Maco 300 gets 47-year tuneup.

[Shockwave]

Gonna need a better way to screen customers' tubes before tearing into an amplifier. A tube tester just doesn't work them hard enough.

The older the tubes get the riskier they seem to get.

73

The most reliable sweep tube tester that I had was an old two tube, cathode driven 6LF6 amplifier. Pulled one tube socket to install a 9 pin 6LQ6 socket. Aligned the tank circuit so the plate tune could hit resonance on a single tube. Switching to a variac as the filament transformer, allowed testing of everything from 6LQ6, to 40KD6. With a little effort you could even add a third socket to test the 6KV6 pinout. Adding a decent glitch resistor prevented tube failures from damaging the "tube tester". You can test for breakdown voltage and emissions at RF frequencies, rather than DC.

Also, letting old tubes sit with just the filament voltage running for several hours, can help raise the breakdown voltage, and prevent internal tube arcs.

 

[nomadradio]

letting old tubes sit with just the filament voltage running for several hours,

We learned to do that to every old russky tube we bought. You can monitor the gas content by putting a 1 megohm resistor in line with a 100 Volt DC supply. The positive side goes to the cathode, the negative side to the grid. A voltmeter across the 1 meg resistor will read one Volt for every microamp of gas leakage current. With the soviet-era metal tubes you could see that reading drop by the hour. When it stopped dropping the tube was ready to try at full operating voltage.

73

 

[Shockwave]

We learned to do that to every old russky tube we bought. You can monitor the gas content by putting a 1 megohm resistor in line with a 100 Volt DC supply. The positive side goes to the cathode, the negative side to the grid. A voltmeter across the 1 meg resistor will read one Volt for every microamp of gas leakage current. With the soviet-era metal tubes you could see that reading drop by the hour. When it stopped dropping the tube was ready to try at full operating voltage.

73

The only Russian Surplus military tube amplifier that I have that still runs reliably, is one that I built for a pair of GI7bs, that required a GS35b to be installed in order to make it reliable. I understand that others have had a better experience with them. Mine were all extremely gassy. I even had one with so little vacuum left, the filament wouldn't light. It just sat there and drew a lot of current.

 

[Amperex]

When the ultra expensive 8950 tubes die, use cheap 6LB6 with two tubes each filaments wired in series. 6LB6 is same tube as 8950 with different filament voltage.

 

[nomadradio]

all extremely gassy

I guess we just lucked out. YMMV. And yeah, I did have one GI7B go kapow. The others worked okay.

73

 

[Shockwave]

It's in the way, blocks proper air flow, doesn't contribute enough additional power to justify blocking the way for a proper plate choke with one parasitic choke for each tube. The power lost by using four final tubes instead of five won't move the S-meter at the other end by more than the thickness of the meter needle.

Pretty much.

73

Forgive me for having a difference of opinion here however, in my mind it's not the extra power the 5th tube makes. It's removing 20% of the load off of the other four finals, that are already being pushed and essentially still receiving the same amount of drive power that was feeding five tubes.

I also understand that the layout in this amplifier sucks in every aspect from cooling to self oscillation. In other words, you have to work with what was available to start with. It's essentially a Palomar 300A now, with improved stability. You put a lot of time into this amplifier and it looks infinitely better than it did to start with.

 

[Shockwave]

When the ultra expensive 8950 tubes die, use cheap 6LB6 with two tubes each filaments wired in series. 6LB6 is same tube as 8950 with different filament voltage.

The 6LB6 is close to an 8950 however, it has 300 milliamps less peak cathode current and slightly less output capability as a result.