DIY Audio Projects Forum

300B SET Design Project
Page 13 of 53

Author:  Suncalc [ 11 Nov 2010, 00:14 ]
Post subject:  Re: 300B SET Design Project

As I intimated previously, I have completed the filament power supply design for the 300Bs. The 300B filament supply has to do several things. First it has to supply two separate supplies of 5vdc@1.2A, one for each output channel. We decided on DC filament supplies for the 300Bs because we are going for a very high fidelity amp. Balanced AC secondaries simply can’t provide the ripple rejection necessary for high fidelity. Secondly, the supply has to maintain at least 70dB of isolation between the two supplies (The standard stereo channel separation specification used in most high fidelity equipment). This is to ensure that channel crosstalk is kept to a minimum. Without this isolation, the channels would bleed together to some extent, and the width of the amp’s perceived soundstage would suffer greatly. The third is that the supplies must not pump “too much” (definition to follow) noise into the signal chain.

The first requirement is easy to meet. We simply design two identical supplies, one for each channel. The second requirement is a little more difficult to meet. If the supplies are passive, then getting 35dB of isolation in each supply is going to be difficult. This implies separate transformers. However a single transformer with dual secondaries may be used if we use an active regulator to provide additional ripple rejection. The third requirement is easy to specify. The supplies must meet the noise requirements of the B+ supply. However, this may be easier said than done.

Working backwards, we’ll deal with the ripple specification first. The ripple from the filament supply is superimposed on the cathode bias voltage; in our case ~71v. Now the ripple specification from the power stage is -90dBv (or ~0.003%). Applying this to the cathode bias voltage yields a ripple spec for the filament supplies of Vr = 71*10^(-90/20) = 2.2mV. Obtaining this ripply level using only rectification and passive filtering is going to be almost impossible. However, using a modern linear (i.e. non-switching) voltage regulator it should be possible to meet this. In addition, most linear regulators provide excellent line regulation such that getting the channel separation should be possible as well. Finally, getting 5v@1.2A out of a modern linear regulator is easy.

So, we have arrived at a set of dc supplies using modern linear regulators, running off of separate secondaries on the same transformer. It’s time to select some components. The first should be what regulator to use. Now one of the things to keep in mind about tube filaments is that when cold, the presented resistance is about 10% of what it is when the filaments are up to temperature. This means that whatever regulator we choose must be able to effectively start up with this very low resistance and must current limit without “crowbar-ing” back to some low voltage state. We also need a regulator designed to operate with a very low input-output voltage differential (i.e. a “low dropout” regulator). This is for two reasons; first, we want to limit the voltage differential to limit the power dissipated by the regulator (to limit thermal design problems) and second, because we will likely be using 6.3vac filament transformers which when rectified and filtered will supply less then 7.9v (i.e. 2.9v max regulator input-output differential). An excellent regulator that meets all of these requirement is the Linear Technologies LT1085 low dropout 3A adjustable linear regulator. (,C1,C1003,C1040,C1055,P1283) Line regulation is better then 70dBv so channel isolation shouldn’t be a problem.

For the rectifier, the ability to attach a heat-sink will be important. The current of 1.2A means that the regulator may well be dissipating 1.2W. Also seeing as how the primary filter cap will be relatively large (~6800µf) the startup surge current will be high and the conduction angle will be small. I settled on the Micro Commercial Components GBU4A. ( This is a 4 amp 50volt bridge rectifier with a 150A surge rating, 1v forward drop, and a case with a heat-sink mounting hole.

Finally, for the transformer, I chose the Hammond 266L12B (or the 266L12). This is a dual secondary 6.3v 2A (2.5 for the 266L12) filament transformer. The 300B heater current load of 1.2A with derating requires at least a 2A secondary. The final circuit is shown in the following figure.
Schematic Fliament Final.png

The circuit includes a trimmer for setting the final filament voltage and the primary can be wired for either 240v or 120v mains (50Hz or 60Hz). The supplies are not grounded except at AC via the 300B cathode bypass capacitors. Also, the positive side of the supply is connected to the same side of the filament as the cathode resistor and bypass capacitor such that the DC supply does not artificially affect the bias on the 300Bs.

There is one more thing to discuss with this supply; the thermal design parameters. Using the stated Vf of the GBU4A from the data sheet we get a power dissipation of 1.2W. Using the bridge Vf we can get a worst case regulator dissipation. The worst case input voltage is Vin=6.3v*sqrt(2)-1=7.9v which gives a power dissipation of (7.9v-5v)*1.2A=3.48W. These “back of the envelope” numbers indicate that a more complete thermal check is in order.

