Help! Tube Bias, Plate Current, Plate Voltage, Wattage>>>

[Resonant Alien]

OK, after thinking that biasing the Renegade was easy based on what the manual says, I have researched a little more about biasing in general, and now I'm confused. Hopefully you guys can help clear this up. (Sorry for the long post)

So, the Renegade manual says that KT66, 6L6 and 5881 type tubes should be biased such that the meter readings at the Renegade bias points read between 60-70mV (which would translate to mA in the tube due to the 1ohm resistor on the bias points of the Renegade). The Renegade bias points are measuring both tubes, so this means that each tube would be biased somewhere between 30-35mV, if I followed this rule of thumb.

What I have read elsewhere is that the 100% maximum current limit of a tube is found by dividing the tube's power dissipation in Watts by the amp's plate voltage. Bruce posted in another thread that the plate voltage for the Renegade is around 460VDC. Most sites also seem to recommend running the tubes at between 70-80% of the max wattage to find the sweet spot, which would translate into 70-80% of the max current, as calculated above. Since the Renegade's bias is done on a mV basis, this should be my target value.

I am currently using a pair of JJ KT66s and a pair of Tung Sol 5881s. The KT66s are rated 25W. 25W/460VDC=54.35mA. So, I am assuming that the maximum plate current a single KT66 can handle is 54.35mA. If I follow a 70-80% rule of thumb, that means I should bias each tube at 38-43mA, or when taking the reading at the Renegade bias point across both tubes, I should be shooting for a reading of 76-87mV.

Likewise, the TS 5881s are rated 23W, which divided by 460V would give a max current rating of 50mA. 70-80% of this would give 35-40mA per tube, or 70-80mV when read at the Renegade bias point across both tubes.

I also have a set of TAD 6L6WGCs and TAD 6L6GC-STRs which are rated 30W. Applying the same calculation gives a max of 65mA per tube, and taking 70-80% of the max would give 45-52mA per tube, or 91-104mV at the Renegade bias points.

All of these values are significantly higher than the recommended settings in the Renegade manual (60-70mV). These suggested settings would correlate to 55-64% of the JJ KT66’s max, 60-70% of the TS 5881s max, and only 46-54% of the TAD 6L6’s max.

Adding to my confusion, but seeming to back up the suggested settings in the manual are the rated plate currents stamped on the tubes. (or at least what I think that number is). The KT66s have “30” written on the box – I assumed this meant that they should be biased to 30mA each, which is in line with the manual’s suggestions. The TADs are marked with a PC=32 and PC=33 rating, respectively. Again, if these are meant to be suggested bias settings, then they line up with the Renegade manual. I can’t find a PC rating on the TS 5881s.

So, what’s the deal? Is this “70-80%” rule of thumb setting I keep seeing on various websites not a good target? Am I doing the calculation wrong? Or should the Renegade just be biased to these lower levels (around 50%) of the tube’s max.

Thanks for any insight!
Rick

 

[Resonant Alien]

Edit - corrected to reference mV in context of Renegade bias settings, since the reading at the Renegades bias points are taken in mV, not mA.

 

[hunter]

at 460V and 70%:

KT66: 38mA
6L6/5881: 29mA

You can verify Plate voltage on the 600V DC setting of your multi meter measuring pins 3 and 8 without tubes in the amp.

 

[Resonant Alien]

at 460V and 70%:

KT66: 38mA
6L6/5881: 29mA

You can verify Plate voltage on the 600V DC setting of your multi meter measuring pins 3 and 8 without tubes in the amp.

38mA for KT66 matches my calculations at 460V and 70% (assuming 25W power dissipation).

How did you calculate 29mA for 6L6 and 5881? My TS 5881s are rated 23W, which at 460V and 70% should give 35mA. My TAD 6L6s are rated 30W, which at 460V and 70% should give 45mA.

Thanks,
Rick

 

[hunter]

at 460V and 70%:

KT66: 38mA
6L6/5881: 29mA

You can verify Plate voltage on the 600V DC setting of your multi meter measuring pins 3 and 8 without tubes in the amp.

38mA for KT66 matches my calculations at 460V and 70% (assuming 25W power dissipation).

How did you calculate 29mA for 6L6 and 5881? My TS 5881s are rated 23W, which at 460V and 70% should give 35mA. My TAD 6L6s are rated 30W, which at 460V and 70% should give 45mA.

Thanks,
Rick

You're right, 6L6GCs are 30W, I was looking for 6L6, my wrong

 

[Resonant Alien]

Bump. Would really like to understand the low down. Thanks!

 

[jkdsteve]

Bump. Would really like to understand the low down. Thanks!

Not 100% sure what there is left to understand.

I've thought the same as you with tube PC readings but it'd be hard to go by any tube sellers recommended numbers without knowing the plate voltage they tested 'em and what % they are recommending right?

For biasing I've seen recommended values in the range of 60-80%..I've never gone higher than that intentionally...there's no saying it produce a tone you are happy with at 50%....you already have the guideline you need...

