Tube Amps / Music Electronics
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|5/25/2000 9:10 PM|
||Lead dress ( again)|
I've got an old Kalamazoo Model I that I am restoring. I decided to go ahead and really re-do this simple little amp ( 1 12AX7, 1 EL84, 6X4 recto, volume, combination tone/on-off switch). All components are either mounted on terminal strips or directly to the tube sockets and pots. Currently, the flimsy filament wires are twisted and laid down against the chassis. I'm thinking I want to use solid core filament wire, and elevate the filament wires above the chassis ( ala Fender). Will this cause a noise/oscillation problem, since many of the components are elevated well above the chassis as well ( on terminal strips) ?
Also, the filaments are referenced to ground by a single wire from pin 9 of the preamp tube to the ground lug of the input jack ( yikes ?). What would be a better way of doing this ?
I'm also replacing the super-cheapo original tube sockets with some better ones I have scavenged.
Finally, can somebody give me some scoop on the 6X4 rectifier tube ?
All comments and suggestions greatly appreciated !
Thanks and regards,
|5/25/2000 9:20 PM|
Fred, Just make sure that the filament leads are twisted tight and not real close to the signal or B+ wires. On my DIY 5E3, I elevated the heater wires for the power tubes and ran the preamp tube wires flush because this gave me the best distance apart. If the filament winding on the PT has no center tap, then just reference each filament wire with a 100ohm 1/2W resistor to ground.
|5/25/2000 11:29 PM|
The 6X4 is a seven pin rectifier, it will work fine in the amp. You can get the specs from Duncan's tube data.
|5/26/2000 2:49 AM|
Sheesh, I'm really posting a lot today.
The Kalamazoo Model 1 is the same circuit as my '67 Gibson Skylark. I did a total mod, considering only the original tube types, pots, and transformers for inspiration. It really is so much sweeter clean or usually played at least a little dirty since there is so little headroom with just a single 6BQ5. I fixed it so it is useful at any volume, especially maxed where it screams and grinds unbeliveably. A Kendrick Blackframe 10inch speaker works phenomenal for this amp IMHO.
I only replaced the power tube socket that had arced, probably due to running it when the speaker lead had come undone. I am obsessed with low noise to the point where I can not accept the S/N of most Fenders & Marshalls, but I left this original heater wiring as is, poorly twisted and all. It does work best when the twisted wires are very close to the chassis. No need for thicker wires for such a small amount of current. I am still only using a 2-prong line cord, but I carefully check the polarity of how it goes in the wall socket so that any failure of the (fully tested) original death cap will only allow the neutral AC line to leak to the chassis, never the hot line. My power switch is a separate knob besides the volume control and tone knob, so I left the line cord, pilot, power switch and fuse area as original.
I also test amps with a very low noise humbucking guitar on which the strings and hardware are not grounded at all. Perhaps you might not want to try all this at home.
Anyway, there was a lot of hum until I soldered a matched pair of resistors (330ohm) directly to the heater pins of the 6X4, and joined their free ends in space (I do a bit of mid-air soldering, since all I was trained on was point-to-point wiring like this). The junction between these resistors was then wired to the local star ground at pin 2 that is unused by the 6X4. I also had to run a wire to the other star ground located on the input jacks and nearby leads connecting to them. Of course I had to first remove the original connection of one of the heater lines that went directly to ground. Major improvement, these were used resistors pulled from a PCB with only short leads (ideal length for the application), but well matched.
The 6X4 is an ordinary rectifier not completely unlike a 5Y3, except it has heaters isolated from the cathode like a preamp or power tube. Therefore it can use the same 6V heater winding from the PT that is shared with the other tubes. The rectified DC HV comes out of a separate cathode pin, unlike an ordinary 5Y3 where the cathode IS the 5volt filament and develops high voltage DC riding on its 5VAC filament supply.
6X4 pins 3 & 4 are heaters (green wires), 1& 6 are plates for connecting to the HV PT winding (red wires), and 7 is the cathode for output of the HV DC to the filter caps and rest of the amp. Pins 2 & 5 are not connected electrically to the inside of the tube, but 2 is used in this amp as a substitute terminal strip for ground.
