6505+ bias.

[razzmire]

hey guys, so the 6505+ has a bias pot, and probe spots on the back, though i have read that the ports on the back are useless, can someone explain to me why this is, or why peavey would put them on there if they are useless,and if there is a way, how can i use my multi meter to check with them.

i dont want to get a bias probe, is there a way to check the bias without a probe? chassis out?

im considering doing the bias mod on it, meaning just changing the resistor that effects the sweep of the bias pot, but ive heard of using a 6.8k ohm 1/4 watt, and a also a 5.1k ohmm resistor.
which resistor am i supposed to use? if there is a way to check bias without a probe, what would be a good range that i should stay within?

thanks for all the help guys, razz.

[mylilss]

the test points on the amp arent a good way to get the proper readings. you will want to get a bias probe. i know it isnt what you wanna hear but it is the best way. they are only like 20 bucks and can be connected to any multimeter. they come in handy

the resistor i used for the bias mod was a 6.8k ohm and it gave me a ton of adjustment. i would suggest using this resistor, as it should allow you to make any adjustment needed to get them in spec

[Box]

http://www.ehow.com/video_4467495_use-v ... e-amp.html Were some articles off Google too. "Check tube bias with voltmeter" was what I searched. Peavey wouldn't put ports there for no reason, I'd think. The rest I can't help you with sadly.

[Rock...Master]

Boogers have the same thing. I've cross referenced readings with bias probe.
Completely inaccurate. Why would they do that?

[razzmire]

hmm.i guess i will get the bias probe. any good links?
can i use my analog multimeter? its not digiital.

also, where can i get a resistor without spending 4$ and getting 100 of them. lol
i have a radio shack in a town near me. no electrical stores really. havent been to radio shack yet.

[Box]

hmm.i guess i will get the bias probe. any good links?
can i use my analog multimeter? its not digiital.

also, where can i get a resistor without spending 4$ and getting 100 of them. lol
i have a radio shack in a town near me. no electrical stores really. havent been to radio shack yet.

Don't see why you couldn't use an analogue multi-meter. Could check Radio Shack, not really sure where else you could get a single resistor. Found some on eBay that were 99 cents and free shipping for 50. Hell, spend more in gas trying to get one at Radio Shack... Links below. Could get both for $2 and choose which you like the most.

http://cgi.ebay.com/50-x-Resistors-5K1- ... 1262wt_905

http://cgi.ebay.com/50-x-Resistors-6K8- ... 3cb91b9333

[Rock...Master]

R Shack sells packs of 5 for a buck or 2.
Bias probes are around 20$, maybe a little more.

[Enzo]

The factory test points are not useless, they are there so you can set the bias to FACTORY SPECS. And that spec is 55 volts for 6L6s. What they ARE useless for is setting the bias current to some percentage dissipation, to some particular current. This is fine, but not the factory spec. I know this is how many would rather do it, but when you want to do something other than the factory recommended practice, you will have to modify the amp or buy external equipment.

Bias probe adaptors simply put a resistor in the tube cathode lead, and you measure the voltage across the resistor. It doesn't matter what kind of voltmeter you use. 38 millivolts is 38 millivolts. Assuming your meter can read that small a voltage.

[WashburnTabu]

Hoffman amps.com sells a "bias checker" kit. I bought the two checker kit for $20 some time ago, now they are $30. The single kit is $15. You must assemble the kits but the instructions are pretty easy to read. Some soldering skills are necessary though. Works great by measureing the actual milliamps through pin 8 of an 8 pin socket (9 pin too). Basically it is a ceramic tube socket and a tube base wired and soldered together to piggyback the tube in the original socket. two leads come out to the voltmeter.
Operation: Insert bias checker in amp socket, and insert tube in top of bias checker (if there is room, you may have to remove the chassis). Turn amp on and read milliamps display in meter, adjust as needed.
Near bottom of page. Download instructions and see if you can assemble it.
http://www.hoffmanamps.com/MyStore/perl ... =403527953

[razzmire]

Hoffman amps.com sells a "bias checker" kit. I bought the two checker kit for $20 some time ago, now they are $30. The single kit is $15. You must assemble the kits but the instructions are pretty easy to read. Some soldering skills are necessary though. Works great by measureing the actual milliamps through pin 8 of an 8 pin socket (9 pin too). Basically it is a ceramic tube socket and a tube base wired and soldered together to piggyback the tube in the original socket. two leads come out to the voltmeter.
Operation: Insert bias checker in amp socket, and insert tube in top of bias checker (if there is room, you may have to remove the chassis). Turn amp on and read milliamps display in meter, adjust as needed.
Near bottom of page. Download instructions and see if you can assemble it.
http://www.hoffmanamps.com/MyStore/perl ... =403527953

thanks! that was a good link!

