Fender Deluxe Hot Rod amp

[carlosraj7]

Hi

I am new to this forum and I badly need some help. I recently bought a used Fender Deluxe Hot Rod amplifier from a secondhand store. The amp is working but I decided to check and set the bias correctly for this amp. Downloaded the service manual from the net. According to the manual, the bias setting at TP30 should be +60mv. As instructed, without any signal applied, I measured the voltage at this point. My DC multimeter shows a reading of approx 30.5mv. I tuned the variable pot R82 to get a reading of +60mv, but the highest reading I can get is approx 46mv at the pot fully clockwise. I do not understand why I am unable to tuned to 60mv. Is it because the mains voltage shown on the manual is for 120V AC, but locally we use 240V AC or is it due to some component failure or problem with the tubes. Please advice.

Thanks

Carlos

[stratcat55]

I would look for cold solder joints on the power tubes. My HRDlx had the same symptoms. Turn it on and look and see if both power tubes are glowing. If only one is, try swapping them. If the same socket still isn't lit. That could be your problem. It sounds like you're only reading the voltage from one of them. My HRDlx was a reliability nightmare so I got rid of it. It was from around 2005. Juston Holton's Unofficial Hot Rod Deluxe manual is still floating around the Internet and describes all sorts of problems and fixes for them.

[NTBluesGuitar]

Okay, here's the skinny...the cathode current in mA (mV) number means NOTHING without the corresponding plate voltages. Take your bias test point current (voltage across the 1ohm resistor) and at the same time, take the plate voltage (power tube pins 3).

What you're measuring is the cathode current FOR THAT PARTICULAR TUBE. There are a LOT of factors that affect that.

So, here's the math:

Plate Voltage in V x (Cathode Current x.001)

So, let's say plate voltage is 400V and your max cathode current is 46mA (converted from 46mV):
400 x .040 = 16

That's the watts being dissipated. 6L6GC tubes are rated at 23W Max, so take the watts dissipated and divide by 23W to get the percentage of MAXIMUM to get what percentage they're dissipating. So:
16 / 23 = .695 which is 69.5% of maximum plate dissipation which is actually in a good spot.

Again, it all depends on plate voltage.

The schematics are assuming a specific brand of power tubes, so it depends on the health of those tubes or brand of those tubes.

[NTBluesGuitar]

But I forgot the most important part - your measurement is for BOTH tubes' cathode current, so, following my previous example:

400V plate voltage
40mV (mA) at test point is really 20 mV per tube

So:

400v x .020 = 8
8 / 23 = 34.7% which is EXTREMELY cold.

All of this is like drinking from a fire hose, so just get the two measurements: Pin(s) 3 and mA (mV) and we'll go from there

[stratcat55]

But I forgot the most important part - your measurement is for BOTH tubes' cathode current, so, following my previous example:

400V plate voltage
40mV (mA) at test point is really 20 mV per tube

So:

400v x .020 = 8
8 / 23 = 34.7% which is EXTREMELY cold.

All of this is like drinking from a fire hose, so just get the two measurements: Pin(s) 3 and mA (mV) and we'll go from there

Or send the amp to you for one of those beautiful builds you do on them.
This was what exactly happened to mine. I couldn't get my bias voltages to spec. I had nothing on one of the sockets so I was running on one 6L6. one tube wasn't even glowing at all.
Sounded horrible too. I had all the stuff happen to mine like the random channel switching, unstable operation, weird bias problems. I finally sold it. I did disclose all the issues with it and transferred the warranty. TJ I wish I knew what you were doing with them back then.

[NTBluesGuitar]

Ha! I haven't done one for a few years now.

I vowed to get my last stalled projects done before I take them up again.

[Offshore Angler]

Being the obsessive compulsive engineer, it appears you lost 6 mV in your calculations?

[NTBluesGuitar]

Perhaps...I was making up the numbers as I typed.

Just wanted to get the formula out there.