Need help wiring a PT without any CT taps

[dude]

I replaced all the filter caps in an old Alamo amp and it came back to life.

The amp has two 6V6s, one 12ax7 and solid state preamp.

I decided to take out the groundless power cord and replace with a grounded one.

The amp has a bridge rectifier and I remember reading somewhere that I don't need a CT for a PT with just two red wires..?

Anyway, I cut out the grounding switch, death cap and wired up as usual using the on/off and standby switch.

The amp has a 120v indicator light.

The fuse blew immediately when I turn on the power.

I've been reading all over the place and can't get much info on how to wire this PT, there are no CT's for the two red wires nor the filaments. I used the two 100 ohm resistors for an artificial CT and elevated the ground.

Is there a special way to wire this PT without a high voltage CT?

The PT has: two red wires, one solid, one red/stripped
                One black and one white wire
                Two green filament wires, one green the other stripped greened.   

Six wires.

I wired as follows:

standby switch - high voltage off the bridge rectifier to the standby, other leg to B+.

On/off switch - black wire from PT to one leg, other leg to center of fuse tip.
                     "Hot side of power cord to side of fuse"
                     "Neutral side of power cord to white wire from PT"

I wired the 120 Volt indicator, one wire to the Neutral side of power cord and the other to the on/off switch.

I've given up searching for a diagram or schematic, any help much appreciated.

See pictures.
   

[Willabe]

Is this helpfull?

Are you using a lamp limiter to test for shorts? There's plans for one in here.

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

        Brad      


 

[dude]

Thanks you, the schematic is slightly different than I wired.

I wired the same but off the bridge rectifier I only have the wire to the B+ ,  I don't have the ground on the other end of the diodes.

What voltage would the 22uf cap be?

Would wiring without the ground and cap on the opposite side of the B+ off the diodes cause the fuse to blow.

I didn't have any shorts till I cut the ground switch and cap out.

The cap that was off the grounded switch was .22uf @600v, can I use that.

Thanks,

al   

[Willabe]

I wired the same but off the bridge rectifier I only have the wire to the B+ ,  I don't have the ground on the other end of the diodes.

You have to have a ground for the diodes. It should go directly to the ground end of the 1'st filter cap.

There is a wire coming off the diodes ground end , but I can't quite tell from the picture where it's going to.

What voltage would the 22uf cap be?

The cap that was off the grounded switch was .22uf @600v, can I use that.

It depends on what the OP tube plate voltage is plus you have to add a little ( 30v to 50v?) extra for surge voltage at start up, 450v might be fine.

No, 0.22uF, is not enough filtering, some common uF values for that 1'st cap would be 16uF, 20uF, 32uF, or 40uF.

I didn't have any shorts till I cut the ground switch and cap out.

Yes, but you have one now if you blew the fuse. It would be best if you build and use the lamp limiter untill you get it fixed, so you don't blow up your PT.      


       Brad      
    

[Willabe]

Hi Al,

After looking at your pics some more, I think your amp does have the FWB grounded, it's wired to one leg of the standby switch with the other leg going to ground.

And it has a voltage/power droping resistor (25R?) before the 1'st filter cap. Which is fine.


      Brad      

[sluckey]

I didn't have any shorts till I cut the ground switch and cap out.

Then you need to be looking at the primary side of the PT circuit. Maybe disconnect the indicator lamp if it is new.

[dude]

There is a wire coming off the diodes ground end , but I can't quite tell from the picture where it's going to.

That wire goes to the standby and then from standby by to B+

I can see from your schematic and info that the problem is I don't have a ground off the rectifier. I'm assuming it was there off the old grounding switch I disconnect.

I not quite clear on where to put the ground, I have no ground. I've attached two pic's to be more clear.

I do have that 25 ohm 5 watt R as you can see in the picture, I assume that creates sag...? the lead off the 25 ohm resistor goes to B+ and the 22uf 500v filter (of course it grounded) and then to the standby to get current when I turn on the standby.

I don't have any grounds off the standby, the problem is I'm not sure where to add the ground. I think this is the problem as there was a ground off the old grounding switch I disconnected, now I cut out that grounding switch and need to put that ground back , somewhere.

As you see the rectifier in the pic, one wire goes that 25ohm r and then to B+ and filter, from there to top of standby.  The other half of the rectifier goes to the bottom leg of the standby, nothing is going to ground.  Maybe I need to attach an additional wire to the bottom leg of the standby with the lead from the rectifier and put that to ground?

