How to improve Mojotone 18 Watt TMB bass tones

[wadeeinkauf]

I have recently finished this amp. It is a Mojotone 18 Watt TMB into a Epiphone Valve Standard chassis keeping the Valve Standards transformers and my upgraded Ted Weber 12 Sig A-S speaker. After the normal first-time builder problems of leads and dressing the amp works great. I have a very dated education in avionics, beginning a new hobby in amp building. Thanks in advance for sharing your knowledge.

This is my first non-Fender amp. I am not happy with the bass. No problem with less bass on the overdrive channel I understand the problem with too much bass when overdriven. I plugged into my ’81 Twin Reverb and really realized how bad (compared to the rich full bass on the Twin) the bass is on the 18 Watt. Ran the 18 watt into the Twins speakers and same problem…Ran the Twin into the Sig 12 and had the same great Twin bass….not a speaker problem…actually there is not a problem with the 18 watt amp I would just like to re voice it a bit.

I would like to get a more full-bodied bass out of the 18 Watt’s clean channel.

What I have done.

1. I changed the tone cap from .01uf to .022uf. This helped a lot.

2. Keeping the .022uf cap as a blocking cap I bypassed the tone and volume control. First just disconnected the ground on the tone pot. And then to be sure bypassed completely. No change. So the loss of bass is not coming from the tone control.

3. I changed the coupling caps from the phase inverter to the power tubes from .01 to .022. No noticeable improvement on the bass in the normal channel but noticeable mudded up the tone in the overdrive channel when in overdrive. Put the .01 back in.

4. V1 cathode bypass cap is the same value as Fenders. The bias resister is 820 rather than Fenders 1.5K. Ran this through some calculator and it made no real change in the bypass frequencies.

What is blocking the bass frequencies. What can I change to experiment with the voicing?

Let me say one other thing. I love the tone of all the other strings except the E bass string. It just does not have that full bass tone like the fender amps….

Below is the schematic.

 

[dan40]

It may simply be a limitation of the circuit, power tubes and transformers. With a small bottle power tube, smaller output transformer and lower B+ voltage, the 18 watt circuit will not produce the strong healthy low end that a Twin does. A larger OT and a PT with a bit more voltage and current capability may help but you are still limited by the EL84's basic voice.

 

[wadeeinkauf]

It may simply be a limitation of the circuit, power tubes and transformers. With a small bottle power tube, smaller output transformer and lower B+ voltage, the 18 watt circuit will not produce the strong healthy low end that a Twin does. A larger OT and a PT with a bit more voltage and current capability may help but you are still limited by the EL84's basic voice.

Thanks for responding. These are the questions that are only answered by experience. Who knows where that Epiphone OT came from. I will do some research on output transformers.

 

[telemnemonics]

I have three 18w, two of the TMB.

They don’t have “a clean and an overdrive channel”.
The TMB channel does have a little more gain but both are clean at low volume while the power section gets overdriven at full volume with either channel.

The bass is wonderful on the TMB side!
And yeah not much bass on the T&B side.

I use the TMB for cleans or dirt pedals but to get a loud clean and same volume dirt I run dirt pedals into the bass shy T&B channel and turn that down to match the clean TMB.

 

[telemnemonics]

It may simply be a limitation of the circuit, power tubes and transformers. With a small bottle power tube, smaller output transformer and lower B+ voltage, the 18 watt circuit will not produce the strong healthy low end that a Twin does. A larger OT and a PT with a bit more voltage and current capability may help but you are still limited by the EL84's basic voice.

Could be the Epi transformers are off spec and holding it back but a pair of cathode biased el84 can produce nice big bass within the limits of total wattage.

The original Marshall preamp channel here with just volume and tone is certainly weak in the bass for clean sounds but OK at full overdriven volume.
Lots of great crunchy full guitar has been recorded with that circuit which got a whole other channel added for the modern TMB version.

 

[Mexitele Blues]

Are all of your cathode bypass cap polarities correct?

 

[wadeeinkauf]

I have three 18w, two of the TMB.

They don’t have “a clean and an overdrive channel”.
The TMB channel does have a little more gain but both are clean at low volume while the power section gets overdriven at full volume with either channel.

The bass is wonderful on the TMB side!
And yeah not much bass on the T&B side.

I use the TMB for cleans or dirt pedals but to get a loud clean and same volume dirt I run dirt pedals into the bass shy T&B channel and turn that down to match the clean TMB.

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.

 

[wadeeinkauf]

Are all of your cathode bypass cap polarities correct?

Yes, thanks. I believe the amp is operating as designed.

 

[Lowerleftcoast]

I changed the tone cap from .01uf to .022uf. This helped a lot.

Wrong cap. The cap you should have changed is the .0047uF. (This cap and the volume pot etc. form a 2nd order filter with the input RC filter of the PI.)
(Weird wacky stuff going on here.)

It is easy to parallel the cap with another cap to increase the capacitance. Just clip in another cap to have a listen before deciding the final value.

The configuration of the phase inverter may also be messing with the frequencies. The position of the volume and tone knobs of the TMB channel may have an interactive effect on the other channel. (and vice-versa)

It is true the transformers may be a limiting factor.

 

[dankilling]

The output transformer will have a lot to do with the final sound of the amp. A small OT will severely limit the amount of bass an amp produces. I have an 18 watt lite 2b that has plenty of bass, but is running a Hammond 18W OT that is physically bigger than my 5e3 transformers. Not sure what the spec is of the OT from the Epiphone amp but it’s likely not beefy enough to move the current required for bigger bass.

 

[Uncle Daddy]

What speaker are you using?

 

[dan40]

If you are interested in a different OT and you want the best bass response possible, Heyboer does sell a 40 watt/8k OT that works just fine with a pair of cathode biased EL84's. I built a circuit last year based upon the Friedman JJ jr amplifier and this was the OT recommended for the build. It definitely delivers an improved bass response over the smaller 15-20 watt OT normally used in these 18 watt circuits.

 

[wadeeinkauf]

Wrong cap. The cap you should have changed is the .0047uF. (This cap and the volume pot etc. form a 2nd order filter with the input RC filter of the PI.)
(Weird wacky stuff going on here.)

It is easy to parallel the cap with another cap to increase the capacitance. Just clip in another cap to have a listen before deciding the final value.

The configuration of the phase inverter may also be messing with the frequencies. The position of the volume and tone knobs of the TMB channel may have an interactive effect on the other channel. (and vice-versa)

It is true the transformers may be a limiting factor.

I will try this.

 

[wadeeinkauf]

What speaker are you using?

Weber 12 Sig A-S

 

[YellowBoots]

Have you experimented with more capacitance on the cathodes of the EL84s? Maybe add another 25uF in parallel there.

 

[dan40]

I have a Weber 12 Sig speaker that I used in my Tweed Deluxe. It definitely does not have the strongest low end. A different speaker would help you in your quest considerably.

 

[YellowBoots]

Sometimes you have to cut the unwanted frequencies to get an apparent boost of the desired frequencies. No promises this would work (I have trouble with tone stacks)... just a hair-brained idea. Maybe there's a better place for a dull cap.

 

[wadeeinkauf]

I have a Weber 12 Sig speaker that I used in my Twee Deluxe. It definitely does not have the strongest low end. A different speaker would help you in your quest considerably.

Actually I tested the speaker. I ran my Twin Reverb into this speaker. Really could tell no difference in tone between this Weber and the original speakers in the Twin at low level. The Weber is only 25 watts.

 

[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.

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.

 

[wadeeinkauf]

Have you experimented with more capacitance on the cathodes of the EL84s? Maybe add another 25uF in parallel there.

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/