Amp biasing equiptment

[Kopfjaeger]

This is all new to me so I'm looking to see what gear (tools, meters, and tricks of the trade) you employ when biasing your amps. I don't expect to have to do this for quite some time since my amp is brand new but I'm just curious to see what you guys use when the time comes.

Also, where do you purchase your tubes/valves? Who makes the best quality/most reliable ones.

Thanks in advance.

Sepp

[ken_j]

I use a digital volt meter and use this method: http://www.aikenamps.com/Biasing.html Basically I do the math to determine the amperage.

[chrisdski]

I've used these guys before. They have a section on biasing, and you can buy a bias probe there also.

http://www.eurotubes.com/index.htm

[johnallg]

My Ampeg SVT-2 is all tube, but it has an easy-to-use LED comparator circuit. You put in the tubes (or periodically look at the LEDs to confirm the bias is still correct) and turn on the power. There are two LEDs, red and green. You turn an associated pot to light the green LED, biasing the three upper half tubes, then do the other bank of three tubes on the lower half. Easy peasy.

A decent multimeter and an insulated screwdriver is all you'll really need. That, and remembering to keep one hand in your pocket while working around high voltage, and not laying your arm or body against ground.

[Seans]

I use a digital volt meter and use this method: http://www.aikenamps.com/Biasing.html Basically I do the math to determine the amperage.

Very informative read, I use the Plate/Cathode current method already with a home made lead, easy and no need to go below.

As for valves themselves, Mullard GEC GE Tungsol RCA Telefunken etc units are the best, no question, ebay is probably the only source now, but grandfathers and old radio shops can still have some gems.

[Kopfjaeger]

OK, I thought I had a handle on this but I'm a bit confused. What is the difference between plate voltage and cathode bias? I'm getting the impression they kinda one and the same with the exception that plate voltage is the entire full voltage measured at the socket where as Cathode bias is an amperage reading at the socket via a resistor placed in line. QWell, I know voltage and amperage are different but they are measured at the same place in the same fashion? Do I have that right??

It seems both are measured with the use of an adapter that the tube plugs into and then that adapter is plugged into the chassis. Am I on the right track?

If so, which is the more accurate/ better way to set the bias??

I found a bias tested on ebay that seems to do both depending on what probe you use. it comes with two probes but you cab order two more. It has a 4 position switch to between the tubes with out unplugging and moving the probes to do all 4 tubes. Here is the link.

http://www.ebay.com/itm/Meter-ed-QuadSt ... 3511197716

There is an old TV repair shop in the town where I live. The place is a mess with TV carcasses piled all over the place and the owner is as old as dirt. I'll have to stop in to see if he has a matched quad of 6550's. it's worth a shot. Are any older tubes manufacturers better than others??

I have a few ther tube questions but I'll wait until these questions get sorted out before launching those.

Sepp

[Seans]

The best valve manufacturers were Mullard, Telefunken, GEC and Tungsol, all others follow very close, todays manufacturers can only come light years behind.

The best 6550 is the Tungsol, GE is close behind, the best KT88 is the GEC (but you won't get near these unless you're loaded).

[soundmasterg]

OK, I thought I had a handle on this but I'm a bit confused. What is the difference between plate voltage and cathode bias? I'm getting the impression they kinda one and the same with the exception that plate voltage is the entire full voltage measured at the socket where as Cathode bias is an amperage reading at the socket via a resistor placed in line. QWell, I know voltage and amperage are different but they are measured at the same place in the same fashion? Do I have that right??

It seems both are measured with the use of an adapter that the tube plugs into and then that adapter is plugged into the chassis. Am I on the right track?

If so, which is the more accurate/ better way to set the bias??

I found a bias tested on ebay that seems to do both depending on what probe you use. it comes with two probes but you cab order two more. It has a 4 position switch to between the tubes with out unplugging and moving the probes to do all 4 tubes. Here is the link.

http://www.ebay.com/itm/Meter-ed-QuadSt ... 3511197716

There is an old TV repair shop in the town where I live. The place is a mess with TV carcasses piled all over the place and the owner is as old as dirt. I'll have to stop in to see if he has a matched quad of 6550's. it's worth a shot. Are any older tubes manufacturers better than others??

I have a few ther tube questions but I'll wait until these questions get sorted out before launching those.

