Traynor YBA-3 - not very loud, bias testing

[beedoola]

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.

I have a DMM with a MA setting and a bias probe. I wanted to see first if the plates were getting the right voltages and that all tubes got the right amount of voltage.

I don't recall the tube sockets having a 1 ohm resistor installed.

[g1]

If you have a bias probe that should be the easiest way. Some need you meter to be set for mV, others need meter set to mA, so check to be sure. Otherwise, you shouldn't need the mA setting on your meter.
Check your DC voltages on power tube pins 3,4, and 5. Measure voltage across R30, check bias with bias probe.
Post results.

[beedoola]

Did you mean measure the voltage on each side of R30, one end of the probe to ground or just both probes on either side of R30?

measure the plate resistors and probe to ground I get 536v.

MA with my bias probe is only 7.0

There is an internal trimpot that I thought was for bias, but I don't think it is. It goes to V3

[jastacey]

Looks like someone has been in that amp, here's a photos of setting the bias on a later model yba-3

P1020379 by jastacey128, on Flickr

[jastacey]

I have another early YBA-3 and I have to dig those photos out ..... seems someone has added a few items and did some handy work

[beedoola]

Looks like someone has been in that amp, here's a photos of setting the bias on a later model yba-3

P1020379 by jastacey128, on Flickr

It looks like I was measuring the wrong resistor, I was measuring the 10W resistor near the caps on the right side of the board.

There doesn't appear to be R30 in my amp...

Anyone know that pot does? Attaches to V3

[jastacey]

Here is a view of the surge protectors (MOV) that need to be clipped out on a later YBA-3

YBA3CustomSpecial14 by jastacey128, on Flickr

Here's a view of them removed

P1020334 by jastacey128, on Flickr

[beedoola]

mine have been removed, it appears.

[jastacey]

I have a early yba-3 and I'll have to open it up and see what it looks like, I seem to remember it had the same bias pot as yours .does your amp have a choke? ( it will have a small transformer between the two larger ones )

[beedoola]

yes, it has a choke.

[g1]

You no longer have R30, so just bias with your bias probe. It is the type that requires you use mA range? Otherwise, set on mA you will likely be in parallel with a 1ohm resistor and getting an incorrect reading.
That pot is not the bias control, it is wired into the phase splitter circuit and is some kind of symmetry adjustment or something.
Your bias is set by the resistor that is seen across the 2 blue caps, which appears to be 47K.
To increase your idle current, you need to decrease the value of that resistor. Alternatively, you could replace that 47K with a 22K in series with a 25K pot.

[beedoola]

You no longer have R30, so just bias with your bias probe. It is the type that requires you use mA range? Otherwise, set on mA you will likely be in parallel with a 1ohm resistor and getting an incorrect reading.
That pot is not the bias control, it is wired into the phase splitter circuit and is some kind of symmetry adjustment or something.
Your bias is set by the resistor that is seen across the 2 blue caps, which appears to be 47K.
To increase your idle current, you need to decrease the value of that resistor. Alternatively, you could replace that 47K with a 22K in series with a 25K pot.

I don't know if my bias probe or my DMM for it is messed up. I have a cheap Cen-tech meter. I also have a Fluke 117 but I didn't know when I bought the probe that they also offer probes that measure MV - which my Fluke can do, but not MA.

I adjusted the internal pot and got a little more volume. What would the purpose of such a mod be?

[loudthud]

I adjusted the internal pot and got a little more volume. What would the purpose of such a mod be?

Like I posted in reply #4, it's a phase inverter balance adjustment. No word anywhere on how it's supposed to be adjusted.

[Jazz P Bass]

I also have a Fluke 117 but I didn't know when I bought the probe that they also offer probes that measure MV - which my Fluke can do, but not MA.

The Fluke 117 can most certainly measure current.
AC or DC.
http://assets.fluke.com/manuals/115C117Cumeng0100.pdf

[Jazz P Bass]

Like I posted in reply #4, it's a phase inverter balance adjustment. No word anywhere on how it's supposed to be adjusted.

Not knowing what the PI balance mod is, I would suspect that it attempts to set the amplitude of each half of thr PI signal to the 'same' height. (if that's even possible in a LTP setup).