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Power tube bias question.....tone

rob2001

Member
Messages
16,927
What are the tonal characteristics of a hot or cold bias on power tubes? Amp is a JCM 800 with EL-34's.
 

Gtowngearhead

Senior Member
Messages
840
Hot bias will break up sooner and be, well, warm. Cold bias will break up later, tubes will last longer, but may sound lifeless.
 

EastCoastRocker

Silver Supporting Member
Messages
1,499
What are the tonal characteristics of a hot or cold bias on power tubes? Amp is a JCM 800 with EL-34's.
Depends on the amp. With the Marshall DSL, a cold bias is where it's at.
For an 800 a hotter bias will drive the tubes harder, but it'll wear the tubes and the amp.Ah, the cost of tone....
 

rob2001

Member
Messages
16,927
I haven't swapped tubes enough to know the difference and I used to have a tech do it. Now my brother builds amps so he can help me learn to bias an amp.

I didn't want to go under the assumption of a hot bias being what you think would be obvious......warmer,fatter early breakup........ I guess this could be the case. I'm willing to go a bit hotter and sacrifice tube life for sweeter tones if thats the case. For my taste and how I run this 800 (2204) i'm fine with loosing clean headroom. I'll have to search the tech section for bias numbers.
 

jhuse

Member
Messages
366
Sometimes, setting the bias too "hot" will make an amp mushy and undefined.

Agreed. Some amps love being run really hot, others get too rich and turn mushy. Best thing to do is to play the amp at various bias setting and determine how hot/cold you like it. I don't think there's a wrong or a right (let your ears decide), but generally speaking, funky stuff will happen at either extreme.
 

rockon1

Platinum Supporting Member
Messages
13,509
Hotter isnt always warmer. My old Supersonic sounded harsher when biased hot around 70%. Thin when too cold. Around 60% sounded best to my ears. Bob
 

EastCoastRocker

Silver Supporting Member
Messages
1,499
Originally Posted by EastCoastRocker
Depends on the amp. With the Marshall DSL, a cold bias is where it's at.
For an 800 a hotter bias will drive the tubes harder, but it'll wear the tubes and the amp.Ah, the cost of tone....


>Guitar Dude
Its amazing what people say sometimes.

I'm curious as to which part of that you are referring to. Running the DSL bias cold,
which is suggested by Myles and everyone else in the world to tame that amp, or running the bias on an old school Marshall bit hot, which I'm sure no one has ever done before....PLEASE !
Jon has it right See: https://www.thegearpage.net/board/sho...8&postcount=36

Thank goodness that no one has ever done something insane like putting a variac
in front of an amp.......
 
Last edited:

barfoden

Member
Messages
229
There is a clip on youtube with a guy demoing a marshall jcm800 KK with a cold bias or a hot bias.. Playing an ACDC riff..
https://www.youtube.com/watch?v=dFIXOEtjMWI

Pretty big difference..
But again I am not sure of his dissipation in watts.. and I think he is using Beam tubes (KT88) in that amp so it might respond a bit different than a true pentode (EL34).

Marshall recommends a total cathode current of 45 mA pr EL34 tube in their DSL50 and DSL100. Those run at ~ 465V on the plates so even with 5-6 mA screen current the EL34 are run at 18-18.5W plate dissipation at idle.. or around 72-74% idle dissipation at idle.. That is more than the "never to exceed" 70% for a push pull grid bias class AB amp.. ???
I think the EL34 sounds warmer when biased a little hotter.. beam tubes such as 6l6GC sound warmer and fuller even with a more moderate bias..
The El34's G3 supressor grid can have a negative voltage applied to it so the tube behaves more linear and "better" but this is only seen on some old traynors and some hifi amps..
I think the the phase inverter setup with the Marshall introduces some crossover/notch distortion and I think a hot bias at 70% is necessary to avoid this..

PS.
THe JCM800 with EL34 came in two different version.. One came with a low plate voltage of ~390V and a version later on was in the high 400's (~480V).
Lets assume both amps are biased at 65% plate dissipation. The lower voltage one will be "browner" due to having more current than voltage applied to the tube (less headroom) and the preamp voltages would also be lower giving a dirtier but slightly warmer tone..
 

Heady Jam Fan

Member
Messages
9,009
It depends on the amp, your personal preference, etc. My suggestion is to try at the colder and hotter end of the typical range for those tubes (and where people seem to bias that amp), then use your ears to decide the rest.

Also, in my experience, bias depends on volume too. For example, if your running an amp flat out (cranked), you might prefer a colder bias than the same amp running at bedroom volume (in which case a hotter bias will give the amp some of the richness of being run loud). IIRC, for example, crossover distortion of an extremely low bias diminishes as the volume increases. At one point I was playing with the idea of making minor adjustments to bias depending on the setting (volume), but I started using a Badcat Unleash and I just bias hotter, keep my amp on the edge of breakup and using the Unleash to adjust my volume.
 

ac/dcfan87

Member
Messages
800
It seems like it depend on the tube brand as well. Some tubes like to be biased hotter. That's what I've heard anyway.
 

Jason_77

Senior Member
Messages
7,198
If you bias over 70%, you risk pushing the tubes into failure quicker. If you bias too cold, you get crossover distortion. Between those points is the safe zone you want to be in. Some tubes like being on the warmer side, some like to be on the cooler side. Just stay between those two points and experiment to see what you like. I usually end up between 60% - 65%
 

Blue Strat

Member
Messages
30,418
As with almost everything, it's a matter of degrees...how hot? how cold? What tone do you prefer?

As always, personal experience trumps 10,000 responses on a web forum.

Just sayin'... ;)
 

Structo

Member
Messages
9,555
You really should know the power tube plate voltage to set the bias.

If you just say I like it set at 34ma, then you have no idea where the bias set in regards to the plate voltage.

(W * %) / V = I

Where W is the max wattage of the power tube in question.
% is how hot you want the bias: .7 for 70% and .6 for 60% and so on.
V is the power tube plate voltagte or B1.
I is the bias current that you adjust your bias to.
 

barfoden

Member
Messages
229
My latest experience with bias is on my Laney LC50 which has two TAD EL34B-STR tubes.
According to the schematic it should run at ~ 400V plate/screen and you can adjust the bias voltage from -26V to -36V and I measure the plate dissipation through the voltage drop over the OT pins..

The amp PCB board show that the amp is made for a 230VAC wall voltage, but my house socket seems to be at 238-239VAC most of the time so I can expect higher voltage which is what I see,, I let my amp warm up for an hour with the high voltage on and I get around 413V on the plates with 39-40mA plate current.
That is 16.1W to 16.5W plate dissipation or 64.5% to 66% of max plate dissipation.
The problem is I can't get any lower.. I have the bias voltage at maximum (-36.4V). It sounds ok the first hour or so but after two hours with the band it start distorting quicker and the bass is less defined,, which is a symptom that the power transformer and PCB loaded components are heating up.. The tubes are PCB mounted hanging down which lets the heat rise to chassis..

I will try some regular JJ EL34 (not the E34L model) and see if they bias colder in my amp..

In comparison
An amp very similar to mine (traynor YCV50 with EL34s) is set at (schematic value) 37.5mA total cathode current at 410V.
This value is with screen current which is substantial in a EL34 pentode (14.6% according to the Mullard datasheet of a fixed bias EL34 with 400V and a 50% bias).
The result is a plate dissipation of 0,032A x 410V = 13.12W or 52.5% of max plate dissipation..
 




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