bias issue

[daz]

I have this egnater amp that reads zero bias at the test points where it read around 50 before. Has a pair of 6l6 and a pair of el34 with bias test points for each pair. Whats odd is the bias supply seems fine and theres 40+ to 50+ VDC at all the pin 5's and plate and screen voltages are there. But theres nothing at the test points. Those fuses are fine and the test point and all traces are there. Theres no bad solder anywhere. I don't get it....what am i missing? The amp makes sound but it's horrible and distorted (in a bad way) and the notes crap out as they decay. It's unusably bad. Tubes were also swapped with no change.

[g1]

Do you mean -40 to -50 volts DC at the grids? And test points used to measure .05VDC ?

[daz]

Do you mean -40 to -50 volts DC at the grids? And test points used to measure .05VDC ?

Yes, but the test points don't measure anything significant. It will just sort of vary from nothing to maybe a millivolt at most. When i put + signs in back of the grid numbers i only meant 40 and change to 50 and change. Probably around -46 and -54 if i recall, something like that.

[g1]

You measure 1ohm from pin 8 of each tube to ground?

[daz]

Yes, it's got test points with a 1R. Look at the schematic i posted.

Edit: oh...you mean you're asking if the 1R is good? Yes, i did measure them. They are fine. Also, being that neither pairs i measuring anything it would seem unlikely to be anything thats not common to both pairs.

[Jazz P Bass]

Yes but are the 1 ohm resistors good? ie: not open circuit

[g1]

I was also considering a bad ground connection somewhere.
But the sound symptoms kind of sound like real cold bias, like a bias pot open or bad solder somewhere. Are you sure the bias neg. voltage is not much more neg. than it used to be?

[daz]

I was also considering a bad ground connection somewhere.
But the sound symptoms kind of sound like real cold bias, like a bias pot open or bad solder somewhere. Are you sure the bias neg. voltage is not much more neg. than it used to be?

The resistors are not open either by being bad or bad solder. They are inline and measurable even from pins 1/8 to the test point.

My marshall style homebrew is setup the same with 1R resistors and testpoints on the chassis. The pin 5's measure -40VDC and the test points measure around 35 millivolts which is where i set them. This egnater doesn't even measure 1 millivolt at the test points unless i max the pot. More like 1/10 Mv. I've measure this amp when it was working and it measured correctly, around 40-50 Mv at the test points. Then it had this issue where the tone went unusable and thats when there was no measurable bias at the test point but it appears at the grids. Bizarre.

[olddawg]

Have you verified that the other sides of the 1 ohm resistors are connected to ground at your ground reference TP point? Measure continuity with the amp off.

[daz]

Have you verified that the other sides of the 1 ohm resistors are connected to ground at your ground reference TP point? Measure continuity with the amp off.

Yes, thats what i meant by "all traces are there". I checked all pathways with a meter initially and there are no open places anywhere. You also have to figure that when the test points for both pairs are not reading things like that are very very unlikely because both would have to be open.

[daz]

I think i found the issue. I found a forum post somewhere that said the plate voltage is supposed to be 460.....this one shows about 350 ! I thought that seemed low but i know there are some guys like matchless that run el34's real low like that and i thought this amp was possibly one of those. so i never gave it another thought till i found that post in a google search. So Now i have to check the B+ supply.

[daz]

Bad PT? There are sever rectifiers for different legs of the PSU. But I checked back to the rectifier output that supplies the plates of the outputs and it's 350VDC there. I don't see anything else it could be as long as the rectifier diodes are good and they seem to be.

[52 Bill]

Bad PT? There are sever rectifiers for different legs of the PSU. But I checked back to the rectifier output that supplies the plates of the outputs and it's 350VDC there. I don't see anything else it could be as long as the rectifier diodes are good and they seem to be.

Is there a multiple primary setting or dual primary windings? Measure the ac voltage at the heater pins of the tube sockets and see if they are low as well.

[daz]

Is there a multiple primary setting or dual primary windings? Measure the ac voltage at the heater pins of the tube sockets and see if they are low as well.

6.4vac. Thats correct heater voltage correct? Just single primary with taps for diff countries. (and yes, it's set to 117v)

Oddly, now the 6l6 pair is showing test voltage, tho way low. it maxes at 24mv. The el34n pair is maxing at 1 or 2 mv but maxes at 10mv if i put in a new pair. This just happened all of a sudden but after i put the main psu board back in. I have tightened the 8 million connectors so i don't know if that caused it, but if so who only small max mv? And plates are still 100v low. This is just maddening.

[g1]

What is the ac voltage at the HV winding? Can you post the PSU schematic?
What is the ac ripple on the B+ ?
Bad solder at the filter caps could give you weird problems like this, but I'm assuming there is a speaker connected and no bad hum?