Help biasing Carvin TS100

[grunge782]

I was not sure if this is the right place to ask questions like this, but this is the only site I could seem to find that might help me.

My Carvin TS100 previously had Tungsol 6l6 tubes biased by a friend, but I want to change out those tubes and put in the original groove tube el34's I had with it (yes I did switch the bias switches to the el34 setting).

I had asked here detailing what I did, but have not gotten any replies/help-

http://carvinguitars.com/bbs/viewtopic. ... 298#271298

What I want to know is if I am properly setting up my multimeter to bias the amp (what probe to what wire/pcb part). The manual says-

a) Remove from the printed circuit board the red wires connecting from the output transformers to QC2 and QC16.
b) Two milliamp meters will be needed. Insert a milliamp meter in series with each of the red wires to the printed circuit board. Current can now be measured through both of the output transformers while no input signal is present. Make sure the leads don’t touch anything such as the chassis, each other, or you- VERY HIGH VOLTAGES ARE PRESENT!
d) Make sure the bias select switch is in the correct position for the tube type and the meters are set to “mA” or milliamps.
e) Power up the amplifier – switch the stand-by switch on. Adjust the bias pots accordingly to obtain a 50mA reading on each of the milliamp meters. Leave the amp on for a few minutes making sure the readings don’t change. Turn the amp off, leaving the standby switch on and let the residual high volt- age bleed down. Remove the milliamp meters from the series connection and re-attach the red wires directly to QC2 and QC16 again. The amp is now correctly biased.

When I tried with the amp on standby I noticed I was getting zero reading on the DCA 200m setting to measure the current. I then found out the fuse in the multimeter was bad. I am not sure if this was from the current being too high through the tubes.

I am wondering how to set the bias pots safely (cold I guess?) first so that I don't run the risk of frying another fuse from too much current trying to set the bias.

Sorry for the super long first post. Also if this is the wrong place to post something like this, if someone could direct me to the proper place to ask these questions.

[selloutrr]

http://www.duncanamps.com/technical/lvbias.html

this should help.

[Firestorm]

It won't hurt you read Lord Valve's take on bias, but the most important thing to read is right from the Carvin manual:

"Biasing should be left to a qualified technician due to the fact that lethal
voltages near 500 volts are present inside the amplifier."

Actually, the current will kill you long before the voltage has a chance to do any damage.

If the friend who biased the amp for 6L6s is no longer available, find a tech or a music store that can do it for you.

You can't do it accurately with one meter; you can't do it accurately with cheap meters; you can't do it all if you try to measure current with the amp in standby; and you shouldn't do it yourself if you aren't 100% sure you know what you are doing. Those red wires are just about the most dangerous part of the amp.

Sorry to sound like your mom. I've biased amps this way hundreds of times and it still scares me.

[grunge782]

It won't hurt you read Lord Valve's take on bias, but the most important thing to read is right from the Carvin manual:

"Biasing should be left to a qualified technician due to the fact that lethal
voltages near 500 volts are present inside the amplifier."

Actually, the current will kill you long before the voltage has a chance to do any damage.

If the friend who biased the amp for 6L6s is no longer available, find a tech or a music store that can do it for you.

You can't do it accurately with one meter; you can't do it accurately with cheap meters; you can't do it all if you try to measure current with the amp in standby; and you shouldn't do it yourself if you aren't 100% sure you know what you are doing. Those red wires are just about the most dangerous part of the amp.

Sorry to sound like your mom. I've biased amps this way hundreds of times and it still scares me.

I appreciate your concern for my safety, but I would like to know how to bias it for myself. The only place near me that can do it has broken 2 of my other amps and I am never going there again.

There is a proper way to bias this amplifier and I'm sure with the right precautions and methods I could do it safely. Biasing is not rocket science, I've seen people do it in under 10 minutes.

I actually bought this because I had seen the comment about using 2 multimeters at the Carvin website-

http://www.amp-head.com/product_info.ph ... ucts_id=33

I can't use the probe for the Carvin because the Channel 2 powertubes are so close to a pcb board that it won't fit, but I can use the probe for my Traynor.

http://www.duncanamps.com/technical/lvbias.html

this should help.

Also thank you for this link.

EDIT: Firestorm, after a little reading I see your point and I will be reading more before I try actually biasing the amplifier.

However, is there a way with the amplifier off to set the bias trim pots safely first for the tubes? I would only being turning the bias trim pots and not touching anything else. (being careful not to touch the filter caps)

I don't want to fry the tubes.

[Firestorm]

Okay, you've been warned.

You need two meters. Otherwise changing the bias setting on each pair of tubes will load down the power supply and change the operating conditions of the other pair of tubes.

Unplug the amplifier and put the standby switch in the play position to drain energy stored in the filter caps. Wait a few minutes. Install your EL34s. Read your DMM manuals to see how to set them up to measure DC current: Good meters (like Flukes) require you connect the leads differently; cheap meters may not. Set the meter to measure current in the range you expect to see: more than 50mA; the 200mA setting will work.

With the amp still unplugged and standby still in the play position, disconnect the two red wires from the spade connectors on the PCB. Do not apply too much force or you may break the solder joints. Mark the leads somehow to remember which goes where.

Follow the Carvin instructions about connecting the meters in series between each red lead and its corresponding connector on the PCB. You will have to figure out how to connect the leads to these two points. You may need to buy new leads. If the female part of the spade connectors are not insulated, you can use insulated alligator clip leads on the male and female parts. If the female part is insulated, you will need to insert an adaptor of some sort into it. The pointy tip of a test lead may fit, but do not deform the spade connectors.

