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Post by JChance on May 1, 2007 10:50:01 GMT -7
So right now, my 66 is down. Used it last week on a showcase in the early evening, then took it to use on a late shift that night.
When I got to the second gig, I flipped on the power and finished setting up. Went to turn on the standby, and POP! POP! POP!!!
Looked back, and one of the KT66's had failed... Was flashing white light like a lightbulb... Really a neat little lightning show!!! LOL So I immediately turned the amp off to keep from damaging anything. LUCKILY, I just happened to have my MAZ 38 head with me so I could do the gig...
So I've got replacement KT66's on the way, as the stock ones were still new enough for warranty.
Have the same GT KT66's on the way, with a rating of 5. However, I'm a little worried about the bias on the amp. I've always been told that, with fixed bias amps, even the same number rating with a new set of tubes could throw the bias off in the amp.
I could get a Bias Rite or something similar, but apparently the 66 doesn't have a bias adjust pot. And I hate to take it to a tech & pay the $50+ bench fee just for a simple power tube bias.
So should I just pop the new #5's in there and not worry about it? I'm not brave enough to try the biasing methods Dr. Z describes on his website (shunt method, etc.).
Ideas?
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Post by jb on May 1, 2007 10:58:00 GMT -7
The 66 does have a bias pot. It's the only thing that you can use a screwdriver on, so its easy to spot. The Bias Rite works great. Just stick in the new tubes and rebias. Good luck. John
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gimik
Junior Member
Peace, Love & Music
Posts: 58
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Post by gimik on May 1, 2007 11:27:02 GMT -7
Could someone detail the Route 66 biasing procedure please? An illustrated "how to" with pics and biasing values would be great. I know that it's a real job but I think many Route 66 owners would be grateful to its author for sharing his expertise. It could be post as a "red pin thread" (I don't know how you call those topics with "red pin" at the top of the threads list). It will then act as a reference for any Route 66 owner looking for biasing info. Thanks for your contribution.
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Post by John on May 1, 2007 12:25:09 GMT -7
Yes, the Route 66 has a bias pot.
I use the Weber Bias Rite tool....about $35 (probe only, no meter). Just make sure your KT66's are well matched.
Then pop off one of the tubes, insert the bias rite tool into the tube socket, then stick the tube in the top of the bias rite. Fire up the amp, then measure the current, and turn the bias adjustment knob in the amp until it reads 36ma, then your done. (the bias rite tool converts it to milliamps instead of millivolts)
I too am not brave enough to try the shunt method. And Myles says the weber tool is fine.
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Post by 5sonic on May 1, 2007 13:43:02 GMT -7
If your Route 66 is identical to mine, these instructions should help.
With the back of the amp facing you, flip it up on it's front side. Do not put it upside down or obviously the chassis will come crashing down when you remove the set screws. You'll see 4 large flathead screws on the bottom. These hold the chassis to the cabinet. You'll want to hold the chassis to the bottom of the cabinet with one hand while removing the screws with the other.
After removing those 4 large screws, put the amp right side up again and carefully pull the chassis out of the cabinet from the back. It's heavy, so I've found it necessary to reach around the front with one hand and grab the chassis through the slot just below the grille cloth. From this position you can tilt it up using both hands and slowly maneuver it out. Make sure the front panel doesn't get caught on the bottom of the cabinet when you start backing the chassis out.
The bias pot is a round plastic turnpot with a slot for a small sized flathead screwdriver. It's on the underside of the amp. I strongly suggest a bias probe of some sort, as holding meter leads to the tube socket pins can be dangerous. With a bias probe you don't have to do this.
Make sure your amp is running to a speaker cabinet so that it's properly loaded. Turn the volume and tone controls all the way down. That's really about it. If the amp is just sitting on a table, you'll need to carefully tilt it forward to adjust the bias pot, or have a friend do this. Otherwise it would be nice to have an empty cabinet or some kind of custom bench to hold the amp so you can access it from below. This way you won't have to tilt it.
I'd say that if you're putting new tubes in, you definitely need to re-bias, even if they're the same. So far I've tried my compu bias on 3 different amps, and all of them were underpowering the tubes quite a bit. In my limited experience, I've observed that power tubes seem to drop in bias as they sit in your amp over time. Both my Route 66 and my buddy's Fender Hot Rod De Ville were getting about half power to the tubes.
