Tweaking My 102 Amp: A Massive Resistor Makeover

[ayan]

Gil, I can follow your post on gain and headroom, but this is confusing;

" To recap, I will list again my original claims that a larger plate load resistor results in:

(b) Less harmonic distortion at the output (taken off the plate) before clipping occurs. CHECK (well, unless you have some place you can point me to that suggests otherwise)

When using a larger plate resistor( like 220k or 150k) you automatically are in curved part of the load line of a 12ax7, so the upper half is different from the bottom half of the audiowave, which is harmonic distortion. Or do you draw your loadline different?

Hi Erwin,

I think irrespective of the plate resistor value, when you apply a signal at the 12AX7 grid, the output will get squashed at the top because the constant grid voltage plots (on a plate current VS voltage graph) are curved. They curve up as the plate voltage increases, so that means that the positive excursion of the plate voltage will start to level off before the negative excursion does.

Also, the higher the plate load resistor, the more horizontal the load lines will be -- in the extreme case, when the plate resistor is infinitely large, there is no AC current through the tube and the load line is horizontal. When this occurs, you can see that while moving along the load line the spacing of the constant grid voltage curves is more uniform that in the case of a vertical load line. The output is therefore more symmetrical when the load line is more horizontal, and therefore larger plate load resistors result in less harmonic distortion.

Gil

[martin manning]

RDH4 describes a shift in the harmonic content when changing plate resistor from 2x internal plate resistance from mostly 2nd harmonics to 3rd and 5th harmonics mostly when going to a plate resistor equal of 4-5x internal plate resistance of a tube. So the harmonic content shifts when doing that, and the result is very audible in these overdrive amps.

PS: yes that is the Radiotron Designers Handbook, 4th ed.

jelle

The answer is right here.

I saw this earlier and made note. Here are the results of the simulations I ran, FFT's for 100k and 220k plate loads. The tube model may not be perfect, but good enough to show the trends. As you can see the 2nd is about the same (in dB down from the fundamental) for both, but the 3rd, 4th, and 5th are significantly increased for the 220k (second plot).

[talbany]

So here is clip of the amp loaded with the Dales Gil is loading in his low plate.
The clean channel first (Neck and then bridge) Then 1:47 OD comes on the bridge. Maybe I do another one with some more chord work and use the PAB. Not the best sound quality but not bad either.You get the Idea!
It's a little long but time flies when your noodling

Btw.
Anyone care to guess what value plate resistors are in this monster

(Gil looking forward to you putting them in your low plate your going to love em)

https://www.youtube.com/watch?v=3DWff50_OVY

Tony

[norburybrook]

So here is clip of the amp loaded with the Dales Gil is loading in his low plate.
The clean channel first (Neck and then bridge) Then 1:47 OD comes on the bridge. Maybe I do another one with some more chord work and use the PAB. Not the best sound quality but not bad either.You get the Idea!
It's a little long but time flies when your noodling

Btw.
Anyone care to guess what value plate resistors are in this monster

(Gil looking forward to you putting them in your low plate your going to love em)

https://www.youtube.com/watch?v=3DWff50_OVY

Tony

Tony,

Thanks for posting that. It's a lovely sounding amp really , really nice. I was taken with the fact that on clean it seemed to have 'hair' which is not something I expect on ODS amps unless you dime the input volume. Was I hearing that correctly? the OD tone is perfect for me , lovely and 'singing' without excessive gain, there's something of my 70's second gen about it which makes me think it's a low plate amp. Is it EL34's ?

nice playing too, little Eric Johnson licks crept in there


M


p.s. nice to actually see who you are too and nothing like I expected

[talbany]

Hey Marcus
Really Appreciate the compliments!. For the most part that amp is relatively clean however occasionally if I dig in hard enough some hair will appear. I think this is the byproduct of the X5F ceramic cap in the tone stack. When I had the chance to play 123 some years ago I got similar hair fragments. I suppose some Dumble amps might respond differently though. My input volume was set right about noon 2-35% CTS Fender pot. I too like the gain or drive pot set pretty low and goose the OD master enough to keep the high strings clear and some snarl on the wound strings. Yes those are my cheap JJ 34's. You are correct this is a low plate amp. 107K all around.

p.s. nice to actually see who you are too and nothing like I expected

I'll bet you were expecting some old fat guy huh!. Yeah well I'm not fat

Tony

[martin manning]

Great sound, indeed Tony. I don't believe I've heard a clip of a MM PS ODS before. Lots of sustain there, and on right the edge of FB. What's in the signal chain?

