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Post subject: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Thu Feb 18, 2016 1:05 am
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I have a Pro Tube Twin dropped off for me. Owner replaced tubes when it started losing power (didn't re-bias, he barely knows how to plug in). It was a little better but shortly lost volume again to the point of silence.

Presented with slight 120 Hz hum (US 60 Hz mains),reduced to a very slight 120 Hz hum after bias set. Will not pass audio even from power amp in jack. No HV found in preamp or PI. R205, open appears to be a 1 watt metal oxide and schematic calls for a 2 watt. Same size as R206 and both match physical size of R203&204 both 1 watt on schematic.

I don't like returns and no way to know if he had a shorted preamp tube overloading R205. I have ordered 2 Watt and 3 watt 10K MOS. No parts supply for almost 200 miles so while I wait, was there an update and this unit was built before a change? I probably should have ordered R66 as it appears the same "under size".

Thanks for your reading a longish post. Search was not productive.


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Thu Feb 18, 2016 1:57 am
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Is R205 in the main supply power rail? If so, think you may have other issues. The 120Hz hum is a sign that at least one of the main filter caps may also be bad.

I'd inspect the cap(s) in the first stage of the main power supply. If the linked schematic is the amp --- I'd check C201 and C202, for sure. Prolly best to looks at all in PSU (C201 through C207).

http://bmamps.com/Schematics/fender/pro_tube.pdf


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Thu Feb 18, 2016 10:32 am
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BMW2002Ti wrote:
Is R205 in the main supply power rail? If so, think you may have other issues. The 120Hz hum is a sign that at least one of the main filter caps may also be bad.

I'd inspect the cap(s) in the first stage of the main power supply. If the linked schematic is the amp --- I'd check C201 and C202, for sure. Prolly best to looks at all in PSU (C201 through C207).

http://bmamps.com/Schematics/fender/pro_tube.pdf


Thank you for the response.

That is the correct schematic and I did check the ESR of C206 and C207 first then all remaining power supply electrolytic caps. The ESR and capacitance was within spec.These are low voltage tests of course. I do plan on powering the circuit past R205 with an external power supply, but it was not (as described to me) a quick failure (such as a shorting cap under high voltage). Instead it was a gradual fading with degrading power and amp response. No fuses have blown.


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sat Feb 20, 2016 3:07 am
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ESR ok did not tell if filter caps are good , it is only one test.

Easiest way , you read AC voltage ( ripple ) on filter caps, less than 0.5 volts ( average) filter caps are good.

I did not read carefully your post, check solder if it is a printed circuit.


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sat Feb 20, 2016 11:00 am
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stratele52 wrote:
ESR ok did not tell if filter caps are good , it is only one test.

Easiest way , you read AC voltage ( ripple ) on filter caps, less than 0.5 volts ( average) filter caps are good.

I did not read carefully your post, check solder if it is a printed circuit.


Thank you. My main concern right now is not with the very slight hum as the is no PI, only the output tubes plates and screens have high voltage. The PCB will be a major pain to remove for access to the foil side, not built with service in mind. I will tack the resistor in to regain plate voltage supplies for all small signal valves to test further. The ESR test I use is an auto program that tests at 3 frequencies and displays capacity, ESR, DF, Q and Delta. My concern is with why the resistor failed. Was it under size from factory, was the wattage rating on the schematic increased after this unit was built or was it just a defective resistor?


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sat Feb 20, 2016 12:15 pm
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Jon A wrote:
[ My concern is with why the resistor failed. Was it under size from factory, was the wattage rating on the schematic increased after this unit was built or was it just a defective resistor?


Short 12XX7 tube(s) ?

With ohms law calculator, read what current goes in this resistor, you'll see if amp is alright.

http://www.ohmslawcalculator.com/ohms-law-calculator


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sat Feb 20, 2016 12:47 pm
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stratele52 wrote:
Jon A wrote:
[ My concern is with why the resistor failed. Was it under size from factory, was the wattage rating on the schematic increased after this unit was built or was it just a defective resistor?


Short 12XX7 tube(s) ?

With ohms law calculator, read what current goes in this resistor, you'll see if amp is alright.

http://www.ohmslawcalculator.com/ohms-law-calculator


You probably missed in my post the original set of tubes was replaced by the owner. I asked about them and he doesn't know where they are so no way to know if it started with shorted tube(s). This is a case of repairs made "harder" by user "repairs".


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sat Feb 20, 2016 1:37 pm
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Jon A wrote:
This is a case of repairs made "harder" by user "repairs".


I really hate broom-and-dustpan jobs where I have to clean up a mess caused by somebody else's ignorance.

:x

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sat Feb 20, 2016 1:55 pm
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Retroverbial wrote:
Jon A wrote:
This is a case of repairs made "harder" by user "repairs".


I really hate broom-and-dustpan jobs where I have to clean up a mess caused by somebody else's ignorance.

:x

Arjay



+100


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sat Feb 20, 2016 2:26 pm
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Retroverbial wrote:
Jon A wrote:
This is a case of repairs made "harder" by user "repairs".


I really hate broom-and-dustpan jobs where I have to clean up a mess caused by somebody else's ignorance.

:x


Arjay


On target! Agreed.

Why is more important than what for me personally. I hoped maybe this was a known issue and a definite fix was available. Without the original tubes it is a WAG. I don't like WAGs (Wild $$$ Guess).


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sun Mar 13, 2016 2:24 pm
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I don't like unresolved threads so here is the outcome.

I believe the resistor stuffed into R205 was an error and was a 1 watt. I tacked in a 2 watt as the numbers did not work well and the 2 watt at full power with a relatively cool room (74 degrees f) was at 75% of its operational temp range with no signal (150 degrees c).

I installed a 3 watt as a final and under the same conditions it ran at 55 degrees c. IMO a combination of assembly error and design error and would not recommend a 2 watt as called out in the schematic.


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sun Mar 13, 2016 4:11 pm
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Let the owner perform some operational field-testing. If you don't hear back from him, you'll know your hypothesis was correct (and I'm betting that it is).

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sun Mar 13, 2016 5:20 pm
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Yeah Jon, I knew you'd get it fixed. Thanks for the update.

I looked at the schematic, R205 was it, a while ago, and in the high pwr mode your wattage calculations are right on. I can see companies trying to save money, but using smaller wattage resistors is a stretch, who knows. Then again schematics do have errors. Could be a production error, mfg. recall, factory service note.
And the resistor was mounted off the board?

And thanks for the in-depth resistor temperature readings, that’s a different way to look at it. Excellent repair.


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Post subject: Re: Updates or miss builds on Pro Tube Twin power supplies?
Posted: Sun Mar 13, 2016 6:19 pm
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Thank you folks.

Yes the resistor is specified to be stood off the pcb. When I clipped it out it was quite a bit smaller than a standard 2 watt. Without the original tubes and with the calculations from the circuit, overheating was the only route I had for failure. The original failed open with mild discoloration. The output tubes I could tell were operational, but even feeding a signal into the effects return or power amp in produced no signal. No plate voltage to the PI or any other small signal tube. The 120 Hz hum that was barely detectable is no longer present by the way.

Maybe this could help someone else in the future.


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