Hallicrafters SR-500 - 6DQ5 vs 8236

[Antonio I0JX]

Does any one have experience on using 6DQ5s in place of 8236s in the
Hallicrafters SR-500? Not easy to find 8236s new.

Tubes are electrically identical, but the 8236 has a carbon plate.

Perhaps not pushing the transmitter to maximum power and cooling the 6DQ5
with a fan could be OK.

Another possibility would be to replace the 8236s with larger sweep tubes,
such as 6KD6, 6JE6 or 6LQ6. But apart from changing sockets, I should check
whether the bias and screeen voltages are compatible.

Again, any hands-on experience on that?

73

Tony I0JX
Rome, Italy

[Scott Dorsey]

Post by Antonio I0JX
Does any one have experience on using 6DQ5s in place of 8236s in the
Hallicrafters SR-500? Not easy to find 8236s new.
Tubes are electrically identical, but the 8236 has a carbon plate.

I can't imagine the carbon plate would make a huge difference in ham
radio service where people will often transmit for a minute or two at a
time. It might help somewhat, but it would not be the huge improvement
in power handling that you would get in pulsed service.

Post by Antonio I0JX
Perhaps not pushing the transmitter to maximum power and cooling the 6DQ5
with a fan could be OK.
Another possibility would be to replace the 8236s with larger sweep tubes,
such as 6KD6, 6JE6 or 6LQ6. But apart from changing sockets, I should check
whether the bias and screeen voltages are compatible.

I would consider the EL509, since it's currently manufactured and pretty
reliable. Maximum ratings including the power dissipation are all MUCH
higher. Socket will have to be changed, or you can make an adaptor, but
it's a far more advanced tube design.
--scott

[Antonio I0JX]

I can't imagine the carbon plate would make a huge difference in ham radio
service where people will often transmit for a minute or two at a time. It
might help somewhat, but it would not be the huge improvement in power
handling that you would get in pulsed service.

That's OK. Nevertheless, in the 8236 data sheet, Tung Sol claims "Its carbon
anode and hard glass bulb permit continuous operation at 50W plate
dissipation". Conversely the 6DQ5 (electrically identical to the 8236) has a
24W plate dissipation. Actually, in the SR-500 the two 8236s draw 500mA at
750V, whilst in the SR-160 (identical to SR-500 except for output power) the
two 6DQ5s draw 200mA at 575V



I would consider the EL509, since it's currently manufactured and pretty
reliable. Maximum ratings including the power dissipation are all MUCH
higher. Socket will have to be changed, or you can make an adaptor, but
it's a far more advanced tube design.

Yes, the EL509 would be the most appropriate substitute. Its plate
dissipation it 35W but, looking at it, it is hard to figure out why it has a
lower plate dissipation than the 8236.

73

Tony I0JX
Rome, Italy