Blues Custom 30 Gain Stages...

[CK1]

Hi All,

 

New to the forums...just wanted to say thanks to all the contributors here...I've already read a ton of great information.

 

I have a Blues Custom 30 that I'm trying to dial in to get a good tone. So far I've swapped out the original Sovtek 6L6WXT+ power tubes with a pair of =C= 6L6GC's and I've also replaced the Shuguang rectifier with a '67 Mullard. After doing that the bottom end of the amp really opened up and sounded fantastic on both the clean and dirty channels. I play mostly hard rock along with some blues and jazz so I was still looking for a bit more gain out of the amp on the dirty channel. I ordered two Tung-Sol 12AX7's that were tested to be high-gain and put them in the V1 and V2 spots. I also got a Tung-Sol Gold balanced 12AX7 and put in in the phase inverter (V4) spot. On top of that I also dropped a Gibson 498T into the bridge position on my Les Paul Custom. WOW - I'm getting a TON of gain now on the dirty channel - it's just where I like it - plenty of crunch and sustain but not out of control. Here's the problem, though - the clean channel now has virtually no headroom and breaks up if I turn the level above 10 o'clock. I wanted to keep the clean channel clean until one or two o'clock like it was before I put the high-gain Tung-Sols in. I'm new to tube amps so I'm guessing here but is the clean channel breaking up so early because I dropped that high-gain Tung-Sol into V1? Is V1 the clean channel circuit and V2 the dirty channel circuit? If I put back in the stock EH 12AX7 into V1 will the clean channel go back to having more headroom? I'm going to try it when I get home from work but wanted to get some helpful opinions if possible.

 

Thanks in advance for your help.

 

CK1

 

P.S. - The amp is coming around but it's still bright so I will be doing the bright cap mods in a day or so. Probably just take out C6 and C5. I'd also like to put in the R39 volume mod if it's not too hard so I can practice without blowing the roof off our house at night. :-) If anyone has any links or tips on doing the volume mod I'd appreciate it.

[wildschwein]

Hey Man, I've been through a similar tube swapping journey with my BC30 and play in the same genres you do. In the end I gave up on the clean channel - I just found it was a little too upper-midrangey for me. For pristine cleans I prefer a slightly sucked midrange tone ala Black and Silverface Fender. On my BC30 I just set the dirt channel to the edge of break up and use my guitar volume to dial in a semi-clean sort of sound when required. I then use an ehx double muff fuzz or a marshall guvnor to get more dirt for harder rock stuff. Having said that I would be interested to see how things work out with your clean channel after some modding.

[barry lyons]

Hi, i have been messing around with the caps in my BC, I have just removed one of them but i will have to do more as it is still was to bright,

 

Before i go out and buy myself a new set of valves (tubes) i would like to ask, do they really make a big difference or just a minor one .

 

I play a strat and with the single coils its just way to toppy, it sounds ok with my LP ,but im a strat man .

[wildschwein]

Hey Barry. Changing the power tubes makes a big difference. The bottom end is greatly improved. It becomes very hi-fi-like. I recommend Svetlana 6L6GCs. I just got Reflektor-marketed ones and they sound great to me but winged Cs could be worth the investment. A lot of the toppy-ness in the amp comes from the Sovtek 5881 or 6L6WXT+ power tubes that the amp ships with. Changing the preamp tubes yields less dramatic results but it's probably worth it. I went for EHX 12ax7s in all positions except the two reverb slots (V3 and V5 from memory) were I kept the stock Chinese 12ax7bs. This made things a tad smoother overall. Changing the rectifier yields negligible results in my opinion. They may change the feel marginally but not the sound. Just put something reliable in there - a Sovtek (some BC30s shipped with one) or JJ is fine.

