Another Princeton Reverb Build w/ mods

[rhodco]

Here is the beginning of my Princeton Reverb build using the Hoffman parts (mostly). Some lessons I've learned so far are as follows:

#1). You can see from the photos that I opted to not use a permanently attached power cord. Instead, I used a hacksaw and file to enlarge the round hole on the far back corner for the A/C cord into a rectangular opening for my detachable A/C cord socket. The only problem with that idea is I failed to consider the back cabinet panel will cover that corner and block access to the socket. 

Solution: On this build I will have to mount the back cabinet panel about an inch or so lower than normal. In future builds, I will either need to plug that hole and cut a new rectangular opening as close as possible to the back plate, or try to find a chassis already cut for this.
 

[rhodco]

#2). Use only Alpha potentiometers, or the high quality CTS with brass stems. Shortly after I installed the front panel pots, I started noticing a huge difference in feel between the lone Alpha 250Kl on the far right for Tremelo Intensity, and all the others. The Alpha had the same smooth feel as my original Fender Princeton Reverb pots. But the others were CTS plastic stem pots that not only wobbled when they were turned, but were sticking and gritty-feeling as if something rough was rubbing on the inside.

Solution: After cleaning them with DeOxit D5 and working them back and forth a little, I concluded that these were just not going to be acceptable so I removed them and ordered replacements of a higher quality. Only the Alpha on the far right remains installed at this point.

[rhodco]

I also installed a hinged dust cover on the ext. speaker jack. It's easy to flip open when I want to plug in, and it covers the only rear jack that won't be used much.

#3). The BOM shows I need 14 "#4 Keps Nuts" and 14 "#4 X 1/4 inch" screws for mounting "Tube sockets, Cap bracket". The parts list shows I got exactly 14 each of the #4/40 Keps Nut and #4/40 screws. The only problem is there are 7 (count 'em) tube sockets. 2 screws and nuts each, plus another 2 screws and nuts for the cap clamp = 16 (Sorry Doug) not 14.

Solution: I was able to scrounge up another couple of screws and nuts to finish the job, but the lesson is to order 16 next time instead of 14.
 

[sluckey]

It's funny how the expensive CTS pots have a cheap feel and the cheap Alpha pots have a good feel. I prefer Alphas but you can't always get what you want, especially for a tremolo circuit.

[rhodco]

More progress today, though it was two steps forward and one step backward all day. Lessons from today:

#4). Mounted my Allen TO-20 Heavy Duty output transformer and threaded the wires through the grommets. Mounting screws fit the chassis holes, but just barely. They could be drilled just a little farther apart next time. As soon as I began to work it into place I realized the cap clamp was in the way. First photo shows where I had to unscrew the clamp, turn it 180 degrees, and mount it again. Next photo shows where I realized the clamp screw was facing an awkward direction making it very hard to get to so I removed the screw and nut, then installed them in reverse.

Solution: Next time, pay more attention to the photos provided by others. I could have saved a lot of time by just looking at how Hoffman and Scaggs had mounted thiers. Duh.

[rhodco]

#5). For some reason, I thought the turret boards and chassis came with mounting holes already drilled. Just more work for me to do I guess, but I didn't think it would be THIS much trouble. My idea for how to do this was to estimate where I wanted my holes in the board by looking at photos from the other builds. Drilled holes and mounted hardware, no problem. Now the tricky part... how to drill chassis holes that line up exactly.

My solution was to paint the bottom of each screw with fingernail polish, press the board down into place, remove the board from the chassis, and just drill holes where the red dots were. Seems to make sense until you actually try it.

[rhodco]

Only problem is that the holes ended up being off, way off, like about an 8th of an inch in some spots..... Hence the round file and ugly holes afterwards to get them all to fit in at the same time. I would like to know of a better way to do this if anyone has a suggestion. This took way too much time to do it the same way again on the next build.

[PRR]

The board bottom is near-flat?

The dummy way is to lay it in the chassis and mark the holes with a pencil. Pencil must be perpendicular!

To be more accurate, drill one hole, run a bolt through (still flat to the chassis) and check the opposite corner hole. If OK, drill that and screw it. Now you can drill the other holes through the holes in the board, using it as a guide-plate. (If very fussy, drill 1/32" smaller to avoid wearing the board-holes, then enlarge after you remove the board.)

