Trainwreck inspired amp completed

[Elcabong]

I finished my Trainwreck inspired amp this morning. I can play it but it is plagued with a low frequency oscillation. It is ok when I use an attenuator but as I bypass it, there it goes, OOOMPH!!! That gets hard over your nerves

It seems to originate from the PI / power amp area. When I turn the presence to maximum, it seems it will initiate the oscillation. I will take a break now and look at it later!

[UR12]

Elcabong

That looks great!! I can't believe how many people are finishing up their amp this weekend. Congrats!!!!

[Icetech]

Just curious, this is the second TW i saw with mallory 150's, are they recommended for these amps? I only ask cause i thought that i would share that on some advice on my last amp i replaced all the 150's with orange drops and it made a HUGE difference.. one i wouldn't believe if i didnt hear it...

[Mark]

Dear Elcabong

Nice amp and congratulations on getting it up and working. I'm not too sure what the problem could be off the top of my head. Any problems I had was from silly mistakes I made.

Just out of interest, keeping the heaters straight looks good, but how is noise effected?
I've always twisted the heater wiring as I'm keen to avoid any sort of problem. In fact at work, one of the mods to 767 displays is to twist the heater wires going to the CRT as it causes a flickering fault, which isn't too cool when your up in the air.

I found Icetech's comments about the differences between the Mallory 150's and the O.D. 715's interesting. How did they sound different?

Anyway Elcabong when you fix your problem do let us know what the fix was.

[Legin]

Just curious, this is the second TW i saw with mallory 150's, are they recommended for these amps? I only ask cause i thought that i would share that on some advice on my last amp i replaced all the 150's with orange drops and it made a HUGE difference.. one i wouldn't believe if i didnt hear it...

I'm happy with the sound I am getting from the Mallorys however I guess the only way to tell would be to build another with ODs and A/B them
Nigel

[Dai H.]

here's a bit on motorboating on R.G. Keen's site:

http://www.geofex.com/ampdbug/mtrboat.htm

looking at the chassis, I might check the filters (which not only "filter" the ripple but de-couple stages, or conversely couple the stage/s to it/them selves) for a bad/iffy connection. One eyelet is filled with lots of solder for instance, and leads me to suspect you have inconsistency in your solder joints. And check the cathode bypass caps too to be sure (though the solder joints look good there).

[Icetech]

Mark, I had done a dumble, and was trying to get it to sound like it should.. it was working fine, but didnt have that "IT" ya know... so on advice from guys there, i swapped out the 150's for 715's all same value.. (although PS orange drops were recommended) and the difference was night and day, the amp was more dynamic, more alive... seemed to have a better response to attack and such..

BUT i had built a 18watt with 150's that i thought was fine.. i did a cali dreamer with a 150's and i hate it.. muddy and sorta blah.. same thing with the dumble.. only thing in common.. 150's...

Like i said i wouldn't believe it if i hadn't done it myself.. for a TW though i dunno.. i would like to build one soon though:) im almost outa donor amps and i hate to actually buy a chassis and transformers...

[LeftyStrat]

My experience is completely subjective, possibly even "placebo", which I've always believed was a major part of the audiophile industry, (CD clarifiers?), but oranges tend to be good when you're looking for "sparkly" and mallorys tend to be good when you're trying to tame harshness. I would use oranges in a Fender circuit and mallorys in a Marshall.

But again, the only way to truely know is build two of them and do a double blind test.

I've even considered building a test rig, recording samples and setting up a web site to see what people preferred, but mp3's are shit and full quality would cost a lot in bandwidth.

WRT the osc probs, I'd offer advice, but right now I'm not feeling too good about my amp building skills. I just built a JTM-transition amp that had EL34s (close to a Germino Club 40, http://www.germinoamps.com/club40.htm), that was so close to tone nirvana (even sounded like the clips on that page), and instead of tweaking it closer, I've turned it into a piece of crap. I even have a problem with an almost tremelo effect on sustained notes.

The fact that your problem occurs only without the attenuator suggests it is volume/feedback issues. I'd suggest layout issues, but my chopstick ain't working in my tremelo case , so don't listen to me.

[Icetech]

Left your probably right bout using oranges for a fenderish/cleaner tone... in my 18watt or my marshall 100w i like to kill the treble right out for a more sabbathy thing so the 150's worked great:) was just mentioning the results i had gotten with switching...

BTW... when i get that frustrated working on a amp.. walk away for a day....

[Dai H.]

Dear Elcabong

Nice amp and congratulations on getting it up and working. I'm not too sure what the problem could be off the top of my head. Any problems I had was from silly mistakes I made.

Just out of interest, keeping the heaters straight looks good, but how is noise effected?

hey Mark, take a look at this article re: the heater wiring:
(Wire Orthogonality and Distancing)

http://www.lynx.bc.ca/~jc/hum.html

and check out the Soldano SLO100 chassis as well as the Reeves (Hiwatt) amps' heater wiring. And Paul Ruby's layouts also, where he uses the same idea.

[gnugear]

I noticed you're not using a 47 pf cap above the phase inverter (between the 1M and 100k resisters). Maybe that's contributing to your problem?

[Elcabong]

I noticed you're not using a 47 pf cap above the phase inverter (between the 1M and 100k resisters). Maybe that's contributing to your problem?

I thought about that too. It is not there because I do not have one on hand. I have tried a 100pF in but the problem persisted.

I went differently for the grounding in this built and I think I need to spend some more time to have it right.

[Elcabong]

I found the problem tonight; what a relief. The problem went away when I reversed the OT primary leads. The fact that it was playable with an attenuator got me confused in the first place.

By the way, I followed the A1b Kelly 90 schematic but added a single/parallel triode option at the first stage, a split plate load at the second stage and a pot/resistance is replacing the 150K before the 3rd stage. Things I picked up from the Trainflex schematic.

This amp is really something different, really fun to play. But it is loud!