lego's spin on Sluckey's AC15

[lego4040]

Anyone use Edcor Trannies? Very resonable and Ive read really nice reviews.https://edcorusa.com/

[lego4040]

  Now that I have rest of parts coming in to finish amp, I am at the mercy of putting notes together again for checking volts and the bias. I know this is a cathode bias so it does it itself basically as long as I built it correctly. I spent months researching a creating a manual on this subject for the Blackface Princeton Reverb I was going to build but I went way of the path. Can some one point me in a direction of source for checking this amp? I drew out the pinout for the EL84 so I have a visual, Am I using this as a triode or pentode push pull? can i still put a 1ohm from cathode to ground to measure ma? I seen the other thread hear about the el34 biasing..OH MY ACHING head.
Muchas Gracious

[Ed_Chambley]

Now that I have rest of parts coming in to finish amp, I am at the mercy of putting notes together again for checking volts and the bias. I know this is a cathode bias so it does it itself basically as long as I built it correctly. I spent months researching a creating a manual on this subject for the Blackface Princeton Reverb I was going to build but I went way of the path. Can some one point me in a direction of source for checking this amp? I drew out the pinout for the EL84 so I have a visual, Am I using this as a triode or pentode push pull? can i still put a 1ohm from cathode to ground to measure ma? I seen the other thread hear about the el34 biasing..OH MY ACHING head.
Muchas Gracious

You will be using the pentode push pull.  In triode mode you would not have but about half the wattage.  Lots of people actually make it switchable.  The triode pentode switch.  In triode is great for hifi, but does make a nice half watt switch.

Cathode bias is very simple and there is no need for the 1 watt resistor.

The fixed bias EL34 is not as difficult as reading it seems.  Do it once and you will completely understand how and why.

Then again, I usually bias to sound, but I have ruined a few EL34's doing this.

On the Edcor, I have worked on a low power twin a friend has and it is built with Edcor trannys.  The amp sounds killer.  Very much tweed and he plays a lot of the Stones music and it sounds right to me.

Here is something I had, but let someone else confirm

1. Connect the black meter lead to chassis and leave it there. Then connect the red lead directly cathode of the output tube. This will be the cathode voltage.

2. Divide this voltage by the value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.

3. Measure the voltage on the plates of the power tubes to ground. Write this down.

4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.

5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6V6s, if it is over 12 watts, then you need to install a higher value cathode resistor. If it's 10.5 watts or less, you need to install a lower value cathode resistor.

6. After installing the new cathode resistor, do ALL of the steps again to see what you now have. You may have to repeat this process several times to get it dialed in, but it is worth it, and your ears will thank you.

[lego4040]

Thanks Ed, for step 1 you would measure volts before resistor then after and get your drop from there. Not put leads in front and back of resistor( that would be where u were using a 1ohm resistor say in a fender). If so, got it. The rest of the steps I can follow but will surely ask if stuck. I read somewhere on net when boasting a cathode biased amp the guy used a appropriate potentiometer instead of cathode resistor. When he was happy with the way amp was preforming he shut it down and measured reading on that pot and put a cathode resistor of that resistance. My question would be why not just use a pot there instead? Find sweet spot, put some locktite on it when done and forget about it. If you change tubes then you can tweet again. BTW , UPS screws up my part package and sent it to PA when it was on my normal truck yeaterday I might Rethink the way I have leads thru turrets and do the mil spec way after reading a thread here about to many leads thu turrets

[lego4040]

Here's Uncle doug a explanation of push-pull biasing

[sluckey]

for step 1 you would measure volts before resistor then after and get your drop from there.

You don't have to make two measurements because one end of the cathode resistor is connected to chassis ground. Just connect the black meter lead to chassis and leave it there. Then connect the red lead directly to pin 3 (cathode) of the output tube. This will be the cathode voltage and it's the same as measuring across the cathode resistor.

My question would be why not just use a pot there instead?

A 10W resistor only cost a dollar. A 10W pot would cost 20 or 100 times that!

[Ed_Chambley]

Thanks Ed, for step 1 you would measure volts before resistor then after and get your drop from there. Not put leads in front and back of resistor( that would be where u were using a 1ohm resistor say in a fender). If so, got it. The rest of the steps I can follow but will surely ask if stuck. I read somewhere on net when boasting a cathode biased amp the guy used a appropriate potentiometer instead of cathode resistor. When he was happy with the way amp was preforming he shut it down and measured reading on that pot and put a cathode resistor of that resistance. My question would be why not just use a pot there instead? Find sweet spot, put some locktite on it when done and forget about it. If you change tubes then you can tweet again. BTW , UPS screws up my part package and sent it to PA when it was on my normal truck yeaterday I might Rethink the way I have leads thru turrets and do the mil spec way after reading a thread here about to many leads thu turrets

Sluckey beat me to it, so yea, what he said.  I will revise my instruction sheet as this is much clearer.

