Help! Amp Issue Started Today

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kevster
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Joined: Fri May 20, 2011 12:00 pm
Location: Hutchinson Island, Florida

Help! Amp Issue Started Today

Post by kevster »

All,

Well, I survived Hurricane Irma's 100 mph gust and two feet of rain, but this new problem with my Ceriatone FM50ME may be stressing me out even more than two days and nights without power and A/C. I have a church gig starting at 2pm Saturday and need this amp....

My back-up Bill M Blues Jr also started something two weeks ago, this sudden loud low to mid frequency squawk, so I'm without that to cover me as well. That's something I deal with next month after I move.

So, today I notice this low volume background noise/hum. Changing volume doesn't initially change it, with either the MV or d-lator, but with loop disconnected and MV greater than 60% it does jump in volume. Taking everything out of the loop and the input doesn't impact the noise at all, so it's not a cable. Taking out the power conditioner I use for power didn't change it...

When I power up with power on, leaving it in standby initially so it warms up, I get a noise switching to operate. Kind of a short "Vrruppp" sound, and the steady noise. I can play through it... The cleans are now slightly muddy, slightly fuzzy, and my volume may have dropped slightly.

I retubed in early August with Mullards, but kept the 5.5-year old Sovtek 12AX7 in the PI position. It's been working fine... I thought about tube rolling with some of my older tubes to see if that changed anything, but this was my first "stop."

TIA for any ideas.

Kevin

Any idea what may be the issue and the fix?
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xtian
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Re: Help! Amp Issue Started Today

Post by xtian »

Are you having the same trouble with both amps? Bad, post-Irma wall current?

If it's not an external issue, I would suspect the filter caps. You can measure ripple AC on the HT, and (if SS rectified) you can just clip in an additional 40uF at the reservoir to see if it reduces hum.
I build and repair tube amps. http://amps.monkeymatic.com
kevster
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Re: Help! Amp Issue Started Today

Post by kevster »

xtian wrote: Thu Sep 14, 2017 9:52 pm Are you having the same trouble with both amps? Bad, post-Irma wall current?

If it's not an external issue, I would suspect the filter caps. You can measure ripple AC on the HT, and (if SS rectified) you can just clip in an additional 40uF at the reservoir to see if it reduces hum.
Not related or quite the same symptoms. Haven't touched the Blues Jr since late August, so both are not impacted by the same event. Crappy power maybe, but normal crappy power that I wasn't aware of...

I may have a low power rated cap in that range to test with. I built a treble bleed that I haven't yet installed, and I have some left over caps.

Bad current isn't revealing itself anywhere else, but I'll try the Blues Jr to ensure it isn't a voltage issue (Brown sound/low voltage causing fuzzy cleans...). That doesn't explain all of it, but it could explain some of it.

No change vs. on the power conditioner or off seems to negate the external power as the culprit, but there are plenty of power factors as restoration is still ongoing in areas.

Thanks for your input...

Filter caps are impossible to fix by Saturday afternoon... No parts to do it with...

Thanks again!
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67plexi
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Re: Help! Amp Issue Started Today

Post by 67plexi »

#1 check your bias then voltage chart the amp starting at B+1 to V-1
kevster
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Re: Help! Amp Issue Started Today

Post by kevster »

67plexi wrote: Fri Sep 15, 2017 1:44 am #1 check your bias then voltage chart the amp starting at B+1 to V-1
The voltages are all over the place.... First, the one that seems okay... The wall voltage measured at main switch: 122.4. At least it's at a good level no matter if it's clean or not.

The first voltage... Bias: 2.9 and 3.6 respectively with the DMM on 200m. Yes, 2.9 - 3.6... Crap. These were at 38.x in August after the initial tube burn-in. My Pin 3 is normally 498 with the Magnetic Component trannies, and it's now 386.. Pin 4: 380, Pin 5: -43, Pin 6: 386.

OT CT: 392. Definitely off/lower...