[CK1]

I'd agree with wildschwein262 - the single biggest improvement in sound I've heard was by replacing the power tubes. I also used 6L6GC's from =C= and the improvement in sound was tremendous. The BC30 went from having a flabby, uninspiring sound to just a beautiful, crisp, and powerful tone - especially on the bottom end. I spent a lot of money on a '67 Mullard rectifier but honestly I wouldn't do it again as the feel of the amp has not changed that much. Similarly with the preamp changes I made (Tung-Sols instead of EH), you could tell a slight change in tone but nothing as dramatic as upgrading the output tubes. If you're on a tight budget and want to get the best sound out of the BC30 the best thing you can do is to upgrade the power tubes.

 

I've had some more time to experiment and found that the early breakup on the clean channel that I asked about in the original question was not due to the high gain Tung-Sol's I put in - it was actually the Gibson 498T hot alnico pickup that I dropped in the bridge position on my Les Paul. That thing is *extremely* hot! I've been a solid state user my whole life (the BC30 is my first tube amp) and I'm used to just keeping my guitar controls at 10 and using pedals and the amp footswitch to control the gain and sound. With this hot pickup keeping it pegged at 10 there's no way to get a clean sound out of it. Once I dialed it down to 6 or so I was able to get a nice clean sound on channel one and didn't get any breakup until I turned the amp level to about 2 o'clock.

 

I pulled C6 and C5 and it did significantly lessen the 'ice-pick' problem. I don't think I'll lift anymore caps as it's sounding pretty good now. I'm getting a nice clean tone on channel one and channel two is giving me a nice hard rock sound with plenty of gain. I think I may invest in a Boss equalizer pedal to try to shape the tone some more but overall I'm pretty happy with how it's sounding. The only gripe I have is that two of the four bolts that hold the chassis to the cabinet were stripped at the factory and I had to drill them out to get the chassis out so I could pull the caps. Now I've got to try and find replacement bolts so I can get the chassis back in and bolted down properly.

 

I think the only other mod I'd like to do is the master volume mod where you replace R39 with a pot. You have to remove the board from the chassis for that, though, and I'm not sure if it's something I want to try. Did anybody do this mod themselves? Any tips?

[Ti-Ti]

Winged -C- is a great choice.

Note that Tung-sol EH, Sovteck are brands owned by New Sensor Technology NYC usually made in Saratov plant Russia (I don't say that those valves doesn't sound good).

Tissan

[mjlewis22]

I did the R39 mod and I put several pictures in the the BC30 mods thread.

 

I did this without removing the board by clipping the resistor near the body and leaving the legs attached to the board. Then I soldered the sheilded cable to the legs. Kinda ghetto but it saved me a lot of trouble and it works well.

 

I used a 1M pot, and I'm not sure if that's right--I think 250k or 500k might have been better. I also wired it backwards, so turning it CW decreases the volume. I beleive that has to do with R39 being connected to ground--as you lower the resistance, more signal goes to ground?

 

Either way, it allows you to really saturate the preamp tubes and get some of that thick syrupy overdrive at reasonable volumes. I swapped out V1 and V2 for GE 5751s, which I really like the sound of. I prefer a little grit even in the clean channel, and the 5751s break up at lower levels.

 

I use a Boss GE-7 EQ as a boost when I want a little more. It boosts the signal just enough and its pretty easy to move the sliders around if you want your input signal to sound a little different.

 

Good Luck.

[CK1]

Thanks for the info, mjlewis22. I have some 500k pots lying around so I'll give one of those a try. One question - what gauge is the shielded wire you used to wire up the master volume pot to R39? Do I have to use a specific gauge or will anything do?

[mjlewis22]

I used a piece of guitar cable I had lying around. It's still working so I assume its alright. I would guess its about 16ga or so.

[CK1]

I put in the master volume mod with a 500K pot last night and it's working very well. Both clean and dirty channels sound very good. The dirty is still a bit trebly for my taste but I'm getting a nice Joe Perry "Sweet Emotion" bark out of it. Thanks to mjlewis22 and others for posting instructions and pics on how to do the mod.

[Guitarnut]

I put in the master volume mod with a 500K pot last night and it's working very well. Both clean and dirty channels sound very good. The dirty is still a bit trebly for my taste but I'm getting a nice Joe Perry "Sweet Emotion" bark out of it. Thanks to mjlewis22 and others for posting instructions and pics on how to do the mod.