Sepp

Ohms law states that E = IR. E stands for voltage, I is current, and R is the resistor. The equation can be flipped around with voltage always on the top. (I = E/R and R = E/I) Because of this cool law, you can stick a 1 ohm resistor in between the cathode of the tube and ground, and then measure the voltage across it, and this will give you the current draw also. Normally to measure current your meter has to be in series with the circuit rather than parallel like the voltage measurement is, but because of ohms law, the current level can be derived by measuring across the 1 ohm resistor and using ohms law to figure out the current.

All of these meters stick a 1 ohm or 10 ohm resistor between the cathode and ground, and the voltage is measured across the resistor. This gives you the cathode current. The cathode current consists of the plate current, AND the screen current. However the screen current is small and can usually be ignored. Having it in there and adding to the equation adds in a safe direction anyway, so it is ok to figure the equation with it in there.

The plate voltage is measured like any other voltage....i.e....in parallel. So you just set your meter on an appropriate voltage scale and measure between the plate and ground. (CAREFULLY!!) You then multiply the plate voltage reading that you got here times the cathode current reading (in milliamps) and this gives you the wattage. (Ohms law also states that voltage times current equals power.) Every tube has a max dissipation rating that basically says how much heat (in watts) that the plate can dissipate away before it runs into it's limits. For a 6L6GC, this is 30 watts. For the early Tung-Sol 6550 this is 35 watts. For later 6550A's it is 42 watts. KT88's are also 42 watts. EL34's are 25 watts. This number is the maximum dissipation rating for each tube and is found on the datasheet for each tube type. Lots of sites provide you with datasheets or you can also get something like an old RCA tube manual to get this info.

So you can see that different tubes will perform differently in the same circuit. Different brands sound different and last longer or shorter also depending on their quality of construction. For 6550 types, the original Tung-Sol 6550 was made VERY well, lasts a LONG time, and sounds great. However, they are better for guitar amps than bass amps, and are very expensive these days. The GE 6550A is a VERY good quality and long lasting tube that sounds great also, but sounds better for bass than guitar. These were used in the original Ampeg SVT's. These are also expensive these days. The Genelex KT88 is the best of those types ever made and are stupendous quality, very good sound, and extremely expensive. Aside from those, there are quite a lot of modern replacements being made that sound very good and last a long time and are reasonably priced. I am partial to JJ KT88's in my bass amps...they sound great and are very long lasting. I have also used Winged C 6550C's and they sound great and are long lasting too. The Winged C's price has gone up a lot recently so that they are not a great choice these days. The Genelex reissue KT88's and Tung-Sol reissue 6550's are pretty good. The Chinese 6550 is also not bad. Eurotubes (www.eurotubes.com)is a vendor only for JJ but they grade their tubes and you get good quality from them. They are local to me here and I have been over there many times. Bob is a decent guy and sells good product, so if you decide to buy from them, it would be a good choice. Another vendor that is good is AES (Antique Electronics Supply... www.tubesandmore.com). I do suggest to get matched tubes for your application whenever you decide to get new power tubes. Without seperate bias pots, matched tubes are usually best.

When you set the bias, you are setting the idle point for your tubes. There are different ways to accomplish this, but all of the different ways make sure that the grid is more negeative than the cathode. One way to do this is to stick a large value resistor between the cathode and ground. This makes the cathode more positive than the grid, and allows the tube to work. This would be termed "cathode bias." Another method grounds the cathode and injects a negative voltage into the grid of the tube. This is called "fixed bias." Fixed bias will allow more power output and cooler running because with this method, the tubes operate more efficiently. Cathode bias is not the same as measuring the cathode current in order to set the bias.

For an existing amp, using the socket probes to measure the cathode current and plate voltage is the way to go when you want to set the bias. I build tip pin jacks into my amps so that I can measure this with just a meter, but for an existing amp, get the bias probes. I used the Weber Bias Rite, but they are no longer available, so find a different one, or get a used Bias Rite. There are others out there that work just as well as the Weber. Just make sure it measures the cathode current and the plate voltage, and you will be just fine.

Hope that helps with your understanding?

Greg

[BAD RONBO, KiLLeR DWaRfS]

i used to buy the genelex kt-88 from the distributor in mississauga ontario, canada for $ 56.00 a piece back in the early '80's. then they went belly up and could not get them anymore. great tube with lots of clean power for my svts.

[soundmasterg]

Just a small update to my post from above. Weber reissued the Weber Bias Rite with the warning to only use it to set the bias and not while playing as they are not built to withstand the increased current while playing. You can find them on the Ted Weber site if you want them. There are others by other sources too.

Greg