First post tweed twin questions

[tfrost]

Hello all,  I have been lurking here for quite some time and finally decided it was time to join.  I enjoy building amps, but would not consider myself a guru, not even close.  I am probably on par with BobL....  I can read a schematic but prefer the layout diagrams.  I use both actually to help me understand the circuit.  Currently I am building a tweed twin amp 5E8A the low power version.  BobL is a good friend of mine and ex bandmate, he also recently built one and I followed his threads and also looked at others threads regarding this circuit.  Mine is the Mojo kit, minus speakers, cab, and tubes.

With that said I have a couple of questions.  Regarding the power tubes 6L6's....  I have built a few El34 Marshall clones now, but have ZERO experience with 6L6 tubes.  As I was beginning my research I noticed there are several versions of this tube.  For instance, 6L6CC, 6L6G, 6L6GB, 6L6WGB (if these letters are incorrect forgive me, just trying to make the point that there are different 6L6's).  I tried finding information on the characteristics of each but have had no luck finding anything thus far.   Can anyone shed some light on the characteristics of each of the different 6L6 types? 

Also, one more thing.  I am considering adding a bias pot.  I have never had a fixed bias amp.  It seems that it stands to reason, that with different B+ voltages for every single amp (at least in my experience I have built two of the same amp and their B+ were substantially different) that going with a fixed bias won't necessarily give you properly biased tubes/amp?  Right or wrong?  Is the reason for this, once you find what you want your amp biased too you can essentially just leave.  Does bias still drift in a fixed bias amp after some hours of use?  At which point it would need to be monkeyed with again? 

I know the easy answer is just add the bias pot, but what are some advantages of fixed bias?

Thanks guys....

[eleventeen]

The difference between diff designations of 6L6 is more an issue of age and improvement of the type over time. The tube came out in the 40's I guess, and was a powerful tube for the time. You could go buy a straight-sided metal one or a little later, a 6L6G, bowling pin shaped. The glass one could get rid of some more heat than the metal one, so was rated somewhat higher powered (in terms of plate dissipation)  There were two sizes of 6L6G, one big, like an older 5U4G, then a smaller one, both bowling pins.

Later came the 6L6GA, which I myself have never seen, although I *have* seen a 6L6GAY (no comments, please) which was also a bowling pin shape, brown base, I assume ruggedized for military use. Maybe in the mid-late 50's the 6L6GB came out. This is essentially today's tube. Then the "C" = a watt or two more dissipation. In the meantime, 5881's came out, not sure when.

You can always sub a "C" for a "B" and a "B" for an "A" or either type of 6L6G. The only thing one has to know about these is you don't want to be placing 6L6Gs into Fender amps because they were rated in the high 300 volts for plate and any Fender has 400+ on the plates. Plus the early ones probably aren't as rugged so when you kick them around or vibrate them with power chords, not so good. I bot a huge lot of tubes and have a bunch of the smaller 6L6Gs in Sylvania, JAN. I built a tube matcher to try to sell them for better value and tried matching them...they were incredibly all over the place even though they were NOS mil-grade tubes.

As for your bias question, be sure you understand the diff between "fixed" bias and "cathode" bias. Fixed bias makes little sense given B+ and tube variations Cathode bias uses the voltage drop across a resistor to "regulate" the bias of a tube so it is more "set and forget". Plenty of amp mfrs like or liked the cathode bias because it resulted in fewer blown up tubes = warrantee repairs. Plus it doesn't use a [more expensive] pot.

I have no comment nor opinion on the sound value of one over the other. Obviously, if you like a sound you get by having your tubes biased "hot", then you're free to do so, but not with cathode bias.

[HotBluePlates]

... 6L6 tubes.  As I was beginning my research I noticed there are several versions of this tube.  For instance, 6L6CC, 6L6G, 6L6GB, 6L6WGB (if these letters are incorrect forgive me, just trying to make the point that there are different 6L6's).  I tried finding information on the characteristics of each but have had no luck finding anything thus far.   Can anyone shed some light on the characteristics of each of the different 6L6 types?  ...

If you're buying new production tubes, the letters may have no significance other than country of origin, convenience to manufacturer or marketing. That is, different brands can and do take a 6L6-like tube and slap any label on it they please.

But let's assume we're talking old-production tubes from the 30's through 70's. The type number breaks down as below:

6L6        - original 19w metal-sleeved version of the tube.
6L6G      - slightly later 19w version with a glass ST (coke bottle) envelope
6L6GA    - 19w updated vesion of the 6L6G, still with ST glass envelope (I'm unsure exactly what improvement was made to this version)
6L6GB    - 22-23w version of the 6L6GA with straight-sided glass and higher plate voltage rating
6L6WGA  - "W" added to other suffix indicates a military version of the tube; this tube often labeled 5932, a Sylvania 6L6 type with very large base and special rigid internal structure.
6L6WGB  - "W" added to other suffix indicates a military version of the tube; this type is generally the same as an original 5881.
5881      - Industrial-numbered 6L6 type with 23w plate, compact plate structure and envelope, originally intended for high-altitude performance (in bomber electronics)
6L6GC    - final updated version with 30w plate rating and increased maximum plate voltage rating.

