Tuesday, February 22, 2011

Giant Amps, Part One, the Stage Star

Sometimes you get giants

I have collector friends that find very odd and interesting things. This is a Valco "Stage Star" at 27" wide, by 20" high, and 11" deep. I could barely get it on the bench, my tired out old drafting table.

It uses four 6" by 9" oval speakers, like older car speakers. You or your dad had one in the back of that 57 Chevy you were trying to score in years ago. These are "oval" speakers, popular in the 1950's as they were used in cars, because they fit into the dash or the back deck easier, due to their small width.

Here you can see the preamp chassis taking a peek outside. Using a 6SJ7 pentode for the high-gain in, and 1/2 of a 6SC7 dual triode for the rest, Valco was able to create a very adjustable preamp. Their preamps had different sounding inputs, Accordian, Guitar, Treble for a bright sound, and low for more of a bass sound.

The easy way to work on some of these big monsters is to just have it all in front of you. This amp has the output transformer mounted on the baffle board, and a solid cable between the amp and preamp, so it is easier just to take either out by itself, and work on it there

This amp is used strickly for Harp, as the amp and speakers give a nice compression and no harsh highs, good bass. Here is some of the typical simple, but mysterious wiring of a Valco preamp of the late 1950's, that gave a nice Hi-Fi sound, great for Blues or Country

The amp chassis is super simple, a 6sc7 driver, and two 5881 output tubes for a good 25 watts cathode biased, Class-A power amp, that drives the four speaks pretty fine. The 6sc7 is a metal tube, a early 6sl7 that was popular from the late 1930's to the 1950's. So this amp is still using some 1940's design motifs

What you can't see in the pics is the output transformer, which is mounted on the baffle board, so the amp can't go very far from the case unless you want to unsolder the transformer from the speakers and unscrew it from the board. Valco made small power amp chassis so they would fit between the speakers, as making even these big amps as small as possible was the way back then.

Here is the monster with it's natty cover, with the "Stage Star" initials on the left top corner. This was a custom amp made by Valco, a builder, for some small company, probably a short run as a lot of unusual designs were not big sellers. It was not until Fender, Shure, Marshall and others turned the newer music stores into big business that a large quanity of amps were sold. Also, small companies existed on the East coast or the in the midwest that sold gear that was never seen in California, land of surf music and Fender amps, and then later these unique inventions faded away

Friday, February 4, 2011

The Gibson Amp attack from 1947

Three Gibby amps at once

Here is one of three Gibson BR-1 amps from the same owner, who loves the sound of this model for his style of blues playing, so he got two more for spares.

Rebuilding all three as they all had bad caps and damage from shipping and previous fools playing around turned into a real adventure

This is the first one, above, with a new custom cabinet, amp finished.

The caps are all underneath the turret board so I had to used several tools to cut the leads and fish them out.  There were 22 dead caps per amp as the 60 years is a bit too long, although the sellers on ebay sold these pretty high

This is a very early amp, about 1947, and it has 25 watts, lots of wires, two "metal" tubes, a small glass tube,  three big glass ones, and a very heavy field-coil speaker. 
One of the amps came packed very badly, with the wood magnet brace missing, and the speaker pulled the entire baffle board off.

This amp has a 6sj7 early pentode preamp tube, very close in specs to a EF86, and one of the best preamp tubes, a 6sc7, used for  EQ and line input , a very good sounding tube, and a 6sn7 driver. Two veteran 6L6G tubes in push pull provide 25 watts to the massive 12" speaker

The 6j7, the 6sj7, earlier 39/44 pentodes were great tubes and I have used them in Vintage mike preamps I have built, finding the 39/44 to be one of the quietest preamp tubes ever.

Here is amp two, with the baffle repaired and now held with brass screws through the front, and a with a "notch" cut into the top so the 5u4 would clear because the previous owner could not figure out how to get the chassis out without hitting cabinet with the the tall glass 5u4 and 6L6 "G" "Coke Bottle" shape tubes!
This is a good sounding amp, but typical of early amps, a cabinet disaster.
This is number three, with a original aluminum baffle screen. 

Amp number three, I thought would be a pretty easy job as I knew the amp by now and knew what to do, I thought, until I discovered the previous had "Re-wired" the controls and more in a vague attempt to make some kind of new amp.
I was a lot of work  figuring it out and lots of solder work to get all these early heavy wires all with the same brown color back where they were!
These three amps were restored over four months, the last one in October.
Two brothers made amps for Gibson back then and made some really wild amps, one that had six output tubes and almost 50 watts of power

This link will take you to videos on youtube that show early Gibson amp


Info from www.ehow.com


The Strange Jukebox Amps from Seeburg

I  have to be "Jack of all Amps" or  get a normal job!

I have worked on really big old Jukebox amps long ago, and thought I would never see
one again, but I have a new friend that has about thirty of them and knows other
collectors, so I am in big juke-amp trouble now

This is a Seeburg stereo tube jukebox amp. The whole thang is on the back door and
to service the amp, you just swing the door out and there it is.

This amp runs 6973 output tubes, which are like a Hi-Fi version of a 6bq5. The amp also has a "AVC" or automatic volume control circuit that keeps one record from being louder
than another record, or one song from being louder than another.

