I almost didn't post this here. Because to many I may be committing sacrilige by augmenting the TrainWreck design to suit my purposes. But even Omar confessed at the MetroAmp forum that he was putting a switchable tube-rectified/SS-rectified power supply in his build, so if he can commit the ultimate sin then so shall I.
I took the version A1a-'Wreck preamp (with a 250pF treble cap in place of the 500pF) and mated it to a 2-6V6GT power amp with a Vox-style Cut control, and used a 5Y3GT-rectified power supply. I just don't have the need for another super loud 50-watt amp, my Marshalls have that covered.
The chassis layout is still more Marshall than Ken Fischer as well.
The guts need some revision. My original turret board was built from a scrap and necessitated the B+ resistors being mounted on a terminal strip. I screwed up that board, but I was already committed to the mounting hole locations and left the board size the same. I did get taller standoffs with the new board material so I can move the B+ resistors onto the filter cap can under the board at some point.
I named her the "Dirty Girl 20." Brand me heretic if you will....but this thing is gonna be a fun player at a reasonable volume.
Great work. S'funny seeing a valve-rectified one - I've got a JTM45 chassis sitting here and had fantasised about a JTM45/Express hybrid...
Anyway, since there's no clip to listen to, I'll have to pester you with lots of questions: What's with the knob on the top of the chassis? Is that a Hammond chassis gold anodized? Any danger of the joint of the two 10K resistors (grey ones) shorting against the chassis? And what does she sound like?
It's a painted Hammond chassis, I'm on the cheap here. The knob on the top is actually my bias adjustment.
I didn't have the correct value resistor when I did the two joined together, so when I get around to moving them directly onto the cap can above the turret board there will only be one resistor. They're pretty stable though.
How's it sound? I wish I knew.....the setting of the volume knob (about one) is all I got to try out the other night since my wife and son were in bed already. Even then it gave a nice crisp clean sound with my favorite Strat, and was too loud to keep playing at 10:45 at night!!
I'm gonna find a way to sneak home early today and give this thing a burn-in that the neighbors may not appreciate if they're home.......
I'm moving at the end of the month, so all my recording gear has been packed away for a couple of months now to get our house in show-ready condition....I can't wait to get set up in my new room at the next house, I'll have three times as much space and can set up the mics, mixer and all.
The Cut control is inspired - I keep seeing that DIY builds can tend towards trebly, and that could certainly enhance the useability. Like the look and that you struck out for new territory. Keep us posted.
Got to play the amp this afternoon, and of course discovered a couple of issues to deal with.
First off is major headroom issues, volume on one is nice and crisp but you hit three on the volume and sound like your guitar is on ten into a bad fuzz. I think the only issue here is voltages, V1 and V2 aren't too far off (could be a touch higher) but the phase inverter is only running about 160VDC per side instead of the 220 or so you'd usually want to see. So I think an adjustment in the values on the B+ resistors is the cure there, and I might as well get them mounted on the cap can while I'm at it.
But here's the puzzling one. I can play this amp through my Weber MASS just fine, crank the volume as far as I'd like, with any attenuation setting. But the second I hit the bypass on the MASS or plug straight into a cabinet, I get a low frequency howl at any volume setting. I can play notes and they come through OK but as they decay the howl sets back in. Removing V1 and V2 don't change anything but pull the PI tube and it stops (I swapped tubes around to no avail also.)
I used plain old Hammonds for the trannies to keep this on the cheap. The only thing that comes to my mind is that these Hammond OT's use a funky dual-coil secondary that gets connected in various series-parallel combinations to achieve the different impedances -- so maybe I get a positive feedback building up as a result of the wiring of the windings? I don't understand why the MASS changes that if the secondaries are the culprit though. But I may try removing the feedback loop and see if it stops anyway. If the feedback is the problem I don't know how to solve the problem without either running no feedback or try hard-wiring every impedance combination and try to find one that doesn't howl.
Does the howl change withe the presence-setting? Maybe you wired the primary of the OT wrong, so you get positive FB instead of negative. Try interchanging the primary wire on the plates of your power tubes.
Good luck!
Timo
ODwan wrote:Does the howl change withe the presence-setting? Maybe you wired the primary of the OT wrong, so you get positive FB instead of negative. Try interchanging the primary wire on the plates of your power tubes.
Good luck!
Timo
I would have suspected this.....if it howled nonstop and didn't even let a note through -- I remember when I did this on an AB165 Bassman without realizing that CBS Fender had switched the OT secondary leads and the feedback reference point!!
I think my easiest route is still to start by unhooking the feedback altogether and see what it does. At least I'll know it's OT wiring at that point.
Just good ol' Rustoleum Metallic, I think I did three coats. Let it sit, label the chassis and then three coats of Rustoleum Enamel clear. Any Home Depot should have it on the shelf.
Originally I did it in a light blue Hammertone, but I exercised my American Male right to not read the instructions and tried to clear coat it too early...........ooops. What a mess!
Anyway, it's definitely a positive feedback issue of some kind I'm having. I changed the B+ resistors for more juice to the preamp and have much better results there. I disconnected the feedback entirely and can now play the amp either attenuated or not.
It still sounds kinda thin, almost like when you wire two humbuckers out of phase in your Les Paul -- is that another sign that I need to swap the OT primary wires? Never wired one up wrong before to see how it sounds other than getting the big positive feedback hum (and I didn't wait around to unhook the feedback wire on that one and see how it sounded wired wrong.)
I'm still puzzled why I could play it OK through the Weber MASS and only had issues when I bypassed/removed the attenuator.
Flames1950 wrote:Just good ol' Rustoleum Metallic, I think I did three coats. Let it sit, label the chassis and then three coats of Rustoleum Enamel clear. Any Home Depot should have it on the shelf.
Originally I did it in a light blue Hammertone, but I exercised my American Male right to not read the instructions and tried to clear coat it too early...........ooops. What a mess!
Anyway, it's definitely a positive feedback issue of some kind I'm having. I changed the B+ resistors for more juice to the preamp and have much better results there. I disconnected the feedback entirely and can now play the amp either attenuated or not.
It still sounds kinda thin, almost like when you wire two humbuckers out of phase in your Les Paul -- is that another sign that I need to swap the OT primary wires? Never wired one up wrong before to see how it sounds other than getting the big positive feedback hum (and I didn't wait around to unhook the feedback wire on that one and see how it sounded wired wrong.)
I'm still puzzled why I could play it OK through the Weber MASS and only had issues when I bypassed/removed the attenuator.
Hi Flames,
The amp looks great!!!! If you are still haveing problems with it reverse the the leads from the O.T.to the tube sockets..........I have found that they don't always hum real loud............sometimes it sounds weak and thin....almost like an octave fuzz is on.........had the same type of symtoms you report happen on a traynor mk III combo I was moding.Hope this helps!!!
Great job!!!!!!!!!!
Chris
Thanks for answering, Flames. I like the idea to have the opportunity to build something around 20 watts. I just finished an 18w Trem and I cannot really try it in the house. I definitively need some attenuator; are you satisfied with the Weber Mass?
The MASS keeps things feeling more natural to my ears, doesn't get to feeling squashed and thin as fast as other attenuators. Plus since it can be a load box for biasing, etc. it serves me well for two different purposes.
I verified those tranny numbers last night, I had it right. I just need some time to fiddle with that OT primary and see where it takes me, time is at a premium lately.
Great work. S'funny seeing a valve-rectified one - I've got a JTM45 chassis sitting here and had fantasised about a JTM45/Express hybrid...