 

How are you guys draining the filter caps? I plan to short the anode/plate resistor (R4) on V1 pin 1 to ground. I've posted this question in several forums and not gotten any answers. I would sure appreciate knowing how you guys are doing it.

 

 

Peace,

Mark

[mjlewis22]

Thats how I did it. Then after a few minutes I probed around all over the place and didnt get any voltage anywhere, so I think it worked.

[Guitarnut]

Thats how I did it. Then after a few minutes I probed around all over the place and didnt get any voltage anywhere, so I think it worked.

 

Thanks. I saw your MV mod before pics in the Mods thread but I didn't see any of the finished mod. Did you wire it up the same as Dannyboyee82 did here? Could you share a drawing of how you did it?

 

Peace,

Mark

 

 

 

[mjlewis22]

I didnt take any "after" pictures because my ex-gf was here while I was working on it and she seemed somewhat annoyed that I wasnt paying attn. to her, even though I told her that I was going to be busy that night and she insisted on coming over anyway. (Stop for breath after run-on sentence.)

 

She broke up w/ me so now I have plenty of time for taking pics, but I dont have any AA batteries handy so I'll just try to describe it as best I can.

 

On the potentiometer, you just bridge the middle lug to either of the side lugs. I dont know the technical term for it but it makes the pot a "unidirectional" resistor. What I mean is that if you have a 1M linear pot turned 40% CW and you probe middle to sides, then 40% of the signal goes one way and 60% goes the other, kind of like a proportioning valve in a hydraulic system. If you were to switch the leads the proportions would switch accordingly. By bridging it, it takes the directionality away so it acts just like a regular resistor with a variable value.

 

Here is where I messed up: I bridged the pot in such a way that turning the knob CW decreased the resistance, which made sense in my head. However, this particular resistor is connected to ground, so decreasing the resistance shunts away more signal thus lowering the output. You need to bridge the pot in the opposite sense, which should be fairly easy to figure out with an ohmmeter. Then just solder the lead wires to the side lugs.

 

My control works fine, but it just goes to -11. I put my knob on the back of the amp, which sort of canceled out the backwards directionality. When standing in front the amp and reaching behind, I turn my wrist CW to increase the volume.

 

FYI, the correct tool for putting the appropriately sized hole into the chassis is a "unibit" or step drill, if you can afford one. Mark with a pencil, then punch, then drill a 1/8" pilot hole using a high speed, then insert the step drill to increase the size of the hole.

[jefrs]

Hi All,

 

New to the forums...just wanted to say thanks to all the contributors here...I've already read a ton of great information.

 

I have a Blues Custom 30 that I'm trying to dial in to get a good tone. So far I've swapped out the original Sovtek 6L6WXT+ power tubes with a pair of =C= 6L6GC's and I've also replaced the Shuguang rectifier with a '67 Mullard. After doing that the bottom end of the amp really opened up and sounded fantastic on both the clean and dirty channels. I play mostly hard rock along with some blues and jazz so I was still looking for a bit more gain out of the amp on the dirty channel. I ordered two Tung-Sol 12AX7's that were tested to be high-gain and put them in the V1 and V2 spots. I also got a Tung-Sol Gold balanced 12AX7 and put in in the phase inverter (V4) spot. On top of that I also dropped a Gibson 498T into the bridge position on my Les Paul Custom. WOW - I'm getting a TON of gain now on the dirty channel - it's just where I like it - plenty of crunch and sustain but not out of control. Here's the problem, though - the clean channel now has virtually no headroom and breaks up if I turn the level above 10 o'clock. I wanted to keep the clean channel clean until one or two o'clock like it was before I put the high-gain Tung-Sols in. I'm new to tube amps so I'm guessing here but is the clean channel breaking up so early because I dropped that high-gain Tung-Sol into V1? Is V1 the clean channel circuit and V2 the dirty channel circuit? If I put back in the stock EH 12AX7 into V1 will the clean channel go back to having more headroom? I'm going to try it when I get home from work but wanted to get some helpful opinions if possible.

