hum, i didn't do my home work...

[Foxfire]

i put together my first amp this past weekend, and i did not know that putting the OT right behind the input jack was a no no. i am a total beginner with amps so forgive me for not knowing the terms, i have a low hum that sounds like just about every other i've ever owned, and i have a higher pitched hum that i can lessen/get rid of with either the tone control or by moving the OT away from the input jack. unfortunately the second solution isn't really an option as i don't have the room in the chassis to move the OT. i used shielded cable for all the leads to and from the volume and tone controls for what it's worth.

do i have any options or do i just have to chalk it up to inexperience and just remember to plan better next time?

Thanks, Rylan

[Chuck H]

If it's not a cutout type mount on the OT some copper shield and carefull lead dress may work wonders. There are some high gainers that actually do have the OT on the input side of the amp. Though other things must be routed carefully to pull this off well. Can you post a pic of the layout, complete and clear???

[Foxfire]

first of all thanks for your time...

the amp is a slightly modded 5f2a circuit that started out as a music voice 168 auxiliary amp and i did this build on the cheap, real cheap. i spent some today cleaning things up. i moved the OT as far away from the input as i could, i cleaned up my heaters a bit and twisted just about every wire i could, i installed a hum balance, i moved the pilot light away from the tone pot, and i played with the placement of the OT leads. i could get rid of 99% of the hum but then i'd get crosstalk with signal leads that caused oscillation. the zip ties aren't pretty, but they are functional. at this point it's pretty quiet, more than usable and i can get it even quieter with the tone control. if you or anyone sees something i can fix, or do i'm all for it as i learn best by doing. so all comments are welcome...

[Chuck H]

Wow. I didn't realize the OT was inside the chassis and right over the preamp tube socket. And I can see your dilema of having limited space. All I can say is if your getting acceptible performance out of the amp it's a minor miracle, You could try a copper and or steel shield around the OT itself. It won't be pretty but it may help. You should be able to get copper at your local hardware store. It's used for flashing material on upper end homes. It's fairly easy to fabricate with because it's not a heavy gauge (can be cut with sheers) and solders easily.

[Foxfire]

Do you suggest I make a box to slide over it concealing it entirely or do I not need to go that far with it? I have something going on at the 65kr at the input jack. I can hear it when I tap it with my chopstick at higher volumes so it may be worth tryin to shield it.

I had the enclosure for a phaser I was gonna rehouse. But I woke up this past Saturday and got it in my head that I should put together this amp instead. I probably should have put the cap can closer to the PT and V1 closer to the input jack which would have let me put the OT top side...oh well. The enclosur that this was in originally was even smaller, although the input jack was well away from everything.

[Chuck H]

Do you suggest I make a box to slide over it concealing it entirely

Yes.

I have something going on at the 65kr at the input jack. I can hear it when I tap it with my chopstick at higher volumes

That's normal. It's the most sensitive circuit in the amp.

If appearance is very important to you, and you can't achieve acceptible performance with this layout, you may simply want to get another enclosure. They're not terribly expensive. There is also the possibility that as long as the amp is stabil you'll have something unique sounding. Proximity does all kinds of things both good and bad. As you noted, moving the OT did cure the buzz but caused an oscillation. There will be tonal differences as well. I once rebuilt an amp and it had great OD tone, but due to the location of the OT it would oscillate. I moved the OT and the OD lost it's hair. So I put the OT back where it was, twisted the plate leads and did some internal shielding. I got rid of the oscillation and kept the hair. So, a happy accident in that layout created something special. It may still work out.

[Foxfire]

so the last round of clean up got the amp well within the range of what i would call acceptable for my first build. i decided i should try adding a NFP which made me realize that i had to switch the OT primaries which seems to have made the primary leads quieter/less sensitive so i don't need the zip ties anymore. i also double checked the 6V6 bias which was around 12 watts at idle. i tried backing it down to around 8 watts, but i decided to put it back to 12 watts after doing some reading. oh and i through a 470R screen stopper in there. i haven't gotten to play it much since the latest round of tweaks, but i think i'm gonna be pretty happy with it, and looking forward to my next amp with a slightly smarter eye thanks to your help...