Guild Masteramp 66-J questions

Hey gearheads, newb here. I just got a 1959 or 1960
66-J amp, appears to be 100% original. Missing leather
handle and jewel cover/retainer nut.

I am fixing it up, any comments or suggestions appreciated.

Tremolo only works on last third of knob travel, on both speed
and strength controls. It is deep and thick, but hard to adjust
for slow and mild trem. incredible rich sound, sweet & vibey.
I replaced the strength knob (500K) but no help.

The amp hummed bad, I replace all the electrolytic filter caps
and several 25uf caps within the circuit. I put (2) 32uf caps
and (2) 20uf sprague atoms in there, that reduced some of the
hum; later I went back and replaced the first 32uf with a 40uf
sprague. That reduced the hum even more, but still a bit more hum
than I would like. replaced the 2 conductor with a
grounded 3 condutor power cable, and removed death cap.
I would like to lower the hum more if possible, it is tolerable but...

Tubes were original near as I can tell, the 6v6's tested good,
amazing for 50 year old tubes. replaced the preamp tubes with
raytheon usa 12AX7's, no noise or microphonics. The 5y3 recto
tube tested kind of flakey, replaced it. I tried some groove tubes
in their, 6v6's, but the original RCA's sounded way better.

Nice amp, sweet sounding for jazz. No distortion until the amp
is dimed, suits me. I play in a jazz combo and church band and
this looks like it will work great for both. All guitars I have tried
thru it sound really nice.
here is a gut shot
http://s240.photobucket.com/albums/ff13 ... CF0005.jpg

I will take some more pics and post if anyone wants to see them.
Pretty clean little amp, I dig the tone.
peace,
chris dansdill

Welcome.

I suspect someone will be along, soon, with answers for you.

fronobulax said:

I suspect someone will be along, soon, with answers for you.

Click to expand...

Paging Cap't Juan. Paging Cap't Juan!

Amp questions on line two!

On it Kurt :wink:

Hi Chris; welcome to LTG and congratulations on your Guild amp! You may not remember but you and I banged it around in '08 on the subject of the Gibson GA1RV, 6BM8s, and a 6BM8 project your were working at the time. We were leveraging off several posts of yours on the Maggie/Valco BB. There are several other LTG members here who post there including LeonC, Mdevito, and default. I have a couple of copies of schemos for your amp but they're pics of the schematic stapled inside the cabinet; if yours is intact, you are better off but, if you want them, PM me with your email addie and I'll send them along.

Hmmm ... sorry, I meant hummmm; what follows is me thinking out loud ... not suggesting you don't know what you're doing because I know you do but just some thoughts ...

Tremolo: maybe its poor function is the source. The fuzzy schematics I have indicate a odd-looking trem circuit design compared to the more familiar Gibson style. It looks like it relies on an array of resistors and not a network of Rs and Cs. If this were my amp, I'd replace all the passives in the trem circuit. There ought to be a highish value/high voltage trem cap somewhere; it has to go too (maybe the one on the extreme right of your PB pic?). Anyway, doing so will give you better trem (probably) and eliminate the trem as a possible source noise (certainty).

CT/PS Filter Cap Ground: do you have the secondary AC center tap grounded at the same point as the new power supply filter caps? If not, you can induce a mui bad ground loop.

Heater imbalance: if the heater circuit is center-tapped, consider wiring a 100ohm pot in series with the indicator lamp; spinning it back and forth should raise and lower the hum ... if it's coming from there. If it's out of balance, you can sub in a 100ohm resistor on the noisy side and usually kill most/all of the hum.

Otherwise all I can suggest is chop-sticking, cleaning/re-seating the jacks and controls, and lead dress. Hope this helps and again, welcome to LTG! John

John,
Awesome advice, thanks. I will look at this over the weekend. Also thanks for the
great help on the old gibson 6BM8 project. I have enjoyed that little cruncher and it
is hands down the quietest amp I have ever built or rebuilt. Can't tell it is even on.

http://s240.photobucket.com/albums/ff13 ... h%20build/

I found a couple of NOS raytheon 6BM8 tubes, and a fellow fanatic amp fiend
turned me on to a telefunken recto tube. I put a really efficient speaker in there,
80 watts celestion on a whim and it is a nice combo with the amp.

I have the schem for the 66-J, intact in the back cover, I would be happy to unstaple and
scan if anyone wants a pdf. This amp was in amazing condition, I suspect it
was used a bunch when new, you can see the grease marks under the handle,
then put in a closet or wrapped up. Hardly even any dust. I have amps I built
some years ago with more dust on the speaker than this amp had. The bottom of
the chassis gleamed. Thanks for your time.

