Fender Hotrod Amp Bias - Why You're Doing It Wrong

[@dannybush6131]

I have been arguing the parts % thing for years. i have come to a point of using nothing above 5% parts if available and the best caps I can get anymore at tube voltages. Thanks for the lesson on trusting the math and not a factory setting.

[@amp_mechanic]

Exactly correct. There's a reason why amps, especially hi-fi, that use 1% components are expensive. Consistency. There's also a major problem with builders not paying attention to voltage ratings on resistors. Especially many notable boutiques amp builders. It's not a problem until it is but, heating causes tolerance to be even worse. There's always a trade off, money and consistency is a battle.

[@iplayloud2]

The Hot Rod amps should have a plate voltage of around 430V (Deluxe) and 475V (Deville). Did you actually measure 505v on your plates ? 🤔

[@amp_mechanic]

Any values I give are the actual measured values. That plate voltage will be dependent on incoming wall voltage, what tubes you have and what the bias setting is. There are some other factors that can be relevant as well. Values printed on a schematic are only relevant under their specific test conditions and are often ideal theoretical numbers. ( I mean theoretical in the scientific sense, not the common parlance definition.) I'm going to do some more videos on this topic at some point. It's a good discussion to have.

[@LewisShieldsUS]

800, Twin and HRD all need retubing. So I bought a Monoprice 5 watt. Thank you for the education. Rather play high voltage than work on it.

[@tedc6694]

I'd like to hear a comparison of too cold biasing vs right vs too hot. But I think this was an important video for people to see. Thanks and good job

[@amp_mechanic]

Thanks Ted. I've thought about this but I'm just not sure how to do it yet so that it would be effective in showcasing the difference.

[@pronga510]

Nice, did the same thing on my Deville, i also noticed that my TAD 6L6s (some years old) are about 10mA apart from each other, that's a little bit much to call them still matched, (well, that's not the fault of the amp), so one was running very cold (~35%) before setting the biaspoint to about 75mV

[@amp_mechanic]

While there can be bias drift and tubes do wear differently, that can sometime be an indication of a problem with the amp. Give it a careful check over. It's not uncommon for one of the protection diodes on the plate to start to leak and cause problems. Not saying that it is, maybe that one tube just wore more. Either way, you'll likely discover that a mismatched set of power tubes can actually sound pretty good.

[@pronga510]

@@amp_mechanichmm, thanks for your tip regarding the diode, i just can say that it's super reliable and didn't have any problems (since more than 4 years when i changed some filter caps), and even after the hoter biasing now. I think the sound is awesome, so i guess nothing to worry about (: the reason i did check the biasing after all was because i swapped one speaker, so i thought it doesn't hurt to check on that stuff too.

[@geoffbarber2860]

Unfortunately using the on-boards cathode resistor to set bias can be misleading for the following reasons; Firstly you can't rely on the 1ohm resistor being the correct value. Secondly what your actually measuring across this 1 ohm resistor is anode current plus screen current. The screen current can account for around 5mA of the total current giving you a false reading. Measuring the DC resistance of each primary ( as the author suggests) will give you a far more accurate reading as your only reading anode current at this point.

[@amp_mechanic]

Correct. I think so many companies do it this way because of ease. They also don't want to encourage people to probe the high voltage. And it also gives a bit of buffer for those that always want their bias hotter. My thing is that, either way you do it, you do need the plate voltage to calculate the bias and get repeatable results. Biasing to 65mv means nothing unless you use it to calculate the bias. It will usually be a safe number with these amps but the feel and sound can be wildly different. And on the resistor value, I would say that the value being off isn't that big a deal since you can account for it in your calculation. Or at the very least, that will be a static value you will always be measuring through. But in terms of just going by the 60mv thing, you are correct.