Painting the screw-tip "should" work, except they probably aren't quite square or centered. If long, they come down well off the proper place. Also the blob of paint is likely to hit more on one side or the other, making an off-center blot. This can work but may need several tries and corrections.

Do not feel bad. I ordered two bolt-on resistors, then two more. I measured one resistor carefully, marked for all 8 holes, drilled pretty close. The first resistor, I knew I'd slipped and I had to rat-tail file a bit. The next one would not fit, though the hole was dang near perfect. Finally slapping myself into sense, I measured all four resistors. The second batch were a good millimeter off from the first! Between my slips and my idiocy (not checking b4 drilling) it was a long session with the file.

[p2pAmps]

The board bottom is near-flat?

The dummy way is to lay it in the chassis and mark the holes with a pencil. Pencil must be perpendicular!

To be more accurate, drill one hole, run a bolt through (still flat to the chassis) and check the opposite corner hole. If OK, drill that and screw it. Now you can drill the other holes through the holes in the board, using it as a guide-plate. (If very fussy, drill 1/32" smaller to avoid wearing the board-holes, then enlarge after you remove the board.)

I did the drill method PRR talked about myself and it lined up perfect.  of course I drilled out the turret board first :)   Your amp looks good so far and these PR are great.  I have built two and couldn't be happier...

Cheers
Scaggs
Nashville

[EL34]

Good job so far.

My board install instructions tells you how to do the board mounting holes 
That's just one of the documents under each board on the library page
Here's the direct link
http://el34world.com/Hoffman/instructions.htm


There's no one place that is idea for every chassis and so you have to plan where those board mounting holes should be placed
Transformers and other objects can be on the backside of the chassis and you may have to shift those holes arouind to clear those objects

It pays to look at all the build info first before starting

I have most of the Alpha values in stock for Fender type amps
3mra is not available in alphas


The plastic shaft CTS post work fine, but I like the Alpha pots better

[rhodco]

More progress, and more lessons learned. Received my Alpha pots, and the CTS brass shaft 3Meg pot. They all have a much better feel to them. Taking Doug's advice, I have installed some old Groove Tubes in the sockets before soldering up the pins so they will align easier. My cheap soldering iron was too hot and started melting the PVC insulation at a few points so I broke down and bought a Weller soldering station with variable temp like the one I used to have at work years ago. Quality tools are well worth the investment.

Trying to keep my wires running along the same lines as the example photos. Clear heat shrink tubing on the coax cable allows visual confirmation that my center conductor is not touching the shield.

[rhodco]

Laced up the board with solid copper wire, but realized that copper is a soft metal so pulling it tight only stretches it. That's why I also soldered some spots in the middle of long runs to hold the wire pairs together.

[p2pAmps]

Looks pretty darn good to me...  Keep the pics coming...

[rhodco]

Planning to finish wiring up the tube sockets and potentiomters, and double check all the wiring BEFORE I install the board and begin adding components to it. Lots of the wires shown are only connected on one end so far. I had to use a little yellow electrical tape to hold one of the coax leads in place. It probably wont be needed by the time I am ready to install the turret board. I guess it wouldn't hurt to leave it, but trying to be aware of the fact that it will get pretty hot in this enclosed space when it is closed up and rockin hard. Don't want to leave anything that could burn.

Just one note of interest here concerning heat - on my other Fender '65 Princeton Reverb RI: I did some testing after I upgraded the OT and re-tubed it last year. I placed a magnetic base thermometer on the faceplate while it was tilted back 45 degrees to see how hot it would get during typical band practice sessions. Air conditioned environment (about 75 degrees air temp), and run at about middle volume (4 to 5) for a few hours. Over several weeks it averaged out at about 135F degrees after warm up, with some peak times at a little over 140F. I don't know how that will compare to this one, but it gave me some baseline data for what a normal amp should run at with bias set to 23mv.

I love to experiment.

[p2pAmps]

I found that doing all the wiring first before any component install was the best for me.  Mount that turret board and get all your wiring nice and clean then the fun stuff will be easier IMHO.  No one right way just saying what I did...

