6sn7gtb Filament Voltage

[kusojiji]

Hi, me again.

This time, it's overvoltage on the driver tubes. I have my hammond 300BX wired for 100vac on both primaries and get my 800vac and 5vac (rectifier) correctly. I am, however, getting 6.9vac out of the 6.3vac lines (loaded and with a true rms meter). I don't get it!

My questions are:

1. long term effect on the 6SN7's at 10% over.
2. how to reduce the 6.9 to 6.3.
3. if the 800vac and 5vac/3a lines are on the same windings and the other filament secondaries are on the other set of windings. If so, I can wire the filament primary for 110vac and get the 6.3 out (Japan - house voltage is 100vac).
4. would this problem cause the loss of treble that I am currently experiencing. I've tried removing the grid stopper resistor only to get shrill, annoying treble.

Seems like I'm not having a lot of luck with my power transformers.

I've fixed my 300B filament supply by driving a 115vac/6.3vac transformer at 90vac in.

 

[Frank Berry]

You could always install a series resistor on the 6SN7 filaments and drop the voltage to 6.3 volts.

 

[kusojiji]

If the transformer has the center tap grounded, will I need to run a resistor on both legs?

 

[Frank Berry]

To minimize hum, I would put a resistor on each leg.

 

[kusojiji]

How does that work? Is it because if I place a resistor on one side only of a CT xformer, it would cause a DC flow due to the imbalance? Kinda like an unbalanced p-p amp?

 

[Frank Berry]

Not exactly. The grounded center tap reduces (cancels) the ac hum when the load (filaments) are balanced. Placing a resistor on only one leg will disturb the balance and the hum cancellation.

 

[kusojiji]

Cool, thanks Frank!

 

[Giaime]

Why not a Zener on the filament's center tap? Doing so will increase hum? I ask that because it's common practice when you want to lower the B+ to put a power zener on the HT windings center tap, but I don't know what will happen if used on the filament's center tap...

 

[kusojiji]

Giaime,

Thanks for the input, but I"m using AC filament voltage.

 

[ilimzn]

Not exactly. The grounded center tap reduces (cancels) the ac hum when the load (filaments) are balanced. Placing a resistor on only one leg will disturb the balance and the hum cancellation.

A resistor will also limit cold current for the heaters, which is an added benefit.

Why not a Zener on the filament's center tap? Doing so will increase hum? I ask that because it's common practice when you want to lower the B+ to put a power zener on the HT windings center tap, but I don't know what will happen if used on the filament's center tap...

Since the heater current is not running through the ground, and thus thecenter tap, a zener there would not drop the voltage at all. In theory, you could put two diodes in antiparallel, in series with the heater supply, and thus rop about 0.6V, but the waveform would not be sinusoidal, rather like a sine with very high crossover distortion. Such a waveform has a lot more high order harmonics, and these will more easily spill into the circuit, so you will get more hum, andit will be more objectionable. A simple resistor offers best price to performance ratio...

 

[kusojiji]

I've got about 100 hours running like this. Are the tubes hosed? Am I experiencing the loss in performance due to the decline of the tubes? Am I just going deaf?

 

[kusojiji]

Oh, as an added sidebar, my 5U4GTB (Tungsol) seems to have a softstart feature. After I apply my filament power for the 300B's and wait about a minute, I apply the main power and watch as the tubes slowly come up. It takes about 10 or so seconds before I can play anything.

 

[Frank Berry]

The tubes should be fine. 6.9 volts isn't high enough to do damage in only 100 hours.

If you want more B+ delay, you might want to try a 5AR4.

 

[kusojiji]

Awe, that's not what I wanted to hear!!

Must mean that I'm going deaf!!

 

[kusojiji]

I don't think a 5AR4 is enough. This is a stereo 300B.

 

[kusojiji]

So let's see, I need to drop .6 volts total. @ .6 amps current draw for a typical 6SN7GTB, that would give me 1 ohm total or .5 ohm per side @ 1/4 watt.

sound right?

 

[mcs]

No, use 2-5W wirewound resistors. The inrush current (at powerup) could kill your 1/4W resistors.

Best regards,

Mikkel C. Simonsen

 

[kusojiji]

Good point! I forgot about that aspect.

Thanks!

 

[kusojiji]

Update -

I put a .30 ohm 5 watt resistor on each side of the filament outputs. Voltage is now down to about 6.2 vac, which works just fine.

Thanks for the inputs!

 

[EC8010]

I trust that you measured that 6.9V on the heater pins, rather than at the transformer terminals? You'd be surprised at how much voltage drop there can be down a bit of heater wiring.