Carolina Overdrive Special amp

[Apexelectric]

Tubenit,

I wanted to try to make this layout work in a Tweed Bassman narrow panel chassis.  I have a blank aluminum chassis that I can drill and cut to my liking. Let me know your thoughts. Cap can and relay power supply is not shown but would most likely locate the cap can on the back of the chassis as typical and put the relay power supply on the side of the chassis near the power transformer. Capacitors are as shown on the original schematic. Ill install an octal tube socket for the rectifier for a GZ34 tube or possibly a SS rectifier plug.

Power transformer is a Classictone #18029, 305-0-305, 5v 3A, 6.3V 5A  and Output transformer is Classictone #18008, 4.2K. - 4 or 8 ohm, 50-60W and Classictone, #18003 4H, 90ma choke.

My biggest concern is noise and hum issues so any insight regarding the prevention of these issues would be appreciated.

[tubenit]

Your layout looks excellent to me and well thought out.  Your turret board is more the Hoffman style which has a good track record of being quiet.

You may be able to make this successfully work?  My main concern would be the relays and how you will fit those in a narrow Bassman chassis between the board and the pots ?


With respect, Tubenit

[Apexelectric]

What if I put the relay boards on top of the circuit board? I changed the layout to reflect what Im thinking. Was thinking the midboost was just a toggle switch on the front panel or do you think it makes more sense to incorporate it as a foot switchable feature?

I like working in the narrow panel format but one more inch of width would be better.

Thanks for checking out the layout.

[tubenit]

I have two Dumblish inspired amps that each have a 3 button footswitch.  What I find is that I use the clean/OD switch and the midboost and very seldom use the 3rd switch on either.

[Apexelectric]

Any noise considerations with the relay board locations on the turret board?

[tubenit]

I would not anticipate any noise problems but I've never done one that way.

[Apexelectric]

Well I’ve taken the plunge and have started on this project. Turret board is staked and ready to populate.

Tubenit, one question I had was that on some of the cap values, you must have ran some in series or parallel to get the value shown on the schematic. I’m assuming this was as a result of tweaking values to get the tone you were looking for and not necessarily a mix of two different types of cap?

Some of the coupling cap changes in the later revisions reduced the values to the point where you must have changed from using the Musicaps to the Orange Drops instead?

What voltage ratings would you recommend using on the caps for the hi/low pass filters and would it matter much if they were Xicon chicklets instead of Orange Drops?

Thanks for all the help! This should be a good one once it’s all done.

[tubenit]

I tend to parallel caps to get odd values.

Correct.  I went from Musicap to lower value Orange Drop PS caps.  The Musicaps are very clear transparent almost hi-fi-ish and should be used sparingly and don't seem to be available in very many values.

I tend to use 400v ratings for the most part.  I think Xicons would be fine.

With respect, Tubenit

[Apexelectric]

Looks like I’m going to have to shoehorn this one in but it should just make it. Here’s the progress so far.

[tubenit]

That looks great!   

[Apexelectric]

Thanks Tubenit,

Ill post more pictures as I continue to progress.  I installed the switching relay boards yesterday afternoon and am waiting on a bypass cap that I didn't have, otherwise the board is pretty much complete.  Ill get the chassis drilled out for all the jacks, switches, sockets, pots, transformers, etc and follow up with some photos later in the week.

I think I've decided to stand off the power transformer and not recess the end to give me a little more room in the chassis for the relay power supply board and the ground terminals.

[Joel]

Watching with interest.

This one is next on my wishlist to build, so I'll be shamelessly stealing your layout drawings and studying your build pics for inspiration!

[tubenit]

I think I've decided to stand off the power transformer and not recess the end to give me a little more room in the chassis for the relay power supply board and the ground terminals.

I think that is a good choice! 

I'm curious what all you are switching with relays?  Is it clean/overdrive and midboost .............. OR is there more?  While I have 3 relay switches on two of my amps, I seldom use anything but those referenced.

With respect, Tubenit

[Apexelectric]

Tubenit,

Just the two relays for the channel and midboost.

Was starting to layout the controls last night and would prefer to move the input jack to the front panel if I’m going to stick it in the narrow panel chassis. Was wondering if I could get rid of one of the OD control pots. Was curious to know if you found one of them to be unnecessary or at least not all that necessary for getting the most out of the design.

I can make it all fit but a Little less to squeeze in might be helpful.

Joel,

So far the layout is 90% there but I wouldn’t get started on it too soon till I can work out all of the the bugs. I’ll continue to modify the DIYLC layout as I make changes during the build and post a final design at the end.