Which trannies are you using?

[rhodco]

I'm using the power trans and reverb trans that Doug shipped to me (not sure of the brand he stocks), but I'm using the TO20 that BillM and David Allen developed as my OT. I tried it in my other Princeton Reverb and became a believer right away! 

http://billmaudio.com/wp/?page_id=545

It gives a more rich sounding bass and makes the mids seem more 3 dimensional. Hard to describe but I definitely noticed the difference when I put it in. When I took it out to use for this build and put the original stock Fender transformer back in, I could tell the difference again. It sounds kind of flat now. Not as much life to it.

I know now what I'm going to have to do after this build is finished.... I'm getting another T020, another turret board and components, and I'm going to gut that Chinese PC board Reissue Fender. It's going to get a Hoffman Heart transplant!

[Willabe]

......  I'm going to gut that Chinese PC board Reissue Fender. It's going to get a Hoffman Heart transplant!

Now your talkin'!   

[Willabe]

but I'm using the TO20 that BillM and David Allen developed as my OT. I tried it in my other Princeton Reverb and became a believer right away! 

http://billmaudio.com/wp/?page_id=545

It gives a more rich sounding bass and makes the mids seem more 3 dimensional. Hard to describe but I definitely noticed the difference when I put it in.

Probably has more current available in the B+ wind? But there could be a trade off for leads, less sag/compression.   

[EL34]

The build is looking good

[sluckey]

I'm using the TO20 that BillM and David Allen developed as my OT.

I noticed in another thread that you asked mscaggs about the blue and brown wire connections on his OT. Since you guys are using different OTs, his connections for those wires may not be the same as yours. It's a 50/50 chance the wires will be connected properly.

I also noticed you have already trimmed your blue and brown wires. If you need to reverse them they may not reach. That's OK. There is another very easy way to accomplish the same thing in the Hoffman PR. See the white wires connected to pins 6 of the 6V6 sockets? Just reverse them at the board connection.

You may want to keep this thought in the back of your mind when you power up the amp.

[p2pAmps]

Good call Steve

[EL34]

On the Output transformer, if you are not sure which plate wire goes to which plate, leave the wires full length for now so you can reverse them later if they are not correct

I have the note shown above in the build instructions
http://el34world.com/Hoffman/instructions.htm


The link above is below every one of my turret board documents on the library page

[rhodco]

Thanks for the tips. Since it was a drop in replacement for my other amp and the colors matched up with that transformer fine, I would assume the windings are the same. I guess we will soon see. I'm planning to leave the speaker negative feedback wire off until I confirm it is correct (see Doug, sometimes I do follow instructions!).

I'm taking my time wiring all the tube sockets and pots, trying to route the wires around carefully so it follows your photos. One thing I learned with the stranded wire is that if I use a clamp and small magnets as weights to hold the wire in the position I want while soldering the end, it will harden and stay in that shape after it cools. Makes it easier to lay some wires down low, and router others up high. I'm waiting till last to run the green filament wires because I want them to fly over all the others.

I've ordered the speaker, decided to go with the 12" Warehouse WGS G12C/S 8 ohm 75 watts American Vintage and see how that feels.

What do you guys like for tubes? I have some fairly new Tung Sol 6V6GT's, and a pair of EH (rebranded as Groove Tubes) 6V6's that I pulled from another amp. I was going to try JJ's but read comments about them sounding a bit harsh when pushed to breakup. I also have a JAN-Philips 12AT7WC for the reverb, and some other 12ATX7 NOS tubes to try out. I'm using a Sylvania 5751 for V1 in my other Princeton because I wanted something with a lower noise floor, more quiet, but I really can't tell much difference in that amp.

I usually play with a Strymon blueSky for a layer of digital reverb added to the amp reverb dialed in at about 2.5, and a MXR Carbon Copy analog delay with tap temp mod from Molten Voltage. I think it's call a Delaytion. I had to break that open and "mod the mod" because the chorus pot value was way too high to be usable. Changed the 5K for a 500ohm and it's much better now. Me loves to tinker.

[rhodco]

Long day today, but I enjoyed every minute. Finished wiring all tube sockets and pots. Left the tremolo and negative feedback wires loose for now. Vacuumed out all the little fragments and spray washed the interior with flux remover.