[talbany]

Great sound, indeed Tony. I don't believe I've heard a clip of a MM PS ODS before. Lots of sustain there, and on right the edge of FB. What's in the signal chain?

Martin
Thanks!. Signal chain is guitar straight in.Dumbleator feeding my ole LXP-15 I've had forever w/ a little reverb. Thats it

Tony

[norburybrook]

Great sound, indeed Tony. I don't believe I've heard a clip of a MM PS ODS before. Lots of sustain there, and on right the edge of FB. What's in the signal chain?

Martin
Thanks!. Signal chain is guitar straight in.Dumbleator feeding my ole LXP-15 I've had forever w/ a little reverb. Thats it

Tony

ah so there's Dumbleator in the mix .... I might have to re visit the Dumbleator

I've been wet/Drying of late to keep the amp tone 'pure' works very well, just makes for a more complex rig!!!

M

[talbany]

Oh Yeah! and it still has Gil's 50k send pot in there
Marcus my Dumbleator is running Parallel (Mix control) This will help keep the sound more "Pure" if you will.I generally run the output pretty wet and blend in just a touch of reverb. I've found the amp stay's organic this way and still get the benefits of the Dumbleator sound.
Tony

[martin manning]

To recap, I will list again my original claims that a larger plate load resistor results in:

(a) More headroom for the stage. CHECK
(b) Less harmonic distortion at the output (taken off the plate) before clipping occurs. CHECK (well, unless you have some place you can point me to that suggests otherwise)
(c) A greater output voltage swing before clipping (and, quoting myself, "in other words, more gain for the stage"). I'll give myself a CHECK here, the intended use of the word gain was clearly to describe the output compared to the input.

Following up here with some results from distortion analysis along a load line for an actual 12AX7, data collected and analyzed using a uTracer curve tracer.

Actually, all three of the above statements a), b), and c) depend on the bias point chosen as well as the load resistance. In the case of distortion, the signal level is also important. This must be because the slope of the transfer curve along any load line is not constant.

In Merlin Blencowe's example (linked above) he looks at several Ra at constant Bias, with signal level adjusted to hold Vg min constant. The results below for two values of Ra are generally in agreement with Blencowe (higher load resistance produces lower HD2), with the same assumptions.

However, if the bias point is chosen according to the convention seen in ODS amps (a bit cooler for the 220k Ra), the HD2 (and THD) are higher for larger Ra. The cases to be analyzed are specified by choosing a bias point and a signal level. The Rk value is back-calculated from the Ra and Vg. I am purposely staying away from Vg = 0, and using Vg = 0.5 as the limiting signal level. The last row in the table shows that the relationship for the two Ra's at constant Vs remains approximately the same if the common bias point is moved to a more negative Vg.

It's been interesting digging into this; hope it adds some value. At least it provided a good excuse to get the uTracer working again!

[norburybrook]

Oh Yeah! and it still has Gil's 50k send pot in there
Marcus my Dumbleator is running Parallel (Mix control) This will help keep the sound more "Pure" if you will.I generally run the output pretty wet and blend in just a touch of reverb. I've found the amp stay's organic this way and still get the benefits of the Dumbleator sound.
Tony

Tony,

I think that's the key, parallel or wet/dry. I've a Ceriatone C Lator in a box here but the new ones are a PCB, which is very nice. I just need to work out how I can mod it to make it parallel though.

@Martin, that's all gone over my head a little I'm afraid but I'm sure everyone else will be grateful of the info


M

[rogb]

Oh Yeah! and it still has Gil's 50k send pot in there
Marcus my Dumbleator is running Parallel (Mix control) This will help keep the sound more "Pure" if you will.I generally run the output pretty wet and blend in just a touch of reverb. I've found the amp stay's organic this way and still get the benefits of the Dumbleator sound.
Tony

Tony,

I think that's the key, parallel or wet/dry. I've a Ceriatone C Lator in a box here but the new ones are a PCB, which is very nice. I just need to work out how I can mod it to make it parallel though.

