Effects of running 6.3V tubes with 5.5V?

[casssax]

I have an amp I'm working on that does not have the original power transformer.
Someone in the past put a different transformer that only has a 5.5V winding for the heaters.

The amp has two EL84's, two 12AX7's and a 6CA4 rectifier.

What effect does running the heaters on lower than the minimum voltage?
(I think the max/min is 10% so 5.67 would be the minimum. I got a reading of about 5.4 when I checked)

Will it have an effect on the overall volume or gain of the amp?

Does it have an effect on the tone?

I've ordered a filament transformer (6.3V @ 3A) so I will be able to see one vs. the other but I was just curious about to expect.

Thanks.

[pompeiisneaks]

I'm very curious about the output of this as well.  I don't have a clue but a wild guess is that if it's working, it's working, the electrons need enough heat to start boiling off and moving, if they get enough it works.  It may, though, have some negative impact on the tube operation so it doesn't work as efficiently.  Those are just wild guesses, I just want to see what the gurus say so I'm chiming in now ;)

~Phil

[John]

Like Pompeii says, if it works it works. However, in Merlin's book (excellent btw) he says that running heaters too low is just as hard on them as running them too high. But you're installing a correct transformer anyway, so in the meantime you should be fine. 

[Paul1453]

It seems odd to me that you have 5.5VAC

5VAC or 6.3VAC would be the most common secondary taps.

If you take out all your tubes, what does that 5.5VAC read when unloaded?

Does the PT get hot?  5.5VAC makes me think a 6.3VAC winding is partially shorted somewhere?
Does the PT buzz or hum? loaded or unloaded?
There are no caps to be leaking, so if you have a 6.3VAC winding that is dropping .8V somewhere,
It could only be the PT, socket wiring/sockets, or lamp indicator if one is connected to this wiring.

Do you agree?   

[hesamadman]

My guesses would be:

If your running at a lower voltage, I would assume you would have a slight rise in current. For a 12ax7 from 6.3v (.3amps) to 5v you may rise to .375. So this boils down to, can the existing PT handle this? It seems to be I'm assuming. But at what cost? A short life span perhaps?

Also, a heaters job is to boil electrons off at the cathode. With the 5v, the tubes most likely aren't getting the full heat to properly do their job. My guess is you'll see some gain reduction.

I'm curious to hear what you find when you install the filament transformer. I'm just making kind of making assumptions and I would like to learn with you. Good luck.

[HotBluePlates]

I have an amp I'm working on that does not have the original power transformer.
Someone in the past put a different transformer that only has a 5.5V winding for the heaters.

The amp has two EL84's, two 12AX7's and a 6CA4 rectifier. ...

I agree with Paul that the specific voltage sounds odd, unless you're dealing with a 5vac winding that's measuring high because it's not loaded.

But you've ordered a 6.3vac transformer, so this concern is probably moot.

What effect does running the heaters on lower than the minimum voltage?
(I think the max/min is 10% so 5.67 would be the minimum. I got a reading of about 5.4 when I checked)

Will it have an effect on the overall volume or gain of the amp?

Here's the straight-poop on this topic:

The tube was designed to be operated at its rated heater voltage (6.3vac) and will deliver "most satisfactory service" (the common term in old texts), meaning "best chance of meeting/exceeding claimed specs, for as long as possible" when you apply 6.3vac.

"If your running at a lower voltage, I would assume you would have a slight rise in current." The unstated assumption for this assertion is that heater-power stays constant. In fact, it doesn't, but rather follows Ohm's Law. So, if you drop the 6.3vac to some lower voltage, less current flows through the same-resistance of the heater, and the heater consumes less power (volts*amps) from the winding.

The cathode was designed with a certain planned power consumption by the heater, which then heats the cathode. Less power consumption then means less cathode-heating. The possible effect has two facets:
-  Normally, a new cathode has an excess of electron emitting capability in its cathode coating. So under-powering results in no real change of actual electron emission, and the tube operates as it would with less heater power, though the tube envelope may be very slightly cooler.
-  A tube which has already had a long period of use may have partially depleted some of its cathode coating, and under-powering the heater results in less emission than the tube would have under normal operating conditions.

If your tube falls in the latter category, then for the same operating point, transconductance (Gm) will be reduced by some amount. That might translate to a measurable reduction in gain. Whether the measurably-reduced gain is audibly-obvious in use depends on the amp and how severe the gain reduction (or really, tube age/wear) is.

Shifting gears a bit, have you ever seen a Hickok tube tester? Typically, these have a rotary switch with selectable heater voltages from very low (a volt or so) up to 115v. Many of these have a "Life Test" button. When you press that button, the applied heater voltage drops from what was selected on the switch to the next-lower heater voltage. The operator keeps an eye on the meter reading for Gm when pressing the Life Test button, and if it drops very significantly, the tube is judged to have limited remaining life. If the Gm reading stays the same or drops only slightly, the tube is judged to have a strong cathode with plenty of emissive material, and lots of useful life ahead of it.

