Common PC board questions

Just when I had the SE amp board ready for launch, Digi-Key discontinued the line of resistors that I was using. I have replaced the smaller ones with metal film resistors and the larger ones with a different line of resistors. This required re – shooting most of the pictures in the resistors section of the manual. This is almost done. I am in the process of setting up my internet account for payment via PayPal. This can be done by credit card without a PayPal account. When these things are complete, boards will be available.

I have received lots of email lately. much of it related to the PC board. I have answered each one individually, but in the process I have discovered several common questions. These comments are intended to answer some of these questions:

1) The board contains TWO complete amplifier channels, and ONE common power supply. One board is needed along with three transformers and a suitable enclosure to build a complete stereo amplifier.

2) With this board you can build an amplifier that is optimized for 45’s OR 2A3’s OR 300B’s. Since these tubes have different characteristics, it is not possible to build ONE amplifier that is COMPLETELY OPTIMIZED for all three tube types. The primary differences between the requirements of these tubes is load impedance and supply voltage. This means that the transformer set is chosen for each tube. It is possible to build ONE amplifier that will work well with all three tube types. To do this a compromise must be made. I choose a transformer set that supplies the output tubes with about 310 to 320 volts. This is above the recommended maximum for 45’s and 2A3’s. I have built 5 amps with this configuration and ran about 20 different tubes through all of them, without issue. I have not tried any globe 45’s. 320 volts is a bit low for a 300B and you will give up a couple of watts on 300B’s compared to an amp optimized for 300B’s. If you are uncomfortable with these compromises, build your amp for your favorite tube type. There is a chart in the manual that outlines all of the choices.

3) Unfortunately 50’s will not work in this amplifier without modification. They require 7.5 volts for the filament, which is above the maximum for the filament regulator IC. Unlike tubes silicon really does not like being fed too much voltage. It would be possible to use an external power supply if you really wanted to use 50’s, but these tubes are out of my price range, so I have not tried them.

4) I know that the price of 5842’s has gone up several fold since I started using them. I used to get them for $3 to $4 USD each. Several people have e-mailed asking if there is a suitable substitute. Unfortunately in this board there is no substitute other than the WE417 which is usually even more expensive. I have been searching for a different tube that is just as linear, with good gain, for use in my next design, and I have not found it yet.

5) Several people have asked me to put the construction manual on the web so they can determine their ability to build this amp. I can not do this since it is over 50 megs in size, and it would use up my bandwidth allotment in a few days. Web hosting plans are like US cell phone plans, you get charged by the megabyte (minute) when you exceed your plans allotment. The difference is that I must guess how many megabytes YOU are going to look at for a year in advance, and plan accordingly. This also limits how much NEW stuff that I can add each month, since the new stuff gets the most page views. I will put SELECTED parts of the manual on the site in the next few weeks, but I must process EACH picture first to reduce its size since I have been near my bandwidth limit for the last two months.

6) This board can be used as a driver board for a large SE amplifier using 845’s, 211’s, 833A’s or other large tubes. There are two ways to do this. The board can be used without modification by substituting interstage transformers for the output transformers. The interstage transformers then drive the large tubes. A separate power supply is needed for the large tubes. The other way is to use PowerDrive. This requires a PowerDrive board to drive the large tubes. I have been convinced of the virtues of PowerDrive since I discovered it, but others have started e-mailing me, telling me how¬† it did wonders for their amp. The Power Drive board is not ready yet, but you could build it on perf board, or point to point wiring.