Ceriatone Forum

So,
I've been mainly happy with my OTS 100w and had about 6 months to get used to gigging it and to find the right speakers etc. I've found a startling discovery which has really made the amp work for me. I wanted to share it with everyone here in case they may like the improvement.

The issues I had were:
1 - Although the tone controls had a noticeable effect on the tone, they weren't as responsive as other amps I have had: the tone always seemed to have a slightly lo-fi quality for lack of a better word...
2 - The PAB was useless live. Too much bass was introduced unless I cranked the treble control, but this caused problems when PAB was not engaged.... This was especially bad on the clean channel, where the tone sounded flabby at all times with PAB on.

So I played with cathode and plate resistors. Even modded the amp to S&M specs. Also reduced snubbers to 270p. All these made a difference to the overal tone, but did not seem to impact the two issues above.

Today I was looking at a schematic of Dumble 124 and i got to thinking something. The cathode and plate resistors on v1a have been inreased in the ceriatone when compared to this dumble. I have read this helps warm up the sound somewhat. I thought, well I could change them to 124 specs, but would this really be the cause of all my problems? It turns out not.... the solution is more complex to explain, but simpler to implement:

The tone stack in 124 is the same as the ceriatone, so I realised that changing the plate resistor on v1a would have a likely impact on the impedance seen by the tone-stack, but this hasn't changed, so perhaps there is a mismatch, when compared to the original design. Turns out (assuming a 12ax7 output impedance of around 60k) that the change to the plate resistors cause the output impedance of v1a gain stage to increase from around 38k to around 47k... but the tonestack has not been modified, and still has the same input impedance. Guess what this does?  ;D

So, I thought, can I re-create the same impedance match between v1a output and the tonestack input? Yes, I can! The slope resistor is surely the key.... So, what if we scale the slope resistor up by the ratio by which the v1a output impedance changed (47/38). Well, we get around 190k. Didn't have resistors to make up this value, but I had a 180k lying around. Replacing this one resistor made the amp behave beautifully in all scenarios! I couldn't believe what I was missing before I did this!

It seems that someone has introduced a well meaning mod (to change the v1a cathode and plate config) but forgotten to take account of the impedance impact on the next stage. When this is factored in, I think the amp behaves "as originally designed". I must stress that the plate and cathode have not been changed to 124 values, so the well meaning mod has been preserved, its just been taken to its full conclusion and implemented "correctly" (in my opinion obviously).

I urge all OTS owners to try this and welcome and views or comments on the above!

Cheers,
K

Your observation regarding impact on output impedance of V1a verses Anode (Plate) resistors is perfectly valid however there was no mistake in the use of 220K / 150K plate loads.

There are a few different variants of D - Style circuits and the 220k/150k version is more generically referred to as "High Plate" - 100k/100k is referred to as low plate. The values of the components used in the following tone stack within the Ceriatone OTS amps are based on actual examples of Dumble produced amps. Tweaking the amp to suit your personal tastes is not a bad option assuming that you have the necessary knowledge and experience with High Voltage electronics with respect to personal safety.

Hi exocet,
So it seems I have what you call a "high plate" amp as per the stock OTS config. I am deffinitely not trying to suggest that there is anything incorrect about the "high plate" config, nor the "low plate" config. Neither am I saying that the tonestack is incorrect in and of itself. When you say that the tone stack is based on actual examples do you mean that the combination of a "high plate" amp with a 150k slope resistor is based on an actual example?

There is always an impedance mismatch between v1a and the tonestack (assuming the guideline that "input impedance > 10 x output impedance" can be applied). In any case, this is never as high as 10 times, however, I feel its fair to say that the exact ratio should be preserved in all designs. After all, what reason would anyone have for deliberately changing this mismatch?

I know its a strong point of view, but the effect is so dramatic, and the theory seemingly sound, that I feel happy to assert that a "high plate" config requires a different slope resistor from a "low plate" config. I think the effect of taking this on board is so positive that I feel this goes beyond personal preference, and is in fact correct. :o I appreciate this is likely to be controversial in its lack of relativism.

Either way, whether this is a design flaw, or just my personal prefference I reccomend everyone first using this amp to experiment with this mod. I would personally love to see it incorporated into the OTS & S&M designs (maybe others require an adjustment too, I havn't checked all the configs...) I have tried this with a variety of loading scenarios (i.e. loads before the amp), guitars, effects pedals. There is no scenario which is not improved by the mod.

