How to bias a Trainwreck Express

jazzbass · Post by **jazzbass** » Mon Jul 28, 2014 9:52 pm

Hi, can anyone please explain me how to bias a clone Trainwreck Express, I mean the correct method Ken Fischer use step by step. I use 2x Mullard EL34 and/or 2x TAD 6V6. Thank you

vibratoking · Post by **vibratoking** » Mon Jul 28, 2014 10:13 pm

You could look in the files section at the build guide.

http://ampgarage.com/forum/files/expres ... te_187.pdf

Or at the Trainwreck pages:

http://www.blueguitar.org/new/schem/tra ... a2b62733=1

Ken probably biased it where his ears told him to. There are many ways to bias an amp. None of them are 100% 'correct'. You might choose a method and decide that a little tweak from there sounds better. The main thing is to make sure you don't bias beyond the power rating of the tube. Otherwise, choose a method and experiment.

jazzbass · Post by **jazzbass** » Tue Jul 29, 2014 9:45 am

vibratoking wrote:You could look in the files section at the build guide.

http://ampgarage.com/forum/files/expres ... te_187.pdf

Or at the Trainwreck pages:

http://www.blueguitar.org/new/schem/tra ... a2b62733=1

Ken probably biased it where his ears told him to. There are many ways to bias an amp. None of them are 100% 'correct'. You might choose a method and decide that a little tweak from there sounds better. The main thing is to make sure you don't bias beyond the power rating of the tube. Otherwise, choose a method and experiment.

Hi vibratoking, your help was very useful to me, I've been looking everywhere for a proper and convenient way to set the bias correctly in my Express but each autor give a different interpretation, and I was not sure what was the right method.
I have read several times "The Trainwreck Pager" but I have not yet figured out the method suggested by Ken ( the correct bias voltage for a Trainwreck Express in operating position......is -30volt on pin 5 of the output tube).
I forgot that i have already printed the wonderful manual "Trainwreck Express Assembly Instructions" (big, big issue). Thanks for reminding me.
Let me ask for help in case of need.
Thank you

vibratoking · Post by **vibratoking** » Tue Jul 29, 2014 3:11 pm

....is -30volt on pin 5 of the output tube). ...

Glad that info help trigger a memory. I feel like I should state that setting the grid to -30V is 'dumbed down' advice that doesn't tell the whole story. The max plate dissipation, Wa, for an EL34 is something like 27 Watts if memory serves. The plate dissipation, Vak, is the voltage dropped across the tube from plate to cathode times the current from plate to cathode. So, blindly setting the grid to -30V could work for a specific Vak, but may not be correct in all cases. Most techs don't like pushing the spec so they bias at something like 70% of Wa to account for errors in some measurement techniques and to allow for margin.

Here is a quick example:
Wa = 27W, so .7*27 = 18.9W. Vak = 400V, so 400*0.05A = 20W. Biasing at 50mA of current would be an acceptable situation for me with this voltage.

jazzbass · Post by **jazzbass** » Tue Jul 29, 2014 3:37 pm

vibratoking wrote:
....is -30volt on pin 5 of the output tube). ...
....I feel like I should state that setting the grid to -30V is 'dumbed down' advice that doesn't tell the whole story.

Hi vibratoking, I agree with you in not considering a dogma every word written on how to set the bias, can be a point of departure and stimulate our curiosity about this wonderful hobby.
Let me ask you what is your preferred method? can you pretty much describe step by step?
Thank you
Franco

vibratoking · Post by **vibratoking** » Tue Jul 29, 2014 4:18 pm

My preferred method is rather quick and dirty. It is not really my method, but one used by many and can be seen in many schematics. Permanently insert a precision 1% or better 1 Ohm resistor from the cathode of each power tube to ground.

1. Measure Vak
2. Calculate the current needed to get to the 70% dissipation point.
3. Measure the voltage across the 1 Ohm resistor. This measurement is actually the current if you think about it and if the resistor is actually 1 Ohm. Adjust the bias according to 2.
4. Adjust by ear and then double check that my 'ear' setting is not violating the dissipation limit.

Enjoy. Now quit playin with amps and play your guitar.

2tone · Post by **2tone** » Tue Jul 29, 2014 5:40 pm

expresses can be fussy. If you bias one on one cabinet w/ certain speakers, it may act up on another cabinet at the same or different impedance w/ different speakers. I bias mine for the cab I'm using. I don't bother about the voltage etc. I know the Express sound I'm after. I bias by ear on the cabinet I'm using, then check the current w/ a bias probe meter to make sure the current isn't too high. Much over 40 can be too much, and stuff around 10 -15 is too lean. Ken used to say anything between 10 and 40 ma is Ok on the tubes.

jazzbass · Post by **jazzbass** » Thu Jul 31, 2014 9:34 pm

2tone wrote:expresses can be fussy. If you bias one on one cabinet w/ certain speakers, it may act up on another cabinet at the same or different impedance w/ different speakers.

Hi guys, what you write hit me and opened my eyes and ears. I did several test which overshadows the theory and I accept your points of view. I discovered a whole new world, a stargate. I understand that I can get the sound I want and that this sound depends on the tubes and speakers and that it is not a heresy to adjust the bias by ear.

Thank you vibratoking and 2tone for your support.

JPGraphX · Post by **JPGraphX** » Tue Apr 28, 2020 12:08 am

Hello,

I'm going to replace tubes in my Express amp. I would like to adjust bias as simple as possible. This is my first time.

Is it the type of probe I need:
https://www.amazon.ca/Nobsound-Vacuum-A ... th=1&psc=1

There is no mention of a 1 ohms resistor.

Thanks
JPGraphX

martin manning · Post by **martin manning** » Tue Apr 28, 2020 9:03 pm

I'd guess that probe has a 1Ω resistor inside since it says 1mV = 1mA on your DMM. Does your amp have bias test jacks? Maybe it already has 1Ω resistors inside?

rooster · Post by **rooster** » Sat May 02, 2020 7:46 pm

I run Mullard Xf2s in both my Express and the OP mentioned Mullard power tubes....

I run them at 28mas, and, although they can take a lot more than this, I'm happy with this. A friend of Ken's, Gregg Levy, suggested 20mas and I tried this for a while. Nope, not for me, although most ppl here run things much hotter, there's a point where the amp just behaves like you want. I had an Alan Phillips clone over here that was running Siemens El34s and it came untouched from the original owner (directly from Alan, but 10 years old) that was biased at 42mas. After comparing my two Express with this, the owner settled on 32mas. Eh, the Siemens could have drifted high, sure.

I like what 2-tone said, btw, the right speakers are important because they can change the feel of the amp.

Anyway, my 2 cents

Mark · Post by **Mark** » Thu May 07, 2020 10:26 am

I too heard the amps tend to be biased cold rather than hot.

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