Anyway, I have done this only to find the suggested resistor to be very close.  I also am a tube roller and only have 2 cathode bias amps.  One I am going to disassemble this weekend as it had the PT on it I plan to use.  It is a TMB 18 watt Marshall preamp with cathode bias EL34,s it only distorts.  Cannot get much clean at all.

The learning has great value and it is interesting to see how much the tube is dissipating.  Remember, this is all at idle and really does not have much to do when playing, especially the amp I am planning to build.  I usually run smaller watt amps hard keeping the amp volume right at the sweet spot.  Too hard and they usually get a tad too over-driven and muddy.  I do however have an amp room, or closet.  Whatever you want to call it and it is almost soundproof.  I control volume by how far I open the door.

There is only my wife and dogs at my house and neither care as they hear it every day.  I guess I am lucky as many wives do not like loud guitar.  Mine doesn't either, but she lives with it and I live with Dance Moms and a million bottles and brushes in the bathroom.  I believe after 40 in men a new gene appears and I call it the apathetic gene.

You know the difference between ignorance and apathy?
I don't know and I don't care.  I have had waaaaaaaaaaaaaaaaaaaayyyyyyyyyyyyyyyyyyyyyy to much coffee.  I will quit now.

[Ed_Chambley]

for step 1 you would measure volts before resistor then after and get your drop from there.

You don't have to make two measurements because one end of the cathode resistor is connected to chassis ground. Just connect the black meter lead to chassis and leave it there. Then connect the red lead directly to pin 3 (cathode) of the output tube. This will be the cathode voltage and it's the same as measuring across the cathode resistor.

Since you wrote this I assume the resistance from the chassis is of no concern since the resistor is grounded to the chassis anyway.  Correct?

I get confused easily.  What is the difference between measuring across the resistor and measuring from the cathode pin to chassis ground?  Nothing right since they are both connected to the resistor.  Why not just measure across the resistor?

You are just offering another way to do the same thing and clarifying, right?

[sluckey]

You are just offering another way to do the same thing and clarifying, right?

Correct. It's all the same. I like putting the meter probe directly on the cathode at the tube socket because......................

the very next thing I will do is measure the plate voltage. Only a half inch move.   

[Ed_Chambley]

You are just offering another way to do the same thing and clarifying, right?

Correct. It's all the same. I like putting the meter probe directly on the cathode at the tube socket because......................

the very next thing I will do is measure the plate voltage. Only a half inch move.   

Got ya.  Keeps from fiddling around in an open chassis.

 Also, if you use one resistor per tube you can have dual bias?  

Matchless does this.  If you do want to individually bias the output tubes you will want to double the resistance and halve the capacitance to keep the same frequency response.  In this case it would be for EL84 a 260ohm/100uf. 

I have done this before and did not notice any difference and have wondered why Matchless does it.  They also use extremely high wattage resistors, but I guess if you are shelling out the big bucks you deserve extra parts that are very durable.

Anyone else ever found a noticeable difference in splitting up the cathodes of power tubes?

[lego4040]

  Thanks Guys, What I like about Uncle Doug's videos is that they're understandable and I feel like I'm actually sitting down with someone getting learned. I've watched his other vids and really appreciate his time and effort

[sluckey]

Anyone else ever found a noticeable difference in splitting up the cathodes of power tubes?

The only advantage I can think of is you don't have to ASSUME the total current splits equally thru the tubes. You will know exactly how the current splits and if it ain't matched to your specs, then start rolling tubes.

[lego4040]

Like this?
 that's were you would make the changes from original. Where the one cap/ resistor would go I would piggyback and reuse those two turrets

[sluckey]

Like this?

Yes. Is it worth it?   

[lego4040]

Done then

[Ed_Chambley]

Thanks Guys, What I like about Uncle Doug's videos is that they're understandable and I feel like I'm actually sitting down with someone getting learned. I've watched his other vids and really appreciate his time and effort

I clicked over and watched the videos.  The first one on SE amps he really gets a lot of valuable information across in a very understandable way.  That coupled with the second video pretty much wraps up biasing of power tubes.  Good stuff.  A must watch for anyone who wants to understand how to maintain and change tubes in their own amp.  Really, everyone who owns a few amps should learn this.

