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Fall/Winter EL84 Monoblock Project-Some Questions


thebes

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After a long hiatus from tube building due to work, family medical issues etc., I’ve decided on a project for this Fall/Winter.

Several years ago I acquired a 6BQ5 mono amp in a lot with some other stuff. It uses a nice beefy Triad power transformer and an equally massive Triad OPT.  Over the last two years of diligent hunting I’ve finally acquired a spare set of the same, and now have the elements needed to construct a pair of monoblocks.  While poking around the web I came a cross a DIY  build site that led to the discovery that the existing monoblock is an exact build from an article published in a 1961 issue of Electronics world. It’s called “The Compact Hi-Fi Power Amplifier.  Here’s a link to the article:

 

http://diyaudioprojects.com/Technical/Papers/Compact-Hi-Fi-Power-Amplifier/

 

The only difference is my monoblock uses the Triad S31 OPT with 4,8 and 16 ohm taps and the rca input has been moved to the back of the chassis.

Now I’m sure that using the elements I’ve gathered I could build a very nice modern circuit, but I’ve decided to stay with the original build unless you guys advise me otherwise.  I’ll beef up the power supply caps a bit to 32/32, and add a three pronged cord, fuse and probably star grounding.  Although given the simplicity of the build, the latter may not even be necessary.

Now I do have a few questions.

 

 

Should I use a larger project box to better isolate the components from one another?I will be swapping out all of the resistors and should I stick with carbon or go with metallized?  The same holds true for the C1 andC1. Something simple like orange drops or something pricier?I have one original 15H, 75Ma choke.  Should I just buy one more or two brand new ones?  Hammond does make this value of filter choke. Also does anybody got any feel for brands of 6C4 that might sound better than others?

 

Anything else I’m not thinking of?

 

 

 

 

 

 

 

 

monoblock2.jpg

monoblock3.jpg

6BQ5-EL84-Compact-Hi-Fi-Power-Amplifier-Schematic.png

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HTTP Error 404 - Not Found...dead link Marty.

 

Does the existing amplifier work? 

If it works and sounds good, maybe build a copy of the same amplifier.

Maybe the output xformer on the existing amplifier could be turned 90 degrees in relation to the choke and PS xformer, but it may not matter.

 

Stick with orange drops until you get things going, and then maybe upgrade coupling caps down the road. For appearance, two similar looking chokes would look best, but maybe you can mount the chokes under the chassis. You could go with carbon comp resistors like the existing amp, or go with carbon film or metal film. Maybe metal oxide for the larger wattage.

Worry about a good strong 6C4 before rolling them.

 

R6 in the schematic must do the signal inversion...not too sure...

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21 hours ago, jimjimbo said:

Calling Maynard!!  @tube fanatic

I thought I heard a voice in the far distance calling.  The sound must have taken longer than expected to arrive here!

 

That amp is one of many inverterless push-pull types with differential output stage which were popular back in the day.  Although the balance between the output tubes is far from ideal, these amps can sound amazingly nice.  R6 is the shared cathode resistor and must not be bypassed or the amp won't work.  As to the p/s, do not increase C3 to a higher value than 22 uf.  However, since you are using a choke, you can increase C4 to even 220 uf with no concerns.  That will really get the ripple down.  As to the type of choke, unfortunately, the dcr wasn't specified and the true output voltage of the power xfmr is unknown.  That said, a Hammond 156L should be fine.  The p/s output voltage isn't going to be overly critical.  Definitely use star grounding and a 3 wire power cord.  You could move the volume control to the 6C4 grid and simply replace R5 with a 470k resistor.  There are other possible modifications to that circuit, but we can deal with that later if you are not satisfied with the performance.  As to metal film vs. carbon film resistors, I'm not convinced that it really matters except that the former will have much tighter tolerances.  C1/R3 form a high frequency rolloff filter (similar to my famous "ear bleed" filter) which can certainly be useful in shaping the response to your taste.

 

I'm not on the computer much right now, but don't hesitate to ask any other questions that arise.

 

Maynard

 

 

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Gotta love this place.  Thanks for the replies, especially Maynard.

