biasing 7868 tubes question

[edadmartin]

Hello, Id like to bias my ampeg sb12 fliptop which has NOS RCA 7868 tubes.Ive never biased this amp with the new tubes in it because my Weber bias rite isnt set up for it. First, what method is used for this amp?. Any one here able to walk me thru? Ive built some amp kits and Im aware of the high voltages, the caps have been dis charged. and like I said Ive biased other amps using the bias rite. Thanks for the help.

[Structo]

It is fixed bias but doesn't appear to have adjustable bias.

The schematic shows 6L6GC power tubes.

The 7868 is only rated at 19 watts.

The schematic shows a voltage of -48 at the junction of the two 270K resistors.
That is for 6L6 tubes so I'm not sure what the 7868 bias at.

This is a negative voltage. If when you measure it is close to that or a little more negative you are fine.

Schematic:
http://www.schematicheaven.com/ampegamp ... taflex.pdf

[edadmartin]

hi its got 9 pin 7868's stock from 1967 so 6l6 specs dont fly. closest tube is the 8 pin 7591a, close to a 6l6 but not the same to me.
. I just want to know how to adjust and read the adjustment when I do it .fixed bias, everyone says just put the tube in and forget it, what if something is out of spec in the bias supply wouldnt you want to see what is happening to the tubes too? The tubes seem to have redder than normal plates, which way would I slightly turn the bias(hum) pot?? and how can you read it on a multimeter? Id rather learn how, than to take it to a tech. HELP!!

[KT66]

I know you don't want to hear this, but what you should really do is take the amp to a tech. The reason that I say this is because setting the bias requires taking measurements in the power section of the amp where if you make a mistake you could seriously hurt yourself or damage expensive components ( do you know how much a new OT costs for that amp!) .

What you could do is have octal sockets put in in place of the novars and use 7591s or 6L6s and then you would be able to check the bias safely with your bias meter.

If you are determined to check and set the bias yourself, the first thing to do is to measure what the bias voltage is. From your description it sounds as if the 7868s you have in there now are running too hot. This means that the bias voltage is not negative enough. This amp : http://www.schematicheaven.com/hifiamps ... chb35a.pdf has a pair of 7868s at a plate voltage probably around what is in your amp and the schem shows -23 volts. so, your amp should probably be in the same range. To measure this in your amp, set your meter to DC, hook the negative lead to the chassis, and put the positive probe on the junction of the reversed diode and the 100U capacitor. The amp should be on for this. Put the hand that you are not using to hold the positive probe behind your back for safety reasons. If this sounds confusing to you, then please don't do it, call a tech. If you do get a bias voltage reading, post it and I will suggest what to do next.

I wouldn't have even suggested that much, but I know many people are determined to do it themselves. If you post this questions on enough forums someone is going to give you a detailed description of how to measure the plate voltage and check the bias with the transformer shunt method, not really thinking that you could ruin your amp or kill yourself in the process.

Also, if you learn too much about amps, you may become a junkie and will find yourself looking at mailboxes in Home Depot thinking " Hey, that would make a good chassis for the 8 X 6V6 5E3 variant I've been wanting to make"

[edadmartin]

Ryan thank you for the help kudos to you, really! I love old school electronics,and the only way to learn this old stuff by talking and doing. Ill be very careful.Ive used the one handed method and even told my son the same tip.I haven't got a mail box amp like yours and Im jealous now !!.I have taken a AE-hammond amp chassis and with the help of the cool cats at 18watt.com I made what I call a marshall 18 watt lite, didnt quite get the 18 watts out of the Hammond transformers though ,more like 14 watts .My son gets all my projects,He does all the AV and all the demos at, www.proguitarshop.com ,check his site for very cool pedal amp and guitar demos. A couple years back I made him a jtm 45 clone with kt-66 tubes from metro amp, it is a head and 2 x12. I made the cabinets to match the earlier 18 watter with brown tooled leather ish tolex.Im a guitar and amp junkie for sure. Ive just never biased an ampeg sb 12 fliptop amp,which is my current project.Im # 112 in the portaflex amp club,cool guys at that ampeg site. Ill come back here and post my biasing experience after Christmas Heres a pic of the jtm 45 and the goldtop I made in 2002.enjoy

[KT66]

I don't actually have a mailbox amp, though I have seen one. My wife does have a cake pan though that would be good for a HiWatt build...

Anyway, some of the things that you wrote lead me to believe that you are kind of green at this stuff. For example, the hum balance pot is for adjusting the balance of the heater ground reference, it has nothing to do with the bias. Also, I don't know how you arrived at the 14W figure for your 18W, because that is a kind of technical measurement to perform, so you are probably mistaken.

