How to improve Mojotone 18 Watt TMB bass tones

[wadeeinkauf]

Oh the TMB channel has LESS bass?
Very odd, both my 18w TMB's have way more bass on the TMB channel than on the V/T channel.
And the non TMB that's more of a stock Marshall circuit single channel, that has the same lack of bass as the V/T channels on the two TMB versions.
So with bass typically at noon there's still way more bass than on the V/T channel.

Maybe the Mojotone circuit has different cathode bypass capacitors than yours.

TMB channel 2.7K and .68uf
Normal channel 820R and 25uf

I ran the charts with the above values.



To better describe the tone I am after when I say I want more bass.

With my Twin Reverb treble set to 5, middle set to 5, and base set to 3. This is about the tone I am getting with the 18 watt now while still having clear non-muddy treble. It is a bass that sounds more like an E string on an acoustic flat top guitar with new strings.

On my Twin when I bring the bass up to 4ish it brings in a fuller boomyer bass increasing a little until at 7 and beyond there is no change. This is the bass tone I am after.

 

[telemnemonics]

Maybe the Mojotone circuit is has different cathode bypass capacitors than yours.

TMB channel 2.7K and .68uf
Normal channel 820R and 25uf

I ran the charts with the above values.

View attachment 897970 View attachment 897971

Could be, I didn't build them but one is a Weber kit build and the other is a scratch build using a Ceriatone schematic.
Really very much the same basic EQ on both.

For such a great basic design it would seem odd for Mojo to change it to reduce bass to a weak bass result?
Sorry I can't really help.
I think the Ceriatone schematic is easy to find online though, maybe compare to that?

 

[wadeeinkauf]

Could be, I didn't build them but one is a Weber kit build and the other is a scratch build using a Ceriatone schematic.
Really very much the same basic EQ on both.

For such a great basic design it would seem odd for Mojo to change it to reduce bass to a weak bass result?
Sorry I can't really help.
I think the Ceriatone schematic is easy to find online though, maybe compare to that?

The Weber 18 Watt has 1.5K and 50uf. This gives no attenuation on bass frequencies.
Interestedly they state on the schematic to adjust the 1.5K for best overall tone and stability.

 

[telemnemonics]

The Weber 18 Watt has 1.5K and 50uf. This gives no attenuation on bass frequencies.
Interestedly they state on the schematic to adjust the 1.5K for best overall tone and stability.

View attachment 897983

Interesting, I wonder what the Mojotone tech was thinking when they went that way, cutting bass that much on an amp where there's a bass knob for players who actually want shy bass?
Maybe they changed other things in an effort to try to make a single 12ax7 channel be more of an overdrive channel?
Both my TMB are the usual low gain preamp and have no real overdrive coming from the preamp, just adequate gain to afford distortion with whole amp volume.

No extra tube or maybe chips snuck in there like an 800???
(The normal channel 12ax7 and TMB channel 12ax7 are independent so only the one tube is functioning on each channel. Pretty sure a single 12ax7 cannot do input preamp, tone stack recovery, AND add clipping? Here again, I've never before heard the TMB channel called an overdrive channel, so maybe they really redesigned that section and found ways to add clipping. Curious! Cutting bass usually reduces clipping when working with limited gain?)

The circuit is kind of crowd sourced as Marshall never made the TMB but like I said, both the Ceriatone and Weber circuits have very much the same eq range and drive etc.
BTW, did Mojo correct the sort of foolish placement of the PI tube between the two preamp channel 12az7s?

Also have you spent much time at 18watt.com?
Here it seems like Tweeds and BF popularity dominates and not many are building Brutish , sorry, British amps.
Those guys over there practically designed the TMB circuit or maybe they actually did design it there.
Sharper tacks for sussing out this sort of thing.

OTOH if this Mojo circuit gets good distorted sound at lower volume range maybe the bass loss is worth it?
You said it gets distorted at around 4 on the volume, I run mine with the PPIMV up full and TMB preamp around 2 1/2 for a much louder than conversation nice clean sound and screaming feedback with dirt pedals.

I'll have to check later today what volume it starts breaking up at but it seems to me mine both have to be up pretty high to get dirty.
I love the amount of fat clean volume with this circuit!
Fatter than a PRRI or DRRI but I have not compared clean volume to the DRRI.
Some of that is also OT, when it's up loud the OT is a big part of the sound.
One here has a Classictone and the other has a MM OT.
I think the OT might even be bigger than a BFDR/ DRRI?
Part of why it has nice clear solid bass up higher than amps with smaller OT's that saturate half way up, I really really dislike that early OT saturation that prevents solid bass above X volume but for many players that's how they think of crunchy rhythm guitar: bass mush with jangle on top.

 

[schmee]

Power Trans and filter caps can make a big difference in low end. I dont know what yours are though.
But you cant really put low end into an amp that cant push it out. Many low wattage amps lack low end.
EL84's do tend to be a bit middy, but the super simple Z Carmen Ghia circuit produces huge low end & low mid punch. And... those have huge tranny's for an 18 watt amp..

