Saturday, 21 January 2017

1968 Princeton Reverb Repairs

1968 Princeton Reverb Repairs — A reprised post from my old QRP HomeBuilder website

This somewhat rare 1968 silver face Princeton Reverb had aluminum trim surrounding the grill cloth. We verified its age by the output transformer code, the trim and by serial number. This particular amp ran a GZ34 rectifier in the rectifier tube socket.

Boy, it sounded terrible!  According to the owner, it sat in a closet for 15 years; but prior to that, some teenagers had borrowed it to "rock it hard", but brought it back and stated that it did not sound very good. After replacing 4 tubes, the power supply capacitors, the bias capacitor, a B+ chain resistor, and the speaker --- this amp sounded pleasant once again.

During physical examination and testing, I found the following problems:

  1. A 4 amp fuse was in the fuse socket
  2. Blown speaker
  3. There were 2 different brands of 6L6 tubes
  4. Loud hum, noise and microphonics in the AF chain
  5. The rectifier tube was audibly clinking and stinking
  6. Low AF output power
  7. The first 1K resistor on the B+ appeared to be temperature distorted
  8. I heard an approximately ~1 KHz intermittent oscillation

Above — Post CBS Princeton Reverb. The Princeton Reverb was essentially a poor man's Deluxe Reverb with a 10 inch speaker. Rated at  around 12 watts, it ran a much smaller output transformer and as a result, offered much less head room than the Deluxe Reverb. The tolex on this old amp looked in incredible shape. All it needed was a good cleaning.

Above — The look on our jazz cat's face says it all. This amp sounded terrible. He
demanded that I make repairs as soon as possible and return it to the owner.

Above — The chassis removed from the wood. The PR chassis proved very compact, lightweight and easy to work on.

Above — A view inside the chassis. This circuit and its schematic looked very much like the blackface pre-CBS version. A few newer, non-Fender parts gave evidence of previous repair(s). This can get scary. Fixing some else's bad repairs may open a Pandora's box. For this particular amp, the previous repairs seemed OK.

Above — The 4 main electrolytic B+ supply capacitors are housed inside 1 can
which is referred to  as a multiple section capacitor. This is the old multiple section capacitor. Note the heat disfigured 1K ohm resistor to the right of the capacitor. I removed it and measured 1K6 ohms.

Above — Removal of an old multi-section capacitor is never fun. I used an 80 watt soldering iron, solder wick and a flat screw driver to unlatch the anchoring tabs from the main chassis. After capacitor removal, the remaining solder was removed and I cleaned and buffed the local chassis area.

Above — The new multiple section capacitor. It's diameter measured 1 - 3/8 inches. This means obtaining a special part as modern cans are greater in diameter and won't fit in a vintage amp. I got this cap from Antique Electronic Supply. The number = C-EC20X4-475 and it's a 20/20/20/20 uF @ 475 VDC. Apparently they're manufactured on original Mallory equipment.

Above — The new multi-section capacitor is soldered in. I replaced the disfigured 1K resistor with a NOS version. It ran cool to touch during testing. For safety purpose, I palpated it immediately after the power was switched off and the high voltage was bled to ground. Nowadays, I measure temperature with an infrared thermometer.

Above — Replacing the bias capacitor removed the intermittent ~1 KHz oscillation and decreased hum. I only had an axial type on hand and it worked fine.

Above — The old and new 6V6 finals are shown. A matched set of Electro-Harmonix 6V6s were installed. I loved the sound of these Russian-built EH 6V6 offerings. I also put in a new, low-noise, Sovtek 12AX7a in the number 1 preamp tube socket. Further, a new Sovtek rectifier tube replaced the clanking old rectifier tube. The other tubes seemed OK.

Above — The old and new 10 inch speakers. The speaker choice was made to suit the owner. He wanted maximum head room and plays only clean guitar. I chose a Fender 099-4810-004. This speaker is actually made by Eminence, a company I like and whose products I have used on other projects. The new speaker sounds great and most importantly, pleases the owner. The old speaker cone and surround were cracked and separated. Further, the cone felt stiff + brittle --- and a new speaker proved the best way to go. Fender didn't put a great speaker in the PR to begin with.

