Tetanus Project

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dorrisant
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Tetanus Project

Post by dorrisant »

Ok, I want to post this up as a build thread. Here's my plan... An all octal 2204. 6SL7s through the preamp and EL34 output. It's gonna have lot's of mods! Also, I'm building it in an old PA chassis from the late 40s or early 50s... and man is it rusty. I'm trying to keep it rat-rod looking. The knobs are temporary... I haven't made up my mind on what will be permanent yet. I seriously doubt I will do anything to the finish. It does have a top cover that I will introduce later and there is a backplate and front badge to be made. I have yet to do a project on my CNC so this looks like a good place to start.

Anyway, I had to do a small layout for a JCM800 for another customer so I totally squished the board layout and made a second one for this project. The PT is from a 60 watt RedBear, the OT is a Pacific Wreck-style TX from RJ. Schematic with mods comes from the late ampgeek's project (Dave-O). Mile's put a lot of these mods out there at the time. I have tried them before with a customer's amp and now I feel the need to build one of these that will stay at home. Check it out here:

http://ampgarage.com/forum/viewtopic.ph ... sc&start=0

Enough for now... more later.
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Last edited by dorrisant on Tue Oct 17, 2017 3:23 pm, edited 1 time in total.
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

I hope it doesn't have any bugs in it... You can't even see the cathode resistors for V1! :wink:
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xtian
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Re: Tetanus Project

Post by xtian »

Dirt Box Bear Cage. I like.
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lovetone
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Re: Tetanus Project

Post by lovetone »

i think the knobs look perfect, they go with the old rat look
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Re: Tetanus Project

Post by dorrisant »

Updates...

I made the backplate, no labels yet. I'm going to install the Jet City FX Loop. (http://ampgarage.com/forum/viewtopic.php?f=6&t=29741) I want to put in a bypass switch as well as send and return trimmers, I haven't drilled holes for them though. The Mojo loop install guide says:

"For a basic install, solder a wire jumper from
one of the trim S pads to the other. Do the
same for the trim R pads. This will complete the
FX Loop circuit.

For input/output level control, solder the wiper
of a 1M Ohm audio taper trimpot or an
equivalent panel mount potentiometer to the
trim S pad furthest away from the switch, and
solder any of the outside pins of the
potentiometer to the other trim S pad. Solder
the final pin of the potentiometer to ground.
This will complete the FX Loop circuit. Do the
same for trim R.

Resistors can also be soldered into the trim S
and trim R pads if a fixed control of the send or
return is desired.


On paper, the Jet City loop and the Mojo look very similar. So, it looks like a 1M pot connected as a voltage divider before the input cap and one after the output cap will be good for the send and return trims. Sound right so far? I'm posting a wiring diagram, without the loop... feel free to tell me about any errors. :)

Gotta order some more red tip jacks for the bias metering taps. More soon...

Tony
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

Here are some current status pictures...
Effects loop with send/return trims and bypass switch. Preamp tube sockets wired too... Onward!
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romberg
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Re: Tetanus Project

Post by romberg »

I'm really looking forward to how this turns out. Both the looks and guts look good! A real sleeper of an amp for sure. Normally I don't go for "reliced" guitars. But this thing screams to me for some funky vintage knobs. Or some distressed and "aged" newer ones. And a knife switch somewhere would also be cool :).

Mike

P.S. Reminds me of a buddy of mine who used to show up at the local drag strip and race new corvettes with his "tow vehicle". Beat up flat-bed truck with a blown 460 ford under the rusty hood :).
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Re: Tetanus Project

Post by dorrisant »

Thanks Mike! That is exactly what I'm going for with this one. I gotta find the right knobs for this one... Just like you were talking about too. Maybe I should post in the for sale sign in on to request some knobs... I still have some vintage preamp tubes I could trade for an assortment of knobs maybe... ?!?
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Re: Tetanus Project

Post by dorrisant »

Time to fire it up very soon. I may change out the Low jack to a 12A... quantity of 4 - 14B jacks = $30... :shock:
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romberg
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Re: Tetanus Project

Post by romberg »

Hope it fires up smoke free!

