What causes the "bloom"?

[JimiB]

Yes. The low end is absolutely luscious. You feel like you're floating on a football field filled with marshmallows. And it gives a singe to the midrange that puts solos right out there. It works great for chords and solos, but especially well for slide. It's the kind of enclosure that Lindley and Lowell George used.

What enclosure are you refering too?

[Tdale]

Does it matter where the 110 and the 110 are placed?

[ayan]

Does it matter where the 110 and the 110 are placed?

I am sure you meant 100 and 110, right? The greater plate load resistor on the PI always goes on the side that gets the negative feedback added to it, and it has the purpose of giving that side a bit more gain to compensate for the output signal loss due to NFB.

Gil

[Tdale]

Of course, 100 and 110.

Will the trim pot do the trick if I use two 100K and a trim pot, or is it necessary to have the two values and a trim pot?

Tommy

[Structo]

When you use a trimmer across the PI plates, how is that adjusted?

Do you measure the plate voltage and try to get them both to the same voltage?

[heisthl]

You use 100k on the input plate and 110k on the GFB plate and adjust to make the input plate 3 to 6 volts higher. Listen for accented 2nd harmonics (octaves) to adjust.

[Zippy]

Will the trim pot do the trick if I use two 100K and a trim pot, or is it necessary to have the two values and a trim pot?

Given that one of your two 100K resistors will be greater in value than the other, it is usually best to use the unequal values (100/110) and adjust the trim pot to achieve optimum balance. The trim pot is only additive and, if in series with the resistor of higher value, cannot subtract resistance if needed.

[ayan]

You use 100k on the input plate and 110k on the GFB plate and adjust to make the input plate 3 to 6 volts higher. Listen for accented 2nd harmonics (octaves) to adjust.

I have to say that my experiences in this department have been very varied. In one amp, the juciest harmonics would occur when the non feedback side had the higher voltager Unless the PI 12AX7 is screened to be perfectly symmetrical in it performance (transconducatncewise), I have found that I simply can't go by the volatages alone.

Furthermore, unless one hits the jackpot with a given tube in the PI, it pays off to try a few different ones, as the differences can be huge.

Gil

[Buschman]

My defination of bloom has been a note that transends into another even order harmonic. Volume and guitar have everything to do with it. I have a strat that blooms certain notes on any amp I play, especially at gig volume. I also believe that unmatched tubes contribute to it. It is a major reason i would rather play electric guitar than acoustic.

[Structo]

I went with 110K / 120K on my D'lite (6L6).

Would it be better at 100k / 110k?

What would be the sonic difference between the two values?

[heisthl]

I went with 110K / 120K on my D'lite (6L6).

Would it be better at 100k / 110k?

What would be the sonic difference between the two values?

Almost none - the higher values mean lower plate voltages (but very slight at only a 10k difference). In theory I suppose the lower values are a little more aggressive. It was common in old Fenders to see 82k/100k.

[Structo]

Thanks.
Yes, I noticed that with the Fenders.

But they also use a 12AT7 as the PI tube. Does that make a difference in what plate resistors are used?

[Zippy]

Thanks.
Yes, I noticed that with the Fenders.

But they also use a 12AT7 as the PI tube. Does that make a difference in what plate resistors are used?

Whether a 12 AT7 or 12AX7 is/was used in Fenders depends on the build - even tweeds (ala Super 5F4 and Bassman 5F6-A) were using a 12AX7 for the PI.

[ayan]

Thanks.
Yes, I noticed that with the Fenders.

But they also use a 12AT7 as the PI tube. Does that make a difference in what plate resistors are used?

I would guess yes, it does. The long-tail pair PI is designed to behave like a constant current source and I would presume the circuitry is built around the tube that goes in the PI. 12AT7s inherently behave more like current sources than do 12AX7s, so I think one can get away scaling down the network (i.e., lower resistors).

In the case of the BM PI, the tube is a 12AX7, the plate load resistors are higher and the tail is shorter... and there is less negative feedback around the power amp as well. All of that makes that PI sound a hell of a lot different (more aggressive, more overdrive, more harmoniccs, etc.) than the regular Dumble PI, which is certainly much smoother sounding.

Gil

[Structo]

Sometimes I get confused by the terms.

Is #124 a Bluesmaster type amp?

The reason I ask is because it has the 110K/ 120K plates on the PI.

That is what I have on my D'Lite 44.

I don't have the trimmer there (yet) because it will be difficult due to the topology on my board.

I also have the 220K/ 150K plates on both V1 and V2.