Post by asmith9509 on Jun 26, 2010 18:24:52 GMT -7
OK, I lied in my other post- I do have a technical question.
Opened up my Galaxie to bias the tubes. I have an engineering degree and I've built/restored a few tube amps, so this is familiar territory for me.
Many amps have a 1 ohm resistor in series with the bias pot that allows you to get a voltage measurement across it, which lets you get an idle current measurement. Do these amps have that feature? If so, where is the resistor?
Most amps have the bias measurement point clearly marked; I couldn't find it. As an alternate to the 1 ohm resistor, I was going to measure current between the plates and the center tap of the transformer, but I couldn't even find that because the leads were under the board and I didn't want to unmount the board.
Post by mtlrecords on Jun 26, 2010 20:12:07 GMT -7
I was wondering if Z makes schematics public too. That would be really handy and keep with the highly professional nature of Z amps. I mean these amps are built to last a lifetime or more, but with a schematic I can get it fixed forever with almost any tech, anywhere. Just sayin.
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Just to follow on this thread... I spoke to the Doc this morning regarding proper bias of the Galaxie. He recommends 28mA idle current with 6L6GC's, like the tweed Super's that the Galaxie is modeled after. To get a shunt current measurement, measure between pin 3 of either output tube (plate)and either terminal of the standby switch with the switch on (center tap of output transformer). As he correctly notes on his website, these are dangerous voltages- so don't try it if you don't know what you're doing. He was really helpful, and a nice guy. I could tell he was busy, but he spent plenty of time on the phone with me. Two Thumbs Up!
Post by headshrinker (Marc) on Oct 28, 2014 10:31:41 GMT -7
Are you saying that the max you can get out of the amp with the bias pot all the way up is 23.6? If so I would guess the tubes you have are way out of spec. If that's what you are getting before turning the bias pot, just turn it up to get to the correct amount. If you are asking can the amp be run at higher bias settings than 28 the answer is yes. I run mine around 32. The doc does run the amp cold to help get that fender tweed 210 super sound. You are running yours even colder which may give it more of a blackface sound. Fender tended to bias cold back in the day. How hot or cold you run an amp has a lot to do with personal preference and what sound you are going for and how you play. I've got a chart that Myles posted a while ago where you can plug in the dissipation value of the tube and the voltage reading you are getting and it gives you the range of bias that the tube can handle from cold to hot without screwing up the tube. If you want it pm me with an email address and I will send it along.
Thanks Mark. I know that the cathode resistor method is not the most precise of all, and the tubes must be even colder that the reading I got, because there is some grid current that must be subtracted.
The thing is that the MAX value I get after 20 minutes of operation is around 24.8 with a plate voltage of 417. I'm using a set of TAD 6L6GC-STR, according to the literature I found its supposed to be 30W max dissipation, contrary to the normal 25W.
The amp sounds great to me, but I wonder how the sound would improve getting close to the recommended value.
Thanks again mate. CE
Last Edit: Oct 29, 2014 9:10:58 GMT -7 by caesparza
Post by wraparound (Steve) on Oct 29, 2014 23:16:41 GMT -7
Do you have the switch on back set to triode or pentode? I just use a bias rite on one of the tubes and set to taste within the neighborhood of the recommended setting noted on the Z amps site. But the reason I ask the question is that the amp in triode will reveal a lower reading than in pentode. At least that's what i recall seeing when doing this, so make sure the switch is in pentode mode. The fact that it sounds good is a hallmark of this amp and frankly anything close bias wise doesn't make a whale of a difference to me so I tend to be at or below the mark to save tube life. But I appreciate the quest to want those tubes to behave as they should using a method I wish I was qualified to use. Hope you can solve the mystery!
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