Hey anybody out there have a duane eddy sig guild hollow body? I have a friend
here in tucson with one, aluminum bridge saddle and wonderful sounding pickups.
peace,
chris dansdill

Alas...it's only a DE-400...

badansdill said:

... I found a couple of NOS raytheon 6BM8 tubes, and a fellow fanatic amp fiend turned me on to a telefunken recto tube.

Click to expand...

Hi Chris; I always thought it was too bad that the 6BM8 didn't enjoy more widespread popularity; they sound so good and the two-for-1 envelop ... not too many tubes achieve such a high return.

badansdill said:

I have the schem for the 66-J, intact in the back cover, I would be happy to unstaple and scan if anyone wants a pdf.

Click to expand...

... I Do! Will send you PM with email address.

badansdill said:

Hey anybody out there have a duane eddy sig guild hollow body?

Click to expand...

Our member Guild's O Grot tends to have 4 or 5 of everything Guild ever made; the BB's most prolific collector ... click the link to his collection ... better wear a bib ... the collection is rated High Slobber Factor! Hope you fix the hum issues; will help any way I can. John

wow Guilds of grot, that is a nice looking guitar.
What models were produced? I am also aware of
a gretsch Duane Eddy model. My friends guild has the
same pickups, but is missing the pick guard. His
has the bigsby type trem, I think it says guild too.
I am going to borrow that guitar from him again,
he loaned it to me for a month so I could play it
and restring it and set it up for him. standing offer,
it is probably time for a setup again so I will give
him a call.
peace,
chris dansdill

badansdill said:

What models were produced?

Click to expand...

The top of the line DE-500 had gold hardware and were usually finished in blonde.

UPdate time, been fixin' that master amp 66-J.
I got the tremolo working now, changed several
caps and resistors within the circuit, sounds
strong and works througout the whole travel of
the speed and strength knobs.

Hum: I already did ground the filter caps to the same
lug as the PT center tap, so I'm ok there. No heater
center tap on this amp. I will reseat the pots
and jacks tonight, and poke around a bit. I noticed
when I mic'd the amp at my sunday gig the hum was in
the mains and monitors a bit so it is louder than I
want it. I also gigged with this amp 2 different
gigs on Saturday, rock at the former and jazz the
latter: Great tone, it is absolutely one of the coolest
amps I have ever owned. tons of mojo...

What years were these amps made anyway? seems like the speaker
date codes suggest it is a '59 but could be a '69 replacement
I suppose. It was not soldered at the speaker. The pots suggest a
1954 date code, so it could be a mid 50's amp with a
59 or 69 replacement speaker. Seems like it was played
alot for a while, then put away. It is in amazing shape.
it has a tweed top and brown leather tolex bottom, looks just like
the one on cawfeereglah's avatar.

BTW, my gretsch streamliner historic with dearmond single coils is
jazz heaven thru this amp......

thanks again for all help.
peace,
chris dansdill

badansdill said:

... the tremolo working ... changed several caps and resistors within the circuit ... I noticed when I mic'd the amp ... hum was in the mains and monitors ... What years were these amps made anyway? ...

Click to expand...

Hi Chris; my limited experience with trem is that it's nearly always the Rs/Cs in the circuit. The Cs are being hit with oscillating DC all the time ... whether the trem is switched in or not and after a while, it just wears them out. Hum: had a chance to mess with the heaters yet? If you don't want to fool around with a series pot, you can always tack a 100 ohm resistor from one side of the pilot light to ground and see what happens. If worse, it goes on the other side. In either event, if you get an improvement, you can manipulate the R value to see if you can damp it further. Otherwise, you might try looking for a leaky coupling cap.

Maybe somebody else does but I don't know when your amp was made. I believe that the 50J, more of a student amp, was built offshore and maybe introduced after the 66J to bulk up the product line but that's just speculation on my part. You can see some pics of early and later Guild amps in this LTG thread, an informal attempt to collect Guild amp pics in one place.

Can you post some pics of the amp? Best, John

John,
I misunderstood on the A/C heater thing, I ran a 100ohm pot between one
of the 6V tap wires and the light bulb. Turning the pot made the light dim,
but the hum was the same. You were suggesting putting the pot between
the bulb and the chassis ground and applying various resistance to see if
hum reduced. duh. I will try that. and if ya find the spot of least hum you
measure the pot and put that value resistor in permanently..... ;^)

I did not get what you meant, thanks. If that does not pan out, I will
check the coupling caps.
peace,
chris

Hi Chris; my bungle ... didn't make myself clear. If you have a 100 ohm pot, jump from one side of the pilot light to the one of the outer lugs, jump from the other side of the pilot light to the other outer lug, and connect the wiper/center lug to chassis ground. If the problem is AC balance, then swinging pot will increase and decrease the hum.

