Start-up problems

[Aural Robert]

Finished my SEX amp the other day, and the resistances checked out ok. When I plugged it in for the "smoke test" there was in fact smoke. And a couple of snaps, it almost sounded like the caps were settling in or something. Which I don't remember from any previous build. I turned it off immediately and found a questionable connection of the cap on T11/12 (which interesting enough is where the smoke came from too).

Rsoldered the cap, powered it back up, voltages all seem to check out, except that on some of them where I should get 0 I have a reading of millivolts - is this correct?

I hooked it up and the volume is waaaaay low. I have a set of Paramour 2A3's and Fostex Fe116E horns, and when I swap the speakers from the 'mours to the SEX the volume is less than half. With headphones, I can crank it to the top and it is not loud by any means.

Not sure what to look at as everything seems to be ok. HELP!!!!!! Thanks to the 'bot community in advance.

Aural Robert.
... we have a map of the piano ...

[Paul Birkeland]

How are the filament voltages?  (Measure between the black and red wires on the twisted pair that goes from the H terminal strip to the two 8 pin tube sockets)

Since the volume is low on both channels, there aren't a whole ton of things to look at.

[Aural Robert]

A8 and B8 are both at 6VDC and A7 is 10.7mV, B7 is 7.7mV.

Does this help?

Is it possible I blew out a cap when I did my first power up? Are millivolt readings ok?

Thanks for the help.

Aural Robert.
... a thought that keeps me thinking, like a stone inside my shoe ...

[JC]

I would like to verify with you that you have the same symptoms on both channels; is this correct?

I would recommend verifying all of your ground connections first, by unplugging the amp and measuring for resistance between the main safety ground connection to the chassis (next to the AC power inlet jack, IIRC) and every other point that should be at circuit ground.  Start by setting your meter to its lowest Ohms scale and touching the probes together to get a reference reading of what your meter considers to be zero Ohms.

If your voltage readings per the check-out are all withing tolerance, I'm thinking there might be an issue with the signal path; since ground is the common part of the signal path, I am suggesting your verify all ground points first.

[Paul Birkeland]

A8 and B8 are both at 6VDC and A7 is 10.7mV, B7 is 7.7mV.

Does this help?

Is it possible I blew out a cap when I did my first power up? Are millivolt readings ok?

Thanks for the help.

Aural Robert.
... a thought that keeps me thinking, like a stone inside my shoe ...

Those look good.  How about the A1-6 and B1-6 measurements.  Those will be quite revealing.  (It is a good sign that you can get these measurements without the fuse blowing by-the-way)

[Aural Robert]

I would like to verify with you that you have the same symptoms on both channels; is this correct?

Yes, that is correct. I checked all of my ground connections and they all read 0 ohms, no exception.

How about the A1-6 and B1-6 measurements.  Those will be quite revealing.

Hmmm, A1/B1 thru A4/B4 are all within spec, but A5 is 38.5V, B5 is 37.8V, A6 is 240mV and B6 is 240mV. So something is awry. Next?

I also forgot to mention that one of the tubes is burning brighter than the other, but there is no difference in the volume from l to r.

Aural Robert.
... Nepalese Bliss ... blow your head right off ...

[Paul Joppa]

"...A6 is 240mV and B6 is 240mV."

Aha! There's a problem, those should be 2.5 volts.

There's a 1270 ohm resistor from pin 6 to ground (at T17/T37 I believe). That resistor is perhaps the wrong value, or shorted out, or something?

[JC]

Ah, so there is a voltage issue!  And, I see Paul is on it.  You may want to take a resistance reading from A6/B6 to ground (powered down, of course) and see what's going on there.  Those low voltages would definitely impact the ones on A5/B5, and hence your driver stages.

[Aural Robert]

OK, now I know what happened.

On my parts list, there are only (2)1.27K ohm resistors, and (2)120ohm resistors - all of which are blue. I installed the first 2 blue ones between T11/31 and T16/36, checking them with the meter to ensure I had the right ones. The other ones I assumed had to be right as they were the only other "blue" ones and therefore didn't need to be metered. DUHHH yes they are physically different sizes, I guess I shoulda noticed.

I'll be calling Eileen shortly to get the right ones.

THANKS to all. I'll report back once my next soldering session is over.

Aural Robert.
... Happy trails to you ...

[JC]

OK, that would do it.  Before you call, you may want to put your Ohmmeter across the plate load resistors for the drivers, RL.  Those are the 150k 2W ones that attach to A5 and B5 at one end, just to make sure they're still within tolerance.  I expect they probably are, but it doesn't hurt to check.