Sorry for my ignorance with diode rectification, I never wire a PT without a HV CT, hang in there with me.

Sluckey, I'm with you on the light bulb limiter and I'll use it thanks.

al

[Willabe]

Maybe I need to attach an additional wire to the bottom leg of the standby with the lead from the rectifier and put that to ground?

_NO!_   If you do that it will be a direct short. Give me a few minuites and I'll post a new drawing to show you how to ground it.

Sorry for my ignorance with diode rectification, I never wire a PT without a HV CT, hang in there with me.

Don't worry, it's not a problem. You'll get it fixed.


                 Brad       

[dude]

Maybe I need to attach an additional wire to the bottom leg of the standby with the lead from the rectifier and put that to ground?

_NO!_   If you do that it will be a direct short. Give me a few minuites and I'll post a new drawing to show you how to ground it.

Sorry for my ignorance with diode rectification, I never wire a PT without a HV CT, hang in there with me.

Don't worry, it's not a problem. You'll get it fixed.

 

 .      Brad       

I think I got it, I wait to hear from you before I wire

Looking at the picture above, I'll just put the B+ side of the diodes to the standby switch (simply breaking the circuit between the switch).

Then take the other side of the diodes to ground at the first filter cap.

That way one side of the rectifier is always grounded ans the standby switch simple breaks and makes the B+ side.

al

[Willabe]

I think I got it, I wait to hear from you before I wire

Looking at the picture above, I'll just put the B+ side of the diodes to the standby switch (simply breaking the circuit between the switch).

Then take the other side of the diodes to ground at the first filter cap.

That way one side of the rectifier is always grounded ans the standby switch simple breaks and makes the B+ side.

Yes, yes, yes. You got it now.    

With a CT on the PT, the CT get's grounded. But if you don't have a CT you add 2 diodes and their ground side become your ground, instead of the CT.



I think the 1'st schemo is how you're saying it is now, 2'nd schemo is how you were thinking of fixing/wiring it, if so don't do it!

The 3'rd schemo will work which you have already figured out.          
  
              
                          Brad    

 

[dude]

Yes, that's the second schematic you sent and after my last post I started looking at it and realized that nothing I've said is correct.

So I will wire as the third drawing you just sent.

I realize now without a HV CT to the complete the circuit you have to return to ground off the bridge.

I assume with a HV center tapped  transformer I don't need to use a bridge type set-up but a full wave if I go with diodes?

In the pic I attached, the standby is in the ground side, I'll put it in the B+ side like your third drawing.   
al

 

[sluckey]

Brad, we're gonna have to nickname you QuicksDraw! Good work.   

[Willabe]

I realize now without a HV CT to the complete the circuit you have to return to ground off the bridge.

I assume with a HV center tapped  transformer I don't need to use a bridge type set-up but a full wave if I go with diodes?

Yes and yes.    

Please keep us posted on your progress Al.


                Brad        

[Willabe]

Brad, we're gonna have to nickname you QuicksDraw! Good work.   

Thank you sluckey, that means a lot to me.

Feels good to give back here, no to mention getting something right.     


           Brad       

[Colas LeGrippa]

I assume with a HV center tapped  transformer I don't need to use a bridge type set-up but a full wave if I go with diodes?





With a center tapped PT, you can use a bridge rectifier, but you HAVE TO cut the minus leg of the ss recto bridge. Just use the + side, ( with the two ac sides, of course ) the chassis being the gnd reference stated by the ct.  I presume that the heaters taps won't have a CT either, then you have to create an artificial gnd with a 100 OHM from each side of the 6.3V ( 3.15-3.15 ) to gnd ( chassis ), but I think you know that already.

[PRR]

If you can choose your parts, you can use:

1) Two windings (CT) and two diodes

2) One winding and four diodes.

With costly vacuum rectifiers there is great synergy with the 2-diode plan, even though the 2-windings add cost.

With Silicon diodes it is often cheaper to use the 4 diodes and one winding. Simpler yet to buy the four diodes as a pre-package "bridge".

> With a center tapped PT, you can use a bridge rectifier

Yes, you "can" use two of the four diodes in a bridge. This is a "waste" of two diodes; but bridges can be bought very cheap and one bridge can be easier to mount than two diodes.

There is also a 1-winding 2-diode 2-main-cap "doubler" plan.