KT66     - Euopean version of the 6L6; the 6L6 is a beam power tube, but the KT66 is a "kinkless tetrode". Each was produced to sidestep the patent on pentodes. Rated for 25w plate dissipation.
EL37     - Sometimes said to be a 6L6 variety, but it notably different. This is a true European pentode with some notable characteristic differences, but otherwise in the 6L6 power class. Performance is a cross between a 6L6 and EL34.

You can see pictures of the different types on this page.

...  It seems that it stands to reason, that with different B+ voltages for every single amp (at least in my experience I have built two of the same amp and their B+ were substantially different) that going with a fixed bias won't necessarily give you properly biased tubes/amp?  Right or wrong? ...

Inside a single amp, the B+ will stay substantially constant over time, because you have the same one powe transformer and supply.

...  Does bias still drift in a fixed bias amp after some hours of use?  At which point it would need to be monkeyed with again?  ...

But while B+ may stay consistent, tubes may not. Obviously swapping tubes may result in a new set that wants a little different bias voltage than the previous set. Depending on whether the tubes are burned-in, their characteristics may not be very stable, and could drift. To a cetain extent, all tubes will drift over time.

When will it "have to be" monkeyed with? If the output tube plates glow red at idle or during use, then the tubes are running too hot and the bias needs to be adjusted. Other than that, it may not need adjusting. I *might* check idle current with installation of a new set of output tubes in an amp I own, or I might only peek in the back and be sure they're not redplating. You can check idle current as little or as much as you like.

If it were a customer's amp, then I'd check with a tube installation and be sure the bias is set to a reasonable value.

Arguably, old amp designers assume a set (and non-adjustable) value of fixed bias in their designs, adding adjustment only to balance current between sides of a push-pull stage.

[HotBluePlates]

... what are some advantages of fixed bias? ...

One advantage: Peak tube current can be significantly more than double idle current, allowing the output stage to operate deeper into class AB (more output power). Additionally, higher B+ can be used with "deeper AB" and fixed bias allows reducing idle current to stay within tube dissipation limits.

Disadvantage: Tube bias is not tightly pegged to tube current; cathode bias is self-limiting and correct cathode resistor value makes it impossible to have runaway tube current.

[HotBluePlates]

... seen a 6L6GAY (no comments, please) which was also a bowling pin shape, brown base, ...

"Y" with any other American tube type indicates a "low loss Micanol base" which is a light brown color.

"X" with any other American tube type indicates a ceramic base. I once saw a 6L6GX sold on Ebay, and should have bought it just for curiosity value. It was ST shaped with a white ceramic base.



BTW, my post of types doesn't preclude Eleventeen's post; I'd just typed it all up and wasn't willing to just throw it away! 

[tfrost]

I'm not sure I understand what this means?  Or at least why that is a disadvantage?  Thank you for all the reply's that helped a ton, and I went with some NOS 6L6GC tubes that according to the guy I got them from date to the 60's.  I have plenty of old pre-amp tubes to try and I also got some others for this build to try out. 

Disadvantage: Tube bias is not tightly pegged to tube current; cathode bias is self-limiting and correct cathode resistor value makes it impossible to have runaway tube current.
[/quote]

Thanks again guys....  Great resource!

[HotBluePlates]

I'm not sure I understand what this means?  Or at least why that is a disadvantage? 

If everything is working properly, it never comes into play.

A tube's bias is the net voltage between the grid and cathode, and tube current is reduced by making the grid more-negative than the cathode.

When the coupling caps between the phase inverter and output tubes leak, some amount of positive voltage gets through and appears at the output tube grids. That makes the grid-cathode voltage less-negative than it was, and tube current increases. If the leaky cap is bad enough, a lot of positive voltage sneaks through and the tube current increases a lot.

If you had cathode bias, that tube current would have to go through the cathode resistor. More current through a resistor = more voltage across the resistor. If the tube's current increased due to coupling cap leakage, then the increased voltage across the cathode resistor would make the grid more-negative compared to the cathode voltage, and would tend to decrease tube current.

This shows that cathode bias tends to offset a fault condition that would cause tube current to rise.

Let's assume the worst: a short-circuit of some kind causes tube current to skyrocket. The cathode resistor has a definite power rating, and the voltage across it times the current through it is the power dissipated by the resistor. With a big enough current (and a properly-sized cathode resistor), the resistor burns up and becomes an open-circuit, which kills all tube current. You could design that to happen at a current too small to damage the output or power transformers.