Meant to ask if you tweaked the plate resistors or voltage dividers in the preamp a bit to account for the 6sl7s slightly lower mu (compared to a 12ax7)? I suppose this sort of thing could also be done after it is running and if it seems like it would even be needed.
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Re: Tetanus Project

Post by dorrisant »

I suspect I'll have to change things a bit... I have been looking for reading material about the effects of the values of the dropping string resistors. I would like to be able to wrap my head around manipulating the string for desired effect.
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dorrisant
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Re: Tetanus Project

Post by dorrisant »

Well Crap! I powered it up with no smoke. The HT is way too low though. No tubes installed and it only puts out 260 or so to the plates... I should have checked the voltage before I put it in. 190 vac between the primaries and half of that from the center tap to each.

I was under the impression that this PT was good for two 6L6s in push pull. Maybe with a voltage doubler. I recall replacing the PT in a Redbear MK60 that required a change in the rectifier arrangement. I replaced the original PT with a Fender P-TF22798: https://www.tubesandmore.com/products/t ... 5-v-200-ma . I have changed a few PTs out before in the MK120s and never had an issue... just connected everything wire-for-wire and they worked as if nothing had changed. When I swapped in the above mentioned replacement Fender PT into this MK60, the voltage soared up to above 700vdc way before the variac was up to 120vac. I had to reconfigure the rectifier to match what is on the Redbear schematic. This was quite some time ago and I have somehow lost the pictures that documented the before and after... :oops: Funny thing is, after an intense web search, all of the pictures I see of the MK60 seem to have used a bridge rectifier. That would lead me to believe that they were designed for a 325-0-325 PT... right? I have never seen a voltage doubler that looked like a bridge rectifier. The ones I have seen only use two diodes anyway. Arron (Xtian) had one of these for sale a while back on his monkeymatic page... Those pics look like it has a bridge rectifier. Hmm... :roll:

So did Redbear actually use two different PTs with corresponding different rectifier circuits? Or do I have a dud? I bought the Redbear PT from a trusted friend and can always get the money paid back if it is... No worries there. Has anyone ever seen an MK60 that had a doubler in it?

I let the thing stay powered up with no tubes installed for about an hour and checked the temperature of the questionable PT. It was barely warm. All of the tests I have done to it since this discovery indicate that it is in good health, so I'm tempted to change to a doubler and see what happens. I already have the reservoir split, so it would actually be very easy to change it. I'm not in a huge hurry so I can wait a bit to see what you guys think. With the attached circuit would I get VAC x 1.4 x 2 = 190vac x 1.4 x 2 = 532vdc with no load? http://el34world.com/Forum/index.php?topic=14884.0
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Rockwell666
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Re: Tetanus Project

Post by Rockwell666 »

does the PT have different line voltage tabs?
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Re: Tetanus Project

Post by martin manning »

I found Red Bear Cub combo Mkx 50 and Mk 120 schemes, both have FWB rectifiers, and the 120 shows 495V at the OT CT. On both schemes they are connecting the CT to the middle of a stacked reservoir arrangement. Are you sure you have the secondary voltages right?
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Re: Tetanus Project

Post by dorrisant »

Yes Martin, 190 across the secondary leads. I don't know if it is clear in the pictures but I did connect the center tap to the middle of the stacked reservoir. As it would be set up that way, it is not a voltage doubler. It is a rectifier with the PT secondary CT connected there to balance the voltage across the reservoir... I am questioning the bleeder resistors I installed across the reservoir. I don't know that they are needed.

Thinking of pulling the bridge rectifier out and installing the doubler circuit from the above post. I'm reasonably sure that this PT is fine and that I just goofed on how it was to be connected.

What do you think? Would that doubler arrangement give up close to the 495v like you mentioned?

And, no, there are no more primary taps on the PT. Also, the 190v tap is the only HT secondary. So you gotta use that combination to get the needed voltage.
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