The effects of the mod are quite subtle in some scenarios (those which were not previously problematic) but dramatically better in other scenarios (those which were previously problematic). This further makes me feel that this is a design issue, rather than my personal preference. The change is not as simple making everything brighter or everything darker as might be attributed to preference, but seems to "correct" a number of issues

The "high plate" with 150k slope features in many Dumble circuits, most commonly with the "Skyliner" tonestack which is included in the OTS. One of the most famous Dumble amps (Robben Fords) serial number 102 also has same mismatch.

I'm not trying to pick a fault in your observations, mearly pointing out that the Ceriatone OTS replicates the architrecture and component values that are found in actual Dumble amps. I'm all for modifying the circuit as the standard design will not suit all tastes, playing styles or guitars.

My OTS was built in 2009, it was built with a 220k slope, when I noticed this, I changed it to stock 150k value and preferred the resulting sound.....bottom line is that it is a great amp platform.

'scuse the intrusion but...

Wow!  I learned some really nice stuff here.  I ordered my OTS and HRM 50 watters prebuilt.  However, I'm not averse to picking up my pencil, braid, and solder to try something new.   I'm not as familiar as the forum's builders, I am not famliar with where such obvious tagnames are located.  I'd like to try it.  Man, if someone could shoot me a PM with maybe a chunk of the layout with notes on this that would be so appreciated... 

Hi,
The resistor that I changed is on the bottom right of the main board on ceriatone's layout (labelled 150k). The layout is available on Ceriatone website. It's tricky to change without removing the board in my amp due to a large capacitor being in the way of the soldering iron. Obviously only mod your amp if you are very confident that you know what you are doing. Lethal voltages are present on this resistor if this is not done properly.  It may not be as simple as just turning the amp off as there are large capacitors in the amp which can store a lot of power after the amp is turned off. I cannot take any responsibility for this mod. Your life is in your hands! What I can say is, when done properly, I love the result.

Having got that out of the way, what I did was to replace this resistor with a 180k carbon film resistor rated for 2 watts. This works brilliantly in my OTS. I havnt looked at the hrm schematic, so can't comment on the suitability of this mod for that amp.

Thank you very much for your prompt and helpful reply.  Safety precaution needs noted.  I have a way to drain the caps to below dangerous levels before taking the chassis out of the headshell and will follow up with additional precaustions.

Once I got a finger across the OT primary terminals by mistake in my Frank-en-Champ very early on in my tinkering while the amp was running, burned a tiny hole all the way througgh the skin right into the meat, didn't bleed because wound was cauterized as it was being bored by the arc of rampant b+... and my stupidity.

Thanks again, will look at both layouts.  Then get to most carefully researching and then source the part(s). 

Hey guys... interesting topic..!
What`s your method to drain off stored voltage in the caps once the amp is totally shut off.
I`ve read that the OTS will do this if you turn off the mains switch and leave the standby in operate position, while waiting an hour or two.
Does this sound right.. or do we need to earth the caps to the chassis...?
I do have a voltmeter to check & confirm any voltages in the caps.
Cheers..

Hi,
My normal method is to use a 100w 16 ohm resistor. I find it a very convenient component because it can also be used as a dummy load to replace the speaker when doing osciloscope work. Its plenty adequate to absorb any power left in the filter caps. I just touch it in parallel with one of the caps and wait a couple of seconds. I then measure any remaining voltage accross each cap using a multimeter set to a high DC voltage range.

Having said all that, I have found that on my OTS simply powering the amp of in the normal fashion (turn to standby, wait 2 secs, turn off, wait 2 secs) will drain the caps. I always measure across the caps just in case even though it has never shown more than 10v when doing this, which is a perfectly safe level. Bear in mind that your OTS may behave differently and you should always, always, always check. "Measure twice, die never", thats my philosophy.

Same waiver as always, I cannot be held responsible for anyone doing this, your life is in your hands, not mine.... ;)

P.S. - Love the avatar, Roy is an inspiration!

Hi Bluesking
I had my OTS 50 build Nik. From the first time I played the amp, it didn`t sound the way I expected it to sound. It was to flubby (as you discribed) all the time, with, or without PAB. I mailed Nik for help and he immediately replied everytime :-). Finally I ended up taking all the bass down to allmost zero. But still the sound is not nice. Flubby at normal volume on the lower strings (`74 Stratocaster with `69 fender costum shop pick-ups) Do you think I could have the problem as you had? Might your mod be the solution?
Thanks for your answer.

