Issue With Bassman 5F6A Build

[Guitarnut]

After checking AC voltages one more time, I powered up and switched the amp out of standby. Same start up as before. DBT bulb glowed slightly then went out. I let the amp idle for five minutes...no arcing this time.

All 3 filter caps metered at 456VDC.

That's progress, baby!

Thanks for the help guys. I'll continue to post as I go along.

Peace,
Mark

[martin manning]

The 456V is with the bulb limiter out of the circuit, I assume. Good going. You may have some work to do yet tidying up leads.

[Guitarnut]

The 456V is with the bulb limiter out of the circuit, I assume. Good going. You may have some work to do yet tidying up leads.

I haven't powered up yet without the tester. I guess I'm good to go without it. Is 456V too high?

Peace,
Mark

[martin manning]

Seems high for having the limiter in. What's the bulb's wattage? A 60W is a good size to use. I would add just the power tubes and power it up with the limiter in. If you don't have the correct bias voltage say, there could be significant current draw.

[Guitarnut]

I'm following Ruby Amps start up procedure.

I checked for B+ at pin 4 on V4 & V5 and have 440V. So, the output trans is good.

Before adding the signal tubes he says to check that the neg voltage for bias is being generated and that it's reaching the power tubes. I checked pin 5 on the power tube sockets but it doesn't show a neg voltage. I'm still checking against ground since it's a neg voltage...correct?

Where else in the circuit can I test for this voltage?

Peace,
Mark

[Guitarnut]

Seems high for having the limiter in. What's the bulb's wattage? A 60W is a good size to use. I would add just the power tubes and power it up with the limiter in. If you don't have the correct bias voltage say, there could be significant current draw.

The bulb is a 45 watt flood.

Will my OT be okay without a load if the power tubes are in?

Peace,
Mark

[martin manning]

Yes measure to ground. Try at the anode (non-banded) end of the bias diode, at the negative end of the second bias filter (after the 15k), and at the junction of the two 220k resistors near the presence pot.

[Guitarnut]

Yes measure to ground. Try at the anode (non-banded) end of the bias diode, at the negative end of the second bias filter, and at the junction of the two 220k resistors near the presence pot.

Sweet. Will do.

[Guitarnut]

The voltage at the anode of the diode and second filter is -46.7V. The junction of the two 220K starts around 150V and drops to 0V. That's probably not good.

Peace,
Mark

[Guitarnut]

Oops, I measured the wrong cap. After the 15K, negative end of the second cap is 0V.

[martin manning]

I guess I should have said the 220k's near the bass pot... Anyway there is the under-board jumper from the second bias filter to that junction. Is that the one you checked?

[Guitarnut]

Well, you're not gonna believe this...or maybe you will. I didn't wire that jumper. I was so careful when putting this kit together but I guess even with the best of plans, things can get overlooked.

I gots me a jumper to solder.

Thanks for all your help so far. I'm learning a lot!

Peace,
Mark

[martin manning]

Hmm... That doesn't quite make sense. With no jumper the negative end of the second cap should be close to the voltage at the negative end of the first, not zero. If the jumper isn't there, then you'll have to sneak it in.

Check the resistance to ground from the junction of the 15k and the second bias filter.

[Guitarnut]

Hmm... That doesn't quite make sense. With no jumper the negative end of the second cap should be close to the voltage at the negative end of the first, not zero. If the jumper isn't there, then you'll have to sneak it in.

Check the resistance to ground from the junction of the 15k and the second bias filter.

The R at the second filter and 15K is 54K to ground.

Here's something strange. The red/blue bias tap is showing 50VAC. The anode of the diode is showing -46VDC. The junction of the second filter and 15K is showing the same as the tap...50VAC! All of those turrets are so close together, I may have a short is there somewhere.

And so it goes. One step forward, two steps back.

Peace,
Mark

[Firestorm]

The turret where your red-blue bias tap is wired seems much too close to the turret with the second bias cap negative. There must be absolutely no connection between them. I think you were better off with the original eyelet board...