Solid state rectification?

4/19/2012 11:31:55 PMElton

I have a Zenith 10s464 with the 6X5 tubes and was told to replace each of them with a diode and 150 ohm 1 watt resistor.
I thought solid state rectifiers in a tube set was a bad thing due to something called cathode stripping. or am I mistaken?
Thanks
Elton

4/20/2012 12:25:15 AMNorm Leal

Hi Elton

You heard about this problem since solid state rectifiers bring up B+ power within a second or so. The 6X5's took longer. Some radios use an 80 or 5Y3 rectifier. These tubes also bring up power before other tubes have a chance to heat.

You will have far less damage from cathode stripping than 6X5's shorting and destroying a power transformer. Haven't seen a problem from this modification.

Norm

:I have a Zenith 10s464 with the 6X5 tubes and was told to replace each of them with a diode and 150 ohm 1 watt resistor.
:I thought solid state rectifiers in a tube set was a bad thing due to something called cathode stripping. or am I mistaken?
:Thanks
:Elton
:

4/20/2012 2:22:31 PMWarren

Cathode stripping or bombardment would only occur with a lot higher B+ voltages. Consider radios with a selenium rectifier. These never have any problem with instant B+

4/20/2012 9:18:13 PMElton

:Cathode stripping or bombardment would only occur with a lot higher B+ voltages. Consider radios with a selenium rectifier. These never have any problem with instant B+
:
I thought I read somewhere that the 6AX5 was a safe replacement for the 6X5?
Elton

4/20/2012 10:45:43 PMNorm Leal

Elton

6AX5 will replace a 6X5 but filament current is double. Good sub if the power transformer doesn't get too hot.

Norm

4/21/2012 6:47:08 AMBill G.

::Cathode stripping or bombardment would only occur with a lot higher B+ voltages. Consider radios with a selenium rectifier. These never have any problem with instant B+
::
:I thought I read somewhere that the 6AX5 was a safe replacement for the 6X5?
:Elton
:
Hi Elton and the whole forum,
I agree with all that was written above. I commonly restore radios from the 1950's that have selenium rectifiers. Some still have their original power output tubes, not cathode stripped.
To maintain the time delay, though I wonder what you think of this idea. Replace the 6X5G tubes with the diodes as before, but do not use the resistor. Diconnect the transformer center tap and connect it to the cathode of the 6X5G, then ground its plate or take the plate to where the center tap had gone if not ground.
Now the 6X5G tube will still create the timed turn on and drop the extra voltage, eliminating the need for the 150 Ohm resistor. If an H-K short develops in the 6X5G, it will not be a big problem as no voltage on the tube will be much above ground.

What do you think?

All the Best,

Bill Grimm

4/21/2012 7:05:48 AMNorm Leal

Bill

Neat idea. Delays B+ coming up in a radio and does away with an added resistor. One negative, requires some rewiring.

Norm

:::Cathode stripping or bombardment would only occur with a lot higher B+ voltages. Consider radios with a selenium rectifier. These never have any problem with instant B+
:::
::I thought I read somewhere that the 6AX5 was a safe replacement for the 6X5?
::Elton
::
:Hi Elton and the whole forum,
: I agree with all that was written above. I commonly restore radios from the 1950's that have selenium rectifiers. Some still have their original power output tubes, not cathode stripped.
: To maintain the time delay, though I wonder what you think of this idea. Replace the 6X5G tubes with the diodes as before, but do not use the resistor. Diconnect the transformer center tap and connect it to the cathode of the 6X5G, then ground its plate or take the plate to where the center tap had gone if not ground.
: Now the 6X5G tube will still create the timed turn on and drop the extra voltage, eliminating the need for the 150 Ohm resistor. If an H-K short develops in the 6X5G, it will not be a big problem as no voltage on the tube will be much above ground.
:
: What do you think?
:
:All the Best,
:
:Bill Grimm
:

4/21/2012 10:51:30 AMEdd

Sir Bill Geeeeeee. . . . .

A la . . . . .

Replace the 6X5G tubes with the diodes as before, but do not use the resistor.

Disconnect the transformer center tap and connect it to the cathode of the 6X5G, then ground its plate or take the plate to where the center tap had gone if not ground.

Now the 6X5G tube will still create the timed turn on and drop the extra voltage, eliminating the need for the 150 Ohm resistor.

If an H-K short develops in the 6X5G, it will not be a big problem as no voltage on the tube will be much above ground.

With the real final qualifier being . . . . .

What do you think?

I can readily see all of the
DC
power flow aspects, but with a little "iff iness" in the
AC
flow path, in the respect of its negative power path now being held
above
"true ground" by the respect of the forward voltage drop of the 6X5.

