I'm looking to order a custom transformer for a Tubelab SE build.
I'm deviating slightly from the standard build and the forum thread there has been dormant in past few weeks, so posting a thread in the generic forum.
A tube rectified 5AR4 capacitor input supplies the positive plate voltages, and SS diodes for the negative supply. (Both form the HV tap)
Instead of the standard CLC or CRC PS I'm going with a HV Maida-style regulator. The HV reg is connected in place of where generally a choke or resistor would go.
My target voltage is 275V to 45' plate.
Using my temporary power transformer (275-0-275V), I am configuring the reg to output 275V, and that is also what I measure on the tube plate.
In order to spec the power transformer, I have been only accounting for the required headroom the reg needs (15V), including through the valleys. So looking for atleast 290V in the low part of the cycle.
A paragraph in the Bugle 45 amp got me confused:
http://www.wavelengthaudio.com/bugle.pdf
It is written: "The required bias at 275V for 35mA plate current is -55V ... which when added to the 275V plate voltage makes for a total B+requirement of 330V"
I'm not sure if this also applies to the Tubelab, but do I need to account for this as well (so targeting 290+55V instead of 290V)?
Attached is the schematic of the Tubelab, the bugle one can be found in the above link.
Thanks in advance
I'm deviating slightly from the standard build and the forum thread there has been dormant in past few weeks, so posting a thread in the generic forum.
A tube rectified 5AR4 capacitor input supplies the positive plate voltages, and SS diodes for the negative supply. (Both form the HV tap)
Instead of the standard CLC or CRC PS I'm going with a HV Maida-style regulator. The HV reg is connected in place of where generally a choke or resistor would go.
My target voltage is 275V to 45' plate.
Using my temporary power transformer (275-0-275V), I am configuring the reg to output 275V, and that is also what I measure on the tube plate.
In order to spec the power transformer, I have been only accounting for the required headroom the reg needs (15V), including through the valleys. So looking for atleast 290V in the low part of the cycle.
A paragraph in the Bugle 45 amp got me confused:
http://www.wavelengthaudio.com/bugle.pdf
It is written: "The required bias at 275V for 35mA plate current is -55V ... which when added to the 275V plate voltage makes for a total B+requirement of 330V"
I'm not sure if this also applies to the Tubelab, but do I need to account for this as well (so targeting 290+55V instead of 290V)?
Attached is the schematic of the Tubelab, the bugle one can be found in the above link.
Thanks in advance
No you don't need to add the negative voltage to it as well. Your SS diodes will give you a negative voltage equal in magnitude to the positive voltage -so you'll have more than enough negative voltage! You'll be dropping most of it with a resistor, no doubt. The current for the bias supply will presumably be negligible.
That's true for a cathode biased amp, since all the voltage is positive.
Last edited:
Thank you! that makes sense.
Staying on topic of the power transformer, I'm trying to determine the optimal rating for the HV winding.
With the temp PT (275-0-275V @ 144mA)
The B+ draws 100mA (measured at the output of the reg), and total current through one wire of the HV secondary was 115mA.
Is it correct to conclude that the total current of HV winding is 115mA, since it is alternating between one half of the secondary to the other?
Additionally, what would be a good factor of headroom when spec'ing the transformer? Assuming 115-130mA required, should I go for 160mA, 175mA, etc.. ?
P.S. I've been reading through How to design valve guitar amplifiers and it has been very helpful, thanks for creating that resource!
Staying on topic of the power transformer, I'm trying to determine the optimal rating for the HV winding.
With the temp PT (275-0-275V @ 144mA)
The B+ draws 100mA (measured at the output of the reg), and total current through one wire of the HV secondary was 115mA.
Is it correct to conclude that the total current of HV winding is 115mA, since it is alternating between one half of the secondary to the other?
Additionally, what would be a good factor of headroom when spec'ing the transformer? Assuming 115-130mA required, should I go for 160mA, 175mA, etc.. ?
