Guild Masteramp 66-J questions

[capnjuan]

Hi Chris; yes ... default's suggestion is how most heaters are wired ... because most have a center tap; the two resistors create a center tap. Before you rip out what you have, you might want to consider just carefully trouble-shooting what you have. At one time, it worked correctly and the only reason it doesn't now is because of either a loose/faulty connection or the new filter caps having disturbed the existing ground scheme. If you can't make it go away, then maybe think about the rip and re-do. Can you post some pictures?

John

 

[badansdill]

John,
You are most helpful, thanks. I am going to take the amp
apart saturday, get it out in the sun and take 40 or 50 photos.
I hear ya, the amp was likely not humming back in the day,
I bet there is a loose ground on a socket or something. I think
if you read your grounds with an ohm meter you can get some
insight into the less than perfect solder joints.

I will get it figured out, thanks again for all the suggestions.
Did I tell ya how good this amp sounds? there is something
undeniable about a tube amp running thru these old jensen
speakers. Have you found any current mfg speaks that give
this same tone and articualate response? It just chimes and
rings on my strat and my LP humbuckers don't sound a bit
muddy at the neck. cool amp......
peace,
chris dansdill

 

[capnjuan]

Hi Chris; well ... happy to help and we'll both benefit from having BBer default 'the remarkable' in the thread. The pics will be a big help ... we can see what you see. Yes, must check the grounds with meter but, with your third hand, you need to bump the wiring with a chopstick to make sure there's a mechanically tight connection.

You mentioned your pleasure with the amp and I too am a fan of vintage Jensens and especially when connected to a pair of sweet 6V6s. All my amps are 6V6 and have either Jensen P12Qs or P12Ns in them. I have more vintage Jensen alnicos and '58 Jensen C12Q sitting in cartons waiting for me to pay attention to them.

The ringing and chime are a function of the alnico magnet in the Jensen which, besides having very pleasing characteristics, was the most common commercial speaker from back in the day. As a result, having an alnico speaker in a vintage amp makes the tone a little more faithful to the period sound - they tend to sound like they are 'supposed' to sound. At the end of the day though, it's mostly a matter of taste ... you and I can say we're crazy about Swiss Cheese (alnico) but others might prefer Bleu (ceramic) ... so it goes.

I think the two manufacturers whose speakers do the chimey/shimmering thing to perfection are Weber (Vintage series) and Celestion (15 watt Blues and 50? watt Golds). Considering the cost of the Celestions ... that pretty much leaves Weber whose products are complete electrical / mechanical / tonal Jensen forgeries. The Weber Signature series copies the P_R in voice coil and magnet size, the Vintage series dupes the P_Q vc/magnet characteristics. The Honorable Mention here goes to Jim Seavall's Scumback Scumnico Speakers which are tone dupes of pre-Rola Celestions. They are not cheap but their owners - having spent a ton of money on them - are crazy about them.

Looking forward to the pics. John

 

[badansdill]

http://s240.photobucket.com/albums/ff13 ... ?start=all

Pictures, 47 of them on Photobucket. The very last photo was a gutshot of
before I did any work. Have fun, these amp porn pictures always give me a thrill!
;^)

peace,
chris dansdill

 

[badansdill]

Update time. after I took all the photos,
I pulled the tube sockets one at a time,
cleaned the chassis, and resoldered the
grounds for each tube socket. took my time
and did a good job. then I listened, I would say the
hum was reduced, but not completely. I was
very pleased with that, but I thought would try
the resistors off the lamp to see what that would
do. I soldered a 47ohm resistor on each lug of the
lamp, and twisted the other ends together and
hooked to ground. I powered up, and it started
making a snapping noise, snap snap snap so I killed
the power. Wonder what the hell that was.

I am gonna rethink, this ground thing on the
lamp is not working for me...... ;^)

peace,
chris

 

[capnjuan]

Hi Chris; your 6.3VAC winding doesn't have a center tap. As a result, hanging resistors off the pilot light isn't going to work. I see the heater supply ground bus on the preamp tubes and the ground jumpers on the 6V6s over to the hold-down bolt.

6V6s, pins 1: is anything connected there? If yes, what? Since they aren't internally connected, they are often used as tie points in the grounding scheme. If stuff is terminated there and there is no ground jumper, then you need one.

From here; there are only 3 options:

1. Try this/try that: considering that there could be dozens of things that might cause the problem, this one has the least likelihood of success but a high likelihood of taking the most amount of time and generating the most frustration.

2. Rip and re-do the 6.3VAC wiring IAW default's suggestion. Create a center-tap using 100 ohm resistors, and then run one side to one set of heater connections and the other side to the other. Tedious, time-consuming, high chance of fixing what ails but also introduces chance of further confusion with a mis-step the fix for which would rely on option 1.

