12DW7 preamp tube
Cool pre-amp tube swap tip for harp amp geeks...long
Subject: Cool pre-amp tube swap tip for harp amp geeks...long
From: The bernadettes
Posting Date: Wed, 05 Dec 2001 21:32:02
Lately I've been experimenting with a 12AX7 type tube swap, that some of you maybe interested in. That is a 12DW7 tube. This tube has the same pin out as a 12AX7 and is a direct replacement for any [bassman preamp tube swap]12AX7 type tube. What is different about the 12DW7 is that it is a "disimilar" or "unbalanced" dual triode, as compared to the 12AX7/5751/12AT7/12AY7/12AU7 tubes which are balanced or similar dual triodes. I realize many of you know this but for those who don't, I'll explain. 12AX7 type dual triodes are actually two pre-amp tubes in a single tube. I've seen a few guys on this list talk about tube swaps as if a tube performs a single function, but they perform two, depending on the circuitry of the amp. All 12AX7 types, except for the 12DW7 are balanced which means each triode is exactly like the other. One a 12AX7 for example, both sides or triodes have a gain factor of 100. The 12DW7 is different in this respect, each side or triode is disimilar. One side has a gain factor of 100, the other side has a gain factor of 20. You can think of it as 1/2 a 12AU7 and 1/2 a 12AX7. As you can imagine, this offers some interesting possibilities in altering gain characteristics of an amp for harp. Obviously some knowledge of the circuitry of your amp will help you figure where this tube might work best. First the pin out of the 12DW7 is as this: the first triode is on pins 1,2,&3 and this has the 20 gain factor, the 2nd triode is on pins 6,7,&8, and this side has a gain factor of 100. Some examples: first tube position on a tweed Bassman. The 1st triode is the 1st gain stage for the normal channel, the 2nd triode is the 1st gain stage for the bright channel. Place a 12DW7 in this slot and you can A-B between two different gain situations. It's like having a 12AU7 in the normal channel and a 12AX7 in the bright channel. You can choose between either channel, to see which you like best. The 2nd tube position on a tweed Bassman is a cathode follower. To be truthful, I'm not sure what effect a disimilar dual triode would have on this position. In theory the overall gain of a 12DW7 in a cathode follower would be kinda equal to the gain of a 12AY7, but there maybe a difference in the response. Having a gain factor of 100 on one triode may help push the amp into breakup but the much lower gain of the the other triode may help to alleviate the excess gain and lack of play in the volume control. I don't currently own a tweed Bassman so I can't tell you how it would work out in experience. The 3rd tube is the phase inverter and I'll get to this later. BF/SF Fender, I did quite a bit of experimenting on my BF and SF Fenders. The 1st tube position is the first gain stage/tone recovery for the normal channel, the 2nd tube position is the fist gain stage/tone recovery for the vibrato channel. The 1st triode is the first gain stage after the input and the 2nd triode is the tone recovery stage, it recovers the gain lost in the tone controls. In this position the 12DW7 worked very nicely. With a gain factor of 20 in the first position, it kept the mike signal from getting too hot, but having the the gain factor of 100 coming out of the pre-amp stage gave the amp some of that 12AX7 type "bark". In overall gain, again the 12DW7 would roughly equal a 12AY7, but this assymetric way of applying the gain seems to have some advantages. You get a bit of bark but you still get some play from the volume knob. Other BF/SF tube positions: socket 3 is the reverb driver and requires both sides of a dual triode, I would not recommend a 12DW7 in this position, same goes for socket 5 which is the vibrato driver. Socket 4 is the reverb recovery/2nd gain stage. In this position the 1st triode is used to reintroduce the signal from the reverb pan, the 2nd triode is an additional gain stage for the vibrato channel. A 12DW7 in this position would lower the gain on the reverb recovery, or give you more play on the reverb knob, but would still give you the gain equal to a 12AX7 on the 2nd gain stage. I personally don't care for it (12DW7) here. The extra gain on this position causes feedback issues and I usually prefer a 12AY7 or 12AU7 in this position, it lowers the gain on the 2nd gain stage and helps to warm up and remove the harshness of the reverb. In fact, on more powerful amps, I internally modify the amp to bypass this additional gain stage, to help control feedback. But in the first two slots of a BF/SF Fender amp, the use of a 12DW7 is definitely worthwhile experimenting with. My Super currently has a 12DW7 in the first gain position of the vibrato channel, I was happy with the response of the amp with this tube in it. The phase inverter position: this is where it gets interesting. Until recently common practice has been to do everything possible to perfectly balance a phase inverter. That is use a balanced dual triode with both triodes perfectly matched, both coupling caps matched, matched tubes, and hoping that both sides of the output transformer were perfectly matched also (rarely the case with guitar amps.) Recent talk amongst amp geeks has been about slightly imbalancing phase inverters in amps where a "fat", or distorted tone is desired. Gerry Weber also mentioned this in his recent article about harp amps where he discovered that purposely unbalancing the phase inverter fattened up the tone of his harp amp. In his case he played with the values of the "balance resistor" of a paraphase type phase inverter. Lately I've been doing the same with the more common "long tailed" phase inverter found in most later vintage Fender amps, by playing with the values of the "tail" resistor. Use of the 12DW7 in the phase inverter slot has a similar effect. Use of this tube has the signal of one power tube running much hotter than the other. It's no longer a perfect 50/50 push pull arangement. This causes the power tubes to push an assymetric wave form. It fattens up the tone a bit. Of course for those looking for a perfectly clean tone, you won't be happy with this. Some caveats: since one power tube will be working harder, it might be a good idea to occasionally switch power tubes, like rotating tires, so to even the wear. Also you may want to visually watch the amp while playing when you first do this. On one of my amps, one of the power tubes started to glow a bit more. You may wanna check the bias of the power tubes to make sure one of the tubes doesn't get too hot. Visually watching the amp you can actually see one of the power tubes working a bit more. Also, try playing with speaker polarity, you may find you get better feedback suppression by switching the polarity of your speakers. In actual use, this effect is fairly subtle, next I wanna try both playing with a 12DW7 in conjunction with altering tail resistor values, and see how this works out. Princeton guys, this won't work, the Princeton uses a single triode cathodyne type phase inverter (the other triode is used to drive the vibrato effect) and using a 12DW7 in this position would just alter the vibrato effect. Not to worry, due to a design flaw by Fender, the phase inverter of a Princeton is slightly out of balance anyway. The 12DW7 is available as a NOS tube from the usual NOS dealers and JJ is currently manufacturing a 12DW7 under the European designation: ECC832. I've been using the JJ's and are happy with them. JJ along with Electro-Harmonix are now producing current manufactured tubes that rank up there with NOS stuff for a reasonable price. I bought mine through: Eurotubes. Bob at Eurotubes is very nice and knowledgeable guy to deal with, has great prices and very efficient service, highly recommended JJ dealer. His 12DW7/ECC832 is $9, not bad. For those of you who try this tube swap, check back on the list with your experiences and opinions. The usual disclaimers apply to those of you playing with your amps. Work safely, make sure you know what your doing.
Regards, Don D.
Created on ... gennaio 12, 2002