Here is a schematic of my B15NC amp project. It is based on the Ampeg B15NC schematic but reflects changes I have made to the circuit.
I finally got around to addressing the high voltages I was showing in my amp. After considering a few potential fixes (some internal and some external) I decided on a relatively cheap and simple internal solution involving the use of Zener diodes (1N5347BG’s) in the PT secondary center tap lead to reduce the B+ voltage.
Since I had room at the end of my turret board, I opted to use five 10v, 5w Zener diodes to drop the voltage. I used 1/16″ slugs of copper, 3/4″ square, as heat sinks for each diode. I get a voltage drop of 38.4v across the string of Zeners, which has brought my other voltages much closer to where they should be. Here is a chart showing the voltages I was measuring before and after the fix:
Okay, it’s been so long since I’ve posted an update it’s taken me awhile to remember how I was doing it! The strong hum issue with the amp has been fixed. Someone much more observant than I pointed out an apparent problem with my tube socket wiring. According to the schematic, pins 8 of V4 and V5 should be tied together. If you look at pin 8 of V4 in the following photograph you will notice that something is missing:
Since correcting the problem the amp runs much quieter! I’m glad the problem was discovered but I also feel kind of stupid. I supposedly did thorough double-checks of my wiring and yet I still managed to miss something this obvious!
I’m happy with how the amp sounds now, but I still plan to correct some lead dress issues with my AC lines feeding from the power cord socket.
I finished the wiring on the amp just before Thanksgiving, but haven’t had time to update the blog, due to other pressing projects. I was able to fire up the amp briefly to try it out.
Everything seems to be working but there are some issues I will need to troubleshoot once I have time to work on this again. There is a constant, low-level hum that I need to track down. I’ve done a quick check of the wiring by moving leads around with a chop stick, with no effect. I’ll need to check further but I’m hoping that at least means that my leads are run okay. I’ll also try different tubes, and double check my solder joints and component values and see where that takes me.
I also need to do something about the voltages in the circuit – everything seems to be running much too high. I’m assuming this is due to my choice of power transformer. I’ll probably deal with this problem first, on the outside chance that it’s contributing to my hum problem.
I’ve made quite a bit of progress in the last couple of days and I feel like I can see the light at the end of the tunnel. I installed the turret board in the chassis and I finished the wiring on the supply caps, pre-amp, phase inverter, tone controls, ext. amp jack and speaker jacks. I am using a 1/4″ jack for the main speaker out, so I spent extra time with the schematic to make sure I understood what I was doing. The typical standby protection will not be there, since I’m using a 1/4″ jack, so I’ve added a 10W, 470 ohm resistor across the 16 ohm output, similar to what Ampeg did on later B15N’s.
I believe all that remains is the input jacks, volume controls, and the last connection of the standby circuit. Before I proceed, I’m going to go back through the amp and double check the wiring I’ve done so far. I’ve tried to do checks as I completed each stage but a recheck certainly can’t hurt.
Well, it’s been awhile but I have finally made a little more progress on the wiring. Besides taking time off to work on a couple of other projects, I also had to get more Teflon wire. While I had plenty of wire here to do this project, I did not have much of a variety of colors. As I got deeper into this project I began to realize that there was a logic (duh!) to the wire colors used in the original amps. In hindsight this seems rather obvious, but as I’ve mentioned before, this is all new to me, and sometimes there are details just staring me in the face that I initially don’t catch. That is the major point of this project though, so I am continuing to learn.
I am trying to get as much wiring done to the board as I can before I install it in the chassis. I am trying to only do wires that make sense right now. Others, such as some of the control leads, I think will be easier to do after the board is installed.
The “font” used for the Ampeg script logos and Portaflex amplifiers (as shown below) was designed by Henry Konarsky. According to Jess Oliver, Mr. Konarsky was “a good friend, artist and guitar player. He also did several of our catalogs and we kept him in amplifiers“.
Here is a scan of a Portaflex cutsheet. One page explains the operation and benefits of the Portaflex design. The second page shows a flowchart for troubleshooting musical instrument amplifiers.
I’ve been trying to find good information to post in this “misc.” category and I thought that I would share these photos. They are of the modified toaster oven that Jess Oliver uses when repotting single transformers. If I thought I was going to be repotting many transformers in the future I think I would try to find an old toaster oven and do the same. It would be better than having to clean up my real oven if I ever dripped some of the melted compound (again)!
I forgot I had this – it’s a 1965 B15NF schematic that I redrew. It’s pretty much the same as the 1966 version I posted previously. However, I did notice some differences with the speaker cable wiring, as well as the Treble pot on channel 1 being shown as 4 Meg, instead of 1 Meg. That seems odd to me, but who knows? There may be other subtle differences that I didn’t catch. I’ve included a photo of the preamp section of the schematic that I used as a reference.