[@billroberts849]

Hey Jay while your basic premise is correct that the Fender Spec Bias of 60mV is very cold but what you missed is, yes while a 6L6GC has a max plate dissipation of 30 watts which would be in the case of an amp running in Class A. The Hot Rod Deluxe is a Class AB amp and the output tube should only be biased at most to 70% of max dissipation otherwise you will burn up the tube. Its my experience with these amps that even with the bias control fully clockwise you will not reach 70% (21watts) of plate dissipation. So it is pretty much impossible to over bias these amps without modding the bias circuit.

[@amp_mechanic]

Hi Bill, if you watch the whole video I say the hot rod amps usually do well biased around 50 to 60%. I've worked on a few of these amps that would red plate the tubes, so saying they won't reach 70% or more is just not true. If someone throws a set of "hotter" tubes (marketing jargon for tubes that are not in spec) in there it's certainly possible to have them running higher than 70, I've seen it. When I say 150% I'm extending it to all amps that have a specified ma running parameter. But if anyone likes how their amp performs using the 60ma parameter or 70ma or whatever, great! Run with it.

[@millstap]

This is confusing. Is 60mv the sum of both tubes, being 30mv on each tube? I have a friend in Italy that biases his '59 Reissue Bassman the same way and I have a vintage tweed 1959 Bassman and I never heard of going 70 or 80 mv. I set the bias around 39mv. My Italian friend said his tech says it is the sum of both tubes. I hope that is correct because if I ever set my NOS Tung-Sol 5881's at 60mv, they would immediately red plate.

[@amp_mechanic]

Correct, 60mv is for both.

[@klapaucjusz1]

Ok, this is very interesting. I've been just setting the bias between 60 and 75 mA, measured on the bias test point in my HRDx. But what should I do if I want to do it correctly? Could you describe the procedure step by step? To point - at 1:15 you say you measured: - primary resistances - voltage drop across the trasformer primary - plate currents You come up with two sets of values on your notepad. Why are there two lines? Are these values measured for each tube separately? How to measure these values? How to make the bias calculations in the next step? Do you use an online calculator? And after that - how to determine the optimal bias value, or at least a safe bias range? Perhaps you can point me to an easy to understand online resource? Most of the easy resources just say to set bias 68 mA at the test point (higher than Fender's spec). I tried reading the instructions on Weber's website, but I'm afraid it's too difficult for me, considering that I have little knowledge of electronics.

[@amp_mechanic]

Look up Uncle Doug here on youtube, he has a great channel and his 2 or 3 part series on amp bias is the best explanation of the procedure that I've seen. kzbin.info/www/bejne/rWqlYYWep8SMY6c

[@giulioluzzardi7632]

Good tone, thanks for the tips!

[@PeterDad60]

Fender say's to use 60 Ma as a starting point. You can go as high as 80 Ma and even 90 Ma.according to Fender.

[@amp_mechanic]

Again, in my view, somewhat useless information. There's wide enough operating variation in these amps, not to mention tubes these days, that's not an accurate measure of how the tube is performing in the amp. If someone wants to start at 60 and then adjust till it sounds right to them, have at it. Anyone aspiring to do a proper bias job and maintain consistency, you can't rely on that. That's all I'm saying.

[@SandersAmps]

Thanks Jay - good stuff!

[@billwilliams6338]

JAYS GUITAR, on fender amps with a normal chan and vibrato chan. If you REMOVE the First preamp tube from the normal chan, the vibrator channels preamp will have a Higher output because of the Impedance/load of the normal channels first preamp tube. Any reasons why the Unused normal channel preamp tube will LOADS down the vibrato channel preamp output wattage?

[@amp_mechanic]

Because they share the same power supply node. If you measure the plate voltage, you'll notice it goes up on the other tubes when V1 is pulled.

[@billwilliams6338]

@@amp_mechanic , Yes the voltage goes up when V1 is pulled but I thought vacuum tubes had very high input impedance and the plate output impedance "shouldn't LOAD" down the other preamp tube when its shared in parallel. Like you said both preamp tubes plates are shared tied together. The DC plate voltage will raise higher when pulling removing V1 on the normal channel. This also changes the "impedance" to next following stage is use to seeing as a load? What i mean is that is changes the Tone of the preamp section now since you removed the unused preamp tube of the parallel channel. Is this true or not that it changes the tone and impedance?

