97_ODS_HRM_Loop Schematic Question

Decko · Post by **Decko** » Thu Mar 29, 2018 5:15 am

Hello,

I am building my first Dumble style amp after spending many years on the Trainwreck side of Tag site.

I have procured the chassis, boards and parts.

The schematic that I am following is: 97_ODS_HRM_Loop[1] which has the built in Dumbleator.

What preamp is this based from?

And would the author of this schematic be willing to share the Visio layout?

Thanks ,
Decko

norburybrook · Post by **norburybrook** » Thu Mar 29, 2018 8:41 am

It's a Skyline EQ and a high plate clean stage but no LNFB, the OD is not one I can recognise from a particular model.

I'd use that layout and make it with the #102 values across the board. I think the #102 is a great starting point for a classic Dumble ODS.

Obviously use the loop section from this board, but take the rest form the #102 layout in the files section. You can put the LNFB on a toggle switch on the rear of the chassis easily if you want.

Just my 2cents.

Marcus

edit. just looked at the thread title and it says 97-ODS. Maybe it's ODS #97. Not and amp I'm familiar with.

10thTx · Post by **10thTx** » Thu Mar 29, 2018 10:13 am

I've attached an editable ExpressSCH schematic that's reasonably close to the layout that you can edit to match the layout.

The SCH file has a 2nd page showing the B+ rail, bias and other information.

With respect, 10thtx

erwin_ve · Post by **erwin_ve** » Thu Mar 29, 2018 11:29 am

This particular model is based on several HRM models(not the blues master). Search for user Dogears, he hasn't been around for several years, he commented in several threads on Hrm models he has been in to.
I built this amp, it has a slightly stiffer feel compared to the #102(skyline non hrm), but the hrm tonestack is very useable and you can get some great tones with it.
Somewhere in the distant past I made a built thread and last year made a recording:
https://soundcloud.com/erwin_ve/the-brother

The layout is made by Normster, he also hasn't been around for years.

fred.violleau · Post by **fred.violleau** » Thu Mar 29, 2018 12:03 pm

@Decko, I just finished a build that is close to this one (Skyline preamp, HRM with built in D lator).

A few comments:
-I put the bright switch on the volume knob with a push-pull knob
-It allowed me to add a “deep” switch, which is pretty neat I must admit

On the Dlator layout, there are many ways to embed it inside the chassis and many members of the TAG forum have shared layouts that are for some easier to work with. If you need more example LMK.

I made a rookie mistake : I had not noticed that on Normster’s Iayout there was a part of the Dlator he inserted between the OD and the PI on the preamp board. I had to put the missing pieces on an extra board. Had I seen it first hand, I would have tried to put everything on the preamp board and leave more room around the 3rd tube.

Also, on his layout, Normster put a 220k resistor between the Poweramp return jack plug and ground, when on the schematic it is a 250k pot. I am still debugging my amp and it feels like 220k may be a bit high.. I recall Normster saying in his thread he ended up using a 82k resistor. I will eventually try to play with the value of the recovery resistor and post it in my thread.

My 2 cents here, Good luck with your build!

Humble and Dumble,

Fred.

Decko · Post by **Decko** » Sat Mar 31, 2018 12:32 am

Thanks so much for the responses about the amp features/Skyline/#102 and the link to the schematics.

Questions:

Will the high plate version give more headroom versus the low plate?
Would you attribute the "stiffer" feel to the HRM circuit?

To this point I have built up the power board which has the additional network for the d-lator.
I popped holes in the rear of the chassis to add the 250K linear pots at the send/return inputs.

For the preamp board: I would like to employ the circuit being used in this 1980's model amp is see the link below.

Unfortunately the player is speaking in Japanese so I am unable to translate; however the tone is nice to my ears.
He is demoing 3 amps.

https://www.youtube.com/watch?v=sl1cOLA4HVs

1. 1970's Overdrive special with reverb
2. 1980's Dumble Overdrive Special (This is the one I'd like to build)
3. 1990's Dumble Overdrive Special with El34

I am interested in the 1980's model.
This link shows the character.
https://www.youtube.com/watch?v=Cmdyw08ig14&t=37s

What do you think?

