Finished 6V6 Plexi

[dude]

I finally finished the cabinets for my 6V6 Plexi's. I used 1/2" birch ply, half blind dovetails made a very square box. Uncle Doug helped with his lesson on Tolexing. I used 3/8" ply for front and back panel, on the others I will attach the front panel from inside as I don't like the small screws on the front of the amp. To get the chassis out easier the front panel has to be removeable, I used T-nuts from bottom, covered them with tolex and screwed from inside. I won't use this method in the future to attached chassis to cab bottom. I'll have the screws on the bottom going up threw the chassis, using the t-nuts as bolts cutting the sharp protruding t-nut legs off, with a tight hole for the T-nut on top of chassis hole into the wood should keep them from spinning. Open for any better way of fast removal of chassis, can't help myself making little changes,     
Again thanks to Rwood (get back to me on making a cab), for his help on the face plates.     

[sluckey]

Many people and manufacturers use something like this to install a chassis in a head cab. Attach the rivnuts to the chassis flanges and screws go in through the bottom. Hoffman sells them and has info on how to install them.

[Latole]

Very nice job dude, congrat !

[dude]

Learn something new every day, those rivnuts would be great. Now the only issue is getting the holes “exactly in the right place”. I made a template but must be exact..,ain’t easy. But the rivnuts have a little leeway. Using rivnuts no need to remove the front panel, no screw heads showing, fixed front and easy chassis removal, l might have to re-recover the front panel now. l see them here, Thanks Sluckey


I could make unassembled “Stout chassis” size head cabs. Cut to fit, with cleats.  Just glue up, sand and cuts cleats to size...? Just a thought.



Thanks, Latole

[Big_Mike]

Looks great!  Couple of questions - did you go with the Hoffman 6v6 design or something different?  Also, what master volume circuit are you using?

[dude]

Looks great!  Couple of questions - did you go with the Hoffman 6v6 design or something different?  Also, what master volume circuit are you using?

Doug’s and Sluckey’s schematic, except 220uf K bypass cap on normal channel. Both pre-master and PPIMV, LaMar dual 250 pot, balanced, 121p bright cap on PPIMV, both sides.

[Big_Mike]

Looks great!  Couple of questions - did you go with the Hoffman 6v6 design or something different?  Also, what master volume circuit are you using?

Doug’s and Sluckey’s schematic, except 220uf K bypass cap on normal channel. Both pre-master and PPIMV, LaMar dual 250 pot, balanced, 121p bright cap on PPIMV, both sides.

I have just the pre-master, how do you like it with the added PPIMV?  I am thinking of adding a PPIMV to mine.

[dude]

Works well with the pre, use a 250k dual pot, not a 500k,,remove the two 220k R’s, or 100k if you have them. Audio pot if you want to have a more gradual range high to low or Lin if just lower a a small amount, l used a  Lin taper.
The 121p bright caps help to retain the highs when turning down, across each pots terminals to bias pot and tubes, not to ground side.

[Big_Mike]

Thanks for the response.  One issue I have with mine, which is the stock Hoffman 6v6 is that it tends to be very bright with distortion.  I can't put my finger on it, but a regular Plexi seems to sound "beefier".  I have been reading up on Plexi 6v6s, and some claim a tube rectifier is superior in sound to a SS rectifier.  Are you using tube or solid state for the rectifier?

[jordan86]

Did you do this exact schematic?

https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf

If so, the bright cap on the bright channels volume pot (R9) is already removed, which would be the first culprit to look at. You could look at removing C5 across the mixing resistor. Could also remove C6 or replace C6 with something of a bigger value. Removing would reduce high end and gain. Replacing wouldn’t make it less bright but give more fullness to balance.

Aside from that you could increase C4 to .022 to fatten it up.

Another option would be an ice pick cap or treble peaker that Rob mentions here. Lots of great Marshall reading
https://robrobinette.com/How_the_Marshall_JCM800_Works.htm

[jordan86]

It may be worthwhile before changes things to really explore all the controls with each channel and even experiment with them jumpered. The bright channel is voiced to be DIMED on a plexi. If youre not pushing it, it will be bright and thin. Try jumpering and adding in normal channel for lower gain stuff. If you add too much low end back to the bright channel it gets muddy and flubby when cranked.

[dude]

Good advice from Jordan, but do one at a time then the numbered knob settings.
If you were to do one of Jordan suggestions, do C4, from .0022 to .022 first, then fool with the knobs. That one change should lower the brightness considerably but then you might not have a Plexi 

[dbishopbliss]

Looks great. I gotta ask... where did you get your panels made?

[dude]

Looks great. I gotta ask... where did you get your panels made?

Panels.., you mean faceplates..? If so, from a free program on the net, took file to Staples, like $10, to print 20 on heavy weight paper. Sandwiched the paper between 1/16" plastic from Home Depot, took a lot of time but $10 for 20 faceplates, can't beat that. 