[lego4040]

FYI, my caps came in and they'll fit nicely. Those Xicon are small, know I have to order (2)270R and (2)100uf/100v for separating the bias

[lego4040]

If you cant get 260 ohm, I, thinking I should get 270 and 250 and test them both or would one be safer then the other. I understand  the 250 will let tubes run hotter and vise versa. Its one thing when your tweaking a pedal and another with 600+volts

[sluckey]

There is an alternative. Look at the pics of my Lightning then look at the layout at the bottom of the page.

     http://home.comcast.net/~seluckey/amps/lightning/matchless.htm

[lego4040]

  So you took the plate voltage and put it in front off the choke and put that to the OT. Then you took the screens after the choke and sent that to the plates on the power tubes. Not sure if I'm missing anything else but what does that exactly do? I'm interested in understanding this other way of doing this

[sluckey]

We're talking about cathodes. Look again.

[lego4040]

Ah, the 1ohms of the cathodes.

[Ed_Chambley]

It looks as if Sluckey's Lighting just has a common cathode resistor and bypass.  The 2 to the left look like screen as in the build it looks as it they are not grounded, but share the b+.  I cannot see how he has separated them if they are.

Look closely at the end of the the sentence where he has the smiley.

He answered the question is it worth it with this.

Yes meaning you understand it.

Is it worth it?  

Watch the smiley.  It is saying it is an idea which seems good at first.  Then after consideration is shakes its head no.

I do not plan to separate them.  I just mentioned it as another way.  I can tell you this, you can get a precise match and bias this way and you can drive yourself crazy doing it.

I have 2 questions.  Is there a way to check the bias of individual tubes in the amp if the cathodes have a common resistor?
and is there any reason to not lower the capacitance to increase the frequency of a power tube like with a preamp tube?

[lego4040]

Ed, the picture I drew is exactly that. it will allow you to test each tube separately, that is why I was going to use the two turrets for original set up, each turret will have its own cap/res piggy backed and loose the jumper on the positive side turrets

[sluckey]

I have 2 questions.  Is there a way to check the bias of individual tubes in the amp if the cathodes have a common resistor?

Look at the Lightning again. Look at the board layout if you don't see it on the schematic.

and is there any reason to not lower the capacitance to increase the frequency of a power tube like with a preamp tube?

Lowering the value of the cathode bypass cap will not increase the frequency. A lower value will DECREASE the bass response. That may make you think the amp is brighter but it's not. It's simply less bassy.

[Ed_Chambley]

I did not see the link to the schematic.  Yes, I easily see the 1 ohm resistors on the schematic.  I do not see a drawn layout, only photos.  I cannot make it out well, but it looks like they are between the 130ohm and cap the 1k5.  In the photo I cannot see pin 3 of either power tube.

Also, your screen resistors are not 5 watt and your cathode resistor is not 15 watt.  Did you think the matchless values were overkill?

I assume it was the most convenient way of doing it an still keep them on the board and keeping a good alignment with grid for a neater lead dress.  Am I correct?

My eyesight is not what it used to be and hunting down 5 stripe resistors is not easy.

Let me re-phrase.  Lowering the value will change the frequency as will raising.  I just never read about people tweaking tone here and wondered why.  Looking at matchless amps it seem they use a higher capacitance as shown here in the lighting.

[sluckey]

Yes, I easily see the 1 ohm resistors on the schematic.  I do not see a drawn layout, only photos.

If you saw a schematic you should be able to see the layout, unless you have forgotten how to do the scroll! The 1Ω resistors are in plain sight on the layout. They are even clearly visible in the photos, if you know where to look.

Also, your screen resistors are not 5 watt and your cathode resistor is not 15 watt.  Did you think the matchless values were overkill?

Absolutely! That 130Ω cathode resistor has 7.5V across it. That's less than 1/2 watt!

[lego4040]

Sluckey I will keep it as is except for adding the 2 1ohms and test ports since that will let me test both valves. Less work

[Ed_Chambley]

Yes, I easily see the 1 ohm resistors on the schematic.  I do not see a drawn layout, only photos.

If you saw a schematic you should be able to see the layout, unless you have forgotten how to do the scroll! The 1Ω resistors are in plain sight on the layout. They are even clearly visible in the photos, if you know where to look.

