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DIY Pro Audio Q+A

I recently bought a 1959 5E3.. O my... the sound I have only dreamt about came true... I am a crazy Neil Young
I have been using a Hot Rod Deluxe but this 5E3 is crazy, or was! I was enjoying playing my White Falcon and Les Paul, nice and clean.. Get a little excited with the Volume and tone and I was in heaven.. Not for long as my P12R died on me. I was devasteted, it is a way being reconed. I fitted a 2002 c12Q, did the job but nothing like the P12R. I read up and thought I would try a Celestion Blue.. Fitted it this evening. WOW, this thing has some go... just not for me . To bright , does not break up till around 7/8. no good for my practise room at home. The P12R was awesome at anything over 3 for my Neil tone. Sorry to ramble on, would you think a Weber or Gold, or will my P12R be OK when reconed, I'm worried it will just blow again.

Sounds like you have some great gear. Sorry about your P12R, but it should be fine reconed. It will take a while to break-in, so give it some time - depending on how much you play it could take months.

I have a question, in your Fender Deluxe build you talk about an Ampeg mod or Paul C mod. I was wondering what Ampeg amp did the mod come from? I've been looking at picking up an old Ampeg "Universal" series amp (Jet or Mercury) and maybe doing some temp mods to it. I hear their tone is very similar to a deluxe but can be had on the cheap. Any info would be a great help.

Ampeg B25, GV22

What does it do (Ampeg Mod) to help out the tone compared to the stock Deluxe?

For me, it made the amp break-up a little smoother - maybe a little more like a Marshall (when combined with the lower value caps I installed). The new deluxe that I built is completely stock and it's fine without the mod. With the first amp, I was never completely satisfied with the tone - so I tweaked it until I was. (It's like playing five identical amps - each one will sound a little different - that's how small the difference is with the mod.)

Where can I purchase a T4 cell? - search for 'T4B' - $89.66

I noticed that you have also worked on the La2a and was wondering if you have available any sweeps at various gain reductions.

Yes, they're on the site.

I'm wondering if its possible to use a DTDP switch for the Meter -- monitoring just the +4 output or Compressor?

Sure you can do that. Off the top of my head, you would wire the switch like this:

3| |2 (Pin 3 of XLR out | | pin 2 of XLR out)

+| |- (+ to meter, - to R24)

R25| |R4

In other words, the center lugs go to the positive and negative of the meter - including the 3.9k (R24) resistor on the small turret board (that goes to the negative of the meter).

The top lugs connect to pins 3 and 2 of the XLR outputs.

The bottom lugs connect to the limiter circuit by way of R25 and R4.

Question about LA-2A C4 variable 50-380p. Because I havent this cap I will use some mica insteed this. On what value you set your LA-2A?

I set mine very high - around 380pf. Otherwise the high end rolls off too soon. But you should experiment and see what you like.

I have been looking for the schematic or response curves for a Pultec equaliser that used peaking circuits (at 100Hz and 5kHz I believe) rather than the shelving circuits of the EQP-1A (other than the treble boost function) and was wondering if you might be able to suggest where I could look to track it down?

Not sure what model you're referring to, but I posted the original EQP-1A manual today which includes a schematic of the filter in another model - the 1R: Pultec manual

A friend of mine approached me a while back asking if I'd be up for building him a LA-2A clone for his studio. I was wondering how hard this really is to finish. I'd hate to have my friend fork out $500+ only to have me fail at building this. I'm confident in soldering but have never messed around with Turret boards before, but they don't look too bad. Is it tough to track down all those parts in your Excel sheet? What about building a T4B from scratch?? Also, is the schematic on your site the most up to date revision (I thought there were several revisions).

The project is relatively easy, but I don't suggest you try it if you are not experienced. The voltages are very high and can hurt or kill you.

With that said, if you decide to go ahead:

It sounds great. The layout and circuit is a very accurate reproduction of the vintage original.

With regard to your questions:

The parts are easy to source from my list.

The original i/o transformers can be hard to find and expensive. I would suggest you get these first before moving forward. You could also source new replacements from Sowter or others.

The soldering is the easiest part and the most fun. The turret boards are easy to work with - you should solder the components to it first, then the wires, then mount it in the chassis and complete the connections.

The chassis I used was steel and awful to machine, so find something in aluminum if you can - par metal is the place. If you use my chassis templates, just print them out (they're actual size) trim, and glue them to the chassis; then carefully drill all the holes.

The T4 is better bought from JBL. They have sales on occasion and I think they go for as low as $99. It's impossible to find the right metal enclosure, and the el panels are sold only in quantity, so it gets expensive and you're left with extra material.

