Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: nandrewjackson on November 01, 2019, 07:58:29 pm
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A friend brought me a carvin nomad 30 watt with el84s.
It's a 2 sided PC board, very nicely made, good wiring, sockets, etc.
The first 2 (of 5) 12ax7s have DC heaters. One of the diodes for the heaters supply is a few mm away from V3 (12ax7) anode supply DC (r29). It had arced at some point and carbonized the board, and took out the diodes and R41. I pulled diodes and R41, cleaned the carbon and top green layer off the board with diamond tip dremel bit. New components in, and voila' , up and running , AND motorboating put put put put .
I've pulled all the electrolytics and tested them, they test better than my new ones laying around here. I put new ones in anyway. Still motorboating. That's about when I noticed plate voltage is 455 and screen voltage is 445. And that's a wee bit high, in general, for el84s, right?
It's designed this way, as per the carvin schematic, the numbers are right on.
Any thoughts from the board?
I'm thinking new tubes, the ones in it are old, who knows how old. There are 2 types of tubes in it, one pair of sovtek el84s and one pair, I'm not sure, chinese? If I pull the chinese and leave the sovtek in, motorboating is less pronounced. From my google research, the sovtek el84M would be the highest voltage rated el84.
Thanks in advance for replies.
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Are you saying you measured that EL84 plate voltage? …………… OR that the schematic says that?
I believe Carvin did have an EL34 amp with similar plate and screen voltages.
With respect, Tubenit
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Yes, measured voltages, 455 plate supply, 445 screen supply. All tubes installed, measured voltages.
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455V is awful "brave" for EL84, even the 7189-rated ones. You may have trouble with these tubes.
But that's not your motoboating.
The trouble-chart suggests that H2B (connection from driver to power-tubes board) could get swapped 1 and 2. (They should have made that impossible with keys or big arrows.)
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The h2A/B connector has keyed clip sockets, can't have it backwards. I'll try to make it backwards and see what happens though . Ha
I'll get some more specific voltage readings and post them asap
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Measured voltages at the pin
Phase inverter
Pin 1, 215
Pin 3, 7mV
Pin 6, 212
Pin 8, 7mV
El84s voltages at the pins (+/- 2v each)
Pin 2, -27
Pin 7, 440
Pin 9, 420
HOWEVER, while getting these voltages I bumped the 8 wire ribbon going to fx loop / rear panel presence control and motorboating stopped. Grabbed a guitar and played one handed while moving ribbon, can turn on/off motorboating by moving ribbon up or down.
Sheesh
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Phase inverter
Pin 1, 215
Pin 3, 7mV <----- way wrong!
Pin 6, 212
Pin 8, 7mV <----- way wrong!
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Yeah I thought the voltage readings in mV looked wrong. I think my DMM battery is low, reading was jumping around, kept settling on 7mV. Took reading again later and said 40-ish volts , I cant remember the exact number.