Low heater voltage- how raise it up higher?

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Bieworm
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Low heater voltage- how raise it up higher?

Post by Bieworm »

Hey

I wonder ... my amp sounds incredibly good. But reading the voltages I worry about the heater voltages. They're really low. 2.6V on V1 to 3 leg 4+5 and leg 9. But measured between 4+5 and 9 I read 5.6V. ( thought 1+1=2? Apparently not...)
I can only find threads about lowering too high voltages, but can 't find any on raising them.
The previous PT gave the same values and they're both from different manufacturers, which makes me think it is not the PT that's to blame? The PT hums a little.. barely audible. Only when I put my ear on the cab close to the PT. Guess that's normal? I read somewhere adding a resistor from center tap to ground is a way to reduce that hum? Any suggestions? Or leave it alone since the hum is very little?
In fact, need I worry anyway about them heater V's?
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Re: Low heater voltage- how raise it up higher?

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Nobody?
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Re: Low heater voltage- how raise it up higher?

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how about this? applicable for an 18 watt?
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Re: Low heater voltage- how raise it up higher?

Post by Daviedawg »

Hum from mechanical vibration at the PT happens. You can look at fibre washers at the fixing bolts to reduce it. Electrical hum - plenty advice here about that.

What is the voltage across the tabs where the heater supply connects to the PT? It should be close to what the PT spec or even the PT itself says.

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Re: Low heater voltage- how raise it up higher?

Post by Bieworm »

Daviedawg wrote:
Tue 03/24/20 3:22 am
Hum from mechanical vibration at the PT happens. You can look at fibre washers at the fixing bolts to reduce it. Electrical hum - plenty advice here about that.

What is the voltage across the tabs where the heater supply connects to the PT? It should be close to what the PT spec or even the PT itself says.

Dd
That's hard to measure. The wires come directly from the inside of the transformer and the first stop is V5 /EL84. At that point I already read 2.6V each on leg 4 and 5. Isn't it strange that combined measurement over leg 4 and 5 this adds up to 5.6V? Or what am I missing here?
The hum is very little and not bothering me. I am a instruction engineer at HV switching stations..lots of very big 380kV transformers humming like crazy. My guess is that transformers just hum...
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Re: Low heater voltage- how raise it up higher?

Post by Daviedawg »

Are you measuring to ground? If so try measuring the voltage between the two connections to the first EL84.

Sorry about the patronising bits, but it is hard to know everyone's experience.

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Re: Low heater voltage- how raise it up higher?

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Daviedawg wrote:
Tue 03/24/20 6:40 am
Are you measuring to ground? If so try measuring the voltage between the two connections to the first EL84.

Sorry about the patronising bits, but it is hard to know everyone's experience.

Dd
Measure V5 pin 4 to ground= 2.6V
Measure V5 pin 5 to ground= 2.6V
Measure V5 pin 4 to pin 5= 5.6V

Same on V4

For V1 to 3:
Measure pin (4+5) to ground= 2.6V
Measure pin 9 to ground= 2.6V
Measure between pin (4+5) and pin 9= 5.6V

I only worry about the PT drawing too much current in case of low filament voltages on the power tubes. I get paranoid and start hearing things like PT hum, even of I have to put my ear very close to it ;)
I wonder... my PT is wired for 240V wall current. I have an option to wire it to 230V. Is that worth a try? Would rather not mess with my clean wirings ;)
Anyway, the wall voltage swings between 233V and 237V, depends on what time of the day I perform the measurement.
20200313_163110.jpg
You see I have another option (I think). Wire the red wires for the powertubes. That is an extra 8V winding . Maybe adding resistors in series to the PT and also create a virtual ground/center tap. But this too...rather not. It's bound to only increase extra noise

OTOH the values of the power tubes are pretty much on spec. Only the general heater voltages and the cathode voltages of V3 are out of spec.
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Re: Low heater voltage- how raise it up higher?

Post by JMPGuitars »

Which sections do you have to which heaters?

The 3.2A section is what should be feeding all your tubes.

The 2A section should only be feeding your rectifier.
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Re: Low heater voltage- how raise it up higher?

