2 Channel "H" tube questions

[BMiller]

lv4rock - out of curiosity, how do you know that the DW7 is for the effects loop? It seems really odd to me that Sewell would design a low level signal into one of the noisiest parts of an amp (near the power transformer). I have yet to pull the chassis on mine to see the actual routing, but topology wise it doesn't make sense. However, the tube choice does make some sense given how the effects loop on this amp works. I guess I need to wire my rack into the effects loop and see what happens....

Anyways, if the DW7 is indeed not the PI, then in which slot is it? From the tube layout, one can come to a basic conclusion that the two ATs on either side of the reverb transformer are for the reverb driver/recovery section and the effects loop send/return. The tube closest to the power tubes is typically the PI. My curiosity is rising...need some time to pull the chassis and see what's in there!

 

[lv4rock]

Actually the reverb recovery is from the blue channel tube. The tube closest to the power transformer is the PI. The tube before the reverb transformer is the reverb driver. The tube closest to the input jack is the input driver, the tube next to that is for the red channel, then the next tube is the blue channel / reverb recovery. The tube layout picture in the manual shows a 12DW7 in the effects loop slot (same side of the chassis as the power tubes and opposite the effects loop input/output jacks and controls. However, in the tech specs, it says that the amp takes 3 each 12AT7s and 3 each 12AX7s. You definitely would not want a 12DW7 in the PI since it is an unbalanced tube. Generally you want a balanced tube for the PI to drive the power tubes equally.

 

[BMiller]

Actually the reverb recovery is from the blue channel tube. The tube closest to the power transformer is the PI. The tube before the reverb transformer is the reverb driver. The tube closest to the input jack is the input driver, the tube next to that is for the red channel, then the next tube is the blue channel / reverb recovery. The tube layout picture in the manual shows a 12DW7 in the effects loop slot (same side of the chassis as the power tubes and opposite the effects loop input/output jacks and controls. However, in the tech specs, it says that the amp takes 3 each 12AT7s and 3 each 12AX7s. You definitely would not want a 12DW7 in the PI since it is an unbalanced tube. Generally you want a balanced tube for the PI to drive the power tubes equally.

Thanks for the info on the info on the how the amp circuits are laid out! Since I have yet to see a schematic or pull the chassis, I was basing my thoughts on the layout on the physical location of the tubes and what tubes were used were.

As far as the DW7 in the PI, I have read a lot on other forums about how blues guitarists and harp players really dig the sound that they get from such an unbalanced tube in a circuit. But you have to remember that the other circuit components play a huge role in this as well along with the fact that although the two tubes (AU and AX) look similar and are interchangeable, the electrical characteristics of the two are very different. So the gain factor of the two tubes in the same circuit is not the only reference you want to consider.

 

[Bluesverb]

If any of you guys replace the 12DW7 with a 12AX7, I would like to know if you pick up extra noise/hum. Especially when using the effects loop with say a delay or something.

 

[elvis]

This may help.

 

[BMiller]

Elvis - thanks for that pic man! I knew from other posts where the PI is actually located, but did not know how the first three AX's were used. Great info!

 

[elvis]

You're welcome. When in doubt, PRS customer service is the best!

 

[BMiller]

If any of you guys replace the 12DW7 with a 12AX7, I would like to know if you pick up extra noise/hum. Especially when using the effects loop with say a delay or something.

I have the JJ ECC83S in place of the DW7 and I like. No extra noise or hum that I can hear in my rig. There is additional gain and a perceived "power" that I am really liking though.

As far as the PI goes, keep the AT7 in there! I did some tube rolling with various balanced AX7s in that slot and they all tended to sound like crap with the power tubes I currently have in there. The low end was mushy and, how do I describe this, the low end sounded separated from the rest of the signal. Lets take a palm mute for example. The highs and mids stayed together and muted as normal but the lows seemed to carry on in a strange and disconnected way and were loose and farty. It could be the crappy power tubes but honestly I think that the power section circuit really wants to "see" the output from the AT. I swapped the AT back in and there was the awesome again.

So in the end, AX in place of the DW and keep that AT in the PI slot! Experiment as always with various manufactures, but keep the values (AT, AX, Au, etc) the same from Paul, Doug, and friends!

 

[Deleted member 5962]

Thanks BM! So heating up the loop is the ticket, huh? Should give more range to the loop in and out controls as well .

 

[BMiller]

Yes, I find that the AX in the loop makes a huge difference in overall tone and gain. I like the change it made to the amp.....others might not....give it a try though as it is the easiest of the AX tubes to get to. You can try the AX in the PI slot but I don't think that you'll care for what it does to the amp (which you might as well do if you've taken the time to take the back panel off!