Tweaker 40 bias test points?

pryde

New member
Picked up new tweaker 40 head and will be changing the power tubes soon. I e-mailed egnater support a few days ago and no reply yet :confused: Can anyone tell me where the bias test points are on the tweaker 40 circuit board?

I found other info on this board about what to set the bias at (65 mV) I believe.

Thanks in advance for your help.
 
Yes you did,
Thank you Bruce.
I'm not the op but my question would have been the same.
Do you test both points at the same time with the multimeter
Or one at a time.
For example tp1 is for v4 tp2 for v5?
 
Thank you Bruce :thumbsup:

BTW: I think the stock tubes sound pretty good but want to be prepared when I need to, or feel like changing out the power tubes. I am loving this amp the more I play it.
 
Measurement is made between the two test points, not to ground. One probe to one test point. Other meter probe to the other test point. Meter set to read DCmillivolts. Power on and in play position. All knobs full CCW.
 
bruce egnater":2743zawc said:
Measurement is made between the two test points, not to ground. One probe to one test point. Other meter probe to the other test point. Meter set to read DCmillivolts. Power on and in play position. All knobs full CCW.

Wasn't sure of that, as a matter of fact, i was sent
A pair of replacement 6l6 GT for my tweaker,
I had one to tp andone to ground,
On one I would get 24 the other would give me 1.35 mv
Luckily I have a bias probe, still right now typing with one hand
And other is palming my forehead.
 
bruce egnater":1s33fe9v said:
Here you go.......
do you have to take the chasis out completely to bias? and are the test points on the bottom or the top of the chasis? how do i get to the test points?
 
Bruce, if your still there? Why did you put the test points on the inside? The cool thing about your amps, is they're different, with great options. Your test points on the outside, of the chasis, was a great idea, why change it? Tweakers are for tweakers, they're going to want to mess with bias and different tubes IMHO. Is it cost ? I'd pay the extra for the bias on the outside. Some companys are finally including auto bias, regardless of what tube you use (can you add that in the future?).
 
Chassis removed, test points
Twkr40044.jpg


Bias adjust pot
Twkr40011.jpg
 
Well.....make sure to unplug from electricity first, and heed the cautions about high voltage being present inside the chassis. Even when unplugged from electricity, the capacitors can still hold a lethal charge. Best not to touch anything inside the chassis, except for test points and adjust pot.

The way that I do it:
Remove the top and rear metal screens (grills). Remove the output tubes, noting their position, a sharpie is good to mark the tubes (4 and 5). Remove the four screws on the bottom of the amp that hold the chassis in (not the feet, the other four screws), making sure that the chassis is secure and won't fall, ie leave the amp in upright position and undo the screws from underneath. Hang one end of the chassis over the edge of workbench and remove those two screws, then do the same for the other end. Remove the chassis from cab (grip the transformers if necessary), one way to position it as shown in pic (above). Replace the output tubes, connect amp output into a speaker cab, connect DC volt meter to amp test points (read mV), plug amp in to electricity. With all knobs set to zero (not sure if that is necessary but I do it anyway), switch amp on, and after a minute switch standby on and note mV reading at test points. [Edit: if installing new tubes, make sure that the initial mV reading is not excessive, be prepared to turn the bias pot down if necessary]. Wait a few to let tubes warm up. Adjust bias pot to desired level (I set mine at 78 mV), then let the tubes warm up for a while, maybe even plug in a guitar and play for a few minutes, recheck bias before replacing the chassis back into cab, as a little drift might occur.

Once bias is steady at the desired level, switch to standby for a minute, then switch the amp off. Unplug all connections from amp and reassemble in reverse order of dis-assembly. Last step: enjoy! :rock:

Twkr40081.jpg
 
shimmilou":faenynjx said:
Well.....make sure to unplug from electricity first, and heed the cautions about high voltage being present inside the chassis. Even when unplugged from electricity, the capacitors can still hold a lethal charge. Best not to touch anything inside the chassis, except for test points and adjust pot.