I took out the expired paper multisection filter cap (even though it still *worked*) and replaced it with individual Nichicon axials having 450VDC rating, all filter caps minus to pin 2, it gets crowded so plan carefully. This type of wiring might be frustrating if you are not used to soldering three or four things together without having them all come apart. The PT HV center-tap (red/yellow) goes here too, as well as the cathode resistor from the 6BQ5, and I also added a bypass cap across the cathode resistor which grounds here and crowds it even more.
Always remember to remove the tube before you solder on its socket, otherwise you will have bad sounding joints.
I removed the tone (sucking) stack from the circuit, using a smaller coupling cap to take the signal from the first triode directly to the top of the volume pot. I then rewired the tone pot to replace the 330K fixed resistor on pin 2 of the 6BQ5 as a master volume, naturally it sounds best with this knob maxed. Also bypassed the cathode resistor on the second triode with a 3.3mf film cap. Another important addition if you want to use higher volume settings is to add a 1Meg resistor from pin 7 of the 12AX7 to ground. This also improves compatibility with pedals.
Well, that's the tip of the iceberg, it was a two-year mod with extensive testing of each individual change, but this could keep you busy for a while Mine impresses with its new ability to breathe fire.
I recommend a Mullard 12AX7, and Sovtek EL84M or original RCA 6BQ5 for my two primary tones (EI Yugo EL84 is almost as good), and any functional 6X4.
Before you beef it up too much, save the original 8ohm alnico speaker, it will not be able to handle this thing on steroids. Best performance will be noticed when you play through an 8ohm 4x12 cabinet instead of the built-in speaker
Miles O'Neal from Austin has a web page dedicated to the Kalamazoo Model 1, I don't know the URL but he usually has a message or two posted on a.g.a. and he can be reached by email.
Have fun & be careful,
|5/26/2000 5:17 AM|
Thanks for you excellent info !
So far, I have moved the HV center tap to it's own ground point at one of the PT mounting screws. I replaced one of the preamp tube plate resistors which had drifted to 120k with a 1W metal film 100k.
I replaced the first .01 coupling cap with a .001, which helped to eliminate most of the farty distortion in the lower frequencies. All of the filter caps have been replaced with new ones corresponding to the original values.
I have been screwing around with the star ground - the filter caps currently have their own ground point, and the preamp grounds have their own star ground point as well ( this may not be such a good idea).
I left the filament reference to ground at the input jacks - I'd like to change this.
As it stands now, the amp is sounding better tone-wise, but there is still a good deal of hum. There is also a noticeable physical vibration in the chassis, when you touch the face plate, you can feel the vibration. This concerns me - I'm afraid maybe some sort of oscillation is going on here.
I'm going to replace the 2-prong power cord with a grounded 3-pronger.
Any further comments/criticisms/suggestions as to what I have done so far, and where to go from here ?
Thanks and regards,
|5/26/2000 7:24 AM|
1st unground that filament wire and refrence it through two 100ohm resistors to ground at the same place as the power transformer also try grounding the caps here too, that should stop the hum.Another thing I would do is change the tone control to a Fender 5e3 deluxe style and put the .01 back also change the cathode resistor comeing off of pin 3 on the 1st pre tube to 1.5k/25uf cap. The stock tone control cuts mids with that .0047 cap and the tone pot only cuts treble (the 5e3 adds & cutts).You could try cutting out the .0047 cap first and then if u like that go for the 5e3 tone thing.
ps. one more thought, try putting a 25uf/50v cap over the power tube cathode resistor (leave some space betwen the cap & resistor, it does give off some heat)
|5/27/2000 5:31 AM|
||Why the bypass cap ?|
What does the bypass cap across the EL84's cathode resistor accomplish, sonically and theoretically ? Does it have the same effect as bypassing the preamp cathode resistor ? Just curious.
Thanks and regards,
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