[dude]

Well, nice education thank you all.

Thanks for the drawings Willabe and your time. Wish I could pay back but you're probably too far away for me to take out your trash,  

Willabe, I did wire it as your last diagram and yes I got voltage. No fuses blowing but:

I must have blown something else besides the fuse, no sound.

My A/C voltages off the bridge both sides is about 185v

DC at the B+ is 453,  plates of the 6V6s - 442v, cathode & bias r 23v

Screens of 6V6s 389v

To feed the preamp a wire runs from the B+ (453v) all the way to the just before the preamp and is dropped to 189v by a 47K r then dropped to 25v by a 56K large wire wound r. This 25V feeds the preamp which his "solid state". I know nothing about SS pre-amps.
 
But 25v seems way off to me.

Forgetting about the voltage to the preamp being 25v, I can see no reason that the amp doesn't play...? If I turn the volume to 10 I can hear some static guitar very low volume through the speakers.

I tested the speaker and they play through another amp.

So what else did I blow wiring that rectifier wrong..? a 6V6, the OT can't be the PT as the voltages are high but OK even the plates from the OT.

I'm thinking a 6V6 tube..? but I have the correct cathode bias voltage of 23v.  

Update: I tried a know good set 6V6s, it's not the tubes also I have 3.3 on each filament leg not the heaters, I think it must be the OT, could a dead short on the power supply blow the OT or maybe screwing around with out the speaker jack pluggedin ?  it blew it... (Time for a hit of Uncle Jack, ha). I do have another OT but I' need to learn how I blew it. I really appreciate the lesson on diode rectifiers,   

I had this amp singing till I decided to keep from getting shocked and ripped out the ground switch...

If you had enough of me it's OK, LoL

al - BTW this amp is an Alamo Electra 2570 with SS preamp made in 1973 and I can't find a schematic.

[Willabe]

Thanks for the drawings Willoughby and your time. Wish I could pay back but you're probably too far away for me to take out your trash,  

If you had enough of me it's OK, LoL

No big thang, you don't owe me anything. People help each other here and _ALL_ have been good to me with out a doubt. We all learn from each other.

I don't know much at all about SS either, sorry.         FWIW, you did what all of us would do by changing the power cord to one with a dedicated ground for safety. You may still be able to get her up and running, so don't throw in the towel yet. Some one here might be able to help you find whats wrong so you can fix it.

Here's a link about an amp like yours, with SS pre and tubes for OP, might be helpfull to you.

http://www.el34world.com/Forum/index.php?topic=13004.0

You could try and take a resistance reading on the OT primary wires to see if you burned it up. Measure (with the amp off, power unplugged and the filter caps drained of their dcv) from the OT CT to one leg and then from the OT CT to the other and post what you get.

Uncle Jack and an open tube amp chassis don't go together well.    


             Brad      



    

[dude]

Primary resistance readings on OT:

brown leg  221ohms
blue leg    240 ohms

I replaced the  OT, Bang Sound, Fixed !

The bad part is the tone from that old paper waxed OT was much better then this Magic Parts replacement.

Oh, Well live and learn.

I've learned a lot here today, first "always use a light bulb limiter especially when your not 100% sure"
Maybe it was worth the lesson on diode rectifiers.

Anyway, the replacement OT fixed the amp, are the reading above a bad OT? 

I guess I blew the OT along with the fuse with the wrong wiring when I replaced that old cord....?

Thanks again, I'll read that link to solid state stuff, although I never played a solid state amp that I liked except on old Acoustic that sounded real nice, I couldn't believe it was solid state.

Let me know about the readings, I assume there's no way to fix a bad OT?

al

[Willabe]

Well All Right Al!         Nice! Good job!  

I'm not 100% convinced that the old OT is bad, the readings seem ok to me, but...       I wouldn't throw it out yet.

Yes _please_ read that link, PRR posted a reply in it that might be whats going on with your amp, it would explain why your amp is now working after you changed the OT, even though (if?) the old one is fine.

You can always swap another OT in there, our host Doug has very good iron at a very good $$. Nothing wrong with having an extra PT/OT laying around for just this reason if you can afford it. Besides $$ just keep going up,up and up, and iron can sit on the shelf with no problems. Yes sometimes you can get lucky and can fix them, so I've read and you can have them re-wound/re-built, but will it be worth the $$?  

Oh, Well live and learn.