[rhodco]

For the filament wires, I find that twisting a pair of wires using two different colors makes it easier to keep them all the same polarity across each tube. 

[rhodco]

The most time consuming part has been keeping all the wires routed so that they don't lay against each other. When they have to cross, I try to get them crossed perpendicular to each other so that the contact is as small as possible. The pot wires and filament wires fly high, as well as the A/C power input.

[rhodco]

Tomorrow is set aside for double checking all the previous wiring before going on to the next phase.

[Willabe]

Looking good. 

What's covering the 2nd speaker jack?

[rhodco]

It's a Switchcraft 515X dust cover. I put those on jacks that never get used to try and keep dust out of the pots inside.

[Willabe]

Hmm, thanks. 

[EL34]

Looking good



I put the parts on the board and do the pot wires last

Hopefully your wires do not get in the way too much when you add the parts to the board

[rhodco]

I put the pot wires on before components because I thought it would work better for me to get the wires wrapped around the turrets first. Now that it's done, I think it would be better to do it your way and just push the wires down into the turrets from the top. Most places there is enough room for that. Only at the bias trim pot would I have to wrap around the turret low. BTW - I really like your choice of solid core hookup wire. It is easy to work with, but stiff enough to stay put where I've routed it when I'm done.

When I finished mounting all the components, I found that I still have a 33K resistor and a .022 cap left over. It took me a while to figure it out, but I think the 33k resistor was for R2 in case I only had one input jack. I used 2 of the 68k resistors in that position instead (R2 and R3). The cap mystery took a bit longer to solve, but I think now it is because C15 and C22 are actually the same component. On the PREAMP and REVERB schematic, C15 is the last component going out to the PHASE INVERTER, but on the PHASE INVERTER schematic, that same cap is labeled C22 coming in from the preamp. According to the board layout, they are both one and the same. However, since they are both listed on the BOM and parts list, I got them both.

Please correct me if I'm wrong, but I think that explains it.

[rhodco]

Assembly finished. Looking for mistakes now...

[Willabe]

You might already know about these but in case you don't;

Doug put this together and shows a great way to double check a build;

http://el34world.com/Forum/index.php?topic=17701.0

This is from Sluckey's web site, lamp limiter, a must have tool for 1st fire up on new builds, rebuilt amps and 1st test on an old or non-working amp;

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


 It's looking real good!       

[rhodco]

Thank you. Yes, I built a limiter box like that a few years back. Just put a 150 watt clear bulb in it. I can see the filament glow more clearly than with a frosted bulb.

[Willabe]

Great! Good looking build!   

But, 150w is too big for most amps, it will let too much current through before you can shut it down. Go with a 60w or 100w.

The wattage of the bulbs filament matters when used in series with the amps ACV power supply(PSU) input draw, it acts as a resistor(R) in series with the PSU.

So the more current pulled through that 'R' the more it will limit the current draw. We see it as it lights up the bulb, so if there's a short we can shut down the amp before it fry's.

You can use frosted or clear but if the bulbs wattage is too big it won't light up enough with either unless it's a direct short.

A good way to get see this is to fire up a 'working' amp and see how the bulb lights up at 1st start up but then dimes to almost nothing.   

[Willabe]

Don't forget to look at the other link I posted from Doug, to double check your wiring, it works great! 

[EL34]

http://el34world.com/Forum/index.php?topic=17701.0

As you are double checking using my link above, make sure you check this area 

[rhodco]

Wow! I totally missed that ground wire. Thank you Doug!

Willabe, thanks for that tip. I will change out that bulb before fire up time.

You guys are a great help!
Cliff

[rhodco]

OK, first power up had no smoke, no bang, but voltages are puzzling. The light bulb limiter didn't even glow, but the front panel lamp comes on. I get less than a volt DC on pins 1 and 6 of the preamp tubes. I unplug from the limiter and plug straight into normal AC outlet. All AC voltages check normal, but very little DC volts. Then it occurs to me... the "Instructions" page reads:

"Powering up the amp and setting the bias

After all connections have been made and double checked Power up the amp with no tubes in the sockets. Using a multi-meter set on DC volts, check to see if pins 1 and 6 on the pre-amp tubes have high voltage. Check pins 3 and 4 on the power tubes to see if they have high voltage. Pins 7 and 9 on EL84 power tubes"

How can I have high DC voltage when there is no rectifier tube? Looking at the power supply schematic, I don't see how I could unless D1 is acting as a half-wave rectifier. Before I chase this any further, could someone please confirm that "no tubes" means no tubes at all, or is that with the exception of the V7 power supply 5U4G?