[mresistor]

Nice faceplates and amp. Care to share the free program you found on the internet?

[rzenc]

Very nice build!!! Congratulations


Best Regards


R.

[dude]

Nice faceplates and amp. Care to share the free program you found on the internet?

I can't seen to find the program on my PC, I'll post when I find which program I used.

[RWood]

The panels were done with Front Panel Express:


https://www.frontpanelexpress.com/products#typ

[dude]

Many people and manufacturers use something like this to install a chassis in a head cab. Attach the rivnuts to the chassis flanges and screws go in through the bottom. Hoffman sells them and has info on how to install them.

I picked up a few rivnuts at local hardware store. The bolt keeps breaking off, maybe Doug’s rivnuts are aluminum.. and l have steel. Also, a hard time finding a 10/24 harden steel hex nut. I see Doug has a washer and hex machine screw (harden steel?) Any suggestions? I’d order the hex nut but at this time don’t need other stuff, hate to pay $10 shipping for a 25 cent bolt.
Any suggestions? If l do order other stuff, seems maybe Doug’s hex nut might break too. Would need harden steel..? Hate to order a tool off eBay, but the rivnuts seem the way to go. Any other suggestions, turning the hex bolt or nut to keep the rivnuts from spinning is taking a lot of force for a 10/24 machine screw. I used a stainless steel Allen head screw, the head stripped out on the 2nd one. Use channel locks, that held the bolt but it snapped..?

[sluckey]

You need a hardened bolt and use oil on the threads. Or get the proper tool.

[dude]

You need a hardened bolt and use oil on the threads. Or get the proper tool.

I think l figured out the problem, l’m no machinist but the washer l’m using might be the incorrect size, hole too big for a No 10 bolt. The rivnuts edges are only touching against the rivnuts top’s “edges”, maybe should be touching the “whole top evenly”. I’m having to tighten the bolt so hard as it breaks due to the pressure not centered on the rivnut but on the edges, basically not expanding like it should. The downward pressure against the rivnut should be on the entire top edge? Sound logical? I shouldn’t have to apply so much force to have the rivnut expand. A stainless steel 10/24 bolt is breaking, shouldn’t need that much force to expand the rivnut. I need a No 10 washer, not a 1/4” one l used. I’ll post back.

[acheld]

Rivnuts are really good but take some practice and some tools.

Couple of observations:

1.   The rivnut should be snug in the hole it fills.  You can fudge a little on this in our amp world; for automotive or aircraft applications, it's a no-no.
2.   Pick the right rivnut for your application.  The smaller ones are obviously easier to place with hand tools.  Aluminum is easier, but not by much.
3.   As Sluckey mentioned, some oil on the threads helps a lot.  3-in-1 (eg, light general purpose) oil is perfect. 
4.   If you are making your own install tools, I suggest using a steel bolt.  Hardened is better.  Stainless steel will not work -- it is too soft.   When I used my improvised tools, I found some bolts with a socket head that made it easy to turn with a hex key, as using two wrenches just did not work for me.  You can not drive these with a Philips screwdriver -- not enough torque is allowed.
5.   As I used these more often, I ended up buying a tool and it has made it much easier.

As you can tell, I love these little guys!

[PRR]

> Stainless steel will not work -- it is too soft.

Weak.

I tried SS on an exhaust joint. The bolts broke at half the stress of a plain steel bolt.

SS comes into its own above red-hot, where plain steel gives up. High-temp heat exchangers, superheaters, some gas turbine blades. Not a Thunderbird exhaust. Not an amp mount.

[acheld]

Not a Thunderbird exhaust. Not an amp mount.

Especially most of the junk stainless hardware you get on eBay or Amazon.    Not saying that there is not a reasonable use for cheap hardware . . .

For reliable hardware, Aircraft Spruce and McMaster are great.   Actually, there are several other very good fastener companies out there.

[sluckey]

https://el34world.com/Forum/index.php?topic=24655.msg265929#msg265929

[dude]

Managed to get thee chassis done with rivnuts, l got the hardest 10/24 bolt. Could only get three or four rivnuts in before the bolt cross threaded.
Doug’s rivnuts must be softer steel than mine. If l had it to do all over again, l’d go with 1/4” / 24 threaded rivnuts, ton’s of case harden hex machine bolts that size. #10 is too small, cause there isn’t availability of harden steel bolts that size, too small.
Since Doug sells them l surmise his supplier sells softer steel rivnuts then what l have. I went to an automotive store, no case harden #10 screws, they are not considered bolts to the guy behind the counter, he said there screws, only available in normal steel. But he has harden steel the next size up, #12 screw which is equivalent to 1/4” bolt.
Maybe l need to get the proper tool. Getting the rivnuts l have became one of the most difficult jobs l ever encountered, l’ve pretty much done it all too.
I probably should have bought Doug’s rivnuts, tool and bolts and l would have never had this nightmare.