We all make mistakes, sounds like you got off easy. I ordered a Fluke DMM, it took 2 months to get it, $250? I new very little to nothing about this stuff, wanted to measure something with it when I got it finally home, took a champ OT, put a 9dcv battery across 1 side of it to get a reading of what it would be, Smoke, killed it dead.                

I've learned a lot here today, first "always use a light bulb limiter especially when your not 100% sure"
Maybe it was worth the lesson on diode rectifiers.

Yes _ AND ANY TIME _ you first fire up a new build or change/rewire anything in/for the power supply. A good number of guys (much better builders/repair men than me) here have posted about how much it cost them after blowing up their new amp or rebuilding an old amp ($100+ easy) and wishing they would have known to build and use a lamp limiter.  

I lived in Chgo. and played/hung out in some of the blues clubs their when I was younger. I heard and got to play with some really good players. One night a guy named Carlos Johnson was playing and was just tearing it up, really, really on fire!          One of the best guitar tones I've ever heard live, guess what he was playing thru?           Yep, an old (early 70's? with 2-12"s) Acoustic SS amp! I still have a hard time believing it.    

Hope to see you around here some more Al, glad you got it fixed!

(OH and no more testing a tube amp without a load/speaker hooked up.  )

                         Brad              

[dude]

Yeah, I'm still trying to buy that Acoustic amp from this guy, he has no idea what good tone is, It's a two twelve, two channel and sound like ten tubes are in there.

Anyway, if the old OT readings are OK I think I might know what I did. When I rewired the PT to your last schematic I moved the secondaries on the OT to another spot so I could just plug in various speaker configurations.

In doing so I cut out the ext. speaker jack wiring that was wired to the old PT then direct to the speakers, maybe I cut out a ground and was using the wrong wires to the new location...?   

In these pictures I can identify the primary and CT side but not sure about the secondaries..? I remember measuring the impedance of the two tens in parallel, it was 7 ohms. Can you identify the three sec. wires?   Which is ground and what are the other two? How can I test them?

Thanks for your patience's, I'm an old guy too just got bit by all this amp stuff about 10 years ago. I've always been a tone freak and love the sound of a strat through a good tube amp playing the Blues

al.

[Willabe]

Well you did take some good pics. We can figure it out from them. Please post the 2 pics of the OT and speaker ex.jack here that you posted earlier in your first post about this amp.

Dog gone it, I messed this up, went to do a quote and erased over half of the post.            
 
                          Brad      

[Willabe]

I do have that 25 ohm 5 watt R as you can see in the picture, I assume that creates sag...? the lead off the 25 ohm resistor goes to B+ and the 22uf 500v filter (of course it grounded)

I forgot this,
 
Yes it will create some sag, but they put it in to knock down the B+ a little. Measure the dcv where the diodes + side and 25R meet, then on the other side of the 25R at the 1'st filter cap. How much is it droping the B+dcv? Your plate dcv is 442 and that's about as high as you'ld want to go with 6V6's. They might have got a deal on the PT's and it was cheaper to buy them and add the 25R 5w then to buy a different PT.      

Oh and you can't have the 6V6 cathode bypass cap laying on the cathode R, you'll cook it that way. You need to move away a little to keep it cooler, 1/4" at least. Heat rises so if you can move it to the side, that is depending on how the chassis sits in the cab.


                  Brad      

[dude]

I do have that 25 ohm 5 watt R as you can see in the picture, I assume that creates sag...? the lead off the 25 ohm resistor goes to B+ and the 22uf 500v filter (of course it grounded)

I forgot this,
 
Yes it will create some sag, but they put it in to knock down the B+ a little. Measure the dcv where the diodes + side and 25R meet, then on the other side of the 25R at the 1'st filter cap. How much is it droping the B+dcv? Your plate dcv is 442 and that's about as high as you'ld want to go with 6V6's. They might have got a deal on the PT's and it was cheaper to buy them and add the 25R 5w then to buy a different PT.      

Oh and you can't have the 6V6 cathode bypass cap laying on the cathode R, you'll cook it that way. You need to move away a little to keep it cooler, 1/4" at least. Heat rises so if you can move it to the side, that is depending on how the chassis sits in the cab.


                  Brad      

The 25 ohm 5 watt has the same voltage on both sides, I guess 25 ohms isn't enough to lower the voltage on my meter. Yeah, the cathode biasing R is not touching the 250 ohm r but too close for comfort, I'll twist it on it's side.