Above —  A rear view of the final tested chassis back in the wood and connected to the new speaker.

Above —  The jazz cat now mesmerized by the sweet tone of the restored Princeton Reverb. The parts total ran about $200 and much of it arose from shipping/handling/duty and tax costs to Canada. The owner only wanted to spend a maximum of $200, so it felt good to come in on budget for a change.

Above — My bench back in 2008. I built and repaired many tube amps here.


Tube amplifiers operate at high DC voltages. Repairing, modifying or building tube amps can be dangerous, or in some cases, fatal.

Thursday, 5 January 2017

Polytone Amplifier Page

This is a reprise of the Polytone Amplifier Page from my old QRP HomeBuilder web site (2008).

Above — The Mega-Brute has an eight inch speaker and is über-portable. This amp tucks into almost any corner. On the rare occasion when I play for a function, I may use a 12 inch extension speaker, depending on the situation.
I've owned 4 different Polytone amplifiers and like their tone and compactness. I must disclaim this blog post by stating that what I have written is just 1 opinion. Discussing guitar amplifiers is akin to stepping into a quagmire. Guitarists often become quite emotional and speak passionately, or possibly even overly-critical when discussing gear. When you consider that most players don't even perform in front of an audience, or record for mass distribution, the "gear wars" diatribe can get really quite silly. If your tone sounds good to you, then perhaps your amp is suitable (at least for this month).

For jazz guitar, there are countless amplifier choices and the list of of good jazz guitar amplifiers has really grown in the last few years. The trend seems to be towards more hi-fi sound (less distortion-more headroom and more power), smaller/lighter designs at somewhat increased cost.

Consider talking to Michael Biller at Sound Island Music if you wish to talk to someone with considerable knowledge and practical experience regarding modern jazz guitar amplification.  Michael stocks many products -- and his passion about helping you obtain your perfect jazz or double bass guitar tone really shines through when you talk to him.

Polytone Mega-Brute Amplifier

Canadian guitarist, Glenn Murch once told me that Polytone guitar amps basically have one sound and you either like it or not. I agree with him. This sound is not
ultra-high in fidelity ( compared to more contemporary designs ), has a dark voicing, a distinct midrange honk and starts to distort at high volume settings. This is exactly why I like Polytone amps in some situations. To each, their own.

The Mega Brute combo amp has an 8 inch speaker. This is probably not the best amp to use if  you play in a big band, but it works okay with a trio if you are happy with the sound it provides and the drummer uses brushes and/or soft hands with sticks.

I have owned Mini-Brutes with 15 inch and 12 inch speakers as well as the Mega-Brute combo amp and head. The various Brute-series amps are worth a trial if you're in the market for a mid-price jazz guitar amp.

Official Polytone Page  Click

Old Murch Music Polytone Page  Click

Old Murch Music Polytone Schematics Page  Click

Above — Rear view. The tolex work is quite excellent. I love closed back speaker
amps for that bass thump, although, in my opinion, ported designs are preferable. The porting seems to be via the via the low input instrument, pre-amp out and FX loop jacks!

Above — Top view showing the various control pots and switches. The sonic circuit was a great addition to the Brute series. The previous Brute design had an overdrive circuit which failed to merit wide acclaim. I personally do not use the sonic circuit and rely upon the main amp circuit. I like the slight break up of the power amp when driven hard, although it is by no means a Marshall-style crunch tone. Baxandall equalizer plus decent spring reverb.

Above — My Mega-Brute atop a 1 by 12 Marshall cabinet. This proved a pleasant
combination for R & B plus jazz-fusion work.

Above — The other Mega-Brute.  An amp head which became known as the
Mega-Brain. This product was discontinued. The speaker is a Raezer's Edge Stealth 12 speaker cabinet. I sold this head in 2004 and now regret it.