[tubenit]

Yes, I would put the input jack on the front panel.  It's only on the back panel in my drawings and not on the real amps.  Strictly a drawing thing in spacing the drawing & a practice I should probably quit doing or explain.

In the signal chain of the overdrive, you have trim, drive and level.  I seldom adjust drive.  I think on one amp I used a 39k resistor to ground instead of the drive pot and on another, I think it was an 82k to ground?  Can't remember right off & these weren't COS amps but did have a clean/od channel. You may want to try the 27k - 39k  understanding that you may wish to increase or decrease that?    So, yes I have eliminated the drive pot on amps before.  If I were doing another build and needed room, I'd consider eliminating the drive pot again. 

I do use the trim pot and level pot consistently.

You also could have it be an internal pot on the board OR on the back of the amp where you mount the PT and OT. 

With respect, Tubenit

[Apexelectric]

I was originally going to do it in a different chassis so that the back side was an option for the input but it’s obviously not too practical in this case. I think I’ll put a trimmer pot on the board instead of the Drive pot if I can fit it easy enough now.

It might be handy to use some 16mm pots instead of the 24mm but they can’t handle the same amount of current. Any pots that would a higher current consideration that might require the 1/4W to 1/3W rating?

Was also looking for a recommendation on labeling the chassis. I’ve got an aluminum chassis that I’m working with now and have considered having faceplates made instead of transfer decals or printed sticker type. What I really would love to do is silk screen it on but seems like way too much work to get it looking right.

Any suggestions?

[Joel]

Mr Apex,  Don't worry.  I won't be building for a while yet.  The wife keeps asking "how many amps do you need anyway?".  So I'm playing the waiting game for a while.  (the answer is of course "just one more"!)

Faceplates.  The one thing I've stuggled with the most.  I have found one promising solution: Film Free Decal Paper.  I haven't tried this yet!  You slide it on a bit like waterslide decals, but then bake it onto the surface.

https://drdecalmrhyde.com.au/shop/film-free-decal-paper/

When I get some time, I'm keen to give this stuff a shot for amp faceplates and stompbox art. 

[tubenit]

I view my amps like a watch without numbers.  I know what each control does, so I currently don't have any faceplates on my 3 amps. 

I've used waterslide decals in the past or decals of just letters like V for volume.

I've used 16mm pots for tone controls and dwell pots in the past without any issue.

With respect, Tubenit

[Apexelectric]

Mr Apex,  Don't worry.  I won't be building for a while yet.  The wife keeps asking "how many amps do you need anyway?".  So I'm playing the waiting game for a while.  (the answer is of course "just one more"!)

Faceplates.  The one thing I've stuggled with the most.  I have found one promising solution: Film Free Decal Paper.  I haven't tried this yet!  You slide it on a bit like waterslide decals, but then bake it onto the surface.

https://drdecalmrhyde.com.au/shop/film-free-decal-paper/

When I get some time, I'm keen to give this stuff a shot for amp faceplates and stompbox art. 

I’ll give my wife props, she pretty much let’s me follow my passions. It’s not so bad to be surrounded by nice guitars and amps an a wall full of records anyway. She kind of gets it.

I’ll have to look into the decals you mentioned. Otherwise it’s going to have to be custom faceplates. I’m kicking around the idea of getting a small CNC to do the plexi faceplates but not ready to commit to that idea.

I've used 16mm pots for tone controls and dwell pots in the past without any issue.

With respect, Tubenit

You think volume type might be more of an issue?

[tubenit]

You think volume type might be more of an issue?

I used a 16mm pot for an FX send once & a reverb dwell once before without issues.  In both of those, I had a cap in the signal path prior to the pot.   Having said that,  I don't know if it could become an issue or not? 

The waterslide decals are pretty cool and fairly easy to use, IMO.  The make the decal paper both with transparent and in white background. It's important to know which of those you're wanting to use as the transparent does allow some background to come thru.  There are plenty of internet instructions on using them out there.  I've used Krylon clear gloss acrylic spray to seal the decal prior to dipping in the water. 

With respect, Tubenit

[Apexelectric]

Just ordered some of the film free decal paper from Amazon. They had two types, one for smooth and one for porous surfaces. I’ll let you know how that goes since I’ll be getting that part done sooner than later.

I think I’ll get the chassis drilled out this weekend and the transformers mounted.

[MFowler]

Henry from Redplate puts his relays under the board and told me he has never had any relay issues! not sure about that but he still continues to build that way.


Mark

[Apexelectric]

Henry from Redplate puts his relays under the board and told me he has never had any relay issues! not sure about that but he still continues to build that way.