The way that I do it:
Remove the top and rear metal screens (grills). Remove the output tubes, noting their position, a sharpie is good to mark the tubes (4 and 5). Remove the four screws on the bottom of the amp that hold the chassis in (not the feet, the other four screws), making sure that the chassis is secure and won't fall, ie leave the amp in upright position and undo the screws from underneath. Hang one end of the chassis over the edge of workbench and remove those two screws, then do the same for the other end. Remove the chassis from cab (grip the transformers if necessary), one way to position it as shown in pic (above). Replace the output tubes, connect amp output into a speaker cab, connect DC volt meter to amp test points (read mV), plug amp in to electricity. With all knobs set to zero (not sure if that is necessary but I do it anyway), switch amp on, and after a minute switch standby on and note mV reading at test points. [Edit: if installing new tubes, make sure that the initial mV reading is not excessive, be prepared to turn the bias pot down if necessary]. Wait a few to let tubes warm up. Adjust bias pot to desired level (I set mine at 78 mV), then let the tubes warm up for a while, maybe even plug in a guitar and play for a few minutes, recheck bias before replacing the chassis back into cab, as a little drift might occur.

Once bias is steady at the desired level, switch to standby for a minute, then switch the amp off. Unplug all connections from amp and reassemble in reverse order of dis-assembly. Last step: enjoy! :rock:

Twkr40081.jpg


Great job! you'll help a lot of guys out there. Still Egnater save us the time and danger and put the test points on the back, where they belong, please.
 
Well.... sorry for digging up an old thread, but I need some help.

Started following these steps (thanks bruce and shimmilou) when all of a sudden a resistor fell out. Here's a pic (it's ~1/4 inch long:)

IMAG0332-1.jpg




Anyhow, it fell out of the housing when I went to move it out of the way. Makes me think it was stuck to the bit of tolex that was peeled up. After doing some searching, I can't see where this particular resistor came from, and given it has no solder on it and looks cut on both ends, I'm wondering if it's not even part of my amp, and was just accidentally dropped in there at the factory.

Any advice or help will be greatly appreciated!! Cheers.
 
Hi ercguitar,

I think that it's great to dig up old threads, keeping the same info all together so it's easy to find. :thumbsup:

That resistor was definitely clipped and didn't fall out of your circuit, I wouldn't worry about it, but be thankful that you found it before it fell onto the live circuit. I presume that your amp functions well and sounds good?
 
Thanks for the reply, and yeah, it was not part of my amp, and if by some chance it was, well... my amp still works and is sounding better than ever!! :D And yes, I'm very thankful, and a bit surprised that it didn't fall into the circuit, especially considering it's been moved a number of times, to and from gigs, etc.

shimmilou, thanks again for the quick tutorial on tearing it down, it was spot on. However, while it's surely better to test it than not, I would have been fine with just replacing the tubes and not testing. The new tubes tested at 69.6mV at first and at 68.4mV after the fully warmed up. I did adjust it it down to Bruce's recommended 65mV (well, 65.4 was as close as I could get it).

Question: I see you set your's at 78mV. What's the reason behind that?
 
The bias setting is to determine the wattage dissipated by the output tubes at idle (while not playing). This idle wattage is calculated by multiplying the Plate voltage by the Plate current. In the case of this amp, and most other amps, the test points are used to indirectly measure the Cathode current of the output tubes, by measuring the voltage drop across a 1 ohm resistor in series with the Cathodes of the output tubes. Ohms law says that the voltage drop across the 1 ohm resistor will be the same value as the current through the resistor, so 78 mV equals 78 mA. The Cathode current is usually close enough to the Plate current for the purpose of determining the idle wattage. The Plate current equals the Cathode current plus the grid current, and typically the Cathode current is within about 3 mA of the Plate current for a 6L6, and it is safer to measure the Cathode current, since measuring the Plate current exposes the test leads to high voltage, whereas measuring the Cathode current does not.

Typically, in a fixed bias amp such as the Tweaker 40, the output tubes will sound best when biased somewhere between 50% and 70% of the max Plate wattage rating of the tube, which in the case of most 6L6 tubes the max rating is 30 watts. Crossover distortion will happen if the bias is too low, many times resulting in some unpleasant sound, or at least not as good a sound as can be had with a higher bias setting. In the Twkr 40, the Plate voltage is approx 430 VDC, so doing the math we get a range of approx 35 mA to 49 mA per output tube. Since we have two output tubes in this amp, we would want to see approx 70 mA to 100 mA for the desired current range at the test points. As you can see, 65 mA is quite cold and below the desired range of current for this amp with these tubes. Obviously, 78 mA (mV) is in the desired range at just over 55%, still on the cool side compared to what others might like but in the desired range. Some people like their tubes biased higher, even higher than 70%. A lot of time spent experimenting and listening can help each individual determine their desired bias setting.

