Fattening the clean side of a non-HRM

[Today]

Are you using a classic tone stack or skyline?
CW

After consultation with people on this thread, their advice, I think the tone stack is the following (already pots are installed, not yet wired): Bass 250kA (log), Mid 250kA, Treble 250kB (linear).

[Today]

Thank you, gentlemen, for your kind participation/advice. Perhaps a small detail scheme should be drawn (as it is on Accent switch - since I think this solution of Presence is really very useful in general... there is no need for subtle changes in this domain, it is ON or OFF... one could hardly tell in-between postions/variations) ... all in a tradition of ¨a picture is better than 1000 words¨, sort of... it would be useful for a general application purposes. I definitly am going to do it. Once again, thanks. Cheers.

[Today]

For the voltage division, Rf can be changed as required. In this 124 example Rf is 4k7 and Rt is 390, making the divider ratio 390/(4k7+390) = 0.077.

Do I understand this correctly, as it looks like it is stated above at quoted sentence: VERSION A: 4 Ohm tap on OT connection requires Rf 4k7 resistor and Rt of 390? And 8 Ohm tap on OT requires Rf 6k8 resistor and Rt of 390?

I saw somewhere on this forum a while ago (written in a schematics) that it is: VERSION B: 3k3 resistor for 4 Ohm tap and 4k7 resistor on 8 Ohm tap. It looks that quoted version (A) is right as numbers go...

[martin manning]

Do I understand this correctly, as it looks like it is stated above at quoted sentence: VERSION A: 4 Ohm tap on OT connection requires Rf 4k7 resistor and Rt of 390? And 8 Ohm tap on OT requires Rf 6k8 resistor and Rt of 390?

Correct. 4k7 and 390Ω came from the 124 schematic, where the OT is a Twin type with a 4Ω secondary.

[Charlie Wilson]

Here is why I asked what stack you are using and why I asked Martin about the tail resistor. All of the Low Plate Classics I have seen use a 3k3 or 2k7 for a NFB resistor with a 270 ohm tail on a Twin OT 4 ohm tap. So for a Low Plate Classic, a 4k7 NFB/ 270 ohm tail on the 8 ohm tap would be equivalent. A Low Plate Classic stack is a .1 bass, .047 mid, and 330pf treble.
CW

[Guy77]

Hands down my favorite tone stack. I use the 3.3K feedback resistor as well with this stack and for variety I place a 10kL pot in line with it at the back of the amp. There are so many cool sounds available when you have an adjustable feedback resistor. Just something to think about.

Cheers

Guy

[Today]

Here is why I asked what stack you are using and why I asked Martin about the tail resistor. All of the Low Plate Classics I have seen use a 3k3 or 2k7 for a NFB resistor with a 270 ohm tail on a Twin OT 4 ohm tap. So for a Low Plate Classic, a 4k7 NFB/ 270 ohm tail on the 8 ohm tap would be equivalent. A Low Plate Classic stack is a .1 bass, .047 mid, and 330pf treble.
CW

Thanks. I use 0,1 bass, 0,047 mid, while my treble is 370pF (other participant stated that 330pF is favorable - why?). As for the negative feedback, all things considered, for an 8 Ohm tap on OT, I would have to use 270 Ohm tail and a Rf 4k7 resistor, right? So it is stated above. Also, this amplifier has 2x EL34 output valves, supposed to be used so that they give about 25-30 W output (not more than 30 W anyway). Output impedance is for an 8 Ohm speaker. So, what would my Rt and Rf be for a standard guitar NFB?

[martin manning]

Also, this amplifier has 2x EL34 output valves, supposed to be used so that they give about 25-30 W output (not more than 30 W anyway). Output impedance is for an 8 Ohm speaker. So, what would my Rt and Rf be for a standard guitar NFB?

You didn't mention that before. Combined with an 8-ohm output you will be doubling both the primary and secondary impedances, and you can keep Rf and Rt the same as for the 100W power section.

[Today]

Also, this amplifier has 2x EL34 output valves, supposed to be used so that they give about 25-30 W output (not more than 30 W anyway). Output impedance is for an 8 Ohm speaker. So, what would my Rt and Rf be for a standard guitar NFB?

You didn't mention that before. Combined with an 8-ohm output you will be doubling both the primary and secondary impedances, and you can keep Rf and Rt the same as for the 100W power section.

So, what would my Rt and Rf be?

NOTE: Yes, I have seen theoretical paper this morning, but let me admit that it would be much easier if there was an ¨instant gratification¨ in two numbers (instead of getting my learning curve engaged... ... I have been long time away from valve amp theory and practice... as years go by... )

[martin manning]

I am about to build a D amp #124 with CLEAN ONLY preamp (only one ECC83 valve), low plate classic with 100k slope and 100k anode resistors etc.

Per the 124 schematic and layout, Rf is 4k7 and Rt is 390.

[Charlie Wilson]

Yes we have photos to back up that those indeed are the values in 124 as a Skyline. However, the layout for 124 as a Low Plate Classic is based on educated assumptions. All of the actual Low Plate Classics I have seen photos of have a 3k3 or 2k7 NFB with a 270 ohm tail. In fact, I have reason to believe that 124 started out as a High Plate Classic not a Low Plate Classic and I have spoken to Tony about this. Really it’s about making the amp sound they way you like it so I think all possibilities should be tried.
CW

[Today]

I am about to build a D amp #124 with CLEAN ONLY preamp (only one ECC83 valve), low plate classic with 100k slope and 100k anode resistors etc.

Per the 124 schematic and layout, Rf is 4k7 and Rt is 390.

So my 2x EL34 approx. 30W clean-only preamp #124 variation amp shall have Rt of 390 and Rf of 4k7 on an 8 Ohm tap of my OT. Thank you for saving me much time that I would spend ¨refreshing¨ my rusted knowledge of valve amps (I have been on this forum in 2006 for couple of years... and than long pause, and than shortly in 2017... and than a long pause...).

[martin manning]

Yes we have photos to back up that those indeed are the values ([4k7 and 390] in 124 as a Skyline. However, the layout for 124 as a Low Plate Classic is based on educated assumptions. All of the actual Low Plate Classics I have seen photos of have a 3k3 or 2k7 NFB with a 270 ohm tail. In fact, I have reason to believe that 124 started out as a High Plate Classic not a Low Plate Classic and I have spoken to Tony about this.

Looking at the FB voltage ratio, 3k3 and 270 is very nearly the same as 4k7 and 390. Combined with the typical 1u presence or accent cap, the difference in the presence knee is 637 Hz, vs. 442 Hz (for the 4k7 & 390), shifting the presence effect up half an octave. I'm guessing that is the reasoning behind the change in values.

Really it’s about making the amp sound they way you like it so I think all possibilities should be tried

Agreed, 100%!

[Charlie Wilson]

I would like to add that in the Low Plate Classics with the accent switch, Dumble used a .1uf presence cap with a 500 ohm trimmer in series. At least in the ones I have seen.
CW

[martin manning]

I would like to add that in the Low Plate Classics with the accent switch, Dumble used a .1uf presence cap with a 500 ohm trimmer in series. At least in the ones I have seen

What value is the PI tail resistor in those?