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Post by 5sonic on May 1, 2007 18:59:59 GMT -7
I wanted to bump this up because I had written something stupid in the above post. It's been edited now, but whatever you do, do not set the amp upside down and remove the set screws from the bottom of the cabinet. You want to filp it on it's front side, loosen the set screws, then set it right side up and carefully pull the chassis out.
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Post by whitestrat on May 14, 2007 7:28:58 GMT -7
Highly recommend the Weber Bias Rite. Been using one for several years now. Recently installed a new pair of matched Valve Arts KT-66 (Chinese) and biased at 31 and 32 mA respectively. I believe this is the bias range that Doc currently recommends for the Route 66. I have tried higher settings; e.g., 34 -38, but didn't like the sound. With the Valve Arts, the amp sounds much better at the lower settings IMO. It's brighter, more articulate, and cleaner at 31-32 mA. Tubes are supposed to last longer too.
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Post by 5sonic on May 15, 2007 18:04:56 GMT -7
Highly recommend the Weber Bias Rite. Been using one for several years now. Recently installed a new pair of matched Valve Arts KT-66 (Chinese) and biased at 31 and 32 mA respectively. I believe this is the bias range that Doc currently recommends for the Route 66. I have tried higher settings; e.g., 34 -38, but didn't like the sound. With the Valve Arts, the amp sounds much better at the lower settings IMO. It's brighter, more articulate, and cleaner at 31-32 mA. Tubes are supposed to last longer too. I thought I had read that the Doc is currently biasing them around 34-36 mA. In any event, whatever sounds best to you is what you should do. As far as the tube life, this amp doesn't run the tubes really hot, so that shouldn't be a major issue. I have heard a crackling sound a few times since I've installed the new tubes. I'm going to reseat all the tubes and make sure the set screws are tight as recommended by the Doc in another post I read.
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Post by Jan on May 16, 2007 1:22:29 GMT -7
Just a reminder that there is a bias notes page on the Dr. Z web site. Oh, right, you have seen it. Sounds like a good question for the ask Myles forum. www.drzamps.com/biasnotes.html
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Post by whitestrat on May 16, 2007 16:39:04 GMT -7
I recall seeing a post on this forum that indicated that Doc recommends biasing the Route 66 between 28 and 32 mA. He said the Route 66 has less hair and breakup at the lower range, plus the tubes last longer. Not sure if this bias range was with the Chinese KT66s, or the GT KT66-HPs. The two brands have slightly different power output and bias requirements as I understand. I prefer 31-32 mA with the Valve Arts KT66s, especially with an NOS rectifier.
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Post by 5sonic on May 16, 2007 22:51:33 GMT -7
According to the compu bias, my valve arts kt66 biased at 36-37 mA are putting out about 15.7 watts each. So that's right around the 32 watts that the Route 66 is supposed to put out. I would guess that running them at 31-32 mA you're getting somewhere around 14 watts per tube.
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Post by whitestrat on May 17, 2007 5:12:31 GMT -7
According to the compu bias, my valve arts kt66 biased at 36-37 mA are putting out about 15.7 watts each. So that's right around the 32 watts that the Route 66 is supposed to put out. I would guess that running them at 31-32 mA you're getting somewhere around 14 watts per tube. Here's Dr. Z's thoughts on biasing the Route 66. It can be found on page 5 of this subject area. "After realizing that many owners couldn't get the 475 plate volts, i realized you need at least 125 wall Volts to do it. My shop is in an industrial area and the voltage to my shop is a little on the high side. Remember the power tranny in a tube amp is a 4 to 1 step up ( in the 66 anyway). So for every volt under 125 V subtract 4 volts from that set-up, that is voltage you read AC from your wall socket. Some guys could only get about 460 V on the plates and they had 121V from the wall. The shunt meadthod or use of a bias right is better. I shoot for 28 to 32 mA per tube ." Z
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Post by 5sonic on May 17, 2007 10:13:09 GMT -7
28 mA seems awfully low. I would guess that the amp would only be putting out around 25 watts if you run them that low. On the Dr Z Bias Notes page it says the range for a Route 66 is between 34-38 mA. I think that the lower range is applicable when you are using a different rectifier tube, such as a 5U4GB.
[edit] Nope, the lower range makes no difference with a different rectifier tube. I just put a 5U4GB in my Route 66 and had to set the VA KT66 to around 38-39 mA to get them up to 16 watts each. They sound really spongy, but the midrange is kinda weird.....way different than the Mullard GZ34. [end edit]
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