Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on July 11, 2020, 05:23:05 pm
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Hi all,
I've been playing around with a stand alone preamp, much like the one Doug has in his project library. I wanted to try an AC 30 style preamp circuit, and was wondering... Originally I was thinking of having both channels, but using a foot switch to switch between the two, but I don't know how useful the Normal channel would be, especially if the "Cut" control won't be there (it's after the PI, which I won't have!) So if I took Doug's original AC30 schematic and only used the top boost channel, what do I do with the mixing resistors for the 2 channels? If I have only the one channel, I don't really need the one 220k, R20, correct?
Thanks
Mark
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simple answer remove them both, put a cap "out"
EDIT: cap not needed, so just out no mix R's
my take, you have an extra 1/2 tube that just wants to be heard :laugh:
use 1 input jack feed the 2 1/2's and blend like the 6V6 plexi. tweak to taste once it's working
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This way you have an unused triode
have you think how to use it ?
May be a CF after the TS as to drive better also long lines ?
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About the mix resistors you don't need it for a single channel, there isn't noting to be mixed
Franco
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Maybe.
I've built a couple of TB-only versions of Doug's AC30 and I've found that omitting the mixing resistor gives too much gain. On a couple of the amps like this I left out the cap on the cathode of V2a, and on the one I've got right here I just left in the 220k, even though it's not "mixing" anything now, it's function is as a voltage divider to lose some excess juice.
But you should probably try it various ways and see what works for you.
EDIT: I run V1 parallel on these amps so there's a tad more gain on tap there, as well.
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.... it's function is as a voltage divider to lose some excess juice
I agree but I think that a Power Section will have also a MV at input to absolve that function
Franco
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Thanks a bunch guys. I was thinking it would be nice to use the extra triode. In fact, I'm starting to think maybe I'll keep both channels and have a switch to go between the two. Seems a shame to waste that extra triode stage.
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Only a proposal
http://sluckeyamps.com/trem/trem.htm (http://sluckeyamps.com/trem/trem.htm)
(https://i.imgur.com/Vv3kLv0.png)
Franco
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first decide how much output Gain you want from the device. If you want to be able to plug this into the power amp of an Ampeg VT-22 (example that has a power amp input) you need lots of gain, so omit any resistors between treble pot output and output jack.
If you want to use this device to like a boost pedal in front of an amp plugging into the instrument input, then plan for whatever gain you want, +12dB ? +16dB?
If you want to feed a board in a studio, maybe up to +15dB to +20dB?
When I do this, I feed the preamp with a signal generator with some set voltage, like 100mV, the use an o-scope to dial down the output to whatever I'm going for. see image below. Max gain to send to power amp optionally (tube is specified, a SS power amp doesn't want this much gain!) Then R1+R2 is a voltage divider to The size of both depends on the max gain you are trying to attenuate. maybe it would be R1=470K R2=250K, or 47K + 100K pot.
(http://dougcircuits.com/vd-out.jpg)
If you use that extra triode as a CF output, you can use smaller value R1 and R2.
don't forget to pay attention to high pass filter RC networks when selecting R1+R2 total value and preceding coupling cap if you go the CF route. In the case below, I used R1=47K and a fixed R2=10K. For 12AX7 level gain, you'd probably end up with a 10:1 divider. This particular one is a B15 preamp so 6SL7's gain is lower hence the 6:1.
also, output jacks are Power on top and preamp on bottom (image is cut off) The power amp could be plugged into a tube B15 power amp. the pre-amp is to send to another amp, or a mixing board (motu ultralite in my exact case -- also, a better location for the NE2H would be on the 120V mains, but I built this into a small enclosure and using the mains was not so convenient)
(http://dougcircuits.com/db30esxc.jpg)
One last thought. this one above has an unused 12AU7 triode (to the left, both triodes in the 6SL7 are used ala B15). Don't fall into the trap of over complicating your circuit just for the sake of using the triode. I've done it before, and its just not a great idea.
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Someone told Paralleled Tube
You can borrow from the Matchless AC30
(https://i.imgur.com/YMQdScv.jpg)
or to stay on the VOX ballpark, stole it from the VOX AC30CC (V1 Channel Link)
(https://i.imgur.com/XMKRcVw.jpg)
Franco
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You don't need to "waste" a triode to go a simpler route. You can use a mosfet cathode follower. Archives has good information with schematic, layout and photos and a reference to "Mosfet Follies" that explains where and not where to use mosfets. I have used a mosfet CF numerous times and I can NOT hear any difference in that position between a mosfet CF or a triode CF.
https://el34world.com/Forum/index.php?topic=21873.0
NOT saying you "should" do this ………. just giving an option to consider. Using this approach, you only need one 12AX7
With respect, Tubenit
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Thanks so much for all the information!! I've got to stew about this for a while ;)
Mark
first decide how much output Gain you want from the device. If you want to be able to plug this into the power amp of an Ampeg VT-22 (example that has a power amp input) you need lots of gain, so omit any resistors between treble pot output and output jack.
If you want to use this device to like a boost pedal in front of an amp plugging into the instrument input, then plan for whatever gain you want, +12dB ? +16dB?
If you want to feed a board in a studio, maybe up to +15dB to +20dB?
When I do this, I feed the preamp with a signal generator with some set voltage, like 100mV, the use an o-scope to dial down the output to whatever I'm going for. see image below. Max gain to send to power amp optionally (tube is specified, a SS power amp doesn't want this much gain!) Then R1+R2 is a voltage divider to The size of both depends on the max gain you are trying to attenuate. maybe it would be R1=470K R2=250K, or 47K + 100K pot.
