DIY 1624 from Hammond M3 Parts

[Smokebreak]

Should that 10k/10 watt resistor be that hot? I'm noticing a little light smoke rising from it. Looks like it's turning black in the middle.

Thanks.

Yep, like Martin is saying it is supposed to get that hot. It's trying to dissipate 18.5W in the form as heat. Your resistor is only rated for 10W so it's toast.

Previously I questioned its necessity in the circuit, but stated its probably gonna dissipate about 20W at power down, as it's dropping the full B+ acroos it. Best make it a 25W , and give it plenty of breathing room. It shouldnt be close to anything.

Martin, is that 10K even necessary? I was approximating the original series string and V divider.

Nate, without tubes , your B+ readings will be high at every checkpoint. Did you check and see if you had negative voltage before the field coil? Also, sounds like a bad tube. Can you take them back to the guy that sold them to you?

[dawsonaudio]

I got a -92 voltage reading before the field coil, but turned off the amp because one of the power tubes is starting to show a slight orange glow in the middle and the bottom is turning a little blue. Since the other tube isn't doing the same thing, that's why I turned it off.

In that time I also measure 320volts off the first 20uf b+.

I also did a prior test with just the heaters attached to the tubes. The three preamp tubes glowed as they should, barely visible. One of the 6973's filament is quickly lit and twice as bright as the other. So it looks like both tubes are lighting up, just one of them is acting different when b+ is added in.

I did have a guitar plugged in as well and strummed it and got sound, but that was it...all this happened in about 20-30 seconds before I powered it down.

I'm going to go through the amp again and take a 5th and 6th look around.

[Smokebreak]

I got a -92 voltage reading before the field coil, but turned off the amp because one of the power tubes is starting to show a slight orange glow in the middle and the bottom is turning a little blue. Since the other tube isn't doing the same thing, that's why I turned it off.

In that time I also measure 320volts off the first 20uf b+.

I also did a prior test with just the heaters attached to the tubes. The three preamp tubes glowed as they should, barely visible. One of the 6973's filament is quickly lit and twice as bright as the other. So it looks like both tubes are lighting up, just one of them is acting different when b+ is added in.

I did have a guitar plugged in as well and strummed it and got sound, but that was it...all this happened in about 20-30 seconds before I powered it down.

I'm going to go through the amp again and take a 5th and 6th look around.

Ok this all sounds pretty good, actually, minus what sounds like the bum tube.

-92V : good
320V under some load : good
Slight orange and blue glow : normal

So you're saying the tube that didn't glow did just that...nothing?

[Smokebreak]

Also, to answer your other question concerning checking to see if the tube is bad, this is where I use my homemade tube tester...which consists of sticking a known working tube in its place
But seriously, if you can measure the voltage drop accross the power tubes' cathode resistor, you can figure out the current draw, and dissipation, and this could be tell-tale.

[dawsonaudio]

Yep, well other than the filament/heater lighting 1/2 as much as the other. No orange glow or blue showing like the other 6973.

[martin manning]

I don't think the 10k resistor is necessary, but it will reduce the voltage a bit with it's added current draw. At 320V it's dissipating 10.25W.

Up above the voltage on the reservoir is reported at 420V with just the rectifier. A 100V drop seems like a lot.

[Smokebreak]

I was assuming the field coil wasn't dropping the ~100V without the tubes drawing current?

[martin manning]

The field coil DCR is close to half of the voltage drop, I think, and it is in the circuit all the time, so I'd expect a pretty big voltage drop either way

[dawsonaudio]

I did check the 6973's again and noticed a little browning color on the bottom of the tube that isn't working. The other 6973 is clear through the glass. Not sure if that is a tell tell sign of something that could be wrong with the tube. These were both clear as I recall when I bought them.

Thanks,
Nate

[dawsonaudio]

So I'm going to make the switch to 6v6's after all. I returned the 6973's to the business where I bought them. That was interesting, but I got a refund. I'll try the 6973 search for my next amp or my next one.

[dawsonaudio]

So I just put in my 6v6's last night and it works, except the tremolo circuit. I have a couple Tung Sol 6v6's and the voltage readings are as follows. Pin 3 (plate) is at 358vdc, pin 4 (screen) is at 349vdc and pin 8 (cathode) is at 22vdc. Do these voltages seem ok?

How do I go about checking the amp draw to make sure the tubes don't red plate? This is a first for me.

Also, I did take out the 10k/10 watt resistor so the voltage is a little higher than previously.

Thanks.

[dawsonaudio]

So I searched around and did some math and I think I came up with a total of 29.5 watts...so that would mean each tube is at 14.8 watts. I think the tubes are rated at 12 watts max, is that correct?

[dawsonaudio]

So would increasing the cathode bias resistor from 250 ohm to 300-320 ohm lower the plate dissipation?

[martin manning]

In your dissipation calc you are using cathode current, so the plate dissipation is a little less than 14.8W. If they don't red-plate you are ok, but you could increase the cathode resistor a bit to prolong their life. Maybe shoot for 12W using cathode current.

[dawsonaudio]

One of the tubes has a slight red plate appearance to it. I ran the amp for several minutes like this idle. I'll place a larger cathode resistor in there.

Is there a more precise method of calculating plate dissipation vs. using cathode current?