Super reverb high heater voltage

[Underwood]

Hello,
I have a very high heater voltage measurement at between high 7's and even 8 volts. What could be the problem? I have 100 ohm resistors on the heater wires at the pilot light like normal. The plate voltage is 450V. Tubes are biased to 46ma with the negative voltage at -43. Schematic says -51 I think. I lowered the bias but it did not make a difference.  Any help would be appreciated.  THANKS

[sluckey]

What is your line voltage?

[Underwood]

I use something called an "amp maniac" and it is set at 117V.

[Willabe]

But have you actually measured the voltage coming out of the "amp maniac"?


                     Brad   

[Underwood]

Good point. Not recently. It was accurate last time I checked. I will check now.

[Underwood]

Just checked. 117V coming out.

[HotBluePlates]

Numbers don't add up.

Even if the Super Reverb needed a 110vac input to get 6.3vac on the heater winding, 117vac would only raise that to 6.7vac.

Something with one or more voltage measurements seems wrong.

[DummyLoad]

battery in meter need replacing?


--pete

[Underwood]

WHat is the proper way to measure the heater voltage? I set my meter to VAC and measure between pin 4 and 5 with the positive lead. I do not do anything with the black lead.

[Underwood]

Maybe the battery could be due to be changed, but all the other measurements seemed to be accurate and make sense.

[HotBluePlates]

WHat is the proper way to measure the heater voltage?

Put red lead to one side of the heater circuit, black lead to the other. You'll see 6.3vac or thereabouts.

Or put black lead to chassis, red lead to one heater wire. You'll see 3.15vac or thereabouts (half the heater voltage, because you're only measuring one leg).

Black lead hanging measures "nothing" of any consequence.

[Underwood]

I may have measured inaccurately with the one measurement, but I also did the measurement to ground and got around 4v. If it is just measuring half, then it is still very high around 8v.

[sluckey]

Measure directly on a 6L6 socket. One meter probe on pin 2, the other probe on pin 7. What do you get? You may want to use gator clips or pull the rectifier tube while doing this if you are shaky or nervous about making two handed voltage measurements.

If this is a later SR that uses two 100Ω resistors for an artificial center tap and those resistors are damaged, then any voltage measurement between chassis and either heater pin will be bogus.

[Underwood]

Yes, it has the 100ohm resistors. I could swear that I also did a measurement from the heater wires on the 6L6's also and got the same measurement. I also got the same measurement at the pilot light terminals where the heater wires begin. The 100 ohm resistors measure correct.

[Underwood]

I will recheck later tonight or tomorrow and post my results. THANKS!!

[PRR]

> I do not do anything with the black lead.

That just reads stray *meter* leakage. Does not read heater voltage.

> black lead to chassis, red lead to one heater wire. (half the heater voltage...)

When everything is correct, this works OK. When in doubt, it is dubious. (You could have the same-phase 3V on both sides--- unlikely in a working amp, but when looking for trouble, look good.) This may also be the only safe way when using a typical 1965 VTVM with grounded black lead.... but everybody has a floating (battery or passive) meter today.

> Measure directly on a 6L6 socket. One meter probe on pin 2, the other probe on pin 7.

Always the bestest way.

> battery in meter need replacing?

Always a great suspicion.

[Underwood]

I am reading 6.4 on the 2 and 7 pin, so everything is great. I changed batteries also. Don't really know? Thanks for everyone's help! Turned the bias down from 70% to around 63 to 65% and noticed the noise floor drop, but kept the same great tone. How much does line voltage effect the bias? Thanks!

[Underwood]

Also, I read 3.09 at the 4 and 5 pin to ground on the preamps. So I had 6.4 at the 6L6 and 6.18 at the preamp. Is that OK?

[sluckey]

Also, I read 3.09 at the 4 and 5 pin to ground on the preamps. So I had 6.4 at the 6L6 and 6.18 at the preamp. Is that OK?

You don't know what you had at the preamp. Don't measure filament voltage from pin to ground. That's a note that's worth remembering for a long time. Measure directly across the tube socket just as you did for the 6L6 socket. Only diff, one probe on pin 4/5, other probe on pin 9.

[HotBluePlates]

Also, I read 3.09 at the 4 and 5 pin to ground on the preamps. So I had 6.4 at the 6L6 and 6.18 at the preamp. Is that OK?

In that amp, the entire heater string is in parallel. Because there is only wire from point to point, in reality the voltage is the same everywhere along the heater chain.

With very high current, and non-zero-resistance connections, the voltage won't be the same everywhere along a parallel circuit like this. But in that case, the voltage would be highest closest to the voltage source, in this case the PT winding at the power tube end of the chain. All of this are fundamental characteristics of parallel circuits.

For the reasons above, either your line voltage is fluctuating, or the meter has error.