Got me an express

[gearhead]

If you don't feel comfortable reading a schematic, just completely forget about the -30V thing. It is a measurement at a different point in the circuit but is also directly affected by where you set the bias pot. The designer, Ken Fischer, probably used that because he didn't have to add a resistor to measure.

Just measure off your 1 ohm resistor at your test points.

According to the tables I referenced:

For EL-34, bias from 36mA, but can go up to 47 mA to run them hot

For 6V6, I believe you want to bias to 18-19 mA. Others might chime in.

Of course, you will actually be reading volts, so set your meter to mV. (V=I*R, 40mV = 40mA * 1 Ohm)

[dartanion]

The bias supply is setup to give you negative voltage. If you check the voltage from the supply to ground, this should give you negative volts on your DMM.

[blaren]

If you don't feel comfortable reading a schematic, just completely forget about the -30V thing. It is a measurement at a different point in the circuit but is also directly affected by where you set the bias pot. The designer, Ken Fischer, probably used that because he didn't have to add a resistor to measure.

Just measure off your 1 ohm resistor at your test points.

According to the tables I referenced:

For EL-34, bias from 36mA, but can go up to 47 mA to run them hot

For 6V6, I believe you want to bias to 18-19 mA. Others might chime in.

Of course, you will actually be reading volts, so set your meter to mV. (V=I*R, 40mV = 40mA * 1 Ohm)

Cool...thanks fellas.

[skyboltone]

Expressed another way:

Bias Voltage: The negative voltage applied to the grid which is not what you generally measure or care about.

Idle current: milliamps of current going thru the tube with no signal. This is what you generally measure and always care about. One way to measure is to read the voltage drop across a resistor on the cathode. (aka "bias taps.")

Yes, and don't forget that the voltage drop across the 1 ohm resistor also includes screen current. So if you're aming at -30V on the grid best put a voltmeter on the grid, dial in -30 then see what the voltage drop across your 1 ohm resistor is and if it says 34.4 mv then that's the number you use when biasing the next set of tubes.

[Tamadool]

I'm surprised no one has mentioned "The Trainwreck Pages" -

http://www.angela.com/catalog/how-to/tw.html

Ken Fischer writes specifically about biasing amps(starting on page '25'), there is even a portion devoted to biasing a Liverpool.

[gearhead]

Great link, thanks!

Good point, but the methods Ken cited 1) don't match the setup that blaren has (external bias points to measure current) and 2) given his level of knowledge, would be reluctant to recommend him poking around near the plates.

I would be interested to know what Ken's "Special Process" was to ensure little need of bias adjustment for years to come!

[v846]

I would be interested to know what s "Special Process" was to ensure little need of bias adjustment for years to come!

just guessing but didn't Ken use "graded" groove tubes? if so you could pick the same number and drop them in.



David

[blaren]

yeah, thanks for the link. Don't worry fellas, I ALWAYS confirm that caps are "drained" before working on/in ANY amp and I keep one hand in my pocket. I'm safe when checking voltages on a "live" amp too. Here's a tip for everyone....make sure that part of your gold neck chain doesnt fall out of your shirt and into the chassis. I've never had any fireworks but I've had a "potential" close call or two.
I unfortunately havent yet built an amp (wanted to but it was SO cheap to get Nik to build me one that I just couldnt do it) so my understanding of what all is exactly going-on in there is a little weak as is my grasp on amp vocabulary. For instance, tell me to change the output coupling caps (if there's such thing) and I'm lost. Tell me where they are or show me a pic or layout and I can SAFELY do a NICE job.
I've done some pretty fussy work on items like satellite receivers and stuff and have good soldering skills. I've re-capped amps and done some simple mods. I play the guitar the same way. I know absolutely NO theory but I've been playing proffesionally for 30 years and making a very good living at it. I dont know why or how to do stuff but I can do it well. Faking it around 600V is a little more dangerous than improvising a solo but I AM careful and safe.
It's DEFINATELY time for me to build one I'd say.
Thanks again fellas.

Oh...I gigged with the wreck again last night. Dont have time right now but I'll fill y'all-in later.

[Structo]

Yes, never wear the bling when servicing an amp!
Also if you are left handed, take off your wedding ring before using that as your probe hand........

[blaren]

I also know the old trick of standing in a galvanized pail of lukewarm water while working on amps...something about the galvenization and water creating a shield preventing circuit completion.
I've never tried that but it makes sense. I can attest though, to the fact that as long as you're wearing rubber boots, it's safe to bring your stack into the shower and wail away...just dont sing...well unless the p.a. is running off the same circuit as the amp.
JUST KIDDING!!!
If anyone does any of this and kills themselves, dont come running to me.
God...I'm SOOOO funny.
DO NOT TRY THIS AT HOME...if you do, you're even more dumb than I am.

OK so....the gig on Friday. Small room again (couple hundred peoplpe) and I used the express through a 2-12 cab with old G12H30s. Didnt use the hotplate this time. Funny thing about a hotplate. I dont mind it with some amps but with a lot of amps, I hate it. It takes all the "guts" out of my marshalls and this trainwreck. The hp works very well with some of my other amps (traynors especially) but on most of my amps, it adds fuzz and castrates them (and I dont mean at -12 or -16dB...even at 4 or 8 somewhat). ANYWAY yeah...no attenuation this time. Amp has a master on it anyway.
WELLLLLL...even with the master cranked, I cant seem to get near crystal clean WITH punch sparkle, or bite. Granted I'm playing a humbucker equipped PRS and the strat does clean-up a bit better but, not only can I not get an "acceptable" clean tone, I cant get it to bloom and feedback. I have it biased at ~ 43mA. Hitting the front end with my ols TS9 doesnt really add drive but rather just makes it spongy, mushy, and weak.
I dont want to ruffle any feathers but...my 36W RP (shout out to Richie) eats this express-up. It's a different animal, and one that I'm used-to but...with the master and volume at about 5, this thing barks and crunches like crazy. Roll off the guitar's volume a bit and she cleans up well enough to take home for thanksgiving dinner. I'm almost talking twin clean. Then when I hit it (mildly) with the TS, she wails, cries, and sings with incredible harmonics and SWEET tone.
I still think it's just me not really "playing" the TW properly but, either way, it just isnt versatile enough for me. I know that some people call these a "lead" amp or a "one trick pony" but to ME, my 36er does EVERYTHING better than the wreck.
Again, I do realize that I'm just a retard and probably dont know what's good for me, I think it's time for me to give-up on this thing. I think I'll have to put it on the block after the holidays and after everyone has paid down some of their credit card Christmas debt. I'd really like to replace it with a sweet combo. That's probably not true either...I've never met...I mean SOLD an amp I didnt like.
dont forget...I'm not trying to say (although it seems that way) that Richie designs better amps that Ken did, just better for ME.

[dartanion]

blaren,

It sounds like there are probably some other issues with your amp. Wrecks are finicky about tubes, so try a bunch of different tubes to find a set that works. Also, compare the circuit with one of the known schematics here and note any differences.

Photos would also really help. Borrow a camera if you have to.