Traynor YBA-3 - not very loud, bias testing

[beedoola]

I got a YBA3 not too long ago. It doesn't have as much volume as I've read people saying it is supposed to have, and I'm wondering if it has an issue that I, and the seller was not aware or, or choose not to disclose to me.

I also have an Ampeg V4 running 6l6GCs and it is way louder than the YBA3.

The YBA3 has 6AC7 tubes in it. Apparently it was modded/worked on by Skip Simmons. I opened it up awhile ago and saw an internal bias pot.

Could a low bias cause less output? I also wanted to make sure all the tubes were getting power and working as they should.

What bias method is best? - I have a bias probe that I can use to test the MA of the tubes; what is a good MA range for 6AC7? I've read 40-50MA.

Is there another method of testing that is better? - DMM probe on each side of the screen resistors?

[52 Bill]

Could a low bias cause less output? I also wanted to make sure all the tubes were getting power and working as they should.

Incorrectly set bias can cause a loss of power, but your problem could be any number of things.

What bias method is best? - I have a bias probe that I can use to test the MA of the tubes; what is a good MA range for 6AC7? I've read 40-50MA.

The correct way to bias an amp is to set the idle dissipation of the power tubes in watts. So you need to at least know the voltage applied to the plates of the output tubes as well as the current being drawn to set it correctly. A suggested value of 40-50mA may or may not be correct for your amp.

[Jazz P Bass]

Give us an idea of the output power (P) of the amp.

Hook up two test leads to the speaker terminals, attach a DMM set to read Volts/ac, crank everything up & wail away.

Now what is the Vac reading (V) and the dc resistance (R) of the speaker.
P= VxV / R

[loudthud]

The YBA-3 usually has a B+ around 560V so bias current needs to be 30 to 35mA. Output power is around 130W. Many of the early examples do not have a bias adjustment pot although some have a phase inverter balance pot. Some schematics suggest setting the bias to get a certain Voltage drop across a common screen droping resistor. Since this amp has been modified, no telling what it might have without some pics of the guts. Most of these amps have some strange looking MOVs across the primary of the output transformer. They look like capacitors in light blue cardboard tubes. They are usually blown and should be removed.

[g1]

According to this schematic, it has series output jacks. So you need to make sure the switching part of the jacks are doing their job.
Some cleaning/exercise of the speaker jacks may help.

[Jazz P Bass]

dead link.

[g1]

Thanks. Fixed link.

[beedoola]

According to this schematic, it has series output jacks. So you need to make sure the switching part of the jacks are doing their job.
Some cleaning/exercise of the speaker jacks may help.

I think you suggested in another thread I posted of making a dummy jack for the ext jack. I did that and I wasn't getting any output from the amp so I unplugged it.

I'll open the amp tomorrow and clean the ext jack with some Deoxit and I'll read the plate voltages. I mentioned it was serviced and there is an adjustable bias pot added inside the chassis.

[g1]

If your dummy plug was shorted from tip to sleeve, and you got no output, then likely the output jacks have been rewired for parallel arrangement. In that case the jacks are just one less thing to worry about.

[beedoola]

If your dummy plug was shorted from tip to sleeve, and you got no output, then likely the output jacks have been rewired for parallel arrangement. In that case the jacks are just one less thing to worry about.

gotcha.

[jastacey]

The YBA -3 bias is done as follows, find the 10 watt resistor (R30) it will be either 470 ohms or 1000 ohms, measure the voltage across that resistor, ( if the resister is 470 ohms, you want the reading to be -4 volts and if it is the 1000 ohm resister you want the drop to measure/read -8 volts ) the bias scheme in the YBA-3 is very different from other amps that use EL-34 tubes, another cause of low volume is the surge protectors ( they are a tubular looking device, there's 2 of them, and it is at the end of the tag board by the 2 large power supply caps, they have been known to cause problems, like low volume, there was a service letter on them and basically they said to remove them

[beedoola]

The YBA -3 bias is done as follows, find the 10 watt resistor (R30) it will be either 470 ohms or 1000 ohms, measure the voltage across that resistor, ( if the resister is 470 ohms, you want the reading to be -4 volts and if it is the 1000 ohm resister you want the drop to measure/read -8 volts ) the bias scheme in the YBA-3 is very different from other amps that use EL-34 tubes, another cause of low volume is the surge protectors ( they are a tubular looking device, there's 2 of them, and it is at the end of the tag board by the 2 large power supply caps, they have been known to cause problems, like low volume, there was a service letter on them and basically they said to remove them

Are you taking about measuring the resistor at idle or with Standby ON - not in operating mode.

I'll open it up later and take a look and take pictures.

[g1]

Any bias measurement/adjustment must be done in "operate" mode, not on standby.
The screen supply resistor is just one way of checking bias. Traynor did it that way for this model only, they no longer use that method. Other methods can also be used, but this way is quite simple.

[beedoola]

Any bias measurement/adjustment must be done in "operate" mode, not on standby.
The screen supply resistor is just one way of checking bias. Traynor did it that way for this model only, they no longer use that method. Other methods can also be used, but this way is quite simple.

Do I used the formula of dissipation x wattage of tube/ plate voltage?

70% dissipation. 25W usually for EL34 but should I use a higher water? EH says then can go up to 30W. Playing it safe, I'll use 25.

so: .7x 25w/plate voltage = MA.

I have a DMM that reads MA, is that the best way to do it?

[g1]

Yes, play it safe and consider it a 25W tube. Loudthud worked out the numbers in post #4, 30mA is around 70% if plate voltage is about 560V.
Using mA range on DMM required breaking the circuit and placing the meter in series.
Someone may have installed resistors between the power tube cathodes and ground. Usually they are 1 ohm, this is the easiest way to check bias, you just read the mV across the resistor. Did you have a bias measurement method you were planning on using?
Also measure the voltage across R30 as mentioned in post #11. This is a quick simple check that will give us a rough idea whether the bias is in the ballpark.