Thanks,
Cliff

[EL34]

You need to have a rectifier hooked up to get DC on the tubes and do your DC voltage checks

The bias circuit has it's own rectifier, which is a single diode and so you will be able to read the negative bias voltage on the power tubes

[rhodco]

Thank you. That's what I thought, just wanted to be sure before I put the rectifier tube in. So far, so good.

[sluckey]

Expect your B+ voltages to all be high. At this point you're just interested to see that you have B+ on all the filter caps and on all tube socket plate pins. And of course, no smoke or sizzling sounds or funky smells. 

[rhodco]

Cleared the bench of all my construction clutter, got my schematics and charts handy. Plugged in a reverb tank and 8 ohm speaker under the bench.

I took care of that missing ground wire, and rerouted my yellow can cap A wire to run over the star ground lead instead of under it.

Also soldered an alligator clip to the end of my negative feedback wire until testing is finished. Then, I'll cut that off and solder it down.

[rhodco]

Checked my tube stock and realized I have plenty of 5AR4's, but no 5U4G rectifiers. even though it might work, I decided not to chance it.
I've ordered the correct tube for this design and will wait for it before I go any further. Everything else looks GO for launch.

[EL34]

Everything else looks GO for launch.

Hard to say from here, that's what this post is for
You have to do it on your end

The missing ground wire would not be highlighted yellow after you did the check
It would have been easy to spot on your diagram

There could be others you missed also

Here is the link again for the double check routinehttp://el34world.com/Forum/index.php?topic=17701.0

[sluckey]

Checked my tube stock and realized I have plenty of 5AR4's, but no 5U4G rectifiers. even though it might work, I decided not to chance it.

A lot of PRs use a 5AR4 rather than a 5U4. Socket wiring is the same. I would rather have the 5AR4 because it draws less filament current and has a separate indirectly heated cathode which means it has a longer warm up time.

http://el34world.com/charts/Schematics/files/fender/Fender_princeton_rev_gz34_aa1164.pdf

[rhodco]

Thanks Sluckey. This is the schematic I have for my other PR. Power supply uses a 5AR4 but that circuit uses two diodes between the transformer and pins 4 and 6 where the Hoffman design doesn't. I thought the difference in design was the reason why a 5U4G is called for?

[rhodco]

Got home from band practice and found my rectifier tube waiting (thanks for the prompt shipment Doug). I plugged it in, said a prayer, and turned on the power... 437VDC on the tubes... 6.3 VAC on the filaments... but only -41VDC bias voltage. The notes said I should have at least -50 but, ok maybe that's close enough. Power off.

All tubes in, power on, and the bias voltage across the 1 ohm resistor sits on zero. After a few seconds I start wondering, then here it comes! Creeping up... 7mv...15mv... 19mv... It's fluctuating but when I turn the reverb and vibrato all the way down the fluctuations settle to within a range of about .8mv. Once it seems to be settled I tweek it up to about 23mv and leave it there a while. I plugged in my tele and played a while to warm it up... all while keeping an eye on the meter. Finally set it to 26mv and watched a while to make sure it stayed there.

I have to say that the noise floor on this little amp is remarkably quiet, but it gets plenty loud when pushed. The sound is classic Princeton Reverb, with maybe a little too much reverb. I'll pull the 12AT7 from V2 tomorrow and try a 12AU7 in its place. That should bring it down a little I think. I don't hear much of a difference when I pull the negative feedback wire off and put it back on, but I'll do more testing with that later.

I want to thank all the guys that offered suggestions, and thanks Doug for spotting that missing wire for me! It all turned out great and the sound is glorious! My cab comes in later this week so I should be able to put it all together then and post a video here.

[Willabe]