ac ripple on screen grids Ultralinear

[Rattler66]

I think that means you have about 450V B+ and 40mA for about 18W plate dissipation per KT66. Using PSUD2 indicates the B+ ripple voltage should be about 12Vpp, or circa 4Vrms, with a 60uF cap and that power transformer secondary and a 5U4G.

Sounds like you have a bad part, or wiring, or your measurement technique is not right.

If your caps can withstand 550VDC, or you have a variac, then you can firstly check the unloaded B+ for DCV and AC ripple, and a steady reading.

I wish it were the wiring but i have built and rebuilt this amp about 4 times and no matter how i wire it it always presents the same problem. My measuring may be the culprit. I'm going to have to ac ground the screens again so i can bias it right then pull the measurements again. I don't think it's a bad part but i could be wrong

[Rattler66]

I think that means you have about 450V B+ and 40mA for about 18W plate dissipation per KT66. Using PSUD2 indicates the B+ ripple voltage should be about 12Vpp, or circa 4Vrms, with a 60uF cap and that power transformer secondary and a 5U4G.

Sounds like you have a bad part, or wiring, or your measurement technique is not right.

If your caps can withstand 550VDC, or you have a variac, then you can firstly check the unloaded B+ for DCV and AC ripple, and a steady reading.

How did you put this into psud. I always get an error when I've tried modeling the power supply

[Rob's Radio-Active]

No, the PT can't cause ripple by being too powerful, but the filter circuit can be too "small" for a given load current. I hope that makes sense.
cheers
Rob

[Rattler66]

No, the PT can't cause ripple by being too powerful, but the filter circuit can be too "small" for a given load current. I hope that makes sense.
cheers
Rob

I'm going to bite the bullet and get the choke ala sunn amps as mentioned

[frus]

It must be doable without it
I have an UL Twin reverb on the bench, it's totally quiet when you set the bias balance and hum balance pots

[Rattler66]

It must be doable without it
I have an UL Twin reverb on the bench, it's totally quiet when you set the bias balance and hum balance pots

You know might as well go that route first. It's much cheaper and easier to swallow if indeed it doesn't work

[g1]

Try the bias balance pot by ear. Due to circuit anomalies minimum hum may be a setting which does not perfectly match when measuring idle current.

[pdf64]

Are you sure that the taps are definitely intended for UL?
eg given the jtm45 / RS Deluxe thing, they may be 8k taps.

[Rattler66]

Are you sure that the taps are definitely intended for UL?
eg given the jtm45 / RS Deluxe thing, they may be 8k taps.

yes according to the wiring diagram that came with the ot from Heyboer it is

[Rattler66]

Here's the schem, don't mind the voltages mine are quite higher due to PT. Can anyone show me how to go about adding a dual bias or bias balance. I would like to keep the bias adjust also. Thanks guys

[trobbins]

Perhaps it is easier to add a 1 ohm current sense resistor in the cathode circuit of each KT66. Your meter should read about 40mV across each resistor (ie. 40mA bias current in that KT66). If you don't have a good meter that will read with 1mV resolution, then use a 10 ohm sense resistor. That reading should be stable - if it isn't then you have other problems to track down first and foremost.

You could clone the existing bias supply circuit so that you have two seperate bias voltage circuits. Split "D" so that each bias circuit feeds just one of the KT66's grid leaks. That is the least risky path to take. Trying to use some of the existing bias supply components to generate two variable bias voltages will change the voltage levels and time constants and bias ripple, and need some assessment and juggling.

[Rattler66]

Perhaps it is easier to add a 1 ohm current sense resistor in the cathode circuit of each KT66. Your meter should read about 40mV across each resistor (ie. 40mA bias current in that KT66). If you don't have a good meter that will read with 1mV resolution, then use a 10 ohm sense resistor. That reading should be stable - if it isn't then you have other problems to track down first and foremost.

You could clone the existing bias supply circuit so that you have two seperate bias voltage circuits. Split "D" so that each bias circuit feeds just one of the KT66's grid leaks. That is the least risky path to take. Trying to use some of the existing bias supply components to generate two variable bias voltages will change the voltage levels and time constants and bias ripple, and need some assessment and juggling.

cool thanks, I just finished a mockup right before I saw your post

also the reading at the cathode (I did install the 1ohm) is steady but the plate voltage isn't, not when the humming starts

[trobbins]

PSUD2 example

[Rattler66]

Thanks i never knew how to set psud up. Cool

[loudthud]

Grounding

The schematic shows the HV CT (the noisiest ground)(1) connecting to the preamp ground (1). Never a good idea. The HV CT should go to (2) where the reservoir cap grounds. If possible the HV CT should go directly to the cap and a separate wire from the cap to the chassis. The (C) node filter cap should ground to the preamp ground (1). The bias supply filter (3) and the power tube cathodes (5) should go to (2) or close by. The ground connection for the heater CT might cause noise in the preamp if grounded there. This depends on parasitic coupling within the transformer. Try it at (1) and (2) and use the one that is lowest in noise. Ground for the output transformer should go the the speaker jacks with a separate wire to the chassis.