Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: embotone on February 08, 2012, 12:15:35 pm
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I have been working on a Bogen RP60 receiver. Amp section is great, but I have a problem in the tuner section. Things had been screwed up in the receiver by someone in the past, resistors going to the wrong places, jumper wires missing, etc. Everything seems good now except on one tube. I have attached a link to the Bogen manual, with the schematic on page 11. V7, a 6au6 is drawing too much current. I am getting 25.6 volts on pin 7, schematic shows 2.3v. All the voltages in that area are good until I plug in the tube there, then my B+ at that node goes from 160v to about 60 or 65v. The tube tests good, and I have tried other tubes, re-tensioned the tube socket, checked values of all the resistors. Only have the big voltage drop when the tube is plugged in. Of course, I am missing something here, but I don't know what. Is it strange to have the plate voltage on pin 1? The voltage chart on page 9 indicates there should be 0 volts here, impossible the way the schematic is drawn.
Any suggestions here would be appreciated.
http://home.comcast.net/~c.duarte/RP-60%20Operating%20and%20Service%20Manual.pdf (http://home.comcast.net/~c.duarte/RP-60%20Operating%20and%20Service%20Manual.pdf)
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Is there actually a noticeable problem with the actual function of the circuit? N.B.:
* I agree there appears to be a dot between the 2DOT3 volts on the schematic for pin7 of V7. But, you can't trust DOTS (or the lack of a dot!) on old schematics. I.e., maybe it's supposed to be 23V.
* I don't see a plate voltage posted on the schematic for V7. Note that V2a, a triode, and V7, a pentode, seem to both be fed B+ off the same node, ea through a 220R resistor. V2a's plate voltage is 149V, and V7's G2 is 148V (the screen drawing about -0- current at idle, so there's no voltage drop.) So if we assume that the pentode's plate is drawing 2X the current of the V2 triode at idle, then it's plate voltage would be about 75V. Close enough for jazz.
So maybe everything is OK???
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I figured out what was wrong...the schematic. There is no connection at the apparent junction of the 220 and 220k resistors. Not the only error I have found in the schematic. I hope I have found all the errors. For example, cathode and plate are mis-marked on 1/2 of V9. Not the thing to be working on late at night when tired.
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:smiley: