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Old 8th January 2005, 06:52 PM   #1
andy2 is offline andy2  United States
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Default Troubl-shooting heater voltage for filament

I have a tube amp that uses RCA 807 goes down. When I turn on, the tubes no longer light up.

The voltage measured at the cap shows 524V so I guess the voltage at the anode is good.

When I measured the voltage across the tube filaments when the tube installed, the voltage was almost zero. But when I removed all the tubes which include the input, driver, and output tubes, the filament voltage goes back to 6.2V which is about right.

I don't think there is anything shorted out at the filament terminals because when I removed all the tubes, the resistance reading across the filament terminals reads very high, almost open.

The filament voltage supply comes from a secondary output from the main xmfr that goes straight to a rectifier and a couple of caps. The rectifier diode comes in a pre-made, single package, but I could not get the part number unless I remove it from the chassis. I made some quick calculation from the combination all the filament resistance and the equivalent resistance is about .37 ohm which would require about 16A to sustain a 6.3V.

Has anyone seen this before and do you think it is the rectifier diode package that causing the problem?
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Old 8th January 2005, 07:24 PM   #2
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If you are only getting 6.2V with your transformer unloaded, something isn't right. Also, if the tubes aren't lighting up, you don't want to be applying B+ to your tubes. You'll kill them.
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Old 8th January 2005, 07:32 PM   #3
BbbyBld is offline BbbyBld  United States
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When I measured the voltage across the tube filaments when the tube installed, the voltage was almost zero. But when I removed all the tubes which include the input, driver, and output tubes, the filament voltage goes back to 6.2V which is about right.
Did you try each tube individually in circuit?

Do you measure any DC resistance across any of the capacitors?

You said the rectifier was in a single package--is it a bridge package like a 2504?

Please list all tube types and how many you have of each.
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Old 8th January 2005, 07:37 PM   #4
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One remedy is to break the 6.3V supply from the transformer/rectifier and do a quick lash up to a high current bench supply set at 6.3V should find the defect. There is only one component left...the rectifier ......

Heater current surge is quite high on switch-on, so the bridge rectifier would have to stand the cold surge current. Choose a spade terminal type rectifier aqs a replacement not a wire ended one. You don't mention the number of output tubes......I presume a pair...the smaller ones ignore.

rich
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Old 8th January 2005, 08:03 PM   #5
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Thanks for all the responses.

The amp has four 807 output tubes, one driver tube, and one input tube.

I measure the resistance of each filament and here is the result:

807 - 2 ohm
driver tube - 3.3 ohm
input tube - 8.4 ohm

The equivalent would be about .37 ohm.

The driver and input tubes come in one pair per tube.

I measured the resistance when all the tubes were taken out and the reading was in the mega ohm so I guess the caps are OK.

I guess I will do as suggested that I should connect the 6.2V to a separate load on a test bench so that I can isolate the problem to the rectifier.

By the way, I remove all the output tubes but still the same no heater voltage. I'll try to remove the input and driver tube one at a time to see what happen.

The rectifier is not 2504. But what would you suggest a good one for this type of amp?

By the way, this is a used amp I bought that i am trying to trouble shoot.
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Old 9th January 2005, 02:28 AM   #6
PRR is offline PRR  United States
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> I made some quick calculation from the combination all the filament resistance and the equivalent resistance is about .37 ohm which would require about 16A to sustain a 6.3V.

Just so you know:

The resistance of metal (usually) rises when hot. A 120V 60W incandescent bulb is obviously 120V/0.5A= 240 ohms when white-hot, but about 16 ohms when cold. Tubes don't go white-hot, but do increase 2X to 10X in resistance from cold to hot.

You could just look-up the heater demands. Took me 2 Google-seconds to find 807 heater is 0.9A (Duh! it is just a 6L6 in a different bottle!). You won't say what your other tubes are but two 807 and a couple little bottles is 3Amps, maybe 4A if you use a monster driver.

As Rich says, start-up current will probably dominate rectifier requirements. The warm-up period for tubes is long enough to overheat silicon, so that "16A" figure you came up with is probably what you should use, even though 99% of life will be more like 3A. You also have capacitors: as an approximate guide, rectifiers feeding large caps must be rated for transformer full rated current. So if you have a 5A winding, you need a 5A diode, even if your average current is 3A. (This hides several approximations that are mostly-true, but fail for extreme designs like 5-gallon caps.)

And whichever current is most, you should double that. Rectifier calculations are all approximate (or too darn tedious) while high-Amp rectifiers are not that expensive.

I'd go 25A or 30A, 50V. If I could only find 6A rectifiers, I'd buy a dozen because they may fail in a few hundred cold-starts.

> when I removed all the tubes... the filament voltage goes back to 6.2V which is about right.

If you have a 6VAC winding, rectifier, caps, I would expect 7 to 9 VDC no-load. So 6.2V is not-right. I suspect the rectifier is blown-open, but leaky enough to register on a sensitive voltmeter, not leaky enough to make real heat.

If you have 6VAC, why not wire it direct to the heaters? That works fine. It was THE way to do it before selenium and sillycon rectifiers appeared. If you never learned to twist and route heater wires the right way, it will hum a little, though sometimes not much. Well-done AC heat is very hum free.

> if the tubes aren't lighting up, you don't want to be applying B+ to your tubes. You'll kill them.

Very unlikely with these tubes at these voltages. The problem comes with the higher voltage transmitting tubes while the uncoated or barely-coated filament is half-warm. When stone-cold, there are no loose electrons to do any damage. And with full oxide coatings as used on tubes like 807, potential emission is so great that cathode stripping is insignificant.
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Old 9th January 2005, 04:03 AM   #7
andy2 is offline andy2  United States
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I would like to thanks all for your responses.

I've just spent the entire afternoon today trouble shooting and ended up replacing the rectifier and everything seems to work fine now.

The rectifier replaced was bought from Radio Shack with a rating of 25A, 50V. I'll look over on the web trying to get a higher ampere rating since I'll tend to use the amp a lot.

Whoa! That was a nice education in tube amp.
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Old 9th January 2005, 08:38 AM   #8
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Quote:
Whoa! That was a nice education in tube amp. [/B]
Everyone starts this way, you have learned alot from your amp.

That was the low voltage side.......respect the B+!!

rich
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Old 9th January 2005, 05:37 PM   #9
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Quote:
Originally posted by richwalters


Everyone starts this way, you have learned alot from your amp.

That was the low voltage side.......respect the B+!!

rich
Rich:

My amp has two hugh capacitors per side each is rated at 3.3mf connected in series. After turning off, I have to wait for a good 5 minutes or more to get 500V discharge completely before I dare to do anything. I hope none of that stuffs break :-)
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Old 10th January 2005, 04:36 AM   #10
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This is the VTL amp you were talking about a while ago, isn't it?

The 807s might not be that happy in there with 500V B+ on the screens (as in UL or triode mode) since the rated maximum is 300V...

I still think 3300mF charged to 500V is an accident waiting to happen. It's a good thing you're exercising caution. I would suggest that you solder clips to a 5K or so power resistor and place that across the cap terminals while you're working on it... just in case the voltage redevelops all on its own (yes, caps can do that sometimes).
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