GA-40t recap started-?'s

[rafe]

I ordered the caps off the schematic. There are big discrepancies in the voltage (Electrolytics) The cans are 10/10 and 10/20 450v I am going to use 16/16 500v that should be OK.  C20 on the schematic calls for 40 150v I bought a 50 150v the amp has a 40 450v. C1 C4 C15 call for 20 25v I bought 3- 25 50v. the amp has 3-20 150v.
I am not sure this has not been recapped before I may have been do you think there is any reason not to follow the schematic also the amp calls for a 2amp fuse and has a 3amp fuse. I'm not too worried but should it be a slo-blo? I have resistors on the way, so I won't start removal until they arrive.Thanks in advance.

[bmccowan]

Throughout the amp you want to use capacitors that are equal to or higher than what is on the schematic. Unless of course you think the schematic has an error. Its not unusual for people to replace 25v rated caps with 50v or even 100v. The rating is what the capacitor will handle. What you bought should be fine. Given the age of the amp, is a pretty safe bet that some caps have been previously replaced.
Fuse, I would follow the schematic and go with the 2amp. Slow blow fuses are less likely to blow on start-up surges. But I have used both with no issues.

[rafe]

Well, I believe you followed that same schematic for your GA-40t and the only cap that is lower is a 16 instead of a 20, I think that should be fine. All the called for voltages are either the same or higher. Even with the 2 extra caps under the hood there will be a lot of free space added. It just surprised me that they would jump so high on the voltages.

[bmccowan]

Yes I did follow the same schematic and checked it against the other available schematics.

It just surprised me that they would jump so high on the voltages

I think that is just a version of "smoke em if ya got em." If you have the right capacitance (or close) and the voltage rating is spec or higher, and it'll fit in the space; if its in the parts bin, solder it in.

[rafe]

That's what I thought. Resistors arrived today (thanks Doug) I watched a couple of videos on orienting the foil side of the caps. I'm wondering if one can rely on the printing on the orange drops.?? If yes what is the rule?? I tried with a cord from the amp and it was dead quiet both ways. In the past I put them in amps without knowing (relying on 50/50 and keeping them the same rt. side negative) and the amps were dead quiet. Maybe just luck ??? any insight is truly welcomed. Rafe

[bmccowan]

You cannot rely on the line on orange drops; it's random. Some folks test caps and orient the caps. They say they can hear a difference. I cannot and install them so that I can read the text. I think most do the same. But its been argued here and elsewhere, so you can read and choose a camp if you want.

[rafe]

I am going to install them as you suggest. I have had nothing but recording grade (ultra quiet) amps after recapping. I am really going slow on this and breaking down the schematic. I am going to reconsidering shot gunning the resistors and at least test them, a lot of them look like allen bradleys , the ones on the tube sockets are ON the sockets so, I'll give them some breathing room (replace).

[shaun]

The way to check orange drops and other non-Electrolytic caps is to hook em up to an oscillator, if you have one handy.

Simply connect the positive and negative leads from the oscillator to the legs of the cap. Then hold the cap betwixt thumb and forefinger, and set the oscillator with your free hand so you have a regular sine wave on the screen. See how high the sine wave reads on the screen, then switch the leads. When you have the leads connected in such a way that the sine wave is lowest (keeping in mind that you want to hold the cap in a similar manner each time, without touching the leads), then you have the cap correctly orientated, so you mark the end with the negative lead for consistent identification.

As I understand it (and someone please correct me if I am wrong), you want to have the negative end toward the connection that is closest to ground; this is because the neg end represents the outer foil, which then sort of extends the grounding up over the cap, thus giving you about half an inch of extra shielding, depending on the length and location of the cap.

No big whoop, I guess. But I suppose if you add up all those caps in the amp, it probably makes a difference. But just one or two? Probably not. But checking caps could be something to do on a rainy day.

[rafe]

The way to check orange drops and other non-Electrolytic caps is to hook em up to an oscillator, if you have one handy.


