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Old 14th August 2008, 09:24 PM   #1
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Default tube heater voltage, how much is too much?

I just got a pair of 300B monoblocks, which each use a pair of 6sn7s as the input tubes.

My problem is my house AC is on the high side.

Powered them up yesterday, and checked some voltages.

The 6SN7's had 6.96 VAC across the heater wires, according to my meter, did not put a scope on the lines.

The 300B's had just under 6 VDC on the heater lines.

I know both numbers are high, am I going to burn out my tubes by running them like this?

So far, I only ran the amp for maybe 15 minutes like this.

I was thinking about getting the tentlabs filament supplies for the 300b's, with a seperate transformer for it.

For the 6SN7's, I was thinking about making a regulated DC heater. If I rectified the AC, I would end up with just under 10VDC, so I have about 3.5 volts of headroom, before the regulator.

B+ is 520 VDC.

One of the 6SN7's has 297 VDC from the plate to cathode, which seems like it's on the high side too.


One other unrelated issue. I tried driving the amp last night with a modifed squeezebox. The squeezebox has the DAC, a PCM1748 connected to the output via a coupling cap, with no op amps. I hooked it up to an old pair of speakers, and barely got any sound out of it. When it started to get to a somewhat reasonable volume, it would start distorting

This morning, I hooked up my portable player, a Rio Karma, with the headphone output going to the 300B input, and it sounded much better, more volume without the distortion.

So, I am either running out of current or voltage, but I have not figured out which one.

Any advice would be appreciated.
This is my first DHT SET amp.

Randy
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Old 14th August 2008, 10:43 PM   #2
fredex is offline fredex  New Zealand
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randytsuch. quote: "I know both numbers are high, am I going to burn out my tubes by running them like this?"

Not immediately but tube life will be shortened. If you lower the filament supply by what ever method you still have the high HT to lower. Best solution would probably be to shave off some of the mains voltage and fix two problems at once. I suggest using a mains step down transformer.
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Old 14th August 2008, 11:17 PM   #3
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Quote:
Originally posted by fredex
randytsuch. quote: "I know both numbers are high, am I going to burn out my tubes by running them like this?"

Not immediately but tube life will be shortened. If you lower the filament supply by what ever method you still have the high HT to lower. Best solution would probably be to shave off some of the mains voltage and fix two problems at once. I suggest using a mains step down transformer.
So, I started looking for a Variac. I always thought it would be nice to have one, and this is an excuse to buy one.

Randy
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Old 14th August 2008, 11:32 PM   #4
fredex is offline fredex  New Zealand
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Variacs are cool but good ones are expensive. Cheap ones hum and can get hot. Unless your mains voltage varies badly a fixed transformer is good. Most Variacs can also boost the mains voltage so be careful with that knob!
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Old 15th August 2008, 02:26 AM   #5
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Randy,

I was puzzled by some high mains readings at one time (130-132), and at someone's suggestion checked the battery in my DMM. I changed the battery and readings went back to normal. Just a thought.
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Old 15th August 2008, 02:52 AM   #6
rdf is offline rdf  Canada
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Another option when faced with high heater voltages are small resistors in each leg (or better yet hash chokes with the correct DCR and adequate current rating) to bring the voltage down, followed by a cap between windings. Turn that lemon into HF noise filtering.
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Old 15th August 2008, 05:22 AM   #7
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Note that regulated DC supplies are not required for heaters (of course that depends on what you call regulated). All that matters is that the RMS voltage across the filament be stable at the rated voltage.

On using a transformer - better to avoid autotransformers - an "open neutral" problem as it is known can cause the full input voltage to appear at the output defeating the purpose of the transformer.

Another interesting fact - relative to the earth pin, the live and neutral are different for different countries. If yours is the "wrong" way and you are using a plug adapter, then the device sees the mains voltage between earth and neutral rather than earth and live (ouch) and it can instantly blow up any voltage stabilizer you put in the middle that is expecting earth to be at neutral potential!
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Old 15th August 2008, 05:39 AM   #8
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Quote:
Originally posted by fredex
Variacs are cool but good ones are expensive. Cheap ones hum and can get hot. Unless your mains voltage varies badly a fixed transformer is good. Most Variacs can also boost the mains voltage so be careful with that knob!
At first I had a brain fart, and could not figure out how to find the right transformer. Then I figured I just need to find one that would output 100V, for 110v input. Then, I found that hammond makes what I need, they even have multiple taps, something like the 168e, but they are not cheap either.
So, I am going to start watching ebay, see if I can find something there.

Quote:
Originally posted by Jeff Yourison
Randy,

I was puzzled by some high mains readings at one time (130-132), and at someone's suggestion checked the battery in my DMM. I changed the battery and readings went back to normal. Just a thought.
Just put in new batteries a few weeks ago, thanks for the advice anyway.

Quote:
Originally posted by rdf
Another option when faced with high heater voltages are small resistors in each leg (or better yet hash chokes with the correct DCR and adequate current rating) to bring the voltage down, followed by a cap between windings. Turn that lemon into HF noise filtering.
Would that work for AC heating?
For the DC heating for the 300b's, I am really leaning to get the tentlabs filament heater, even if I solve this problem another way.

Randy
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Old 15th August 2008, 06:11 AM   #9
fredex is offline fredex  New Zealand
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Quote:
Originally posted by rdf
Another option when faced with high heater voltages are small resistors in each leg (or better yet hash chokes with the correct DCR and adequate current rating) to bring the voltage down, followed by a cap between windings. Turn that lemon into HF noise filtering.
This is a good solution as you get a soft start for those precious filaments.
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Old 15th August 2008, 06:21 AM   #10
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I really think some of the solutions noted here are a "dirty" fix to a problem that should be fixed in a "proper" way.
Really...given that your mains voltage is prone to wandering, some of the fixes noted here won't deal with an opposite 'problem' that might crop up later.....a voltage sag.
Note that you are from Los Angeles USA...which has very good codes for proper wiring...here in Argentina we don't even have grounds wired up, and chassis grounds of all my gear here has a "biting" voltage floating about.
You really should have all voltages regulated...It's really easy to do, granted you will have to redesign/recalculate your PS but it will absolutely nail your intended voltages, get you a nearly pure Thevenin source, clean up any and all sources of hum and once in place you won't have to worry about it.
Use one 317 (TO220 case) for each 6SN7 and one each 317K (TO3 case) for the 300Bs. The 317Ks for the 300Bs of course won't have the voltage variation capacity that the heater filiments will, but they will suffice quite nicely.
________________________________________Rick...... ....
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