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whittele
I only work on tube amps
Posts: 453
(10/9/03 8:55 pm)
68.60.208.225
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I have made a handfull of the plexi with the Hotswitch amps for folks and everyone asks the same thing.
Can you make the volume difference not so different. I saw the mod to the mod to the mod and have been contemplating this design. Could you compare/contrast your design with Doug's to help me decide to go for it. Thanks, whittele
PHATamps
I will work on all amps
Posts: 375
(10/10/03 12:27 am)
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Hey Whit,
What I wanted to do was tame the "hot" stage a bit while keeping the normal stages as "warm" as I could. I increased Rk to reduce gain, decreased the coupling cap to make it less woofy, and rearranged the resistors into a voltage divider and adjusted their values. There's still some volume jump, but it's not as drastic. I like a bit of a jump for solos, so I tweaked it to my taste, YMMV.
Someone posted a few days ago a parallel pot arrangment that looked interesting. You can also look at the Top Hat Emplexador schem for another take on switching in a series gain stage. The idea is to develop the "hot" waveform with the extra stage, and then just throw away some of the voltage gain with a divider or pot. The Hi-Octane preamp does this with 2 pots and 2 dividers, but of course the extra stage is hard-wired in.
Hope this helps,
Paul
whittele
I only work on tube amps
Posts: 454
(10/10/03 4:54 am)
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Paul, that was not only what I was looking for but what I wanted to hear. I built an emplexador clone and understand what you mean. I will give the PHAT mttmttm a go. What relay did you use for this? I have a P&B relay but it is a bit cumbersome to wire in a turret board type construction. Thanks so much.
jerrydyer
Junior tube assistant
Posts: 12
(10/10/03 10:38 am)
64.52.89.117
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I have one you might want to try. The volume boost is slight but the gain is about 25% more. V1b has 2 options. I use a switch and wire it to a 1/4 inch jack for a footswitch. V1b option 1 is the same as a 2204 which is 10k or you may use a lower as described on many of these sites, 4.7k 5.6k. then the switch changes v1b to a combo 5.6k and a .56 cap. The one I built for my friend has a 5.6k as the stock and v1b option, the 5.6k/ .56 cap and he's is pretty happy. I tried this on my 79 50 watter and it ended up with way too much gain. Can't figure out why there is a difference in the two amps. But my plexi/hoffman clone sounds awesome with this mod. I like the Hot Switch but the volume jump was too much for me as well.
EL34
I got your parts right here.
Posts: 2089
(10/10/03 11:29 am)
66.82.9.46
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Re: A question for PHAT amps
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The hot switch signal is way larger because it is being amplified by one more stage of gain than the stock signal.
If you like the way the hot switch sounds, any altererations will change the sound completely and you will have something very different.
You can play with it and see if it sounds the same and you like it or not.
jerrydyer
Junior tube assistant
Posts: 13
(10/10/03 2:02 pm)
64.52.89.117
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after reading so many posts, I'm pretty convinced that we are all pretty much looking for the same thing. A great tone with a nice gain boost on a switch or footswitch with a slight jump in volume as well. Any other cool ideas ?
Edited by: jerrydyer at: 10/10/03 5:20 pm
PHATamps
I will work on all amps
Posts: 378
(10/10/03 9:49 pm)
24.220.0.48
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Whit,
I did my Plexi/Hotswitch in a Carvin combo carcass. The Carvin PT had a 12-0-12 winding for their op amps and such, so I copied their rectifer/filter circuit to get the 12V. I used a 12V relay I got at R/S, which is about 1.25"L x 0.5"W x 1.0"H.
I cut a piece of cheap perfboard (also from R/S) to about 1.75" x 0.5", so that there were "ears" sticking out off the ends of the relay, the leads of which lined up with the perfs. I drilled a hole in each ear, and matching holes in the chassis between the first two 12AX7 sockets, and mounted the relay leads-up using long 4-40 screws and 1" spacers. From there it was a cinch to wire it up, I had no problem wrapping the wires around the little leads and soldering them.
Paul
whittele
I only work on tube amps
Posts: 455
(10/11/03 1:43 am)
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I just got done wiring up the SS P&B relay and I must have smoked it. For now I will use a dpdt switch and see how she runs. Thanks for your help. I can't wait to fire her up!
whittele
I only work on tube amps
Posts: 456
(10/12/03 12:23 pm)
68.60.208.225
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since I smoked my relay I decided that I could not wait so I hardwired the HOT channel in. It was a bit grainy full tilt so I bypassed the first plate resistor with a 27pF cap. NICE and smooth. I added Doug's "meat" control using a 47uF cap instead of 22 uF. That meat control is very interesting and really brings the cleaner sounds to life. It is unlikely that I will make the HOT channel switchable. It sounds great cleaned up and is really touch sensitive. GOOD CALL PAUL on you mttmttm.
Finally the question: For fun I swapped out the PI 82K Rp for another 100K and found that to be most tasty. Now I hold a note, even fairly clean, and get that smooth musical feedback.
To make sure I put the 82K back in (same type and manufacturer) and that sweetness disappeared. HOW COME?
I have been the Aiken site but just don't seem to have my arms around the whole PI thing. Someone lend this dummy a hand, please.
GroundhogKen
Moderator de forumus
Posts: 1895
(10/12/03 2:02 pm)
66.67.155.173
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I don't have a real answer to the PI question, but...
I think the 82K/100K combination is supposed to improve the balance of the differential signal halves.
Note that Vox used 100K/100K. Of course they used put signal into the PI from both sides, so they couldn't balance the output from one side's input with out messing up the other.
Also note that Hiwatt used 91K/100K. Maybe you could try this one.
Ken
jbrew73
Junior tube assistant
Posts: 78
(10/12/03 2:08 pm)
65.41.107.167
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i believe the hiwatts use 82k and 91k but the you would get similar results either way
GroundhogKen
Moderator de forumus
Posts: 1896
(10/12/03 2:21 pm)
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Sorry, my mistake. Jbrew is right.
I opened a Vox scheme to check the 100/100, I should double checked Hiwatt to verify 82/91.
Ken
PHATamps
I will work on all amps
Posts: 380
(10/12/03 10:17 pm)
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Also, the Vox LTP has a larger Rk (1K2) and a very long tail (47K), which decreases the inherent imbalance. Maybe that Meat control hanging off the tail resistor has something to do with it. Then again, maybe you just like the sound of a slightly imbalanced PI!
whittele
I only work on tube amps
Posts: 458
(10/13/03 6:44 am)
68.60.208.225
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You know Paul, you may be on to something. I have a Tophat ambassador clone and a Matchless Cheiftan Clone and they both use a pair of 100K's as the Rp's for the PI. Hmmm...
Thanks for the info
HotBluePlates
I only work on Fender's
Posts: 659
(10/14/03 4:58 pm)
141.152.184.100
Reply | Edit | Del phase inverter
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alright, back to school...
If you look hard at the LTP phase inverter, you see where the input to the circuit feeds a regular common cathode stage.
One output signal comes out of the plate connection, but there is another output on the cathode. You could look at that output as feeding the other triode section, which is wired as a grounded grid amplifier (signal goes in the cathode, comes out the plate).
The grounded grid configuration has more gain than the common cathode stage, which helps make up for the fact that you lose some gain when the signal goes in the grid and out the cathode of the first triode section (kinda like a cathode follower).
Without getting into the math of it, 82k happens to be the value of plate resistor that balances out the gain of the common cathode stage and
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