3. Take it somewhere: I have a local shop, they treat me nice. I left an amp there. Took them 10 days to get to it ... and 10 minutes to figure out what was wrong. I got it back, the tech said do this and do this and it was fixed. I paid a token fee.

The pics are excellent but load tediously in PB ... it's agony looking at them. Anyway, check the 6V6/pin 1s and see what's going on. I wish I had a better suggestion but I don't.

Best, J

 

[badansdill]

hey john,
sorry for the pain with the pics, 40 some photos, didn't know
a better way to share them. I have made progress again today,
I redid the grounds on the 2 volume pots, and also reflowed
the solder on the center tap connection. This time I fixed
something and the hum was cut in half. I think there is gonna
be little improvement from here considering the PT itself is
humming! I put a screw driver on it and could feel the vib
through the plastic handle! I have seen this before, maybe I
can balance the secondary high voltage winding, perhaps?
ever heard of this? Seems like you could measure the voltage off
each leg of the mains and see if they are balanced.....

I hate to take my amps to techs, I have spent the last 5 years
learning bout tube amps and building probably 2 dozen, 20 of
those from scratch. Old vintage stuff is a totally different
animal, I have no problem trouble shooting my scratch builts,
they are basically new. big diff compared to the insides of
a 50 year old tube amp, all the dust and goo and corrosion,
no wonder the thing was humming! ;^)

Anyways;
It is much better now, not perfect but MUCH better. If I get
a hair up my ass someday I may try to rewire the heaters,
but at this point I have done lots of work and the amp is
cranking. I retentioned and cleaned power tube and recto
sockets, have touched every section of the amp at least
once. I will live with it for a while, now I can mic it without
introducing a ton of hum into our PA at church. I totally
isolate my guitar amp, face it up against a heavy stage curtain
and there are sound absorbing materials sort of behind it that
keep the sound from front of house. I usually use close back
cabs, but I am digging this amp right now. I put lots of my
guitar in a dedicated monitor at my feet, I can really hear my
guit without blowing teh vocalists faces off.

thanks for your help it was appreciated.
peace,
chris dansdill

 

[capnjuan]

Hi Chris; glad to hear about the improvement. Yes; you can measure the secondary AC ... I'd expect it to be a little out of balance but don't whether it can cause hum or how far out is bad ... also don't know of way to adjust.

Better reply tomorrow; I'll sketch out in block form a rip and re-do of the heater wiring although the more strokes you take at the amp, the better it seems to be getting. Anyway, something better thought-out tomorrow.

Regards,
John

 

[capnjuan]

Hi Chris; as you know, there are lots of things that can cause hum. For the time being, this assumes that the noise is coming from the 6.3VAC filament supply. The first pic is how your heater wiring is laid out. Note that there are four independent grounds in one leg and another ground in the other leg ... that many more chances for a poor connection.

Default's suggestion looks like this; adding two 47 to 100 ohm Rs off the pilot light, tying them together, and then grounding them ... doing so creates a center tap, the resistors provide noise snubbing and act like fuses if something goes bad, and limits the # of grounds to 1.

If you decide to re-do it, you really only have to work with the 'near' side, the far side closest to the chassis wall wouldn't have to be changed. This layout is considered best practices for non-center tapped heater circuits. If there is a center tap, then the 6.3 supply still goes to the pilot light and then to the tubes. Of course, if the noise isn't coming from the heaters, then other than generally providing for quieter operation, this isn't going to help. I share your reluctance to take it somewhere ... what the heck, part of goal here is to take the tech out of the equation in the first place.

PT: Depending on how long the hold-down bolts are, you might consider buying some grommets, back the bolts out, and slip the grommets in between the PT and chassis to act as vibration dampers. If the bolts aren't long enough, you can always replace them with longer. I don't know of a way to balance the secondary AC but unless it's wildly out, I don't see this as being a sort of noise.

Anyway, good luck ... hope this helps. John

 

[badansdill]

Wow man, thanks for taking the time to draw all that in!! Yah I am considering
doing just that, so i am torn. would be as easy as lifting the grounds along
the front of the 12AX7's (2 ground points) and continuing that ground rail along the
6V6 tubes to pick up the other grounds, then on back to the other
side of the bulb! Now that you have drawn that out I see how much simpler
that would be. IN fact, because each ground was terminated on an unsued pin
of the 6V6 tubes, I can snip the jumper that goes to ground, solder in 3 wires
and be done. I could do it in a half hour, good show and tell!
peace,
chris

 

[capnjuan]

.... would be as easy as lifting the grounds along the front of the 12AX7's (2 ground points) and continuing that ground rail along the 6V6 tubes to pick up the other grounds, then on back to the other side of the bulb ...

