Help with output transformers

[davesans]

Awhile back I was given a old Grundig in pretty bad shape lots of bad caps. Well I figured that the output transformers could be used to make a new amp. Would like to make a SET amp using EL84's. I searched for exact spec on the transformers but came up empty I was able to fine a schematic of the unit and this is where I get a little confused the OT shows a red wire connected to the B+. I not an expert on this stuff but would think that this OT would be better suited for a Push-Pull design all the single end transformers I seen don't have a center tap. This is where I seek your help is this something worth pursuing could I build a SET amp with these or a low watt P-P. I think it would be a great learning experience for me and help better my DIY skills. 

 

Thanks 

Dave

 

 

[tube fanatic]

Dave, can you post the full schematic so I can see the p/s section?  I want to look at something before I comment.  Grundig did some weird stuff back in the day!  Also, what kind of speakers are in that radio (if you have a model # that would help), and do you have an audio signal generator available?

 

Maynard

[davesans]

Hi Maynard 

Here is the power supply I also added another of the output section so you could see where the where the B+ comes from. The uint was part of a console system I removed it for that because it was in real bad shape so no speakers. The model is  Grundig Majestic 5199WE and yes I do have an audio generator 

 

Thanks 

Dave

[mike stehr]

6 hours ago, davesans said:

I not an expert on this stuff but would think that this OT would be better suited for a Push-Pull design all the single end transformers I seen don't have a center tap.

 

Notice the difference in ohms between each leg of the output transformer. The top lead is 380 ohms, the bottom 78 ohms. I'm thinking push-pull OPT primary leads are a bit closer than that when measured between each other in ohms. A step-down winding on the OPT for lower voltage on the screens of the output tubes?

That and maybe a form of hum-bucking?

 

Okay...the 78 ohm indication is right next to the red B+ center tap, and the upper has the 78 ohm indication right next the yellow screen lead...Nevermind, I'm lost...

[tube fanatic]

Actually, you hit the explanation very well Mike!  The lower section of the opt primary serves as a choke between C1A and C1B to get the ripple down for the screen and front end supply.  The 2k resistor in series provides additional voltage drop and also knocks down the ripple a bit more.  It's a fairly elegant way of getting better filtering than with a resistor alone.  RCA used a similar method in many of their table radios of the 40s.

 

Using the signal generator and multimeter it's possible to take an actual, fairly accurate, measurement of the opt impedance.  If interested, I can explain how this is done.

 

Maynard 

[davesans]

3 hours ago, tube fanatic said:

Using the signal generator and multimeter it's possible to take an actual, fairly accurate, measurement of the opt impedance.  If interested, I can explain how this is done.

 

Maynard 

Maynard 

I would be interested I have both a DDM and audio generator. If there is something I could use these OT's for that would a great starting point for me to get back into tubes.

 

Thanks 

Dave 

[tube fanatic]

Let's start with measuring across the red and blue primary connections as used in the Grundig.  Use the xfmr which you have removed from the unit.  Set your audio generator for sine wave output of 5-10V at 100 Hz to 1 kHz (not critical) and, using the DMM set to an appropriate AC range, take an exact measurement of the voltage.  Write down the number and then measure the exact AC voltage across the brown and black secondary leads.  Divide the voltage across the primary by the secondary voltage.  For example, let's say that you had 10V across the primary and 0.4V across the secondary.  10/0.4= 25.  This is the primary to secondary turns ratio (25:1).  Impedance is the square of the turns ratio, so 25 X 25/ 1X1= 625 (625:1).  So, if you connect an 8 ohm load across the opt secondary, the tube would "see" an impedance of 625 X 8/ 1 X 8, or 5K:8.  You can also measure across the full opt primary (blue-yellow) to obtain another possibly useful load impedance.  Let us know what you come up with.

 

Maynard

[davesans]

5 hours ago, tube fanatic said:

Let's start with measuring across the red and blue primary connections as used in the Grundig.  Use the xfmr which you have removed from the unit.  Set your audio generator for sine wave output of 5-10V at 100 Hz to 1 kHz (not critical) and, using the DMM set to an appropriate AC range, take an exact measurement of the voltage.  Write down the number and then measure the exact AC voltage across the brown and black secondary leads.  Divide the voltage across the primary by the secondary voltage.  For example, let's say that you had 10V across the primary and 0.4V across the secondary.  10/0.4= 25.  This is the primary to secondary turns ratio (25:1).  Impedance is the square of the turns ratio, so 25 X 25/ 1X1= 625 (625:1).  So, if you connect an 8 ohm load across the opt secondary, the tube would "see" an impedance of 625 X 8/ 1 X 8, or 5K:8.  You can also measure across the full opt primary (blue-yellow) to obtain another possibly useful load impedance.  Let us know what you come up with.

 

Maynard

Ok thanks I will report back soon my audio generator is a handheld uint I will have to see how my volts it outputs. If it doesn't output enough I have a better one at my work that I can use. 