[@aplusatlanta1]

Mine only went up to 54 mV all the way up on the bias is that indicating a problem with the tubes?

[@amp_mechanic]

Likely they are just worn. If they have a lot of hours, time for a new set.

[@aplusatlanta1]

@@amp_mechanic that’s what I’m hoping. Ordered a new set a little while ago. Thanks you

[@kieselguhrvonepp9737]

This is very good information. However, I think your calculations are based on Canadian volts and metric current (based on your accent), so they obviously would not apply outside of Canada 🇨🇦 Fender’s 60 mV recommendation is perfect when using American watts. Very confusing. I often have to rebias my amplifiers when crossing the border.

[@amp_mechanic]

Good catch. Keep on doing you Kiesselguhr.

[@campbellmorrison8540]

Be nice if we could actually see what was on that tablet or what ever it is. Just a piece of paper would be better.

[@backlineguy]

I have the Hot Rod Deluxe and I'm running it at about 65mV...what do you r calculations tell you as to where I SHOULD be running this amp? Thanks...interesting video

[@amp_mechanic]

Go look up Uncle Doug on youtube and search for amp bias or something to that effect. He has a great explanation on what measurements to take. You just need the dc resistance of each half of your OT primary, voltage drop across each and plate voltage. With that you can calculate the dissipation of each tube and know exactly how it lines up with what you're hearing. That way no matter what current rating tubes you get, you can have consistent performance, whatever you like to run them at. Also note, wall voltage can have a huge effect so it's no good to set it up at your shop running 124V from the wall if most places only get 118 or the worst I've seen 113 or 112. I use a variac and do everything based off consistent 120V. For ex: If I know someone is playing a lot of bars around here, I'll set it up at 117 because that's what the amp will realistically see most of the time and make sure the 120v is still in spec though it will be on the higher end.

[@backlineguy]

@@amp_mechanic thanks...

[@K_Dallas88]

You need a lead plugged into the input, dude.

[@amp_mechanic]

No, you don't. Makes no difference.

[@EstudioMantraChile]

@@amp_mechanic okay

[@MrBrentHolt]

The recommended 60mv is for a 5881WXT Sovtek tube...

[@amp_mechanic]

Still a 6L6 variant. Plate dissipation specs are a specific value depending on the sub type design. Given that Groove Tubes rebrands them and uses their garbage rating system, and they come from that factory with a wide range, you have no idea what the plate current draw spec is on your particular set is. Hence 60mv doesn't really mean much unless you measure plate voltage and do the math. If you like 60mv and it works for you, great!

[@MrBrentHolt]

@@amp_mechanic the 60 mv was specified by fender for their warranty.

[@amp_mechanic]

Sure..if you believe that, that's what you should do.

[@nedryerson988]

Hey bud, you’re putting way to much thought and effort into it. Who cares what percent the valves are running at, adjust it to taste. If they’re running a little hot and burn up a little quicker than usual, so be it, as long as you get the sound you’re after. Otherwise what’s the point? In the old days it made sense to try to get the valves in their sweet spot because it was a pain to bias them. But nowadays all you you have to do is adjust the potentiometer to your liking.

[@amp_mechanic]

Works for me....and it's a big reason I get lots of repairs in. So power to you and all those out there with a similar mindset.

[@pepuletrz]

@@amp_mechanic totally rite..

[@debutts7723]

I'd rather get the tone I'm after for a while than a tone I don't like forever.

[@newsycpl]

In the old days, the amps were all tube, not these hybrid ones with their “circus boards”!

[@blogsfred3187]

Methinks you confuse point to point vs tube.

[@craigslist6027]

why make it so complicated....you could just show what to do.....duh

[@ronaldmah6595]

what kind of crap is this