 

Thanks in advance for your help.

 

CK1

 

P.S. - The amp is coming around but it's still bright so I will be doing the bright cap mods in a day or so. Probably just take out C6 and C5. I'd also like to put in the R39 volume mod if it's not too hard so I can practice without blowing the roof off our house at night. :-) If anyone has any links or tips on doing the volume mod I'd appreciate it.

 

I think I "invented" the R39 MV mod.

 

If you look at R39 you will see that it is part of a voltage divider into the phase inverter. So it is a natural candidate for a VR potentiometer MV mod before the PI.

 

Needed:

250k VR and shielded cable with at least 3 conductors e.g. as used for humbucker pickups.

Soldering iron and solder

Parent's permission

Present for wife for burning hole in rug

Wire cutters and tweezers/forceps

Multimeter

Assorted plasters and field dressings

Large hammer for re-assembly

 

One end of R39 is ground, note that ground connection = A.

The other end = B connects to signal input from tone stack and reverb. The C27 cap is the input to the PI =C.

Sorry no pics please examine circuit diagram and your board, the parts are numbered.

 

R39 is 220k - replace with 250k VR, I used an Alpha B250k. I put it where the deceased standby switch went on a pair of big washers, you may want to put it elsewhere. The amp does not need a standby switch.

 

Lift the connecting leg of C27 -C- and connect this free leg of C27 to wiper.

Remove or cut out R39

Connect that ground connection -A- to the cold-end connection of the MV VR, that's with an inner conductor wire, not the shield braid.

Also connect the shield braid to the board ground -A-, but do not connect it again at its other end, at the pot or the its can or the chassis.

Connect the signal junction -B- to the hot-end of the MV VR.

 

Route the cable well away from HT, PSU, transformatorizers, output bottles, fan, mains and heaters. If you must cross their lines then do so at right angles.

 

See main thread on Beastly-30 mods http://forum.gibson.com/index.php?/topic/43505-blues-custom-mods/

[jefrs]

Thanks. I saw your MV mod before pics in the Mods thread but I didn't see any of the finished mod. Did you wire it up the same as Dannyboyee82 did here? Could you share a drawing of how you did it?

 

Peace,

Mark

 

 

 

 

Nice pics but the wiring is wrong. Sorry.

 

You have found the hot-signal and the ground connection with your co-ax. You need to use multi-wire shielded like for humbucker.

You need separate inner conductors from the hot and cold to the pot, and to use the ground on the shield - but at the board end only.

 

You need to lift the leg of C27 and take that to the wiper of the pot on a third wire.

 

You have connected up the MV as a variable resistor rheostat, whilst this will work after a fashion, this is better done by making it a potentiometer.

[mjlewis22]

Well, its pretty clear that I did it wrong.

 

Sorry to anyone that I've given bad advice to--it "worked" so I thought it was right.

 

I'm gonna try to redo it the way jefrs has described and I'll take pictures for everyone's benefit.

[jefrs]

Well, its pretty clear that I did it wrong.

 

Sorry to anyone that I've given bad advice to--it "worked" so I thought it was right.

 

I'm gonna try to redo it the way jefrs has described and I'll take pictures for everyone's benefit.

 

Sorry to chide. Both methods do work. The other side of the PI is at ground anyway. I don't think the variable R39 method will even unbalance the PI.

The variable R39 method will work as a volume control because it is still a forms a voltage divider with R31 and R32 feeding it, however it does alter the load seen from the tone stack and the reverb. No problem at full on and grounded at zero but betwixt it might sound a wee bit off, won't hurt the valve just sound a bit funny. If it works then it works. If it is simple it may work even better, just never seen a volume control done like that - not conventional and not as I originally described.

 

Funny thing, we buy balanced PI valves but quite often an unbalanced PI valve sounds better. When vintage amps were new, we didn't have balanced PI valves, we just had valves and stuck them in like they were light bulbs. Personally I like the sound of unbalanced output pairs, you get a bit of a tremolo beat when they overload and sag.