P.S. I've been reading through How to design valve guitar amplifiers and it has been very helpful, thanks for creating that resource!
Hi itsikhefez,
Merlin is absolutely correct. And if you want to read further I can highly recommend his book on tube preamps. It is excellent.
I'm no transformer expert by any stretch, but your B+ current of 150 to 175 mA sounds about right. If I recall correctly, you're looking for cool and silent (no mechanical buzzing). Just be sure your transformer winder builds things in a mechanically, er, "sound" manner in addition to satisfying your electrical requirements.
It's great to see you're experimenting, learning and making connections / parallels. The Bugle schematic is a good one from which to learn the basics, but as I'm sure you're now aware the Bugle 45 and Tubelab SE/SE-II are entirely different beasts. They're both Single-Ended Triode (SET) amps - but that's about where the similarity ends.
The Bugle uses self-bias (aka cathode bias) to set the idle point of the output tubes. This is achieved mainly via the 1k5 10 W resistor from the tube filament to ground (for simplicity's sake ignore the 47R / 50R hum-cancelling network just above it, which is normally found in SETs using raw AC on the filaments). The current through the tube causes a voltage drop through the resistor, which establishes the bias.
Among the advantages of this arrangement are simplicity and safety. There are fewer parts overall in the bias circuit, it is somewhat self-regulating, and it usually fails-safe if the cathode resistor should burn out. One of the disadvantages is that power supply design has to account for the the bias voltage. This has to be added to the tube's nominal B+ to achieve the same power output - which is why the design calls for a 330 V winding on the PT.
The Tubelab design uses "fixed bias" in which the bias voltage is created via a separate supply. In this case it's connected directly to the B+ windings on the PT. One advantage is that this winding can be at the nominal 275 V for the same dissipation / output power. A disadvantage is that an interruption anywhere in the bias circuit can cause the output tubes to go into runaway, possibly destroying the tubes, power and output transformers.
Merlin is absolutely correct. And if you want to read further I can highly recommend his book on tube preamps. It is excellent.
I'm no transformer expert by any stretch, but your B+ current of 150 to 175 mA sounds about right. If I recall correctly, you're looking for cool and silent (no mechanical buzzing). Just be sure your transformer winder builds things in a mechanically, er, "sound" manner in addition to satisfying your electrical requirements.
It's great to see you're experimenting, learning and making connections / parallels. The Bugle schematic is a good one from which to learn the basics, but as I'm sure you're now aware the Bugle 45 and Tubelab SE/SE-II are entirely different beasts. They're both Single-Ended Triode (SET) amps - but that's about where the similarity ends.
The Bugle uses self-bias (aka cathode bias) to set the idle point of the output tubes. This is achieved mainly via the 1k5 10 W resistor from the tube filament to ground (for simplicity's sake ignore the 47R / 50R hum-cancelling network just above it, which is normally found in SETs using raw AC on the filaments). The current through the tube causes a voltage drop through the resistor, which establishes the bias.
Among the advantages of this arrangement are simplicity and safety. There are fewer parts overall in the bias circuit, it is somewhat self-regulating, and it usually fails-safe if the cathode resistor should burn out. One of the disadvantages is that power supply design has to account for the the bias voltage. This has to be added to the tube's nominal B+ to achieve the same power output - which is why the design calls for a 330 V winding on the PT.
The Tubelab design uses "fixed bias" in which the bias voltage is created via a separate supply. In this case it's connected directly to the B+ windings on the PT. One advantage is that this winding can be at the nominal 275 V for the same dissipation / output power. A disadvantage is that an interruption anywhere in the bias circuit can cause the output tubes to go into runaway, possibly destroying the tubes, power and output transformers.
As stated by others, it does not. The Bugle is cathode biased, so that it's negative voltage is developed by the cathode resistor which is in series with the tube, so the B+ must be large enough to develop the bias and power the tube.
The TSE has it's cathodes (filaments) connected directly to ground, and it's negative bias is developed by other means.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.