Try the circuit mod with a scope hanging from true ground across the 6X5 circuitry and evaluate how it affects the bass at the different volume levels of operation.

E.G. . . . . . do you see any waveform activity on the scope in testing?

Phase two in testing :

Use a meter to DC monitor the maximum voltage read across the 6X5 from a cold start up to running temperature.

Then, use that highest voltage figure to select a larger voltage rated electrolytic to place across the 6X5 circuitry, install it polarity respected and use a back up .01 ufd disc ceramic cap shunting across it to give a low Z path for any errant RF on the line, while the electrolytic takes care of lower AC frequencies now getting a low Z AC path to ground.

This just basically smells like the olde tyme procedure of floating the high voltage center tap of the secondary of the power transformer above ground, to acquire a derived negative voltage power source for other areas, BUT, which was using electrolytic shunting /filtering across that supply also.

End . . . .

73's de Edd

One should never spoil a good theory by explaining it.

:Bill
:
: Neat idea. Delays B+ coming up in a radio and does away with an added resistor. One negative, requires some rewiring.
:
:Norm
:
::::Cathode stripping or bombardment would only occur with a lot higher B+ voltages. Consider radios with a selenium rectifier. These never have any problem with instant B+
::::
:::I thought I read somewhere that the 6AX5 was a safe replacement for the 6X5?
:::Elton
:::
::Hi Elton and the whole forum,
:: I agree with all that was written above. I commonly restore radios from the 1950's that have selenium rectifiers. Some still have their original power output tubes, not cathode stripped.
:: To maintain the time delay, though I wonder what you think of this idea. Replace the 6X5G tubes with the diodes as before, but do not use the resistor. Diconnect the transformer center tap and connect it to the cathode of the 6X5G, then ground its plate or take the plate to where the center tap had gone if not ground.
:: Now the 6X5G tube will still create the timed turn on and drop the extra voltage, eliminating the need for the 150 Ohm resistor. If an H-K short develops in the 6X5G, it will not be a big problem as no voltage on the tube will be much above ground.
::
:: What do you think?
::
::All the Best,
::
::Bill Grimm
::
:
:

4/21/2012 5:39:08 PMBill G.

Ah, Sir Edd,
I am most impressed with your well thought out response and it has me thinking.
Back in the back of my shelf I have the only radio with a 6X5. If I modified it this way and placed the scope across the 6X5 and turned it on,I am sure I would see no AC. The primary of the power transformer is open. That always kills humm. It is in the back of my shelf for a reason.
Little doubt that 120Hz would be noted on a live radio, as expected. The ampitude would be based on the forward voltage drop of the 6X5, which would never have the pleasure of being reverse biased.

All I can still see from the change would be that the HV winding of the transformer would float at a little higher DC potential, as the forward drop is now near ground not near B+.
If an attempt is made to use the negative voltage at the 6X5 cathode, all bets are off. It is unfiltered. The cathode only connects to the center tap. The 6X5 plate would connect to where the center tap had been. On those sets that use a negative bias, that would not be ground, but to a voltage divider.

Am I missing something?

All the Best,

Bill Grimm

4/21/2012 8:05:42 PMJohn Kogel

Why not just install a standby switch on the B+?

We were checking out a high end two channel Scott amplifier the other night at a radio group meeting, and noticed a solid state full wave rectifier and a resistor. Nothing fancy about that design. I think if the Scott designers were concerned about cathode stripping, they could have come up with a timer switch.

:Ah, Sir Edd,
: I am most impressed with your well thought out response and it has me thinking.
: Back in the back of my shelf I have the only radio with a 6X5. If I modified it this way and placed the scope across the 6X5 and turned it on,I am sure I would see no AC. The primary of the power transformer is open. That always kills humm. It is in the back of my shelf for a reason.
: Little doubt that 120Hz would be noted on a live radio, as expected. The ampitude would be based on the forward voltage drop of the 6X5, which would never have the pleasure of being reverse biased.
:
: All I can still see from the change would be that the HV winding of the transformer would float at a little higher DC potential, as the forward drop is now near ground not near B+.
: If an attempt is made to use the negative voltage at the 6X5 cathode, all bets are off. It is unfiltered. The cathode only connects to the center tap. The 6X5 plate would connect to where the center tap had been. On those sets that use a negative bias, that would not be ground, but to a voltage divider.
:
: Am I missing something?
:
:All the Best,
:
:Bill Grimm
:

4/21/2012 8:58:26 PMBill G.

Hi John,
Yes, cathode stripping is off the table, not an issue.
I am just getting fancy, using the 6X5 in place of a resistor.

All the Best,

Bill Grimm