Biasing a Crate VC50

[baddog]

How do i check the bias on this amp ,their is no cathode bias resistor . I noted V6 is alittle brighter than the rest of the el84s and V5 is a little dimmer .

thanks
keith

[Enzo]

The brightness of the heaters has absolutely nothing to do with bias. The only part of the heater that matters is the part inside where you can't see anyway. If the heater pokes out a little farther on one tube compared top another, it doesn't much matter.

Unless I am thinking of a different amp, these are not adjustable.

To measure the current, you either need to add a sensing resistor or use the shunt method across the output transformer. or get a bias probe.

There should be a spsot on the circuit board where the large power resistor would have gone if they had made it cathode biased. (Other models that ARE cathode biased use this same circuit board) SOmewhere nearby should be a jum,per wire that takes teh place of that resistor. Replace the wire jumper with a 1 ohm resistor, then use that resistor for current readings. it will read for all four tubes, so divide by four for per-tube current.

[baddog]

biasing a crate vc50

Thanks Enzo thats what i thought but wasn't sure. I changed the v2 tube to a 5751 and v4 to a 12at7 will my voltage reading change at test points 13 & 14 because of the tube swap? If i do voltage DC voltage checks on my amp should i put the 12ax7s back in it or doesnt it make a differance?


thanks
keith

[hasserl]

Hey Baddog, there's another method you can use to get the current draw of all four tubes together that requires no resistors to be added. Simply measure the current draw across the standby switch with the amp turned on and at idle. That will give you the current draw of the entire amp. Then measure the voltage drop across R52 and divide that by the value of the resistor (470), that will give you the current draw of the power tube screens and all the preamp tubes. Subtract the result of that from the total current draw and you'll have the current draw of just the four power tubes. Quick and easy.

To measure the current across the standby switch I always connect one lead of the DMM to one side of the switch with a clip-on adapter, so it's hands free, then turn the amp on but leave it in standby and let the tubes warm up, then touch the second lead of the DMM to the other side of the switch in effect bypassing the switch with the meter. I actually use clip-on type adapters for both leads, but I attach just the adapter to the second side of the switch before turning the amp on, then once the amp is turned on and warmed up I just slip the prode into the adapter. This keeps me from slipping with the probe while making my measurements shorting the circuit to the chassis resulting in a blown fuse. Also, I don't attach both leads to the standby switch then turn on the power as the current inrush will typically blow the fuse in the DMM. That may sound a little more difficult than it actually is. But once you do it you'll how easy it is.

[baddog]

Thanks i will check it out tonight and let you know what i find.

baddog

[baddog]

biasing a crate VC

The voltage across the standby switch is 400v with a 10v drop across the 470 ohm resistor . I also was doing a voltage check on the amp at the test points in the schematic and found som interesting stuff. Test points 13 & 14 had 288v and should have 215v & 209v . I am also getting a lot of transformer hum in the clean channel (channel A) . I dont now if this a result of installing the carbon composit resistors i am prety sure it was there befor . Is there somthing i can check or do to quiet the hum other than crank the amp up.


thanks

[hasserl]

Note: you should be measuring current across the Standby switch, not voltage, and for that amp it should be somewhere around 90 to 100ma.

[baddog]

biasing a crate vc

soory ! I wasnt using the meter wright . I will test it again tonight .

[baddog]

biasing a crate vc

I used the DC ma setting on my meter and read 150ma across the standby switch. so with a 10 v drop across the 470 resistor that should give me a current draw of 103 . does that sound right?

[hasserl]

What's the plate voltage? Power dissipated is the sum of volts x the current. 103ma would be fine for 326 volts or less on the plates.

[baddog]

I messured pin 7 to ground on 2 of the el84s and got 374v and on the other 2 i had 377v . I have been doung some DC voltage checks in my amp and all of the readings are high.What could be causing this problem or is it nothing to worry about?

thanks
keith

[hasserl]

At 377 volts I'd say the tubes are running a bit hot. I would cool down that bias a bit if I were you. Note that the plate voltage will go up as you reduce the current, so you've got to remeasure it as you make adjustments.

Check your line voltage and see what it is, if it is higher than what the schematic shows that would explain the higher voltages you see.

[baddog]

By line voltage you mean the 120V coming from the power cord? Im assuming that is what you mean.

[hasserl]

By line voltage you mean the 120V coming from the power cord? Im assuming that is what you mean.

Yep, what is the actual measured voltage. It usually is not right at 120, in fact I never see 120 around here. It's always low, sometimes down to around 114 on hot days when everyone's running AC. Check it when your checking your voltages so you have a reference. Don't just assume it's always the same, or even that it's 120. In some places it's a lot higher.

[baddog]

biasing a crate VC50

The voltage is at 120v coming from the outlet.