COS 5879 plus

[Auke Jolman]

Hi All,

I'm building the Carolina Overdrive Special 5879 plus (see: https://el34world.com/Forum/index.php?topic=21804.0) version 4-2019 revised. I drew up the layout and Tubenit was so kind to help me getting it up to par.

I've added some photo's to show the progress I made sofar.

One concern I have has to do with the relay power. This afternoon I've completed the powersection and all B+ have voltage on them as do the filaments. I also added the DC-power section for the relays. After rectification the voltage is 8.2 V. I will be using 6v relais so this voltage has to come down a bit. I orderd a few IC 7806 TO-220 so I think it will be ok. If there are other solutions then I would like to hear them.

[sluckey]

8.2V is gonna be right on the edge of the threshold for that regulator chip to operate smoothly. The chip will probably be dropping in and out of regulation which may cause some switching noise. I would be very tempted to just put a 2.2V Zener diode in series with the relay coil. Use a separate Zener for each relay.

[kagliostro]

Nice Job







Franco

p.s.: Those feet are a classic in many tube amp build 

[tubenit]

That looks like it's going to be a great amp!  Very nicely done so far.

with respect, Tubenit

[sluckey]

Good pics. Very nice layout. Looking forward to seeing more.

[PRR]

8.2V won't burn a 6V relay right away.

Most *small* relays can take *twice* the rated voltage for thousands of hours.

Try it. The 8.2V may sag down significantly. (Making the regulator a dubious idea.)

Then throw a 100 Ohm resistor in series. If that puts the relay coil down near 5.0V, try a smaller resistor. But really 5.1V-7.0V should work fine forever.

[Auke Jolman]

Thanks for all the kind words.

The treshold is 8 V so I can see that that is going to be a problem with the voltage regulator.

Maybe the voltages will sag when all the tubes are in. I will see. I have used a kit from Tube-Town in the past with basicly the same setup and it switches just fine for a few years now. The PT has 2 6.3 V secundary windings. The first one is used for the filaments. I'm using the second one for the relais.

Will keep you posted.

@kagliostro: Always trying to make a nice tidy build and then mess up the photo's with a pair of feet

[Willabe]

Very nice work as always. 

p.s.: Those feet are a classic in many tube amp build 

@kagliostro: Always trying to make a nice tidy build and then mess up the photo's with a pair of feet

       

[kagliostro]

This is not an amp, it is part of a cigar box guitar build by *********

(the name obviously is secretated  )



This is an amp (feet are ...... nobody will know  )



 

Franco

p.s.: Lot of times the same happen to me 

[Joel]

Looking good!  I have a layout drawn up.  But life is getting in the way of actually building it.

What size is your chassis?

[Auke Jolman]

@Kagliostro: Always nice kwnowing you're not alone in this world

@Joel: My chassis is 645x175x65 mm    (25.4 x 6.8 x 2.5 inches)

[tubenit]

Here is a PDF copy of Auke's well thought out excellent layout design!  I attached a JPEG screen shot of it, but the PDF version is clearer and easier to read especially when enlarging it.

With respect, Tubenit

[Auke Jolman]

Hi guys,

Finished the wiring today and fired it up without any tubes in it. Checked voltages and all seemed ok. Put the tubes in and then things got very ugly!

No guitar sound was coming throug, only a very loud hum that did not changed with any volume. Did some voltage measuring and noticed there was een lot of sag. With no load B+1 = 422 V, with the tubes in 220V. Then one of the output tubes blew up. The glass was completely in two (see the attached picture).

Any hints as to were to search for problems are welcome. There's also a gut-shot attached.

[Auke Jolman]

The voltage chart is attached. The preamp tubes are inserted, the power tubes ar not.

I messed up with measuring the sag-voltage in the previous post. It was taken at B+4 and not B+1.

[sluckey]

That broken tube was not caused by the amp. Most likely rough handling. I would start by comparing the schematic with your layout and the actual wiring. I did notice the power tube sockets were wired up backwards on the layout diagram but looks like the actual chassis wiring is correct.

[Tony Bones]

Did you try disconnecting the NFB?  I've never seen a tube detonate from oscillation, but I have seen them redplate.

[Auke Jolman]

@slucky: Good to hear that is is not caused by the amp. I did indeed notice the backwards wiring of the powertube sockets. I will edit the layout when the amp is done.

@Tony Bones: I will give it a shot later on in the evening.

In tracing the wiring I discoverd that I had the tailresistor of the PI wired directly to ground instead of going to the Presence control. I will continue with the build comparing to the schematic.

[Auke Jolman]

One part of the puzzle is solved. It turned out that the labeling on the OT does not match with the actual windings of the secundary. Normaly I would guess that black is common, but not so according to the labeling. I wired it up according to the labeling.

I asked Hammond Europe as to the colours and labeling and they replied as follows: "I have just heard from our transformer specialist, they have said the following:

We don't guarantee a colour match. It's no big deal but yes, I think The Green is correct. At the very worst just reverse them. It won't damage the speaker or transformer."

So I switch the green and black wires (black being common now) and now I've some noise out of the amp with my guitar, but that's about it. I've got a lot more troubleshouting to do, but I've got the OT sorted out at least.