Yes; that would collect up all the grounds and satisfy the need to ground one side of the pilot light provided that, when finished, there is only one point at ground on the one side of the 6.3V service. If the studs holding up the ground bus in front of the 12AX7s are grounded, you'd have to insulate them or else you are back to mulitiple grounds.

... because each ground was terminated on an unsued pin of the 6V6 tubes, I can snip the jumper that goes to ground, solder in 3 wires and be done.

I think I'm getting confused ... if you don't go 'center tap', then you have to carry a ground from each 6V6 pin #2 (or pin #7 ... whichever was formerly grounded) or jump from tube to tube and then back to the pilot light. Is that what you mean?

I think anything you can do to reduce the number of 6.3V grounds is a good idea. just speculation but I wonder if the grounding scheme in this amp isn't something left over from the 98RT which used 6__7s in the preamp; heaters ran on 6.3V but were not internally wired 'humbucking' like 12AX7s. Anyway, good luck with the amp. J

 

[Bill Ashton]

Hey there...

Been away from this forum for a while, but as a 66-J owner ('61 or so ???) I will chime in.

Got my amp originally in '68, tremolo was loud and throbing then; after a long period of disuse through the late 70's and 80's, tremolo disappeared. Complete cap job did not help. Recently, the effect has come back on its own, not as deep as I recall, but gives a nice surfy shimmer to my Jag.

As to the speaker, I vote for a Weber Signature Series AlNiCo. Have one in use in that very spot, so as to save the (date coded) '59 Jensen. Sounds great.

This amp also does not break up, or just barely does, until full volume...even with old Delco 6V6's...a recent try with reissue Tung Sols brought quite a bit of dirt to her, not sure if I liked it or not, but I went back to old GE's and Amphenol 7025's.

Good luck with your amp!

 

[capnjuan]

... after a long period of disuse through the late 70's and 80's, tremolo disappeared. Complete cap job did not help. Recently, the effect has come back on its own, not as deep as I recall ...

Hi Bill and welcome back! I wonder if re-tubing might have knocked some corrosion off the pins of the oscillator tube or socket? Usually when stuff goes, it doesn't come back but, who knows. This is an excerpt from the 66J schematic; if you want a copy, send me a PM with your email address. The small-value caps are often the no-trem problem. The circuit is always 'on' whether the trem is invoked or not so these caps are constantly exposed to fluctuating DC which, over time, wears them out. Assuming the footswitch and input jack are clean and the tube is fresh, you might consider looking here for a more pronounced trem effect. John

 

[Bill Ashton]

Hello Cap'n! Good to be back. Long period of computer problems...

Anyway, I have the schematic that came with my 66-J, inside the top back panel...well, a copy anyway...seems little brother had to use the original for a note on our room door back in the old days...

My tech says the original schematic was note completely accurate, so I have a copy that he marked up as well. Will look at both to find those capacitors. As I recall, I did clean the contacts on the footswitch jack, and although I rarely ever used it, I also tried that to see if the tremolo would come back. Nada...

Interesting to hear the OP speak of his search for hum. Mine is dead silent, except for a sound not unlike the rustling of a fieldmouse inside your wall in wintertime on one channel. I've determined its not a preamp tube, so presume it to be a noisy resistor which will probably take me time to find. Not really a problem at volume, but a little annoying at front-porch level.

 

[capnjuan]

... My tech says the original schematic was note completely accurate, so I have a copy that he marked up as well ... Interesting to hear the OP speak of his search for hum. Mine is dead silent, except for a sound not unlike the rustling of a fieldmouse inside your wall in wintertime on one channel ...

Hi Bill; chances are that Guild, like everybody else, made subtle electronic changes that never made it to the documentation ... rev 1, rev 2, rev 3 on and on. Chris' amp and yours might not be grounded the same way or share that bus bar in front of the 12AX7s - do you have that bus too?

To me, it looks like something left over from designs using 6SL7s / 6SN7s / 6SC7s that Guild used in the 98RT and a few other models where these types also had 6 volt heaters but weren't internally wired humbucker like the 12A_7s that are in his amp. Anyway, this might have been one of those deals where our well-intentioned meddling produced unintended muddling ... what with the road to hell being paved with good intentions and all.

If you followed the yack above, one side of his 6.3VAC fed the pilot light and then went to ground; the other leg fed all the heaters from one side and then over to ground ... more like DC in a model train ... instead of having pins 4 and 5 tied and one leg hitting 4/5 and the other hitting pin 9. I can appreciate him wanting to hang onto the original scheme but if the source of the hum wasn't in the new caps, then the next best guess was the heaters ... but who knows.

That mouse must get around ... it rustles in my stuff from time to time too. :wink: J