Plexi project using 6973's

[1blueheron]

I think I have decided to use my Thomas Organ chassis/iron to build a "plexi"

It was brought to my attention by a member over on the 18Watt forum that my voltages are almost identical the those used in the Mark Huss 6V6 plexi build.  The only real difference seems to be the 6973's vs. the 6V6's.

My son has been begging for something more Marshall.  I think this will be a fun but challenging build.

I put this together based on a "modular" approach.  I used Sluckey's channel switch from his Supro build.  The preamp stages, tone controls and Phase inverter are straight from the Huss 6V6 "plexi" schematic.  I left the output section pretty much as is from the Thomas Organ amp with the transformer and bias setup for the 6973's as they were. (not sure if that is correct to do but I figured it worked that way originally so it should be OK) 

Power supply section is pretty much stock from the Thomas organ with the exception that I have changed out the 2 wire cord for 3 wire, added a power switch and pilot light. 

I have removed the original pre-amp section and the old wafer style sockets and have new 9 pin sockets to install.  I am considering replacing the larger wafer style sockets for the power amp section with 8 pin sockets and then using an adapter for the 6973's.  That way if I don't like them, I can revert back to 6V6 or something else with a simple rewire or adapter removal.  I have plenty of octal bases socket on hand.

On the fence as to whether to do a turret board or keep it point to point.

I do not have the expectation of this sounding just like a Marshall from a tone perspective.  I realize it is off the reservation as far as being a clone.  If it just works, I will be happy.  Anything beyond that would be icing on the cake

I don't know if I correctly coupled the PI and power amp sections. 

Does this look like a reasonable plan?

[bmccowan]

I think Sluckey is the guy to help you with this build, and I'm sure he will. But, a couple of thoughts. Valco built some model 24 amps with 6V6s for a time and then switched to 6973s with very minor changes, so this should certainly work out ok. Me, I would go ahead with the 9 pin sockets and trust that you'll get it right. But I do find that I don't really like 12Ax7>6973s amps that much until I turn them up too loud for home use. I built a pentode preamp amp (5879) with 6973s and like that a lot. Its basically a GA-40 preamp into a Valco power amp. Just an interesting option if you find that you need to turn the amp up to scare the dog levels. Should be fun.

[ALBATROS1234]

i think thats a great idea i have a handful of rca 6973 tubes and they sound great. kinda in between a 6v6 an a good european el84. they should sound phenominal in a 6v6 plexi style build. probably better than a nos 6v6 pair. post sound demo please

[sluckey]

Sounds like a good plan to me. If you build this in the Thomas chassis then do a point to point because the tube layout is not conducive to a turret board. One thing I would change... Remove the 12Ω bias resistor and use two 1Ω 1 watt resistors as shown in Mark's schematic.

[1blueheron]

Sounds like a good plan to me. If you build this in the Thomas chassis then do a point to point because the tube layout is not conducive to a turret board. One thing I would change... Remove the 12Ω bias resistor and use two 1Ω 1 watt resistors as shown in Mark's schematic.

Change noted and will update the drawing.  BTW, thanks a bunch for helping me out on the "as was" of the Thomas! 

[1blueheron]

Updated the schematic to show two 1Ω 1 watt resistors .
Sketched panel layout.

Stripped and painted chassis and bell housing covers.

All new holes have been drilled and most of the unused holes blanked.

Installed power switch and light jewel.  Pretty much ready to heat up the iron.

Removed the multi-cap that was used for off chassis.  Gutted the aluminum canister.  Might put new caps in it and put it back on the chassis in original position.

Need to order some parts.

[shooter]

NICE!
be careful, this might be addicting 

[sluckey]

What does your socket adapter look like?

[1blueheron]

What does your socket adapter look like?

Well, I don't have one yet.  My plan is to wire as if it were a 6V6, then use an octal plug and a Noval socket with the wire conversion inside and pour it full of epoxy unless someone has a cheaper, easier method.  The existing sockets were a large wafer style. (the kind everyone has told me are no good and should be replaced) The octal sockets were pretty much drop in.