Above —  Inside the Raezer's Edge Stealth 12 speaker cabinet. Foam and
fiberglass insulation absorb standing waves + reflections, lower the box Q and hopefully smooths out the bass response.

Above — Another view of the Raezer's Edge Stealth 12 speaker cabinet.

Above — I like Eminence speakers; including the Eminence Patriot Swamp Thang mounted in another cabinet for my stereo amp rig.

 Above — My old Mini-Brute with a 15 inch speaker on its side ready to carry. Bass for days!

Sunday, 1 January 2017

Fender Telecaster Jazz Box

Leo Fender invented the solid body electric guitar. Eventually, his invention became the legendary Fender Telecaster®. This basic guitar: withstood the test of time; exudes versatility; sounds great; and most importantly -- proves fun to play.

I tried out a number of Fender Telecasters at local music stores and 1 stood out. A blue American Special Telecaster®. This particular guitar sounded nice acoustically and the neck felt great under my fingers. I play jazz fusion, or clean-only, jazz chord style blues on mostly the front pick up.

While rolling the guitar tone knob back about 50% provided a decent jazz tone, the 60 hertz hum plus the front pick's thin sound left me wanting to place humbucking pickups in this axe.

I 've replaced a lot of pickups over the decades and the aftermarket choices today seems incredible. Still, too, I tend to go with what I know. I bought a pair of Seymour Duncan hot rails for tele pups and mounted them. Click for a link to those pickups on the Seymour Duncan web site.

Above — The stock Telecaster® pickup just sitting in its cavity. The protective cellophane still lies in-situ.

This guitar came stock with Texas Special™ Single-Coil Tele pickups. They sound good and would likely work well for the classic rock, blues and country player. When I removed the pick guard, I felt surprised to see the guitar body was already routed for a full size humbucker pickup, although the routing seemed more shallow than standard.

Above — The new Duncan humbucker sitting in the neck cavity.

Above — Alternate view of Duncan humbucker just sitting in the neck cavity. I removed the bridge pickup. You can see the black ground wire I added to ground the bridge since the Duncan pickup lacks the classic Telecaster brass grounding backing plate used with stock Tele single coil pickups.

Above — The front pickup screwed into the guitar wood. I lacked the hardware to mount it in the pickguard and so just screwed it to the guitar. In my 22 fret telecaster, in order to put the pick guard back on, I had to remove the neck. I also had to sand my pickguard's front pickup slot a little to better fit the pickup width.  You can see, that an early attempt to place the pickup in the pickguard shaved off a little of the tape that covers the pickup front.

Above — To unsolder and solder new pickups, the tele's control plate gets removed and set on a towel to avoid scratching the guitar.


Above 3 photographs — Control panel, switch, and the deep control cavity. Wiring guitars seems so simple compared to the microwave circuitry I've toiled with lately.
I also changed the stock Fender tone circuit to my preferred tone circuit: that used by Gibson® guitars in the 1950s.

Above — The bridge pickup mounted in the vintage style Fender bridge.
Above — Both pickups installed and the bridge screwed in and grounded with my new ground wire. I then temporarily set the pickguard and neck on top plus fitted the E string shown to determine the ideal front pickup height. The tele rails pickups feature a strong ceramic magnet so I opted to keep the front pickup low to avoid decreasing string vibration. Once the neck pickup height seemed correct, I re-attached the neck and, screwed down the pickguard.

I played this guitar for about 2 weeks, and while I loved the sound of the bridge pickup, I only liked the neck pickup for my particular playing style. So, then, I ordered my all-time favorite neck pickup: The Duncan 59. Click for link.

I ordered a new aftermarket Tele pickguard cut for a pickguard mounted humbucker and waited for parts to arrive.

Further, I got some Bourns 500K panel potentiometers specified and distributed by Seymour Duncan and their dealers.

Above — (Bourns) Seymour Duncan pots that turn like butter. For me, the most important determinants in the guitar/amp system = the guitar's volume and tone pots.

