Vox Dual Channel Full Plate Voltage Pre-amp

[MattOcaster]

Dude this is sick!

Did you consider running it off a SMPS (nixie-style) power supply to avoid the transformer?
Or is there an advantage to the transformer?

Very impressive job!

[JakeFuzz]

Thanks for the info! I may choose a fancy color it makes a really long time that i did'nt bought a painted enclosure.

Yup, it's that amp, my first amp project

Yeah I've seen that amp kit. It actually uses one 12au7 as a self split kind of deal so it does run in push pull. That has about 1 watt output right? Man that is cool, let us know how it sounds. I think one of the miniature vox night train heads was like that. I am also slowly putting together a deacy amp with an 8"; that should be an interesting one.

Did you consider running it off a SMPS (nixie-style) power supply to avoid the transformer?
Or is there an advantage to the transformer?

I've never seen those type of power supplies before. Is that MOSFET and 555 acting like a high voltage charge pump? That is a wild idea. I wonder if it can get up to around 350 volts. It can deliver enough current too. Ill have to look into this in the future. Thanks!

[MattOcaster]

Yep, it'll get up to 350V - and without a bulky transformer!

Im actually working on a Bassman preamp with a SMPS psu.
All on breadboard at the moment but it works!

There have been other DIY designs using the SMPS, like the Frequency Central "Superfly" amp and the Analog Sound "GTFO" overdrive. I used the GTFO power section for my project.

Check it out

[JakeFuzz]

Yep, it'll get up to 350V - and without a bulky transformer!

Im actually working on a Bassman preamp with a SMPS psu.
All on breadboard at the moment but it works!

There have been other DIY designs using the SMPS, like the Frequency Central "Superfly" amp and the Analog Sound "GTFO" overdrive. I used the GTFO power section for my project.

Check it out

That is ridiculously cool. I see how it works now, very much like a high voltage charge pump. The Weber tranny isn't too bad as far as cost ($20) but I agree it is a pain to lug all that weight around.

[Ettore_M]

Yep, it'll get up to 350V - and without a bulky transformer!

Im actually working on a Bassman preamp with a SMPS psu.
All on breadboard at the moment but it works!

There have been other DIY designs using the SMPS, like the Frequency Central "Superfly" amp and the Analog Sound "GTFO" overdrive. I used the GTFO power section for my project.

Check it out

I've actually wanted to build an amp using the nixie SMPS, but I heard it may cause hum, so I rejected the idea. I may try it myself to see if I have any issues.. But please, keep us posted on your project! 

[nzCdog]

SMPS is generally not recommended for amplification... particularly high gain.  A world of transients and ripple awaits you... which translates to whistles, whines and other intermittent weirdness.  Linear transformer based is the way to go.

[GrindCustoms]

Thanks for the info! I may choose a fancy color it makes a really long time that i did'nt bought a painted enclosure.

Yup, it's that amp, my first amp project

Yeah I've seen that amp kit. It actually uses one 12au7 as a self split kind of deal so it does run in push pull. That has about 1 watt output right? Man that is cool, let us know how it sounds. I think one of the miniature vox night train heads was like that. I am also slowly putting together a deacy amp with an 8"; that should be an interesting one.

I was on PPP site yesterday night and none of the DD sized enclosure listed where in the «Tall» sizing.......i may be stupid or just not getting it..........

And yes it's a 1watt amp, also have a 8inch speaker on his way for a little cab for this amp, really excited about this project.

[timbo_93631]

Thanks guys. Demos this weekend when I get a chance. I went through the TB channel with a fine tooth comb and can't find anything totally obvious   Ill have to start doing component checks.

Misha, the regulators are for the tube filaments. I could probably run the 12ax7 filaments on 12VAC without the center tap but I don't want tons of high current AC lines running all over the place. The other regulator is tuned under load (LM317) to 6.3 volts to run the EF86. Those suckers get hot too, I hope they last...  

Oh and thanks to Timbo BTW for suggesting this project. You were dead on about the EF86 sounding killer!

Man Paul, sheesh.  You get the credit for this one, it is SO STINKING NEAT!  Really good work here.

[stecykmi]

SMPS is generally not recommended for amplification... particularly high gain.  A world of transients and ripple awaits you... which translates to whistles, whines and other intermittent weirdness.  Linear transformer based is the way to go.

off topic but...
peavey actually released a 300 watt bass amp with a switching power supply called the VB-3. it runs 8 x el34's and 5 preamp tubes. http://www.peavey.com/products/index.cfm/series/813/Pro%E2%84%A2%20Series

SMPS are possible for audio, but they need to be purpose built, primarily the switching frequency needs to be well above the audio range, and they need to be well regulated.

[midwayfair]

I'm fairly certain this is a switched power supply.

http://milbert.com/gaga

[nzCdog]

off topic but...
peavey actually released a 300 watt bass amp with a switching power supply called the VB-3. it runs 8 x el34's and 5 preamp tubes. http://www.peavey.com/products/index.cfm/series/813/Pro%E2%84%A2%20Series

That looks pretty cool

SMPS are possible for audio, but they need to be purpose built, primarily the switching frequency needs to be well above the audio range, and they need to be well regulated.]SMPS are possible for audio, but they need to be purpose built, primarily the switching frequency needs to be well above the audio range, and they need to be well regulated.

Fair comment Misha... In theory its got to be possible, its just my experience in the lab at school with switched mode means I wouldn't recommend it at all... even my TC Nova Delay SMPS wall wart gives intermittent noises...  

[pickdropper]

I love this build. 

The outside looks great, but the wiring is particularly cool.

[JakeFuzz]

Thanks!

Just a little update. I finally got some time to do some basic checks on the TB channel and I still cannot find whats wrong. All voltages and current draws are normal. Component values have been triple checked. The issue is that as I turn the gain knob past about 12 it goes into a fizzy crackling mode, where the fizz sits in the back of a low volume clean signal all the way up to maximum on the gain. Audio probing tells me that directly off of the plate of the first tube I am getting this issue which means it can only be from a weird ground connection with the V1 cathode and the gain pot or there is some kind of strange output impedance problem with V1 as I turn up the gain. I have disconnected V1 from the circuit and will probe it like this next. Could it be that I used a 1.5K cathode resistor instead of around 3K because the real Vox is a shared cathode/twin triode setup? It sure doesn't seem like it but it is the only thing I can see that is different at this point.  , another strange thing I was seeing is that as I increase the gain up to maximum the current draw into the cathode follower increases and the plate current into V1B decreases...

[jkokura]

Is it possible that the problem is with V1 and not with the circuitry Paul?

Jacob

[JakeFuzz]

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.