Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: guitarsan221 on December 31, 2016, 02:45:12 am
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Hello Guys, New to this forum. I hope you had a good Christmas, looking forward to the new year. I recently purchased Princeton reverb kit. I just finished soldering all the components, I used a 200 watt lamp limiter to power up the amp. I smoked the 100 ohm/3watt resistor (bias tap). Turned it off, went back and started checking what could have caused it. I had a problem following the layout provided when it came to connecting the can capacitor 40/20/20/20 to the board and the rectifier. Specifically rectifier pins 6,2 ? I know one goes to the 40uF and the other to the bias range resistor. I'm not clear on which goes to which. So if someone can please give a hand, I would greatly appreciate it, Thanks.http://el34world.com/Forum/Smileys/default/dontknow.gif (http://el34world.com/Forum/Smileys/default/dontknow.gif)
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When uncertain about a fenderish clone, it is useful to look at the schematic/layout of the original.
http://el34world.com/charts/Schematics/files/fender/Fender_Princeton-Reverb-AA1164-layout.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_Princeton-Reverb-AA1164-layout.pdf)
With respect, Tubenit
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I smoked the 100 ohm/3watt resistor (bias tap).
That resistor should be 100,000Ω, not 100Ω. If you used a 100Ω then that's why it smoked. While it was smoking, D1, R65 (bias pot), R66 (22K), and C32 (47µF) were being severely stressed and may also need replacing. Remove the 6V6 tubes and don't put them back in until the bias circuit is healthy. To check that it is healthy, measure the voltage on pin 5 of each 6V6 socket. It should be approx. -30vdc to -40vdc. Set this voltage to the maximum using the bias pot.
I had a problem following the layout provided when it came to connecting the can capacitor 40/20/20/20 to the board and the rectifier. Specifically rectifier pins 6,2 ? I know one goes to the 40uF and the other to the bias range resistor. I'm not clear on which goes to which.
The cap can only connects to the board. It does not connect to the rectifier socket. Connect 4 wires from the cap can to the board points A, B, C, and D. Use the attached pic. Connect the negative terminal of the cap can to a lug at a power transformer bolt.
The rectifier pin 2 only connects to a yellow wire from the PT. It connects to nothing else. The rectifier pin 8 has the other yellow wire from the PT and it also has a wire that connects to the STBY switch. The other side of the STBY switch connects to the board point A. Point A will have two wires connected, one to the 40µF, the other to the STBY switch.
Connect a wire from the rectifier pin 6 to the board at the turret called "Bias Tap".
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Sluckey,
Your post made me notice something that I've never caught before. Ironically, the original Princeton Reverb schematic shows the rectifier pin 2 connecting to B+ rail (which is incorrect) . Where the Princeton Reverb layout shows rectifier pin 8 connecting to the B+ rail (which is correct). (That incongruency is more typical of old Gibson schematic errors then Fender, IMO)
The original Deluxe Reverb layout also confirms that pin 8 is correct.
http://el34world.com/charts/Schematics/files/fender/Fender_deluxe_reverb_ab763_schem.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_deluxe_reverb_ab763_schem.pdf)
guitarsan221, follow Sluckey's advice. He is remarkably knowledgeable & has helped me many times when I've made an error or gotten stuck trying to resolve some issue on an amp build.
With respect, Tubenit
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"Ironically, the original Princeton Reverb schematic shows the rectifier pin 2 connecting to B+ rail (which is incorrect) . Where the Princeton Reverb layout shows rectifier pin 8 connecting to the B+ rail (which is correct)."
OK, pardon the question, but is there an electrical reason for the pin 2/8 distinction, or is connecting pin 8 to B+ the convention or best practice?
By the way, while we are critiquing Fender's schematic, I noticed that it shows the heater center tap being grounded while each lead is also grounded through a 100-ohm resistor. That's redundant, right?
Ed
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OK, pardon the question, but is there an electrical reason for the pin 2/8 distinction, or is connecting pin 8 to B+ the convention or best practice?
For tubes such as 5Y3 or 5U4 the cathode is just a coating on the filament element. Makes no difference in this case. But a 5AR4/GZ34 has a completely separate cathode element. The cathode is indirectly heated. That accounts for the long warm up time of the 5AR4 as compared to a 5U4. But, in order for the 5AR4 to be pin compatible with the 5U4 the cathode element is internally connected to one end of the filament at pin 8. That's the difference. So, in order to connect the cathode directly to the power supply filters you must use pin 8. If you use pin 2, then the B+ must flow through the filament element to reach the cathode element. It will still work either way, but if you use a scope to look at pin 8 and pin 2, you'll see a difference.
So, use pin 8 and always be technically correct. Or use pin 2 and just be happy! :wink:
By the way, while we are critiquing Fender's schematic, I noticed that it shows the heater center tap being grounded while each lead is also grounded through a 100-ohm resistor. That's redundant, right?
Correct
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guitarsan221, if what I posted is still confusing, just look at the pics of Hoffman's PR. Wire yours exactly as shown in the hi-rez pics on this page...
http://el34world.com/Hoffman/Princeton_Reverb_Board_Build.htm (http://el34world.com/Hoffman/Princeton_Reverb_Board_Build.htm)
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Steve:
Ah! Glad I asked. So far I've dealt only with 5U4GB's. I just learned something.
And your explanation seems to confirm my suspicion that there would be some 60 Hz AC riding on the 5U4GB cathode/B+, whereas not on the GZ34/5AR4 pin 8.
Thanks,
Ed
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And your explanation seems to confirm my suspicion that there would be some 60 Hz AC riding on the 5U4GB cathode/B+, whereas not on the GZ34/5AR4 pin 8.
No, I don't think that's it, that's what the filter caps are for.
It's that the heater won't like the stress of having all the B+ dcv/current going through it. It will shorten the tubes life.
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Thanks a million! will do on advise, great help. Now all I have to do is follow it. Thanks again, and have great new year!