Fender-style valve pre-amp using a 12VDC supply in Hammond box

jpfamps

Here are some snaps of a valve pre-amp I made recently.

Before (ish):

After:

It was for a jazzer who wanted a clean valve pre-amp, and is based on the Fender AB763 circuit. 

It was designed to run on an easy to source 12VDC supply; a 12AX7/ECC83 can be wired for 12VDC operation (12.6 VDC to be really accurate), and I used a charge pump kit from Tube Town for the HT, which we adjusted to 140 VDC. The kit is well-designed and gets my recommendation.

There where two issues I thought needed addressing; firstly the voltage output from the pre-amp could potentially be way too much for a normal amp input (or indeed a line input), so I attenuated the signal from the second triode using a split anode resistor, and then added a master volume.

The second issue is that output impedance would potentially be quite high, so I used a high voltage MOSFET as a source follower buffer to give a reasonably low output impedance (probably around 150-200 ohm) so the unit can drive a typical 10k line input impedance.

spacecadet

I would be very interested in hearing that.

spacecadet

In all seriousness, there could be a market for these. If you did a Fender, a Vox and a Marshall, I reckon you could sell some units. I'd certainly look at the Fender one.

jpfamps

It sounded (predictably) like a BF Fender amp, but we didn't have very long to play with it as the owned was keen to get his new toy.

I was concerned before embarking on the project about noise from the 12 VDC supply, however this ended up not being a problem and the unit was very quiet.

I should (hopefully) get some sound samples in the near future.

spacecadet

Like I say, I would definitely be in the market for one of these. I'd be happy to do some pro demos for you too.

sweepy

Very neat unit  there. definitely a market as a recording pre amp

ecc83

Nice one J!

Did the ht generator use the UC3843A chip? If so I hope you used a higher rated resistor in the feedback than "we" did at first. It went high to O/C and HT soared to 4-500V!

Yes there are some high signal voltages present in the pedals, 100V+ pk-pk. They used a conventional attenuator to bring them down to drive standard op amps but since there are  +&-15V rails there is still a lot of headroom!

"They" had not thought of a CLEAN valve pre amp AFAIK. To late mow mate!

Hot cakes I reckon.

Dave.

jpfamps

ecc83 said:

Nice one J!

Did the ht generator use the UC3843A chip? If so I hope you used a higher rated resistor in the feedback than "we" did at first. It went high to O/C and HT soared to 4-500V!

Yes there are some high signal voltages present in the pedals, 100V+ pk-pk. They used a conventional attenuator to bring them down to drive standard op amps but since there are  +&-15V rails there is still a lot of headroom!

"They" had not thought of a CLEAN valve pre amp AFAIK. To late mow mate!

Hot cakes I reckon.

Dave.

The charge pump is essentially this circuit which has been knocking around the DIY community for a while sans the capacitor coupled -ve rail generator (the circuit values and components are identical which is a bit of a give away):

The chip is a common or garden 555 configured as an oscillator, and the grunt work is done by the IRF740  MOSFET (not an especially cheap part). Tube Town have been selling this kit for a while now, and it appears to be very dependable.

You don't really need that sophisticated a charge pump here are the load is quite light and an extra RC filter cap be added at little expense. Really tight regulation is not really needed.

And it's still F by the way......

Misterg

Neat build!

I don't think yours uses it, but I hadn't seen the -ve voltage add-on to that circuit before - interesting stuff from Rick. I'd also vouch for the basic voltage converter circuit being pretty robust and reliable - and it's a lot easier to live with than the MAX1771 designs!

jpfamps

Misterg said:

Neat build!

I don't think yours uses it, but I hadn't seen the -ve voltage add-on to that circuit before - interesting stuff from Rick. I'd also vouch for the basic voltage converter circuit being pretty robust and reliable - and it's a lot easier to live with than the MAX1771 designs!

I've not had great experiences with chip-based voltage converters, although I've not (yet!) used the MAX1771. 

The heavy lifting in this circuit is done by the IRF740, which is total overkill in this circuit, so I anticipate no reliability problems.

Using the oscillator out as a "free" way of generating a negative rail is OK as long as you don't need to draw significant current, so would be fine for say a bias supply (which I assume was the intention), or for switching JFETs, but I would use it to power anything significant as there is a good chance of damaging the 555.

steamabacus

Very nice.

How possible would it be to add a parallel speaker-emulated output to that? Maybe an fx loop as well?

jpfamps

steamabacus said:

Very nice.

How possible would it be to add a parallel speaker-emulated output to that? Maybe an fx loop as well?

Well anything is possible....

Seriously, the pre-amp was constructed using PTFE eyelet board and a chassis mounted valve socket, and with the charge pump on a separate PCB, so there wasn't much spare space inside, which would make a more complex design challenging from a space point of view.

I could have made a more compact design by mounting the valve socket(s) through the eyelet board and using smaller components, for example I used 630 VDC poly caps, when much smaller 250 VDC caps would have been fine.

My thoughts are to use a PCB for the whole design, and thus you can mount the charge pump on the same PCB, thus saving more space.

This would make construction easier and 2 valves could easily be accommodated.

I was thinking of a Fender-style pre-amp with the Vibro-Champ tremolo circuit.

marauder

2 valves is possible.  Me and Juansolo recently did this one.

That PSU works great, and seems very reliable.

http://juansolo.demon.co.uk/stompage/images/btt-i2.jpg

Heartfeltdawn

One of these into Repear with some suitable IR impulses = win. 

Please consider making more