Above — A rear view of the Duncan 59 neck humbucker. It gives that old PAF sound with some tweaks: slightly scooped mids + peaked highs on an ALNICO ( aluminum - nickel - cobalt ) 5 magnet. Sweetness indeed.  I measured the pickups DC resistance at 7.56K - right on spec.
Your DMM comes in handy for guitar wiring -- and can help you clearly establish correct grounding, plus help you detect short circuits.

Above — The 59 pickup mounted in a new aftermarket pick guard.

Above — Because the factory carved front pickup cavity loomed shallow, I got in there with a spade bit and created room for the Duncan 59 pickup mounting plate and screws.


Above — Testing to ensure that my spade drill bit holes were deep enough to allow the pickup hardware to clear and not bottom out. After a couple of gentle drillings, It fit perfectly.

Above — Time to bolt on the neck and secure the new pickguard.

Above —  I soldered the control panel in reverse fashion. I prefer the switch at the back and pots up front. This gives  easier access to the volume pot since I employ a  lot of volume swells and mostly just play on the front pick up so the switch is much less important. I oriented the switch so it points to the chosen pickup per normal.

I ran (black) shielded RG-174 wire to connect the switch output to the 500K volume pot. The tone capacitor value = 0.018 µF per my personal preference.

Above — You can see the reversed Tele control plate well in the photo. I'll enjoy boosted access to the volume pot.

Treble Bleed Network

As you roll down the volume pot, the sound grows muffled due to low-pass filtration of the pick up(s) output signal. Some builders/players place a "treble bleed" circuit across the volume pot's high impedance lug and the center wiper to restore high frequency at lower gain settings.  If you search this topic you'll find a large number of websites and videos showing simple C, plus parallel, or series R C networks to help solve the loss of treble that occurs as the volume pot's resistance gets increased.

Above — To audibly test various treble bypass circuits on a Tele, 1 way is to solder in a couple of wires with the control plate gently screwed down. Then you can connect your treble bypass network to find your favorite. Here,  I've got a 250K pot and a parallel 1 nF capacitor attached to my wires. It's important to test your "bleeder network" with your normal patch cord, pedal board and amp(s) since the entire signal chain exerts reactance that may skew your results.

Once you find your magic C or R C network, then remove the wires and solder these part(s) to the volume pot and you're done.

Above — My final guitar schematic after many listening tests. For treble bypass, I chose a 270 pF NP0 ceramic bypass capacitor. Some networks ( especially some parallel R C combinations ) can wreck the taper of your volume pot. As a player who swells the volume knob frequently, I prefer a simple bypass capacitor as shown since this modification often skews the volume potentiometer taper the least and makes me happy. 

Above — Guitar strung and tuned.  I'm not into hyperbole: it sounds great and noise free. The pots turn easily and I've got a new toy to enjoy.

QRP —  Postdata         Addition of a Hipshot Bridge on January 13, 2017

In January 2017, I placed an aftermarket bridge from Hipshot

Above — Hipshot's compensated stainless Telecaster replacement bridge overcomes the 2 strings per saddle intonation glitch in the vintage bridge.  Further, it lacks the Leo Fender bridge lip that may boost your ability to finger pick and palm mute. I love not having that metal lip to contend with.

Above — Installation proves easy. Remove your strings, then unscrew the bridge pickup and old bridge. Here, I've placed the rubber pickup spacers over the 2 pickup mounting bolts.

Above — Everything installed, I hooked up a strobe tuner. Now each string pair lies perfectly intonated thanks to the compensated bridge. I saw a compensated saddle bridge on 1 stock, higher end, American-built Fender Telecaster in a local shop this Winter.

I tested the action at a few settings before properly setting the guitar up for my normal flat-wound strings.

Above — After intonating the saddles + setting up the action came the best part --- the playing test. Lovely bridge and intonation. Final guitar photo.

Above — To mellow out my Tele for jazz-fusion and clean jazz-blues, I installed my favorite flat wound strings. In stock, I've got this and the .011 to 50 set; both with a wound G string.