PTP Tremwreck

[jjman]

I finally finished the PTP Tremwreck. I used lastwinj’s posting of his PTP units as a starting point to help develop my layout jadeDSCF0005.JPG. (Thanx for getting me started!)

Yes I installed a PPIMV as a trial since I’ve never had one. It is staying since it works well. That bundle of ceramic caps is the tremolo oscillator. I’ll replace the group of 5 with a .05 single cap later. It took much tweaking and insertion-point moving to get an acceptable result. I initially injected it into the (cathode) bias of the 6V6s but the MV affected it’s intensity when the MV was not cranked. I ended up injecting into the grid of the 3rd stage. The Harmony H410 was the guide for that as supplied by Andy Le Blanc.

Here are my voltages:

6v6s P-352, S-347, C-23.7 (9.6watts ea.)
PI P1-187, P2-175
#3 P-223
#2 P-178
#1 P-157

Although I didn’t use the normal fixed bias, I figured I would check on which stages clip before which. I’ve read here that the output should clip 1st, then the PI, then the 3rd stage, as one turns up the volume. I left the MV cranked and did some scoping.

Out_PI_50%vol.JPG pic shows the speaker output on the top trace and one leg of the PI output on the bottom. This is at around ½ volume which is where the clipping became visible. I think my signal generator was probably set on the low side since the guitar distorts before ½ volume.) It’s hard to see but both are clipping although the speaker’s peaks are “slanted” whereas the PI’s are flat. My guess is that the output is also starting to clip itself or the PI is not perfectly balanced or something. I really don’t know what the “slant” means. (Can you tell?) I should have scoped both PI outputs separately. So I don’t think I can confirm than my setup is clipping on the 6v6s first.

PI_#3_50%vol.JPG pic shows one leg of the PI output on the top trace and the 3rd preamp stage’s output on the bottom trace. Also at 50% volume. This one clearly shows that the PI clips before the 3rd stage. This would be one of the reasons to use a PPIMV since one could utilize the PI clipping while keeping speaker volume lower. Assuming PI clipping is more “desirable” than stage 3 clipping.

#3_#2_maxvol.JPG pic shows stage 3 on the top and stage 2 on the bottom. This is with the volume cranked to show that my stage 3 appears to clip only on the top half of the wave. The stage 3 clipping actually starts around 75% volume. No clipping in stage 2 with volume cranked. Tone settings were at 50% each and I used an ~800hz signal. A stronger input signal could have possible clipped the 2nd stage. I tried to match it’s volume by ear to my guitar since I don’t trust the amplitude calibration of my scope in that vicinity. I then switched to a dummy load to avoid deafness and insanity.

I haven’t made a back panel yet. I may want to use a “mesh” type. Cab is red oak stained “red-mahogany” then gloss poly. I have a 4x10 Marshall cab for mating. The individual cathode bias adjusters are 100ohm wire wound pots and vary the idle by one watt for each tube. Probably unnecessary but fun to have. Tubes are all JJ except V2 which is EH. The EH was a little bright in the V1 position. Haven’t tried more tubes yet. The chrome chicken heads are hard to find and not cheap!

Tone? This head rocks! With the cathode bias and the 6v6s, I can crank it and not cause an earthquake. Yet there is more than enough volume for anything I would need. I built a half-open-half-sealed rear panel for the Marshall 4x10 to have the best of both those worlds. Previously the Marshall was too focused for me. I have an outdoor “gig” next month and may switch back to the sealed cab for that.

Loud and clean mode? –turn the guitar’s volume down. As most of you have probably seen and heard, this is one of the great aspects of the Express. For it’s 1st run at a jam, my homebrew fuzz pedal crapped out. No problem.

I’m looking for a little more bass which is think may be resolved with higher value caps on the MV. I already increased the #2>#3 coupler and the value of the bass cap, so the MV is the next area to hit on that. I tried a 50k mid pot at 1st but that was crazy and went to the 25k which works nicely. Generally I pulled from the values on the schematics here with a 2watt CC plate resistor on the 3rd stage, 1watt CF on #2 and 3wt MF on #1. The 3wt-MF is the “flameproof” one in the Mouser catalog that has no color codes on it, and had the lowest noise level. There is no hiss from the #2 stage and a little hiss from the CC on #3. I can eliminate the hiss from the #3 by turning the MV down to around “8” which is a nice bonus. No hiss from the PI which currently has 1/2wt MFs on it. I will likely change those to 1watt CFs.

I’ll try to post a quick mp3 but I’m much more of a tube geek than a player as you would quickly hear.

[jjman]

More pics:

[lastwinj]

cool beans!!! kinda fun to build that way, eh?

question for you, i see a couple of 100k resistors hanging off a dual ganged pot (my guess is PPIMV). how did you connect the resistors to the wires? formed "J" hooks on each, put them together in tension, crimp, and then solder? best way to do that kind of connection, IMO, and will give you less hassle in long run.

germ

[jjman]

Hey lastwinj

Yes, that's how I connected those. I forgot to include them in my layout initially so I had no standoffs ready for them. It worked fine w/o them too, and I don't really know if they are doing anything, but I left them in.

I spotted an extra screen door guard thing in my garage left by the prior house owners. After much nipping, bending, cleaning, and painting it made for nice front and rear grills.

Couldn’t find adequate stick-on letters in metallic silver at Michael’s so I’m using the “rope” design for now. I like that they match the color of the wood well.