Thanks,
Danny

erwin_ve · Post by **erwin_ve** » Sat Mar 31, 2018 7:39 am

The Low plate has more headroom, high plate has more compression and more overtones.
Both are dynamic in response to the volume control on your guitar.

Difference between a Non-HRM and regular HRM circuit: Non-Hrm is more forgiving to play. While potentially very different in overdrive sounds, when doing a side by side comparison it's no question both amps have the typical Dumble overdrive sound. For me it's a feel thing rather than a sound thing.

One of the things I like about the #102 is overtones, compression and easier sustain. I think high plates are giving the goods.
Big thing to the tone of #102 is the presence cap, while on most ODS it is 1uF, the one on #102 is 2,2uF. Due to that, the Presence is shifted more towards the mid rather than the high. In fact I like that so much it's on every single ODS I built. YMMV

The cap on the master volume: when not using a Dlator omit it. When using a Dlator anything between 15pF-68pF according to taste is fine.

Another thing that contribute to a good sounding ODS are the 6PS orange drop( or equivalent 418P and 225P), but you need to get the outer foil connected to the low impance side of each stage( see http://ampgarage.com/forum/viewtopic.php?f=4&t=24654 ). Otherwise you might end up with a fuzzy and Buzzy amp. The first ODS style for me was a Ceriatone OTS, the outer foils were not checked on this amp and I was fighting the amp with bandaids for giving to much fuzz in the lows. After marking the outside foils of all 6PS in that amp and place them accordingly it became a very nice and tonal balanced amp.

Electrolytics: for filtering F&T are good. For cathode bypass caps; superimportant: electrolytics also have a different ESR when choosing different manufacuters; NIchicon vx, Sprague are good and have a nice roll off at highs. Some caps are great for clean( Elna Silmic) but tend to have too much highs when using it for overdrive and will destroy your overdrive tone. Again my experience YMMV.

If you check pictures on #124 you get a good impression on parts that are used and on lead dress(wiring, important).
It's like a Trainwreck you're familiar with, sticking close to the original will give you the desired tone you're after.

martin manning · Post by **martin manning** » Sat Mar 31, 2018 10:42 am

erwin_ve wrote: ↑Sat Mar 31, 2018 7:39 amFor cathode bypass caps; superimportant: electrolytics also have a different ESR when choosing different manufacuters; NIchicon vx, Sprague are good and have a nice roll off at highs. Some caps are great for clean( Elna Silmic) but tend to have too much highs when using it for overdrive and will destroy your overdrive tone. Again my experience YMMV.

I'm not buying this. You might be hearing the effect of differences in capacitance value (given that tolerances are typically +/-20% for these things), but ESR will only provide a bit of negative feedback, with little effect on frequency response. Typical ESR for a 4u7, 25V aluminum electrolytic is a few tens of ohms, say ~50. Nichicon lists DF instead of ESR, but when converted they are in the same range as others.

erwin_ve · Post by **erwin_ve** » Sat Mar 31, 2018 11:14 am

martin manning wrote: ↑Sat Mar 31, 2018 10:42 am
erwin_ve wrote: ↑Sat Mar 31, 2018 7:39 amFor cathode bypass caps; superimportant: electrolytics also have a different ESR when choosing different manufacuters; NIchicon vx, Sprague are good and have a nice roll off at highs. Some caps are great for clean( Elna Silmic) but tend to have too much highs when using it for overdrive and will destroy your overdrive tone. Again my experience YMMV.
I'm not buying this. You might be hearing the effect of differences in capacitance value (given that tolerances are typically +/-20% for these things), but ESR will only provide a bit of negative feedback, with little effect on frequency response. Typical ESR for a 4u7, 25V aluminum electrolytic is a few tens of ohms, say ~50. Nichicon lists DF instead of ESR, but when converted they are in the same range as others.

