Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubenit on March 06, 2019, 08:05:27 am
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I've been playing my Tweed BluezMeister for a few weeks now using the 12DW7 where the 12AU7 side of it is the reverb driver and the 12AX7 side the recovery. I am enjoying the amp and the tone I am getting.
I was checking voltages on the amp yesterday and discovered a whopping 98v drop between node C and node B. Turns out that huge drop is connected somehow to the 12DW7 being used and the 12AU7 side being connected to node C. When I put a 12AT7 in there, it was about half the voltage drop?
I prefer the sound of the 12DW7 for the reverb. It's "softer" and the high's are so ping-y.
Any problem continuing to use the 12DW7 when it's creating that much of a voltage drop?
The V3-3 cathode is 10.78 v
I've also thought about using a 4.7k/2w dropping resistor instead of the 10k dropping resistor between node B and node C. Thoughts?
With respect, Tubenit
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The 12DW7 is sucking more current that the 12AT7 or 12AX7. Not only is node C affected. Nodes D and E should have dropped also. If you like the way the amp sounds just let it be. But the lower voltages are affecting more than just the reverb driver. Node C also feeds the PI, not to mention whatever nodes D and E are feeding. You can bring all those voltages back up to pre-12DW7 voltages by lowering that 10K dropping resistor. But first, just move the reverb transformer red wire to node B (like Fender does) and see how you like that. Easy to do.
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Sluckey,
Thanks, my friend! As always, I appreciate your information and giving me some direction. I think I will get those voltages back up for the preamp & see how that sounds.
with respect, Tubenit
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> whopping 98v drop ... 12AT7 in there, it was about half the voltage drop
I build a dog-porch for the little dog. It has a little sag. I get a big dog. The dog-porch sags more.
It may be fine like that. Doesn't look like that 10K resistor will break (go up in smoke). You say the dog (reverb) sounds fine.
As Sluckey points out, your cat-perch (preamp) is also sagged. I don't know if the cats mind.
A stiffer porch-beam may be worth playing with. Or not, if nobody complains.
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PRR,
Your word pictures are priceless. Good information. Good perspective and always some humor with them.
Bravo! and thanks!
With respect, Jeff
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He does have a way with words. So do you.
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As PRR alluded to, the 12AU7 draws more current than the 12AT7 in the same circuit. If you really want it to drop try a 12AV7!
Greg
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I've also thought about using a 4.7k/2w dropping resistor instead of the 10k dropping resistor between node B and node C.
That's what I did as it was easier. I would've had to splice more wire in to reach node B
Changing from a 10k to a 4.7k increased the voltages from 98v drop to 53v drop. This increased V1 & V2 plate voltages to more where I like them also.
The 12AU7 side of the 12DW7 is now at 12.5v on the cathode. It does draw a lot of current.
With respect, Tubenit
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Just for reference, look at the reverb driver on this amp, especially the B+ supply and the cathode resistor. Plenty of verb in this amp...
http://sluckeyamps.com/hammond_2/AO-63.pdf
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Sluckey,
Thanks for the information and the reference! I'm fairly close to that now. :thumbsup: :icon_biggrin:
Your (reverb) amp has 240v/send (9v cathode) & 190v/return. Mine is 288v/send (12.5v cathode) & 193v/return.
I appreciate your help and direction on this! With respect, Tubenit