New build: 6v6 Marshall in a Trainwreck Chassis (up and running)

[strelok]

Started another build today. Switchable 2203/1959 style amp using either 6v6's or 6k6's. Using a 4 pos impedance switch to allow for compatibility between the two tube types with only a need for re-biasing. Plate voltage will be somewhere between 320-340v, which may be a little high for a 6k6 but we'll see. The amp will have a VVR in it once the boards I'm working on with JazzGuitarGimp are complete so I can always regulate it down a bit if its a problem. Here's some pics of what I got done today:



Doing the headphone tuning of the transformers. The quietest spot was not one I would have suspected to begin with so I'm glad I did it. One downside to the 125 series from hammond is its open design makes it slightly more prone to picking up hum.

Test mounting transformers.


Ran into a bit of a problem with the PT mounting. The hole I drilled was a little to far forward away from where the wires come out of the end bell on one side. Ended up having to widen the hole so that the wires wouldn't get pinched by it. Just something to be mindful of if you're using edcor transformers. Had to learn that one the hard way. The end bell goes down to about as low as the mounting brackets and leaves very little wiggle room for the wires to come out. Also the snap bushings I ordered were a bit too small for how many wires were coming off each side of the PT. They fit but its a pain in the ass to get them all through. 4 are unused but again since the endbell goes all the way to the bottom of the transformer I can't heat shrink and cable tie them up top.

Once the hole was widened I fixed in the snap bushing with a bit of hot glue. I was thinking about using something stronger like jb weld or some other epoxy but I would have a hell of a time with it if it still somehow didn't line up properly. The bottom of the transformer along with the tension of the wires holds it in place. Still I'd rather not have to resort to using hot glue in an amp. Its always something lol.

Heaters wired up and terminal strips installed. Those with a sharp eye may notice I somehow managed to wire the pre-amp heaters one pin off. Didn't catch it till later and I was too tired at that point to fix it. Will correct it tomorrow.



Pre-amp grounds installed and starting on the power supply.

Power supply partially completed, transformers installed. Was at about this point I noticed my mistake with the heaters.

[davebolden44]

Sounds like a neat project. I just got a box of old tubes with a couple 6k6 tubes in there. I put one in my champion 600 which is what I use to bench test guitars and I watched it cook that tube. I have never bothered to check the voltages in it and it is single ended so I am sure the tube was working a little harder than a push pull. I digress, happy building.

[strelok]

Thanks! The B+ in the champ 600 is quite a bit higher than what I'm using, around 370v. Though I've heard people say a 6k6 will hold up to that, for how long I don't know. I'm guessing the reason it failed in yours is more likely due to bias than anything. The champ 600's stock bias resistor value is 470 ohms and if I remember correctly a 6k6 in SE needs something in the region of 800 to 1200 ohms to be in the right range.

[davebolden44]

it didnt last long but to be fair it was from a box of old tubes found in a barn. i cleaned them up and put the ones i could use on the tester to see if they were any good. it said they were good so i slapped it in the champion which was right there. from there it was one of those hold my beer and watch this moments, i dimed it and played for like a minute until it started crackling and losing volume. it wasnt a very responsible or controlled experiment

[strelok]

Well thankfully 6k6's aren't valuable tubes, which is why I'm willing to take some chances with the voltage, I do hate to blow them up though as they sound quite nice. In any case I finished the amp up tonight. Here's some more pics of my progression:


Heaters fixed, also I had wired the screen supply of one tube to the wrong side of the resistor fixed that and shortened the leads. Transformers hooked up, PS mostly done.

Caps and resistors installed, starting on pre-amp wiring.

Push-Pull pot wired up. That a was pain in the ass.


Starting on Tone stack and rest of the Pre-amp.

Gettin' Close.

And with the OT Secondaries, power cord and NFB wire installed the amp is complete, more or less.


Got it on the test bench, came up just fine on the variac. Using 6k6's at the moment. Biased fairly conservatively for now, around 6w a tube or about 17ma. Played for a bit at low volume but didn't have much time and its much too late to crank it up so will have to wait till tomorrow. Only problems I found was I wired the presence pot backwards (go figure) and if the presence is all the way up it will sustain an oscillation if its excited, otherwise it will sit there stable. NFB isn't in the best spot however it was the quick and dirty solution to route that wire. May find a better way to thread it past everything. Although I could probably just try a slightly smaller NFB resistor and that would probably shut it up. Will have to see how it goes when I crank it tomorrow.

[norburybrook]

Dave,

point to point beautiful. Loved watching this come together.


great project.

marcus

[strelok]

Thanks! I was able to crank it for a bit yesterday. I also measured the power output. 6 watts clean with the 6k6's. Sounds a lot louder than that. Its also a bit on the bright side. I may remove the bright cap on the volume control and possibly the treble peaking cap for the second gain stage. Plexi mode has plenty of gain with everything dimed and I like the sound with the bright cap totally out of the circuit in that situation. The amp is pretty dead quiet. If I have the master down I have to put my ear up to the speaker to hear any hum. Of course when you turn it up the usual hiss comes in but that's to be expected. Pre-amp seems to be totally stable despite only using shielded wire on the input.

Output section does have a slight oscillation when the presence is turned up far enough. I may try routing some of the wires differently to the PI socket to see if that cures it. Its not particularly loud or anything but still needs to be fixed. Overall I'm pretty happy with it so far.

[strelok]

Fixed the oscillation. Turns out the feedback polarity was just reversed. Swapped the grid wires and now its fine. Granted I wish I had done this before I had reversed the presence pot because I actually had it right the first time. I happened to be running just enough feedback to cause the amp to oscillate with the presence turned down. It just seemed like it was reversed since the positive feedback caused the amp to have more gain at the high frequencies as you turned the pot down. Haven't had a chance to play it yet as the pickups in the shop guitar are currently sitting on the table letting paraffin wax harden in them as I got tired of the terrible autistic screeching destroying my hearing.

[studiodunn]

Awesome play-by-play pictorial. Thanks for sharing!

[joeboo88]

nice work, that p t p looks great.
did you follow a schematic or just create this on your own?

[strelok]

Thanks guys! The pre-amp is a pretty standard Marshall 2203/1959. Pretty much copy and pasted that, coupled with a smaller output section. The layout I created myself. It works pretty well. Sounding much better now that I fixed the issue with the NFB loop. I may yet have to put in some shielded wires. I get quite a bit of hum with the volume/gain dimed and the bass control up. Though I will probably double check the solder joints first as it really shouldn't hum at all. I was careful to keep the signal wires away from anything that could cause problems. With the gain down its dead quiet so it has to be coming in on one of the early stages. Will have to investigate it further this weekend if I have the time.

[strelok]

Fixed the hum, was just a leaky tube in V1. Swapped it with the PI and all is good. I think this may be the quietest amp I've ever built. Everything dimed in 1959 mode and its just a very mild hiss. Of course its alot more when you engage the additional gain stage but its about as good as you could ever hope to get with a high gain amp. Tempted to try some larger output coupling caps as I think it might stand a little fattening up.

[JazzGuitarGimp]

Really digging the true point to point layout. I've been wanting to try this assembly style ever since I saw the video of Mr. Carr (forgot his first name) talking about it. He says true ptp adds "air" to the tone that just can't be had from eyelet / turret / pc board construction.

[strelok]

Thanks, I've never heard that one before. I mostly just use PTP because we have the terminal strips on hand at work and if you're building 1 or 2 of a particular amp its just easier to do it that way I think. Especially if you're building new circuits or layouts that haven't been tried before, if something needs to be change in terms of layout its much easier to do with terminal strips, or you decide you want to add something else in etc.