B+ too high?

8/25/2005 8:16:09 AMjim

Hello I have a GE with a 6v6 push and pull output.
It was rewired to replace a 6j5 with a 6n7. I rewired it for the 6j5 according to the schematic GE G105. My plate and screen voltages are about 13% higher then the spec. I think the culprit may be the voltage divider resistor. It has four tabs and is mounted on the chassis. It gets quite hot too. I have no idea what is inside. Carbon resistors can change value. What about these strip voltage divider resistors? Any tips on bypassing it?

8/25/2005 9:45:22 AMNorm Leal

Hi Jim

Rewiring a 6N7 as 6J5 wouldn't effect overall voltages.

What is your line voltage? Do you have taps for different line voltages? High line voltage will increase B+ voltage.

Has someone replaced voltage divider resistors? Most are wire wound and don't usually change value.

Carbon resistor can change. Usually they go up in value. They aren't in higher current circuits so shouldn't effect overall voltage.

Do your 6V6 tubes have good emission? Older tubes lose emission so draw less current. This causes B+ voltage to be high.

Norm

:Hello I have a GE with a 6v6 push and pull output.
:It was rewired to replace a 6j5 with a 6n7. I rewired it for the 6j5 according to the schematic GE G105. My plate and screen voltages are about 13% higher then the spec. I think the culprit may be the voltage divider resistor. It has four tabs and is mounted on the chassis. It gets quite hot too. I have no idea what is inside. Carbon resistors can change value. What about these strip voltage divider resistors? Any tips on bypassing it?

8/25/2005 10:51:23 AMjim

Hello Norm, I think the line voltage is 122. I'm glad to learn that the voltage dividers are wire wound. They were not replaced. 90% of the resistors are wire wound despite the GE service manual which shows most as carbon. I think you probably hit it on the 6v6's. As I remember they tested close to "weak" on my tube tester. I will see if I can dig up some better ones before going further. You know I found something interesting in a box of disgarded shop parts- line voltage regulators. They are tubular and plug into the outlets. The graphics on the box show a happy housewife plugging in her radio, television and refrigerator into them. I'm curious to test one out and see what kind of voltage drop it brings about.

:Hi Jim:
: Rewiring a 6N7 as 6J5 wouldn't effect overall voltages.
:
: What is your line voltage? Do you have taps for different line voltages? High line voltage will increase B+ voltage.
:
: Has someone replaced voltage divider resistors? Most are wire wound and don't usually change value.
:
: Carbon resistor can change. Usually they go up in value. They aren't in higher current circuits so shouldn't effect overall voltage.
:
: Do your 6V6 tubes have good emission? Older tubes lose emission so draw less current. This causes B+ voltage to be high.
:
:Norm
:
:
:
::Hello I have a GE with a 6v6 push and pull output.
::It was rewired to replace a 6j5 with a 6n7. I rewired it for the 6j5 according to the schematic GE G105. My plate and screen voltages are about 13% higher then the spec. I think the culprit may be the voltage divider resistor. It has four tabs and is mounted on the chassis. It gets quite hot too. I have no idea what is inside. Carbon resistors can change value. What about these strip voltage divider resistors? Any tips on bypassing it?

8/26/2005 3:30:14 PMThomas Dermody

A line voltage of 122 is fine. Voltage dropping resistors in the B section of radios often run warm to hot. It is normal for radios to run +/- 10% on B current, so a combination of weak output tubes and higher line voltage may be the trouble. Check all resistors in the radio for drifting (which will cause some sections to not draw enough power, which will cause the chief B current to be higher). If using new 6V6 tubes brings down the voltage, do not assume that you cannot use the old tubes anyway.

It is wise just as preventative maintainence to check all resistors for drifting and all condensers for leakage (anything less than infinity, with electrolytics as the exception--anything less than about 10 meg). Remember to disconnect each component you test, for accurate measurement.

Thomas

8/27/2005 1:21:55 PMBilly Richardson

Jim, a lot of restored radios run a high B+, and I believe the cause is a combination of two factors. Higher line voltage and new electrolytic condensers. These old sets were usually designed for 110-115 VAC, and most of us have 120 or more today. Where a modern electrolytic is concerned, I always expect it to yield a higher B+ than an early one, especially if the original was a wet type, and would be surprised if it didn’t. Evidently, the new ones of today just work better than those of yesteryear.

You didn’t say if your set was working or not, so I am just guessing here. If it is working, the voltage dividing resistor is probably good. These wire wound resistors usually run hot. They use the chassis as a heat sink, which is the reason for mounting them there. If your set is not working, then part of this resistor could be the cause. One section of this resistor is generally used for the bias or negative voltage, and if it is bad, the B+ may run high. Most of the time, a bad section can be repaired by soldering a suitable power resistor across the tabs.

Occasionally, a restored set with a high B+ will exceed the manufacturer’s specs for the tubes, capacitors, etc. This is usually an easy fix, and has to do with whether the power supply has a capacitor or resistance input.

A power supply with a resistance input yields a lower B+ and better regulation than a capacitor input type. For this type, just increase the resistance. If a choke is used instead of a resistor, install a resistor in series ahead of it. For anyone restoring a very early AC set, the reason for this placement ahead of the choke is because some of them had a tuned power supply. However, I don’t remember seeing any tuned power supplies once the electrolytic came into use.

Most manufacturers used a capacitor input to save cost, so your set is likely to have this type. Therefore, install a resistor between the rectifier tube and the capacitor, usually the first electrolytic. This will convert the power supply to a resistance type, which should be better than original.