Tetanus Project

Marshall Amp Discussion

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martin manning
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Re: Tetanus Project

Post by martin manning »

You can hook it up as a doubler and expect to see ~500 volts. Be aware that the DC current available will only be about 30% of the AC current rating for that winding. It seems odd that a transformer designed for that would have a CT. I suppose it's possible that it was intended to be a multi-purpose unit.
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

That seems strange... The 30%. I have been living under the assumption that the rating would be 50%. Good to know but I guess I don't understand.
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

Built the doubler... simple enough. Just rearranged two of the diodes in the bridge I had in there, take the others out, rewire the secondary taps... Bring it up... 511 vdc... Oh wait...why are those filaments only 3.5vac... Doh!!! :shock: That explains it. The PT is made for 220vac primary.

Hmmm... I do have another Stancor PT. It is a PC-10148. Looks like it will bolt right in. 300-0-300 @ 200mA, 5v 3A, 5v 2A and 6.3v 12A! I was gonna use it for whatever project down the road with a bunch of preamp tubes and some 6V6s. Looks like a full wave rectifier will get it down to 411 on the plates. What do you guys think? I mean it already has a wreck OT. It would probably solve any issues getting the voltages down for the 6SL7s and for the loop too. If I do... any advice on tweaking the plate voltage for EL34s? 6L6s? Or anything else that may work?
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martin manning
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Re: Tetanus Project

Post by martin manning »

Ah, filament voltage, the question nobody asked!

The other transformer sounds like it will work fine, except the filaments may run a bit high without some fix. 400V is JMP 50 level. I built a 2204 with JMP PT and it sounds great.

Re current ratings, the often referenced Hammond rectifier .pdf references FW CT rectifier at Idc/Iac = 1, and 0.62 for FWB, so if you use a doubler you will indeed halve the current that would be available using a FWB, or 0.31 re FW CT. Referencing FW CT Idc/Iac as 1.0 is their choice, I think, since I find Idc/Iac (RMS) for a FW CT is actually 0.82 (using a simulation). That means the relationship I get between FWB and FW is 0.71. A doubler gives Idc/Iac (RMS) 0.29.
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

Thanks again for the confirmation. I think I will put the RedBear PT in the F/S section and see if any of our European builders will want it. I'm liking the idea of lower plate voltage anyway. I will figure out what to do with the filament voltage when the time comes. In the meantime, I will have to wait for a new nibbler tool comes in to make more progress. My old tool broke trying to open up the hole for the Stancor PT. More as it comes... 😊
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

I got the PT in there and it fired up just fine... the filaments are fine running at just under 7vac. The voltages for the 6SL7s worked out damn near perfect. The only problems I was having was some squealing when I engaged the clipper circuit and some random rumbling and crackling from one of the preamp tubes. The tube issue has to be V2 or V3 since it is unaffected by the preamp volume control. I will roll some tubes to solve this later. As for the squealing, I replaced the wire from V1a to the volume pot with coax and the problem disappeared.

Next I verified my voltages and connected the power to the Jet City FX Loop. Quoting from here: http://ampgarage.com/forum/viewtopic.ph ... 1&start=75
"I have about 280vdc feeding it from the final filter node. I put it on a bypass switch so I could hear any differences. As is, the loop adds some gain to the signal. I have installed send and return trims to mine. I had to turn the trims to about 3:00 to get unity between bypass and engaged. With the way it is right now, I can't really tell a difference between bypass and engaged as far as any coloration... a bit more time may reveal the difference though. I ran a Strymon El Capisatan in the loop with glorious results. I also tried a Moer Shakey Jimi in there and instantly found myself playing Bridge of Sighs... I think the truth is in the fact that I kept forgetting that the effect was in the loop and I was just leaning on the natural character of the pedal to do its job.

Btw... I did a small mod to the loop. I replaced R7 with a 47k 3w instead of the 1w that was in there. This was one of the mods that Ten Over suggested. I may try changing the 10M to 4M7 later, but I'm pretty happy with this baseline so far. I really like the loop."

I really love the tone of this amp. Very much like a JMP at times. It really likes my '95 Classic Goldtop Les Paul. The slope control is awesome... there is just so much that can be dialed in with it.

The only thing that I'm not really digging is the Jose Master. I may remove it and replace it with a Lar/Mar type PPIMV instead... I'l have to see if I have a pot for it in stock.

More later...
Last edited by dorrisant on Fri Dec 08, 2017 12:36 am, edited 2 times in total.
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

Ok... I removed the Jose Master and replaced it with a Lar/Mar PPIMV. This will demo better. Up all the way for those who aren't familiar... then down to suit the need.

I also changed the bias pot. I love the ten-turn type in my own builds...

Looks like I could move my FX loop feed to B+4 if I need the headroom. The wire is long enough to move over if need be. I'll wait and see.

DC Volts:
B+1 = 440v
B+2 = 437v
B+3 = 331v
B+4 = 250v
B+5 = 200v

V1 - 6SL7
Plate 1 = 138v
Cath 1 = 1.65v
Plate 2 = 138v
Cath 2 = 1.66v

V2 - 6SL7
Plate 1 = 249v
Cath 1 = 156v
Plate 2 = 156v
Cath 2 = 1.79v

V3 - 6SL7
Plate 1 = 212v
Cath 1 & @ = 44.2v
Plate 2 =196v

V4 - EL34 (biased @ 80mA for the set)
Plate = 438v
Screen = 432v
Grid = -37.1v

V5 - El34
Plate = 438v
Screen = 432v
Grid = -37.1v
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

A few more...
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romberg
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Re: Tetanus Project

Post by romberg »

Bumped into your amp while browsing one of my favorite youtube channels last night. Saw the thumbnail and thought "There is no way in the world two of these exist!". Sounds good!


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Re: Tetanus Project

Post by pompeiisneaks »

I fixed your link romberg, and yes I to saw the video! Is that you dorrisant in the video? Sounds great!
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

Yep, that's me. Thanks guys! ... 😮
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M Fowler
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Re: Tetanus Project

Post by M Fowler »

Thanks Tony great amps that little SE kicks ass buddy.

Another video on pedals perhaps!

Mark
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

Thanks again guys.

I think I'm about finished with this one. Good thing, it ships out on the 6th. Still need to put the new tubes in it. I think I will try to stamp some new labels on the front. Maybe mix up some rusty looking paint to try to match. Waiting on a part for my 3D printer, so I may redo the badge. The back plate may be the very last that I do completely by hand. The CNC I have here is about to sling chips... :D

I will be recording some clips before it goes as well. I finally got all of my recording gear together and I don't want to miss the chance before it's gone. I will build another one before too long. I already have two more old PA chassis that I bought from Brad, The Guitologist. He actually posted a video of the teardown of the two, if anyone is interested... https://www.youtube.com/embed/0xVLeJd9HyA

Also, I added an IEC socket... Nice! :wink:
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joeboo88
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Re: Tetanus Project

Post by joeboo88 »

awesome stuff Tony: I listened to the youtube video and learned alot..... just shows me how little I know.

keep making something out of nothing or rusty stuff!
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Guy77
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Re: Tetanus Project

Post by Guy77 »

The YouTube video was great! Thanks for sharing.

Cheers
Guy
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