New Amp Day - StewMac Mojotone '65 P-Reverb Princeton Reverb Assembly & Review

[jonfields45]

Originally Posted by redir

So for you guys that know this stuff, when you solder a high voltage, or any I suppose, wire to a lug so you want first, a mechanical contact? IOW maybe not wrap the wire around it but at least be sure there is wire to lug contact before locking it in place with solder?

The StewMac instructions call out a mechanical connection (wrap the lead or wire) before soldering. This is clearly impractical when soldering to the back of a pot, for example.



I think the probability of a cold solder joint is greatly reduced if you first have a mechanical connection. So for this kit, wires that are attached and soldered later have mechanical connections. Wires that are being soldered right after connection, it depends on how stable it is prior to soldering. Stable being good :~).

[Glennwillow]

I don't have anything to contribute here, I have never built a guitar amp kit, but I am enjoying reading through this thread.

- Glenn

[redir]

Originally Posted by jonfields45

The StewMac instructions call out a mechanical connection (wrap the lead or wire) before soldering. This is clearly impractical when soldering to the back of a pot, for example.



I think the probability of a cold solder joint is greatly reduced if you first have a mechanical connection. So for this kit, wires that are attached and soldered later have mechanical connections. Wires that are being soldered right after connection, it depends on how stable it is prior to soldering. Stable being good :~).

Ok yeah makes sense. For the back of pots I press and hold the wire tight to the 'can' and melt the solder on.

[jonfields45]

I also spent a little extra for US made eutectic, 63% tin, 37% lead, Kester solder. 60/40 is cheaper but possibly very slightly more likely to form a bad joint as (I understand…) the two metals solidify at different temperatures. I trust that Kester’s no clean rosin (meaning it doesn’t accumulate moisture to corrode the joint over time) is the real thing with no compromise to compete with the huge selection of inexpensive imports.

[jonfields45]

A little progress today. The tube sockets, short of the heater wires, are complete. I broke one wire from the eyelet board, and another was too short. The nice thing about real eyelets, now that the backside jumpers are secured by the fiber backing board, is they will take a lot of abuse, and no problem reheating, sucking out the solder, and doing them over. Many of the jumpers were too long and I cut them a little (should have taken more) and took advantage of the pushback insulation. I verified every connection with my meter before moving to the next.

I'm thinking a job offer from Headstrong is not in my future :~).

[Bluside]

Originally Posted by jonfields45

A little progress today. The tube sockets, short of the heater wires, are complete. I broke one wire from the eyelet board, and another was too short. The nice thing about real eyelets, now that the backside jumpers are secured by the fiber backing board, is they will take a lot of abuse, and no problem reheating, sucking out the solder, and doing them over. Many of the jumpers were too long and I cut them a little (should have taken more) and took advantage of the pushback insulation. I verified every connection with my meter before moving to the next.

I'm thinking a job offer from Headstrong is not in my future :~).

I'm not an expert but I think you are doing a fantastic job. It looks very professional. Nice and clean.

I built one of the Champ amps during covid. I put it in a bigger cabinet with a 12" speaker. It's my favorite amp.

Your amp looks much more complicated.

[jonfields45]

Tube filaments and power chord still to be wired. I'm giving a little thought to rewiring the input jacks to the first tube grid with a short shielded cable, instead of the longish jumper running through two holes in the eyelet board.

[leew3]

Jon, just another note of thanks for your starting this thread and sharing your hard work on this journey. As you noted earlier, this helps to remind us why the factory made amp costs what it does. Much like wondering why a professional level mandolin can cost $10k until you learn what's involved in building it.

[jonfields45]

Originally Posted by clintj

Whoever wrote that likes doing things the hard way. I just push it back a quarter inch or so, so the cloth is clear, then slide it back down to the joint when it cools.

At this point, I’m in complete agreement. I’m paranoid that stripping the wire, as instructed in the assembly manual, left slight damage to the core and made broken flying leads more likely. Hopefully at this point not relevant to future reliability.