 

Sounds like it should turn out well and I'll be following Maynard's approach, including the minor modifications he proposes.  Sounds like I'll just get a new pair of chokes, and new project boxes or plates and put the power supply cans underneath.

 

I do have a couple more questions but I have to do a little research before I ask them.

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3 hours ago, thebes said:

Sounds like I'll just get a new pair of chokes, and new project boxes or plates and put the power supply cans underneath.

 

If you are going to do that, you might to use as large/deep a project box as you can to keep component layout simple with plenty of room.

 

Does that existing 6BQ5 push-pull amplifier work?

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Well I don't want to go too large.  There are two of them after all.

 

Yup it worked the last time I used it. I did recap it a few years ago.  I've also got a pair of bran new 5Y3's coming.

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2 hours ago, thebes said:

Well I don't want to go too large.  There are two of them after all.

 

Yup it worked the last time I used it. I did recap it a few years ago.  I've also got a pair of bran new 5Y3's coming.

 

Within practical limits or course. 

 

Cool. I like the looks of the GE box plate 5Y3, not the weeny looking plates of the RCA 5Y3 types...I have a bunch of them...

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  • 5 months later...

Well I finally got around to playing with this.  Drilled out the box for one of the amps .  Don't want to drill out both at once in case modifications are needed. Fortunately I have a small drill press which is great for these types of jobs.  Also those step drill bits are the cats meow when it comes to drilling out the holes for the tube sockets.

 

Starting with the power supply and heater wiring first.  Been a couple of years so I just and have two questions.

 

1.  3 cord wall plug hookup.  Please correct me if I am wrong, but the way I have it, is the hot wire from the wall plug goes to the back of the fuse holder, then a wire runs from other side of the fuse holder to the on/off light then to one side of the on.off switch.  Then the other side of the switch goes to one lead of the primary side of the power amp.  Then the other primary lead of the power amp simply goes to the neutral wire of the wall plug.

 

Correct?

 

2.  I have several amps with either a .05 or .1uf wire wound resistor between the 6.3 volt secondary leads.  Is this designed to lower the voltage to 6.3 volts due to higher household voltages?  I notice the schematics never seem to show this resistor.

 

 

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4 hours ago, wdecho said:

1. If I understand you correctly it is right. Here is an example of a tube power supply. http://diyaudioprojects.com/Tubes/6EM7-SET-Amplifier/6EM7-SET-Amp-Power-Supply-Schematic.png   I have never used a power light on a tube amplifier. The tubes themself are a good indicator of having power. There are other examples of PS circuits for tube amplifiers here as well.  http://diyaudioprojects.com/Tubes/tubes.htm

 

2  I am sure you meant .1 ohm not uf  wire wound resistor. I would have to see the schematic to give you an answer but all voltage dropping resistors I have seen on the filaments are in series with one of the supply wires. 

 

Perfect!  That schematic is just what I needed,.  Since you called the wirewound a dropping resistor I'm correct in my assumption. Yes it is wired in series on one leg of the heater wires.

 

Now the original monoblock has run for about 57 years without a lamp, fuse or three cord plug, nor star grounding, but I like the light and Maynard feels strongly about a modern plug and a fuse, the latter which brings me to another question. Any guess what size fuse I should use?

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Hey Marty!  In the picture above I see a pilot lamp/lens.  Being a screw-in type it's probably a #40 (6.3V/150 ma).  You could, if you have a bayonet socket, use a #47.  Alternatively, a NE-2 neon bulb with a 100k to 150k resistor in series with one leg, could be put in parallel with the primary of the power xfmr. 

 

As to fuse size, it would have to be based on the initial turn-on surge and steady state current draw.  You could measure the former with a peak-hold multimeter if you have one.  Another way is to determine the fuse value empirically by using the lowest value which doesn't pop when you turn the amp on!  I can't imagine that you would need more than a 1A slow-blow type with an amp like this.

 

 

Maynard

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Thanks guys.  I think I'll go with a one amp slow-blow to start.

 

Sorry about the confusion Maynard, that picture is the original amp.  I tried re-capping it about a decade ago when I was just getting used to soldering and now after looking at it afresh, I'm afraid to fire it up!  I'll strip it for the build on the 2nd monobloc.

