JTM 45 100.tubes

[bmcash]

I have seen a lot of people.drop.6550s in their plexi superlead or bass but never a JTM 45 100. Any reason for this? Is it possible to use anything other than kt66s? Just curious.

B $

[Reeltarded]

Yep. 6L6 is pretty much straight swap and most of the other big tubes that bias up nice at whatever your voltages are. You only need to mind bias supply and that you don't go over the capacity for current on your PT and make a smoke signal.

(s o s my amp is dead)

[Stevem]

I do not think the 45s PT can output the needed extra heater current for a 6550 or a 34.

[Littlewyan]

Also I believe 6550s like to see lower value grid leaks than KT66s, 6L6s, EL34s etc. Marshall used to lower them to 150K from 220K, but I think even 150K is pushing it.

[pdf64]

My understanding is that there's a balance between the tube, especially cathode, temperature and the control grid circuit resistance limit; as the cathode temperature increases, the grid tends to pick up heat and this somehow results in positive grid current.
This results in the grid becoming more positive (WRT the cathode), the cathode then runs hotter and thermal runaway ensues.
So for hot running tubes, the max grid circuit resistance tends to be low; 50k in the case of the 6550 http://www.mif.pg.gda.pl/homepages/fran ... 6/6550.pdf
However, if the tube is never going to reach it's max dissipation, eg when subbed for EL34 in a Marshall, a higher grid circuit resistance may be tolerated without much risk of thermal runaway.
Note that the control grid circuit resistance limit for 6L6GC is only 100k!
The above is the case for fixed bias; in cathode bias, higher control grid circuit resistances can generally be tolerated, due to the self regulating nature of cathode bias.
However, if cathode bias resistors are shared by 2 or more tubes, the self regulation effect will tend to be diminished.

[Littlewyan]

Thanks for the info PDF64. I never knew any of that!

[pdf64]

I remember someone clever, Gingertube maybe, putting forward the theory that the cool operating conditions (eg idle plate dissipation<70%) of guitar amps were the reason they were able to tolerate grid circuit resistance that are often way above the limiting value.
The GEC KT66 Apr 77 info sheet may be seen to back this theory, ie that higher tube dissipations may tend to necessitate lower grid circuit resistances http://www.mif.pg.gda.pl/homepages/fran ... k/KT66.pdf

[Littlewyan]

Thats interesting as I know Ken Fischer believed that EL34s may have a longer life in Marshalls if the grid leaks were reduced to 100K from 220K.

Whats also interesting is how in that datasheet it specifies that a 100-300Ohm Resistor is required for the Screen grid to prevent parasitic oscillation. I know Marshall omitted this resistor when they moved to EL34s so maybe they thought it was only required for KT66s.