 

I took some pics today but I posted my updated mods circuit diagram on the big BC30 Mods thread, and used up most of my allocated space doing so.

[mjlewis22]

As promised, I repaired the MV mod and took a picture:

 

 

Sorry its blurry, my camera batteries were dying and I was in a hurry.

 

I have to thank jefrs for posting the directions to do this properly. I think the amp actually sounds better now.

 

I didnt have any 3-conductor shielded cable and I couldnt find any in town, so I made one out of an ethernet cable and some bare wire strand. I followed the directions exactly and voila, it works perfectly.

 

Here is my comparison of the potentiometer versus the rheostat:

 

• 
   
  
• The rheostat caused some hum that I didnt know I had until it wasnt there anymore.
  
• The 1M rheostat went from "0" to "10" in about 1/4 turn, so it had to be in precisely the right spot to achieve the desired attenuation without being silent, not to mention that turning it CW lowered the output. The 250k pot handles the attenuation as expected, and the amp actually sounds more natural.
  
• The rheostat did not attenuate the output as much as the potentiometer does. It was almost like the rheostat went from "0" to "7" and then you had control of "7" to "10". As stated previously, the potentiometer produces a consistent gradient over its entire range.
  
• Because the potentiometer works consistently throughout its entire range, the level controls on each channel interact with it better. Better is not much of a description, but I just cant seem to explain it in a concise manner right now. What I'm saying is with the combination of master volume, channel volume, and guitar volume, its possible to dial in any amount breakup/overdrive.
  

 

jefrs, thanks again.

 

--ML

[jefrs]

As promised, I repaired the MV mod and took a picture:

 

 

Sorry its blurry, my camera batteries were dying and I was in a hurry.

 

I have to thank jefrs for posting the directions to do this properly. I think the amp actually sounds better now.

 

I didnt have any 3-conductor shielded cable and I couldnt find any in town, so I made one out of an ethernet cable and some bare wire strand. I followed the directions exactly and voila, it works perfectly.

 

Here is my comparison of the potentiometer versus the rheostat:

 

• 
   
  
• The rheostat caused some hum that I didnt know I had until it wasnt there anymore.
  
• The 1M rheostat went from "0" to "10" in about 1/4 turn, so it had to be in precisely the right spot to achieve the desired attenuation without being silent, not to mention that turning it CW lowered the output. The 250k pot handles the attenuation as expected, and the amp actually sounds more natural.
  
• The rheostat did not attenuate the output as much as the potentiometer does. It was almost like the rheostat went from "0" to "7" and then you had control of "7" to "10". As stated previously, the potentiometer produces a consistent gradient over its entire range.
  
• Because the potentiometer works consistently throughout its entire range, the level controls on each channel interact with it better. Better is not much of a description, but I just cant seem to explain it in a concise manner right now. What I'm saying is with the combination of master volume, channel volume, and guitar volume, its possible to dial in any amount breakup/overdrive.
  

 

jefrs, thanks again.

 

--ML

 

Cheers and glad you like.

The new pic does look something like mine but I cannot post it up because the schematic has used all my available up.

 

Whilst a screened connection to the pot might be better, they are really only essential for the input.

 

The pot can is at ground, it's bolted to the chassis, a connection from here to the PCB might create a hum-loop.

But, a screen from the board (one of the wires is at ground anyway) that covers the other three conductors until they reach the pot is the preferred method.

It might help with the RF spat out from that reverb transformer ;-)

 

However, the three-wires you have there are going to approximate "twisted-pair" anyway and the signal they are carrying is large compared to noise level.

 

I was a little concerned that the pot (from my bits bin) might not be rated high enough as the output is quite high at this point but have not noticed any problems, it does not even get warm.

For the wire, the usual 4-conductor shielded as normally used for humbucker pickups may be a little thin, so I used a thicker version which I prefer for guitar wiring, it's a type often used for stereo line signal cables. You could butcher a suitable cable but I had some on a drum, 'cos it's cheaper to buy it that way.