[Auke Jolman]

After comparing the wiring etc. to the schematic, I discoverd that I made a mistake with the CT of the 6.3 V filament going to the cathodes of the powertubes. Instead I used the CT of the HT-wires.

I'm not sure how bad that is, but what I found, after making the right connections, the amp played fine for a few moments, then suddenly it gave out. Only when I hit the guitar realy hard, some sound is coming through.

I realy could use some advice as to how proceed.

[sluckey]

After comparing the wiring etc. to the schematic, I discoverd that I made a mistake with the CT of the 6.3 V filament going to the cathodes of the powertubes. Instead I used the CT of the HT-wires.

  OUCH!

Never seen that one before. That basically put a negative voltage on the cathode of the output tubes causing them to draw very high current. The higher the current, the higher the negative voltage on the cathode, causing the tube to draw even more current. This runaway condition likely continued until the tubes died. Hopefully the OT did not give up too. That cathode cap is probably toasted also.

Edit... Hope I'm wrong.

[Auke Jolman]

Thanks Steve, I too am hoping you're wrong. And I think I'm in luck. Didn't get the amp to play yet, but I can hear hiss coming out of the speaker when I turn the volume up. Also I get some noise when I'm measuring the plate voltages.

The miswiring explaines the sag in the voltages I mentioned before. I will change the cathode cap with a new one, just incase. Futhermore I will change the preamp tubes one by one, sust to see if it makes a difference.

[Auke Jolman]

I'm in luck. I connected another amp in the fx-loop and the output section works! So that narrows it down to the preamp section. Tube swapping did not do anything. Will continue the search. I f anyone has some ideas I'm all ears.

[Auke Jolman]

I narrowed the problem down to the second gain stage.

After knowing the PA was working, including the second part of the FX-loop, I wired the input directly to the preceding stage and kept on doing so until I got no sound anymore. This came down to the first preamp tube. I then wired the output of stage 1 to the input of stage 3. Playing the clean en overdrive section seems to be without any problem.

I then wired the second stage back in. The amp works, but after a few seconds the volume drops. When I hit the guitar hard, some distorted sound is heard that dies out very quickly. I have replaced the first preamp tube without any success. I also replaced the coupling cap and cathode bypass cap of the second stage with no result.

At this point I'm not sure what to do next and could use some pointers.

[Tony Bones]

I then wired the output of stage 1 to the input of stage 3. Playing the clean en overdrive section seems to be without any problem.

Obviously, the problem is somewhere in the part of the circuit that you jumped over.

I then wired the second stage back in. The amp works, but after a few seconds the volume drops. When I hit the guitar hard, some distorted sound is heard that dies out very quickly.

That suggests that the problem is probably either a bad solder joint or a bad resistor, both of which can act like a bad/intermittent connection depending on how you push or pull on it. What's likely happening is that the bad connection (solder or resistor) is making good contact when the amp is cold, but as things heats up it pulls the connection apart.

I would reflow all the solder joints in that part of the amp, adding a little fresh solder to each joint as I go and making sure the new solder flows well. The new solder will help fill any dry spots, but most importantly it introduces a little fresh flux.

It that doesn't fix things, then start tapping on the resistors and caps with something insulating like a wooden chopstick while strumming on the guitar. It's helpful to have someone else do the strumming, but I've done both and operated a kick drum at the same time.

[Auke Jolman]

@Tony Bones: Thanks for the advice. Did all you said, but with no positive effect. Also the tapping revealed nothing I'm afraid.

Will continue tomorrow. At this point I'm not sure were to look.

[tubenit]

If a output tube red plates, it can distort in a garbled tone and then go silent. Have you tried disconnecting the NFB?  Oscillation problems can do odd things.  OT wires hooked backwards can do odd things.  Tubenit

[Auke Jolman]

Success!

A good night sleep can do a lot. Went over the schematics again and discovered that I omitted the groundwire for the grid leak resistor of stage 2 (were the boost switch is drawn).

Next thing to do is put the relays in (after I played it for a while of coarse  )

I would like to thank everybody in helping me solve the problems so far.

[tubenit]

Congrats on getting the issue resolved!  Hooray!  Thanks for sharing your success.


With respect, Tubenit

[Auke Jolman]

Hi all,

I've added the relays and made a footswitch for it. The switching is without any noises. Very pleased as to how that turned out. I did use a regulator chip (IC 7806 TO-220 ST) to get the voltages a down to 6V.

I will be uploading some pictures later.

Now it's time to get to know the amp.

[Auke Jolman]

Hi all,

The amp is up and running. I had some trouble with it because I found that it was to bright. Tubenit made some suggestions on how to deal with that.

This evening I decided to take the MV in the clean channel out. I don't know why, but that made a huge difference to my ears. At this moment I don't see the need to change up anything. I did add a post PI MV though.

Thanks all for the interest and replies!

[tubenit]

That is a beautifully wired up amp.  Much neater job then mine is.  Congrats on an incredibly well laid out design. IF you make a sound clip at some point, please share it.

Good job!  With respect, Tubenit