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JMPGuitars wrote:
Tue 03/24/20 8:00 am
Which sections do you have to which heaters?

The 3.2A section is what should be feeding all your tubes.

The 2A section should only be feeding your rectifier.
The 3.2A are used as heaters. So the 2 brown wires as F1 and F2,with the black CT to ground
20200317_145909.jpg
20200317_145915.jpg
Should I move the diodes over? I have one pair of diodes from the support to the EZ81..serving as backup in case the tube shorts. And from the EZ81 I have another pair in series that conjunct at the 120R which goes to the rectifier switch. I'm pretty sure it's wired correctly for du@l r*ct*f**r mode ;)
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Re: Low heater voltage- how raise it up higher?

Post by JMPGuitars »

What I'm assuming is your rectifier switch is insufficient for the voltage you're putting on it. It should be as heavy duty as the power and standby switches.

What are your voltages without any tubes?

With just the rectifier?
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Re: Low heater voltage- how raise it up higher?

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JMPGuitars wrote:
Tue 03/24/20 8:27 am
What I'm assuming is your rectifier switch is insufficient for the voltage you're putting on it. It should be as heavy duty as the power and standby switches.

What are your voltages without any tubes?

With just the rectifier?
Crap! Was thinking just the same thing when looking at the photo when I posted it. Could that Be the culprit of the low heater voltages?
Kinda hard to believe. Ok the switch needs to be upgraded, but a voltagedrop on the heaters? Well, I'm far from being an expert..so maybe?
As for voltages without tubes I will check them when I get home tonight
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Re: Low heater voltage- how raise it up higher?

Post by JMPGuitars »

Bieworm wrote:
Tue 03/24/20 8:38 am
Crap! Was thinking just the same thing when looking at the photo when I posted it. Could that Be the culprit of the low heater voltages?
Kinda hard to believe. Ok the switch needs to be upgraded, but a voltagedrop on the heaters? Well, I'm far from being an expert..so maybe?
As for voltages without tubes I will check them when I get home tonight
Not likely related at all. Just trying to not let you kill yourself. ;)
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Re: Low heater voltage- how raise it up higher?

Post by Bieworm »

The heater voltage without tubes is 5.9V
With only the power tubes it's 5.8V
And with the preamp tubes 5.6V
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Re: Low heater voltage- how raise it up higher?

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JMPGuitars wrote:
Tue 03/24/20 8:48 am
Bieworm wrote:
Tue 03/24/20 8:38 am
Crap! Was thinking just the same thing when looking at the photo when I posted it. Could that Be the culprit of the low heater voltages?
Kinda hard to believe. Ok the switch needs to be upgraded, but a voltagedrop on the heaters? Well, I'm far from being an expert..so maybe?
As for voltages without tubes I will check them when I get home tonight
Not likely related at all. Just trying to not let you kill yourself. ;)
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The heater voltage without tubes is 5.9V
With only the power tubes it's 5.8V
And with the preamp tubes 5.6V
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Re: Low heater voltage- how raise it up higher?

Post by JMPGuitars »

Bieworm wrote:
Tue 03/24/20 12:50 pm
The heater voltage without tubes is 5.9V
With only the power tubes it's 5.8V
And with the preamp tubes 5.6V
Was the tube rectifier also removed? If you're getting less than 6.3V with no load at all, then either your PT isn't correctly rated, or there's possibly a short somewhere. My guess is that it's not rated correctly since 2 of them were exactly the same, but I could be wrong. ;)

5.67VAC would be 10% below the tubes rated heater voltage. It sounds like you're there, or just under. So MAYBE you're okay, but possibly also reducing the level of gain, and life expectancy of the tubes.

You can expect current draw to drop your voltage, so the small drop you're seeing is normal. Usually a PT would start higher though. I think typical current draw for an EL84 is .76amps and 12ax7s are .3amps. If you have 5 tubes (2 el84, 3 12ax7), then you' should be under 2.5 amps on that heater string, and should be fine in that regard.

There shouldn't be any difference, but for the heck of it take the voltages with no tubes at all, and take them again after flipping your rectifier switch. Use a wooden chopstick, I don't trust it. ;)
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