 

Here's some pics of the just completed first monobloc.  I used the three cord plug and star grounding and the lamp I'm using (the same one we used on the Little Gem build Maynard) is rated for 125 volts so I just wired it into the p[imary.  It's pretty tight quarters so the layout's a little bit off.  I'll let it sit for a day (working tomorrow) and then look everything over with again for cold joints, mistakes in wiring etc.  It's amazing to me just how small resistors and caps have become in the last few years.

 

So one last set of questions, I've completly forgot how to do the electrical testing.  Want to put a load on it and check things out before I move onto making the second monobloc. Things like primary power voltage , heaters voltage, voltage test points on the schematic etc.  I really do have to write down the methodology and keep it somewhere. 

EL84amp.jpg

el84amp2.jpg

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Obviously, the usual precautions apply:  no bare feet, one hand in pocket, work on non-conductive surface, etc.

 

Start out by verifying filament voltage with no tubes in the amp.  Clip the meter lead onto one of the 6.3V supply legs and verify voltage (AC) at each tube socket's filament pins.  Unplug the amp and clip both leads across the 5V filament pins (2 and 8) of the rectifier tube socket.  Then plug in and verify that voltage.  Unplug again, set the meter to an appropriate range (at least 500VAC), clip the leads across pins 4 and 6 of the rectifier tube socket, plug back in and verify that voltage.  Unplug once more, ground one meter lead, and clip the other onto pin 4 of the rectifier tube socket, plug back in, and verify voltage is approx. 50% of the previous measurement.  Unplug again, switch the meter lead to pin 6, plug back in and verify once more (the power xfmrs are rarely symmetrical on both sides of the center tap so you are likely to measure a difference).  

 

Now insert the 6C4 and both 6BQ5s.  As above, measure the voltage across each tube socket's filament pins.  If you are using a dropping resistor you may have to adjust its value for 6-6.3V (a 3 watt resistor should cover any anticipated requirement).  Check the voltage on the rectifier tube filament as well.  That may have to be dropped if too high.  If all is well, unplug the amp, and insert the 5Y3.  Set the meter to DC (300V or higher range), ground one probe, and clip the other onto pin 7 of one 6BQ5.  Plug in the amp and look for around 250V.  Unplug again, switch the lead to pin 9 of the same tube.  Plug in and look for around the same voltage.  Do the same for plate/screen voltage on the other 6BQ5, and the 6C4 (pin 1 or 5).  And, lastly, check for around 10V on pin 1 or 3 of each  6BQ5.

 

Some of the steps outlined are redundant, but are good for a learning experience.  I also recommend adding a 220k to 270k resistor ( 1 watt min.) across C4 to act as a bleeder.

 

I typed all of this rather quickly so, if anything doesn't make sense, I invite William, Mike Stehr, or any of the other "tube heads" to make the necessary clarifications or changes.  I'll try to check in later this afternoon or in the morning if time permits.

 

 

Maynard

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Just got home,. and thanks for the detailed institutors. Probably I won't be able to do the testing until tomorrow.

 

It seems to me that this methodology, with obvious changes for types of  tubes, pinouts etc. can be used with many builds.  I'm thinking maybe down  the road I can cobble together a build -an-amp tutorial, have you guys refine it and post it as a sticky .

 

Obviously I could have touched base with Maynard privately for this build, but by these type of posts I hope I can get some folks interested in do-it-yourselfing some tube gear.

 

It's fun and you can set your hair on fire! 

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I've checked my parts stash and I have several one watt 250k resistors so I'll add one across C4 and mark up the schematic accordingly.  I think I've seen this before but can you explain the reason behind it? And what your saying by across is soldered from negative to positive on the can?

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When you turn off the amp the tubes will stop drawing current before the p/s electrolytics are fully discharged.  The bleeder will ensure that the caps will not retain any charge.  Keep in mind that the very high value I recommended is intentional- it will only draw about 1 milliamp so we don't waste precious p/s current availability.  The downside is that it will take quite a while to fully discharge the caps.  So, if you are going to test the amp, and then want to work under the chassis, you will need to verify that the caps are fully discharged before putting your fingers in there.  If you want to test and then immediately work on it, use a 3K (2 or 3 watt) resistor with one end grounded to manually discharge the caps in a few seconds (and be sure to use your meter to verify that they are truly discharged).  The other reason for the bleeder is that you don't want someone to pick up the amp a week after you have used it and mistakenly assume that everything under the chassis is "dead."

