Accent Switch Discussion from Hoyt Axton's Dumble for sale in Nashville topic

[martin manning]

Here is where I show how boneheaded I am at the technical stuff. It just seems to me that the feedback frequencies have to travel across either a 4k7 or a 3k3 resistor before it gets to the junction of the divider and that would have some effect on frequency response. I get that because of the divider the voltage would be the same at the junction point.

You are right, but it's the parallel combination of Rfb and Rtail that sets the lower 3dB point, and since Rtail is around 1/10 of Rfb, it dominates in the result. The voltage division is a function of frequency, so this FB voltage ratio that we've been calculating is only correct for DC, strictly speaking.

[talbany]

Besides the speaker load, the difference in the gain of the PI and of the output tubes is in that number.

Yep.. Aiken say's it right on the paper. He also adds in plate voltage and output transformer.
Aiken

For the circuit shown above, the forward gain, G, to the 16 ohm output, when loaded with 16 ohms, is around 41, depending on the plate voltage, the output transformer, the tubes, etc.

Tony

[Charlie Wilson]

So I have been thinking about all of this tonight and what Martin said about the change in frequency response from the presence cap when changing those negative feedback resistors may be what I am hearing. When I have the 4k7/390 the top end and upper mids seem a bit harsh and kind of jump out at you. The 3k3/270 seems to soften it and pull it back a bit. I could see with a Skyline or High Plate Classic where the more aggressive top end would help it cut through all those lows and mids. I keep my presence control usually at twelve o'clock so the effect from the presence cap is subtle but still there. I also think with the earlier Low Plate Classic that the presence circuit is not as big a part of the sound of the amp since it is either completely out or switched in with a 500 ohm trimmer in series with the cap. Also, from what I can see, the cap is a .1uf not 1uf. So it adds maybe a little sparkle to the top end. (the drawing was done by Ultraphonix)
CW

[martin manning]

When I have the 4k7/390 the top end and upper mids seem a bit harsh and kind of jump out at you. The 3k3/270 seems to soften it and pull it back a bit.

Exactly. With the same presence cap value, the smaller Rtail delays the roll-off in the FB to a higher frequency, extending the range over which the gain is held down. Hence, more mids with the larger one.

[talbany]

So I have been thinking about all of this tonight and what Martin said about the change in frequency response from the presence cap when changing those negative feedback resistors may be what I am hearing.
CW

He is talking about the R/C time constant between the cap and the 2 shunt resistor values. It's hard to say that would be enough of a roll-off there (390-270) to shift the -3dB knee low enough to bleed off those frequency spikes your hearing (whatever those are?) Maybe he can run the capacitance reactive numbers. You could always just raise the value of presence cap on the 390 (ala 102) to get the same result and smooth out those upper mids you are hearing(after you run the numbers)


You 2 can take it from here

Tony

[Charlie Wilson]

Hmmm, well maybe have come full circle on this. Maybe a little frustration setting in. Dumble used the 3k3/270 in this Low Plate Classic and all of the others that there are pictures of yet I seem to be getting the vibe that there should be no sonic difference between that and the 4k7/390. It is kind of the feeling when ALL of the evidence points one way yet something to the contrary is being defended (and I don't know why). My understanding of part of the goal of the forum is to study what Dumble did in actual amps.
CW

[martin manning]

CW, what two configurations are you comparing? I know 3k3 and 270, and 4k7 and 390, but are the presence caps the same, and if so what value? 1uF or 0.1uF?

[Charlie Wilson]

Martin, with the 3k3/270 (at least in the amp in this thread) it is a .1 and from the size of the cap in the other Low Plate Classic photos, that seems standard for the accent switch set up with a 500 ohm in series. Also, standard for a Low Plate Classic. Borderland has the conversion to a presence pot. 3k3(sorry 2k7)/270 with a 1k pot. The presence cap on that amp does look larger, so that may be a 1uf.
CW

[talbany]

Martin, with the 3k3/270 (at least in the amp in this thread) it is a .1 and from the size of the cap in the other Low Plate Classic photos, that seems standard for the accent switch set up with a 500 ohm in series. Also, standard for a Low Plate Classic. Borderland has the conversion to a presence pot. 3k3(sorry 2k7)/270 with a 1k pot. The presence cap on that amp does look larger, so that may be a 1uf.
CW

Accept for 102 I have only seen a 1uF used. I think the hand-drawn layout the period was used as an abbreviation for Tantalum.

Tony

[Charlie Wilson]

If you look real close at one of the photos in the original Hoyt Axton thread you can see 104 on the cap.
CW

[talbany]

If you look real close at one of the photos in the original Hoyt Axton thread you can see 104 on the cap.
CW

I skimmed quickly through the thread and don't see it. Can you post the picture? I see the post UP mentions it but I dont see a close-up of a Tant. cap that says 104?


Tony

[Charlie Wilson]

Tony, at my phone right now but find the photo with both preamp boards and the blue tant. and 500 ohm trim right in the middle at the top. If you zoom in real close to the cap you can see 104.
CW

[talbany]

This one? you gotta be kidding me

tant1.jpg

[Charlie Wilson]

That’s it. Zoom in on the cap. Right on top.(on a computer)
CW

[talbany]

That’s it. Zoom in on the cap. Right on top.(on a computer)
CW

You mean to tell me you can actually see 104 in black printing on top of this little blue dot
(Anybody got any spy satellite software I can borrow )
Really it's not that big a deal if you can see it. I trust you . 104 it is.. Next

tantzoomed2.jpg

edited w/ MacbookPro & Photoshop
Tony