Author Topic: Vox Dual Channel Full Plate Voltage Pre-amp  (Read 6845 times)

jubal81

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #30 on: September 30, 2012, 08:01:10 PM »
Mind. Blown. Amazing work. Just amazing.
I see in my crystal ball a rack of hot-pluggable preamps with a microcontroller switching scheme. It's only a matter of time ...  ;)

I don't know if you've been there, but the Hoffman amp forum is loaded with tube gurus. I have no doubt they'd be able to help you out.
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jkokura

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #31 on: September 30, 2012, 11:54:38 PM »
Is it possible that the problem is with V1 and not with the circuitry Paul?

Jacob

Yep I thought that too. I swapped V1 with a few different tubes and I still get the same results.

Worth a try suggesting. Hmm... I'm not an amp guru, but my gut says that your though about the wrong plate resistor might hold true. Is it also possible you're using a bad resistor in there somewhere, one that's introducing the noise?

Jacob
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timbo_93631

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #32 on: October 01, 2012, 12:01:50 AM »
^sounds likely to me as well, when I was messing with the preamp on my Supro V1 and V2 have a shared cathode and it did not like my experimenting there at all!  V1 is preamp, and V2 is Tremolo in that amp.  I'd try to see if increasing the 1k5 cathode resistor helps.  You have a schematic I can look at?
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JakeFuzz

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #33 on: October 01, 2012, 12:19:16 AM »
Thanks guys! I think I will try a 2.7K in there tomorrow if I get a chance. It has got to be something simple like this. That is definitely a possibility Jacob about the noisy resistor, Ive gone through and checked almost all of the values that would be suspect. Man it would be tough to find if it were an intermittent thing. I've pumped myself up watching AC-30 videos on youtube! I've rewired the ground for the first stage and increased the plate voltages into most of the pre-amp (V1s were sitting around 120 at the plate which is a little low but it is 220K so...). We'll see if one of these works over the next few days, then demos I promise  :D

timbo_93631

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #34 on: October 01, 2012, 12:44:06 AM »
Paul, it is probably well worth chopsticking all around in there if you haven't yet.  Intermittent stuff usually reacts fairly noticeably to the chopstick tap-tap-push-push-tap.  Did I mention how neat this project is yet?  Serious.
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dbharris

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #35 on: October 01, 2012, 09:30:47 AM »
Looks really cool!!!!  I hope you get this figured out soon.

-Dan

JakeFuzz

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #36 on: October 04, 2012, 10:57:52 AM »
Okay I think I've got this figured out. What I was hearing from the TB channel was a very low volume clean signal with some interference riding along. I adding grid stoppers on V1B (56K) and a large one on the PI approximation (100K); this killed all weird fizziness I was getting. I still had extremely low volume though and barely any drive at full tilt. Checking out single channel schematics, most people omit the 220K ground reference which acts as a voltage divider right before the PI. This 220K/220K divider acts as a blend between the bright and normal channels. I typically don't omit these because they reference ground when the opposite channel volume is all the way down. I am positive this will allow me to drive the last triode stage much harder if I remove it; Ill try it when I get home. It is also a little concerning that the pre-amp was producing such little drive up to this point though. I've been seeing mixed schematics on the web whether or not people put a 25uF cathode bypass cap on the second stage triode. This will definitely increase the gain and I think it is worth a shot.

I am also going to re-design the PI approximation. I have been able to match the frequency response of a real PI by going through some basic calcs. It is really just changing the values of the coupling caps around to get the same bass cut response in and out of that last stage. I can't match the gain of the real PI though without adding a huge voltage divider between the tonestack and this last stage. I think we go down by about 6-8dB from a standard triode stage to a LTPI! I am going to test values of the grid resistor feeding into that last triode stage with a pot to see what sounds best. 

JakeFuzz

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Re: Vox Dual Channel Full Plate Voltage Pre-amp
« Reply #37 on: October 07, 2012, 06:03:02 PM »

I've still got issues with the TB channel. To me it doesn't sound like it is saturating anywhere near the amount it should be. EF86 channel still sounds great. A little bit of the fizzies that will probably come out will slightly larger input grid stoppers. Another demo when the TB channel is working properly.

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