Strange observations in start up of amp (rebuild)

[IggyT]

I have gone through the RobRob startup sequence as I have before but stopped at a strange (to me) observation. To note this is a rebuild and only fundamental change has been move to tube rec instead of the diodes rec I had before. In those days the amp worked fine just a bit more of a hum level than I could accept. So here are the observations and figures:
1) All steps through the bulb (40W) limiter. House voltage 123 in all expected locations. flipped the Power On and bulb quickly dimmed. 4.4 V AC across rec pins 2-8 and 5.7 v AC across all preamp and power tube heaters (2 x 12ax7 and 2 x KT66). 330 V AC on rec pin 4 and 8. So far so good. Installed GZ 34 rec tube....
2) power on and limit bulb glowed faintly and became dim. ok. Flipped standby switch and limit bulb quickly went dim. First weird observation: the 120V neon pilot light went dim too and came back to full brightness. Hmmm... observed voltages: 390V DC at all filter cap reservoirs as well as pin 3 on the power tube sockets (fed by blue and brown from OT). also 390V DC on pin1 on v1 and v2. OK. the pilot light phenom is something Ive never encountered before. on to next step.
3) installed the two preamp tubes and Power on....limit bulb went dim and pilot light flickered. Standby ..ON..limit bulb went dim and pilot light flickered briefly before lighting steadily. B+ 347VDC , B2+ 343 VDC, B3+ 277 VDC. Also measured 346 VDC on Power tube socket pins 3, 4 and 6. Readings on V1 were as follow: p1=160V, p3=1 volt, p6 (jumpered from p1 also 160. p 7 = 0, p8=1 . On V2 : p1=196V, p2=18V, p3=38V , p6=185V, p7=20V and p8=28V. all heater voltages were similar to previous readings. OK...other than the funky pilot light behaviour I then installed the 2 KT66 beasts and hooked up output to a speaker and...
4) Power ON....limit bulb went dim but NO Pilot light hmmm... I did NOT proceed to turning Standby On but did quickly check a few points. Only 200 VAC on rec pins 4 and 6 ??? and 200V DC on pins 2 and 8 ??? Also house voltage measured at fuse and Power switch are reading only 60 V AC...maybe explaining why the pilot light isnt on.
I would appreciate any guidance to help me figure this all out. I thought it would be easy to go from ss rec to tube !
thanks
Iggy

 

[Peegoo]

60vAC means you're reading only half-wave AC. Are you testing across both legs of the supply?

 

[IggyT]

60vAC means you're reading only half-wave AC. Are you testing across both legs of the supply?

When I first powered up at the very start I read 123 V to ground at the supply from the house. And in the final step I again made the 60 v reading from that point to ground . Its not a point Ive ever actually measured before! Is that why the pilot is not lighting ??

 

[Tele_Tom64]

When I first powered up at the very start I read 123 V to ground at the supply from the house. And in the final step I again made the 60 v reading from that point to ground . Its not a point Ive ever actually measured before! Is that why the pilot is not lighting ??

What's a pilot light? The jewel-covered light on the front?

 

[Snfoilhat]

I think a tube rectifier isn’t always a drop-in replacement for ss diodes — so showing a drawing of how the earlier version of the amp was wired might help, and of course providing a clearer picture of how it’s wired now too.

Side note, [email protected] and [email protected] and a couple preamp tubes are going to draw a lot of current just heating up and a fair bit more at idle, and it’s all gotta pass through that 40W incandescent bulb. Does neon even work in situations far outside the voltage it’s made to operate at?

 

[Lowerleftcoast]

Make sure the KT66 are in the sockets correctly. Sometimes the key on the tube can fit into the socket in any position.

 

[YellowBoots]

All steps through the bulb (40W) limiter.

Oh no…. Not again. (Do you have any higher wattage bulbs?)

 

[YellowBoots]

Does neon even work in situations far outside the voltage it’s made to operate at?

No, it has a minimum strike voltage and then a minimum voltage for continuous operation.

 

[IggyT]

No, it has a minimum strike voltage and then a minimum voltage for continuous operation.

This same pilot light worked fine when I was using diode rectification... help me understand whats different now? sure I can put a 120 W bulb in the limiter

 

[IggyT]

Make sure the KT66 are in the sockets correctly. Sometimes the key on the tube can fit into the socket in any position.

yes sir they are

 

[IggyT]

I think a tube rectifier isn’t always a drop-in replacement for ss diodes — so showing a drawing of how the earlier version of the amp was wired might help, and of course providing a clearer picture of how it’s wired now too.