Also, your screen resistors are not 5 watt and your cathode resistor is not 15 watt.  Did you think the matchless values were overkill?

Absolutely! That 130Ω cathode resistor has 7.5V across it. That's less than 1/2 watt!

I have forgotten how to scroll.  

I can easily see them on the layout and the tip jacks.  I will shut up about this now, but I will remember it.

In another note, I was going through some old boxes here at work and ran across some chassis mount 9 pin and 8 pin ceramic tube sockets and other boxes of electrical parts from the early 60's.  All US made.  I scrounge old parts a lot, but it was surprising to find such nice new parts I am sure we will never see again.
However, I have some kewl tube sockets I will be using for a while.  I was sad for a moment, but everything changes.

Anyway, on my progress for the build.  I have decided to make a head box and a 1, 12 cabinet to hold a Celestion Blue.  I really like white/blond tolex with Oxblood, so I ordered the Tolex, Oxblood grill cloth, the kind without the gold stripe.  Like the early 60 Fender Bassman/Showman amps (even added some ampeg blue grill cloth to finish up my last cabinet).  I am going with short and squatty with the head being the exact width as the cabinet.  24 inches wide with a 14 inch tall cabinet and 11 inch deep.  I have built one this size before with 3/4 pine with a half closed back and the size makes for a real nice sounding cabinet if you are not wanting modern metal.  It helps to pad each side, but that is all since the pine makes for a nice resonance.  I prefer Birch in 3/8 for the baffle.  I think if make a faceplate to match the grill cloth and use cream fender knobs it may look 60's.  I have the picture in my mind, now if I can pull it off.  I decided to break all the rules and use Marshall style Head and Cab with Fender colors for a Vox amp.

Now I have everything but the Output Transformer.  Tone Junkie Bill is correct.  I have a very bad case of tube fever, but I am having a blast.  Be easy on me you Alabama Wild Man.  "Unless you forgot how to scroll"

[lego4040]

You build chassis and cabs, curses. I just don't have time and way to rusty in my wood working skills. I did manage to finish populating board before I left shop. I changed the 3 watt 100ohm to 5 watters and here is a picture of the small .003,.01&.002 red chicklet caps. They fit really nice, tomorrow I hope to get the iron hot

[Ed_Chambley]

Looking good my man.  Yes, I must say my metal skills are superior to wood.  I can make a cab, but I really should let someone else do it.  I have only made one that was good enough to wood finish and it was a easy tweed deluxe style.  They are not that great, but they are strong and I have a good buddy who does car interiors that puts on the tolex for me.  When he is done, they look great.

I am finishing up a complete build while I am doing this one and I also have another one I am working on as well.  When I get it done I will post final photos.  It is a KT88 build and I used red tolex.

[HotBluePlates]

... I changed the 3 watt 100ohm to 5 watters ...

Why make that change?

The EL84 data sheet says the screen is rated for 2w maximum. If you wind up with voltages similar to the 1960 AC-15, you'll have something like 310-320v on your screen. 2w / 310v = ~6.5mA.

Let's assume you used a 5w resistor because you didn't want it warm while dissipating 2.5w (i.e., derating). Current = √(Power/Resistance) = √(2.5w/100Ω) = ~158mA. Verify by finding voltage drop for that current across 100Ω: 0.158A * 100Ω = 15.8v, and 15.8v * 0.158A = ~2.5w.

So the resistor will drop 15.8v, reducing screen voltage to 310v - 15.8v = 294.2v, and Screen Dissipation = 294.2v * 0.158A = 46.5w.

What does that mean? The screen resistor's function is to protect the tube's screen. It does that by dropping voltage as screen current increases, because increased screen current would imply more screen dissipation. The drop in voltage counteracts the rise of screen dissipation directly in the form Volts*Amps, and also indirectly because a drop of screen voltage reduces plate (and screen) current.

Another way the screen resistor could protect the screen is by being rated such that excessive screen current (due to some fault)burns open the screen resistor. Once this happens, screen voltage drops to zero and the tube passes almost no current, which saves the tube. The burned resistor also points you to the section of the amp that malfunctioned.

If you start with the screen current implied by the screen dissipation rating and the known screen voltage (which we found to be ~6.5mA above), you can calculate the dissipation of the screen resistor. 6.5mA² * 100Ω = ~4.2mW. So even a 1/4w resistor is over-rated enough that it might not prevent screen damage.

The situation is different with larger resistances; if you use the ohm's law and the equation for power, you'll find larger resistance dissipates more power when carrying a given current.