Talking about the bias in this amp it originally had a 200 ohm -5 watt w/cathode at 25uf. I though that to be too low for 6V6s so I only had a 10 watt 250 ohm. Tubes never red plated with the 200 ohm maybe I should put it back?

I luckily cut the wire from the extension jack leaving a half inch or so, now I can identify how the jack was wired for clues. Looks like when the jack was being used it shorted out the white secondary wire to the OT (middle one) so that was probably 8 ohms to the enclosed speakers. The lighter blue on the extension jack was hooked up to ground so that's probably the "ground". I'm assuming this OT has two taps, 4 and 8 ohm so the other dark blue is probably 4 ohms. If this is true I diffidently wired the new located jack "wrong" I wired without a ground - just using the two taps, ha nothings going to play doing that. So the PT is most likely good and I can't see that dead short in the PT causing my OT to blow, the fuse went in a second.

The only thing is looking at the OT pic, there are two tabs with wire soldered to them and one goes inside the windings so I would think the tabs are 4 and 8 ohms and the wire inside is ground but the blue wire is going to the ground on the extension jack...?

I think I'll hook the OT back up using the blue as ground and see what I get from the others, I can always change the wiring around if I get sound.

I wouldn't expect anyone here to follow all this, it's all on me, I took it apart without taking a lot of notes.

One other thing, the magic parts OT has one sec @ 8ohms, from spec sheet. One wire is green and one is black, which is ground the Green or Black. I know it doesn't make a difference playing one speaker but I have two - wired parallel.

The voltage is too high for my comfort, could I use a string of three 15v 5 watt Zeners off the B+ to standby to lower the voltage? That would probably effect the 25V to the SS part but not too much.

I'll practice on this circuit learning and will probably gut the amp in the end, get rid of the rats nest - true PTP and put in an 18 watt two channel tone/vol with tremolo all on a turret bd. like a real amp, all tube.

Maybe a VVR too in the grounding switch hole.

Any answers appreciated, I know I'm all over the place though.

Thanks again for all your help.

PS - in the pictures of the ext jack it looks like they used some of the primary wire scrapes but the light blue sure looks like ground, it went right to the OT. Nothing soldered to the OT just a tag strip that broke off.

al          

[Willabe]

Well you did take some good pics. We can figure it out from them. Please post the 2 pics of the OT and speaker ex.jack here that you posted earlier in your first post about this amp.

The pics from this 1'st post.    

http://www.el34world.com/Forum/index.php?topic=13041.0

I luckily cut the wire from the extension jack leaving a half inch or so, now I can identify how the jack was wired for clues. Looks like when the jack was being used it shorted out the white secondary wire to the OT (middle one) so that was probably 8 ohms to the enclosed speakers. The lighter blue on the extension jack was hooked up to ground so that's probably the "ground". I'm assuming this OT has two taps, 4 and 8 ohm so the other dark blue is probably 4 ohms. If this is true I diffidently wired the new located jack "wrong" I wired without a ground - just using the two taps, ha nothings going to play doing that.
The only thing is looking at the OT pic, there are two tabs with wire soldered to them and one goes inside the windings so I would think the tabs are 4 and 8 ohms and the wire inside is ground but the blue wire is going to the ground on the extension jack...?

I don't think that's right. There's 3 wires on the secondary. 2 go to the extension speaker jack, 1 to the switch contact and 1 that goes to the ground on the extension speaker jack, when nothing is plugged into the extension jack it shorts to the tip contact and there's an orange(?) wire that goes back to the speakers hot terminal, this will work.  

But when you plug in to the extension jack it opens the switch and now there's no signal to the speakers + (hot) terminal.

Plus there's also another/different 3'rd wire on the OT secondary that goes to the ground of the speaker.

I don't see how this works. I see 2 ground wires, (1 to the ex-jack and 1 to the speakers ground) and 1, 8R tap, not 2 taps, (a 4R and an 8R?) and 1 ground, can't be right.  

If you take an ohms reading of the OT secondarys it will help to figure this out. Measure the resistance of all 3 pairings and post them with their colors.


             Brad      

[Willabe]

Unless the speaker is/was wired backwards?     