 

[Mexitele Blues]

On my Twin when I bring the bass up to 4ish it brings in a fuller boomyer bass increasing a little until at 7 and beyond there is no change. This is the bass tone I am after.

A twin has a ton of power to devote to the low end, even at low volume. The 18w lacks that reserve and may need to be cranked up a tad.

IMO you have a number of factors working against your desire for twin-level bass. 18 vs 85 watts, 1x12 vs 2x12, cathode vs fixed bias.

 

[wadeeinkauf]

Interesting, I wonder what the Mojotone tech was thinking when they went that way, cutting bass that much on an amp where there's a bass knob for players who actually want shy bass?
Maybe they changed other things in an effort to try to make a single 12ax7 channel be more of an overdrive channel?
Both my TMB are the usual low gain preamp and have no real overdrive coming from the preamp, just adequate gain to afford distortion with whole amp volume.

No extra tube or maybe chips snuck in there like an 800???
(The normal channel 12ax7 and TMB channel 12ax7 are independent so only the one tube is functioning on each channel. Pretty sure a single 12ax7 cannot do input preamp, tone stack recovery, AND add clipping? Here again, I've never before heard the TMB channel called an overdrive channel, so maybe they really redesigned that section and found ways to add clipping. Curious! Cutting bass usually reduces clipping when working with limited gain?)

The circuit is kind of crowd sourced as Marshall never made the TMB but like I said, both the Ceriatone and Weber circuits have very much the same eq range and drive etc.
BTW, did Mojo correct the sort of foolish placement of the PI tube between the two preamp channel 12az7s?

Also have you spent much time at 18watt.com?
Here it seems like Tweeds and BF popularity dominates and not many are building Brutish , sorry, British amps.
Those guys over there practically designed the TMB circuit or maybe they actually did design it there.
Sharper tacks for sussing out this sort of thing.

OTOH if this Mojo circuit gets good distorted sound at lower volume range maybe the bass loss is worth it?
You said it gets distorted at around 4 on the volume, I run mine with the PPIMV up full and TMB preamp around 2 1/2 for a much louder than conversation nice clean sound and screaming feedback with dirt pedals.

I'll have to check later today what volume it starts breaking up at but it seems to me mine both have to be up pretty high to get dirty.
I love the amount of fat clean volume with this circuit!
Fatter than a PRRI or DRRI but I have not compared clean volume to the DRRI.
Some of that is also OT, when it's up loud the OT is a big part of the sound.
One here has a Classictone and the other has a MM OT.
I think the OT might even be bigger than a BFDR/ DRRI?
Part of why it has nice clear solid bass up higher than amps with smaller OT's that saturate half way up, I really really dislike that early OT saturation that prevents solid bass above X volume but for many players that's how they think of crunchy rhythm guitar: bass mush with jangle on top.

Here is the link to Mojotone 18Watt schematic.
https://www.mojotone.com/Amp_Kits/British/British_18W_TMB_SCH.pdf

This thing is CRAZY loud. It is a great amp. But there is very little head room on the "dirty (Mojo calls the TMB channel the dirty channel)" but thats OK as that is what this amp is all about. I would like the 1st stage of preamp channel, Mojo calls this the normal channel V/T more like a Fender on the bass if possible. I am certainly not dis-satisfied with the amp. And I am using it as a learning tool.

Anyway here is something very interesting to me. Because this thing has so much gain I thought I would drop in a 12 AT7 in the first preamp stage(the 12AT7 is a lower output tube. Fender uses this tube as the first preamp tube in the my ’81 Twin Reverb) It is the only preamp stage for the normal channel and the first of 2 preamp stages and then a 3rd stage for the TMB tone stage. Anyway, with this tube in as the first preamp stage it is a different amp. In the normal channel I can not move the volume up to near 5 with the 12AX7 it would start to hurt your ears at 2.5. The tone is I would call it sweeter. More like the 60’s R&B, Al Green tones at lower volume. It breaks up at around 7 but it is painfully loud there. Still playing around with it on the TMB channel. My first impression is to keep a 12AX7 there.

 

[YellowBoots]

Ran the numbers in the Cathode Bypass Capacitor Calculator. 25uf bypass cap passes 82hz (guitar low e) and above.
https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

Are those numbers useful for EL84's in P-P? The effect of the bypass cap is tube and circuit dependent.

I perused the schematics here. I see 50uF to 150uF on these designs. One design split the cathodes and put 270R/50uF on each.

 

[wadeeinkauf]

A twin has a ton of power to devote to the low end, even at low volume. The 18w lacks that reserve and may need to be cranked up a tad.

IMO you have a number of factors working against your desire for twin-level bass. 18 vs 85 watts, 1x12 vs 2x12, cathode vs fixed bias.

You are right of course. I do not expect to actually make this sound like a twin. The one coupling cap change to .022uf helped. I will work at bit with the cathode bias resister. The charts say I can get a bit more bass changing to a 1.5K resister there. I want to try a 12AT7 in the phase inverter as well. That is what the Twin uses….I do not have another 12AT7 at the moment.

