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Modify English Muff'n?

higgi

higgi

Mar 28, 2005
1,351
4
Eureka Springs, Arkansas
www.redambition.com
#1
I have an English muffin that I was thinking about adding a buffered blend. I was also thinking about changing the tone stack to lower the q on the bass eq, and to convert the mid to parametric. I was also looking for the input and output capacitors that are supposed to block DC signals.

If I can only do one thing with this it would be to add a blend so that I can increase the punch with a solid state signal. Is it possible to tap into circuit just before the signal goes to the tubes but after what appears to be a buffer preamp? Then take that signal past the tubes and mix it back in at the EQ? Also I don't see anything that looks like input or output capacitors, do tube circuits not need these? I am pretty good with a soldiering iron but a poor amateur at figuring the circuits out. Any help would be greatly appreciated.

Here's the circuit from EHX email.
 

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#2
Do you mean you want to change the center frequency of the mid control or actually the Q? This is pretty cool. I've not seen the English Muff'n schematic before. It appears to have a basic passive Fender/Marshall amp type tone stack. If you download Duncan's Tone Stack simulator software (google it) you can play with the values to see how you might want to mod it. Also, this tone stack is a passive one which means you can't make it truly (semi-) parametric. I dunno about the input cap, doesn't look like it has one.
 
#3
Thank you.

I studied it a bit more this weekend. I haven't figured out an input capacitor but the outputs are the three in the eq. The middle one .1u farad controls the lowest frequencys. I guess I should take a signal right at input and use an opamp and add that back in the EQ section. I have never studied anything on tube circuits so I am still learning.
 
#6
The most dangerous parts of the circuit are C16, C17 and C18. There's about 320k (DC wise) between the 220V and ground which would mean it may store charge pretty well. I would probably use a 10k discharge resistor and clip it across C16 for a minute.

Moving to a parametric mid is not trivial, particularly if you want full parametric with cut & boost.

The caps in the tone stack are there to set the frequencies of the tone stack, not strictly to block the DC, although, they do! C8/C9 also help. The input is direct coupled - there are no input blocking caps.

I'm no tube expert either, but I'd take the signal from the point marked "15" and mix it back in with the solid state "bypass" signal right at the output jack.
 
#7
Whoa, this thing is build like real tube preamp!! :eek: Just add the driver stages and powertubes after the tonestack and you are rock! But this also means the electrical shock hazard is very big.

Before you open this beast up, google everything you can about safety while working on tube amps, discharging caps etc. Then read all the info at least four times. Seriously, this may save your life or at least prevent some painfull schocks!

niftydog said:
I'm no tube expert either, but I'd take the signal from the point marked "15" and mix it back in with the solid state "bypass" signal right at the output jack.
Click to expand...
No expert either, but at "15", before C2, the DC Voltage is 129V. Wouldn't be the audio signal somewhere in that range too? Or at least amplified by the tube as per gain ratio? Ex: 0,5V P2P times 50 (gain ratio) = 25V P2P? That would create signal matching issues. And you need buffers for both signals: adding another tube or sand to the tube path.

I wouldn't bother mixing clean signal in. I'd work on bass response to suit my needs. Changing C2 or at least C4 to something higher might do the trick well. And there is also C5 to look at to.

Parametric mids again means adding another tube or sand to the tube path. Not worth it IMHO. The strenght of this design is it's simplicity. Like vintage tube amps.

I'd ditch the mids control and put a fixed 10k resistor there. After all, with this type of tone stack you can only cut mids. I checked with Duncan Amps Tone Stack Calculator and in it's neatral position (Bass Treble fully CCW, mids fully CW) the stack is perfectly flat. You can still have midscoop by cranking bass&treble.

Having now a knob spare gave me an idea for a cool mod: to replace the R11/R12 voltage diveder with a 1M log pot (and in this case, change C4 to something higher). Now you can control the gain in the third stage. Which can be very cool if you put different tubes in there. You could control the gain for each type of tube (almost) independently to achieve different mixes of various textures and tastes. Yummy! :)

Consult your local tube amps expert before doing anything inside though.

Good luck and take care,
John
 
#8
Zeemno said:
No expert either, but at "15", before C2, the DC Voltage is 129V. Wouldn't be the audio signal somewhere in that range too? Or at least amplified by the tube as per gain ratio? Ex: 0,5V P2P times 50 (gain ratio) = 25V P2P? That would create signal matching issues. And you need buffers for both signals: adding another tube or sand to the tube path.
Click to expand...
I really don't know either, I thought that maybe C2 was blocking the DC from V1B, so the only DC there should be the bias voltage for V1A. I'd be surprised to find any DC across the "gain" pot, pots abhor DC, especially high voltage DC. I don't even know how to calc. gain in a tube stage... it's like trying to read a document that half Russian and half English!

