18 Watt TMB Combo Build w/Greenback - Too middy..Solutions?

[Yamariv]

Hey Guys,

I built a 18 Watt TMB Combo with a 25 Watt Greenback (used the layout on found on the dowload part of the site) and find the amp to be very thin and middy, no real warm bass tones at all, just not my cup of sonic tea..I was hoping this amp would replace my DSL40C for my main gigging combo but as it sounds now that's not happening..

Now, from what I've read, this middiness is a characteristic of the 18 Watt sound but I won't want to play it like it is right now and at this point want to mod it so it sounds good and don't really care to stay with a traditional 18 watt sound.

Any suggestions on how to change the voicing? I'll attach the schematic below for reference.

- should I compensate with a less mid focused speaker? Would that make enough difference?
- or should I change out some caps, like the .022uf's for .1's more like a Fender?

Any and all suggestions appreciated! Cheers

[crgfrench]

Perhaps you could get more from the tone circuit by:
1) replace the 56k tone slope resistor with a 33k
2) replace the 250p treble tone cap with a 500p
3) replace the 10n bass tone cap with 22n

The .1 coupling caps will also help.

[crgfrench]

What output transformer are you using?

[Bieworm]

Personally I'm not a fan of greenbacks. I find them dull. The g12h30 is a much better option. More sparkle and boomier... overall the better choice for the 18w in the celestion domain.
As for tonal tweakings Craig gave some good options. Maybe replace one cap at a time and use alligator clips. Adjust to flavor.
But I'd start with the g12h30 and go from there if needed.
There are good tones in the 18 watt!!! Crank the normal channel and you'll know what I mean...

[colossal]

Regarding a speaker choice for the 18W combo, I have tried both the 30W Heritage G12H30 55Hz and 20W Heritage 75Hz, and prefer the 20W. But I think this is an individual choice and will also depend on your guitar and pickups, and what style and at what volume you play. I play Les Pauls and absolutely enjoy the interaction with the guitar's Volume and Tone controls and extreme touch sensitivity of the circuit. I have found the stock circuit to be very well balanced with either speaker. I built an 18W for a local player and he started with a G12H30 but he too migrated to the 20W Heritage. He is a Tele player and the tone he is getting is plenty fat and full.

As Craig asked "what output transformer are you using", this, in my opinion is an extremely important part of the whole 18W equation, and probably the most important. The original amps used an interleaved Radiospares hi-fi transformer. There seems to be a fair amount of variance in what is offered as "18W transformers" and different builders get different results. I use the Radiospares wound OTs exclusively and my amps seem to have a fair amount of headroom before they distort in typical 18W fashion. I also use a 10% taper pot for Vol and Tone on the amp. The only mod I make is using a 10nF coupling cap on the Normal channel preamp.

The reason I mention all this is that, with the RS output transformer and 20W speaker, I find the low end to be plenty, the mids well balanced, and the highs clear and sparkly and delicate. The 30W 55Hz cone, for me, can be a bit boomy and the top end just a bit syrupy. It instantly reminds me of Hendrix and while that's not at all a bad thing, I just preferred the more delicate, woody nature of the 20 watt cone. The top end sparkle is killer, IMO. Again, this choice is also influenced by my guitars and my playing style. My Les Pauls all have unpotted and underwound PAF pickups. These comments only apply to the Ipswich UK Heritage 20 and 30 watt speakers and the Lite IIb Normal channel preamp, not the 25W reissue Greenback or 30W 75Hz reissue cone or cascaded circuits (brighter and more cutting).

My point of all this comparison and background gets back to the output transformer. If you find your amp sounding thin and you want to change the circuit, the red flag for me is "what output transformer are you using"? GDS and Mercury are the only two that I know of that offer an actual Radiospares winding pattern. I think if you need to increase the output coupling caps on the Normal channel (Lite IIb circuit) or do other work arounds, something might not be right. Try other preamp tubes as well. JJ short plate 12AX7s can be a bit too middy and strident in the top end for my taste. But of course, eveything may be just fine and you might just prefer one speaker over the next.

[JMPGuitars]

should I compensate with a less mid focused speaker? Would that make enough difference?
- or should I change out some caps, like the .022uf's for .1's more like a Fender?

The speaker makes a HUGE impact on the tone curve. I suggest trying a few different speakers before making any circuit changes. Once you find a speaker that suits you, then tweak the circuit if you still feel the need.

Are your voltages all within spec?

I wouldn't generally characterize the tone as thin. There could be an issue with the build, or you could just not like it with the speaker. Or, is it a brand new speaker? Maybe the speaker sounds like crap because it needs to be broken in?

A friend of mine I built an amp for hates enhanced mids. One of the speakers that was recommended to me was the WGS Reaper HP. I haven't tried it yet, but I imagine it should be tamer in the mids.