Hi Bluesking
I tried your 180Kohm resistor mod to the tonestack input and I also replaced the  bias bypass cap from 5uF to 2.2uF, as you described in you pm to me. It’s really a very big difference. Now my amp sounds as I want it to sound. I can get the John Mayer sound and also other smooth lead sounds, which I couldn’t get before. Your mods definitely worked out for me. Thanks a lot for your help and advising.
 :) 8)

hi everyone
i just tried the 180k mod and found it very good. up till now i had hardly used the clean channel as it found kind of "dry" or boxy as other have put it, only to be exacerbated with the PAB.

the 180k mod definitely makes the clean channel more fender like; although with a slightly higher insertion loss to the tone stack.

as for others who mention flabby bass, yes i know of the problem. since i got the HRM some years ago i have almost exclusively used it in jazz mode as i found the bass in rock mode too flabby and unfocussed. this was using the amp as a combo with an EVM12L. recently, i made some Thiele boxes for my HRM each with EVM12L. the difference is extreme, bass is tight , mids much more focussed and sweet highs.

i was hoping the thiele boxes would rectify the issues i had with the clean channel and it did to some extent, however the fundamental tone did not change. but the bottom end issues were none the less resolved.

BUT, with the 180k resistor and the thiele boxes, the clean channel is now much more useable, and the deep switch works really well with it.

i am of the opinion that the deep bass of the HRM was made for a tuned cabinet which really makes a HUGE difference.

thanks for the mod suggestion, and i like it.

PS: in earlier threads pickmaster suggested LOWERING the slope resistor to 82K, i cant recall exactly the improvments he got, i will find it and report back

i have performed a detailed circuit analysis on the skyline eq, using different resistors.

results are as follows:

for a given eq setting (ie bass middle treble), and varying the slope resistor only, the effect is to slightly reduce the bass, and make a larger skoop in the mids, dropping approx 1db at 1.5 kHz,

... hence, the more fender like tone!!

i may try increasing it to 200k and see what happens.................

I'm interested in this, too. I did the 68k slope pickmaster mod and my amp is now quite bassy, although I thought it was the lifted LNFB who caused that. What exactly do the different slope values to the sound?

i am happy with the 180k mod and am not keen on trying the 68k

however i have simulated it... you get more signal to the rest of the amp and more bass and mids making the sound less scooped

overall i would say it is not the direction i want to take it; and if anything try slightly increasing the 180 k to 190 or even 200k

however being cautious, i want to try it in a live situation as it is now

the overdrive is much smoother and refined, especially with a les paul and the PAB engaged

This is great overall but I'm now more confused

Pickmaster indeed said that lowering the slope resistor to 68k does wonders in "bloom" and "touch responsiveness"

Now just what exactly does modding the other way (increasing slope resistor) do to the sound? Please elaborate to me kindly and thanks a lot.

And about the "high plate" and "low plate" thing. COuld you guys explain that concept and what made the difference between the two and how they impact the response given the slope resistor values. ..I've read that "high plate" versions yield a stiffer feel. Now those who know me know that I am rather allergic of anything stiff. What can I do for more responsive compression then? You know, so I can strum colorful chords and yield a glassy pleasing chirpy sound as opposed to ear fatigue inducing stiffness.

Kinda like John's tone here (he is indeed using one of his early Dumbles here through a Fender 2x15 most probably)
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I'm sorry fo r asking these I have a rather lower average knowledge in amps.

THanks for all the well meaning modders and tweakerses you guys.

I would say, just try it out. I like the 68k resistor, although I don't know if the difference I hear is the resistor or the LNFB... :) Anyways, it was an easy mod. We just snipped the old resistor and didn't have to get the turret board out. If you do it, make sure you record soundclips of both variants ;) :)

This is a cheesy fix, but you don't HAVE to remove the other resistor.  Stack the right values in parallel you get the same thing.  IF you don't like it later, you haven't disturbed the board.

Thilo didn't you make a switch for LNFB? Why not switch it on and see what the 68k slope did?

Oh... you're right :D Totally forgot about that, cause I never switched it on again  ;D
I'll see if I can trust my memory enough to make out the difference... The only thing I can say atm is, that I didn't get a significant increase in bloom. (Maybe a strat is the wrong guitar for that anyway... I'll try it with an ES-335 :) )
Well, I think if I use the settings I always did before the mod and still have more bass and low mids, we know what the slope resistor did.

I should re phrase my post. I read back the thread and I think I get it better how the 180k slope resistor concept could work well. Perhaps I'm just a bit baffled cause I am about to do the new Dr. Ika (Pickmaster) FM ME / OTS "TR" mod with the suggested 68k slope.

I just hope bluesking could read this and shed some light into other aspects of the mod like is it more punchy or what other than being more scooped. And also I hope Pickmaster could chime in too so he could share his ideas on this.

