Peavey xxx super 40 to 6v6 plexi conversion.

[Captain chunkulus]

So, been thinking about what to do with this old peavey xxx super 40 combo i have. I've been wanting to build sluckey's 6v6 plexi for awhile. The combo took el34s or 6l6s. It was 40 watts. Anybody have any thoughts or reasons this might not work? I cant find a schematic but I was planning on just using the power and output transformer and the adding a classic tone choke. The chassis is cut for 4 12ax7s and two power tubes so I would have to cut for external can caps and the rectifier tube. I'll post dimensions and pictures when I get home today.

[1blueheron]

Join the Plexi conversion craze!

[Captain chunkulus]

Join the Plexi conversion craze!

Ikr! Lol. Now I'm thinking about just building sluckey's  dual 50 except with 6v6s.

[sluckey]

The combo took el34s or 6l6s.

So why mess with 6V6s?

[Captain chunkulus]

The combo took el34s or 6l6s.

So why mess with 6V6s?

Idk, really. I like 6v6s in my deluxe reverb. I've always like el34's for distortion. Just curious how 6v6s would sound compared to el34s in that circuit. I might just leave well enough alone and do your dual 50. The chassis already has plenty of front panel holes for the pots.

[shooter]

just leave well enough alone

good advice 
EL 34's Need to be in plexi's, 6V6's are for champs n such 

[PRR]

6V6 should really be worked at lower B+ voltage and higher impedance than 6L6/EL34 are normally worked at.

Don't put motorcycle pistons in your plow-truck. Keep the big tubes.

[Captain chunkulus]

So, maybe just use the 6v6 plexi layout with the dual 50 el34 power/output section.

[dude]

I've run EL34s in my 6V6 Plexi. If the PT can handle the extra draw, try them but I say the 18/20 watt OT in the 6v6 plexi will choke the big bottles, not going to get the beef. But the fifty watt Plexi will give you the beef and you can still run the smaller bottles but ton of headroom.

[Captain chunkulus]

Well, the peavey was rated 40 watts with 2 6l6s or 2 el34s. I'm guessing it could handle just staying el34 and do the plexi preamp. I'm planning on starting it soon. Got the itch. Lol. Never owned a plexi but I love classic rock tones. I was thinking however  about maybe doing sluckey's dual 50 but adding a second separate eq to make it 2 channels. That seems pretty  usefully to me.

[dude]

Go for it, I'm sure that OT in the Peavey can handle EL34s without saturating. I should have read your first post more clearly, OT would be fine for EL34s. Set the bias range resistor low enough for 6V6s too, you have a lot of headroom, push those boys. Sounds like a nice build, I personally love the preamp in the plexi  . Start building.

[Captain chunkulus]

So, got started on this project today. Decided on Sluckey's dual 50. I was curious about your guys's opinions on if I should relocate the output  transformer or leave it as is. I was thinking of moving it mid chassis so I could drill new holes for the preamp tubes. I don't want to use the preexisting preamp tube holes. I would rather have all the tubes near the back of the chassis. Thoughts?

[Captain chunkulus]

Building the board.

[sluckey]

So, got started on this project today. Decided on Sluckey's dual 50. I was curious about your guys's opinions on if I should relocate the output  transformer or leave it as is. I was thinking of moving it mid chassis so I could drill new holes for the preamp tubes. I don't want to use the preexisting preamp tube holes. I would rather have all the tubes near the back of the chassis. Thoughts?

Good idea

[Captain chunkulus]

So here's kind of what I'm thinking. Also going to mount the phase inverter tube in the other power tube hole after I fill it in.

[Captain chunkulus]

Just an update. Almost finished with the board. I hope to get it all done in a couple of weeks.

[Captain chunkulus]

Hey guys question, I was looking at the schematic for the Marshall dual 50 and it has a filter choke. However it doesn't say what value in Henry's. I was thinking about using a classic town 20 Henry Plexi choke. Thoughts and suggestions welcome.

[sluckey]

Looks good to me.

[Captain chunkulus]

So here's some initial ruff pictures of where I am at. Waiting on my choke to come inext before I go crazy wiring it all up.

[Captain chunkulus]

Okay, so my plan changed about halfway through this design. I was originally just going to do a Plexi but decided on Sluckey's Marshal dual 50.  After building the circuit and looking at my power Transforme, I realized that the schematic calls for a center tapped Transformer and the bias circuit  is taken directly off of the power transformer HT secondary. So the power transformer I'm using specs are,
300vac @.318amps
41.5vac ct @ .318amps
6.19vac @ 5.18amps.
So my thought was just to do a regular bridge rectifier power supply. And get my bias voltage from the 41.5 volt AC Ct secondary. The question is, could I pull enough voltage to bias the tubes from there? Or is there another way to bias the power tubes while using a bridge rectifier circuit for the b+rail. Thanks. C

[sluckey]

Yes to both questions. I would just use the separate 41.5vac winding without the ct. That's plenty for EL34s.

