Single Channel 6G3

Che_Guitarra

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Hi folks. New project time.

My last project was a 6G2 (with LTPI and trem delete... so maybe closer to a 12W 6G3?). I built it mainly to experiment, and use up a bunch of Princeton spec parts I had lying around. It turned out to be a killer amp. So much so, I plan on building the exact same amp with some Deluxe Reverb spec trafos that I also have lying around.

I did have half a plan to build a 20W JCM800 w GZ34, but put a RAT in front of a 6G3 and i'll probably end up in the same ballpark.


First blows have been struck. Chassis has been bent up, and power trafo and IEC socket holes sorted. Pausing on the remaining holes to consider layout.

XE7TU9n.jpg
 

Che_Guitarra

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One mod I want to carry over from my 6G2.5 is a switching/pot option to reduce gain going into the power section. Beyond the single volume control. With hopes to flesh out a wider palette of tones at lower gain.

The 6G2.5 build thread is here.

@andrewRneumann gave me this idea to wash off a bit of pre-PI signal - it works really well - I chose this method as it was the easiest to integrate into the existing PCB I had built.

1642025189023-jpeg.939527



@Lowerleftcoast came up with this PPIMV idea. I didn't follow through with it as I would have had to build a completely new PCB. But now i'll be designing/building from scratch, it's definitely an option.

897470-23c8db45a97212a29a4006d934bb8599.jpg



Is either of these methods a superior option? Or other ideas to investigate?

I guess I could potentially rig up both, although I do have a design preference to keep layouts as simple as possible.

Any input on this would be greatly appreciated.
 

2L man

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That works but it is Pre Phase Inverter Master Volume which is called just MV! The PPIMV is known as Past PIMV. MV does not necessarily need any changes to circuit board and PPIMV neither.
 

FenderLover

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It looks like you might be most interested in an adjustable method, but you could also consider small changes to the PI section to dial in a 'just right for me' response.

Look at the Twin - the PI is fed with 450V but a huge chunk of that is dropped by the tail resistor. By contrast, the 6G3 drops a much smaller portion across the tail. Messing with the tail, and the voltage string feeding the plates, can get to a 'just right for me' place. Add a 50K pot (or whatever you have) to the 6800 tail resistor, then turn the pot up. If that gives favorable results, consider increasing the plate voltages (reduce the 10K PS resistor) and try it again.

Otherwise, adding a MV could satisfy, but we often think the amount of signal is the obstacle. Sometimes adjusting the way the circuit is biased produces the individual response we are looking for.
 

SerpentRuss

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Hi folks. New project time.

My last project was a 6G2 (with LTPI and trem delete... so maybe closer to a 12W 6G3?). I built it mainly to experiment, and use up a bunch of Princeton spec parts I had lying around. It turned out to be a killer amp. So much so, I plan on building the exact same amp with some Deluxe Reverb spec trafos that I also have lying around.

I did have half a plan to build a 20W JCM800 w GZ34, but put a RAT in front of a 6G3 and i'll probably end up in the same ballpark.


First blows have been struck. Chassis has been bent up, and power trafo and IEC socket holes sorted. Pausing on the remaining holes to consider layout.

XE7TU9n.jpg
I like your homemade chassis. I'm assuming by the relationship of the IEC and transformer cut-out that this is going to be top mounted in the cabinet. I've made a couple like this and so far I haven't shielded the side or top when I put them in a cabinet. Do you, and if so what do you do?
 

Che_Guitarra

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It looks like you might be most interested in an adjustable method, but you could also consider small changes to the PI section to dial in a 'just right for me' response.

...

Otherwise, adding a MV could satisfy, but we often think the amount of signal is the obstacle. Sometimes adjusting the way the circuit is biased produces the individual response we are looking for.

Thanks FenderLover. I think this is the right approach.

Stock 6G3 is pretty close to where I want to be. If I can tune the gain staging and breakup point a little more precisely for my style, i'll end up with a firecracker of an amp.

Looked into the master volume thing a little more - not sure I will implement this - seems to hollow out the sound more than I would prefer. And I want a dead simple layout - one volume, one tone.


I've made a couple like this and so far I haven't shielded the side or top when I put them in a cabinet. Do you, and if so what do you do?

I never knew shielding cabinets was a thing? Shielded wire in the preamp is about as far as I go.
 

SerpentRuss

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Thanks FenderLover. I think this is the right approach.

Stock 6G3 is pretty close to where I want to be. If I can tune the gain staging and breakup point a little more precisely for my style, i'll end up with a firecracker of an amp.

Looked into the master volume thing a little more - not sure I will implement this - seems to hollow out the sound more than I would prefer. And I want a dead simple layout - one volume, one tone.




I never knew shielding cabinets was a thing? Shielded wire in the preamp is about as far as I go.
I don't know how important it is either, but fender puts foil on the back panel of tweed-loaded cabinets and I've seen several threads where people put foil on the underside of the top panel in front-loaded cabs.