Originally Posted by clintj

I believe Fender would fully populate the boards, then flip them to solder and add the flying leads as they went. The shape of solder joints on vintage eyelet boards are backwards from what top soldering would produce.

The instructions are near Heathkit class and a rewrite to this strategy would be prohibitively expensive, but I think in hindsight a better way.

[jonfields45]

But will it work?







Through out the entire assembly I checked every wire, component, and solder joint with a meter and my hopes are high. I started with 1 oz. of 63/37 Kester solder and only a few linear inches left. Half way through I bought a pound of Kester and discarded all the various no-name solder I had lying around. The green hook up wire that came with the kit is down to the last two inches.

[jseth]

Looks great! Looking forward to "The Test Drive"... are you going to keep the standard speaker size or go with a 12"? Always thought that one of the attractions of a Princeton Reverb was having a 10" speaker, as opposed to my Deluxe Reverb with a 12"...

[jonfields45]

At first the 5AR4 rectifier filaments did not light up, but wiggling the tube fixed that problem after checking all the voltages at the tube socket [EDIT: I later figured out that I had not soldered one of the filament connections from the transformer. There were two wires at that pin and I only soldered one]. Everything worked but the reverb, which just hummed when turned up and no noise when banging the tank (so the return was likely not working).

Next I spent way too long verifying the spring was good, the cables were good, the 12AT7 send and 12AX7 return tubes were both biased correctly... Then a good long time figuring out how to use a really bad hand held scope (and signal generator), which ultimately showed the signal getting to the spring. But I could not get it to trigger to the output of the spring (in hindsight duh...). And then it would not trigger properly in simple loop back. Uuuggghhh!!! That scope is going in the trash (actually, probably eBay).

As I was giving up for the day I accidentally hit the spring and got the appropriate boing. I plugged my guitar in and the reverb seems fine. Maybe similar to the rectifier tube, the contacts on the RCA jacks to/from the amp/spring might have needed the extra work out to clean off oxidation (the RCA jacks themselves are not nice and shiny). Not sure it really sounds super wonderful as the speaker is pointed at the wall, but enough for today.

[Bluside]

Fantastic. I remember the first time my Champ kit actually made sound after I finished the build. It's a great feeling.

Well done and thanks for taking us on your journey. I have really enjoyed following this thread.

[jonfields45]

To the various posts about the 10" speaker, I built this amp for the fun of it and don't plan to do anything more than put it by my guitars in the living room and practice with it at home. My QSC CP-8 and Spark Go are my traveling solution. I ordered this off eBay...



BTW, it did seem yesterday useful to discharge the power supply caps. I was shocked that StewMac wanted $38 for a banana jack, 4 watts of 100-150K resistance and an alligator clip. I looked in my parts box for the oldest worst tolerance Radio Shack carbon film resistors and made a ~3 watt, 110K ohm version (still more than fine for this problem). I threw it together using (4) 100K and (2) 120K 1/2 watt resistors. I assembled it with lugs to quickly crimp it together before soldering. Three layers of shrink wrap gave it a decent degree of mechanical stiffness.



Today I plan to clean the reverb tank RCA jacks with Home Depot heavy duty contact cleaner (don't spray this stuff in pots), move the cabinet away from the wall, and practice my finger style repertoire (about half an hour without repeats) through the amp to see what I think.

[jonfields45]

Today was not the best... I discovered I wired the filament wrong (one pin off same direction both sides) on one of the 6V6's. Fixed that and the amp sounds great, no issues now pointing the speaker at the wall :~), except still no reverb. The reverb return works fine (I get a bang when I hit the tank) and I think that was oxidized RCA contacts. The send side 12AT7 cathode voltage is 25% low, I dismissed that yesterday (the two triodes are in parallel). All the other pins have the right DC bias and I've (re) checked the wiring to all the pins. I'm going to borrow a known good 12AT7 and give that a try tomorrow.