Captainbackfire,

For determining what is the right voicing for the slope resistor, you can bench test lower values without a soldering iron.  If you are uncertain as to the effect of the mod or which value is "you", calculate and parallel a resistor with some alligator clips.  Be careful doing it, but it saves time...

THats very interesting on the alligator clips. Could I just clip it on and off while amp is operating ? Better to turn off?

Definitely OFF!  Yeah, you want the test resistor in place, stable, and isolated from adjacent components when you power it up.  To determine the right values for testing, just use the formula:

               1/R1 (Existing Slope) + 1/R2 (secondary test value) = Total R.

That will allow you to hear before you commit.

If you aren't comfortable moving around in an amp, you might want to get around somebody with experience.  It can hurt you quick.

a couple of points:

1. placing a resistor in parallel will always decrease resistance. therefore to get an increase in resistance you either need to  change the resistor or add another resistor in series
2. a correction on the parallel resistor formula posted earlier:

 1/R1 (Existing Slope) + 1/R2 (secondary test value) = 1/Total R.

lowering the slope resistance will decrease the overall insertion loss of the tonestack and therefore provide more signal to the overdrive section. hence giving more bloom, however also with an increase in bass and mids
increasing the slope resistor has the opposite effect, it increases the insertion loss and there the result will be a more pristine signal to the overdrive section, with less bass and mids, but the overall tone is much more natural. to compensate for the loss, i have turned up the overdrive somewhat when needed, however the sound is tighter and clearer.

each to his own.... however a customer has heard my amp many times before, however after hearing it last week with the resistor mod, has decided to place an order.

i cannot stress enough for those using EV12L's, putting them in a tuned box such as the TL606 makes the world of difference to the overall sound and response too ....tightens the bass up without loss of highs

Thanks for elaborating.

Looks like lowering the slope resistor is my cup of tea. I have one concern though. If the sound "blooms" with more signal hitting the OD stage does it stand to reason that there will be more flub to the bass particularly when you leave your clean EQ settings ( I usually put treb and bass on 7 with mids on 3 ) and use the drive?

I don't believe so though cause I thought the lifted LNFB and bass cap mod will introduce more signal to the rest of the amp and cause bass distortion/flub but nooo, my OD became thicker but more defined with much less flub.

Hi there tone lovers,

Once again:
Low slope resistor (not very low though!). Fat sounding Bluesmaster has 47k but there is a  0,02mf after it which cuts down to much bass in comparison to 0,1mf on OTS.
68k slope on OTS is MY PERSONAL favourite which introduces little bit more bass and low mids but most importantly adds natural clean compression to the first tube, so clean gets so called CHIRP and singing quality. I’m a very light touch player and I use very thick pick 3mm Stubby. Compressed clean sound allows me to have less drive on OD trimmer which is kind of clean but fat singing, bloomy, touch sensitive drive tone which does not need PAB to sound great.
In short lower slope resistance and less or no  capacitance on NFB gives me more TWEED tone enhanced in high frequencies with higher capacitor value on the clean volume pot. Personally I always prefer TWEED to Blackface, more correctly tweed fatness with blackface glassy tone added.

Mr. Fabulous is right in case that higher slope resistor gives you more clean, less fat non compressed (more classic blackface) tone and I’m sure many players love this tone but not me. I like rubbery, spongy, fat, wet, syrupy, singing clean and drive tone.

But I totally agree!   Tone is very individual and you have to try to find the best for yourself, tone which makes you play and feel better.

I love my amps. No D-lator in the loop !!! just Digitech RP200

http://www.youtube.com/watch?v=80FM7kbeM30 
 

thanks dr ika
yes music and sound is such a personal think and we all have our own perceived conceptions of what a great tone is

its is wonderful that Nik has buils such a wonderful platform upon which with a little experimentation and tweaking you can make the amp sound perfect to your own tastes.

i reckon if we compiled a list of all the mods done by various people in this forum, it would read like a nice little cookbook of dumlesque tones

Thanks I think I will indeed go with the 68k and I will try to do a before and after sound demo.

That sounds fantastic Doc. Are you using the cut control on the OD channel?

Hello Hywel,
Yes I do. 12 way rotary. It is on 200pf on this clip, plus V2’s double 330pf.

By the way I have new invention – 4x10 cab which eliminates the beam and same time spreads the sound on 180 degrees. I’m trying to patent it or obtain some kind of copyright licence before it goes on production.
Cheers.

Best keep schtum then!!!

