Tweaker 40 bias test points?

  • Thread starter Thread starter pryde
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Hi notryt,

The only 6V6 tubes that can be operated safely in the Twkr 40 are tubes that have specs high enough for the higher voltages in an amp designed for 6L6. Tubes that I'm aware of that are rated high enough are, JJ 6V6, or GT 6V6 S (S version only), and maybe the newer EH 6V6. Check the tube data sheet for any particular tube specs. The reason is that the Plate voltage in this amp, when using 6V6, will exceed the specs for most all 6V6, which can lead to tube failure and possible damage to the amp.

If you are willing to risk using the Tung Sol, the formula for setting the idle wattage of output tubes is the same for any type of tube, as described previously in this thread. Since most 6V6 are rated 14 watts max Plate wattage dissipation, and the range of operation in a fixed bias amp is 50% to 70% of max Plate wattage dissipation, and Plate voltage x Plate current = Plate wattage, just plug in the numbers. Measure your Plate voltage, it definitely will increase to probably around 450 VDC to 470 VDC with the 6V6 installed, and use the formula to determine the test point range. The test point will be for both output tubes.

Let's say that the Plate voltage with the 6V6 installed is about 470 VDC, and you want to set the idle to 50%. The math is; 14 watts x .5 = 7 watts, and 7 watts/470 VDC = about 15 mA. The test point is for two tubes, so 15 mA x 2 = 30 mA. So the test point reading to shoot for in this case would be 30 mV.

You have to actually measure the Plate voltage at pin 3 of the output tube, and this voltage will change as you adjust the bias pot, so adjust, remeasure and recalculate with each adjustment.

Again, to be clear, make sure the specs of a tube will allow its use in any particular amp. Tung Sol are great tubes, but I'm not sure that their 6V6 are a good choice for the Twkr 40.
 
BUMP

any info about the tweaker 88 test points and values?
 
I am guessing that the 88 is the same as the 40 as far as bias adjust, except a KT88 is a 40 watt to 44 watt tube, check the specs for your particular brand of tube.
 
Yeah? That's what I was going to find out when I opened mine up. Just compare it with these pics of the 40. Can you tell me what the red object is that your black probe is sticking into? Is that a part of the amp or your multimeter probe?

In the past Ive biased my Fender Hot Rod by clamping the black probe on the chassis. These pics have me assuming theres an actual port to insert the probes into. Am I correct?

As far as the bias values, Ive dug up some info from this site....

They should be set to 90mV - 45mV per tube.

Thanks for your help.
 
There are two of those red objects, sockets, test points, or ports as you called them, one for each meter lead. For many other amps, you do indeed connect the negative meter lead to ground (chassis), and the positive meter lead to a test point, but with the 40 you put both leads into the provided sockets, neither of which is ground.
 
oh yeah?...cool I'm assuming theyre clearly marked which is which?...haha. I'll be opening it up tonight possibly and didn't want to go in there blind.

thanks for your help though. I think when I find where things are I'll post some pics so people in the future will have something to reference. Cause theres not much on the web about these things.
 
I Have a Tweaker 40 combo. I ordered some GT 6L6-R power tubes. I emailed Egnater to ask them about what the bias should be set to. I have the printout from this site to show the test points, and on that sheet it says to set it to 65mV. The Egnater guy said don't set it over 45mV cause they will run too hot. Then on this site, I see someone said they set it to 78mV. I am now confused as to what I should set it at. This will be my first time biasing anything, and I want to do it myself because I live in a remote area and no one here to do it. Plus, I would like to learn how, so I could play with tube changes in the future! Any help would be appreciated! Thanks
 
Hi stratman1970,

The Egnater recommended setting for 6L6 in the Twkr 40 is 65 mV. You might be confused, as the 6V6 setting might be closer to 45 mV.
 
Thanks for the info. So I will set them to 65mv then. Do I need to have the volume on the amp turned up when I take my readings? I also saw on here something about 78mV. Should I put it to that or stick with what Egnater says at 65? Thanks again for your help
 
All pots turned to zero. I bias mine based on the factors that I've explained above previously, and I prefer 78 mV myself.
 
One last question before I embark on my maiden voyage of rebiasing my amp. Do I have standby switch on or off? Do I need to have a cord plugged in to jack? Thanks so much for all your help.
 
Yes, standby switch on, in the play position, as if you were going to play, nothing plugged into the input jack, speaker connected to speaker jack.
 
Well, I successfully changed my first set of power tubes, it went well. Except for the fact that it didn't fix what I thought was the problem. Before I changed tubes, the last time I played the amp, there was a crackling and a big crunch noise!! The volume on channel one went very quite, and the volume on channel two went really loud. I thought it was the power tubes that screwed up, but now that I changed the tubes, it is still the same in reference to the volumes. Has anyone else had this issue? I emailed the contact site on egnateramps.com but haven't heard back yet. Bruce if you still check out this site, could you help? I would be willing to ship it back to Egnater if they would fix it for me. Thanks to anyone that could help or shed some light on my issue. And Shimmilou thanks so much for the info on how to bias and what I should do, it really helped, and now I know I can do it anytime I wish!! I also at the same time replace the preamp tubes with brand new GT12ax7 tubes!
 
I'll be the next to dig up the old thread. My tweaker 40 power tubes have gone, that PDF Bruce posted, very useful. I noticed though, it said it could be biased for EL34's. I've been emailing a (the least helpful I've ever spoken to) tech from the egnater websites help email, who keeps saying to only use what the amp came with. But obviously that PDF says otherwise. I'd like to try EL34's, anyone done this before/know if its safe for the amp/tubes?

Ta
Oli
 
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