                      Brad       

[sluckey]

Maybe this will help...

http://s670.photobucket.com/albums/vv61/Big-Eds_photos/1974%20Alamo%20Electra%202570/

[sluckey]

After looking at the unhacked pics on photobucket, I see several differences in your wiring. First off, the standby switch should be connected to the negative side of the bridge. That means the other side of the switch should be connected to ground. It seems that the OT secondary common 'may' also be connected to ground on the STBY switch. This may explain why the OT died when you connected B+ to the STBY switch.

Maybe these pics will help you get it wired as it was. I highly recommend you spend $15 for the schematic at this website...
http://www.musicparts.com/products.asp?Company=Alamo

[dude]

After looking at the unhacked pics on photobucket, I see several differences in your wiring. First off, the standby switch should be connected to the negative side of the bridge. That means the other side of the switch should be connected to ground. It seems that the OT secondary common 'may' also be connected to ground on the STBY switch. This may explain why the OT died when you connected B+ to the STBY switch.

Maybe these pics will help you get it wired as it was. I highly recommend you spend $15 for the schematic at this website...
http://www.musicparts.com/products.asp?Company=Alamo

Before I hacked the amp the OT was not hooked to any part of the standby but your right the bridge ground was hooked to the standby. I had changed the standby wiring to switch the B+ line and the ran the other half of the bridge to ground as Willabe's schematic that says this will work.

But after wiring Willabe's rectifier diagram, I cut out the ext. jack wiring and wired the OT to a jack on the back of the amp. I'm pretty sure I wired it wrong and that's why the OT didn't work. Now that I disconnected the wiring on the OT, I have no idea what's what. I read that applying low voltage ac to the primary I'll be able to distinguish the seconday's ohms.

It's always the simple amps that can give me problems....

I either get the schematic for $15 or just build an 18 watt Marshall type vol/tone and tremolo channel which has the same face plate as this amp.

I'd like to use the this old OT, I don't think it's blown

I have about 240 ohms from each plate wire to CT
And the two plate wires have 462 ohms 

dDes this say anything about the OT working?   

The secondaries are all .4 ohms between them except the `one has .7 ohms

al

[sluckey]

Refer to the attached pic. Connect one meter lead to the common wire. Measure resistance to the other two lugs. The lug with the lower resistance will likeky be the 4 ohm tap. The lug with the higher resistance will likely be the 8 ohm tap. As you've already seen, the resistance readings will both be verrrry low.

You can connect a voltage source (6.3VAC) to the primary. Connect one meter lead to the common wire. Measure AC voltage on the other two lugs. The higher voltage will be the 8 ohm tap.

Just from your resistance readings so far, I think the OT is good. I'd put it back in to be sure. Especially since you like the sound of it better than the replacement.

[Willabe]

sluckey, did you see these 2 pics in here? Reply #1. They show clearly how the ex-jack and OT were wired to the speakers.

   
http://www.el34world.com/Forum/index.php?topic=13041.0       

                 
                 Brad     

[sluckey]

I saw those. Good pics, but they don't clearly show all I'd like to see.

[Willabe]

I saw those. Good pics, but they don't clearly show all I'd like to see.

OK.

I'm not 100% convinced that the old OT is bad, the readings seem ok to me, but...       I wouldn't throw it out yet.
                         

     

[Willabe]

This is how I think it's wired from Al's pics.

If the WHT and BLK are 4R and 8R taps and BLU is common/ground, I don't see how this works and I don't see how this works when you plug into the ex-jack. I'm missing something.


       Brad      

 

[dude]

This is how I think it's wired from Al's pics, don't know if the second speaker is in parallel or series.

If the WHT and BLK are 4R and 8R taps and BLU is common/ground, I don't see how this works and I don't see how this works when you plug into the ex-jack. I'm missing something.


       Brad      

 

That blue wire did go to the ground on the ext. jack.

Two speakers in parallel for 7 ohms (each speakers is 13.5 ohms)

Another thing, I remember plugging in speakers in that ext. speaker jack and both internal and external speakers played....?

The stand-of on the OT used some scrape wire to go to the ext. jack so the colors may be off.

Again the Blue wire ran directly to the ground on the ext. jack

al 

[Willabe]

Again the Blue wire ran directly to the ground on the ext. jack

I belive you Al, I just don't understand how/why it worked wired up like that.    

Sluckey will figure it out. Please post the readings he asked you for.


                  Brad    

[sluckey]

Just to be sure I'm caught up with you guys...