Look at pages 8 and 9 to see how to get negative bias from a PT HT secondary when using a FWB rectifier...

     http://sluckeyamps.com/misc/Amp_Scrapbook.pdf

[sluckey]

Here is a slightly revised layout that will accommodate your Peavey PT. Look closely at the wiring changes for the rectifier diodes and bias input. Don't overlook the board jumper changes! Also notice the bias range resistor value is now 470Ω/3W to work with your 41VAC bias winding (shown as purple wires). You may have to juggle the bias range resistor value to get the best adjustment range of bias voltage.

I've also included some notes for the grounds. Basically there are two chassis ground points... POWER GROUND near the PT and PREAMP GROUND near the input jack.

[Captain chunkulus]

Also notice the bias range resistor value is now 470Ω/3W to work with your 41VAC bias winding (shown as purple wires).

On the revised layout The color code for that resistor comes up 740 ohms. So 470 for sure, right? That was just a mistake on the revised layout?

[Captain chunkulus]

Also notice the bias range resistor value is now 470Ω/3W to work with your 41VAC bias winding (shown as purple wires). You may have to juggle the bias range resistor value to get the best adjustment range of bias voltage.

Also, on the revised layout , 1 of the purple wires is going to ground. . So the secondary Is Center tapped. Am I supposed to wire One side to the bias resistor and the other side to ground? And leave the center tap Open?

[sluckey]

On the revised layout The color code for that resistor comes up 740 ohms. So 470 for sure, right? That was just a mistake on the revised layout?

That's a mistake on the drawing. I'll make the correction. Use 470 to begin. May have to change it later.

Am I supposed to wire One side to the bias resistor and the other side to ground? And leave the center tap Open?

That's correct. Also notice the bias turret no longer has a jumpered to the HT rectifier diode turret.

[Captain chunkulus]

Yeah, I saw that it was No longer connected to the Ht. So the the 40ish volt secondary get grounded on one side and the center tap floats? Just wanting to clarify so I'm Absolutely certain of what I'm doing. LOL. And thanks so much Slucky, you Rawk!

[sluckey]

READ REPLY #24 100 TIMES.

Are you ABSOLUTELY certain now? 

[Captain chunkulus]

Sorry, lol. I got it.

[Captain chunkulus]

So I got the amp wired. It works. However I need some assistance.  The bias voltage is -46.1. The voltage across  the 2 1ohm resisters is only 7.8ma. So, I'm having a brain fart and can't figure out how to bias this amp correctly.  I'm just trying to learn so I hope you guys understand that I'm a noob. Thanks for your help.

[shooter]

only 7.8ma

so 7.8mA * Plate vdc ~~ = dissipation per tube
you're using 6v6 push-pull so start around 65-70% off Max dissipation by adjusting the -46 SLOWLY less negative.

monitor the 1ohm, repeat math til you're ~~ 65-70%, play it quiet, play it hard, watch tubes REAL close

[Captain chunkulus]

Actually I'm using el34s. I was wondering what the DC voltage should be in milliamps at the 1 ohm resistors. So like maybe 17.5 milliamps per tube?

[sluckey]

We need the whole picture. What are the voltages on pins 3 and 4? I suspect the voltage on pin 5 will be between -35 and -40 once it's biased to your liking.

[Captain chunkulus]

Pin3 408vdc
Pin4 400vdc
Pin5  -41

[sluckey]

The voltage across  the 2 1ohm resisters is only 7.8ma.

Maybe you need to set your meter to measure milliVOLTS.

[Captain chunkulus]

The voltage across  the 2 1ohm resisters is only 7.8ma.

Maybe you need to set your meter to measure milliVOLTS.

That's what I meant, sorry. It was set to mv.

[sluckey]

Turn the bias pot while monitoring the voltage across the 1Ω resistor. Can you get up to about 40 to 45mV?

[Captain chunkulus]

Turn the bias pot while monitoring the voltage across the 1Ω resistor. Can you get up to about 40 to 45mV?

No, the highest it goes on either tube is about 14ish and the amp starts to get super  noisy at max bias.

[sluckey]

Monitor the voltage on pin 5 of the output tubes while turning the bias pot end to end. What is the min and max voltage?

[Captain chunkulus]

Monitor the voltage on pin 5 of the output tubes while turning the bias pot end to end. What is the min and max voltage?

Minimum is-44
Maximum is -38

[sluckey]

Replace the 470Ω bias range with a 1K. What is the min/max on pin 5 now? Any better? BTW, what does the amp sound like?

[Captain chunkulus]

Replace the 470Ω bias range with a 1K. What is the min/max on pin 5 now? Any better? BTW, what does the amp sound like?

Minimum is -44
Maximum  is -36
At -36 it hums really bad.
But it sounds great. It plays like it should.  Just trying to see where the ideal bias is. Thanks for all yourights help.

[sluckey]

What is the cathode current with the bias set to -36V?

I suspect you have other issues too. Show us some hi-rez pics of your amp.

[Captain chunkulus]

Pin8 correct?
22mv on one tube and 17.2mv across the other.

[Captain chunkulus]

I think my meter isn't  working  right. I'm going to buy a new meter and see.