That Hickok Life Test would drop a 6.3v tube's heater to 5v. That's below your 5.5v, which I'd offer is close-enough to the 5.7v of "10% low operation" to just call it 10%-low. So an average tube, and definitely a new tube, should operate just like normal at that heater voltage.

"Getting the Most Out of Vacuum Tubes", found on Pete Millett's site, was written by an engineer working for a tube manufacturer. After detailing all the typical ways in which tubes can fail, he discusses some means for extending tube life. He notes that while getting the "most satisfactory performance" would happen while operating the tube at its rated heater voltage, if you can't apply rated heater voltage then going lower is better than going higher. While he doesn't explain it, he does say that with lowered heater voltage, "... they may possibly fail to give quite as many hours of service at the remote end of their life span...," but are less likely to have complications or sudden catastrophic failures as compared to higher-than rated heater voltage.

Tomer's wording in quotes above is a short way of saying what I did earlier, that electron emission will tend to drop off more as the cathode is depleted compared to providing rated heater voltage.

[drgonzonm]

What voltage is your b rail running?   If it is also lower, than the schematic suggests, then you might consider an voltage adjuster using a 120/12v transformer to boost your voltages.  (Steve Luckey "Sluckey") one of the moderators has on website, a vintage voltage adapter, designed to lower voltage for vintage amps.  By wiring incorrectly you can boost the voltage. 

[casssax]

If you take out all your tubes, what does that 5.5VAC read when unloaded?

I took the PT out of the amp completely.
At 120V on the primary I'm seeing 6.16 on the filament winding. When under load I think there would be about 3Amps.




This PT may have come out of an old radio or something. You can see below it has '55' in red stamped on the top.
I'm not sure if that is 5.5 or 1955 (although it kind of looks like there is a dot between the two numbers to me)



I think I'll try A/B testing with the 5.5 and the 6.3 when I get it to see if there is any real difference.
If there is no perceivable difference I might just leave it as is.

The cathode was designed with a certain planned power consumption by the heater, which then heats the cathode. Less power consumption then means less cathode-heating. The possible effect has two facets:
-  Normally, a new cathode has an excess of electron emitting capability in its cathode coating. So under-powering results in no real change of actual electron emission, and the tube operates as it would with less heater power, though the tube envelope may be very slightly cooler.
-  A tube which has already had a long period of use may have partially depleted some of its cathode coating, and under-powering the heater results in less emission than the tube would have under normal operating conditions.

If your tube falls in the latter category, then for the same operating point, transconductance (Gm) will be reduced by some amount. That might translate to a measurable reduction in gain. Whether the measurably-reduced gain is audibly-obvious in use depends on the amp and how severe the gain reduction (or really, tube age/wear) is.

The tube would definitely fall into the latter. They are old tubes, but I have no idea how much they have been used.

[Paul1453]

This PT may have come out of an old radio or something. You can see below it has '55' in red stamped on the top.
I'm not sure if that is 5.5 or 1955 (although it kind of looks like there is a dot between the two numbers to me)

I think I'll try A/B testing with the 5.5 and the 6.3 when I get it to see if there is any real difference.
If there is no perceivable difference I might just leave it as is.

You may be correct, there are some 5V tubes that were used like a 5AQ5 output tube.
If the PT came out of something with a full compliment of 5V tubes that might explain that winding.
This would seem to be a more rare PT/radio that it came out of.
I have a few 5V tubes like the 5AQ5, and even though I buy a lot of cheap junk tube gear,
I have yet to run across anything with 5V tubes installed, besides 5V rectifiers of course.

If it is working and sounds good, I definitely wouldn't change the PT either.   

[PRR]

> unstated assumption for this assertion is that heater-power stays constant. In fact, it doesn't, but rather follows Ohm's Law.

Close-enuff for small changes.

Filaments actually tend toward a somewhat constant current. It falls, but not as fast as voltage. So power drops a little slower than voltage squared.

But temperature falls as a high power of Watts, and Emission falls as a fast power of Temp. Emission goes way down.

All oxide cathodes have way excess emission when new and hot.

12AX7 have been deliberately run at 5.5V (see Dynaco). Relative to useful plate current 0.2mA-2mA, the rated heater power is very generous.

I would not be so happy about a power tube working ~~15% down on rating.

Also change your DMM battery. We sometimes see "wrong reading" reports and next day the meter punks-out with a dead battery error.

> PT may have come out of an old radio

For *radio*, the only likely voltages are 2.5V, 5.0V, and 6.3V, reading 10% no-load, reading 5%-15% high for 110V gear on modern 125-130V wall outlets. Do you have a 7V winding? (Really a 6.3 but no-load and hi-volted.)

[casssax]

The transformer only has two secondary windings. The high voltage and the filament.

Is it possible this is a 5v winding but designed for a higher load like 5A and under only 3A its not dropping all the way down to 5.0?

The DMM battery is new and the reading match what I took several days ago so I don't think it's that.