Seeking suggestions to modify Ceriatone HRM Bluesmaster 50

[mwdavis14]

Hi All,

I recently completed this build and, unfortunately, am far from impressed with the amp. I find it to be extremely dark and muddy. Must dial the mid and bass controls near zero and crank the treble to get a useable tone. I'm running through a C-Lator and generally have to use one or both of the bright switches on that as well, depending on the guitar. Brighter guitars are, obviously less of a problem. The magnitude of the issue was somewhat improved when I installed better tubes. Scanning the myriad entries on this site & ceriatone forum, I found suggestions on tubes and currently have the following installed (all NOS): V1 - RCA 7025 short plate, v2 - mullard 12AX7, V3 - mullard CV4004 (matched), and a matched set of NOS Sylvania 6L6GC. PI is set w/a 6VDC difference b/t the A&B points per manual. Power Tubes are biased at 35. Speaker is an Austin Speaker Works KTS-70 in a 1x12 cab

This was my first build and a real learning experience. I can follow a layout and build the thing, but as far as really understanding how tweaks work - beyond my current understanding. I'm looking for suggestions on how to reduce the amount of mid & bass (particularly mids) in this thing. It would be nice to have the "default" signal produced in the amp at a point where the treble, mid & bass pots can all produce a useable signal when positioned at Noon so as to allow more effective tweaks with those controls. As noted above, I currently need to have the treble b/t 7-9, the mids b/t 1-2 and bass around 2 with the pull bright on the vol active in order to get a halfway decent sound that doesn't turn to mud.

Here is a link to the ceriatone layout for this amp. Any suggestions would be very much appreciated:

http://ceriatone.com/images/layoutPic/O ... BM-50W.jpg

[ToneMerc]

Well, you not the first person to build a D style amp and then discover it's not what they thought it would be. IMHO, the BM HRM is one of the more difficult circuits to get right off the bat, assembling the kit was the far easy part!

In order to tune it, answer this, why did you choose this model in the first place? Oh yeah, don't get too attached to those preamp tubes.

First things first:

1. How are the HRM trimmers set?
2. What's the V2 plate voltage?

TM

[rogb]

Are you talking clean, OD or both?
If OD, add an HRM lift switch at the back, when I had an HRM, I had it switched off most of the time

[roknroll]

As ToneMerc said, building was the easy part. Tuning is the time consuming part. In the past when I first started becoming intrigued by the Dumble sound my first build sounded so bad it was not tolerable. It took months of reading, experimenting, sourcing parts, soldering, desoldering, listening and combing thru TAG and other sources , purchasing speakers, mounting the speakers, dismounting the speakers, listening again, sell them on ebay, buy more speakers, cabinets, all inorder to acquire a personal knowledge base that would allow me to finally dial in what made me happy. I have gone as far as being so pissed of at a build that it goes on a shelf for months at a time and then finally I come back to it again.

I don't know why people get all bent out of shape because they can only get a good tone when the bass is on 1 or the treble is on 9 or whatever. For me, wherever along the rotation of the pot the magic happens, I am just happy to have found it whether it is on 1,2,3,4,5,6,7,8,9 or 0.

One of the simplest non technical tweeks you can make is having the right cabinet and most importantly the right speaker. Of course there is no correct combination that makes everybody happy, so make yourself happy starting by trying other peoples recommendations for cabinets and speakers and go from there. While it is a non technical tweek it is an expensive time consuming tweek.

So I hate to rain on your parade but take it from someone who has been there and done that (not with a Ceriatone), until you have put in the time to find what makes the amp do what u want it to you will be frustrated. There is no one cure all. It is usually a number of things. There is no replacement for time put in, experimentation, research.

I don't think this is what u wanted to hear or read, but for me and I am sure many others involved in this pursuit, it is reality.
Good Luck

[martin manning]

It's possible that you have lurking wiring issues too, based on the history of filament string and impedance switch connectivity. A good place to start would be a full set of voltages... previous measurements had V1 plates at ~197. Note it is best to stick to a single thread so that anyone trying to help you out has all relevant information available without having to go searching for it.

Further to TM's comment, I question the wisdom of using expensive parts in a first-time amp build by persons without any previous electronics experience. Only in very rare cases does the build quality come out at a comparable level, and the more complex the circuit is, the more likely that is to be the case.

Love this: http://ceriatone.com Not only are the kits FULLY unsupported, but there is NO tech assistance available too!

[M Fowler]

Here is some instructions I was given.

The HRM trimmers in the BM are finicky to say the least (much like all HRM's in nature) , if I were you I'd get in there and try theses settings,

Bass: 200k
Middle: 9K
Treble: 125k

Then, adjust the PI with a matched 12ax7 ( I dig Long plates )
And a short plate in V1 & V2 (Mullard or Amperex short plates sound killer)

And set the OD Trigger at around 27.4k

Also, some install a 3-way mid boost or 6-way treble bleed. Also, some add a switch to defeat the HRM.

Mark

[mwdavis14]

Thanks.

Jack, what you describe is what I'm dealing with presently. I'm exploring different speakers & cabinet designs as well, but at this point as a Noob I'm trying to gather sufficient understanding of how these things work to make intelligent choices re: changes.

