ceriatone 18w tmb issues

[charlo489]

So I've had this amp for a few years now and while it's great with cleans, I just cannot stand the ugly bass of the tmb channel when fully cranked. It lacks definition, you cannot hear the grinding on the fret for example...it's all muddy and lacking clarity. Is this to be expected with those amps or could it be improved with a better output transformer ? I was about to order a classic tone OT but I just want to get your opinion

[geoff 1965]

Hello,
don't order the OT just yet! you say you're happy with the cleans so if you plug into the normal input and crank it do you have good bass with definition and clarity? if so then it might be the filtering of the TMB channel you need to tweak.
replacing the el84's 100uf cathode cap with a 50uf will give a tighter bass response but will affect both channels.
just confirm this layout & schematic are for your amp.

18wattTMBceriatone.jpg

27767572173_2b442802e2_b.jpg

[charlo489]

Well this weekend I did exactly that ! The 50 uF cap does help. I'm also using a 12AU7 to lower the gain on the tmb. Those 2 mods improved the bass end a little bit but it's not to my taste.

I do like the normal channel, even when fully cranked it's tight but it doesn't have a lot of gain. So that's why I'm thinking about upgrading the OT

[geoff 1965]

try swapping the cathode cap on V1 Pin8 from 1uf to .68uf,this will cut some bass into the cascaded signal and should get rid of some of the "mushiness" you describe.

[charlo489]

Perfect I need to order some parts from mouser, I will get that .68 cap. Anything else I should order from mouser for any upcomings mods ?

[geoff 1965]

get good quality mallory or sozo polyester film,these improve the clarity more than electrolytic.
it's handy to have some spare various coupling caps i.e. .0022uf,.0047,.01 etc but that's up to you to decide.

[geoff 1965]

i'll be interested in knowing what your el84 plate & grid voltages are with that 130R cathode resistor?
plus if you click on the "forums" then "18watt-technical discussion" and type "ceriatone TMB" in the search box there is over 100 pages of posts!
i'm sure there's some good info in there if you have the time to browse through.
good luck

[charlo489]

What do you mean by that ? I'm really a beginner when it comes to electricity but I do know how to solder and my step father is an electrical engineer, big help haha !

[geoff 1965]

i'm no senior amp tech myself! the cathode resistors value determines the dissipation "watts" of the el84's and the max for an el84 is 12 watts.so with my amp i originally had a 150R but this sent my dissipation above the 12W max so i raised the resistance to 180R which lowered the dissipation to 11W,this is with 340 volts on the plates.
so i was looking at the ceriatone schematic thinking either you must have lower plate voltage or them el84's are biased very hot!

[Crabman]

Have a look at some of the 18W Trinity amp schematics as well since there are at least 3 TMB variants to consider that have various levels of bass and gain attenuation between the first and second stages.

As Geoff suggests .. putting a 0.68uF bypass cap on the the first TMB cathode resistor is a good place to start playing around. Also consider playing around with the first bypass cap (C5 on your schematic). Try a 0.01uF or 0.0047uF in that position. Most of the boutique builders are using lower values at that position to remove booming or mushy bass early in the pre-amp. I find that 0.022uF just makes things a bit too rubbery on the low strings .. at least for my taste.

On some of Trinity amps 18W versions, the 470K grid stopper (R12 on your schematic) is removed and a 100K grid leak resitor is placed at that spot instead. From what I can tell, this grid leak sits in parallel with the first gain pot. Just a different way to attenuate the signal arriving at the grid of the second stage. It will load down the first stage a bit more though. However, I have tried it both ways and I prefer it with the grid leak .. I think the 470K grid stopper kills more of the chime.

You can also up the value of the second cathode resistor to reduce the gain a bit if you prefer it that way. Or you can put a pair of resistors in parallel on a switch. 2.7K to give you less gain and tighten things up a bit and then switch in 1.2K in parallel to get you back to around 820R .. the way you currently have it. I currently have this type of set-up on my 18W and find it very useful.

Anyway, some good starting points to consider.