[tubenit]

Have you seen this thread out of ARCHIVES on making a socket adaptor?

http://el34world.com/Forum/index.php?topic=16786.0

With respect, Tubenit

[66Strat]

Have you considered a socket plate to convert from octal to noval sockets?

https://www.mojotone.com/amp-parts/TubeAccessories_5/Octal-socket-to-9-pin-socket-adapter

https://www.angela.com/octalto9pinnovaltubesocketconversionmountingplate.aspx

https://www.tedweber.com/ap1

[1blueheron]

Have you seen this thread out of ARCHIVES on making a socket adaptor?

http://el34world.com/Forum/index.php?topic=16786.0

With respect, Tubenit

No, I had not seen that post but it is exactly what I had planned to do.   If you don't mind take a look at the following pin conversion and check my work...

[1blueheron]

Have you considered a socket plate to convert from octal to noval sockets?

https://www.mojotone.com/amp-parts/TubeAccessories_5/Octal-socket-to-9-pin-socket-adapter

https://www.angela.com/octalto9pinnovaltubesocketconversionmountingplate.aspx

https://www.tedweber.com/ap1

Yes, I could do that, but part of this project is to be easily able to revert to a 6V6 without a lot of effort.  I'm willing to accept the additional labor now in exchange for versatility later.  Thanks for the links though and I will keep them in mind for future GP.

[bmccowan]

Just throwing out another idea.
There is a 9 pin tube (6CM6) that is close in performance/sound to 6V6s. By jumping two pins in the socket, it can be interchanged with a 6973. You need to watch plate voltage. I built a GA-40 Lite with that tube in a Hammond AO-39 chassis. It sounds pretty much like channel 1 in a GA-40. I have swapped 6973s, 6CZ5 and 6CM6s in and out of that amp. The 6CZ5s and 6973s sound alike. All sound good, but the 6CM6 is grittier at lower volume. 

[SoundmasterG]

Just throwing out another idea.
There is a 9 pin tube (6CM6) that is close in performance/sound to 6V6s. By jumping two pins in the socket, it can be interchanged with a 6973. You need to watch plate voltage. I built a GA-40 Lite with that tube in a Hammond AO-39 chassis. It sounds pretty much like channel 1 in a GA-40. I have swapped 6973s, 6CZ5 and 6CM6s in and out of that amp. The 6CZ5s and 6973s sound alike. All sound good, but the 6CM6 is grittier at lower volume.

That's a good idea. I've used the 6CM6 for years and they're nice tubes and pretty cheap in comparison to vintage OS US 6V6's. They are pretty stout too, but yes watch the plate voltage.

Greg

[1blueheron]

I will have to scrounge through my stock piles and see if I can find a 6CM6.  Thanks for the suggestion!

I finished up the heater wiring yesterday.  Hope to get some work done on the power switch today.

[sluckey]

Why do you have two filament strings?

[1blueheron]

I ran the preamp section on a second run.  Seemed like a good idea to separate from power amp filaments.  Is that a bad idea?

[sluckey]

Not a "bad" idea, just unnecessary. I would have tied the three little tubes onto the power tube string.

I'm still curious why "Seemed like a good idea to separate from power amp filaments." What are your thoughts on that?

[1blueheron]

This probably sounds crazy.  I was thinking initially terms of ampacity, and the two separate runs being twisted in opposite directions, which in theory should cancel any noise on homeruns to the PT.   Long term if I decide to change the sockets for the output tubes in the future,or make repairs, I won't have to worry about any impact to the pre-amp section, which will be difficult to access in a PTP layout. 

[NewYorker]

So we’ve eliminated the 12R resistor associated with power tube bias and substituted two 1R resistors. This would make it possible to check bias on each power tube individually. Just add two long yellow wires. Just kidding.  Seriously, have you considered putting a couple banana plug jacks on the chassis for this purpose?  It would save having to go inside if you ever need to recheck bias.

[1blueheron]

So we’ve eliminated the 12R resistor associated with power tube bias and substituted two 1R resistors. This would make it possible to check bias on each power tube individually. Just add two long yellow wires. Just kidding.  Seriously, have you considered putting a couple banana plug jacks on the chassis for this purpose?  It would save having to go inside if you ever need to recheck bias.