Your chassis construction (and mine) have open ends, probably because your metal is stiff enough that from a structural standpoint you don't need them. It's also a bit of a chore on a sheet metal brake to set up to bend the ends. I don't own my own brake, I use the one at work, so I don't mess with removing fingers.

I just wondered if you lined the ends and top of your cabs with foil. I might try it on my next build.
 

Che_Guitarra

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It looks like you might be most interested in an adjustable method, but you could also consider small changes to the PI section to dial in a 'just right for me' response.

Fenderlover - thanks for this suggestion - I gave it a try - immediate results!

Using my 6G2.5 as a guinea pig, shifted the PI tail value 6K8 to 18K (all I had on hand) and cathode to 470. Wow! A solid move toward deluxe reverb territory, but still sounds juicy and chewy like a tweed. The Marshall-y bark is less pronounced on the high side of the dial, but got it right back with a booster pedal. Really cool.

Upcoming build - 10K seems to be the universal standard for a PI tail - I might start a touch higher (13K) and go from there.
 

FenderLover

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Using my 6G2.5 as a guinea pig, shifted the PI tail value 6K8 to 18K (all I had on hand) and cathode to 470. Wow!
Good to see a little success. Nothing wrong with results, but I might caution to change one thing at a time, specifically the bias resistor at 470 is quite hot. I would go as high as 22K on the tail, leaving the bias resistor closer to 1500, evaluate, then make more decisions.

Look at two more LTPI examples: Trainwreck and Dr Z. The TW has a short tail (10K) with a hot bias (470) following their famous cold clipper. That combo produces very high touch sensitivity given all the asymmetry forced by the bias conditions. The Z-28 has a long tail (47K) with a mid bias (1200) for a higher clean headroom approach.

Edit: The Bassman/Marshall also use 10K tail/470 bias, but let's not throw oranges into the apple barrel. Their PI's follows high attenuation from the tone stack, and require less tail than if following gain stages.

Baby steps will get you there, and is much more personalized than throwing on a master volume.
 
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Che_Guitarra

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Been waiting on parts for a while - they've finally arrived (postage in Australia - nothing is fast!).

Now that I can eyeball and measure dimensions, it's time to start planning circuitboard design. Rather than a multi filter cap can, i've opted for individual filter caps - gifts me the ability to experiment with values, and also makes future serviceability that little bit easier.

I was hoping to use Wima caps, but couldn't find all the values/dimensions I wanted at any one source unless backordered. Went for these orange ones instead. I won't lie - I only bought them because they look cool! I've used them before in a 1176 build and they seemed pretty noisy, but I doubt a 60yo guitar circuit is going to be too fussy.

VaIGC5e.jpg



Starting point for design will be my 6G2 board design. I made a couple of boo boos that I remedied off the board - lessons I can hopefully transfer to improve this design.

ZhZOrXS.jpg



I'll take a couple of evenings to get the design right, and then hopefully cut the board later in the weekend.
 

Che_Guitarra

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Tackled the chassis this weekend. My toolkit isn't really suited to metal work, but between a step drill bit and a dremel, I think I have it done. Have done a mock assembly - all seems OK. Now looking to the next phase - hate soldering in these tight box chassis, but prefer the small footprint once up and running.


Next step - wondering if I should paint the chassis while I wait for the last few parts to arrive 🤔 will have to take a few youtube lessons if I go that route.

0vZ0URw.jpg
 

Che_Guitarra

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Conducting science. Looking to clean up the chassis - get rid of the mill scale as can be seen in the above pic and clean back to bare metal. Acid to the rescue.

Not something I would recommend unless thoroughly familiar with best handling practices - get it wrong and you'll wear the scars for the rest of your life.

Found an open area in my yeard, filled up a few plastic bottles with water to keep the bucket volume as low as possible, then i'm guessing about 5 litres of water and maybe a can of coke worth of HCl. And another bin just out of shot, loaded with diluted baking soda to neutralise when done.

2CxwpL1.jpg



Already know the results - turned out really well. Better than expected. Will post with next upload, once daylight returns.
 

Che_Guitarra

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After 5 years waiting for its' moment, i'm finally putting use to a dusty old Classictone 18017 PT.

Checking out a schematic, no problems understanding the primary side (wired for 240V - Australia). But i'm not sure I fully understand the dual centre tap arrangement on the output side.

Can anyone advise on correct wiring?

aMuhRKQ.jpg
 

NTC

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The red-blue wire is for the bias voltage circuit in a fixed bias amp.

Edit: and the red-yellow is the ct for the high voltage red wires. You can confirm this with resistance readings between wires.
 
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Che_Guitarra

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Ahhh - got it. So red-blue is essentially an optional tap, if I don't want to run to the rectifier socket.

Cool. Might as well use it and keep the tight box chassis that little bit neater.
 




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