The trade-off is that a lower bias setting will usually result in longer tube life, and a higher bias setting will yield a better sound. There are many things to consider, tube life, sound, tube type, etc. Some tubes will last longer than others and can be biased higher with no detriment to their life, some tubes aren't as good quality, and each person has differing tastes, so there really is not one correct way to do it. The above explanation only encompasses my experience and taste, but seems to suit the majority of users quite well. I typically like my bias setting between 55% and 60% for most fixed bias amps. The term "fixed bias" is simply the way that the circuit "fixes" or "sets" the bias level, and many fixed bias amps will have an adjustment pot, and some only have a non adjustable resistor to set the bias level. For fixed bias amps without a bias adjust pot, the resistor can be changed to a different value, or a pot can be added in order to adjust the bias level. Some amps have another type of bias for the output tube(s),which is called "Cathode bias" or "Self biasing" and usually no adjustment is necessary when changing output tubes to a different type or brand. The preamp tubes in your Twkr 40, and most any tube amp, are "Cathode biased", and that is why you can simply swap your preamp tubes for most any other type with no adjustment needed. I hope that I have sufficiently explained the biasing to you without boring you to sleep or confusing you.
:rock:
 
HA! Not bored at all. I have a general understanding of ohms, amps, volts, etc, took enough physics classes, but the actual application of it to the amp is very helpful. So THANKS!! And yes, I'm familiar with the different types of bias controls in amps, and I'm glad Egnater put this amp together to be easier adjusted. On the flip side, my buddy is having a hell of a time finding out anything about his Vox Night Train 50. He wants to swap the tubes, but can't find the first bit of info on it.

So, can we take this one step further and get into specifics on my amp/tubes? I'm running Tung-Sol 12AX7s (brand new), and TAD 6L6GC-STR (also brand new, hence this conversation... :) ). I previously had JJ 6L6GCs with the factory Tung-Sol 12AX7s. Anyhow, so far I'm loving the sound with the new power tubes, much more crisp, and dynamic than the JJs. Also a bit more percussive, which I like, particularly when playing with single coils. What I gathered from your last message is that I should pull it back apart, and play through it at some different bias settings, but is there anyway to take the info I've put here and say, "Bam, you should target XXmV!"? Not looking for a shortcut, but really just wondering, is there more to the science of specific tube and amp combo.

Also, not sure if this is related or not... my amp starting picking up radio broadcasts after the tube swap. My buddies old amp, in the same room as mine, used to do it, and mine never did. Now his Night Train doesn't, but mine does. :confused:

Thanks again for all the info on this.

Hopefully this looks normal... LOL

IMAG0339.jpg
 
ercguitar":22pnwma4 said:
...So, can we take this one step further and get into specifics on my amp/tubes? I'm running Tung-Sol 12AX7s (brand new), and TAD 6L6GC-STR (also brand new, hence this conversation... :) )...What I gathered from your last message is that I should pull it back apart, and play through it at some different bias settings, but is there anyway to take the info I've put here and say, "Bam, you should target XXmV!"? Not looking for a shortcut, but really just wondering, is there more to the science of specific tube and amp combo.

Also, not sure if this is related or not... my amp starting picking up radio broadcasts after the tube swap...

That's the same tubes that I had in mine, I liked the sound, and most of the pleasant sound in this case comes from the Tung Sol preamp/PI tubes (great tubes). The preamp section is where the tone is first created after all, and the output tubes mostly just amplify the created sound, although the output tubes do play a part in tone shaping also. The parts that I covered about biasing are really where the science ends. Knowing the range of current is one thing, but your ears will make the final determination as to where you end up with the bias. Only years of experience will tell you a specific target current for a particular set of tubes in any particular amp. You might like your current tubes biased low, and the next set being a different brand might sound better biased higher. In Marshall amps for example, the sound is usually much better, or more "Marshally" when the output tubes are biased very high.

Amps will produce sounds from radios, CBs and other sources, but the guitar and cables into the amp are the likely suspects as far as what is picked-up. It can happen with any amp, and depends largely on the power and location of any broadcast, as well as the position of your guitar. I can get my guitar next to the wall in my music room and pick-up the sound of the radio in the living room and hear it through my amp.
 
what should the bias be set at on a tweaker 40 using tung-sol 6v6gt tubes.. all i see is 6l6.levels. which is already in there to begin with and if using a stock multimeter what should the setting be? :aww:
 
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