[KTB]

Milli amps and millivolts aren't the same thing. Don't get a probe and go turn that thing to 55ma's or you will toast that amp bigger than dodo. The jack on the back of the Peaveys are for reading the negative grid bias voltage only which they recommend to be -55 vdc.

A bias probe reads voltage across a 1 ohm resistor and turns that into ma current reading as a ratio of 1volt = 1ma so if it reads 35 mv than that equates to 35 ma's however that is not where the negative grid bias voltage may be.

To calculate bias current you have to select a dissipation % in watts you want to run your tubes at. So lets say a safe range of 80 %. So a 6l6GC at 30 watts would be 30 X .80= 24 watts so that's where we want to set out wattage so we take the 24 watts and divide it by the plate voltage which is say 450 volts = 53 ma's. Take note that it is not -53 volts of grid voltage but 53 ma's of tube current and that the negative grid voltage would be probably around -45 to 48 to get that ma reading.

To measure current through the tube you either have to use a bias probe, a current sense resistor from cathode to ground or the shunt method which is dangrous and not recommended.

[purpledc]

I would have to suggest the probe and the digital meter. The meter can be had as cheap as $5 and is much much easier to read than an analog meter. So much in fact that when using some analog meters its pretty difficult to get an exact reading. Its more of an approximation meter IMHO. You can test the amp without a probe but its more dangerous and its not going to give you the exact reading at the tubes. I know spending money sucks but in this case I think its worth it. IMO the test points on the back are for setting to factory spec but little of anything else. The ma at the tube can vary widely with a small amount of voltage change at the test points. Especially if like on my amp one socket seems to get more power than the rest. You could have 3 tubes sitting pretty and the last one could be getting toasty.

[razzmire]

ok. well see if i do the mod. if i dont do the mod ill just leave the pot at max as it is in stock trim.


my eq pedal does wonders. i have it in the loop. its only a 7 but i think i may move up to a higher band one. ebecause if i drop off the highest band totally all fizz is gon but i also lose some of my tone, im thinking a more precise eq would do it. may try one of the rackmount ones we have for our pa and see if i can find what band its on.

[KTB]

I would have to suggest the probe and the digital meter. The meter can be had as cheap as $5 and is much much easier to read than an analog meter. So much in fact that when using some analog meters its pretty difficult to get an exact reading. Its more of an approximation meter IMHO. You can test the amp without a probe but its more dangerous and its not going to give you the exact reading at the tubes. I know spending money sucks but in this case I think its worth it. IMO the test points on the back are for setting to factory spec but little of anything else. The ma at the tube can vary widely with a small amount of voltage change at the test points. Especially if like on my amp one socket seems to get more power than the rest. You could have 3 tubes sitting pretty and the last one could be getting toasty.

Don't know why that happens if it's not the tubes not being an exact quartet. If you switch tubes with one you biased up and it changes than there has to be one parameter that is off and I doubt the screen current is one of them. My Bias probe reads plate voltage,current and power through the display so it's easy to tell if they are the same. Heater voltage could also be the issue as if one tube is emitting more electrons from the Cathode it will run slightly hotter. These things usually turn out to be either a cold solder joint or connection problem at least when using the 6L6GC.
EL-34's do draw quite a bit of screen current so a screen resistor could effect that type of tube.
I would monitor the negative grid bias voltage,plate voltage,wattage and heater voltage and see if there is a difference. Hell doesn't hurt to even monitor the voltage drop across the screen resistor and check the if there is any voltage from Cathode to ground which there shouldn't be with the bias probe out of circuit.