Help! please… The good news is it works.

[Jlanguirand]

Hi Robin,

The cap to the right of the 40uf is a 16uf 475 volt and should be connected as you mentioned. We used a star ground buss wire but did not attach the volume/tone pots into this ground. The reasoning was that the pot was attached to the chassis and already therefore grounded. This is how I did it on my 5e3 and that thing has no white noise at all.

[Robin]

So, is the issue oscillation or noise (hum/hiss)? The 2Stroke amps I know of have neither as a problem, so it must be something in your construction or parts. It could be a lot of things, so here are a couple of guesses: Be sure that the non-electrolytic caps have the outermost shielding going toward ground. Check to be sure that the heater wires (twisted pair) are away from the other wiring. Ensure that the signal does not cross back over itself in the circuit. Double check that all leads are short in length. Check out Alan D’s 2Stroke chassis photo (the orange one) here on the TAN site. His circuit layout is clean, organized and exhibits correct lead dress. Note that the heater wiring is dressed away from the rest of the leads.

[beelzebum]

in one of the pics you posted, it looks like you have a 68ohm (blue,grey,black,gold) resistor in your nfb loop. it should be a 68k (blue,grey,orange)

[Robin]

Beelzebum is right, that should be 68k, not 68 ohms, do both of your amps have the 68 ohm resistor installed? While your at it, go through and be sure the other component values are correct.

[Jlanguirand]

Wow! Nice catch Beelzebum!! Everywhere we should have a 68k, we have a 68ohm.

They must have sent us the wrong values and I never checked it.

Question, are the resistors from radio shack the same quality as from antique electronic supply?

Thanks so much!!!

[beelzebum]

radio shack sells carbon film resistors. They are about the same quality as any other carbon film you get, so they should work fine.

[Jlanguirand]

Thank you guys so much! I changed out the resistors to their correct values and now this is the best sounding amp I have ever heard!

Along with it’s great sound, I have a hum though. I think it is grounding related so I’m going to clean that up a bit.

Question: Do any of you have resistors from the pilot light going to ground on the 6.3v filament supply? There are a couple on my 5e3 circuit and that amp is totally silent.

Thanks!
Justin

[Robin]

Congrats! The 2Stroke really is a great little amp.
Those 100r resistors create an artificial center tap for the heater circuit to reduce hum. If your 2Stroke PT has a center tap (going to ground)for the 6.3v circuit, you don’t need the resistors. If you do not have a center tap for the 6.3v pair, you should add the resistors. They don’t have to connect to at the pilot light, but they need to go from each side of the 6.3v pair to ground. Again, if the 6.3v pair is center-tapped, you don’t need the resistors.
Here’s a link: http://www.freewebs.com/valvewizard/heater.html
Do not ground any extra center-taps on the PT that are not indicated in the 2Stroke layout.

Please let us know if this resolves the issue, if we don’t hear from you, we don’t know if our advice was helpful or not, and others don’t benefit from the topic. If the center-tap is not the issue, there are other things to check.
Thanks

[beelzebum]

The resistors on your pilot are your artificial center tap. You could also use a balance pot. With a balance pot you could find the lowest possible hum. On some amps I have worked on, the balance pots were all the way to one side which is equivalent to grounding one side. On the 5E3 schematic one side of the filament is grounded. That is not always a bad thing if that is your point of lowest hum. But remember just because a schematic shows one side being grounded, it doesnt mean you can ground either side with the same results. If grounding one side lowers hum then grounding the other will increase it. If you do use the artificial center tap method make sure you use 100 ohm resistors, not 100k. Many people make this mistake.

[Jlanguirand]

Okay, so I have a red/yellow center tap wire that is grounded. This should be for the High Voltage AC Supply. I do not have a green/yellow wire, as described in the book. Sooo… I need to create an artificial center tap with two 100 ohm resistors correct? Should they be half watt or 1 watt?

Thanks so much!!

[Jlanguirand]

I think this guy on youtube has the same problem with the hum. I just don’t think he realizes it yet. He put in a Mojo PT as well.

Another quick question, what resistor values should I use to get rid of the popping on the boost switch?

[Robin]

1/2 watt will do. Don’t go by the colors spec-ed in the book, use the wiring schematic you got with the transformer. If you didn’t get one, check with the manufacturer. Even the same transformer manufacturer changes the color of the leads sometime, so be sure you have the correct info for your specific PT (and OT too, for that matter). The popping “boost” switch in the amp project video is easy to fix, I think it’s addressed in another forum topic on the TAN site. Also, some of the hum may go away when he gets it in a cabinet with a shielded back. By the way, those Jensens would sound a lot better with Ted Weber’s break-in process applied. The 10″ jensen is really transformed with that treatment.

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