# Biasing a 5150 iii 50w?



## imnotnollynollynolly (Apr 17, 2014)

Hey guys,

I've been trying to look into how to bias a 5150 iii and haven't found much info, although apparently these amps have a bias pot on the inside. I know they ship with JJ tubes and was thinking of ordering new power tubes and a few preamp tubes from Eurotubes since the amp has acted up a few times recently. Does anyone know if changing the bias will void warranty, because from a few pictures I've seen the bias pot looks glued in place. I might just install a matched power tube pair and leave the bias where it is regardless if that's the case, and have the preamp tubes on hand just in case something funny happens.

Thanks.


----------



## Underworld (Apr 17, 2014)

The bias is adjustable on these amp. I did bias mine last month when I installed the new tubes. You will need a bias probe and a small flat screwdriver. I biased mine at 35mA which is about 60% of plate dissipation.


----------



## imnotnollynollynolly (Apr 17, 2014)

Underworld said:


> The bias is adjustable on these amp. I did bias mine last month when I installed the new tubes. You will need a bias probe and a small flat screwdriver. I biased mine at 35mA which is about 60% of plate dissipation.



Is there a test point somewhere/ can you just use a multimeter?


----------



## JustinG60 (Apr 17, 2014)

be weary of biasing your own amp if you don't have any idea of what you're doing... i know it's a generic oooh be scared statement but you really can die if you're wreckless.

first thing you should do before you replace the tubes is get your plate voltage from your old tubes...

so the formula to bias is...

tube dissipation (usually 25w) divided by plate voltage 

ex. a lot of amps run around 470 volts...

25/470 = .053mA

the .53 is the max the tubes can be biased at. that's 100% power. you should bias your amp in the 70% power range. soo...

.53 x .7 = .37mA (70% of the maximum allowable bias)

so if your amp runs at 470 plate volts your bias should be 37mA

...how do you read mA? well there are ways by reading off certain points on the circuit board but i very highly discourage that, buy a tube probe! the Eurotubes Pro One reads plate voltage and mA. they're awesome.

it's not a lot but if i lost you then you should probably pay a shop to bias the amp


----------



## imnotnollynollynolly (Apr 17, 2014)

JustinG60 said:


> be weary of biasing your own amp if you don't have any idea of what you're doing... i know it's a generic oooh be scared statement but you really can die if you're wreckless.
> 
> first thing you should do before you replace the tubes is get your plate voltage from your old tubes...
> 
> ...



I've biased my Hot Rod Deluxe before with the test point, I just had an easier time finding material online and there were more guides and schematics available, though I haven't looked all that hard yet.


----------



## JustinG60 (Apr 17, 2014)

https://sites.google.com/site/jimification/5150_iii_schematics

TP36 is the test point. it's located on the schematic between the power tubes... i haven't personally used this method to vouch for it, i use probes to bias my amps. good luck


----------



## imnotnollynollynolly (Apr 18, 2014)

JustinG60 said:


> https://sites.google.com/site/jimification/5150_iii_schematics
> 
> TP36 is the test point. it's located on the schematic between the power tubes... i haven't personally used this method to vouch for it, i use probes to bias my amps. good luck



Perfect, thanks!


----------



## wakjob (Apr 18, 2014)

I'm not trying to step on any toes here but...

First off, it's a 6L6 based amp. So... most 6L6GC's have a 30 watt max dissipation.

Second, the whole 70% thing is utter nonsense. It's completely an amp dependent issue. Some amp circuits respond well to a _slightly_ hotter bias, but most do not. Mush city.

Seeing as this is essentially a 'preamp' amp, meaning the bulk of it's tone is coming from the preamp (like 90% of all modern amps) keeping the bias as cold as possible without hitting the crossover notch will give it maximum headroom. Which is what is needed for the preamp to do its thing.

P.S. "power tube breakup" is a myth regarding modern amp design. People are confusing it with phase inverter conduction and speaker cab resonance.


----------



## imnotnollynollynolly (Apr 18, 2014)

wakjob said:


> I'm not trying to step on any toes here but...
> 
> First off, it's a 6L6 based amp. So... most 6L6GC's have a 30 watt max dissipation.
> 
> ...



For sure, the whole point of this thing is for the saturated preamp distortion. I don't want to bias it hot at all, hell I never put the gain past like 4, I don't need any more


----------



## JustinG60 (Apr 18, 2014)

wakjob said:


> I'm not trying to step on any toes here but...
> 
> First off, it's a 6L6 based amp. So... most 6L6GC's have a 30 watt max dissipation.
> 
> ...



well, some 6L6's still have 19 watt dissipation if we're trying to fill this thread with useful info... but yeh that was my mistype of 25, I was thinking of EL34's.

though i am not sure people people bias hot for power tube break up? i bias hot or cold depending on amp for compression. it is quite noticeable on my TSL100 when i bias it to the opposing ends of the spectrum but minimal on my Rockerverb (in my opinion).

anyways, thanks for the correction


----------



## wheelsdeal (Apr 18, 2014)

Not so relevent but does the EVH 5153 take other type of tubes besides 6L6??


----------

