I recently fitted the power stage of my Fender Hot Rod Deluxe with a pair of Ruby 6L6GCMSTR tubes. I gave a full review of the Ruby tubes in an earlier blog post. What I want to discuss here is the bias setting for the power stage in a Fender Hot Rod Deluxe.
I did a lot of research before deciding to replace the Hot Rod’s factory-fitted Groove Tubes GT-6L6 C(HP) power tubes with a pair of Ruby 6L6s and learned much about 6L6 variants and their different power requirements along the way. It’s all a bit of a mine field. But thankfully help is available in the form of an old article by Justin Holton at The Unofficial Fender Hot Rod Deluxe Owner’s Guide.
When you replace the power tubes on a Fender Hot Rod you should always have the bias setting adjusted to ensure that your tubes are operating within safe limits and you’re getting the kind of tone that you want from your amp. Unless you know what you are doing, never attempt to adjust the bias on your amp yourself. Without a good working knowledge of electronics you could blow your amp, or your tubes, and possibly kill yourself in the process.
When you adjust the bias on your amp you are adjusting the level of current available to the power tubes. Setting the bias on your amp is like setting the idle on your car. The higher you set your bias, the hotter your tubes will run and the earlier your sound will break up. They will also burn out sooner. This may be just what you want. But never set the bias current so high as to exceed the plate dissipation limit set by the tube manufacturer at any portion of the tubes operation. Beware that different types of 6L6 tubes have different operating level ratings and recommendations. The lower you set your bias, the colder your tubes will run and the longer they will last. You’ll get more headroom and a cleaner tone. But set the bias too low and your tubes could sound flat and sterile. You don’t want your tubes running at one extreme or the other. So long as your bias is set within the safe operating limits defined by the manufacturer of your tubes, there is no single “correct” bias point. It’s all a matter of taste.
Trawl the web and you’ll find that recommendations for setting the bias level of 6L6GC tubes vary greatly. This isn’t a problem. You can run your bias at what ever millivolt (mV) level you want, so long as you set it within the safe operating limits of your tubes. The problem is that many sources cite Fender as recommending this, that or the other particular bias level. But take a close look at the schematics from the old Hot Rod Deluxe or the newer version III and you’ll see, in very small writing which you may need to magnify a few hundred percent, the following:
Set bias adjust pot R82 to obtain a reading of +60mV at TP30
On the electronics schematic, R82 is the bias adjustment wheel and TP30 is the location of the test point for your multimeter.
Fender has never recommended any setting other than 60mV. Now, if you know anything about amps and tubes, you’ll know that 60mV is a little on the cool side. Fender recommend 60mv to ensure your safety and tube longevity. But Hot Rods tend to leave the factory with their bias set somewhere between 60mV-70mV.
I run my Ruby 6L6GCMSTR power tubes at 68.3mV. I find that this setting offers maximum headroom, warmth, shimmer and chime on the clean channel. I’m not too interested in driving the tubes for early breakup. I tend to use my Suhr Riot Pedal for crunch and higher levels of distortion. 68mV works as the sweet spot for 6L6GCs and similar power tubes, but is unlikely to work so well for other types. And it may be way too hot for some power tube varieties.
Fat Sound 
I personally set my Ruby 6L6GCMSTR power tubes at a rather reasonable 65.7mV – I find that for me it hold the tone at the correct level. Great article.
Try around 68mV. You’ll find that it’s a sweet spot. Get too close too 60mV and your tone is likely to become flabby.
I just got my Fender Hot Rod Deluxe III. Went to bias the amp, just to see where it was at and the reading was all the way down to around 48. Is this a sign that the power tubes are busted? If that is where it began, will pushing up to 60/70 be too high?
I’ve never heard of anyone running a Hot Rod Deluxe that low – so long as it is definitely fitted with 6L6 type tubes, and not 6V6 tubes. If it’s new, they should be 6L6. Before tinkering with it, I’d recommend contacting the dealer you bought it from. If it’s that low, they should adjust it for you. But if you bought it 2nd hand, someone may have fitted 6V6 tubes – which need to be run lower. Biasing an amp is a dangerous activity. If in any doubt about how to do it, or if you are unsure about the type of tubes fitted, seek professional advice.
