Help on new #40 build

[andresound]

Hi Guys,
I have finished wiring up an ODS (#40). Everrything is cramped because I am using a gutted Bassman 50.

My problem is:
1) passes bulb limiter switch on test with no tubes
2) does NOT pass limiter test when tubes are in. (bulb stays bright)
3) if i remove just the power tubes, then bulb goes back to dim and heater voltages are 6.35v and preamp tubes light up

Whats up? I have checked the wiring around the output tubes 100 times. All seems correct. bias v = -45v. B+ is 450v

[JazzGuitarGimp]

With the output tubes removed, measure the DC voltage at pin 5 of the output tube sockets and adjust the bias pot for the most negative voltage at pin 5. That voltage should be at least -50V to -60V

Then, temporarily disconnect the negative feedback, and try the light bulb limiter with the power tubes installed.

[dinkotom]

It looks like the NFB resistor and output grid leak resistors are not soldered?

[andresound]

just tried that.
Bias = -50v
Feedback (going to the 820r) disconnected, but still the limiter stays bright

[andresound]

just tried that.
Bias = -50v
Feedback (going to the 820r) disconnected, but still the limiter stays bright

[mhartman]

What is the wattage of your lightbulb? If the wattage of the bulb is too low, it might seem stay bright even though everything is ok.

[andresound]

It looks like the NFB resistor and output grid leak resistors are not soldered?

Sorry, Im a newbie!! Do you mean the 1k5w going to B+2 ??
Also, was intructed by JGG to disconnect NFB and try

Andre

[andresound]

What is the wattage of your lightbulb? If the wattage of the bulb is too low, it might seem stay bright even though everything is ok.

Light bulb is a 40w.
The thing is... when the output tubes are removed, the light bulb dims the way it should. I also has fused the OT CT with a 250ma fuse and have blown nothing.

Andre

[andresound]

It must have something to do with the output section.? Only happens with power tubes installed. Tried an old set of RCA 6L6 black plate and a NOB NOS set of 5881. Still the same

[martin manning]

Could be just the additional 1.8A filament current from the power tubes causing the bulb to glow. How is the standby switch set, standby or play?

[andresound]

Could be just the additional 1.8A filament current from the power tubes causing the bulb to glow. How is the standby switch set, standby or play?

I will check, but the stanby position does not effect the outcome I believe.

BTW, how long does it take for the bulb to dim? With or without the preamp tubes, it is about 1 sec and settles into a dim glow. I might be being a little over cautious??? Will I damage anything if I leave it a bit longer and wait for the power tubes to warm up? I haven't tried a power up without the limiter AND power tubes in!

Andre

[martin manning]

Normal behavior with the standby switch in standby mode is the bulb will glow at about half brightness for a brief period (~1 sec) and then reduce in brightness. What you are seeing is the initial current surge from cold filaments and the charging of the reservoir caps.

When you switch out of standby into play mode you should see another quick pulse of a brighter glow and then perhaps a slight dimming, but most of the increased brightness should remain. The remainder of the power supply caps are changing, but the tubes will now be drawing current, which accounts for the increased glow.

[andresound]

Martin,

I think yoou are right and I AM being over cautious!!!! Just cant afford to blow anything up!!!

After leaving the amp with limiter on, AND power tubes installed, NO SMOKE All heaters are glowing

However, the volltages (with bulb limiter) at the B+ = 196, then B2 - B5 are all 183v !!! Shouldn't they be dropping in increments?

I havent tried without bulb YET!

Andre

[andresound]

Normal behavior with the standby switch in standby mode is the bulb will glow at about half brightness for a brief period (~1 sec) and then reduce in brightness. What you are seeing is the initial current surge from cold filaments and the charging of the reservoir caps.

When you switch out of standby into play mode you should see another quick pulse of a brighter glow and then perhaps a slight dimming, but most of the increased brightness should remain. The remainder of the power supply caps are changing, but the tubes will now be drawing current, which accounts for the increased glow.

Yes it powers up as you have just stated

[mhartman]

I find that 40W bulbs glow too bright for larger amps (50W and 100W). While it isn't as safe, I typically use a 60W bulb in my limiter so I can tell the difference between normal current draw and s short.