Now, I'm not getting any useable PT readings. My OT isn't putting out right. I stopped taking pin readings there since the cascade downstream are going to be whacked. This is where I need to stop with this since it isn't a "by tomorrow" fix...

So, any ideas based on those levels?

Thanks again!
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67plexi
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Re: Help! Amp Issue Started Today

Post by 67plexi »

Is your power transformer getting hot ?
Next pull your power tubes and recheck voltages make sure you don't have a weak ground
I got a bad batch of Belton octal tube sockets in the past. I will never use them ever. They were shorting out inside the socket.
I was at my wits end on that issue thanks to Jimmy Page's amp tech to help me sort out the tube socket's
22 6L6GC's and a power transformer later problem solved that's why I say pull the power tubes.
The Octal tube sockets tested fine with an ohm meter it was only when high voltage was applied.
Check your standby switch on my 150 watt SSS build the standby switch was weak changed to a 15 amp switch.
kevster
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Re: Help! Amp Issue Started Today

Post by kevster »

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Last edited by kevster on Fri Sep 15, 2017 11:36 pm, edited 1 time in total.
dpin
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Re: Help! Amp Issue Started Today

Post by dpin »

Is your first filter cap after the rectifier a single cap or 2 series stacked caps?
kevster
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Re: Help! Amp Issue Started Today

Post by kevster »

67plexi wrote: Fri Sep 15, 2017 6:03 pm Is your power transformer getting hot ?
Next pull your power tubes and recheck voltages make sure you don't have a weak ground
I got a bad batch of Belton octal tube sockets in the past. I will never use them ever. They were shorting out inside the socket.
I was at my wits end on that issue thanks to Jimmy Page's amp tech to help me sort out the tube socket's
22 6L6GC's and a power transformer later problem solved that's why I say pull the power tubes.
The Octal tube sockets tested fine with an ohm meter it was only when high voltage was applied.
Check your standby switch on my 150 watt SSS build the standby switch was weak changed to a 15 amp switch.
I have noticed the PT getting warm, but it is only because I reach in to turn on the reverb pedal nearby... I wouldn't call it HOT.

I reinstalled my worn out power tubes to test, and nothing changed. Stripping everything out of the loop accentuated the volume of the noise, but nothing changed.

I'll check the Standby switch.

Thanks!
kevster
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Re: Help! Amp Issue Started Today

Post by kevster »

Image
This my layout. I modded the slope and one other resistor by a stacking parallel resistor on it. Other than that, my model does not have a half power switch and I used Magnetic Component Classic Tone trannies. They're slightly hotter than MM, until now anyway...
dpin
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Re: Help! Amp Issue Started Today

Post by dpin »

What AC voltage reading are you getting feeding the rectifier? (345-0-345 on the layout)

Clip one meter lead to ground, set to AC and measure each leg going into the diodes with the amp in standby
kevster
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Re: Help! Amp Issue Started Today

Post by kevster »

dpin wrote: Fri Sep 15, 2017 11:53 pm What AC voltage reading are you getting feeding the rectifier? (345-0-345 on the layout)

Clip one meter lead to ground, set to AC and measure each leg going into the diodes with the amp in standby
It was 372 and 373, as well as a 52v reading from the 50v PT output, which was right before the HT fuse blew...
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Phil_S
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Re: Help! Amp Issue Started Today

Post by Phil_S »

I'm late to this party and I'm an amateur. I've built about 15 amps and done a decent amount of trouble shooting. Take what I say with a grain of salt. My general sense is that good diagnostic technique starts with the basics.

Reading through, this is what I think we have as cogent facts:
1) There's a buzz or a hum. I'm guessing 120 Hz.
2) There's almost no bias voltage. This is a likely cause of the hum, but it's not a certainty. Restore the bias voltage to see if that cures the problem. However, that's easier said than done.
3) Plate voltage is well below what's expected. You're losing something between the AC feed of the secondary and the plate. There's not a whole lot of stuff in the path and it begs for close examination. Look for a short. There is plenty of discussion on this point in this thread.
4) The AC HT secondary voltage seems a bit high compared to spec, but not unreasonable. The difference is not accounted for by the wall voltage being 122.4 vs. 120 spec for the PT. When you put a PT under load, the voltage will drop about 5% and the other 2% is accounted for by the high line voltage. I'm thinking the meter reading is spot on.
5) The fuse blows when metering for #4, above.