Definitely 6L6 (stock), the amp is brand new, and I just tested how high it would go and the bias wheel stopped at 71.
I hope your multimeter is measuring correctly. If it is, setting the bias anywhere between 60 and 70 is safe and will prolong the life of your tubes. My preference is for 68, which gives great tone and the tubes won’t be driven too hard. It’s interesting that your bias wheel won’t go any higher than 71. My personal preference would lead me to keep under 70 anyway. However, I recommend feeding back to your dealer. You never know, Fender may have changes the spec so that users can’t drive their tubes too hard. But I doubt it. The bias range has been the same for years. If they haven’t changed the spec, and you want flexibility in your bias range, I’d ask your dealer to swap the amp. When I bought mine, it had a set of power tubes that rattled, so I took it back to the dealer the same day and they swapped the amp for me. Your amp should have been supplied with a wiring schematic and manual. Check the schematic. As my blog article points out, you can see the recommended bias setting in the schematic. If it is still 60 on the schematic then you should be able to turn the bias up to anywhere around 95 and beyond. I’d be interested to hear what your dealer says.
Well, unfortunately, I’m on my own with this cause 30 days have passed since purchase. It was only when I wanted more headroom that I checked the Bias/Tubes. The schematics are as you said in your article, 60. It could be a faulty multimeter so, I’ll take it to a trusted tech, see what he makes of all this. I’ll post updates in about a week or so. Cheers for the help.
Keep me posted. I’d like to know what the issue is.
My HRD III, just out of the box, has 68 mv of measured bias
That is outstanding. The sweet spot. Enjoy.
It is possible when retubing an amplifier to find the bias reading that you desire is either too far to the left or too far to the right of the bias pot.
In other words, you max-out before you hit your desired number.
The bias on the amplifier is set at the factory with whatever tubes are in it on the date it was born. Tube values can vary greatly from one amp to the next. So the bias point (on the bias wheel) from one amp to the next will vary quite a bit to arrive at the same reading, say 60mv.
If the amp was born with a pair of “hotter” tubes, the setting on the wheel will be far to the left. If the amp was born with a pair of “colder” tubes, the setting on the wheel will be further to the right to achieve the same 60 mV. I owned a Bogner Shiva that was “born” with a pair of tubes so hot, that when the first set of replacement tubes went in, my bias wheel maxed out before I could even hit the number that I wanted. After swapping in several sets of tubes to confirm this, I called Bogner. The tech had me solder in a resistor of a predetermined value over the bias pot. This ensured that when I grabbed a set of tubes that were rated “in the middle” somewhere,
The bias wheel wood then provide me with enough sweep to the left and to the right to dial-in the sweet spot. In conclusion, if you run out of bias pot travel before you hit your desired value, try a different set of tubes. If you get basically the same results, you may have one of the odd amplifiers that was born with a set of tubes at one extreme or the other. It is rare, but it happens. The only remedy being, to make sure you order a specific value on your tubes, either very hot, or very cold, or you will have to end up modding the amp like I did. I repeat, I believe this to be a rare condition.
Hi dennis here. Have a hot rod deluxe with same problem u had on biassing. Came set at 33 an now has rubby 6l6 tubes. Will not go past 62 on bias. I need 68. Wut was the rissister value u ussed to get urs to set. At least it will give me a starting point. Thank u.
60mV for the hot rod deluxe is too low in my book… If you have a real strong 30W plate rated 6L6GC that bias should be around 28.8mA plate dissipation pr tube.. (I am assuming the screen are drawing 4% of the current which is a fairly low estimation) . THe plate voltage with this cold bias should be around 430V according to the schematic. ( i would recommend you to measure the plate voltage on the output tubes to be sure) You will need this number to calculate total dissipation..
Ok back to the math..