Hi everyone,

this thread took the words right out of my mouth, as a famous rockstar would sing...  :)
I'm not quite shure, which resistor is meant in this thread, is it the one I pointed with the red arrow? Thanks for helping!

yes thats the one

i have to say after a few gigs that the amp sounds much better with the resistor change to 180k

pedals  sound much better, reverb more lush in the loop
and the clean channel is bouncier *ie more scooped

A big thanks to bluesking and mr fabulous for the info and your input.
Very helpful indeed.. Cheers Guys.

Have you had time to experiment with a 200K mr fabulous....?
Please let us know if you do...!
The fine tuning aspect of this amp is becoming very interesting..! Cheers

Changed to 180K. Hmmm... sounds a bit thin to me. Tone controls (esp. the mids) are somewhat flat... Not quite shure if I like it. Playing some gigs with it, lets see...

yes it can sound thin depending on your speakers.... my amp is set up with 2 x thiele boxes with ev12l.... bottom end is way huge....and punchy. like a quadbox with clarity. i have tried the amp with an open back cab and i agree it can sound thin.

this amp really needs a special cab with it, i also found the bottom end too loose and flabby in an open cab (rock mode), but now a totally different animal with the thiele cabs.

blueprints for ev thiele cabs can be found on the web and in my opinion are simply fantastic, they also work really well with my 69 fender dual showman

no i havent, i dont want to take it too far as you also pay with extra insertion loss in the tone stack (ie more loss)

i may bring it up a tad, say to 170k...just to hear the difference. my simulations indicate approx 1db more scoop in the mids

Yeah I've tested it also with a thiele box, with evm 12l. Sounds thin anyway, with strat and Les Paul. Got also a 2x12 box with two evm 12l. Semi-open back. This babe was a bit bassy, lets see how it sounds with this one.

As i understand it, the higher the value if the slope resistor, the less responsive ceratin aspectsof  the tonestack will be, as a rule.
Typical slope-resistor values range from 33 kΩ to 100 kΩ. A larger value yields a sound with more of a midrange scoop (i.e., where treble and bass frequencies are louder than the mids). Smaller values accentuate midrange.

I consider the slope resistor to be like a voltage divider that splits up the signal into the treble and non-treble frequencies. By using a higher value resistor, less of the signal goes to the bass and mid caps so the overall sound is brighter and usually cleaner. With a lower value resistor the sound is thicker and heavier, with much more of the lower frequencies.  Of course the value of the coupling cap has an effect as well...
 

I have been thinking in recent weeks (after having tried a Redplate Astro Dust, a very impressive amp indeed) about how to implement switching of the slope resistor. In essence what you have to do is switch after the caps following the slope resistor, which led me to the conclusion that that was probably how Henry does it with the 6 position Blackface->Tweed rotary.

So does anyone know what the effect of having dual slope resistors would be? ie one for bass and one for mids. How to model this in terms of tonestack behaviour. Or would it just be a matter of trial an error.

I did find a 12 way rotary switch on Mouser which can be  ordered as a 12x1, 6x 2, 4x3 way etc. The only problem with trying to implement this is where to put all the components. The Redplate amps have a set of additional turrets in a cluster which I would assume would be for these connections.

i guess i should qualify my results.... i almost exclusively use the amp in jazz mode. and with this slope resistor change i am extremely happy. and i still have plenty off bass and mids in my controls anf if i need more i use the mids boost switch)

i previously found the mid boost and rock mode way to much for my likings and so always used the amp in jazz. but after this mod..it seems to have tamed these setting to make them more useable and controllable

fab

ps, if there are and members here in melbourne australia, feel free to get in touch to come and try it out.

Well... changed back to 150K. Much better (for me!) now. Especially starts don't sound so thin. But I must honestly say, I don't use the jazz-mode.
BTW: HAPPY XMAS TO YOU ALL!!

So this could be the reason, why I like my amp now really much with the 68k slope. It's really full sounding.

I told ya!  :) :D ;D
And I NEVER use JAZZ setting.

WOW!!! This OTS sounds amazing!!! IS that just the result of changing out 1 resistor?

I have a stock S&M OTS50 - what would i need to change to make it sound like Your blue one?!

No Olaf, you have to change much more than that.
As I’ve mentioned to you on You Tube our good friend here on forum and your countryman  ERWIN is a great master of modifications and awesome guitarist as well. Possibly he can help you with the mods I’ve mentioned in recent threads.

Cheers

Hi
I swapped the 150k to 180k and it is an improvement, thank you. I also changed the resistor on the PAB relay from 10M to 6M6. (Mine was 10m not 22m) This also made a difference.
In an earlier reply someone mentioned changing the bias bypass cap(s?) from 5uf to 2.5uf. Just wondering which one? There appears to be four of them.
Thank you for this.