Starting with the GBU4 I have made the following assumptions. Total dissipation is 1.2W at load. I am going to allow a maximum ambient temperature (where the rectifier is located) of 75˚C (167˚F). This is actually a pretty standard design number for vented enclosures without forced air cooling. I am also using a thermal resistance for the case to whatever heat sink used of 1.5˚C/W. This is reasonable for a plastic case to anodized aluminum heat sink using a good quality thermally conductive paste. Finally. I am assuming a junction to ambient thermal resistance of 50˚C/W for the rectifier without a heat-sink. This is also a pretty typical number for plastic cases of this type. The results of the thermal calculations are shown in the following spreadsheet.
Thermal Design GBU4A.png

You’ll notice that with these assumptions, the maximum thermal resistance of a heat sink to ambient is ~38˚C/W. For my prototype board I used a Comair Rotron 822202B00000 heat-sink with a thermal resistance = 13˚C/W. ( This gave almost 25˚C/W margin in my design. And given that it was in open air, this may even be rated as overkill. However, please note that the maximum allowed thermal resistance junction to ambient is 41.67˚C/W which is less then the 50˚C/W without a heat-sink so, even at this load, a heat sink of some type is required.

For the regulator, the design is a little more critical. I used much the same assumptions as made above. However, I calculated the regulator dissipation directly from the circuit parameters assuming an input DC voltage of 7.9v. The thermal design spreadsheet is shown below.
Thermal Design LT1085.png

Here the maximum allowed thermal resistance junction to ambient is only 14.82˚C/W. This means that with the other parameters we need a heat-sink with a thermal resistance to ambient of no more than 10.3˚C/W. In this case for my prototype I chose the rather substantial Aavid Thermalloy 529802B02500G heat-sink with a thermal resistance = 3.7˚C/W. ( This gave me only 6.62˚C/W thermal margin so clearly this heat-sink is a good choice for this application. It should also be noted that if the trimmer is used to lower the 300B filament voltage to less then 5Vdc, the regulator dissipation will go up and the thermal design will have to be revisited.

This is it for the 300B heater design. Although I may also post an alternate design post for those who would like to try the build using a balanced AC heater approach. It’s not quite as clean as the DC heater option, but it is a much simpler circuit to be sure.

Well, this is the last of the circuit design work for the 300B stereo amp. :thumbsup: I hope that people have gleaned some good information from everything posted on this thread. In my next post (probably tomorrow) I’ll post a summary with all the design schematics and the overall specifications for the amp.

Comments anyone?

Author:  mwhouston [ 11 Nov 2010, 03:03 ]
Post subject:  Re: 300B SET Design Project

I think building the amp will be easy compared to sourcing all the bloody parts!!

Author:  Gio [ 11 Nov 2010, 15:45 ]
Post subject:  Re: 300B SET Design Project

Thanks for the octal relay tip! ;)
Suncalc wrote:
Schematic Fliament Final.png

With the higher build quality being requested you could also add Protection Diodes on the regulators. Tanalum caps are typically preferred at the 25uF and 200uF positions.

Author:  Suncalc [ 11 Nov 2010, 17:20 ]
Post subject:  Re: 300B SET Design Project


I actually considered the use of protection diodes. However, the LT1085 data sheet has this to say on the subject:
Protection Diodes

In normal operation, the LT1083 family does not need any protection diodes. Older adjustable regulators required protection diodes between the adjustment pin and the output and from the output to the input to prevent over stressing the die. The internal current paths on the LT1083 adjustment pin are limited by internal resistors. Therefore, even with capacitors on the adjustment pin, no protection diode is needed to ensure device safety under short-circuit conditions.

Diodes between input and output are usually not needed. The internal diode between the input and the output pins of the LT1083 family can handle microsecond surge currents of 50A to 100A. Even with large output capacitances, it is very difficult to get those values of surge currents in normal operations. Only with a high value of output capacitors, such as 1000μF to 5000μF and with the input pin instantaneously shorted to ground, can damage occur. A crowbar circuit at the input of the LT1083 can generate those kinds of currents, and a diode from output to input is then recommended. Normal power supply cycling or even plugging and unplugging in the system will not generate current large enough to do any damage.

Given this information, I decided to leave the protection diodes out of the circuit. As for your suggestion about the capacitors normally being tantalum, in general I would agree. However for the output, I felt that the additional smoothing provided by the larger value capacitor would offset the low ESR of the tantalum capacitor. It might be beneficial however to include a 10µf tantalum in parallel with the 200µf output capacitor. That way we get the best of both worlds; stability and extra smoothing. :up:

Author:  Suncalc [ 12 Nov 2010, 13:46 ]
Post subject:  Re: 300B SET Design Project

As promised, here is the summary post, complete with schematics, for the entire 300B stereo amp.

Peak power out : 7.5W/channel
Input sensitivity for peak output : 5.1Vpeak (3.6Vrms)
Frequency response (-3dB BW) : 20Hz to >20KHz
Distortion at full output : 3.3%
Distortion per watt (Ds) : 0.44%/W
Output impedance : 8Ω
Input impedance : 200kΩ + 69µµf

Amplifier Schematic (2 channel)
Schematic Final Amp.png

Power Supply Schematics (120v and 240v mains versions)
Schematics PS final.png

300B Filament Supply Schematic
Schematic Filament Final 240.png

Thermal Design Note:
The bridge rectifiers need to be heat sinked to a heat sink with a thermal resistance of no more than 39.5˚C/W (including interface layer thermal resistance). I recommend the Comair Rotron 822202B00000 heat-sink with a thermal resistance = 13˚C/W ( The LT1085 rectifiers need to be heat sinked to a heat sink with a thermal resistance of no more than 11.8˚C/W (including interface layer thermal resistance). I recommend the Aavid Thermalloy 529802B02500G heat-sink with a thermal resistance = 3.7˚C/W ( The thermal design is based on a peak ambient temperature of 75˚C at the heat sink. As such, the case containing the filament supplies should be vented but forced air cooling should not be required.

