6550's as a 6L6 replacement

[greiswig]

Hi,

I was wondering about trying a pair of 6550's in a D'Lite-type build. The PT is easily capable of supplying the filament current, but I'm wondering about another parameter that seems to be a concern: the grid circuit resistance.

I'm not sure how one adjusts this parameter, or whether it really does need to be adjusted? I am guessing that it is the total of several different resistors in the circuit, but which ones?

Any other parameters that would be a concern?

Thanks!

[Structo]

Here is the data for a 6550 and a 6L6GC.

The pinout looks to be the same.

You can download the Duncan Tube Data Sheet Locator here
Very handy program. Just enter the designator in the search area.

http://www.duncanamps.com/tdslpe/

[Alexo]

Grid circuit resistance is just the value of the resistor between the grid and the bias supply. 6550's spec this at 50k in fixed bias, which is very low, relative to most power tubes. But if you look around, I'm sure you'll find examples of guitar amps with higher values used.

[Andy Le Blanc]

some types of 6550 have significantly higher plate dissipation ratings
over 40 watts on the GE specs for a 6550-A....... a pair can draw over 290 ma.
Id double check your power tranny and OPT ratings if you have notions of
trying to push the tubes....... you might put your amp into a condition which is
very taxing upon the PT and the OPT......

[greiswig]

Thanks, gents. Most of the Ampeg circuits I saw had the 50k in there, not close to the 220k that is in my current build. But at least now I know where to tweak to decide whether I like it or not.

Andy, the OPT should be fine, as it's rated for 4 6L6's. But the PT is another story...I'll have to check. Thanks for that heads-up.

Happy Thanksgiving, all!

[greiswig]

Okay, the (EDIT) PT is rated at 200ma. Plus there's 3 AX7's in there as well. But I'm not sure how/whether the draw of the 6550's is related to the voltage. The 6L6's are running at 430V. What are a pair of 6550's going to draw at that voltage?

http://www.mclink.it/com/audiomatica/tubes/6550.htm undoubtedly shows me what I need to know, but I don't know how to interpret it.

[Andy Le Blanc]

assume 4 6l6 run at 19w max dissipation at 430v
19x4= 76......... 76/430=.176

so with 6l6's your in the clear with around 180ma. being demanded by the
power tubes worst case, leaving some margin

assume 4 6550 run at 35w max dissipation at 430v
35x4= 140.........140/430=.325

and with 4 6550 youll find issue with a worst case demand of over 300ma.

lets say 70% rule 35x .7 = 24.5w

so 24.5x4= 98......... 98/430=.228 or around 230 ma. so it looks possible
but your not going to be able to take full advantage of the 6550
if your looking to run a quad.

assume 2 6550, 35x2=70........ 70/430=.162 or 162ma
2 6550-A, 42x2=84..................84/430=.195 or 195ma.
so a pair looks good.

these are all based on assumptions, the plate voltage will drop under load
you should also look at the OPTs effective impeadance to see if its a close match for the 6550.

[greiswig]

Thank you, Andy. I'm only looking to run a pair of 6550's. I thought the 200ma figure you were stating was for a pair, but from what you wrote here, a single pair of 6550s should be in the tolerance of this, am I right?

[Andy Le Blanc]

yes. the bits of information were an opt for 4 6l6 and then later 200ma. .
thats the trouble with assumptions.

whats the pt rated for?

if you have a signal generator you can futher test the opt, for impeadance and turns ratios.
itll help to figure an optimal loading.

[greiswig]

yes. the bits of information were an opt for 4 6l6 and then later 200ma. .
thats the trouble with assumptions.

whats the pt rated for?

if you have a signal generator you can futher test the opt, for impeadance and turns ratios.
itll help to figure an optimal loading.

Andy, I meant to say before that the POWER transformer was rated at 200ma. I put in OPT instead by mistake. The OPT is rated for 4 6L6's or EL-34s.

[Andy Le Blanc]

I have a similar project waiting on me. It looks like I can put an 8 ohm load
on the 4 ohm tap of a marshall 100w opt and end up with an effective plate
load thats very close to RCA quoted load for a pair 6550, around 3.5k.
the GE tube data says I should get around 75 to 80 w out of a pair of 6550-A
with that load and fender-ish voltages. the opt for a twin is around 4k with
a 8 ohm load too. in the ballpark for 6550.

[greiswig]

Thanks, gents. Most of the Ampeg circuits I saw had the 50k in there, not close to the 220k that is in my current build. But at least now I know where to tweak to decide whether I like it or not.

Andy, the OPT should be fine, as it's rated for 4 6L6's. But the PT is another story...I'll have to check. Thanks for that heads-up.

Happy Thanksgiving, all!

Okay, I got a pair of 6550's to try out, used, and wanted to plug them in to make sure they work. Briefly. I turned my bias pot all the way down, but even there the tubes settled into nearly 100ma bias! Clearly too hot.

I have not changed that grid resistor yet. If I do that (220k down to 50k), will the bias current drop to within range of the adjuster? I feel pretty dumb for asking the question, because I feel like I should know, but...there it is.

[Ears]

Watch out for heater draw.
6550 = 1.6 A
6L6 = 0.9 A
edit: whoops, see you are aware of that.

[Ears]

Although for 6550A GEC give 440V as max for Vg2 Kevin OConnor (Principles of Power) and RCA (for 6550) rate it at 400V max.

So if Vg2 s close to your Va at 430V you will, at the very least, be running very close to limits of Vg2 for 6550.

Also beware of the max plate currents quoted for typical pentode operation in the data sheets. For 6550A they are usually given for a lower Vg2 of around 300V.
You need to scale the curves if operating at higher Vg2. I'm unsure of the % of change to expect. Kuehnel provides a method to do this in two of his books, the one on Guitar Power Amps and the one on the 5f6-A

[greiswig]

Although for 6550A GEC give 440V as max for Vg2 Kevin OConnor (Principles of Power) and RCA (for 6550) rate it at 400V max.

So if Vg2 s close to your Va at 430V you will, at the very least, be running very close to limits of Vg2 for 6550.

Also beware of the max plate currents quoted for typical pentode operation in the data sheets. For 6550A they are usually given for a lower Vg2 of around 300V.
You need to scale the curves if operating at higher Vg2. I'm unsure of the % of change to expect. Kuehnel provides a method to do this in two of his books, the one on Guitar Power Amps and the one on the 5f6-A

Thank you, Ears. I'm still learning this stuff, and I consider this to be one of the parts of tube technology that I am least aware of. (and this should be basic stuff!) Terminology also sometimes screws me up...

I think I can figure out which pins to check Vg2 (Pin 5?) and Va (Pin 3?) voltages on. If the grid resistor is the 220k that feeds Pin 5, then reducing it from 220k to 50k will raise that voltage even higher, yet that grid resistance is limited to 50k according to what I've read. So do you just have to play with the dropping string to get this into line?

Also, as I said, the bias reading is way off. I'm still not clear on how to get that more into line. Can you help me out there?

I see I can get the book you mention from pentode press...I'll see if I can get that.