I don't have one available . I would check them if I did . I understand the potential for noise, and one is on the lists of things I'd like. Thanks

[rafe]

Moving along at a snails pace.....7 down 13 to go. 

[rafe]

I'm as far as the other cap can and I'll pass on that tonight. I almost made a mistake rewiring the first 16/16 cap, so I don't even want to think about doing the next one while tired. It's going good I like it. Looks neater than what was there. I'm pretty sure it was gone over in the past, It was an OK job. I put the speaker under the bench, It's kind of funny but I'm doing this recapped literally on a bench for the AC it's been hot here no AC in the shop. When I fire it up I'll use a 2x10 period correct alnico cabinet until I sort out the speaker .......

[rafe]

Caps are wrapped and the 3 prong is next. I had a 2amp slo-blo so I put that in. I think ( ) Hot to fuse tip, through fuse over to switch to tranny, Neutral direct to tranny. This is what is there now. (see blueprint)

[bmccowan]

+1 on the 3-wire. You may want to secure the filter caps if they seems floppy. But those F&Ts have fairly stout leads - so if a stay at home amp, maybe not needed. Noticed that v7 is not seated all the way in its socket. You'll be getting sound soon! I'll be interested to see how you like it. And if that finicky trem works out of the gate. I had some trouble with mine. but got some help here and fresh caps, resistors, and a strong 6V6 got it going.

[rafe]

I didn't have tremolo .But the foot switch is not with the amp. I fired it up and had it singing for a while and then it faded out (it sounded great but wasn't very loud) something failed, so I'll have to go through the tubes. It's making noise, I pulled the first four tubes and it still makes the noise. it could be one of the power tubes, I hope. Great tone for a spell though.    All the caps are well secured.

[rafe]

Checked all the tubes all were good (The 6sn7gt was ?able) I replaced that. Backing up a little here. I noticed a few bright flashes from behind the output trans when I fired it up full power, up until that point I was firing it up through a limiter. (explains low volume) , I looked and noticed that a ground wire from one leg of the speaker connection(on op tranny) to chassis ground was broken I fixed that and fired it up on limiter, it was ok and sounded good but low volume. I fired it up full power and it would pop pop pop (not motorboating ) kind of quick but random so I shut it down and left it until today when I checked all the tubes for shorts . That 6sn7 was dropping back and forth on the tester when I tapped it so I thought it was that. (But No) I fired it up and pop pop pop and I saw flashes behind the output trans. I turned it around and it was arcing out of the #5 lug . I have 5.4 k ohms across the op tranny from plate to plate of the P tubes.  It temporarily had good volume and tone except for the arcing. I am pretty sure it was replaced long long ago . I'm not sure if it ever worked though. Any thoughts?

[sluckey]

I looked and noticed that a ground wire from one leg of the speaker connection(on op tranny) to chassis ground was broken

Not saying it did but this could have caused major damage to the OT and power tubes/sockets.

[rafe]

I'm pretty sure the ot doesn't have a future! The tubes tested good and hopefully it is confined to the op transformer. It was a solid wire and probably should have been braided.It separated in the middle likely from vibration. Schematic calls for Z3533 10k ohm primary 6 to 8 ohm Secondary and a center tap on the primary

[bmccowan]

That's a drag. I don't use a lightbulb limiter as I rely on a variac on startups. But you may want to wire up a limiter for further testing. It can save some beer and pizza money.
BTW, it'd likely be possible to find a Triad, Thordarson, or Stancor OT on eBay, or the OT from a Hammond organ AO-39 amp works really well in 2x6V6 amps. Those amps used to be damn cheap on eBay, but things have changed.

[rafe]

I am in the process of picking up a variac, I have a lightbulb limiter and it did not indicate a short so I proceeded. I have an old 6v6 chassis out in the garage I'll check it out and see if I can sort it out.

