I bought this guitaramplifier used 1985.
The only information I had before was that it received very good
marks in a number of the magazine HiFI & Musik in the beginning
of the -80's. The
company Guyatone still exists today and are mostly known for
their manufacturing of effectpedals. Their homepage about amplifiers
are still without contents (may 23 2000). Nowadays totally reworked
and nothing about amplifiers (nov. 20 2000).
Earlier I had a Marshall 50W amplifier with a 4x12" celestionbox
which sounded well but I needed equipment that took less space.
I played dancemusik in a band at that time on smaller hotels and
restaurangs in Norway and thought that the Marshall stack was
to big and awkward.
Strolled around at Musikbörsen in Gothenburg and saw this
piece used with the price label 1500 SEK (about 163 US$) ! It
costed nearly 6000 SEK new at the time ! The shop took my Marshall
equipment in change and I also got a new digital Ibanez echo for 1500 SEK in the barter
transaction. Consequently I got 3000 SEK for the Marshallstuff
which I bought used for the same price 5 years earlier. Nice business
I think.
Reading about the amplifiers further history is under the heading restoration below.
A hybrid on 55 watt. 2SK30 FET's in the prestages and tubes for the endstage. Outlets for headphones and footswitches for clean sound or overdrive and with or without reverb. The clean/overdrive switching is done electronically with two 4016 IC chips. The endstage is fairly conventional "Fenderlookalike" with an ECC81 as phaseshifting tube and two 6L6GC in pushpull class AB. What confuses me is the end tubes which is by the manufacture RCA and also the "blackplates" ! All well-informed knows that this tubes production was stopped at the beginning of the -60's and new tubes on stock (NOS) commands a very high price today, more then 100 US$. RCA 6L6GC is nothing but the tubes Rolls & Royce. Did the former owner equipped the amplifier with these tubes or have the factory bought an old stock before this model was sold in series ?
After a couple of year when I nearly haven't
played anything on the amplifier it fly to me to restring my old
Aria and take some tones. It crunched from the potentiometres
and sounded ill with crackles on higher volumes. Giving exercise
to the tubesockets. and spraying the pots fixed that part. It
sounded as it should again. However I was more curious than that.
I did some research on the net last autumn and found lots of information
about tubes, amplifiers, technique, adjustment of the bias current,
modifications etc. In particular triodcoupling of a penthode endstage
to get another sound and lower output effect interested me. I
also wanted a standby switch which was missing in the construction.
Preferably a servicemanual or schematic as well.
My research showed that the amplifier was imported by the accordion
and instrument importer, today discontinued, Hagström's music
in Älvdalen. After some phonecalls I got in contact with
the company Amtech
which had a schematic
to send me.
 The front with new switches |
The old on/off switch is here replaced with
two others, the left is for standby with anodvoltage indicator.
The resistor supply the LED with the right current and drains
the voltage away when the amplifier is turned off. Nice if one
should work with it and want to get away from the 500 volt VDC
charged electrolytic's shock. What's not viewed here is a nowadays inbuilt timerelay with delay on operation, set at 30 seconds. Accordingly the tubes are not fed with platevoltage before they are heated by the filament voltage. Maybe exaggerated but lenient to the tubes. |
 Bananacontacts and resistors viewed from the inside |
A nice tip from the newsgroups and homepages is to lead the cathodes via two resistors on 1 ohm each to earth instead of directly as, in my construction. This resistor has no meaning for the amplifiers construction but implies that you can measure the bias current indirect with a millivolmeter trough the banana contacts I've mounted them on. E.g. 35 mA bias current corresponds to 35 millivolt. Smart and undangerous. One can also put in separate potentiometres for this adjustment, one for each tube. Updated 16 june 2003. Now it's done !y tubes idle current is now individual adjusted to about 40 mA respectively, from about 55mA before which is somewhat to high for the supply voltage on 500 volt, 480 volt on the plates, which this construction have. I would'nt willingly destroy my RCA -tubes to soon. The last update also includes a switch for cathode autobias or fixed, as it was original. When in autobias I also have a switch for bypass of cathodecapacior. I now can chose high gain, clear and crisp sound up to clipping, with fixedbias, or a really round mellow sound with autobias. This modification you can find as a tip on Kevin O'conners faq which was taken from his book "The ultimate tone - vol2.". |
 Almost finished |
This picture show a nearly finished reconstruction
of the amplifier. As you can see theres still room over at the
left back on the chassis for more switches and modifications. The one-polar switch I use to connect a resistor which can change the feed-back coupling from the output of the output transformer back to the cathoderesitors on the phase shifting tube. Less feedback gives higher gain and a more ragged sound and smaller frequency response. Different sounds to experiment with and test by the eager one. |
The modifications are done for this time.
I know that numerous of you have done similar operations and I
would appreciate comments and historys about your own experiences.
Large portions of the text above was updated june 16 2003.
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