 

[wadeeinkauf]

Are those numbers useful for EL84's in P-P? The effect of the bypass cap is tube and circuit dependent.

I perused the schematics here. I see 50uF to 150uF on these designs. One design split the cathodes and put 270R/50uF on each.

Thank you for posting that link. I see some really useful info there. Will look more closely when I have time.

 

[wadeeinkauf]

Power Trans and filter caps can make a big difference in low end. I dont know what yours are though.
But you cant really put low end into an amp that cant push it out. Many low wattage amps lack low end.
EL84's do tend to be a bit middy, but the super simple Z Carmen Ghia circuit produces huge low end & low mid punch. And... those have huge tranny's for an 18 watt amp..

I will check it out...Thank you.

 

[telemnemonics]

I see what you are saying. I have come to think of an overdrive channel as one with a gain control and a master volume. This Mojotone calls the dirty channel. The other channel they actually call the normal channel. I take your point. Neither one is very clean above about 4 on mine. There is less bass on mine in the TMB channel.

I did a quick test of volume on 4 with the Ceriatone based TMB and the amp is fully clean at that setting.
Even full chords but if I slam them it will clip.
Just overall nice & clean at that volume.
Didn't crank it up today though!

So maybe the Mojo circuit has a bunch of changes?
How about the rectifier?
Tube or SS?
My non TMB 18w is a Lil Dawg Pug with octal rather than nine pin mini rec socket, and he provides I think a 5v4.
Going to GZ34 gets too damn bright and 5y3 drops voltage down to a warmer tone and earlier breakup.
One way of the other the voltages provided by the Epi PT will of course determine tone and clean volume. Here it shifts the bias too so there's that.
The ez81 is a surprisingly potent tube rec.
Drives home the fact that looking at the small bottle and assuming tonal character is a bit of a mistake. FWIW one of my TMB's has 6v6 rather than el84 and the sound is clearly mostly the circuit not the power tubes.

 

[printer2]

Uh... ...how about changing the tonestack values to the Blackface values? Mind you then you would have to change pots and we may be able to push you in the right direction with a few different capacitors and resistor. Let's play with the online tonestack calculator... ... got something. Change the 500pF to 250pF, change the resistor to 220k. Oh, and you may have to turn the mid pot down to 1.5-2 to get the Fender dip in the response. So 5 treble, bass 4, mid 1.5-2.

 

[jjlemon]

The suggestion to look at the output transformer is a good one. Having had an epi valve junior, the stock OT was quite weedy. Maybe it's the same on the standard.

I would try changing the cathode resistor on V1a the clean input valve, maybe try 1.5k, and go for a centre biased input stage. I've found that centre bias seems to yield the best overall clean tone, but ears vary...

Someone else suggested looking at the 0.0047 coupling cap. That could be worth trying a 10n in it's place.

 

[wadeeinkauf]

The suggestion to look at the output transformer is a good one. Having had an epi valve junior, the stock OT was quite weedy. Maybe it's the same on the standard.

I would try changing the cathode resistor on V1a the clean input valve, maybe try 1.5k, and go for a centre biased input stage. I've found that centre bias seems to yield the best overall clean tone, but ears vary...

Someone else suggested looking at the 0.0047 coupling cap. That could be worth trying a 10n in it's place.

Thanks for taking the time for your suggestions. Any amp other than a Fender silver face is new to me. This amp really requires you to take the time to play with the tone and volume controls to control the tone. And the tone is dramatically different with different pickups. Fortunately, since I build guitars I have a few to choose from. I am almost back to square one in my testing. Will update later today.

 

[wadeeinkauf]

Trinity Amps 18 Watt Builders’s Guide Version 21.3 Page 56 has a great section on 18 Watt tone Tweaking. It is very practical and useful including changes that can be made to the inverter section for tone. Below is the link. This is useful for any similar amp.

(link removed)

I will go over some problems I found in the hopes it may be of help for anyone making a Mojotone 18 watt TMB. My differences may have been a result of the Epiphone Power and OT transformers.

I had read that different bias settings (running the power tubes at different plate Dissipation levels) affects tone. I had checked the bias using the Tube Dissipation Using Plate Current method after my build and found one tube was running at 125%. I figured I had somehow done something wrong. The amp was working. I rechecked it, used the Tube Dissipation apps at robrobinette.com. It was running high for sure. I started by lowering my B+ 340V (generally considered to be about right by the 18Watt.com guys. Still running high. I adjusted the bias resister…see my schematic for more info on this if interested. Anyway after I got the bias set at around 87% (Fender sets it 70 they say, some say 85% is ok for AB circuits), at any rate this tamed the beast. Bass response is better, the cleans are better. I was looking at Fender circuits and noticed that Fender uses the 12AT7 for the inverter..more checking…some say this creates a cleaner tone as well. I tried this. The tone is a bit different. I would call it more tamed, sweeter…I know what does sweet sound like. A bit more Fender like. It does give you more headroom for sure on the normal channel.

The attached schematic has my measured voltages, MojoTones voltages from their tech support, and the description how I checked the bias.
myamp.pdf