It works!

[Danhei]

Hey Guys,
Thanks for the help and having this site. I got my 18 W running this afternoon and it sounds really great. 74 jailbreak was the first song I played...seemed fitting.

There is some 60 Hz hum which I suspect is because the transformers are too close. The hum is very slight but present even before the B+ is on and is louder when on and cranked. But not noticeable when playing. I was pretty careful about using shielded wire for every signal carrying wire until reaching the output tranny wires. Also star grounded and the heaters are referenced to about 50-60 V. I'll experiment with moving the tranny's later or maybe shielding the output one.

I experimented with dif preamp tubes. EI sound great but they flash brightly when first turned on (heaters only). None of the other tubes do this. Is this an EI problem or something else. The heater is prob running 7 VAC because my power here is 123 instead of the 115 hammond rated for.

Again, thanks for all your help.

Dan

[Wrongdog]

Dan, ! you are having a great day I bet!

[Danhei]

I sure am! I've been playing all evening!

Dan

[zaphod_phil]

I experimented with dif preamp tubes. EI sound great but they flash brightly when first turned on (heaters only). None of the other tubes do this. Is this an EI problem or something else. The heater is prob running 7 VAC because my power here is 123 instead of the 115 hammond rated for.

This is normal for many 12AX7s - even Mullards flash at power up. The idea is for them to get their cathodes to working temperature almost instantaneously.

Completeley apart from that, carefully check your heater voltage to make sure it doesn't get any higher than 7V. 10% above/below 6.3V are the limits. If it's any higher you may need to install a pair of balast resistors in the heater supply or else build a "vintage voltage adapter". That's basically a small PT (say 9V AC out) with it's secondary wired in seried with the primary of your main PT, but out of phase. That will reduce the voltage the main PT primary sees coming in by 9V.

[Gabi]

Dan, what is your PT? Is it a Hammond? Does it have the center tap on the heaters wiring? If not, than that can be the cause for that hum.

There is a workaround for this called "virtual center tap" using 2x 100 ohm rezistors connected from each side of the filament winding to ground.

Here is the picture of how I did it (on P1):

http://www.18watt.com/modules.php?full= ... _photo.php

Gabi.

[zaphod_phil]

Personally, I like to elevate the heater supply by connecting the heater winding center-tap (or virtual center tap) to the EL84 cathodes. I know this isn't usual with the 18W amps, but it's still good practice to help reduce hum a little more.

Having said all that, the fact that there's some hum even when on Standby, suggests that there may be some magnetic coupling between the PT and the OT, as Danhei suggested. Hopefully, the two Xformers have been mounted at right angles to each other - if not, they need to be.

[Danhei]

Hey,
Thanks for the tips. Interesting about the flash. I looked at my Ampeg V4B and the EI 12DW7 in that flashes as well. Those EI 12AX7's are great sounding tubes. Better than the Groove, JAN Phillips (NOS), RCA (OS), or Tung Sol (OS) I have. I used to work in a physics lab (in college) and ran across a bunch of old tubes. Alas, no Mullard 12AX7's or RCA 6L6's!

The heaters are referenced to a virtual center tap, thorugh 100 ohm resistors, to a voltage divider off the B+ (to about 50V).

The tranny's are at right angles but physically close (1/2 to 3/4 inch apart). The power tranny also has some mechanical vibration. This afternoon I'll unbolt and pull it away from the output and see if that helps. Could you take some sheet metal or thick foil and wrap the coils of the trannys to cut down on hum? I'm using a Heyboer output and a Hammond 261M6 (I think) power. Both of which have exposed coils. The reason they are close is because I'm using a pretty small chassis and wanted stuff away from the preamp.

Dan

[Danhei]

Here's some outside pics:
http://briefcase.yahoo.com/dan636

Dan

[Danhei]

OK, I did some poking around and couldn't figure out the hum. The transformer proximity isn't the issue, though.

I'm running 310 V on the plates, 277 V for the preamp, and 125 on the screens. SV83 needs less than 200 V but 125 seems low. Would this cause problems?

With the preamp tube removed it still hums at the same vol and the vol and tone controls affect the hum. With the PI removed it is significantly reduced. So I'm thinking there must be a problem between the preamp and PI tubes but I don't know what. Moving the wires around doesn't affect it at all and all those wires are shielded. I've doubled checked with the schem but don't see any issues. Maybe I'll take out the tone control (would rather have big muff style anyway) and see if that's the problem. Any other ideas?

Dan