Now I'll be handing this thread off to Mark so he can keep us up to date as the amplifier comes together. I'm really looking forward to seeing how this comes out. :headphones:

Author:  mwhouston [ 12 Nov 2010, 17:44 ]
Post subject:  Re: 300B SET Design Project


I think you wanted to know the DC resistance of the 193M choke. It is a low 60ohms.

The Hammond OPTs, choke and the wrong filament transformer arrived today. In all 30lb of iron. I now have a double hernia after carrying it box to the car..

The OPTs are bigger then the biggest power tranni I have ever used the choke is so big just standing near it has reduced my tinitus!!

Author:  Suncalc [ 12 Nov 2010, 18:18 ]
Post subject:  Re: 300B SET Design Project

Cool! The big iron has arrived! :thumbsup:

The 3Ω delta on the choke shouldn't be an issue.

I would like to know the DC resistance on the primary side of the output transformers (to check the 200Ω assumption I used in the design). I would also like to know the DC resistances of the primary and HV secondary of the power transformer. These values affect the source resistance going into the smoothing filter and hence the DC voltage out. If I have all of these numbers then I can check the power supply design and confirm (or recalculate) the value of the voltage dropping resistor (currently 150Ω 6W in the 240v mains power supply).

Funny you should mention a "double hernia".. I've been at home for the last three days on muscle relaxers and steroids after I threw out my back last weekend. This week I've been asking my wife to move the heavy stuff.

If you get a chance post some pictures of the iron. I'm interested to see how big it all is. Maybe a separate power supply chassis for this amp in a good idea. :lightbuilb: Didn't you mention that you were thinking about going that way?

Author:  mwhouston [ 12 Nov 2010, 20:49 ]
Post subject:  Re: 300B SET Design Project

I was just zapped by the primary of the Hannond OPT. I use an analogue multi-meter. While measuring the primary DC resistance of the Hammond OPT I was holding the meter probes on with my fingers. Of course as I let go the back EMF got me. It was a fair belt. This has to be the first time I have got a shock from something not even plugged in!! Ouch..

Matt: DC resistance of the OPTs is 120ohms. The power tranni is still being wound.

Author:  Gio [ 13 Nov 2010, 12:27 ]
Post subject:  Re: 300B SET Design Project

Ouch. :hot: Remember that a multi-meter puts out about 2 to 3 V when measuring resistance. This could have something to do with getting zapped.
Suncalc wrote:
As promised, here is the summary post, complete with schematics, for the entire 300B stereo amp.

Great. I added a link to the summary post from the first post. Perhaps when this is all together we can consoidate it into a single page on the main site.

Author:  Suncalc [ 13 Nov 2010, 20:32 ]
Post subject:  Re: 300B SET Design Project

Oops, I found another one. :blush: On the filament schematic posted above, I said that the mains supply was 240 (234) but I wired it like it was a 120v main. So here is the corrected schematic with the proper primary side wiring.
Schematic Filament Final 240.png

I hope that it's a simple enough mistake that most people will catch it.

And to keep one other promise which I made. I am going to include here the alternate schematic for the AC 300B filament heaters. When the 300B was introduced, the state of music reproduction was somewhat different then it is today. The 300B was introduced in the late 1930s. At that time it was accepted that there would be some slight hum in the background of most amplifiers. The thought was that the hum/noise level will be many dB down from the audio so it will be hidden in the music. For this reason, the 300B was introduced as a DHT. It really wasn't until later that the unipotential cathodes became standard on most audio tubes.

In the post on the filament supply I calculated the unbalanced ripple level on the cathode of 2.2mV. Frankly getting this level of balance using a "hum pot" is going to be nearly impossible. This is the equivalent of getting a single turn (270˚) pot adjusted to within 0.12˚. If we go to a ten turn potentiometer (3600˚) then this 2.2mV requires getting that adjustment to within about 1.6˚ (still very difficult). So I decided that I'd specify a ten turn pot. On this potentiometer, getting within 16˚ will balance the ripple to about 22mV (-70dBv). So dependent on the skill of the builder, the hum should be able to be cancelled to somewhere between -70dB and -90dB. The circuit is obviously much simpler, but the sonic results will not be quite as good.
Schematic Alternate Heater.png

In theory you could put a single turn potentiometer between two 100Ω 1% resistors and get essentially the same result as the 10 turn potentiometer, but I decided to minimize parts count (That and the 10 turn pot is only $10.25 USD at Allied Electronics Supply).

Gio; Could you put a note on the summary post about the mains wiring mixup? Thanks. ;) - Changes made - 15 Nov 2010 :cop:

Page 13 of 53 All times are UTC - 6 hours [ DST ]
Powered by phpBB® Forum Software © phpBB Group