1. The amp is fixed now, right?

2. You are just curious about the status of the old OT, right?

[dude]

Refer to the attached pic. Connect one meter lead to the common wire. Measure resistance to the other two lugs. The lug with the lower resistance will likeky be the 4 ohm tap. The lug with the higher resistance will likely be the 8 ohm tap. As you've already seen, the resistance readings will both be verrrry low.

Using the blue wire on the secondary as common: I get .3 ohm (white) and .6 ohm (black)so 4ohm (white) and 8 ohm (black).

I'm going to wire the OT back in using the Blue as common and the 8 ohm tap

Brad, I posted all the readings here.

I do have an 6.3vac transformer I use to break in speakers so I could hook that to the OT and get better readings but at this point I;m go with what I posted above and let you know what I get.

Thanks, all

[dude]

Just to be sure I'm caught up with you guys...

1. The amp is fixed now, right?

2. You are just curious about the status of the old OT, right?

Yes, it plays OK but some ghost notes on the new magic parts OT that weren't there before but switching the OT plate leads may cure that...?

But I'm going to put the old OT back now and post.

Thanks you, Sluckey and I promise no more hack jobs from me, ha. (Pictures if needed and a limiter on start up).

[PRR]

> If the WHT and BLK are 4R and 8R taps and BLU is common/ground

BLACK is "common" but NOT grounded.

Taps are:

Black: "zero ohms"
White: 8 ohms
Blue: 16 ohms

The internal speaker does not have to be grounded.

The external speaker jack bolts to the chassis ground (or even if plastic it should be grounded).

So they grounded the 16-ohm tap, let the "zero" tap float, and floated the speaker on the zero and 8-ohm taps.

The connect back to 8-tap is through the EXT jack switch. When you plug-in, the 8 is broke, the current is SERIES through internal speaker and external speaker.

Assuming 8-ohm external, this gives the nominal 16 ohm loading; if external speaker is 4 or 16 the _series_ connection keep the load between 12 and 24 ohms which is close-enuff.

If you wish to omit an EXT jack, then simplicity suggests grounding the "zero" tap and tagging Wht=8 and Blu=16 for connection to internal speaker or impedance switch or jacks. Do note that this flips polarity so if the amp has NFB from speaker side you need another polarity flip elsewhere.

[Willabe]

Thanks PRR. I think I get it now.

Is there a reason why the designer choose to wire it this way, other than personal preference?


          Brad     

[dude]

> If the WHT and BLK are 4R and 8R taps and BLU is common/ground

BLACK is "common" but NOT grounded.

Taps are:

Black: "zero ohms"
White: 8 ohms
Blue: 16 ohms

The internal speaker does not have to be grounded.

The external speaker jack bolts to the chassis ground (or even if plastic it should be grounded).

So they grounded the 16-ohm tap, let the "zero" tap float, and floated the speaker on the zero and 8-ohm taps.

The connect back to 8-tap is through the EXT jack switch. When you plug-in, the 8 is broke, the current is SERIES through internal speaker and external speaker.

Assuming 8-ohm external, this gives the nominal 16 ohm loading; if external speaker is 4 or 16 the _series_ connection keep the load between 12 and 24 ohms which is close-enuff.

If you wish to omit an EXT jack, then simplicity suggests grounding the "zero" tap and tagging Wht=8 and Blu=16 for connection to internal speaker or impedance switch or jacks. Do note that this flips polarity so if the amp has NFB from speaker side you need another polarity flip elsewhere.

Thanks PRR but someone needs to decipher that info for me.

I did wire up the old OT, it does work. I used the Blue as common (ground ?)  Thought the white would be 4 ohms and the black 8 ohms but maybe 8 and 16..?

I have strange tones over riding the notes as they ring out, especially on the high bass notes on the sixth string.  They weren't there before I started the hack, ha.

Perhaps I should wire the Black as common or ground or zero ohms, and white as 8 and other as 16 as PRR suggested. Before I try to get the over-riding notes out, I'd like to be sure I have the  OT wired correct.

I had these over riding notes with the other new OT too, would switching the plates leads from the OT help?

al


EDIT... untangled quote...sluckey
    

[PRR]

> strange tones over riding the notes as they ring out

Bad connections, sick filter caps.

[dude]

> strange tones over riding the notes as they ring out

Bad connections, sick filter caps.

I'll be: I wired the old OT - Black (zero ohms, ground?) to common on the speaker jack, white wire to tip (for 8 ohms) no ground to chassis (just a hanging fender type jack) and the amp played. I think you nailed it. The white is 8 ohm the blue is 16 ohm.