[shooter]

it did not indicate a short

that's because you had an open.
the open possibly caused a fly-back spike that would be hard to see with a meter.  The duration would be small, but the volts would be MANY 

[rafe]

 Z3533 10k ohm primary 6 to 8 ohm Secondary and a center tap on the primary    This is what is called for on the schematic, how can I source this ? I have no way of knowing if the output transformer that they replaced on this amp is the correct one (Triad S-57z Universal Output Transformer 15w.) I have a transformer on an old pa chassis but it was 4 6v6's not 2 .....I'm going to do a crash course on Uncle Doug's channel to help  my understanding on these. I believe it needs a 35.3:1 turn ratio @15watts ??

[rafe]

I just noticed that the transformer called for on the schematic matches the math I used to figure the windings ....A lightbulb moment!It was 35.33xxxxxxx

[AlNewman]

Do uncle doug.  I think the video is called explaining output impedances.

[bmccowan]

I'm not discouraging you from the direction you are taking. But, a shortcut is to look at OTs for guitar amps with similar power tubes and ratings. A replacement OT for a Deluxe or DR would work fine in that amp IMO. As will a vintage OT for 2x6v6. Mercury Magnetics will supply you with the "exact" OT, but at a price.

[rafe]

I really appreciate the input you give on this amp. I have been scouring all the transformer avenues online and The information on them is sketchy at best.I'll re-check the Hammond replacements for Deluxe and D-R again today thanks!

[rafe]

This is probably my best option, I am opting for the paper which is a few dollars more. Decent price 60ish  8.5k primary instead of 10k should drop right in what do you think?

[sluckey]

It will certainly work. But you may need to drill one hole. This OT needs mounting holes spaced 2.813” apart. What's the spacing on the original OT?

[rafe]

Looks like 2-13/16, looks pretty good to me.
I just went ahead and ordered that one and hopefully my variac arrives around the same time. Thanks Steve

[bmccowan]

Hey Rafe - should work great.

spaced 2.813” apart

I can't find that on my tape measure and I don't think it converts to a standard metric measurement either. How does Hammond come up with this stuff?
Rafe, are you going to try to recone that speaker yourself? It seems I need to do mine - its sounding a bit raspy. And I just picked up a 1939 National Dobro amp that has the original field coil speaker - I suspect that might need some TLC too.

[rafe]

Mac, I'm not sure yet about the speaker. My concern would be with the magnet. There was some talk about loss of it. Iwill look into a recone kit for the P12 Q though. I've had some 8" jensens reconed (1959ish) and will compare prices. There is a resistor off the ungrounded speaker leg of the OPT and I have to chase it down. I don't think it will belong on the new OPT It's taped up so I haven't checked the value nor can I see the bands. I see a 470K (R29) it looks like that on the schematic ,but the layout hook-up confuses me.

[bmccowan]

I believe that is a negative feedback resistor - although 470K is unusually high. The next generation GA-40 does not have NFB, but there are a lot of other changes too.

[rafe]

My previous post was 1. concern about the loss of magnetism 2. compare the cost of the 12" kit to the cost of re-coning the 12" . In case that was confusing.  I am confused by R29 as to the way it is portrayed on the layout as opposed to the schematic. In my mind(concept) they are showing 2 different things. I'm sure it's me, but can you explain it ?? R 29 goes back to the cathode on V3, and it does call for a 470k but on the layout it appears to be (Integral?) to the speaker side of the OPT' however on my amp the resistor (which is a 470k y v y) is on the ungrounded speaker wire to the lug of the OPT. I don't understand what the layout shows.

[bmccowan]

reconing speakers - One more option is to buy the speaker parts from Ted Weber - parts have their own page. I haven't priced it out in a while, but was much cheaper than buying a kit from others. Of course you need to figure out exactly what to buy and what to do. YouTube.
R29 - that resistor should go from the junction of the positive (ungrounded) side of the OT and speaker lead to the cathode of that preamp tube. It is taking a tiny bit of that output signal and feeding it back into the preamp. It should not go between the OT and the speaker terminal. If you look closely at that schematic and layout you will see that they agree. If that amp had a speaker jack, rather than a direct connection to the speaker, the NFB loop would connect to the positive tip of the speaker jack, and run back to the preamp. The past replacement of the OT might have involved misplacing that resistor.