On the basis of #5 alone, I'd want to rule out the PT as the problem. Pull all the tubes. This will allow the filament winding to be completely unloaded -- no need to disconnect anything there. Disconnect the HT and the bias supply. Now, turn on the power and check all the voltages. Are voltages reasonable? Does the fuse blow? Assuming voltages are reasonable and the fuse doesn't blow, that's a good sign, but not definitive. It is still possible there is an internal short due to insulation breakdown that doesn't show until you put the PT under load. Do the "super secret" transformer test. https://www.premierguitar.com/articles/ ... r-tester-1 (Thank you to R. G. Keen, a regular here.) Assuming it passes, then you can look for trouble elsewhere.

Elsewhere I might look for a spot where an unintended ground was made that didn't exist when the initial build was completed. Maybe something melted...whatever. When a fuse blows, the first thought I've got is there is an excessive current draw due to a short.

I'm doubtful you'll get this one fixed quickly. Look for a loaner, or fix the Blues Jr. first.
kevster
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Re: Help! Amp Issue Started Today

Post by kevster »

Phil_S wrote: Sat Sep 16, 2017 2:08 pm I'm late to this party and I'm an amateur. I've built about 15 amps and done a decent amount of trouble shooting. Take what I say with a grain of salt. My general sense is that good diagnostic technique starts with the basics.

Reading through, this is what I think we have as cogent facts:
1) There's a buzz or a hum. I'm guessing 120 Hz.
2) There's almost no bias voltage. This is a likely cause of the hum, but it's not a certainty. Restore the bias voltage to see if that cures the problem. However, that's easier said than done.
3) Plate voltage is well below what's expected. You're losing something between the AC feed of the secondary and the plate. There's not a whole lot of stuff in the path and it begs for close examination. Look for a short. There is plenty of discussion on this point in this thread.
4) The AC HT secondary voltage seems a bit high compared to spec, but not unreasonable. The difference is not accounted for by the wall voltage being 122.4 vs. 120 spec for the PT. When you put a PT under load, the voltage will drop about 5% and the other 2% is accounted for by the high line voltage. I'm thinking the meter reading is spot on.
5) The fuse blows when metering for #4, above.

On the basis of #5 alone, I'd want to rule out the PT as the problem. Pull all the tubes. This will allow the filament winding to be completely unloaded -- no need to disconnect anything there. Disconnect the HT and the bias supply. Now, turn on the power and check all the voltages. Are voltages reasonable? Does the fuse blow? Assuming voltages are reasonable and the fuse doesn't blow, that's a good sign, but not definitive. It is still possible there is an internal short due to insulation breakdown that doesn't show until you put the PT under load. Do the "super secret" transformer test. https://www.premierguitar.com/articles/ ... r-tester-1 (Thank you to R. G. Keen, a regular here.) Assuming it passes, then you can look for trouble elsewhere.

Elsewhere I might look for a spot where an unintended ground was made that didn't exist when the initial build was completed. Maybe something melted...whatever. When a fuse blows, the first thought I've got is there is an excessive current draw due to a short.

I'm doubtful you'll get this one fixed quickly. Look for a loaner, or fix the Blues Jr. first.
I fixed the Blues Jr. Deoxit to the pots didn't do it all. This a Bill M modded amp. Swapped out a preamp tube, and it's 95% sweet. After at least nine years, it's time for a retube.

Good game plan! The trannies are higher than spec, but many people have used them with no issue in these clones. However, it does skew the voltage chart at a time like this...

If it proves to be the OT, then I'll consider going to spec with MM PT and OT... I played through a nearly identical clone with the MM trannies. Sounded slightly more mellow. I'd be okay with slightly more mellow and a pedal for extra umph. That extra 8% may mean just enough...

Thanks so much.
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