Plus everything Colossal said, and I'll add that the most popular preamp tubes are the new reissue Tung Sol and Mullard. Though I think Mullard pushes mids more than the Tung Sol, so you might prefer the Tung Sol (I like mids, so I like the Mullard).

Thanks,
Josh

[JMPGuitars]

Here's the typical values I use for TMBs, based on ZP's Jalapeno tone stack:

tone-curves.jpg

You can see by that sweep that it's plenty versatile. You can experiment yourself with what changing different values does here: https://www.guitarscience.net/tsc/marshall.htm

Thanks,
Josh

[Yamariv]

Wow, love this site, lots of awesome advice and knowledge boys! I have an update..And to answer the OT question, I'm using the 18 Watt Hammond transformer.

Ok, spent part of the morning playing the 18 Watt through various cabs, I tried Celestion Blues from my AC30, my 212 Geenback Cab and my DSL40C's 75 watt Creamback. My analysis from that would be this:

- The normal channel sounds full and thick through all the speakers, no issues on that side of the amp. That channel is very Fendery and sounds great, crisp, clean and nice bass.
- 212 Greeback cab sounded a little fuller but still thin, not much improvement
- 212 Alnico, sounded a bit fuller but not much of an improvement on the TMB either
- Single 75 Watt Creamback sounded much nicer and a little more full. What a great sounding speaker but still something is off..

Just before I shut everything down I noticed that the Bass and Treble knobs when turned full either way do pretty much nothing Yes, how did I not notice this before..well I set them to where I like them and played with the middle knob most assuming that the Treble and Bass would be working..ugh! The middle knob varies the sound like crazy when turned full left and right.. I do have an effects loop inserted in the TMB but it seems to be working fine.

So, given this I obviously have a wiring issue on the tone stack. I've attached some pics in case someone with a keen eye can spot the issue as I won't be able to open the amp up for a few days.

[crgfrench]

The Hammond is the issue. Swap in a Heyboer from Mojo or GDS.

[JMPGuitars]

The Hammond is the issue. Swap in a Heyboer from Mojo or GDS.

Except he said one channel sounds good, and the other doesn't, so that points to a problem with the tone circuit. Plus (assuming the correct model) the Hammond 18 watt OT is supposed to be good.

[Yamariv]

Yeah, the Normal channel sounds great so I'll hold off on any OT changes cause there's definitely something wrong with the TMB tone stack. Anyone know which wire would specifically cause the treble and bass knobs not to work at all of the top of their head?

[Bieworm]

Yeah, the Normal channel sounds great so I'll hold off on any OT changes cause there's definitely something wrong with the TMB tone stack. Anyone know which wire would specifically cause the treble and bass knobs not to work at all of the top of their head?

I never heard an RS OT but both my 18w sport a hammond OT and they sound awesome. The classic is very bassy (nice!) and the tremolo TMB has a little less, but still plenty. Over-middyness is not the word I'd use. Mid pot is maxed out on mine: look -->

15980834022363337134209928925231.jpg

[JMPGuitars]

Yeah, the Normal channel sounds great so I'll hold off on any OT changes cause there's definitely something wrong with the TMB tone stack. Anyone know which wire would specifically cause the treble and bass knobs not to work at all of the top of their head?

You need to do a highlighter test with the schematic AND the layout, make sure they both match, and make sure your amp matches the docs. Verify every component and connection. For anybody else to do it, is a lot harder, unless something is glaringly obvious.

[Yamariv]

For sure, I'll definitely go through the tone stack wire for wire shortly, just thought maybe someone might spot something glaringly wrong from the pics

[Yamariv]

Yeah, the Normal channel sounds great so I'll hold off on any OT changes cause there's definitely something wrong with the TMB tone stack. Anyone know which wire would specifically cause the treble and bass knobs not to work at all of the top of their head?

I never heard an RS OT but both my 18w sport a hammond OT and they sound awesome. The classic is very bassy (nice!) and the tremolo TMB has a little less, but still plenty. Over-middyness is not the word I'd use. Mid pot is maxed out on mine: look -->
15980834022363337134209928925231.jpg

Interesting pic and settings you run on your TMB! Good to know a normal TMB circuit allows you to do that, mine's definitely got something wrong if you're able to run it that way!

[Bieworm]

Yeah, the Normal channel sounds great so I'll hold off on any OT changes cause there's definitely something wrong with the TMB tone stack. Anyone know which wire would specifically cause the treble and bass knobs not to work at all of the top of their head?

I never heard an RS OT but both my 18w sport a hammond OT and they sound awesome. The classic is very bassy (nice!) and the tremolo TMB has a little less, but still plenty. Over-middyness is not the word I'd use. Mid pot is maxed out on mine: look -->
15980834022363337134209928925231.jpg

Interesting pic and settings you run on your TMB! Good to know a normal TMB circuit allows you to do that, mine's definitely got something wrong if you're able to run it that way!

Definitely if you know I play jazzmaster and jaguar guitars. They are pretty trebly by themselves