As far as where the issue lies, its actually a bigger problem w/the clean than w/the OD. OD I tend to use the bridge PU anyway, which is brighter and thus not as big an issue. The HRM trimmers are set per Ceriatone's default instructions: Midrange is at appx 12-1:00, Bass is at appx 10-11:00 and treble @ 11:00. I installed the HRM lift switch, but get a better OD sound with the HRM active vs. lifted.

I would actually appreciate it if someone could help me understand how changes to the values in the tonestack work, and could confirm that I have correctly identified the parts. I have attached photos of what I believe are the slope resister, and the treble, bass & mid caps on the ceriatone layout. The 5th photo shows resisters feeding V1 from the input jacks the purpose/function of which I have no concept (so help there would be appreciated). The photos are listed.

Here is my understanding from research to date:
Slope Resister - Adjusts the frequency point for the midrange. Increasing value will decrease the amount of mids

No idea of how changing the values on the treble/mid/bass caps will impact the sound. I do note, however, that the values for these caps on the regular HRM and on the OTS series are of different value from the BM. Specifically, the BM uses .02uf for the bass & mid caps whereas the other models seem to use .01uf (combined with higher value slope resisters). BM uses a .002 for the treble cap, which is same value on the regular HRM & all OTS models except for the OTS FM Modern Eagle which uses a .001. However, each of those other models have a higher value for the slope resister.

Would changing the bass & mid cap value to .01uf increase the impact of a change in the slope resister?

I see also that the BM uses a unique value on the mid pot. It uses a 25k (linear) pot, whereas the other models use a 250k (audio) pot. How would this change impact the frequency response?

I see that the BM uses a 68k resister to ground off the middle lug of the mid pot. This seems to be a unique aspect, which is not shared by any of the other models. Can someone help me understand what that does?

Also, someone advise of what the 47k resisters b/t the input and pin 2 of V1 do? The values for this resister seem to vary a bit by model, and I have no idea how that impacts the tone.

Finally, can someone confirm that I have correctly identified the slope resister & the treble/bass/mid caps? Pics are attached.

Thanks in advance. I'm looking to learn & understand, so I greatly appreciate the time in responding.

Mark

[mwdavis14]

Martin,
Thanks for the reply. I changed out the impedance switch which now works fine. The old switch was defective. I also located a broken filament wire once I removed the tubes and re-checked for pin voltage w/o them conducting. That quickly located the break which has been corrected. Sorry for placing questions in different strings. I will certainly avoid doing that in the future!

Honestly, I don't expect wiring issues. The amp is wired exactly per the layout with careful attention to lead dress. With the exception of the broken filament wire and bad impedance switch, which took a while to locate, there have not been any real issues. I had a bias problem, but it turned out to be a bad power tube. Bias is spot on with the new power tubes. It runs dead quiet now, and plays fine w/lots of volume & plenty of OD on tap, its just the muddiness issue which persists. I'll provide a pic of the overall build for reference & completeness as soon as my e-mail uploads

Voltage readings are as follows:

V1
1 184.2
2 0
3 1.53
4 3.46 vac
5 3.46 vac
6 189
7 0
8 1.51
9 3.46 vac

v2
1 195.8
2 0
3 1.84
4 3.46 vac
5 3.46 vac
6 202
7 0
8 1.74
9 3.46 vac

V3
1 232.3
2 268
3 43.0
4 3.46 vac
5 3.46 vac
6 231.5
7 27.25
8 43.2
9 3.46 vac

V4
1 35.3mv
2 3.47 vac
3 451
4 449
5 -49.3
6 450
7 3.47 vac
8 35mv

V5
1 38.7mv
2 3.47 vac
3 451
4 449
5 -49.3
6 450
7 3.47 vac
8 38mv

[martin manning]

So you are down about 10-12V on V1's plates now... that must be related to the bias and more idle current. V3 pin 2 is out, or there is a typo.

You have correctly identified the slope resistor and T/B/M caps. The two 47k's on the input stage are a combination of grid stopper and input attenuator which is typical of Fender hi-low inputs. Can you read a schematic? That is a skill worth acquiring. Layouts are useful for placing parts, but not so good for understanding a circuit. FWIW, I would never claim "The amp is wired exactly per the layout" without adding "but I could be mistaken."

[mwdavis14]

Here is a pic of the overall build

[mwdavis14]

Martin,

You are correct re: V3 pin 2. Typo. 26.8, not 268

[mwdavis14]

FWIW, I re-measured plate @ V1 and got 187.3

[mwdavis14]

Martin,
I have a general understanding of schematics, but would not profess expertise, and thank you for confirming correct ID of the caps/resister. Can you (or anyone else) recommend good books to get a grip on understanding how the amps work, beyond just the "big picture".

Can anyone help me w/big picture of impact changing the T/M/B caps would impact tone, and the mid pot issue? Is my understanding of the slope resister (stated previously) accurate?

Many thanks.

[martin manning]

I'd recommend that you play with Duncan's tone stack calculator, http://www.duncanamps.com/tsc/ which will show you what the effects of changing the component values is in dB. Then you will have to experiment yourself to hear it. Swapping a 100k in for the slope resistor would be a very good place to start. Visit the Valve Wizard pages http://www.valvewizard.co.uk to begin to get an understanding of how the circuits work.

[mwdavis14]

Thanks Martin. Much appreciated!