The schematic is from 1968 and represents what UA is basing their reissue on. There are just a few components that UA adds to the reissue, but I decided to stick with the original vintage circuit.

Lastly, the parts cost will probably run $700+.

My Pultec has finally been powered on after a few months of waiting on parts and transformers and squeezing build time in between sessions. I was wondering if your voltages lined up with the original schematics. I am running a bit low out of my rectifier (325V versus 355V) and that voltage difference is carried through the rest of the unit. Also, my unit passes audio exactly the same as when the unit is bypassed or the knobs are set flat, but there is a slow dropoff in the bottom end similar to the graph provided by sowter for the input tranny but magnified so that 20 Hz is 6 dB down. I am going to check my work again but I wondered if you had experienced any of these symptoms. Also, does your 12AU7 get wicked hot?

Glad to hear you built the circuit.

With regard to B+ voltage, as long as you're within 10% of the schematic you should be fine, in fact the old Fender guitar amp schematics are 20% tolerance. Low AC mains voltage will also reduce B+. Check your heater voltage for 6.3 AC volts.

It's been a while since I built mine, and I didn't bother to take notes on voltages, but I'm sure I would have checked and made sure they were all in line as per the schematic. However, I do think my B+ was a bit low also. Also, your 6X4 might be weak. The important thing is that all voltages are proportional down the line, that suggests everything is right and that the B+ is just a little low. Again, low AC mains or a weak 6X4 would cause this. Either way, I wouldn't worry about it.

Regarding frequencies, my example is flat across the board, eq in and out. Make sure your input impedance is 600 ohms as this can have an effect on frequency response. Also, make sure your source signal is flat. In any event, a few dB's down at 20hz is not anything to worry about, unless it is indicating a mistake in wiring - but if your curves look right in all eq configurations, then I would think you're wired correctly.

I will say that the only anomaly in my build was that the unit passes signals about 2 dB down from source across the spectrum. This could be a result of the reduced B+ on my unit. The feedback resistors (360 ohm) can be adjusted to control gain - but I didn't bother.

Generally, the tubes should be too hot to touch.

I'm trying to build an La-2a now and I was wondering what the specifics of your second switch were. Is there a make? I know that it is supposed to be a 2 pole 3 position switch?

The original switch is made by Electroswitch - very hard to find. There should be a Grayhill equivalent though.

Many people do without the +10 position and just use a DPDT toggle switch.

What should I be looking for in an insulated solder terminal, in other words what type of quality? What did you use to raise your turret board off of the chassis?

I used nylon spacers to elevate the turret board. I understand the original and some DIYers used fish paper - available at Antique Electronics. Fish paper is an insulator that would negate the need for spacers.

The insulated terminals I found were pretty expensive, you could just use terminal strips. All you want to do is provide an insulated point to connect wires. Just make sure no connections are being grounded through the terminal itself.

One of our guys here may build a couple of LA2as, using your reference info. Do you know where to get a machine shop to punch the metal for front and chassis?

Machining the chassis is a real chore, and I have no idea of any facility that is currently setup to do it.

Do you happen to have a LA2a front panel and chassis "layout" showing not only the hole sizes, but their locations on the respective sheets of metal? Know any place I could get those measurements?

As far as I am aware, no mechanical drawings exist on the net for the La2a chassis.

I can tell you that my drawings when printed are essentially actual size - I glued the prints to the chassis and used them as guides. They're not exact to the original, although they are derived from photos of same. However, you might consider creating mechanicals from them. The tranny mounts could be trouble though as there is not much room for error.

The best advice I have is to source all the parts that mount to the chassis, then make sure those mounts are represented accurately on the mechanical. Possibly mock-up on flat sheets of aluminum to get it perfect.

How good is that redd 47? the retail price is 3000 US..whew! Are the others new or vintage that you would compare it to?

Not a pro engineer so I'm not in a position to evaluate the Redd against any other preamp. Sounds good to me though. Vintage modules originated from 3 consoles built by EMI - one or two for Abbey Road, so they're very rare.

I'm considering building the BoHansen DI; Wondering what the dimensions are on your turret layout ( is the layout on your site the actual size of the turretboard? ).

The turret layout is actual size when printed. It could be made much smaller (and cheaper) if you use perfboard and lower voltage caps.

Thanxs again... another La2a is born -- I couldn't have done it without you.

Nice job, make some music with that stuff.

Just wanted to say thanks for all your hard work on the lay out for the Pultec, incredible work, it really has inspired me to do my own lay outs of future projects. I have built two Pultecs, all worked first time! Of course these are just for me and I never will let go of them! Thanks again.

Glad to hear you built the circuit. Checking out your music right now at Sine Star Project - interesting and very ambitious work. Recordings sound terrific; production and arrangements as well.

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