[davesans]

Ok I hope I did this right I put my audio gen leads on the blue and red wires and my DDM across the secondary leads generator set to 5V @ 1000kHz the reading was 0.156V So 

5/0.156=32 (32:1) turn ratio 32 X 32/ 1X1=1,024 1,024 X 8/1 X 8 or 8K Math not my strong point so if I did it right the primary would be 8K. 

 

Regards 

Dave 

[tube fanatic]

Dave, I presume that you meant 1000 Hz when you measured.  Your numbers are fine and the xfmr appears to be an 8k:8 unit as you said.  That impedance is pretty good for the EL84 and, since I imagine that you want to use them as pentodes, the required negative fb will take care of things nicely.  Since you have all of those Grundig parts, why not pull the power xfmr and simply reconstruct the p/s, driver stage, and output stage?  Sure, it may not be the best amp in the world, but it will sure give you a nice taste of tube sound.

 

Maynard

[davesans]

Thanks Maynard 

I could do that also I am not opposed to using the OT's for something completely different. Seeing the Grunding was in such bad shape when I got there was something living in the console. The other thing are these OP best suited for P-P or SE? I am not completely locked into the EL84 I just have a few of them on hand. 

 

Thanks again 

Dave 

[tube fanatic]

Dave, those opts are single ended only.  For push-pull use, the windings would have to be symmetrical on either side of the B+ feed in the middle.  As to other tubes, off the top of my head an excellent choice for the 8k load would be the 6K6.  It is a terrific audio tube and can provide around 3 watts +/- depending on the opt efficiency.  Another possibility is the 6CL6 which can probably give you around 2.5 watts +/-.  The EL84 has the highest output, delivering around 4 watts.

 

Maynard 

[Marvel]

Maynard, isn't that Grundig done as an SEP? There is only one el84 per channel.

 

Bruce

[davesans]

1 hour ago, tube fanatic said:

Dave, those opts are single ended only.  For push-pull use, the windings would have to be symmetrical on either side of the B+ feed in the middle.  As to other tubes, off the top of my head an excellent choice for the 8k load would be the 6K6.  It is a terrific audio tube and can provide around 3 watts +/- depending on the opt efficiency.  Another possibility is the 6CL6 which can probably give you around 2.5 watts +/-.  The EL84 has the highest output, delivering around 4 watts.

 

Maynard 

Maynard 

The 6K6 looks like a cool tube I especially like the coke bottle ones plus they seem to reasonable priced. 3 watts could be perfect I have a pair of Klipsch Forte's in a small bed room that i use a my audio/hobby room it's only 10'x10'. So where do we go from here?

 

Thanks 

Dave 

[mike stehr]

3 hours ago, Marvel said:

Maynard, isn't that Grundig done as an SEP? There is only one el84 per channel.

 

Bruce

 

Yes Bruce. It's a single ended pentode connected EL84 amp, with feedback. The extra winding on the output transformer is for the screens of the EL84s. (pin 9)

The winding helps with voltage drop and filtering. (ripple) There is the classic feedback loop (with 3.3K R) from the speaker secondary to the cathode of the 12AX7.

[tube fanatic]

14 hours ago, davesans said:

3 watts could be perfect I have a pair of Klipsch Forte's in a small bed room that i use a my audio/hobby room it's only 10'x10'. So where do we go from here?

 

Thanks 

Dave 

No question that 3W is more than enough for your system.  So, do you have experience in working with high voltages in a safe manner?  And, have you built any diy tube amps in the past and have familiarity with setting up the layout?

 

Maynard 

[EMRR]

Yeah, that's an interesting choke section of a SE transformer, and the size looks sufficient to suggest good bass will be possible.  You have lots of power tube choices with 8K:8, so long as current aspect is observed with care.  

[davesans]

I done a few diy projects over the years I rebuilt pair of Dynaco Mk 3 and ST70 with the Curcio mods I also built two Pass diy B1 buffered preamps one is in my system now. Also I am working on a tube pre that I started years ago but put it on the shelf now getting back to it. I can read schematics and should have all the tools I would need. Here a photo of the preamp I build a few years ago. 

[tube fanatic]

It seems like you can handle a build with no problems Dave!  Do you want to use the Grundig power xfmr and, if you do, is it open frame like the opts?  With an open frame power xfmr location will be critical in relation to the opts in order to avoid hum issues.  I recommend using a chassis which allows the former to be at least 6" away from the opts.  And, orientation to minimize induced hum pickup will be important as well.  Also, do you want to use a separate driver for each channel, or a dual tube like the 12AX7?  I'm very tied up with projects right now and won't be able to draw out a schematic for 2-3 weeks, so I hope you're not in too much of a rush.

 

Maynard 

[davesans]

Here is a photo of the Grundig power xfmr I have yet to fully test it I could use it. I also have a Thordarson T 70R61 385-0-385 VA 60 DC ma 70 5v @ 2A 6.3V @ 2.5 CT and a Hammond 270bx So whatever one you think that would work best I am fine will.