Mark

Im afraid that it would be a service nightmare in this case if I did that. There’s no easy way to remove the board to get to the relay. Will have to see if there is any unwanted noise with it mounted on top. However, I am a bit concerned with it being so close to the power rail.

[tubenit]

That is a well thought out and neat layout.  I think you'll be fine with the relays where they are. 

I definitely would want easy access to them because I mod things so much and may want to switch something else in/out.  IF I mounted them under a layout board, I think I'd try to rig up some type of sheet metal door on the other side of the chassis to have access to them.

With respect, Tubenit

[Apexelectric]

Thanks Tubenit!

Was re-considering the transformer layout on the back of the chassis and was wondering if it would make more sense to mount the choke next to the cap can and land the choke leads at the caps instead of on the circuit board? This would position the output transformer in a little better spot for bringing the leads through where I want them and I can have the reverb transformer and the choke front to back and not take up as much room on the width of the chassis. This also gives a little more room with nothing around the perimeter of the output transformer to hopefully keep it from picking up any interference. There is currently only about an inch of space between all of the transformers in order to get them to fit side by side with the proper orientation.

I have the PT and the choke rotated 90deg from the OT and reverb transformer.

Thoughts?

[tubenit]

I think what you're suggesting sounds reasonable and I would give it a try.  You are sort of "out there on your own" in that I am not aware of anyone who has attempted this particular build in this confined of a space.    Having said that,  what you're doing makes sense to me and it looks right.    I am rooting for your success with this!

With respect, Tubenit

[Apexelectric]

If it makes sense to you, I respect your experience enough to move forward in the direction that seems to be a logical option to me. If it works out then that gives options that might not have seemed worthwhile before, if it doesn’t then I guess it’s a lesson learned.

No risk, no reward.

I’ll keep you posted on the outcome.

Thanks again!

[sluckey]

I don't envy you working inside that skinny chassis.

[Apexelectric]

I do love a challenge though. Preferably one that works out in my favor at the end.

I’ll admit that if the chassis was another inch wider it would be 10X better to work with.

[Apexelectric]

Hers a shot of the latest progress. Mostly done on the tube side of things and the back side. Just need to flip it over and get the control panel side wired up and the relay power supply board wiring done and then ready for testing.

It’s definately been a bit of a challenge cramming this all in the Bassman sized chassis. Not sure I’d go this route again.

[Apexelectric]

Just need to finish the switching relay power supply wiring. Any reason to isolate the 5V power supply ground reference from the 1/4” jack for the foot switch or will a standard Switchcraft jack suffice?

I’m using a non switching stereo jack to control the two relays and sending the ground through the jack before it gets to the coil on the relays.

[sluckey]

Any reason to isolate the 5V power supply ground reference from the 1/4” jack for the foot switch or will a standard Switchcraft jack suffice?

That depends on where you got the AC power for your 5VDC power supply. If you used a separate transformer to supply AC to the 5V supply then there is no need to isolate. But... if you stole 6.3VAC from your existing filament winding ***AND*** that filament winding has a center tap that is connected to chassis ground ***AND*** your 5V power supply uses a bridge rectifier, you must isolate the entire negative side of the 5VDC supply from chassis. Use a Cliff style isolated jack or buy a couple fiber bushings for your switchcraft jack. (You'll have to drill a 1/2" hole to mount the switchcraft using these fiber bushings.) Hoffman sells the bushings.

If you are unsure about your filament wiring, then always float/isolate the 5V negative (you called it ground). There is no requirement to connect any part of the 5V supply to chassis.
 

[Apexelectric]

Guess I better isolate it then seeing as I meet all the criteria. Glad I asked.

It’s a center tapped filament winding on the PT. I’m using the Hoffman 6.3VAC to 5VDC board with the bridge rectifier.

What would happen if you connected the ground/negative from the 5V power supply board to the chassis ground?

[sluckey]

What would happen if you connected the ground/negative from the 5V power supply board to the chassis ground?

In your case, two of the diodes in the bridge will immediately fail! Hopefully they will burn open and not cause any damage to the PT.

[Apexelectric]

Glad I asked. Thanks again Steve!

[Apexelectric]

All the wiring is complete and am now waiting on a couple of 12AY7’s to show up. None on the shelf. I’ll probably try to get it up and running tomorrow and use a 12AT7 or a 5751 for V1 in the meantime. Keeping my fingers crossed.

[Apexelectric]

Gut shot

[tubenit]

Wow! What a remarkably well done job !   Wiring, layout and everything else looks very neatly and professionally done.  I think you stand a good chance to have a reasonably quiet amp at idle even with the small chassis to work with.  I am genuinely impressed with your amp building skills. IF it sounds as good as it looks, I think you will have a sweet sounding amp there.