[rafe]

 Hey Mac, I see my point of confusion (My Eyes) I blew up the layout and realized that what I thought was a connection were the letters nc and although I haven't really come across them before that I recall, most likely they mean "No  Connection" It is hooked up correctly on my amp I am really digging in to the schematic along with the layout and the amp itself. after I get this up and going. I will have to rely on the schematic and the amp itself (GA-50T) to order parts and find problems. I will check out the weber sight to see if I can get the parts for the speaker from them. It would be awesome to get this amp back up and singing "In Shop" thanks for your help and insight. Rafe

[bmccowan]

NC - It's either that or North Carolina. I think you are safe with no connection.

[rafe]

NC - It's either that or North Carolina. I think you are safe with no connection.

It was "NO  COMPRENDE" until I enlarged it, lol

[sluckey]

When you see NC written beside a switch or relay contact it means normally closed.

No connection or normally closed are the only electronic references I've ever seen.

[rafe]

When you see NC written beside a switch or relay contact it means normally closed.

No connection or normally closed are the only electronic references I've ever seen.

Thanks Steve, Whoever did the computer schematic and layout has provided me a great source of information. I'm indebted, but then they put R29 very close to the OPT and sideways, so with the tiny nc at the center of the transformer it appeared connected to (what I now know is nonexistent) a center lug,  now I feel like the guy who was handed a pair of binoculars to look through and spent the rest of the day with charcoal circles around his eyes !! I gotta go wash my face

[sluckey]

but then they put R29 very close to the OPT and sideways

Yep. Could have been drawn a lot better.

[rafe]

OP transformer came in today and it fits perfectly. I removed it and noticed that the "L2" looked questionable, it appears that when installed (and it appears original) one lead was stressed (not quite long enough) it looks like it pulled through the waxed paper and it also has a lot of play (it's hard to describe this) I can move it up and down about 3/16 " Is there some way I can test its function easily? it's 30henrey@.030 amp. I hope to keep it if possible as it appears to be from 1951 and was sitting around for a good while (Could have been damaged but still used and functional) thanks R

[bmccowan]

I think that its pretty easy to test to see if that lead broke off. Test for resistance across the leads and see if you have an open circuit. Move the lead around a bit to make sure. I think testing a choke for accurate inductance is more involved - beyond my limited knowledge. Others here will know.

[rafe]

First off . I retested the speaker with a 9 volt and it functions fine . I'm going to hook it up to a ga5 tomorrow and see how it sounds. I worked a little on it(GA-40) this evening and hooked up the OPT. I tested the choke and JB welded it for protection and will put a piece of masking tape over it when it hardens. There was a loose connection on a resistor, and I resoldered it and a few other cold solder joints. I am waiting on a variac which should be here mon. I am not sure if the ground on the speaker was broken before I started working on it or arc'd apart when I fired it up. I'm leaning towards existed before.. I also don't know if the OPT shorted out on my watch or was purchased that way .....I will be more observant on the GA-50t when I start working on it. I may(will) remove all the tubes and fire it up on the LB limiter then put them back one by one starting with the rectifier??? Any tips on that and the variac. You know like what approach to take . I have no idea what kind of voltages I am going to be looking for {any ideas.} Mac, I know you repaired yours and Pinball also redid one ....any advice and experience is appreciated and sought. Thanks R

[bmccowan]