More of this interesting pasta mess of wiring to figure out. There is a high-voltage
supply, a bias and DC turntabe  motor supply, two 18 watt power amps, two preamps,
two tone control amps, a and a stereo AVC circuit all in there somewhere

The 6.3 volts AC for the filament comes from another transformer that is in the "Coin
Reciever" unit, so I had to sneak the filament volts off another amp!

The way one of these amps works is this. The whole amp is dead, customer walks up
and puts coins in, the filament volts is turned on and the amp warms up.
Then when the arm starts moving to the first selection, the 115 volts AC to the high voltage transformer is turned on and the 5u4 cranks up the high voltage to the tubes just in time before the needle hits the record. These tricks save the tubes and amp as it in idle, just warm mode until needed, so the jukebox can sit ready all day long.

This is the "Testing method" a stereo turntable and a record of oldies.  
I had two of these amps to get running, so we listend to the same oldies while I set the bias on the 6973 tubes, which was a adventure as one tube out of the four was at maxium hot idle and one of the four was at the low idle point. without balancing adjustments, tubes have to be matched for same idle, which means out of ten tubes,
maybe four would really match up

This kind of job is the "Yeah Sure i can do it", then you just have to pile through and use your wits to make it all happen, so the customer can get a good sounding amp back,
and you can get paid, then pay the rent, and go on to another month, more work.

The Wah Pedal, Caps and Coils, and Germanium Transistors

The Wild Wahs

I have a friend that is really into gear, and he was after a sooper Wah pedal for his setup

After re-building several, I started using Vintage germanium transistors and just going
hog wild with the circuit, until I came up with a "Sooper-Wah" that has a very clear, no
grainy sound, lots of tone, and very warm strong "Wah".

Also the volume level when the wah is kicked in is actually a bit higher and the wah is quieter with the mods and "Germ" transistors for a real "Hi-Fi warm rich wah sound

This version above has three tone ranges and a bright switch, which gives six tone ranges,
and runs on 12 to 15 volts from a Wall Wart supply which gives more a more big full tone sound from the voltage starved transistors, and a clear, rich "Wah"

The low 9 volts is not enough for the transistors to generate all the hidden tones and power in the sound, and 9 volt batteries are an ancient idea that was bad from the beginning, and still bad today, when re-chargable batteries are easy to use.

Transistors are the key to really changing the sound of a lot of these vintage circuits,
and it takes research and testing to learn what transisors really give that special sound.

I have had a lot of fun trying various germanium transistors to see what there sound qualities are and learning about the circuits and what they can do sonically

Modded wah pedals are right around $125 for complete mod, just email me for info on custom work

Class-A Williamson Vintage Hi-Fi amps

I finished a repair of a Knight KB85 stereo amp, and found it to be a great warm amp, 
very magical sound, typical of the great 1950s era of Hi-Fi

These older Hi-Fi amps have fantastic output transformers and circuits designed to put
out a wide range of frequencies, flat response, and small amounts of phase shift.

I am surprised when I crank one of these amps up and hear really deep bass notes come
out of the Wharfdale as most guitar amps have no real bottom end and are tuned
toward highs out mostly as guitars need lots of high boost.

Here is some of the fun wiring underneath, and me measuring everything without
melting one of my test leads while around the 485 to 500 volts on the plates of the KT66
tubes. I tested the amp with 6l6gc and these amps will run with EL34 tubes, but vintage KT66s
are super smooth and very rich magical sweet sound.

These tubes were invented by the English as a high voltage version of the 6l6g for their
super-williamson circuis that became the Heath W-5M amps and others we know

So the KT tubes are High-voltage versions of 6bq5, 6ca7, and 6l6 tubes and EL tubes are
European versions of American tubes for serious High-Fi gear.

Really OLD amps of all sorts

Here is a 1930's Wurlitzer Jukebox amp, one of my early 
vintage restoration jobs 

This is more like the kind of gear I work on, a very old tube amp that might work after
spending hours of hopeful work shaping all the vintage parts back into some possible
working shape, and hopefully all will stay together and keep running, at least for a while

Here you can see the wild, running all over the place wiring job, and now all the wire colors have
faded so they are all brown!

This amp really surprised me when I brought it up on the Variac, quiet, great smooth
tone, and very well behaved amp. The speaker sounded great also, with solid bass and
very good highs.

This amp like other all triode amps of this era, uses triodes with no cathodes. Then very clever wiring of the filaments seperates the tubes so each has it's own "cathode" grounding and biasing.

This amp uses two #30 triodes as preamp stages, a $45 driver, and two #45 output tubes, very much a "SE" amp.

This amp also devolops negative bias for the two #45 triodes in the power stage by
placing the feild coil of the speaker in series with the center tap of the high-voltage coil.

So this amp has all the same circuit functions of a modern SE amp, three triode tube
driver stage and driver transformer, and push-pull triode output stage with fixed bias, way before diodes of any kind. One circuit has a #45 backwards as a bias rectifier, very clever for 1933!

The amp comes up very fast also as they selected #30 tubes for the preamp, and the
#45 driver and the 5Z3 rectifier tube come up fast also.

Jukebox amps have to come up fast as the time between the customer putting in the quarters and the record playing can be very short, and the amp has to be ready before the needle hits!