I don't understand why the amp played using the blue as ground too but when I used blue as ground or common I also grounded the jack to chassis but not when I use the black as common. Should I ground the black, as when I put the jack back on the chassis it will be grounded?

The strange notes are gone. I ran the cathode and biasing R to ground at the first filter and used the original biasing set up, 200 ohm - 5watt and 10uf @35 watts. I had changed them out as I thought the 6V6s would run too hot. A 250 ohm worked OK but going pass 150 uf across the r causes the strange tones....? can't figure that one out. Usually the higher the cathode cap the tighter the amp gets.

Anyway, I want to hook up the ext. jack again as it was originally but can't follow your explanation, at least not just yet.

Maybe I can twist Brad's arm on a another "quick draw"


Thank you,

al

[Willabe]

Al, it was wired like this;

BLACK is "common" but NOT grounded.

Taps are:

Black: "zero ohms"
White: 8 ohms
Blue: 16 ohms

The internal speaker does not have to be grounded.

        Brad      




Edit; I added the ohms values to the schemo.

[dude]

Al, it was wired like this;

BLACK is "common" but NOT grounded.

Taps are:

Black: "zero ohms"
White: 8 ohms
Blue: 16 ohms

The internal speaker does not have to be grounded.

        Brad      




Edit; I added the ohms values to the schemo.

Thank you so much, everyone.

Brad, the schematic's were fantastic. The amp plays great.

The over-riding notes were the speakers. Here, I'm changing components and the speaker is the problem. We forget what we take for granted.

This thing sounds great and to top it off. I plugged in a Celestion Hellatone that I blew before and didn't expect to work, it did.! So, two things fixed, can't beat that.

Thanks again,

al       

[Willabe]

Glad you got it fixed Al.      

Sluckey and PRR are both top shelf guys, be very hard pressed to find any better.  Those 2 guys have both probably forgotten more than I'll ever know about amps and electronics.    

FWIW, if it were my amp (and it's not) I would leave it alone other than maybe trying a few different speakers.

If it sounds even pretty good as is, and you say it sounds great, then I'd __leave it__ as is.

I'd be worried about messing up the ss pre and not being able to fix it. No schemo, might not be able to find drop in replacement transistors, could be a mess.

And/Plus I would _not_ try to run a big jolt from a boost pedal or pre amp in to the front end, might blow up/over load a transistor.

If you still feel you would like or need something different, then build something else.          

Lots of pre-punched chassis around, and our host Doug has a good number in his store. He also has a new program on the site here where you can put in where the turrets/eyelets would go and he will drill the holes on with his CNC on a fiber glass board cut to the dimensions that you send him. Nice!

Here's the link;

http://www.el34world.com/Forum/index.php?topic=12317.0

I had a Musicman RD50 (?) years ago, SS pre and tube OP. It was a pretty darn good sounding amp. I have an old friend, Jimmy Johnson, Chgo. blues guitar man/singer/band leader, he's 80 and he still plays out a lot. He's been playing through a Mm RD50 for a good time now and he sounds great through it. The Mesa boogie he played through for years got to heavy for him to carry out to gigs. He told me he really loves the Mm.
            

                 Brad      

[Willabe]

The over-riding notes were the speakers.

Save those speakers, they might sound great in a different/lower power amp. Just make sure you store them in a dry (heated/AC) place, not a damp basement or an unheated garage or shed, to protect the paper cone and the paper voice coil former.


             Brad      

[dude]

The over-riding notes were the speakers.

Save those speakers, they might sound great in a different/lower power amp. Just make sure you store them in a dry (heated/AC) place, not a damp basement or an unheated garage or shed, to protect the paper cone and the paper voice coil former.


             Brad      

I'll keep the amp as it is now, sounds nice and Fendery clean and when you start to crank it, it slowly starts to crunch, a great blues amp just a little past when the distortion starts.  When I got it the surrounds were shot and mice had eaten part of the cones, I got a couple of new surrounds and fixed the one hole with a coffee filter and glue. I didn't expect them to work as well as they did but after a while they were breaking down, voicecoil rub I was hearing. The GH1230 sounds great.

The custom turret bds are nice I didn't know Doug made them, years ago he made a 5F6 loaded board, it was nice.

Thanks all again, such a cool looking amp too

al