Mixing signals of different amplitudes is just a matter of selecting the right resistor ratios. And yes, a proper mixer requires another active element, either a tube of an op amp. Passive mixers cannot make up for the gain you loose in the resistor summing network.
 
#9
Thank you guys for the replies.

I have gigged with this rig as my "sansamp" a couple of times. I love the tone and the smooth overdrive. But from low D down B(5string) there seems to be some weakness.

So do you think that C2 is blocking some low frequencys? It has the same value (22n) as the midrange bleed off cap C12 in the EQ. Seems like all I should be hearing is midrange and up because of C2.

I think I will forgo the paramid. But I would like to put some power back into my B string. A capacitor change would be very easy.

I wish electroharmonix would build a tube version of the MXR bass DI+ with a built in russian muff and a true bypass effects loop with blend and a 9v dc out port to run external effects.

NIfty dog why would you use point 15? So that I wouldn't need a buffer amp? I was thinking about mixing the bypass signal in back at "1" just before the EQ so that my clean signal would be EQed.
How about C11 at .1uf? Shouldn't that cap be bigger also? I got a looper bypass schematic from Seanm that worked very nicely. I sold the looper when I went digital for a while. I can't find my copy of it now but from my memory the DC blocking caps where 10uf.
 
#10
higgi said:
So do you think that C2 is blocking some low frequencys? It has the same value (22n) as the midrange bleed off cap C12 in the EQ. Seems like all I should be hearing is midrange and up because of C2.
Click to expand...
Filters rely on more than just capacitance, it's also dependent on the resistance in the circuit, so just because it's the same value doesn't mean the filter cut off frequency is the same.


higgi said:
NIfty dog why would you use point 15? So that I wouldn't need a buffer amp? I was thinking about mixing the bypass signal in back at "1" just before the EQ so that my clean signal would be EQed.
Click to expand...
Yeah, but don't listen to me because, although I'm an EE I know next to nothing about tubes!!! Personally I'm not a big fan of modding where it can be avoided, and if I were you I'd be building an external sidechain like the paralooper... but that's just me.

One thing you would want to be careful with when mixing in at point "1" is that you don't want to upset the impedances in that area or you might have unwanted impact on the EQ circuit.

If you've got a multimeter, measure the DC volts at point 15. If it's low like I think it is, then it's probably a good spot. If it's high like Zeemno said, then it's probably not a good spot!

Mang, I wish I could help you more but all the guys I know who are good with tubes are old enough to be scared of internet forums!!!
 
#13
It's about the same price as a sansamp, and sounds better to me. I paid $160. It's just a little weak on the B string, but not bad. Actually the D sounds strong just the 3 notes below it are weak. For regular tuned 4 string bass I think this thing is awesome.

I have a Zoom B2 that I alternate gigs and practice with. The muff is so smooth and perfect compared to the tube modeling. But the Zoom is strong all the way down to b and a little more punchy. Plus it's so nice to have everything in one simple little box. What a freak I am to switch between real tubes and dsp all the time.

I do have a problem with modifying things. I have three basses, all modified. I even modify my cars. Is there a support group out there for modoholics?
 
#16
damn i'm gonna have lots of fun with that schematic this summer!!!... i had a EHX Black finger and i loved to overdrive it. Now i'm planning on building a tube stage an experimenting with it and then jumping on buldiong a custom english muffing or a SWR IOD
 
#17
hey
so i started having alot of fun with this schematic, but i cant get it to work
ive got all the right voltages and resistances everywhere, but i dont know what those points marked 1,2,4,6 etc up to 15 mean. can anybody help me?
oh and if anyone is curious, ive found the best way to drain the power caps are by connecting the the circuit to ground and the intersection of resistors 22 and 24

thanks
 
#20
If yo add a blend between 15 and the output it will just oscillate, as you will be introducing a positive feedback loop around two inverting gain stages. If you wnt this to work with a 5 string bass, install a .022uf across C4 [.0068uf]. This was designed for guitars not to be muddy.

Disclosure- I did not supply the schematic, I would never do this to EHX. However I did design and build it for them years ago.
 
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