[geoff 1965]

yeah good advice!
i've done a lot of testing with the cascade mod on my lite2b and .68uf on the cathode with the .022 cap is a good match and you will be suprised how much difference the polyester cap makes!
Dave's right about the 470K,i was using 400-470K to get the right attenuation but i got the best tone/clipping from 220K so i ran a 220K to ground "voltage divider" to lower the signal.
i would appreciate if you could "safely" take some voltage readings from the following: rectifier V6 Pin7 & Pin3, el84 V5 Pin7 & Pin9, also the first connection of the 32/32 can capacitor position "A" on the layout, i'm trying to work out the unusual B+ supply!

[charlo489]

Okay thank you very much for all this help ! The 470K resistor you're talking about, is it on pin 2 of v3 ? I have a hard time with the small size of the schematic. So this one in circled below ?

What kind of resistor do I need for the best upgrade ? Film or carbon ?

I'll try to take some readings for you Geoff, gonna ask my stepfather !

[geoff 1965]

Thank's, yeah be safe and don't put yourself at risk if you are unsure!
yes that's the 470K and you will need "1W carbon film" but i have'nt done that mod so Dave "crabman" will be best to advise you on that.
good luck

[Crabman]

The 470K you circled on your layout is the correct one. However, before you go down that path can you confirm what capacitor you have in C5 that is circled on the layout below?

18wattTMBceriatone.jpg

The reason I ask is that your layout and schematic are different with respect to that capacitor. If you already have the 0.0022uF as shown on the layout then it would appear that Ceriatone have already gone to some lengths to reduce bass in the pre-amp since they also have a 0.0047uF going into the phase inverter at C7. These two coupling caps are already considerably less than the old traditional Marshall's values.

If your build follows the layout values then I'm not sure why you are getting the boomy bass and lack of note articulation.

If you have a 0.022uF in the C5 position (like the schematic) then the first thing that I would do is to replace it with the value shown on the layout and see how you like it.

[JMPGuitars]

Well this weekend I did exactly that ! The 50 uF cap does help. I'm also using a 12AU7 to lower the gain on the tmb. Those 2 mods improved the bass end a little bit but it's not to my taste.

I do like the normal channel, even when fully cranked it's tight but it doesn't have a lot of gain. So that's why I'm thinking about upgrading the OT

Upgrading your OT will probably improve the quality of your output sound, but not likely the shape of your sound. Follow these gents' advice and you'll find your way to happy tone.

One question: what kind of speaker are you using?

[JMPGuitars]

Well this weekend I did exactly that ! The 50 uF cap does help. I'm also using a 12AU7 to lower the gain on the tmb. Those 2 mods improved the bass end a little bit but it's not to my taste.

I do like the normal channel, even when fully cranked it's tight but it doesn't have a lot of gain. So that's why I'm thinking about upgrading the OT

Upgrading your OT will probably improve the quality of your output sound, but not likely the shape of your sound. Follow these gents' advice and you'll find your way to happy tone.

One question: what kind of speaker are you using?

[charlo489]

I'm using g12h30 and vintage 30. I checked my amp and it's got a 0.0022uF cap in C5 so I don't need to change it.

I also ordered a classic tone OT yesterday so with all those changes, I think I'll get what I'm looking for

[geoff 1965]

well spotted dave!
i'm very suprised at C5 being .0022 with your description of the bass especially with C11 being .0047 like dave mentioned.i would still replace the 1uf with a .68 polyester for better clarity.
i have .68uf and .022 and have good detailed bass with no mushiness but my tonestack is lite2b not TMB.
it might be worth checking the rest of your component values in the amp against the layout in case someone has done a previous mod.

[charlo489]

Well it was bought brand new straight from ceriatone. Nik from Ceriatone told me that the bass mushiness is to be expected when driving the tmb channel at the max. Right now, with the AU7 tube and the 50uF cap in place, it's much better. The .68 cap and classic tone will probably solve everything hopefully !

[geoff 1965]

okay,so if you've bought the new OT then fitting that is the first job on the list,then we can look at tweaking the amp to your taste.