@Martin, that's all gone over my head a little I'm afraid but I'm sure everyone else will be grateful of the info


M

Hey Marcus, I'm certain parallel is the key too, and is so easy and great sounding with a Suhr MiniMixII ! Saves having to mess around with the D-lator. I did the parallel mod but never sounded as good as the MMix. It just sat in the bottom of the head cab out of the way.

[talbany]

Oh Yeah! and it still has Gil's 50k send pot in there
Marcus my Dumbleator is running Parallel (Mix control) This will help keep the sound more "Pure" if you will.I generally run the output pretty wet and blend in just a touch of reverb. I've found the amp stay's organic this way and still get the benefits of the Dumbleator sound.
Tony

Tony,

I think that's the key, parallel or wet/dry. I've a Ceriatone C Lator in a box here but the new ones are a PCB, which is very nice. I just need to work out how I can mod it to make it parallel though.

@Martin, that's all gone over my head a little I'm afraid but I'm sure everyone else will be grateful of the info


M

Marcus here is how I did mine. You to have use a dual 500k pot reverse wired to eliminate any null spots you would normally get with wiring in a single pot which makes the pot too touchy which I discovered when it tried it. This method also keeps the effx unit in phase. I have converted all my amps either internal or external D-lators to this configuration. Some people say they have done the parallel mod and preferred the minimix. There are several parallel mods out there and I am not exactly sure which ones they have tried but can tell you I have no problems with this method and have ever felt that I needed to haul around another piece of gear to put into my chain and complicate things further. Therefore have no opinion on the minimix!
I am a simple set and forget reverb/ slapback player
Tony
BTW. On my external Lator I converted it from a 3 knob (Drive control) to a 2 knob and used the extra pot for the mix control.

Serparallel (1).jpg

[donvan]

3. OD HF Taper

Through the years, I went back and forth on this a few times. Early on, I used it. After getting the caps’ outer foils orientation correct, I got rid of it. When I added a larger bright cap on the Master volume, because I liked what it did to the clean, I reinstalled it and it stayed there. I had also tried using a 180K OD output resistor, instead of 150K, but I didn’t like that it decreased the available OD volume somewhat. In the end, I had opted for staying with the 150K and using the HF trim. This time around, I disconnected the HF trim and used a 180K Draloric resistor from Jelle. I will put it this way: I found that the HF trim, which for me was a .001uF cap and the 1 M trimmer dialed in at about 350-400K to ground, has more of an effect on the OD volume than using the 180K output resistor. The overdrive is on the bright side but it is very musical at the same time.

Gil

I know I'm going to feel stupid for asking this but what is the HF Taper and HF trim?

[jcsifu]

Oh Yeah! and it still has Gil's 50k send pot in there
Marcus my Dumbleator is running Parallel (Mix control) This will help keep the sound more "Pure" if you will.I generally run the output pretty wet and blend in just a touch of reverb. I've found the amp stay's organic this way and still get the benefits of the Dumbleator sound.
Tony

Tony,

I think that's the key, parallel or wet/dry. I've a Ceriatone C Lator in a box here but the new ones are a PCB, which is very nice. I just need to work out how I can mod it to make it parallel though.

@Martin, that's all gone over my head a little I'm afraid but I'm sure everyone else will be grateful of the info


M

This is what I use and recommend:



From bluesmanfenderblues:
The Claus-ulator

Dumblelator + 25k linear pot!!

After trying a lot of different parallel loop designs and checking out a lot of schematics, I stumbles on the latest MESA iteration of the effects loop. They used this idea (a simple 25k pot) in the rectifier series 20 years ago, and they still used it in current amps like e.g. the NOMAD series.

I really think the parallel loop below, is the best solution if you want

- Original Dumblelator with
- Effects added in parallel (and in phase with the direct signal)

No effects unit connected:
The 25k pot is totally out of the equation.

With effect unit in the loop:
Turn the pot all the way down, you get 10% wet signal and 90% direct
Turn the pot all the way up to 10, you get almost 100% wet signal, like a traditional Dumblelator.

If you cant stand the idea of the pot being in the loop, you can simply add a switch to take it out of the circuit.

Very simple addition to the Dumblelator and IMO it works like a charm.