28.8mA x 430V = 12.4W idle plate dissipation or 41.3% of max plate dissipation for a strong GE 6L6GC tube.. You should be above 50% at least.. If you have 5881 tubes in the hot rod deluxe you need to use 23W as the max plate dissipation as the 5881 is a lower output tube with smaller plates.
You can add the JJ 6V6 tubes in the amp as they take the high voltages in the amp.. But use a value of 14-15W max plate dissipation pr tube when you adjust the bias… The JJ6V6 will need more negative grid bias to be biased correctly in the hot rod deluxe, especially as the primary impedance is 4.2K in the hot rod deluxe which is a little too low. The JJ6V6 would probably prefer around 7K with those voltages…
Checked my hrd iiic (2012) bias the other day and found it was set at 68mv from the factory….the “sweet spot” from all I have read.
Also decided to swap out the pi to an at7 after much reading, and have been very pleased with the results. The od channels are now useable….even though I still use pedals for dirt. I’m planning to do a accurate tone comparison of the at7 vs ax7 in the pi position using reamping, which is really the only way to properly a/b two sounds quick enough to really compare.
It’s very interesting and
thumbs up for all of of you.
I have a good supply of 6L6GC tubes (JJ, GE, RCA, Sylvania, Sovtek and Jan Phillips) but the Ruby Tubes 6L6GCMSTR has the best tone in my Fender Hot Rod Deluxe and this was without change to the factory bias setting. I can only assume the bias setting is in a range that works for this tube. I have changed the 12AX7 tubes in V1 and V2 with a 12AY7 to reduce gain. I have also changed V3 to a 12AT7. The change in the preamp tubes makes the amp sound like an old fender tweed. The volume is greatly reduced but loud enough for studio and live use. The drive channel now goes from clean to drive and the spring reverb no longer sounds brittle. I changed the harsh Jensen C12N speaker for a Jensen Mod 35-12 which adds a little speaker breakup when driven. I bought this amp years ago because I liked the idea of an amp that was fairly light and had every feature I liked but to me was too loud, difficult to control and the reverb was like an ice pick so this amp was useless to me. Now this amp with these changes is a pleasure to use.The question remains to be seen if I have created something unstable or short lived. Thanks to you for a lot of details of tube biasing.
Hey, Gordon. That is some mod. Thanks so much for the detail. I have always felt the Hot Rod would benefit from less volume so that I could get that saturated valve sound without making my ears bleed. If I can ever find the time, I’ll try your modifications.
Hi Joe, I have a HRD III that’s about a year old the amp works fine but sounds a little brittle. So I decided to see where the bias was set. It is at 43 mv at tp 30 if I try to get to the sweet spot 68mv the amp starts to hum around 53 mv ….It has the stock 6L6 GC HP any ideas…
Hi Terry. 43mv is very low. Did you get the amp new or secondhand? I presume it is humming at low volume. If it starts to hum at 53mv I would recommend getting it checked out by the dealer you got it from, if new. It could be any number of issues and I wouldn’t recommend poking around unless you really know what you’re doing. If you have a spare set of valves to hand, my first instinct would be to swap them to see if that makes a difference.
Hi Joe, thank you for the quick reply. The amp was bought new over a year ago, I do believe the warranty is expired but I will check with Fender on this. The amp works and sounds fine ,never had a problem with it, but I have been having issues with my strat just sounding a little too thin… I have installed other pickups that helped a little but just not what I was looking for. I ran across several articles about biasing tubes and swapping out the (V1 )12ax7 with a 12at7 to help with the volume. So I thought I would have a look at the bias, as some articles suggested this may help with the ice pick highs. That is when I discovered how low it was. The tubes are 6L6GC(HP) when I slowly increase the voltage from the 43mv and I start getting around 51 or 53 mv it starts to hum this is with the amp volume all the way down. I will swap out the tubes (valves) later when I get home from work and see if that helps. Any other ideas are most welcome. Thanks Terry
Hi Joe, I have an older Hot Rod Deville 410 I bought used with Sovtek 6L6’s in it. When Biasing the amp the wheel is maxed clockwise at 50.8 mV. Before biasing the wheel was about 12:00 which was about 30mV. Is it time to buy new tubes or would this amp require a mod? Being as the setting is so low running the bias wheel wide open shouldn’t hurt the amp if it’s only reaching 50 mV would it?