 

As for bleeder placement, you can solder it directly across the positive and negative terminals of C4, or from the positive terminal of C4 to ground (if more convenient, you can even go from the screen of either output tube, pin 9, to ground.  C3 will be discharged along with C4 as long as the choke isn't open or disconnected). 

 

Maynard

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Uh Oh. I let some magic smoke out and now I've got troubles.

 

Plug it in and poof, spark smoke at the star ground where everything including the neutral wire for the wall plug comes together.

 

Look everything over very closely and I discover that the lead from the amp to the rectifier that is supposed to be on pin 8 is actually on PIn 2.

 

Fix that.  It powers on but I have voltage on only one side of primary circuit.  Thinking I could have damaged the on/off light or wired the on/off switch incorrectly I simply bypass them and still only voltage on the live (black) side and nothing on the neutral (white side) side. So I'm guessing I blew out one side of the power amp primary.  I test for resistance and get about 7.4 ohms. Do the same for my other power amp, and get about 10 ohms or basically the same. So I should have power on both sides? Right?

 

 

 

My heads hurts, and my pride, and my...

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I need some clarification about things.  Looking at your pics I see the blue power cord lead (neutral) connected to one side of the power xfmr primary.  And, the green power cord ground is connected to the red/yellow striped wire from the xfmr secondary center tap.  I'm assuming that the terminal to which the latter two wires are connected go to the star ground point.  The hot lead of the power cord (brown?) should be going to the spst on/off switch or fuse (it doesn't matter which comes first), and the other side of the switch or fuse to the remaining power xfmr primary lead.  So, if you put your meter between the blue and brown power cord leads you should be reading line voltage.  Likewise, if you measure between the blue lead and the remaining power xfmr primary lead you should also be reading line voltage if the fuse is good.  You are  measuring a low resistance across the power xfmr primary which tells us that the primary is intact.  In your pic of the power xfmrs it looks like the primary is not center tapped, but correct me if I'm wrong (if you give me the model # of the xfmr I'll look it up). 

 

The red power xfmr secondary leads are going to pins 4 and 6 of the rectifier which is correct.  The p/s filter can be connected to either pin 2 or 8 of the rectifier.  Did you verify that the electrolytics are correctly polarized?  Are you getting any filament voltage on the green leads to the driver and output tubes?

 

As a guide for future builds, print a copy of this in case you ever have to figure out which wires are for which winding on an unknown power xfmr:           http://www.radioremembered.org/xfmr.htm

 

 

Maynard

 

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Sorry Maynard, I think I was measuring things wrong. I'm pretty sure I had the wall plug hooked up correctly, but in case I had the color coding wrong I went to the store and bought a regular cord. I'd been re-purposing a computer cord.

 

With the switch and light bypassed I'm getting regular voltages everywhere but in one instances:

 

First the AC voltages without and with tubes plugged in:

 

Heaters:  7.3vac and 6.8vac with tubes plugged in (6Bq5's and 6C4)

Rectifier voltage across both pins:  564 vac dropping to 540vac with 5Y3 tube in, and 282vac on both pins (very close on both sides)

dropping to 540 and 272 each with tube in.

 

However, when it comes to measuring DC on the various tubes I'm getting the following results:

6BQ5's: 8.4dc on  Pin 7 on both tubes

               281dc on Pin 9 both tubes

               8.6 and 8.2DC on Pin3 on both tubes

 

6C4:  36.4 dc on Pin 5

 

So within range except if Pin 7 which contains the leads from the output transformer with the center tap going back to the plus side of c4.  Let me look at that closer in the morning.  Just had a glass of wine with dinner so more electricity for me today.

 

Again sorry about the confusion, but I did get magic smoke.  Oh, and yes the green wall plug wire goes to a grounded lug and that's where I ran all the other ground wires to.

 

 

 

 

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