(http://dougcircuits.com/vd-out.jpg)
If you use that extra triode as a CF output, you can use smaller value R1 and R2.
don't forget to pay attention to high pass filter RC networks when selecting R1+R2 total value and preceding coupling cap if you go the CF route. In the case below, I used R1=47K and a fixed R2=10K. For 12AX7 level gain, you'd probably end up with a 10:1 divider. This particular one is a B15 preamp so 6SL7's gain is lower hence the 6:1.
also, output jacks are Power on top and preamp on bottom (image is cut off) The power amp could be plugged into a tube B15 power amp. the pre-amp is to send to another amp, or a mixing board (motu ultralite in my exact case -- also, a better location for the NE2H would be on the 120V mains, but I built this into a small enclosure and using the mains was not so convenient)
(http://dougcircuits.com/db30esxc.jpg)
One last thought. this one above has an unused 12AU7 triode (to the left, both triodes in the 6SL7 are used ala B15). Don't fall into the trap of over complicating your circuit just for the sake of using the triode. I've done it before, and its just not a great idea.
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As always, thanks Tubenit! Interesting approach, using a mosfet CF. It would elimniate one tube. I'm building this on a test harness with 2 tubes already, so I may keep both for now
You don't need to "waste" a triode to go a simpler route. You can use a mosfet cathode follower. Archives has good information with schematic, layout and photos and a reference to "Mosfet Follies" that explains where and not where to use mosfets. I have used a mosfet CF numerous times and I can NOT hear any difference in that position between a mosfet CF or a triode CF.
https://el34world.com/Forum/index.php?topic=21873.0 (https://el34world.com/Forum/index.php?topic=21873.0)
NOT saying you "should" do this ………. just giving an option to consider. Using this approach, you only need one 12AX7
With respect, Tubenit
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Thanks, kagliostro. I'm really interested in the Vox idea of linking the channels. I may try this route first.
Mark
Someone told Paralleled Tube
You can borrow from the Matchless AC30
(https://i.imgur.com/YMQdScv.jpg)
or to stay on the VOX ballpark, stole it from the VOX AC30CC (V1 Channel Link)
(https://i.imgur.com/XMKRcVw.jpg)
Franco
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Our friend Sluckey did something of intriguing linking channels
on this pdf you can see the standard solution and Sluckey's improved solution (Channel A or B or A+B)
http://sluckeyamps.com/VAC15/Vox_AC15_mods.pdf (http://sluckeyamps.com/VAC15/Vox_AC15_mods.pdf)
Channel Link
(https://i.imgur.com/GAbhsBc.jpg)
Channel Select - A or B or A+B
(https://i.imgur.com/sKKQ1Xr.jpg)
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On the Normal Channel you can give a try to this Treble Control (as you say you have not the Top Cut as it is on the Power Section of the amp)
(https://i.imgur.com/R9fwYd1.jpg)
Franco
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As always, thanks Tubenit! Interesting approach, using a mosfet CF. It would elimniate one tube. I'm building this on a test harness with 2 tubes already, so I may keep both for now
OK, two tubes. Another consideration using a 5879 (where Vox would use an EF86). Two of my 3 amps use a 5879. One of my favorite tubes.
With respect, Tubenit
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Nice Suggestion Tubenit :thumbsup:
Franco
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Wow, I like the idea of the ON ON ON switch allowing you to combine the channels. I was going to use a footswitch DPDT, maybe I'll do that first and then try the ON ON ON.
Kagliostro - thanks for the link to Sluckey's post. Does the treble control mimic the Cut control from the power amp? (That was one concern, not having that control because it's a great way to dial back any harshness.)
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That Treble Control Is from Merlin, I don't know if is a Merlin idea or he take it from an existing circuit, however the purpose is that, to have a way to manage the Treble level
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May be you'll be able to arrange a Foot Switch (A - A B - A+B) with an ON-ON-ON control ?
Franco
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That Treble Control Is from Merlin, I don't know if is a Merlin idea or he take it from an existing circuit, however the purpose is that, to have a way to manage the Treble level
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May be you'll be able to arrange a Foot Switch (A - A - A+B) with an ON-ON-ON control ?
Franco
Thanks, Franco.
Yes, I was thinking of a footswitch ON-ON-ON (A, B, A+B), but I can't find one yet. Worse case, I use a 3PDT footswitch to be A or A+B, or I can use a toggle ON-ON-ON.
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There are plenty of A/B Y switch pedals out there. They all do the same thing --- select A or select B, or select both. Some more elaborate than others, some as simple as this...
https://www.ebay.com/itm/ABY-Switch-Box-Pedal-by-Yooper-Pedals/353127555771?hash=item52380ae2bb:g:4okAAOSw6FVe~owk
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+1
AB/Y is the easy option
Still want a on-on-on the guitar stores have 'em;
https://guitarelectronics.com/double-pole-on-on-on-bat-handle-mini-switch-chrome/
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I've find this, but they must be verified
(https://i.imgur.com/e7EvkWg.jpg)
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(https://i.imgur.com/4kEuwLc.jpg)
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Franco
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Thanks, guys! I like the idea of an AB/Y. I was thinking of that. Fortunately I have a bunch of those 3PDT foot switches...this is getting fun :)