Great idea, and since i have some homeless ones sitting on the workbench, I think I will give them a loving home in this build.

I finished wiring up the PT last night.  I'm reading 7.0V to all the filament pins with all tubes pulled out. 

[NewYorker]

In order for this scheme to work reliably, your need to ensure that the two 1R resistors are closely matched and within close tolerance of 1 ohm.  Not as simple as it sounds, I think, and in fact I've never dealt with that.  I'm sure others on this forum have more experience and can comment more meaningfully.  Or search for past discussions on the topic.

Ed

[sluckey]

In order for this scheme to work reliably, your need to ensure that the two 1R resistors are closely matched and within close tolerance of 1 ohm.  Not as simple as it sounds, I think, and in fact I've never dealt with that.  I'm sure others on this forum have more experience and can comment more meaningfully.  Or search for past discussions on the topic.

Ed

It's very simple. 1Ω 1% tolerance resistors are very common and cheap too. Lot's of us do this in all our projects.

[1blueheron]

In order for this scheme to work reliably, your need to ensure that the two 1R resistors are closely matched and within close tolerance of 1 ohm.  Not as simple as it sounds, I think, and in fact I've never dealt with that.  I'm sure others on this forum have more experience and can comment more meaningfully.  Or search for past discussions on the topic.

Ed

It's very simple. 1Ω 1% tolerance resistors are very common and cheap too. Lot's of us do this in all our projects.

Have 1 ohm 1W 1% tolerance metal oxide film resistors on the shopping list!

[1blueheron]

Started building out V1 last night. (using the resistors I have in the parts bin so ignore the 68k resistors being oversized)   Does this look to be a reasonable approach using a wire ground bus?    I have seen this done in other amps where it is soldered to the back of the pots but don't know the particulars of it and when it is good or bad.  Plan to solder in caps tonite and complete soldering the resistors if this looks like a reasonable layout. If not, I will start over.

Also, I know a lot of Marshall amps use isolated jacks.  When is this neccessary? Am I ok not to isolate the jacks from the chassis?

[1blueheron]

Answered some of my own questions.  I won't solder the bus to the pot casings.  Aiken says it can cause ground loops and induce noise.

Continued work on wiring V1 & V2.  Still have a couple caps to install before moving on to PI. Found a couple errors in my previous work and corrected.  Might re-work a few more things as I go and think it through.  All comments positive or negative welcome. 

[sluckey]

Looks very tidy.   

[1blueheron]

So here is where I am at...

Still need to wire/install input jack and input selector switch, output jack, bias adjust, bias check resistors and check points, and finish wiring the pots.  Ordered some Marshall style knobs but they shipped me knurled rather than smooth so have to re-visit that.

Have some holes to blank off but I can see the light at the end of the tunnel and barring any big surprises should have it finished up soon.  Any critiques, warnings, observations welcome.   (I did remove that second filament home run as things were starting to get crowded)

I ordered a Mallory 150 .1uF cap to replace that blue molded .1uF as it is really too big to fit well.

I used a 500pf in place of the 470pf as it was what I had... Bad choice?

The original 2500ohm power resistor had a lead break off in the build process so I replaced it with that mallory 3000ohm I had on hand.

Since that should lower all the downstream voltages a little bit more than original, I went to using 2-10k resistors for the next stage rather than the 22K on the schematic.  Will see what it looks like for voltage with a load.  May need to tweek?

Not sure of my B+ lead routes.  May need to alter for noise?

[bmccowan]

1blueheron - I am interested to know how this amp turned out, and especially which power tubes you like in the amp? I ask because I built an 18W TMB a while back. It does what its supposed to do, but I guess I do not like what its supposed to do. The sound is tiring to my ears. I am thinking of rewiring the power amp for 6CM6/6973/6CZ5 tubes. I've built a lite amp like that and like it. I've also built a Plexi 6V6 and like that better than the 18.
I thought others would also be interested in the final outcome, so replied to your thread. If I decide to move forward I will start a new thread to get some advice on that conversion. I know i have to watch voltage on the 6CM6, but I'm around 300V now on the EL84s.