• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tesla PCC88 7DJ8 dual triode - Help Needed.

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Shoog

Shoog

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Joined 2002
Vincent77 said:
The PCC88 has a 7 volt heater! It will work at 6.3v with reduced life and performance...
Click to expand...

I have been running a pair for over 7years in my preamp at 6.3V.
So no reduced life (in fact underheating will significantly extend tube life) , and they are reputed to operate more linearly at undervoltage.

Think of them as cheap NOS ECC88, so they are a bit of a bargain (cheaper than I paid for Philips 7yrs ago).

Shoog
 
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Vincent77

Vincent77

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Joined 2011
From "Getting the most out of vacuum tubes" by Robert B Tomer, page 150:

"Tubes are designed to be operated at their rated filament voltages. To get the best over-all satisfaction from their use, they should be operated as near this value as possible. However, where this is not possible, it is safer to err on the low side than it is on the high side. In other words, if they are operated at 5 to 10 percent below their rated center value, they may possibly fail to give quite as many hours of service at the remote end of their life span [...]"
 
Shoog

Shoog

Member
Joined 2002
From valve Wizard;

Voltage considerations: The heater voltage specified in the data sheet is the optimum value (usually specified as +/- 10%). Running them at higher voltages will considerably reduce valve lifespan and should be avoided. Running them at lower voltages, within reason, will increase their lifespan and reduce hum but also reduce their emission, although the grid curves and general performance remain much the same;- only the saturation current is reduced. Running heaters under-voltage is therefore perfectly acceptable, and provided the voltage isn't too low there will usually be no noticeable difference in sound. Normal heaters rated at 6.3V can be run quite happily between 5V and 6.9V, maybe even lower, but not higher. The exception to this rule is rectifier valves, which should not be run below 10% of their rated voltage since they usually operate very close to saturation.
Click to expand...

Shoog
 
Michael Koster

Michael Koster

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Joined 2008
redwoodcoastmusic.com/studioblade.html
Vincent77 said:
From "Getting the most out of vacuum tubes" by Robert B Tomer, page 150:

"Tubes are designed to be operated at their rated filament voltages. To get the best over-all satisfaction from their use, they should be operated as near this value as possible. However, where this is not possible, it is safer to err on the low side than it is on the high side. In other words, if they are operated at 5 to 10 percent below their rated center value, they may possibly fail to give quite as many hours of service at the remote end of their life span [...]"
Click to expand...

From "Getting the most out of vacuum tubes" by Robert B Tomer, page 12:

"Tubes, when operated at ten percent above their rated heater voltage, will suffer up to a fifty percent decrease in heater life. A voltage under the rated value on the other hand, while having some disadvantages as far as certain other characteristics go, tends to increase heater life substantially".

Without data, what is one to believe?

A far as the sound goes, you will need to try it in your own equipment to know for sure: 1. too many circuit variables to assign a sound to a tube, and 2. Other peoples ears are even less reliable than your own ;-)
 

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