DO NOT ATTEMPT TO HOLD THE TEST LEADS IN YOUR HANDS. You want to connect the leads so they stay put, not touching anything else, all by themselves.

Wear rubber-soled shoes. Do not stand on concrete.

Put the standby switch into standby and make sure the power switch is off. Turn the controls all the way down. Plug the amp in and put one hand in your pocket. LEAVE IT THERE. Turn the power on. After about a minute, flip the standby switch to the play position. Read the current on each meter. Depending on how the leads are connected, this may be positive or negative. It doesn't matter, just look at the number. If it is something other than 50mA, use an INSULATED (plastic) adjustment tool to set the bias pots until each meter reads 50mA. Wait awhile to make sure the bias doesn't drift.

When done, turn the amp off, but leave the standy in the play position. Unplug the amp. Wait a few mintes. You can take your hand out of your pocket now. Reconnect the red leads to the PCB.

Reassemble. Done.

[Firestorm]

However, is there a way with the amplifier off to set the bias trim pots safely first for the tubes? I would only being turning the bias trim pots and not touching anything else. (being careful not to touch the filter caps)

I don't want to fry the tubes.

Afraid not. Carvin seems to want 25mA per tube regardless of whether it's EL34 or 6L6 so it shouldn't be that far off unless your friend screwed up. You're running with no signal so it wouild take a major mis-setting to fry a tube. Probably not even possible unless the trim pots go all the way to ground.

[xtian]

In other news: remember the power capacitors can hold a lethal charge long after the amp is off and unplugged.

[Firestorm]

Fortunately, this amp does have bleeder resistors on the caps.

[grunge782]

Okay, you've been warned.

You need two meters. Otherwise changing the bias setting on each pair of tubes will load down the power supply and change the operating conditions of the other pair of tubes.

Unplug the amplifier and put the standby switch in the play position to drain energy stored in the filter caps. Wait a few minutes. Install your EL34s. Read your DMM manuals to see how to set them up to measure DC current: Good meters (like Flukes) require you connect the leads differently; cheap meters may not. Set the meter to measure current in the range you expect to see: more than 50mA; the 200mA setting will work.

With the amp still unplugged and standby still in the play position, disconnect the two red wires from the spade connectors on the PCB. Do not apply too much force or you may break the solder joints. Mark the leads somehow to remember which goes where.

Follow the Carvin instructions about connecting the meters in series between each red lead and its corresponding connector on the PCB. You will have to figure out how to connect the leads to these two points. You may need to buy new leads. If the female part of the spade connectors are not insulated, you can use insulated alligator clip leads on the male and female parts. If the female part is insulated, you will need to insert an adaptor of some sort into it. The pointy tip of a test lead may fit, but do not deform the spade connectors.

DO NOT ATTEMPT TO HOLD THE TEST LEADS IN YOUR HANDS. You want to connect the leads so they stay put, not touching anything else, all by themselves.

Wear rubber-soled shoes. Do not stand on concrete.

Put the standby switch into standby and make sure the power switch is off. Turn the controls all the way down. Plug the amp in and put one hand in your pocket. LEAVE IT THERE. Turn the power on. After about a minute, flip the standby switch to the play position. Read the current on each meter. Depending on how the leads are connected, this may be positive or negative. It doesn't matter, just look at the number. If it is something other than 50mA, use an INSULATED (plastic) adjustment tool to set the bias pots until each meter reads 50mA. Wait awhile to make sure the bias doesn't drift.

When done, turn the amp off, but leave the standy in the play position. Unplug the amp. Wait a few mintes. You can take your hand out of your pocket now. Reconnect the red leads to the PCB.

Reassemble. Done.

Thank you so very much for your help. It seems rarer these days to get it.
I will be reading some more about how amplifiers work and biasing before I try this. This seems to be a good site-
http://www.aikenamps.com/

I do have 2 questions though.

1. I have wired speakers before in series, however I do not know how exactly that would work with a DMM and a PCB. Would it be red wire(previously connected to q2) connected to red lead of DMM1, then black lead of DMM1 connected to Q2 PCB? Then the same hookup for between DMM2 and Q16? Or is that incorrect?

2. When you say insulated spade connectors do you mean these kind-
[img:140:123]http://thumbs3.ebaystatic.com/m/mkNeEml ... hQ/140.jpg[/img]
Because it is not like the one above so I am guessing I could use alligator clips.
[/img]

[Firestorm]

Without seeing the amp it's just a guess, but the red wires are probably the center taps of the output transformers. So the DMMs positive lead (red) goes to the PCB and the negative lead (black) goes to the wire. If you connect it backwards the only thing that will happen is the current will read negative.

The question about connectors is whether the one on the end of the red wire is insulated or not. If the connector is bare metal, alligator clips would work fine. The male/spade part sticking out of the PCB will always be bare metal.

Be careful.

[grunge782]

Without seeing the amp it's just a guess, but the red wires are probably the center taps of the output transformers. So the DMMs positive lead (red) goes to the PCB and the negative lead (black) goes to the wire. If you connect it backwards the only thing that will happen is the current will read negative.

The question about connectors is whether the one on the end of the red wire is insulated or not. If the connector is bare metal, alligator clips would work fine. The male/spade part sticking out of the PCB will always be bare metal.

Be careful.

Thanks again for explaining that.

The female spade connector from the output transformer/red wire looks like this-
[img:483:361]http://www.binbin.net/photos/generic/mi ... emale-.jpg[/img]

There are 3 different kind of spade connectors I found online, which are completely bare metal, the one above and the one I posted earlier. So would an alligator clip be okay to use with the one above or no?

[Firestorm]

Yeah. You just need metal to clip onto. Some connectors sit entirely in a plastic sleeve.