Side note, [email protected] and [email protected] and a couple preamp tubes are going to draw a lot of current just heating up and a fair bit more at idle, and it’s all gotta pass through that 40W incandescent bulb. Does neon even work in situations far outside the voltage it’s made to operate at?

This PT is a Hammond 273BX is rated at 201mA so should be ok. the rec heater spec is 3 A and the other filament rating is 5 A so I think Im in range. The amp worked fine with ss rectifier ; it was just a bit noisy and a bit stiff so I am trying the tube rec. I expected lower voltages anyway. The pilot light worked fine before yet I agree adding the rec tube has changed the draw ; My first concern is to "do no harm" to the parts.

 

[YellowBoots]

This same pilot light worked fine when I was using diode rectification... help me understand whats different now? sure I can put a 120 W bulb in the limiter

It worked fine with a 40W LBL installed?

Also house voltage measured at fuse and Power switch are reading only 60 V AC...maybe explaining why the pilot light isnt on.

Bingo. 60VAC—a perfect 50/50 voltage divider between the LBL and the amp! Try the higher wattage bulb and let us know.

 

[IggyT]

It worked fine with a 40W LBL installed?



Bingo. 60VAC—a perfect 50/50 voltage divider between the LBL and the amp! Try the higher wattage bulb and let us know.

Thanks Andrew; the pilot light in this build is not different from any Marshall 120 v and connected through the main power and the transformer primary. Ive used the same bulb limiter and bulb for years and never had issues. I used it when I first started up the amp years ago and that pilot light did not flicker HOWEVER at that time the rec was ss diodes.
I just installed a 100w bulb in the limiter and started again with main power on..Yahoo. Limit bulb glowed and went dim and ...Pilot light turned on and stayed on. Voltage now about 100 on main supply at fuse holder so I flipped the standby switch and the pilot light went out.

 

[dan40]

You're fine. Once you remove your limiter, the bulb will light properly. With the added current draw of the tube rectifier and the KT66's installed, you are losing a lot of voltage and current through the bulb limiter. Some of these pilot lights will not operate properly while on a limiter.

 

[YellowBoots]

Well I learned something:

https://www.powerelectronictips.com/the-humble-neon-lamp-down-but-definitely-not-out-part-2-faq/

Q: What’s the failure mechanism for a neon lamp?

A: Neon lamps gradually decline in light output as electrodes evaporate and condense inside the glass envelope. This situation is gradual, with failure defined as a 50% decrease from the original brightness. As neon lamps age, the firing voltages slowly increase until the supply voltage value is reached. At this point, the lamp flickers and becomes erratic, indicating the end of the useful lifetime.

 

[King Fan]

Good fix. Cool insight about the neon lamp.

Since we've recently had both a Moby Dick thread and a War and Peace thread here on LBLs, lemme just say 40W is OK for a 'most protective' first start, but even if you're that cautious, I'd move up to 100-150W for the rest of startup, and you may need to go as high as 150-250W to see the bulb go dim on really thirsty big-bottle amps. And of course as everyone knows (except when I forget), measuring voltages on an LBL may show 'relative' values, but is not accurate.

 

[IggyT]

thanks to all so far for the good intel. Ive learned a few things with this one!

 

[2L man]

One KT66 filament power is more than 8W. One pre amp tube 1,9W.

When filament does not get much heat the current is proportionally higher.

Your test voltages look normal. You can power amp without LBL

If bias is adjustable turn it max negative voltage on both power tube pins 5. If there are 1 ohm bias resistors on cathodes Install voltmeter to other and calculate the max voltage what there should come. When bias is turned max negative there possibly come very small voltage loss to bias measure resistor.

If power tubes are cathode biased calculate the voltage there come when bias current is 90%. If both tubes share same resistor the current comes double and the voltage comes higher. Then put voltmeter over bias resistor and then switch HV and look the reading hand on HV or Mains switch.

Before switching high voltage measure high voltages again and filaments too. They should come bit lower now when power tube filaments use current.

Do play test and if there is sound adjust power tube bias hotter.

 

[IggyT]

You're fine. Once you remove your limiter, the bulb will light properly. With the added current draw of the tube rectifier and the KT66's installed, you are losing a lot of voltage and current through the bulb limiter. Some of these pilot lights will not operate properly while on a limiter.

done! everything is good and amp sounds fine. I will tinker around a bit with grounding now to reduce a the faint noise. which is unnoticeable when Im playing, cheers .

 

[dan40]

Great news! Glad to hear that the startup went well.