If you get a quiet spot, read the AC voltage between each outer lug and the center lug; one side should read higher than the other. If the pot is full over to reduce the noise, then the 100 ohm resistor goes from the side of the pilot light with the higher AC voltage to ground ... the idea is to waste some voltage to ground to achieve balance. This amounts to wiring in a temporary Hum Balance pot; the next generation 'Thunder' series amps were fitted with Hum Balance pots.

While you're at it, you might check the incoming AC; it varies all over the country but the power transformer was designed for 110VAC supply but, in most places, it can run as high as 117V-125V. Amp has 3-wire cord, no? and .05 AC line-to-ground cap is gone too, no? John

John,

A/C at my home is 117VAC, is there anything you can even do bout that?
I added 3 conductor power cord and removed death cap, first thing I did.
The ground leg of the new power cable is attached to a convenient chassis ground.
I ran the power cable into the chassis with a rubber o ring, and tied a knot
in the power cable for strain relief. That knot is sitting right next to the Recto
tube socket, wonder if that is contributing. Maybe I should move the ground
leg of power cable to center tap ground point.

Lead dress looks good, have reseated all jacks and pots. Sounds like an A/C hum
for sure, running out of options.

Thanks for clarifying the 100 ohm pot thing, got the same info from the net and
gave it a try. When I powered up the amp with pot tacked on to bulb and ground,
there was a flash crackle pop in the pot and it smoked it! maybe defective pot,
dunno and I didn't have another one lying around. This heater scheme feeds both
legs of the 6V tap to the tube heaters, and chassis grounds at each socket. THere
is a wire coming off the first 6V6 tube socket that feeds the light bulb, and the
other side of the bulb is a wire to chassis ground. I will pickup a couple more
100 ohm pots and try again, I did try tacking a 100 ohm resistor to each side of
teh light bulb and then to ground, neither seemed to lessen the hum to my ear.
Guess i don't fully understand the balance thing, should the two legs from the
6V tap feed the light bulb directly?

If I cannot get this a little quieter I may have to sell it. Love the tone, hate the
hum..... I ended up replacing all the coupling caps, in fact replaced every cap
but the ceramic caps and poly caps on the tone pots. No change to hum.
Any suggestions appreciated, thanks!
peace,
chris dansdill

One more thing, from the moment you switch this amp on
you can hear a hum, even in standby (with center tap lifted)
through the speaker! how does that work?!?!? when you
flip the standby to 'on' the hum is a bit louder.
peace,
chris dansdill

Hi Chris; eek! Unless you blow money on a Variac, you won't get the line voltage down to 110V. It's high but I don't think high enough to be a problem on its own. Didn't realize they are supplying power to the pilot light with only one leg of the 6.3 ... sheesh ... no way the pot trick is going to work with the light wired like that.

Schematic says no center tap on the 6.3V winding, please confirm. They are running everything off the hot side and the other side is grounded ... just like you said on the other side of the pilot light ... ugh.

That is; taking the hot wire to one heater on a 6V6, jumping to the other side, and from there to ground ... commonly pin 1 on 6V6s, it's not connected internally. The supply continues to the next 6V6, jumps to the other heater, and then to ground.

From there, the supply runs to the 12AX7s; pins 4 and 5 jumpered together; the supply will hit them (or pin 9), jumper to pin 9 (or hit 9 and jumper to 4/5) and then to ground.

Assuming all the heater grounding at each tube is intact (that goes on the to-do list), chances are the new filter caps with their ground disturbed what was already there. Like I said, in some designs, filter caps are grounded at pin 1 of a 6V6 and from there to ground.

You are not going to be able to re-wire the pilot light as you suggested and the hum balance pot won't work without a center tap.

Must check the continuity of all the tube heater grounding. If they used pin 1 in some common grounding scheme then you have a ground loop the fix for which is bringing all the tube heater grounding back to a central 'star' ground. ... :wink: J

How about a 100 or 50 ohm resistor running from the 6v heater wires to ground?
I did that on the T-32 and it helped.

Default said:

How about a 100 or 50 ohm resistor running from the 6v heater wires to ground? I did that on the T-32 and it helped.

Click to expand...

Hi Steve; sent you the schematic via email .... see whatcha think. J

I'd disconnect the ground and run the resistors off the lampsocket, if I could, but you get the idea.

hey john, default. Thanks for the input. I will check the grounds on the
tube sockets, that is a good idea. The solder connections look pretty
funky. Seems like looking at the schematic you would wire both legs
of the heater winding to the bulb and then on to the tube sockets
like on a fender. I could redo the heater wires like that, couldn't I?

I see what you are saying default, just use a resistor to make the
ground leg off the lamp. I will tack in a couple resistors to try that
and see what it does. I think I can reach the center tap chassis ground
with the resistor off the ground side of the lamp.

I will have a few hours this weekend to poke around some more.
peace dudes,
hope the guitar playing goes well for ya.
chris dansdill