6550's as a 6L6 replacement

[greiswig]

Andy, how does all this relate to the manufacturer's published spec of "3.5k primary?"

Pentode press...interesting site!

[Ears]

,<snip>

Pentode press...interesting site!

Yes, get his books if you have an interest from a rigorous engineering angle.
Those, plus the Valve wizard articles are amongst the best I've read specific to guitar amp theory. K O'C too, but his writing style is a little too informal for my taste.

[greiswig]

One of the items at Pentode Press talks about bias excursion. It seems to be at least part of the reason why the 6550 might need a 50k grid resistor, where a 6L6 needs something higher.

But I'm too naive to make much sense of what they're saying. Can someone tell me, in layman's terms to start, why the grid resistor is reduced in value for the 6550? Does this have a sonic effect, or is it just for stability of the tube, or...

[Ears]

One of the items at Pentode Press talks about bias excursion. It seems to be at least part of the reason why the 6550 might need a 50k grid resistor, where a 6L6 needs something higher.

But I'm too naive to make much sense of what they're saying. Can someone tell me, in layman's terms to start, why the grid resistor is reduced in value for the 6550? Does this have a sonic effect, or is it just for stability of the tube, or...

It will affect loading on previous stage so will have an effect in that regard. Read one of my previous posts where I quote valve wizard site for the reason the grid resistance has an upper limit, it's down to reasons of stability.
Edit - the difference between the tubes will be due to internal architecture and grid dissipation limits.

[Ears]

Here are relevant extracts from radiotron designers handbook.

pgs 18-21 are on the make up of grid current in tubes
pgs 82-85 discuss max ratings for grid circuit resistance
pgs 487-4789 are discussion on max resistance for triodes in cathode and fixed bias
pgs 501-503 are same for pentodes.

I included pgs 18-21 for completeness and because the figures are referred to in the later discussions.

I took liberty of highlighting some stuff. Of particular interest to me was the discovery that screengrid resistance had bearing on max control grid cct resistance.

[Andy Le Blanc]

This seems to be a very old discussion of preamp tubes, the values dont seem
to apply to power pentodes?...

[Ears]

I suppose it is old but at 1474 pages long RDH4 is still widely regarded as the most extensive and thorough treatise on tube technology ever written.

The discussion applies to the devices themselves and to max grid resistance in general cases, yes the values may have been for voltage amps. There is also a further two thirds of a page specifically on on power amps that I omitted, so thanks for the heads up. Here it is.

[Andy Le Blanc]

All the equations cited use a grid to cathode transconductance figure.
How do you differentiate this for your tube type when the most common figure
is the total transconductance for your tube?

[David Root]

I built a 2x6550 amp with approx 100k grid resistors. Works fine sounds fine. I am thinking of taking them up to 160k for a bit more power.

I have seen a website, it is www.turneraudio.com.au where he reduced his grid resistors in a 2xKT88 hi-fi design from 220k to 68k to "give better dc stability since less dcV is generated across the 68k as the tube ages". Implying that it worked fine with 220k resistors.

That is a real good website too, I learned a lot from it.

[Bob-I]

I built a 2x6550 amp with approx 100k grid resistors. Works fine sounds fine. I am thinking of taking them up to 160k for a bit more power.

I built a bass amp with 2x6550. I started with 47k as recomended. I measured the output at about 42 watts. After upping the grid resistors to 100k the power went up to almost 80 watts. Really added a lot of punch to the sound as well.

[greiswig]

I built a 2x6550 amp with approx 100k grid resistors. Works fine sounds fine. I am thinking of taking them up to 160k for a bit more power.

I built a bass amp with 2x6550. I started with 47k as recomended. I measured the output at about 42 watts. After upping the grid resistors to 100k the power went up to almost 80 watts. Really added a lot of punch to the sound as well.

Why would upping the grid resistors boost power? And if higher values like this are preferable for guitar amps, why does the data sheet call for 50k max?

[Andy Le Blanc]

its the conditions that were being used when they generated the data.
if want to keep the tube in the safest area of operation when you up the plate volts.
other wise the grid resistors are also part of the plate cicuit of the preceding
stage.
and are counted upon when finding the gain figures of that stage.
Most of the examples presented are also based on minimum distortion .
so that is you have an identical tube, conditions, and load you can replicate the
figures presented.

[Bob-I]

Why would upping the grid resistors boost power? And if higher values like this are preferable for guitar amps, why does the data sheet call for 50k max?

I think, but I'm not sure, that it's the voltage/current swing across the resistor. I only made the change because I was having some clipping issues so I compared to a Marshall schematic. It didn't fix the clipping which turned out to be a bias issue, but suddenly I had much more power.

[greiswig]

Why would upping the grid resistors boost power? And if higher values like this are preferable for guitar amps, why does the data sheet call for 50k max?

I think, but I'm not sure, that it's the voltage/current swing across the resistor. I only made the change because I was having some clipping issues so I compared to a Marshall schematic. It didn't fix the clipping which turned out to be a bias issue, but suddenly I had much more power.

Interesting. I ended up installing a switch to change resistances according to which tube type I was using. I thought the KT88s sounded better with around 55k than with 220k, and there was no discernible volume difference. Had you changed anything else in the circuit at the same time?

[Firestorm]

Lower grid circuit resistance can have a subtle impact on sound and output volume in a push-pull amp because you have essentially reduced the resistance isolating two out-of-phase signals. But to get a big change (like 42 watts going to 80 watts), it's likely that the amp shifted into AB2 operation and began drawing grid current (if the source can provide the current, the amp will be louder).

But this also means that ions are building up on the grid which destabilizes the bias. Gassy tubes can destroy themselves very quickly this way, but even tubes with a solid vacuum can runaway.

Some of the published grid circuit resistances are overly conservative; I think the 6V6 is limited to 100K, but Fender ran them at 220K with no ill effects. The internal geometry of 6550s seems to be less forgiving. I have killed several of them with too much grid resistance.

The other spec that gets ignored too often is the requirement that each tube have it's own bias supply (or it's own cathode resistor). If you do that, you can probably exceed the maximum grid circuit resistance with less danger.