HotelHog Fender 135W Twin Reverb Blowing Fuses

[dorrisant]

I have a Rhodes era Twin that I've converted to a Hotel Hog here on the bench. It is loaded with a quad of EL34s.

I biased it up at 30 watts per tube... 120 Hz hum is present even with the volume all the way down, (both volumes and master). I noticed that the hum got louder as I turned up the volume on ch 2, so I pulled the tube (v2) to see if it would go away with the tube out. I had previously lifted the filaments from the hum balance pot on the back panel. The two 100Ω 2W resistors that I used to replace that pot immediately shot out sparks and burned out. Also, the mains fuse blew.

Any ideas as to why this happened? The only thing I noticed as a deviation from the Fender schematic is that the 100k balance resistors across the reservoir capacitor set are 270k in the original build. This may cause a higher HT with less voltage bleeding to ground across them. Plates on the EL34s was about 507 before the fuse popped. Also, all electrolytics have been changed, all resistor values been checked. Some have been replaced either by mod or just because they were off. So all resistors are within tolerance.

I'm kinda hesitant to replace the fuse and fire it back up until I have a better understanding of what just happened. I was in the middle of adjusting the resistor hanging off of the bias pot in order to get it in the correct range. It had been sitting there warming up for a good 30 minutes just before this happened. Then all of the sudden... POW!!

Maybe I wired something wrong...

[Stevem]

You can just drop in el34s even with dropping the bias voltage down enough for them as compared to a 6L6.

El34s are a 25 watt tube.

Pins 1 and 8 must be jumped and you need over a extra 2 amps of heater current that the PT does not have!

[dorrisant]

Thanks Steve, I forgot about the filament current draw. I did all the other stuff you mentioned, but that one thing is probably it.

[Stevem]

Looks like you will be making that Beast even heavier with a added 4 amp transformer just to power the heater in 2 out of the four 34s!

How's your back? Lol!

[R.G.]

120 Hz hum is present even with the volume all the way down, (both volumes and master). I noticed that the hum got louder as I turned up the volume on ch 2, so I pulled the tube (v2) to see if it would go away with the tube out. I had previously lifted the filaments from the hum balance pot on the back panel. The two 100Ω 2W resistors that I used to replace that pot immediately but someot out sparks and burned out. Also, the mains fuse blew.

Random thoughts:
> 120Hz hum? Have you done an AC voltage measurement on the B+ line to see how big the 120Hz ripple is? For a given capacitance, the size of the ripple voltage is a direct indicator of the current load.
> It's rare, but some electro caps are bad when new.
> Did any of the series resistors in the dropping chain get hot?
> It's rare for any circuit past the PI in the B+ dropping chain to be able to pull enough current to cause an AC fuse blow. Generally, only things in the power supply, power output stage, or the AC wiring can possibly pull enough current to pop the AC fuse.
> Do you have a light bulb limiter? If so, that's a GREAT next step.
> How solid is the bias generator and any bias pot?

Any ideas as to why this happened?

Lots, but a disciplined debugging approach of measurement is more likely to fix it than guessing.

The only thing I noticed as a deviation from the Fender schematic is that the 100k balance resistors across the reservoir capacitor set are 270k in the original build. This may cause a higher HT with less voltage bleeding to ground across them.

Hmmm. Georg Ohm says that a balance resistor of 100K with 253V across it pulls 2.53mA. A 270K pulls 0.9ma. Neither of them nor the difference between them is likely to affect the power drain much. Probably not that.

I'm kinda hesitant to replace the fuse and fire it back up until I have a better understanding of what just happened. I was in the middle of adjusting the resistor hanging off of the bias pot in order to get it in the correct range. It had been sitting there warming up for a good 30 minutes just before this happened. Then all of the sudden... POW!!

I think it's time to get out the light bulb limiter.

[JD0x0]

Why are you biasing a 25W tube to 30 watts in a class A/B fixed bias amp? You're likely red plating the tubes, and the overbiasing may be causing some of that hum.


If there's over 500V on the plates, chances are the screens are also running about 500VDC, which is too hot for modern EL34s, this coupled with the bias at ~120% dissipation could've blown a power tube and taken out the fuse and maybe the heater balance resistors, as well.

[Stevem]

Good point in the last post!
When a 34 shorts out they near always seem to do it from screen to heater, and this will burn up the humdinger pot or the 100 ohm hum balance resistors before the main fuse goes south!

This is yet another reason why these 500 volt Fender’s should have a internal HT fuse added to them!

Also if it where me I would up those 470 ohm screen resistor’ s to 1K to better limit the current to those EL34 screens.

[dorrisant]

I discussed this with the owner. He wants to go to 6L6s anyway, so this should avoid a lot of issue.

I do have a bulb limiter, my variac runs through it or not depending on a switch. I generally bring amps up on the bulb limiter then switch it out of circuit when at full ac voltage and everything seems normal. My variac feeds through an ammeter which has an ac voltmeter mounted next to it. This way I can watch the mains current and voltage as I bring the variac up to 124 vac though the bulb filament.

This was how I brought this one up. It had been up to 124 vac at less than 2A for more than 10 minutes and seemed to be fine until I pulled V2, then the show started. It didn't blow the mains fuse on the amp... it blew a fast acting 10A fuse that protects the variac. First time that has happened in years. I always run amps up on a variac as a habbit. Sure does save on fuses.

I will get in a fresh set of tubes as soon as possible. I did fire it up/run it up with the power tubes out and repairs done to the virtual center tap. That seemed fine. No hum and clean signal all the way to the grids of the power tube sockets. I could put in a set of power tubes to make sure of function, but it is off the bench for the moment and tubes will be here soon enough. I will be putting in a known good test set of power tubes before I even unbox the new ones.

I'll report more later.

[pdf64]

...The only thing I noticed as a deviation from the Fender schematic is that the 100k balance resistors across the reservoir capacitor set are 270k in the original build...

Just to note that I don't think they actually are balance resistors, ie their values could be way dissimilar without affecting things, because the winding CT defines the VDC at that node.
I suppose they were included to discharge the caps.