London Power ad

[-]
Search the Forum








(Advanced Search)

Bogen CHB-50 Rebuild
#21
Hi Guys

With a traditional CTed plate winding on a PT designed to support a tube amp, there are two options when using an integrated bridge rectifier.

The first option is basic and the AC terminals of the bridge merely tie to the ends of the winding. The bridge has to be rated for the full winding voltage. For example, if the winding is 300-0-300Vac, that is 600Vac total which produces a peak voltage of 848V. The rated voltage is at full load and the regulation rating for the PT will suggest what the unloaded voltage will be, where lower-VA devices have worse regulation and their unloaded voltage will be higher. Say it is 20%, then the unloaded peak is now just over 1kV - 1,060V - so the bridge must be rated for >1kV.

In this example, because the CT is tied to ground, the transformer produces +/-420Vdc (loaded; up to 470Vdc unloaded0, once filter caps are added, with the usual bleeder resistors to ground from each DC output. Of course, you are unlikely to need such a high negative voltage although you now have full-wave pulsating DC to generate a bias voltage from.

Further with this same example, if there is a bias tap on the winding, it will still look like its AC value, usually around 50Vac, and can be half-wave rectified for bias or other uses.

In a cathode-biased amp, the negative end of the bridge simply ties to ground through a bleeder resistor to protect the bridge. No cap.

The alternate bridge wiring would be for the bridge to span from the CT to one end of the winding. The free winding end is insulated and stored. The DC output of the bridge is handled in the usual way with a cap and bleeder resistor. In the case of a plate supply, the negative end of this supply ties to ground. Bias should be derived from a separate PT if needed, or the winding and bridge can be rewired as above.

With regard to an aux winding or PT generating bias: There is no worry about which voltage rises faster. None of the voltages used in typical guitar amps are high enough to cause cathode stripping, as TUTs state, so this is a non-worry. Even were B+ to rise ahead of C- the tubes are not warmed enough for current to flow. Usually the supplies rise at about the same rate despite their seemingly different impedances.

The reverse-wired auxiliary PT for bias is usually low-VA which results in a voltage loss of twice the regulation rating. Tiny PTs are a compromise of design and even when used with forward connections, the loss is significant and certain adaptations are made to get useful throughput of energy. The reverse connection is more lossy and is very useful in some cases to achieve a more controlled output - another topic. In the bias supply context, it has sufficient output and remaining VA capacity to do the job well.

I have expanded on this and made two new threads in the Power Supply Design section.
Reply
#22
(06-25-2024, 10:20 AM)K O'Connor Wrote: Hi Guys

With a traditional CTed plate winding on a PT designed to support a tube amp, there are two options when using an integrated bridge rectifier.

The first option is basic and the AC terminals of the bridge merely tie to the ends of the winding. The bridge has to be rated for the full winding voltage. For example, if the winding is 300-0-300Vac, that is 600Vac total which produces a peak voltage of 848V. The rated voltage is at full load and the regulation rating for the PT will suggest what the unloaded voltage will be, where lower-VA devices have worse regulation and their unloaded voltage will be higher. Say it is 20%, then the unloaded peak is now just over 1kV - 1,060V - so the bridge must be rated for >1kV.

In this example, because the CT is tied to ground, the transformer produces +/-420Vdc (loaded; up to 470Vdc unloaded0, once filter caps are added, with the usual bleeder resistors to ground from each DC output. Of course, you are unlikely to need such a high negative voltage although you now have full-wave pulsating DC to generate a bias voltage from.

Further with this same example, if there is a bias tap on the winding, it will still look like its AC value, usually around 50Vac, and can be half-wave rectified for bias or other uses.

In a cathode-biased amp, the negative end of the bridge simply ties to ground through a bleeder resistor to protect the bridge. No cap.

The alternate bridge wiring would be for the bridge to span from the CT to one end of the winding. The free winding end is insulated and stored. The DC output of the bridge is handled in the usual way with a cap and bleeder resistor. In the case of a plate supply, the negative end of this supply ties to ground. Bias should be derived from a separate PT if needed, or the winding and bridge can be rewired as above.

With regard to an aux winding or PT generating bias: There is no worry about which voltage rises faster. None of the voltages used in typical guitar amps are high enough to cause cathode stripping, as TUTs state, so this is a non-worry. Even were B+ to rise ahead of C- the tubes are not warmed enough for current to flow. Usually the supplies rise at about the same rate despite their seemingly different impedances.

The reverse-wired auxiliary PT for bias is usually low-VA which results in a voltage loss of twice the regulation rating. Tiny PTs are a compromise of design and even when used with forward connections, the loss is significant and certain adaptations are made to get useful throughput of energy. The reverse connection is more lossy and is very useful in some cases to achieve a more controlled output - another topic. In the bias supply context, it has sufficient output and remaining VA capacity to do the job well.

I have expanded on this and made two new threads in the Power Supply Design section.

Thanks for the responses here and in the other mentioned section! After reading your response I realized that I forgot to factor in the rest of the RC network for the plate supply, the bits for the screen, PI, and the pre-amp nodes. I added those to the simulation (I've been using https://falstad.com/circuit/ to double check calculations and expectations, to make sure I'm roughly on target, to the extent that the simulation model is), and indeed the rise time for the plate supply ended up being about the same as the bias supply despite the plate node in isolation having a much shorter time constant. I assume the rest of the network outside the plate supply is why you said they rise at about the rate same usually? If so it would seem like another case of me simplifying more than I should.

My next focus with the amp is the bias set pots. I originally had some 250k pots I was planning on using, with some capacitors to drop from the wipers to ground to make a low-impedance AC ground. I had grabbed 250k because I thought that I saw 220k pots used like this in one of the TUTs as an example of modifying a bias supply, but I'm having a hard time finding that figure again. Maybe I misread. Anyhow, doing the math now, that configuration would give me an RC of a couple seconds for the caps on the wipers, which seems out of whack, so I'm working out the details for using lower value pots, like what I see in the TUTs, in the ballpark of 25k. Currently sorting through the output impedance of the bias supply and grid circuit resistance, I'll create a thread in the TUT FAQ section for my question related to that.

Getting close to a complete power supply!
Reply


Forum Jump:

[-]
Come in where it's warm!
A warm welcome to tube amp modding fans and those interested in hi-fi audio! Readers of Kevin O'Connor's The Ultimate Tone (TUT) book series form a part of our population. Kevin O'Connor is the creator of the popular Power Scaling methodology for amplifiers.
Please remember these three principles: respect, sharing, community.
Not familiar with The Ultimate Tone book series? See discussion topics, or click here to visit London Power/Power Press Publishing.

[-]
Tube Amp Forum Hosted by London Power
London Power logo