Yet another 124 build

[GAStan]

Chassis is basically complete. All components installed, voltages look good. Replaced 715P with 6PS (current production) paying attention to outer foil placement. OD and PAB switches corrected. New power and standby switches installed. Voltage dropping resistors installed for heaters. Replaced makeshift 250K trimmer with proper one.

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Was unable to find .06 ohm resistors of acceptable wattage but found .12 ohm 5 Watt current sensing resistors so used these in parallel, a pair on each leg to keep balanced as was suggested-thank you Martin. The have a combined total of 20 watts dissipation. There is now 6.7vac to heaters at 126vac wall power.

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Tube heaters now have nice orange glow rather than bright yellow.

Have not had a chance to plug in a guitar since all changes were made but should sometime today.

[martin manning]

Good progress. Still wondering why the heater voltage is still high. 125V primary is on the high end of the range, but it seems like the transformer over spec'd.

[GAStan]

Good progress. Still wondering why the heater voltage is still high. 125V primary is on the high end of the range, but it seems like the transformer over spec'd.

Thank you Martin.

I have to agree about the transformer spec. I may have spoke too soon on the voltages, how do these look?

B+= 463.7; 461 at 6L6 plates
B2= 461
B3= 440; 296 & 303 PI plates
B4= 298; 193 & 197 OD plates
B5= 290; 190 & 193 CL plates

[martin manning]

Those look good. Where is your bias set?

[GAStan]

Those look good. Where is your bias set?

25 mA still. Was leaving it cooler while working on the other things.

[martin manning]

Ok so your voltages will drop a bit, which is fine. V1 plates should be 180 or so.

[GAStan]

Was adjusting bias, checking actual plate voltage, and....oops!

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Letting it discharge and cool a bit before assessing damage. I know I lost the 120 ohm artificial ground and 1 ohm resistors. Hopefully that's all.

[GAStan]

Pulled tubes. Clipped artificial ground and 1 ohm resistors on V4. Isolated 4 leads to HV windings. Have no continuity between HV, heater or primary windings, do have continuity through each winding, all looks good. Checked heaters on each tube as I pulled them, all look good.

Is there anything else I should check before I move on to powering it up to check voltages?

[Guy77]

Pulled tubes. Clipped artificial ground and 1 ohm resistors on V4. Isolated 4 leads to HV windings. Have no continuity between HV, heater or primary windings, do have continuity through each winding, all looks good. Checked heaters on each tube as I pulled them, all look good.

Is there anything else I should check before I move on to powering it up to check voltages?

Hi. Was curious what wattage your heater balance resistors were, the 120 ohm ones you mentioned and did they get destroyed?
I know Two Rock uses 100 ohm 3 watters in there amps.

Thanks

Guy

[GAStan]

Pulled tubes. Clipped artificial ground and 1 ohm resistors on V4. Isolated 4 leads to HV windings. Have no continuity between HV, heater or primary windings, do have continuity through each winding, all looks good. Checked heaters on each tube as I pulled them, all look good.

Is there anything else I should check before I move on to powering it up to check voltages?

Hi. Was curious what wattage your heater balance resistors were, the 120 ohm ones you mentioned and did they get destroyed?
I know Two Rock uses 100 ohm 3 watters in there amps.

Thanks

Guy

1 watt 1% 120 ohm resistors. When I slipped off the plate the probe went between pins 2 and 3, putting ~440 volts DC on that 120 ohm resistor. Doing the math that's a bit over 1.6 KW. A 3 watt resistor wouldn't have helped in this case.

Once it started arcing it popped the input fuse and continued to arc until the filter caps had discharged. It destroyed one 120 ohm resistor and covered the 1 ohm and other 120 ohm resistors in carbon soot. The latter two could probably be used again but I'll replace them just for my peace of mind.

I'm actually glad for the 1 watt, it provided a discharge path for the high voltage.

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[GAStan]

Everything else checks out ok. Powered it up sans tubes and HV came up, all voltages look good. Now have to wait on more 120 ohm resistors from Mouser.

What a wake-up call!

[Stephen1966]

Everything else checks out ok. Powered it up sans tubes and HV came up, all voltages look good. Now have to wait on more 120 ohm resistors from Mouser.

What a wake-up call!

Fingers crossed for your tubes. Maybe wrap some heat shrink around your probes or get some of the grabby type.

[ijedouglas]

Everything else checks out ok. Powered it up sans tubes and HV came up, all voltages look good. Now have to wait on more 120 ohm resistors from Mouser.

What a wake-up call!

I had this happen to me as well. It definitely caught my attention I just lost the 1R cathode resistor... replaced it and everything was fine again

[GAStan]

Everything else checks out ok. Powered it up sans tubes and HV came up, all voltages look good. Now have to wait on more 120 ohm resistors from Mouser.

What a wake-up call!

Fingers crossed for your tubes. Maybe wrap some heat shrink around your probes or get some of the grabby type.

I have my fingers crossed as well, thank you.

Everything else checks out ok. Powered it up sans tubes and HV came up, all voltages look good. Now have to wait on more 120 ohm resistors from Mouser.

What a wake-up call!

I had this happen to me as well. It definitely caught my attention I just lost the 1R cathode resistor... replaced it and everything was fine again

I hope resistors is all I have to replace as well.

[GAStan]

I've noticed several values used for the artificial ground on the heaters. 100 ohm common on other amps, 120 ohm for #124 and 150 ohm on #183. Is there any guidelines for this resistor value? I ask because I do have several 470 ohm 1 watt that I could put in.