Power = Current * Voltage

Bias Current = %output * Rated Power / Plate Voltage

 

[Resonant Alien]

Bump. Would really like to understand the low down. Thanks!

Not 100% sure what there is left to understand.

I've thought the same as you with tube PC readings but it'd be hard to go by any tube sellers recommended numbers without knowing the plate voltage they tested 'em and what % they are recommending right?

For biasing I've seen recommended values in the range of 60-80%..I've never gone higher than that intentionally...there's no saying it produce a tone you are happy with at 50%....you already have the guideline you need...

Power = Current * Voltage

Bias Current = %output * Rated Power / Plate Voltage

I guess I'm just concerned mostly about the 6L6s. I'm an Electrical Engineer by trade, so I understand the theoretical calculations, but when I calculate out the 30W 6L6s, that tells me that 70% is 45mA per tube, or 90mV at the Renegade bias points. That's just so much higher than the 60-70mV setting that the manual recommends that it just had me concerned that I was missing something, or that it wasn't as straightforward as it seems.

 

[natfan]

Hi all, new guy here,
I realize this is old thread but it is a great question.
Now I am wondering if I am running my old GE 6550's in my Renegade to cold.
At 42 watt pd the formula comes out to about 63ma per tube. (120ma total)?
What is the low down?
Love my Renegade

 

[C1-ocaster]

Hi all, new guy here,
I realize this is old thread but it is a great question.
Now I am wondering if I am running my old GE 6550's in my Renegade to cold.
At 42 watt pd the formula comes out to about 63ma per tube. (120ma total)?
What is the low down?
Love my Renegade

For my Vengeance head I am using Egnaters recommended mV for the 6L6 tubes I am using.
The bias test points and adjustment are external on my amp so I can actually play the amp while the meter is hooked up and make adjustments as I play.
After doing the initial bias I left the meter hooked up and played the amp, while playing I tweaked the bias hotter and colder to hear what settings I liked best.
To my ears I like the lower end of Egnaters recommended range for 6L6's. I even went really hot a good bit above the recommended range just to hear what ti would sound like, but it wasn't better. I also went a good bit colder and it too wasn't as good.
I find the recommended range to be right in the tonal sweet spot.

I don't have an explanation of the electronic formula numbers to give you the low down.
But I can tell you that the recommended range does sound best.
I find that when it comes to tubes and tweaking, it's best to use the numbers to get you in the range and then use your ears to make the final setting.
Different tube types and even same tube type among different brands sound different, and there isn't even one bias setting that works for every 6L6, KT88, EL34, etc...

For me that's part of the appeal, it's the "magic" of those electrons doing their dance in the vacuum of the tube.
It's sonically and electronically cooler over solid state amps.

 

[catfish777]

Having played tube amps for over 30 years(maybe longer) this is what I know. With newer tube amps(mass produced/foreign manufacture) you had better bias them as cold as you can get away with or you will soon be dealing with transformer and screen grid resistor etc...problems.Good Luck and be careful!

 

[jamme61]

Having played tube amps for over 30 years(maybe longer) this is what I know. With newer tube amps(mass produced/foreign manufacture) you had better bias them as cold as you can get away with or you will soon be dealing with transformer and screen grid resistor etc...problems.Good Luck and be careful!

plus 100%

 

[jamme61]

read this Bruce explains

bruce egnater
Post subject: Re: Renegade bias GE 6550s Help BrPostPosted: Sat, Nov 16, 2013 11:02am

Yes, it can be confusing. Not because of the math but because you get different information from everyone. The issue is that tubes in general are "all over the place" and the specs for modern tubes cannot always be trusted. There....more useless information. 6550 tubes specs historically range from 35watts (SED6550C 1998 and JJ) to old GE specs at 42 watts. FYI, running the tubes at lower idle current, as I often do to extend tube life, is in no way harmful to the amp or the tubes. This hopefully makes things more reliable by causing less stress on everything. The "hotter" your tubes idle, the more stress you put on the tubes themselves, shortening their life, and also the power transformer etc. Notice most of the manufacturers tube specs rate their tubes at a lower screen grid voltage than typical guitar amps so that power rating is not exactly what the spec says. As an amp manufacturer, tube reliability is always an issue. We like to be on the conservative side in hopes of reducing tube failures. There are tradeoffs in everything. You will find the hotter you run your tubes, the more lively your amp may feel to the point where you really crank the bias current up and your amp really comes alive, while you burn everything up. While this may be true, there is a point where you will be shortening your tube life. So, from my experience, the settings we suggest are clearly on the low end with the goal of avoiding these issues. I have adjusted and listened to the amps with a range of settings and found the suggestions were a good compromise between tone and reliability. Your are welcome to try other settings though I do suggest keeping on the lower end of things if possible. Heat is the worst enemy of any electronics and the hotter you idle your tubes, the more heat is produced.