Hey, you're getting there. The amp likely had multiple issues when you got it. Every old Gibson I have bought - same thing. So don't kick yourself.
LBL - I've never used one, relying on a variac, and carefully going over the circuit before powering. I'm not suggesting you do the same, just that I don't have that particular experience.
Variac - this is what I do, assuming the amp has fresh or known good filter caps. I plug in the speaker and a guitar. With no tubes in I check to see that I have the right heater voltages. Then I put all the tubes in and plug the amp into the variac. I start at about 40V and if nothing scares me, I bring up the power in stages; 10-20v at a time; at about 80volts I turn up the volume a bit and make sure I'm getting sound. If my most common mistake shows itself now the amp sounds like a dive bomb - missed/cold solder joint. All ok I continue up to about 110 and play the amp for a bit, testing the controls. All ok, I go up to 115v as that is my typical house current here. Then its off the variac and plug er in.
So that's what I do. Some others I know bring an amp up much more slowly on the variac. The idea, I think, is that it is good for the filter caps, even if they are new. But, I am kind of an impatient bloke, probably only taking about 5-10 minutes total for the variac procedure. If I am trying to reuse old filters (I often find old Mallory cap cans to be OK) I take my time with the variac.

[rafe]

I only got 10 minutes out of the speaker. It was distorted but sounded pretty good. It was worth the effort for sure. I guess it felt it owed me that much 

[bmccowan]

Ha - I've been there. I had and early GA-5 with an old Jensen. I replaced the filter caps and fired it up. I thought it was the best sounding speaker I had ever heard. A few minutes later it was toast.

[rafe]

I fired it up last night, on the light bulb limiter and didn't like the brightness, so I shut it down. Tonight I went from the wall through the LBL into the variac and brought it up slowly. I started on 20v and turned on the amp the light came on dimly and went off, 40v on and died down to off, 60v light on and down to dim. Guitar in  (Celestion vintage 30 12") speaker sounded. sounded great but overdriven I played it for a while 5 minutes ..left it on...and then dialed it up 20v's each time, played each stage a few minutes finished up at 115v it is quiet but not clean . I like the tones they are Great but they don't tame down. Tried both channels and they worked as the should mic a little louder and swampy (No Tremolo) but they removed the footswitch. Amp is almost silent at full volume (almost nothing at all) Tomorrow I'll try again without the variac to see if the bulb gets bright but then dims. This will go in while I have original P12Q re-coned.1952 JENSEN P12S 12 INCH 8 OHM ALNICO SPEAKER 

[bmccowan]

be careful with the P12S. I have two that came with a square dance caller's amp. Even he managed to blow them both. They are only rated for around 10 watts I think.
Great that its working. Not sure about using a variac with a LBL. No experience there, but I've read that they do not play together all that well. Might have that confused though.
Not clean - tubes? drifted resistors? I think you changed all the caps, eh? Probably good to make a voltage chart.

[rafe]

I checked all the tubes for shorts and strength they were OK all caps are new I still want to change thePlate resistors and will check them all for drifting (I think it's going to be resistors.) Thanks for the heads up on the speaker, I'll see if I can find some specs. on the speaker as far as wattage goes. I'll post up some voltages over the weekend. I want to see how they disconnected the footswitch. I have a footswitch from the GA-50T and will clone the wooden block and make a new one. Looking at the schematic it should not have affected anything to disconnect it. But shouldn't doesn't mean didn't. I should be able to short it to test it (Kaboom). I'm thinking I should have picked up a Vintage 30 for the amp .....

[bmccowan]

I checked all the tubes for shorts and strength they were OK

That's a good step, but they can still sound crappy. Unfortunately, only way to know is swap in other tubes. But I think you are right to check out the resistors first - they are a hell of a lot cheaper Of course that amp will never get as clean as a post tweed Fender - just not in the genes.

[bmccowan]

I just saw your other thread about the light bulb limiter. I might suggest that the preamp tubes that amp uses are pretty inexpensive NOS on ebay, and even at AES. I'd get a set, and a pair of JJ 6V6s. I always use JJ power tubes when testing amps, so I don't screw up valuable old tubes. Once running right, I might switch to a set of NOS power tubes. But truthfully, those JJs sound pretty darn good.