OK. Even with no rectifier tube installed, you should still be able to power up the amp and check the heaters and the B- supply. B- uses the FREDs. WRT to the B+ winding on the transformer...maybe you are using a bias tap as the center tap? Are there any extra wires that you are not using? If so, check for resistance between any extras and one of the B+ wires. If you measure some low resistance, try using it as your center tap when measuring the B+ winding (do this without the PCB attached).
Russ
Russ
mpilley22 said:
He is talking about the heater winding, not the high voltage winding for the B+ and B- supplies. Nothing will work without the high voltage center tap and since you are reading uneven voltages between the two side, it sounds like you might be using a bias tap for the center tap.
It seems like you have multiple problems in your amp. Lets look at the filament problem first. Connect the red-yellow wire. I am not sure what may happen with it disconnected. Remove the rectifier tube (5AR4) to disable the B+.
Connect a voltmeter to the filament pins on one of the 300B sockets (Yes it is the fat pins). Insert the 5842 tubes, power up the amp. Do the 5842's light up? Do you still have 4.97 volts. Turn the amp off and insert one 300B, power up again. Does it light up? Do you get 4.97 volts? Does the voltage remain or dissapear after a few seconds? Repeat the test with both 300B's. Do you have 4.97 volts with the tubes in the sockets? Does the tube still light up for a few seconds then go out. If so the filament regulator IC is probably overheating and shutting down. 300B's usually require some extra heat hink to be added to the regulator IC especially if your transformer is generous with filament voltage.
I have needed to add some extra heat sinking to the filament regulator on most 300B builds and a considerable sized heat sink if the air circulation is poor.
It may help if you take a photo of your board and post it. It is hard to troubleshoot what we can not see.
Connect a voltmeter to the filament pins on one of the 300B sockets (Yes it is the fat pins). Insert the 5842 tubes, power up the amp. Do the 5842's light up? Do you still have 4.97 volts. Turn the amp off and insert one 300B, power up again. Does it light up? Do you get 4.97 volts? Does the voltage remain or dissapear after a few seconds? Repeat the test with both 300B's. Do you have 4.97 volts with the tubes in the sockets? Does the tube still light up for a few seconds then go out. If so the filament regulator IC is probably overheating and shutting down. 300B's usually require some extra heat hink to be added to the regulator IC especially if your transformer is generous with filament voltage.
I have needed to add some extra heat sinking to the filament regulator on most 300B builds and a considerable sized heat sink if the air circulation is poor.
It may help if you take a photo of your board and post it. It is hard to troubleshoot what we can not see.
Yeah, that totally sounds like the filament regulator is overheating. It has built-in thermal protection and will shut down until power is removed. That little heat sink can't dissipate all the heat generated by supplying the current requirements of the 300B. Mine would shut down after a minute or two. For 45s, it works fine. To move on with testing, all I did was clamp my Squangle (an adjustable aluminum square) to the heat sink and then it worked fine. There is only a few volts on the heatsink, so you don't have to worry about it. Just don't ever touch the CCS heatsink!
As Russ stated you need to add some heat sinking to the regulator. The regulator is likely shutting down from heat. After the amp shuts down you will find the regulator too hot to touch. You may be able to test the amp with a strong fan blowing on the existing heat sink.
I usually use heat sinks collected from old computers the ones from 486's and Pentium 1's work great. One can be seen inside the Lexan amp on my home page. It is the green thing behind the 5AR4. I will post a better picture to this thread tonight or tomorrow.
I usually use heat sinks collected from old computers the ones from 486's and Pentium 1's work great. One can be seen inside the Lexan amp on my home page. It is the green thing behind the 5AR4. I will post a better picture to this thread tonight or tomorrow.
If you can set the bias on the output tubes to the correct value it is OK.
Every tube is different, if the amp sounds good at 165 leave it there. If it doesn't, I can tell you which resistor to change, but I am at work now and I don't have access to the schematic, so I don't know the part #.
mpilley22 said:
I now have 14 Raytheon 5842s and I can say that they are all over the place. The 4 RCAs I have are all pretty close. Even with the tweak in the cathode resistance, it's not always enough to make them match-up. Honestly I can't hear any difference when it is right-on or 10 or 20 volts off, but YMMV.