/ask advice),
 2) To contain everything to one spot so I'm not creating a bunch of seemingly random posts for questions I have along the way.


I was given a Crate V18 with a blown switch mode power supply. My goals are to
 1) Replace the SMPS with a transformer
 2) Reconfigure the pre-amp
 3) Rewire for all tube signal path when not using effects loop
 4) Add N.F.B. loop and presence
 5) Add a P.P.I. master volume


I've made a lot of progress so far, but I'm planning to go back and show pics/schematics of what I've done so far(probally should have done that in the first place)


Is this something anyone is interested in?
More to come(pics/schematics) soon.


Peace

[jeff]

Here are the original schematics(errors n all)
I'll be drawing up schematics for my conversions and posting soon.
https://www.thetubestore.com/lib/thetubestore/schematics/Crate/Crate-V18-Schematic.pdf

[jeff]

This is the pre-amp I’ll be using. I used this on my “sewing machine” amp with 6V6’s and like how it sounds

[rake]

Are you going for an all tube circus board / solid state free version?

[jeff]

Nope,
Just rewiring/modifying the existing board. I see no need not to do this unless there's a problem.
   Also, this amp has a line out but no line in for some reason. It uses one half of a TL072 chip for the line out and another half is always in-line with the signal path. I don't think this is needed with the preamp I'm going to use.
   I'd like to use a switching jack to bypass this and only engage the 2nd half of the TL072 when using as an effect loop return(unless something is plugged into the loop, the TL072 will be switched out of the circuit).
That's the plan.

[jeff]

•WARNING•
Don't take these as instructions yet, I've not finished and got the amp up and running. If/when this conversion is a success, I will edit post.


First thing I did was to convert the 4th stage to a cathode follower. This does involve lifting traces.
I removed C11, R17, R14
Lifted trace connecting pin 1 to C11
Lifted trace connecting pin 6 to R18
Lifted trace connecting pin 7 to R17
Replaced R14 with jumper
Connected pin 1 to pin 7 with jumper
Removed R15
Replaced R16 with 100K
Removed C13
Lift left side of R18 to solder jumper wire to it then resoldered to original hole
Connect other end of jumper to + of C13 hole
Remove R21, R19, R26
Jumper LEFT hole of R19 to RIGHT hole of R26(may revisit for effects loop)
Removed R22, C16
Jumper replaces C16
Jumper replaces R62

[jeff]

Before/After

[jeff]

When I say piggy back I mean solder components together in parallel so the two components have two leads

Value changes:
Replace C6 w/ .68uF
R2 - 2K7
R5 - 10K
R10 - piggy back 100K+500pF
R3 - 100K
C9 - Remove(optional plate snub, 2nd stage)
C7 - .022uF
R51 - piggy back 470K+500pF
R13 - piggy back 1K+.68uF
R12 - 470K


Remove: C8, R7, C1, C2, C5, C4, R6, C3, R8, R9 & R10


C1 - .022uF
C2 - jumper
LEFT hole of R6 to LEFT hole of C3 - .001uF(skipping over and NOT connected to RIGHT hole of R6)*
C3 - .002uF(could be connected to toggle for .002uF/500pF switch)
R10 - 470K


Jumper wire connecting LEFT hole of R9 to be soldered to wiper of gain pot
Replace 250K gain pot w/ 1meg Audio taper pot

[jeff]

This is where I could use some advice.
I like the idea of bypassing the TL072 chips when the effects loop is not in use and using switching jacks to use as a line out buffer/recovery gain stage when using loop. I want to dupe this:
https://www.tangible-technology.com/schematics/fender/hrd/hrd-3.htm
"Tu be or not Tu be" section


I believe the HRD uses a 50K volume pot, where this mod will be using a 1meg pot
 My question is, the HDR uses a voltage divider47k/2k7 to the input of the send buffer. Not a drag on the HRD's 50K volume pot but looks like it'd be a drag on the 1meg pot I plan to use.
Could I just scale up the values?
Instead of 50K vol pot with 47K/2K7 use my 1meg vol pot with a 470K/27K to the buffer's input?

Also, OK to just dupe all other values for the return?

[jeff]

Any advice for drilling a round hole with a flat spot(fuse holder)?

[bmccowan]

Not trying to be flip, but drill a round hole and use a round fuse holder. I'm not aware what fuse holder requires a "round hole with a flat spot?"
I am not able to respond to your other questions as I don't deal with PCB boards and such. I suspect that is true of many here. Faced with your amp, my choice would have been to pull the boards, determine a schematic and layout, and build a turret board to fit that chassis, adding a suitable PT and filtering.
But, best of luck with your project.

[jeff]

Yes, most fuse holders do fit a round hole, but the fuse holder itself usually has a flat spot(where there are no threads) on one side or on two opposite sides. This acts as a 'keyway' to stop the fuse holder from spinning or twisting in the unlikely event it gets loose.

[jeff]

Or loosen when twisting on and off the cap when replacing fuse. As you twist the cap, the holder is locked in place by the flat spot 'keyway' so if the cap is too tight you're not unscrewing/loosening the holder while trying to unscrew the cap.
I think that's the general idea

[jeff]

 Faced with your amp, my choice would have been to pull the boards, determine a schematic and layout, and build a turret board to fit that chassis, adding a suitable PT and filtering.
But, best of luck with your project.

I dig it. I much prefer that method too. All the amps I've built up to now, I've taken that approach. I guess what led me down this path was someone gave me this amp for free because the switch mode power supply failed so, I figured, I have an extra P.T. let's see if I can get it up and running. Then that led me to think, as long as I'm in here, let's see what I can do.


Much prefer other method over PCB, and that's what I'd normally do, but I didn't want to put any money into it(spare transformer anyway) and figured, hey, it was free, I got nothing to lose, why not mess around, maybe learn something new along the way.


Peace.

[bmccowan]

fuse holder - I recognize now what you mean. I guess I am just so used to chasses with round holes that I've been ignoring the flat sides. I think the punch for those is called a "double D" - not sure, but the rubber washer and/or a toothed washer seems to do the trick.
I hope the project goes well - fun to experiment. My experiences with pedal pcbs with traces have been good, but no so with amps. but my eyes and fingers are both on Social Security, so that doesn't help.

[jeff]

Wow, Double D punch, that's it.
Thanks!
I was planning on drilling a hole no bigger than the flat sides and using a rat tail file to expand the "D"s. That'd've been a pain.
Much better solution


Peace
Tip: When you google 'Double D' and want the tool, include 'punch'...

[bmccowan]

or just use a fuse so big that it never blows - sorry, just a stupid joke.

[PRR]

Anybody still in the dark: the standard fuseholder for 3AG has a mild "D" shape to make it harder to rotate (and tear itself up if forced).
https://www.littelfuse.com/~/media/electronics/datasheets/fuse_holders/littelfuse_fuse_holder_342_datasheet.pdf.pdf