Hot Rod Deluxe Burned Up Screen Resistor

[Seedpod1]

My Hot Rod Deluxe was getting increasingly distorted on the clean channel until it was unusable after about an hour. When I checked bias, it was around 53 mV and when adjusted it maxed out at 58 mV before I could reach the recommended 60 mV. Then I noticed one tube was much hotter than the other. Checking bias with a tube pulled out, it measured 58 mV with only V4 installed, and 0.0 mV with only V5. So only one tube was working, and that's why the bias was so low. It didn't matter which tube was used, so it had to be a circuit problem.

Checking with an ohm meter found an open screen resistor, that looked like it had been very hot. 6L6s need a high voltage on the screen grid or they don't work. I replaced R61, a 1 watt 470 ohm resistor, which fixed it. When first turned on the bias was around 120 mV because now both tubes were working. It was easily reduced to 60 mV (in a 1 ohm resistor this means 60 mA idle current). But I still have no idea why it burned up, so I'm concerned it could happen again. The amp sounds really sweet now with an acoustic or electric and I'd like to keep it.

Anybody else have a problem with a Hot Rod Deluxe burning up screen resistors?

[Rich_S]

Re: Hot Rod Deluxe Burned Up Screen Resistor

Screen resistors have always been a problem in ALL guitar amps. I think it's tempting for manufacturers to cut corners here because they need to be relatively high-wattage and therefore expensive. While you're changing one the burned-out one, it's worth doing the other, and increasing both their wattage to 2x or 3x the original rating.

[Chickenwings]

Re: Hot Rod Deluxe Burned Up Screen Resistor

yep they do that a lot - especially in hot rods. ive had it happen in a few other amps where they sit on top of the big power tubes, and i had it happen in my hot rod (when i owned one). Hot rods have them mounted away from the tubes on the pcb, but they still can cook pretty easily.
Just replace them with some nice chunky 5 watt ceramic (the big white rectangular ones) ones, bend the legs so they sit further off the board than the original ones and put a dob of silicon undeneath them too. The 5 watt resistor will have no problems throwing off the heat.
The reason often that hot rods are known for this problems is not actually just the resistors themselves burning out, but the heat from the resistors melting the pcb and also causing melting of the solder join which can lead to a cold solder join when it cools again. Big chunky resistors will throw off the heat more easily, and the dob of silicon under them will give you some distance between the body of the resistor and the pcb, increasing the reliability significantly.

[Seedpod1]

Re: Hot Rod Deluxe Burned Up Screen Resistor

Thanks for the prompt reply Rich! If it's a common problem then I can substitute a higher wattage part without fear of masking another problem. I'll replace with a couple of 5 watt resistors.

Since you've obviously been down this road before, does it seem like poor tube quality to you, if one Chinese 6L6 CHP draws 37.5 mA idle current by itself, and the other 6L6 only draws 30.0 mA? They say they are a "matched" duet of tubes when you buy them, but nobody knows exactly what that means or how matched they should be. These are fairly new, but because of the burned out screen resistor, one of these tubes was pushed hard and I'm considering spending a little more to get better ones.

[Seedpod1]

Re: Hot Rod Deluxe Burned Up Screen Resistor

Thanks Gibson,

At this point I'm looking for reliability. The last few gigs I've wound up plugging direct into the PA because I hadn't found the problem yet. It's a big help to know this is a common problem.

[Chickenwings]

Re: Hot Rod Deluxe Burned Up Screen Resistor

there used to be a website dedicated to hot rods and their idiosyncracies and also offered a whole lot of repair advice and modification stuff.
It was called the "unofficial hot rod deluxe owners guide" by justin holton. It was awesome, but sadly defunct.
you can get most of the resources from it here tho:

http://www.stratopastor.org.uk/strato/amps/hrdx/hrdx.html

[gordon_39422]

Re: Hot Rod Deluxe Burned Up Screen Resistor

I had an HRDx and the same problem occurred with mine. This was back when I knew much less about amps than I do now. Fortunately I stumbled across a page named "The Unofficial Hot Rod Deluxe Website" or something to that effect. The creator was Justin Holton and he offfered general information about tube amps and a great deal of info specifically relating to the HRDx. That is the first time I had even heard of a screen resistor and he said due to cost, the use cheap parts to keep cost down. I purchased some higher quality screen resistors and changed them all. After a retube, and bias, I NEVER had problems with the amp again! I also performed the volume pot replacement from linear to audio taper. This allowed me to used the ENTIRE volume sweep, instead of 0-3... Way easier to dial in perfect volume then. They are some good gigging amps, too!

I don't have the link, but I found someone who reposted the website in articles. I just did a search for it and found the reposts...

Hope this helps restore some faith in your amp.

[gordon_39422]

Re: Hot Rod Deluxe Burned Up Screen Resistor

Sorry, I missed Gibson's post about Holton's website.

[Seedpod1]

Re: Hot Rod Deluxe Burned Up Screen Resistor

"Sorry, I missed Gibson's post about Holton's website"

Gordon and Gibson,

Thanks a whole bunch guys. Justin Holton's defunct website has about everything I would ever want to know.

I took my HRD to rehearsal tonight and it sounds awesome! The clean channel with my Strat sounds like Knopfler's guitar on Once Upon a Time in the West. My friends even commented how great it sounds. I don't think I ever really liked this amp before, probably because it was running on one tube.

I never would have imagined that half of a working push-pull amplifier would sound usable at all, but I've been running it this way for a year! My first clue was when I found a cracked 6L6, and it wasn't lighting up. So I replaced both tubes but it still sounded bad, and the hotter it got the worse it sounded. Now I realize the bias was way too high for one tube, and it didn't take much signal to exceed maximum plate dissipation. That resistor was likely burned out when I bought the amp used.

Forums like this are a big help - Thanks!