Question about trainwreck performance....

[Glyph]

Let me clarify. I am not looking for an identical name plate. I heard that there were name plates that read " Trainwreck Clone" that you could buy. The amp I have was given to me by a friend of Ken's that had a few amps built with Ken's parts that he had given to him.

[paulster]

It's also polite not to dive into someone else's thread and start a completely new topic.

[Glyph]

Very sorry. Lets just cancel the whole line of thinking. This whole thread thing and forum stuff is new to me. I apologize to anyone I may have irritated. I would ask that no one reply to my initial question till I can do the thread thing properly.
Thanks

[paulster]

No problem! The button you want is the 'New Topic' button within the appropriate forum. That will create a new thread you can title how you like.

[jaxx]

I'll take a stab, measuring the bias voltage is not the best way to bias an amp.
It doesn't really take into account the plate voltage which is half of the equation.

Best to either use the shunt method or solder 1 ohm precision resistors on the cathode to ground.

Then use this formula.
---------------------------------------------------------
(W * .7) / V = I

Here W is multiplied by .7 to give us 70% of the tube's maximum plate dissipation.

This number is divided by the actual B+ voltage in the amp to determine what idle cathode current to adjust the tubes for in AMPS.

An example:
In an amp with 480V B+ and using EL34 output tubes (25W max plate dis.).

(25W * .7) / 480V = .036A

17.5 / 480 = 36mA

Cathode current at idle should be adjusted for 36 milliamps.

-------------------------------------------------------------------------

The guys here can tell you where they like to bias there amps in relation to the tubes used and voltage present.

this is the right way but not totally/properly corret as my english too



if we place a 1ohm resistor from katode to ground we can misure Ik current:

Ik=Ia+Ig2

to have a correct Ia or Ibias reading we've got to sottract Ig2 from Ik.

Ia=Ik-Ig2


the best way to have a perfectly Ia reading is to place a digital multimeter from OT center tap to anode power tube tap, and now divide it by the DC ohmic res of the OT primary (center tap to one lead only res).
this is possible/correct only if our OT has not the UL tap for screen greed supply, like TW amp hasn't.

in this way all the current flow is only anodic current.

so we can calculate the static dissipation:

Va*Ia=Wa



best regards,


Eddie

[vincenz]

What is it in the circuit that creates that thing of mean to clean with not a lot of dB's lost?

I'm bringing back the thread because I have a question regarding this famous dynamic behavior.

Does the bias setting have any impact on the dynamic (compression) of the power stage?
In other words, would a cold bias give more or less volume difference between clean and dirty sound (when controlling with the guitar knobs) ?

[redshark]

Hard to tell, the diference bias will give the amp a diferent "feel" not so sure how it does contribute to the clean to mean.
I asked this question in 09 before I built my express. Now 2 years after I can tell you that everything matters to create this effect. Also how you wire your guitar controls can be a lot of help. 50's wiring in a Gibson style guitar makes this effect more dramatic.
I think you should set the bias around the 40 mA if you are using el34's and your plate voltage is around 400vdc. I have 44mA. This is just to be in the ball park. I haven't experimented that much changing bias. All I wanted is just to have a good sounding amp. Once I got it I have no need to mess with it more.

[vincenz]

Yep. I play a Gibson Les Paul with PAF style pickups and 50's wiring (RS kit).
Actually the clean to mean on my amp seems to work quite well.
I recently change power tubes for NOS Siemens and I note that I can't bias warmer than 33-34mA (trim pot at maximum).
Amp is sounding good as is, I was just wondering if I am loosing some "compression" ( or mean to clean capabilities) with this "not so warm" bias set up.

What do you think about that?

[Big Jim]

Thanks for the answers guys!!
Keep them coming, I would like to understand this thing as much as posible.
Hey Glen!!
Well that is interesting that you think speakers contribute also. I've heard before that TW amps like low watt uneficient speakers. the only cab I own is an old jcm800 2x12 with 2 G12/65's in parallel.
Have you ever tried or heard a TW with G12/65's? they seem to be a more powerful greenback design.
If anyone tried this speakers with clones also i would like to know your comments about them.

Hi Redshark, There is a Youtube video of my Express through a cab with G12-65's. I have since done some tweaking on the amp (mostly tubes), but it should give you an idea what those speakers sound like. Keep in mind this was recorded with a Sony handycam. Search for "Trainwreck Express clone" by Stratpaulguy86.
Jim

[redshark]

Big Jim I saw your vid. I know you were playing a 4x12, mine is a 2x12 so a little diferent. You have more Box resonance plus you move more air with 4 instead of 2.
Anyway after doing a lot of research of this "little things" that matter let me run a couple of bias "facts"

When Kenny wrote in the trainwreck pages that the correct bias was -30vdc for the express we need to know a couple things.

At that time (early 80's) Ken was a groove tubes dealer. The Co. was being run by Aspen Pittman and at that time some of the tubes being labeled GT were the famous Siemens EL34's and the tungsrams and the Ei's. The grading of the tubes was accurate at the time (first method developed by Pittman) so if Ken said, the correct EL34 for the express is let's said GT no. 4 that meant that as long as you put that number you could set the bias at -30 and that number was gonna draw probably the current to be in perfect bias setting or in the "ballpark"
Later the Co. was sold and then other tubes (russian, chinese etc.) were the EL34's being labeled GT and other grading methods came out like the red, white and blue.
So at this time if you buy a NOS set of Siemens EL34's which are not graded like the old GT's a need to bias by current and dissipation considering the screens is what could be more accurate.
Times have changed from the early 80's to now that is why Ken wasn't very optimistic about reissuing his amps after 2000.

[vincenz]

Yep. I play a Gibson Les Paul with PAF style pickups and 50's wiring (RS kit).
Actually the clean to mean on my amp seems to work quite well.
I recently change power tubes for NOS Siemens and I note that I can't bias warmer than 33-34mA (trim pot at maximum).
Amp is sounding good as is, I was just wondering if I am loosing some "compression" ( or mean to clean capabilities) with this "not so warm" bias set up.

Someone here might have an opinion on this?

[rooster]

Well, it is clear to me that you would be better served using the Siemens in the 40ma range. You will have to change that bias resistor off the power supply diode chain. 150K is typical although the schematic shows a higher value. Eh, if you are running 150K here then those are some cold tubes you picked up maybe.

I fought the 40 ma thing at first but now I agree, afterall it's only 400VDC on the plates - if you are running the typical spec Express PT?