Thank you for sharing the photos!

Somewhere down the line, if it's possible for you to record both the clean and the OD channel without too much in the way of effects ........ I'd sure love to hear some sound clips from you.

My guess is that you might enjoy the amp more with the 12AY7  in V1?  As an FYI,  I have not found a 12AT7 that sounds very musical for me in the V1 position for some reason?  I'll use them in the LTPI, reverb or an FX though.

with respect, Tubenit

[Apexelectric]

Thanks Tubenit!

Really appreciate the kudos. I was double checking the schematic vs my layout last night and found a wiring discrepancy when I pulled the OD drive pot and replaced it with a trimmer on the board. So I think I’ll take some time today to go through my layout with a fine toothed comb and triple check it all before I fire it up.

The player friend of mine that I think might like this amp wants to check it out next weekend so maybe I’ll record him playing it instead of me which would certainly be a better way to show it off.

Thanks again. Hopefully this fires up without too many issues to bug you about.

[Apexelectric]

One question I meant to ask. I am not going to bother with a separate standby switch but I was considering a simple way to silence the amp during breaks and Guitar changes would be to just short out the signal somehow. I was thinking about using a 3 position power/standby switch and using the extra set of terminals to short the signal but I don’t want to introduce any noise by taking it back to the input. Was wondering about doing it at the MV control?

[Apexelectric]

Can use a little help on the amp.

Not sure why Im not getting plate voltage on V2b. I have it at the plate resistor but its not making it through. Doesn't seem to matter whether the tube is installed or not. I get audio out of the amp but its weak.

Attached is the layout as it stands

[sluckey]

Remove that 120pF cap from the socket. Got voltage on pin 6 now?

Is there a schematic for this layout?

[tubenit]

This schematic probably isn't accurate to the layout, but perhaps you can edit the SCH version attached to match your layout?

With respect, Tubenit

Can you clarify what you mean that you have voltage at the plate resistor?   Do you mean on the B+ side of the resistor or the socket side of the resistor?    FWIW,  I once had a brand new resistor that looked perfect & would measure OK, but it had a broken wire inside the resistor itself inside the enamel body.

[tubenit]

Look at your layout and how the reverb is hooked up.   Look at the schematic I posted.   Focus on the topology of the layout in relationship to the topology of the schematic.

V4-2 should go to dwell.    V4-6 has C-19 cap ............. where does that go?  It should go to the reverb pot.

Where is the insertion points in your layout for the reverb?

Double check stuff and see what you have there, please.

With respect, Tubenit

[tubenit]

IF you remove V4 tube and safely use an insulated alligator clipped wire to go from the OD level wiper to the entrance cap into the LTPI  C20, then you do you a good volume to the amp?   Illustrated with orange jumper.

IF that doesn't work, then jump from OD to C20.  Does that have good volume?   Illustrated with green jumper.

IF so, then perhaps your reverb wiring needs to be redone? 

You'll get there and have a working amp and I anticipate a good sounding one!   It's somewhat within normal limits to have something amiss initially on a build that is this complicated.

With respect, Tubenit

[Apexelectric]

Remove that 120pF cap from the socket. Got voltage on pin 6 now?

Is there a schematic for this layout?

No change

I’ve been working off Tubenits schematic for this amp

V4-2 should go to dwell.    V4-6 has C-19 cap ............. where does that go?  It should go to the reverb pot.

Where is the insertion points in your layout for the reverb?

Double check stuff and see what you have there, please.

With respect, Tubenit

Looks like I got some issues with the reverb layout. I’ll look into it some more and get back to you. I seem to have the insertion points backwards on the pots but there might be more. I’ll check in after I look at it.

{EDIT- lost quote fix -PRR}

[sluckey]

Not sure why Im not getting plate voltage on V2b. I have it at the plate resistor but its not making it through.

Please clarify what "not making it through" means. Do you have voltage on both sides of the 220K resistor on board but the voltage does not make it to V2 pin 6? If so, then the wire between the board and pin 6 is open. Easy check with ohm meter.

Or do you mean you have voltage on only one side of the resistor and it's not making it through to the other side of the resistor? If so, the resistor is open, or has a bad connection, or is shorted to ground. Easy check with ohm meter.

Or does "not making it through" mean something else? A good close-up pic showing that part of the board and wiring to V2 will be helpful if you can't figure this out.

[Apexelectric]

No voltage is passing through the resistor to the plate. Does not appear to be an issue with the resistor itself.  Its shorting somewhere after the resistor.

[sluckey]

Check resistance between V2 pin 6 and chassis.

[tubenit]

Any update? 

Tubenit