Martin, as this might result in a longer discussion shall I make a new topic on this one?

martin manning · Post by **martin manning** » Sat Mar 31, 2018 11:23 am

Well, Decko kind of derailed his own thread to a general discussion of ODS tones and circuits... It's an interesting topic, and I'm sure of interest to others, so why not continue?

erwin_ve · Post by **erwin_ve** » Sat Mar 31, 2018 11:45 am

martin manning wrote: ↑Sat Mar 31, 2018 10:42 am
erwin_ve wrote: ↑Sat Mar 31, 2018 7:39 amFor cathode bypass caps; superimportant: electrolytics also have a different ESR when choosing different manufacuters; NIchicon vx, Sprague are good and have a nice roll off at highs. Some caps are great for clean( Elna Silmic) but tend to have too much highs when using it for overdrive and will destroy your overdrive tone. Again my experience YMMV.
I'm not buying this. You might be hearing the effect of differences in capacitance value (given that tolerances are typically +/-20% for these things), but ESR will only provide a bit of negative feedback, with little effect on frequency response. Typical ESR for a 4u7, 25V aluminum electrolytic is a few tens of ohms, say ~50. Nichicon lists DF instead of ESR, but when converted they are in the same range as others.

Yes it is intersting, so here we go.
I agree difference in tolerance has a effect. I measure before putting it in.
Besides Esr, impedance should be mentioned?
The difference for me was a perceived roll off on the high freq. But might be a mid bump as well.
I don't' have a freq analyser so can't tell you.
It was very noticable when switching between Elma silmic and Nichicon vx, I tried different caps from the same series and got the same results.
Correct me if I'm wrong but I think the impedance at 10kHz is different to 1kHz at 4,7uF?
Spec sheets do often higher capacitance graphs, so it's hard to compare.

martin manning · Post by **martin manning** » Sat Mar 31, 2018 12:59 pm

Impedance Z is the vector sum of reactance (Xc = 1/(2πfC) and resistance (ESR). Xc varies with f (it's a capacitor), ESR not so much. So the variation in Z with frequency is mostly due to C. I'm ignoring inductance here since we are at audio frequencies. I don't have a bunch of different brands of caps around, but I measured a 4u7 Nichicon TVX and found the following:

120Hz C= 4.6u ESR= 8.8Ω
1kHz C= 4.4u ESR= 3.0Ω
10kHz C= 3.4u ESR= 1.8Ω

The variation in ESR is not significant, but I wonder if the reduction in C is more more or less in different brands? The decrease in C would reduce lows, and give a perceived increase in highs. The difference there is 26%, or half an octave.

erwin_ve · Post by **erwin_ve** » Sat Mar 31, 2018 3:55 pm

That was quick Martin! What did you use to measure?

So; 120Hz C= 4.6u ESR= 8.8Ω Xc= 1 / (2 × π × 120Hz (4.6x10^-6) )=288 Ohm Z= square root (R^2+Xc^2)= 288 Ohm.
1kHz C= 4.4u ESR= 3.0Ω Xc= 36 Ohm
10kHz C= 3.4u ESR= 1.8Ω Xc= +/- 5 Ohm

I get your point on ESR, the outcome difference between reactance and impedance is in milliOhm.
The C in combination with the frequency is surprising for me.
What are your thoughts on the calculation and it's audibility?

Decko · Post by **Decko** » Sat Mar 31, 2018 4:41 pm

I have notice that even though the value might be the same the tone does change from brand to brand. Sprague’s are a bit “darker” and at the first stage this gets amplified through the whole circuit. From an engineering perspective, it would be good to understand why. Cool stuff, please continue!

martin manning · Post by **martin manning** » Sat Mar 31, 2018 5:02 pm

erwin_ve wrote: ↑Sat Mar 31, 2018 3:55 pm That was quick Martin! What did you use to measure?

B&K 879B LCR Meter

erwin_ve wrote: ↑Sat Mar 31, 2018 3:55 pm What are your thoughts on the calculation and it's audibility?

I was off-base above- the change in capacitance at high frequency is negligible (inaudible) since the Rk//Ck roll-off is way down at a couple of hundred Hz. In other words, that's where the capacitance value becomes important. The specs for max ESR in e-caps (including this one) are much higher than I measured (~5x), so perhaps other brands are closer to their upper spec limit? Here's how it plots:

The Amp Garage