Check this article http://www.eurotubes.com/store/pc/hotrod%20deluxe%20how%20to%20bias%20video.htm as far as I know the bias on a Deville runs lower. Apologies for the late response, I’ve been bonkers busy at work.
great article, thank you joe.
i have a hot rod deluxe (i think version 1, built 2004), tweed, limited series (stock speaker is a jensen c12n).
the guy from eurotubes in the video you’ve linked recommends “anywhere between 85 and 90” mV on the meter at tp30 without referring to a specific set of power tubes.
the great margin between your recommendation of 68 mV and his 85-90 mV irritates me.
i’d really appreciate it if you could shed some light on this.
cheers
i didn’t read the text below the video which frankly explains it all. 🙂
I think that many of you are actually missing the point here….
Only one other person seems to have it correct. Anders Barfod!
First let me say that the number one thing you are trying to achieve with checking and adjusting bias is that you are making sure that your valves are dissipating the the proper amount of power. POWER IS MEASURED IN WATTAGE! We dont want the tube to dissipate more power than what it has been designed for, so we look to the TUBES manufactered specifications to find out what is its normal designed maximum dissipation. There exists a general rule of thumb that in a PP amp we dont want to exceed 70% of that maximum dissipation. For that tube to work or for it to dissipate power it requires voltage and current. You cant simply measure the idle current and know at what wattage your tube is dissipating! Why because the tube isnt dissipating current, in fact that current wont be there unless you also have the voltage, after all, current is measuring how fast the flow of voltage is entering the tube and changing forms in the way of power dissipation. You can have a very large amount of volts and it wont require as much current to replenish what energy is being used up or changing forms or it might be a smaller voltage that would require a greater replenishment and would demand a greater flow or current to dissipate the same amount of power. So before you simply measure the current to find out how fast the voltage is being consumed, shouldnt you first know how much voltage the current actually has to move first? You can make wattage by either having a large voltage and less current or you can have lower voltage and more current. If your voltage is high then you might require less than a certain x amount of mA or if your voltage is lower you may require more current to make the same power. You should always check plate voltage first before relying on some arbitrary number given for the current. Ok maybe not totally arbitrary, Fender knows what the target design voltage is for the plates of the amp, however its not always the actual case because there are too many variable that can affect what voltage the plates are really being subject too!
YOU SHOULD BE MEASURING FOR THE MAXIMUM POWER DISSIPATION OF YOUR TUBE @ IDLE CURRENT. REMEMBER POWER IS MEASURED IN WATTS….
To further confuse you is the fact that the factory test point is not really even measuring current but is measuring the voltage drop of a one ohm resistor between the cathode of the tube and ground. One ohm resistor is used because it is very easily divisible unit and if we know the resistance and we know the voltage measured we can arrive at the amount of current Now we have the current we can measure the voltage of the plate and divide that to arrive at the wattage or power being dissipated by the tubes plate. However another small fly in the ointment is that the voltage drop measured at the one ohm resistor to ground off of the tubes cathode isnt just seeing the dissipation or consumption of voltage from the resistor and the plates but also the grids of the tube too… but the amount is negligible enough and also fudges the accuracy in our favor of keeping those tubes happy and at less dissapation.
So measure voltage first at the plate and then measure your voltage drop at the testpoint convert the voltage at the test point to current by dividing the voltage drop by the resistors value take that current in mA and divide it by plate voltage and arrive at your actual idle tube dissipation, compare this to the tubes published specifications . Is it below 70% of tubes max published specification?
Much more difficulty having to explain this than to actually do this!
Justin Holton at The Unofficial Fender Hot Rod Deluxe Owner’s Guide. link is down
http://www.justinholton.com/hotrod/bias.html
Any way to obtain the lost information?
Looks like Justin has built a new site at http://hotroddiy.com . I have updated the link in my blog post.