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Monoprice 15 watt tube amp problems
Monoprice 15 watt tube amp problems
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Old 16th May 2018, 09:04 PM   #11
Printer2 is offline Printer2  Canada
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The screen resistors go to the high voltage supply. Look at the schematic, it says 300V on one side of the resistors and 290V on the other side which goes to the screens.



Quote:
If power tubes were plugged into those sockets when the amp was turned on then shorted within a few seconds, then turned off with the wrong resistors. Is it safe to say I should get a new pair of matched power tubes?

What was shorted for a few seconds?The tubes, the power supply, the resistors?



Does any of the above tell me to get a new pair of tubes? No, I have no idea what is going on without more information.
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Old 16th May 2018, 09:18 PM   #12
Reptile93274 is offline Reptile93274  United States
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Quote:
Originally Posted by Printer2 View Post
The screen resistors go to the high voltage supply. Look at the schematic, it says 300V on one side of the resistors and 290V on the other side which goes to the screens.






What was shorted for a few seconds?The tubes, the power supply, the resistors?



Does any of the above tell me to get a new pair of tubes? No, I have no idea what is going on without more information.
Thanks for the reply. The amp still sits as it did as described in the first post of this thread. Its still waiting for the new parts. I am still studying up on the repairs and I noticed the R2 & R12 went to pin 9 on the circuit board V5 power tube socket.

And I was just confirming that the two resistors I had in question were in fact the screen resistors. Thats all. And I was wondering what kinda damage could it do with the wrong screen resistors installed, as some were in this poor lil guy.


Also I am thinking that the replacement screen resistors should be at least 350v units huh. Anyone know an easy place to find these guys as replacements? I am going to hunt high n low on google.

Last edited by Reptile93274; 16th May 2018 at 09:38 PM.
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Old 16th May 2018, 10:10 PM   #13
Reptile93274 is offline Reptile93274  United States
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Can and should I run a pair of 500v 1k 1 watt fusible resistors for the screen resistors R2 & R12?

Reason I ask is the resistors I ordered were only rated at 250v. And the pair listed on the schematic are surrounded by 290 & 300 volts.

and the stock resistors measured out to be 1/2 a watt.
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Old 16th May 2018, 11:48 PM   #14
Reptile93274 is offline Reptile93274  United States
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I was thinking of getting these vishay resistors: CMF601K0000FKBF64
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Old 16th May 2018, 11:49 PM   #15
PRR is offline PRR  United States
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Quote:
Originally Posted by Reptile93274 View Post
...I ordered were only rated at 250v. And the pair listed on the schematic are surrounded by 290 & 300 volts.
...and the stock resistors measured out to be 1/2 a watt.
Yes you have 300V one end and 290V on the other end. The resistor FEELS just ten Volts! Bird on a wire. It is fine until it puts its other foot on ground.

The plan clearly says 1W FP. Were they really 1/2W from the factory? Then you have a case where a prototype was Safety Tested with /!\ parts which would not start a fire..... then in production they changed to cheaper parts, safety be danged! This happens. This is not a Good Sign.

These should be sturdy but not too sturdy resistors. The normal strain is small, well under 1W. But a common failure is power tube G2 short to Cathode. "Infinite" current wants to flow. I have seen a power transformer burn-up from this. With some resistance and ample but not excessive dissipation rating (and preferably no flame), a shorted tube is a minor repair instead of a major rebuild or a panicked crowd.

Last edited by PRR; 16th May 2018 at 11:53 PM.
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Old 16th May 2018, 11:54 PM   #16
Reptile93274 is offline Reptile93274  United States
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Quote:
Originally Posted by PRR View Post
Yes you have 300V one end and 290V on the other end. The resistor FEELS just ten Volts! Bird on a wire. It is fine until it puts its other foot on ground.

The plan clearly says 1W FP. Were they really 1/2W from the factory? Then you have a case where a prototype was Safety Tested with /!\ parts which would not start a fire..... then in production they changed to cheaper parts, safety be danged! This happens. This is not a Good Sign.

These should be sturdy but not too sturdy resistors. The normal strain is small, well under 1W. But a common failure is power tube G2 short to Cathode. "Infinite" current wants to flow. I have seen a power transformer burn-up from this. With some resistance and ample but not excessive dissipation rating (and preferably no flame), a shorted tube is a minor repair instead of a major rebuild or a panicked crowd.
Where on that schematic does it say 1w FP? What I am seeing is 1K FP. On R2 & R12?
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Old 16th May 2018, 11:57 PM   #17
PRR is offline PRR  United States
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You are right.
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Old 17th May 2018, 12:00 AM   #18
Reptile93274 is offline Reptile93274  United States
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Thanks for your replies. I am just wondering if a pair of 1 watt 1k fusible flameproof 500v resistors is a better idea as replacements instead of the
half watt 1k 250v fusible flameproof resistors I was thinking of using.
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Old 17th May 2018, 12:37 AM   #19
Printer2 is offline Printer2  Canada
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Quote:
Originally Posted by Reptile93274 View Post
Thanks for your replies. I am just wondering if a pair of 1 watt 1k fusible flameproof 500v resistors is a better idea as replacements instead of the
half watt 1k 250v fusible flameproof resistors I was thinking of using.



Let's start again. PRR has said that you have 300V on one side of the resistor and 290V on the other. So there is only 10V across the resistor. (actually I said that also) You seem to be hung up on the voltage rating. If there is 1000V at one end of the resistor and 990V at the other end there is still only 10V across the resistor.



That is one hint that I am concerned about troubleshooting the amp at a distance. The other thing that worried me was your saying the resistor goes from the screen to ground. I thought either you are not reading the schematic or do not comprehend how things are hooked up.



As far as the resistor you selected, the Digi-Key web page says that the resistors are out of stock and there is a 14 week lead time. That is the normal wait time but it seems that there is a part shortage going on and it may be longer. It also says that the standard package is 100, so you will be paying $3.0795 per unit and a total price of $307.95. The good news is that Digi-key will probably offer free shipping if you order this part. Probably could get a new amp for less than $307.95.



This one may be a better selection at $0.83 a piece in quantities of one. Vishay Dale CPF11K0000FKEE6



CPF11K0000FKEE6 Vishay Dale | Resistors | DigiKey


They will probably ding you for shipping though. It would be good to know what the health of the amp is otherwise, if you are going to do multiple purchases from them shipping charges are going to hurt. Oh wait can you see, (never mind, my mind has a different sense of humour) these prices are in CND, your price may be less.



Or if you are looking for 1/2W there is the PPC1.0KBCT-ND.


NFR25H0001001JR500 Vishay BC Components | Resistors | DigiKey


I don't know if they want the resistor to burn up fast or not, my money would be on the 1W though. Since you have an amp that seems to like eating them I might get more than two.
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Old 17th May 2018, 12:50 AM   #20
Reptile93274 is offline Reptile93274  United States
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Quote:
Originally Posted by Printer2 View Post
Let's start again. PRR has said that you have 300V on one side of the resistor and 290V on the other. So there is only 10V across the resistor. (actually I said that also) You seem to be hung up on the voltage rating. If there is 1000V at one end of the resistor and 990V at the other end there is still only 10V across the resistor.


That is one hint that I am concerned about troubleshooting the amp at a distance. The other thing that worried me was your saying the resistor goes from the screen to ground. I thought either you are not reading the schematic or do not comprehend how things are hooked up.

I understand the volts of the R2 & R12 now lol, thanks for clearing that up for me.

And I did misslook at my board, the first two leads from R2 & R12 go to the pin 9 and the other two leads go to a pin of the largest filter cap.
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