Catastrophe, HELP!

[ioginy]

Ok, this is about the amp in this thread, but on a totally different vein.
http://www.diytube.com/phpBB2/viewtopic.php?t=3958

I just did the dim bulb test and everything went off without a hitch. Then I setup the bias and ran into a little curiosity...
One would max out at 53v across the cathode resistor and the other would bottom out at 54v across the other cathode resistor... Oh, wait. I just realized I installed 10k resistors instead of 10ohm. Ok, I'll fix that soon.

Anyway, I tried it out and it sounds GREAT, but has absolutely no volume. Also, the power light doesn't come on, which is in parallel before the tube heaters. I replaced the bulb and still no lighty light.

So I am in need of help. Does anyone have any ideas of what could be going on. Major issue is the volume. If the 10ohm resistor will fix that, cool. the power light isn't a major worry unless it sparks something in your minds that could be a major issue.

Also, I just realized I need to drain the caps. 4.5k 15w resistor? will that be fine?

[ioginy]

I just did some research and it looks like the 10k resistors off the cathode are the culprit. I also checked the voltage going into the power lamp and it's a steady 6v AC. I have a feeling it's the socket.

[TomMcNally]

Yep a 10k instead of 10 ohm cathode resistor would do that,
the tubes would barely work. As for your power light, I have
a feeling it's the same problem. The lamp socket you used
is "recycled" and was meant for a Neon bulb, like a NE-51,
and has a high-value internal dropping resistor, so a normal
# 47 type bulb will draw no current and stay off.

... tom

[ioginy]

that makes so much sense! Tomorrow, I will fix that. Thanks.

[ioginy]

Alright, I swapped out the 10k for the 10ohm resistors and gave it a whirl.

Now I pick up every radio station from here to Guadeloupe (tried a .02uf shunt cap off the input ground with no change), have intermittent squeal (tried inverting the output transformer with no change) and so much hum I can't figure out if I am actually strumming my guitar or not, which warmbled up and down as I adjust the controls. When I flick the switch to pull the ground from the tone stack everything just goes LOUD.

I don't even know where to start.

Also, I hooked the lamp to the main 120v line and get a whee tiny glow from it. On the back of the lamp casing it says from 110 to 125V. Something is very a-miss.

[ioginy]

Alright, I did a little adjusting and the hum has gone down, however my volume, treble and bass knobs act as independent radio frequency adjustments. Anyone have any ideas on this one?

[TomMcNally]

Do you live under a big radio tower ?

You may have to run a heavier ground buss around the amp
and connect all of the stages together. I don't think your
cluster of ground wires at one point is gonna work in the
plexi chassis. Experiment a little and see what you can
do. Depending on how strong the RF is, it could be coming
in on the power lines, speaker lines, or on that long antenna
called a guitar cord.

[ioginy]

Nope, no power lines around here. I'm in my basement with the nearest high tension wires miles and miles away.

I'll give the ground buss a whirl. I did however do a check up and from all my ground points there is no resistance to the earth ground point.

The radio comes through CLEAR! and the volume adjusts the frequency which really throws me off, and it squeals . All of this with no guitar cable plugged in, just the speaker cable.

[TomMcNally]

I asked about a radio tower ... not power lines.
What kind of stations are you hearing? Local AM stations ?
In any case, with audio, it's not about how solid a ground
connection is, it's about shielding. Your plastic case with
no shielding is picking up signals in the air. That's the
reason most amps are built in metal boxes. You may be
able to calm it down with a heavy ground - like # 14 gauge
or so, that would run from module to module. Those small
wires you used are like a bunch of antennas.

You might try shorting the input connector to see if the RF
goes away. Do you have a schematic link for the amp ?

[ioginy]

Oh, no radio towers either. Waaaaait, there is a radio station about 4 km from here.

I don't recognize any of the stations, though I wasn't really listening to them, I was more glaring and grumbling at the amp :)

I shall try out some heavier guage wire.

I'll put up a schematic asap. I'll have to draw up the revised version.

[ioginy]

This is the most recent one I have. I didn't have the energy to draw up a whole new one. The top, errrr bottom, corner is torn off but it's just the other side of the preamp tube.

I also put two 20k pots in parallel to adjust the tubes bias individually. Kept the 22k resistors but I have them going directly to the bias pots without a connection in the middle. I am thinking this was a bad idea because I have them bottomed out and am still getting around 60v over the 10ohm cathode resistors. I changed the 10ohm cathode resistors to 820ohm and have an approximate reading 32v now. I am thinking of putting the two tubes together on a single bias pot and using the other as balance.

[ioginy]

Also, I tried shorting out the input connection and the radio noise really cut down. However that made me realize how much hum there was as well. The bass control really aggravates that as well, and if I touch my finger to the mounting nut (i haven't put knobs on yet) the hum goes through the roof.

I also measured the voltages across the filter caps and came up with some oddly low numbers compared to what the fender deluxe ab763 says.

http://www.schematicheaven.com/fenderamps/deluxe_ab763_schem.pdf

first cap before standby - 283v
second cap - 281v
third - 126v
fourth - 113v

Also, I measured the voltage before it enters the power transformer and it's around 116v, which I can't imagine would make that much of a difference.

[dhuebert]

Are the 68K grid stoppers right at the input tube? Without shielding they can't do their job from a distance.

Read the article here: http://www.aikenamps.com/ about grid stoppers if you havent already.

Are all the pot bodies soldered to ground? I just worked on a Japanese guitar amp that was dead quiet. The ground buss went from the input across to the backs of the pots and everything was tied to this. It was so quiet it was sickening. Damn. Used 6AV6s and 6BQ5. Beltone. Nice piece.

Don

[ioginy]

There is only one grid stopper and I have it soldered from the line to the switch on the input jack, using the switch as a jumping post. I figured it would work fine as the switch disconnects as soon as a guitar is plugged in, which should be always.

Only the bass body is soldered to ground, as I used it as a jumper for the 6.8k mid resistor to ground.

I'll connect all the bodies of the pots, use 14g wire for all my ground circuits and move the 68k grid resistor directly to the 12ax7 socket. Great reference to the Aiken site by the way. It really did a good job of explaining what the resistor is for.

[ioginy]

Alrighty.

Realized that I did in fact have the 68k in the right place, it was the 1M that was across the switch on the jack. BUT it has moved to a more suitable location now going directly to the tone stack buss.

Good News! There is NO radio anymore. All gone. No brittany spears or talk radio coming through my baby. It's GREAT.

Bad News! It still hums really badly, and it squeals when I turn the volume up to a good playing level. I think I am going to try and swap the output transformer leads to the 6l6's and see if that was actually the problem, just covered up by all the noise before. Also I am going to redo the bias as I said before. Single adjust with balance control.

Does anyone think a hum pot on the filament line would help?

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edit - 1:10
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I just did a little reading and some fellah knocked his amp over and it started to squeal. He shut it down and checked the tubes to make sure they were seated, fired it up and it worked fine. Another guy said that it could be a loose connection in the socket. Soooo I get to thinking, maybe I suck at soldering and I should check my contacts again.

See ya'll tomorrow. Thanks for the help today. One problem down, two more to go.