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Old August 18th, 2012, 02:14 PM   #1 (permalink)
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12AU7, 12BH7, ECC99 and OTs?

I currently use two Vox Lil Night Trains, one "clean" and one "dirty".

The "clean" head is all stock (for now), except for the tubes. I replaced the Stock Chinese 12AX7s (V3 Pre-amp, V2 PI) and 12AU7 (V1 output) with an EH 12AY7 in V3, TS 12AT7 in V2 and an EH 12AU7. This gives the amp a nice Vox jangly clean and a slightly warmer tone.

The "dirty" amp has had the tubes replaced with a TS 12AX7 in V3, a TS 12AT7 in V2 and an EH 12BH7 in V1. The C19 "bright cap" was placed on a switch to turn it on and off as needed. The R10 slope resistor, was replaced with a 0-100 KΩ potentiometer, to adjust the bright/dark slope. The OT was replaced with a Musical Power Supplies OT5PP for added depth. This head had decent depth and can go from Bright jangly Vox Clean (barely!) to Vox's overdriven crunch.

I know with a low wattage amps it can be more difficult to pull a lot of bass, with smaller OTs and the output tubes usually used in such amps, let alone the typical smaller speaker. All of the mods to the "dirty" amp added a bit more bass response, as did the speaker swap in the V110NT cab and the Kustom Defender 112 cab (still debating what speaker will go in this one).

As you have seen, I have used a 12BH7 in place of the 12AU7 for the output tube (as has been done by many others numerous times), but have just found only a handful that are using ECC99s, with very little information available as to compatibility in amps designed for the 12AU7/12BH7s. The LNTs handle the increased heater amperage draw of the 12BH7's, is there much of a change going to the ECC99s? How about the OTs? Not the stock ones, but aftermarket manufacturer ones, such as the MPS OT5PP, Hammond 125c, or the Mojo Clone of the Wright 1580 ? The MPS is the correct input impedance, the Hammond 125c is adjustable, but the Mojo Clone is fixed at 10 KΩ, instead of the 22.5 KΩ of the original and MPS OT5PP. Would this be a problem, If I find I want to go with the Mojo?

Theoretically, if my math is correct, going from the 2watts of the 12AU7 to the 12BH7 should bump my output to 2.5 watts. Going to the ECC99 should put me to 2.75 watts. I know these are negligible gains, volume wise, but should assist in bumping up the drive for better bass response. Does this sound correct? (My experience with tube/valves comes from the early '80s on military radars, not civilian audio and we used only certain valves, tube rolling not allowed. I trying to recall nearly forgotten information.) What kind of output is available with these tubes?

Although the LNT is the platform I'm working with, How does this apply to other amps that use the 12AU7/12BH7 valves. Any information concerning these tubes/valves and/or OTs or any other compatible models please feel free to chime in.


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Old August 18th, 2012, 04:06 PM   #2 (permalink)
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Well the 12aAU7 heater is 300mA at 6V, the 12BH7 is 600mA and the ECC99 is 800mA. If you try it and the heater voltage does not drop too much you may be OK. The people at ax84.com have a lot of experience with the ECC99 and have found an increase in bass response. They seem to like the 22k transformer impedance.
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Old August 18th, 2012, 04:41 PM   #3 (permalink)
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I tried both a 12au7 and ecc99 in my Firefly. The ECC99 sounds much better and more bass response. I needed to rebias for ECC99.
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Old August 20th, 2012, 07:59 AM   #4 (permalink)
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I have been one of the ECC99 tinkers over at AX84. Maybe I still am.

From 12AU7 to ECC99 you would halve the cathode resistance, if it is cathode biased.

I never tried an output transformer smaller than a Hammond 125e.
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Last edited by Wattie; August 20th, 2012 at 08:02 AM. Reason: Doh! then, than, this, that, the other
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Old August 24th, 2012, 05:42 PM   #5 (permalink)
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I replaced the EH 12BH7 in the Dirty head with the JJ ECC99. It got a little bit thicker toned, just noticeable. After playing for about half an hour the PT was slightly warmer than with the 12BH7, which hardly got warm at all. My Tele sounds better, but it also got new pups a couple of days after the tubes came in. I use it with the C19 (bright cap) lifted, about half of the time. My LP I now have to start adding a moderate amount of treble to the mix and leave the C19 in. I now turn the Bass down a bit, since there is enough low end response. I'm just about where I want to be with this amp. I put a JJ 5751 in there, that mellowed it out, so I'm going back to the TS 12AX7 in V.3/Pre-amp. Still looking to get a speaker for the Kustom Cab, still most likely a Weber Ceramic 1225 (their version of the G12M-25) with the Hemp Cone. The Kustom speaker's smoothness is a good balance with the dirty tones from this head.

I put the EH 12BH7 from the Dirty head and put it in the clean head. It help the low end response without any negative effects on the brightness, aside from maybe smoothing it out just a tad bit.

*Note: When putting these tubes in the Lil' Night Trains, you need to stretch the tube retainer springs to fit the longer tubes. The ECC99 is the longest tube that will fit under the chrome top. The 12BH7 is slightly shorter than the ECC99, but longer than the 12AU7.

The swap appears to be the right move for both heads. The ECC99 is about as far as I dare go. I was contemplating a fat boost mod, but I don't think I need to now. Will be doining the C19 lift switch to the Clean head for a bit more tonal range.

Retain the Twang!

Last edited by MadJack; August 24th, 2012 at 05:43 PM. Reason: Forgot link to speaker.
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