Also of this competes with the fact that amp designers often keep the screen resistance low (or zero) to minimize its impact on maximum possible power output.

[lego4040]

There is no emoticon here for a scanners head explosion but it happened. I had to read your response a few times and go back to BYOC and find my discussion with Sluckey. Thank you for bringing that mistake to my attention, I reread it and realized he made the 3w over the 5watt suggestion and I had it backward. I will undo that tomorrow. Hopefully I can get under board wiring and flying leads ready so I can solder it up.

[HotBluePlates]

... I reread it and realized he made the 3w over the 5watt suggestion and I had it backward. I will undo that tomorrow. ...

It's not necessarily something you have to undo...

Since even a 1/4w or 1/8w might be too big to act as a fuse to protect the screen, I wanted to get you thinking about why we choose certain parrts for certain applications.

The 5w Dales certainly look cool (so do the lower-watt versions), but they're more expense than is necessary in this spot, in this amp. My thought is you could think about this in future builds and evaluate which part is the best for the role it plays, rather than buying the most expensive parts and thinking that assures the best performance.

[lego4040]

  thanks. I will have to write those equations done and chew on them for a while to grasp it more. When I was in the apprenticeship we had to pass both the local and international electrical test. In the field you don't use any of that, we don't design the equipment. I'm licensed for operating and repairing, a part acts up I evaluate, test and if it needs replacing I rip it out and put new in. I have HVAC control boards that go every now and then and cost close to $500 bucks a pop, usually it's the dale resistor after the SS rectifier that allows us to run on 24v DC. I usually change the cap and resistor on board and change step down transformer from 120/24v@40ma to a 70ma and that solves the problem

[sluckey]

Finally had a rainy day. Here's the long board...

EDIT... big pic removed.

[lego4040]

How  you get that done so fast? It's gorgeous and you'll sure have it done before the rest of us.  I don't have the long board layout in front of me but I only see 1 100ohm 3 watt resistor?  I will look at it now

[Willabe]

Man Sluckey, that's beautiful work, the layout and the soldering. Even have the output tubes K R up in the air for air flow around it to keep it cool.


            Brad   

 

[lego4040]

You threw in one Carbon Comp for some extra Mojo I see

[lego4040]

I cannot find the single board layout you posted, am I blind? Could you repost please

[sluckey]

but I only see 1 100ohm 3 watt resistor?

There are two. You just can't see one because the big white resistor is sitting high off the board and is blocking the view from this angle.

You threw in one Carbon Comp for some extra Mojo I see

Not mojo. Nobody had a 560K metal film. 

I cannot find the single board layout you posted, am I blind? Could you repost please

I haven't posted a layout yet. But I did post a quick screenshot of Visio to show where the long board was headed. It's had several minor changes since then. See Reply #45 in this thread...    http://el34world.com/Forum/index.php?topic=16851.0


 

[lego4040]

Thanks sluckey, but you still only have  1 100ohm 

[sluckey]

Thanks sluckey, but you still only have  1 100ohm 

You only SEE one. I promise there are two. How's this angle?

EDIT... big pic removed.

[Ed_Chambley]

Sluckey, killer work as usual.  I really kine the long version.  I guess you had time to finish up the visio files as well?  Got a link to the longboard version?

I would really appreciate it.

I went north to the mountains and spending tonight cutting wood.  Should be able to glue up the head and speaker box tomorrow and I will be ready to start my board.

It looks like some nice work coming out on these.  I will be proud to show my finished cabs, but my board is probably not going to look as nice as you guys.  I am going to try.  I do have some red board material tho.

The forum has new colors and stuff.

[sluckey]

Got a link to the longboard version?

It's scattered right now. I'll pull everything into one file and post a link very soon.

[Ed_Chambley]

Thank you.  Got a little tube fever yourself!

I appreciate you work.

[lego4040]

Yeah that's incredible work. Soldering technique is off the hook, something I plan on finishing this week. Thanks for everything. I noticed forum got spruced up

[archaos]

Finally had a rainy day. Here's the long board...

Fortunately (for us) you had a rainy day. 

Your populated board is a real pleasure for the eyes, very neat, awesomest ! 



P.S. : BTW have you forgotten to solder 2 silver mica + 1 poly caps ?
 

[sluckey]

P.S. : BTW have you forgotten to solder 2 silver mica + 1 poly caps ?

I was wondering when someone would catch that! There are two 180K resistors that go parallel with the silver micas. They should arrive in a couple days.

[archaos]