If you are going to build and repair tube amps, you really should have a more solid understanding of how basic systems work, like power tube bias. here is a link to an article by Lord Valve about how to check the bias in your amp : http://www.duncanamps.com/technical/lvbias.html. Use the cathode resistor method, it's easier and safer.

Be sure to read the "TYPES OF (FIXED) BIAS CIRCUITS" section, as it lays out how to change your bias circuit so that it has an adjustable pot. The resistors in the bias circuit in your amp are probably different than those shown in the schematic posted for the 6L6 version of your amp, so be sure to measure the one you are going to sub with a resistor and pot.

To learn more about amps, see this thread : http://ampgarage.com/forum/viewtopic.php?t=9023.

[edadmartin]

ahhh! yes . Green as Molsen beer and willing to learn from the ones who know !
Time to go read thanks

[Sonny ReVerb]

Mmmm, beer... [H.S.]

[edadmartin]

the duncan amp link is bad FYI

[edadmartin]

it googles the same , from outside the forum it works fine Anyway I got it

[tictac]

Unless your experience with electronics, soldering, using a multimeter and soldering you have no business inside a tube amp unless you want to kill yourself. There are lethal voltages present!

Reading the negative voltage is not a good way to set the bias, it doesn't tell you what the tube is doing and if you've got red plates then you're already in an over biased condition so your -B is too low.

The Duncan website has a nice bias calculator you should download and play with. If you plug in the plate voltage, for an example let's use 450V then plug in 19W (for a 7868/7591) you'll notice the optimum ma per tube is going to be around 30 ma. The safest way to read current for each tube is to life the cathode ground and solder a 1 ohm resistor between cathode and ground. When you set you DMM to Milli-volts and read the voltage across the resistor volt is converted to amps so you get the approximate ma draw per tube. With this method you are also reading screen current which for a 7868/7591 could be as high as 4ma. but 30ma across the 1 ohm will get you in the ballpark.

So you need to read the plate voltage with a DMM then you can determine the proper current setting with the Duncan calculator.

TT

[edadmartin]

Thanks everyone. seems we all are concerned about high voltage me to Iam gonna learn how to do this and safely. Your experience and info is appreciated

[KT66]

Unless your experience with electronics, soldering, using a multimeter and soldering you have no business inside a tube amp unless you want to kill yourself. There are lethal voltages present!

I think the building an 18W lite and a JTM45 probably covers those bases.

Reading the negative voltage is not a good way to set the bias, it doesn't tell you what the tube is doing and if you've got red plates then you're already in an over biased condition so your -B is too low.

it is true that using the bias voltage to bias an amp is not the best method ; However, I think it is a good place to start for a newbie - It involves measurements away from the power tube socket, helps to learn how to trace out the schematic in the actual amp, and does provide some useful information. From Ed's description, his tubes are redplating. That means the bias voltage needs to be more negative. As long as it is not so negative for crossover distortion to occur, and not so positive that the tubes are exceeding the maximum rated dissipation, a usable bias point can be found by observing the tubes and listening to the amp. Furthermore, I posted the Bogen amp as a reference as to what the ballpark bias voltage should be, because it can give an indication that something is wrong if the bias voltage differs substantially from similar operating conditions in an amp with the same tubes.

If you plug in the plate voltage, for an example let's use 450V then plug in 19W (for a 7868/7591) you'll notice the optimum ma per tube is going to be around 30 ma.

This assumes that the optimum bias is 70% of maximum dissipation, there is no optimum, it's a matter of taste as long as the 2 conditions I mentioned above are met.

Here's the working link for the bias article I referenced above :
http://www.duncanamps.com/technical/lvbias.html

In the article, L.V. mentions that you should use 1% tolerance resistors, which is important to get the most accurate reading with the cathode resistor method. He also has a nice little lethal-voltages-can-kill-you type warning at the top in case you haven't heard that before.

The tube data sheet for the 7868 http://www.nj7p.org/Tube4.php?tube=7868 shows that the idle screen current draw can be as high as 7.2 ma at a plate voltage of 450, so this means that if you are shooting for some magic bias number, say 70%, and ignore the screen current, you are actually biasing the amp at about 55% of max dissipation, according to Duncan's anode load calculator. If you want real accuracy, then you could measure the voltage drop across the screen resistor at idle and use Ohm's Law (Votage = Current X Resistance) to find the actual screen current draw( of course, the amp in question has no screen-grid resistor, so unless you install them or put a 1 Ohm resistor in the screen supply to measure this which sounds like a PITA waste of time to me, then you will just have to guess). Anyway, it seems better to me if you want to have a safety margin to take the screen current into consideration, and then shoot for a lower percentage of max. dissipation, at least that way you know where you are at.

The purpose of my original post was to try and give a novice a safe way to begin, because I think most people who post his question are going to attempt to do something.

Oh, and BTW, there are lethal voltages in a tube amp, so be careful.