Fixed-bias is not inherently self-limiting, and will cheerfully let the amp go up in smoke.

[tfrost]

Ok....  So finally finished this Tweed Twin build.  First off let me say I think the amp sounds awesome.  Very woody, organic, and full!!!

BUT...   With that said I have an awfully strange bias measurement?!?!

My B+ is 490.  

All other voltage measurements seem to be pretty much on the mark.  

With my meter set to millivolts, black lead grounded, Red lead probing pin 8 of V5 and V6 and with a 1 ohm MO resistor from ground to pin 1 to pin 8 on each V5 & V6.  I get 17.9 mvdc at V6 and 16.4 mvdc at V5.  

It seems I should be getting a larger reading here if I am doing this correctly.  And here is how I am coming up with my number.  6L6GC power tubes at 30 Watts max plate dissipation times 0.7 for 70% of max plate dissipation and then divided by a B+ of 490 should be 42.8 milliamps or in this case 42.8 mVDC.

Where do I start looking?  Should I even worry about it being that it is technically reading a cold bias and the amp sounds great?

Again with that said the amp sounds incredible.  Very clean with a good break up the farther up you crank it.  It is loud but nothing even close to my 100 watt superbass that I built last year.  It should be said that I am running Weber 12A125A's in this amp, so 30 watt per speaker.  

Thanks guys

oh and P.S.  I went with fixed bias due to a screw up of sorts (lets just say I had installed the board prior to adding an adjustable bias pot and I didn't want to take it all apart again to get under the boards.  So I have the 56k bias resistor in series with the diode and a 3.3k resistor.

In order to get a hotter bias do I just replace the 3.3k resistor?  and what direction value do I go to?  Higher or lower?

[sluckey]

Yes, that's biased very cold. How much negative bias voltage is present on pin 5 of the 6L6s?

If your bias circuit looks like the 5E8A bias circuit, you would need to increase the value of that 3.3K to run the tubes hotter.

[HotBluePlates]

My B+ is 490.  

...  I get 17.9 mvdc at V6 and 16.4 mvdc at V5.

... I have the 56k bias resistor in series with the diode and a 3.3k resistor.

Are you sure that resistor is Orange-Orange-Red (3.3kΩ) and not Orange-Orange-Brown (330Ω)? Is the 56kΩ a Green-Blue-Orange and not Green-Blue-Yellow (560kΩ)?

What is the bias voltage that you measure at pin 5 of the output tubes? Is it reasonably close to -32vdc as shown on the schematic?

490vdc for you B+ is high, but may be due to low tube current (hopefully you don't have a transformer with too much voltage output).

Assuming you find that the resistors are the correct values (3.3kΩ, 56kΩ), then what you need is less-negative bias voltage to get more idle current. You could either make the 3.3kΩ bigger or the 56kΩ smaller. The easiest way to do this is wire a pot across the 56kΩ or in series with the 3.3kΩ. Read on...

You might try a 1MΩ pot across the 56kΩ resistor, with the left lug (looking at the back with lugs pointing up) connected to ground and the middle lug connected to the junction of the 56kΩ and rectifier diode. With the amp off, make sure the pot is turned "full-up". If you measured resistance with a meter from the pot/diode/56kΩ point to ground you should get closer to 56kΩ, not 0Ω. Connect your meter to measure millivolts across the 1Ω resistor. Turn the amp on and *slowly* turn the pot "down" until the tube current increases and lands on your desired idle current.

Now turn the amp off. Carefully measure resistance of the pot/56kΩ and find its present total resistance. Remove the pot/56kΩ and replace with the nearest standard value to what you measured.

You can do the same thing with the 3.3kΩ, but since its value needs to go up, the added pot would be in series, not parallel. A 1kΩ pot would probably do nicely.

Pick one or the other (adjust 3.3kΩ or 56kΩ); you don't need to do both.

[tfrost]

Thank you...  I will head down and check it all over to make sure I didn't misread a resistor, and double check all my voltages again, then report back!

And yes i copied the 5E8A schematic and layout diagram precisely with no deviations.  I will try and get a pic up for viewing as well.  On a side note I built the cab installed the tweed and lacquer all myself and it turned out looking pretty dang cool.  The cab is bigger than a normal twin, and I put the speakers side by side like the high power version.

Thanks again gents!

[tfrost]

"What is the bias voltage that you measure at pin 5 of the output tubes? Is it reasonably close to -32vdc as shown on the schematic?"


OK so...  I measured -55vdc at pin 5 of V5 and V6.  That seems way out of range as well....

[Willabe]

I measured -55vdc at pin 5 of V5 and V6.  

That's why it's running cold, too much -bias. Bring it up to ~-32 or so and see what you get for a current reading then. Make sure you watch the output tubes as you do this, if the tubes plates start to turn red shut down the amp. Plates turning red means the tube is drawing too much current.

Figure out why your -bias is high and after you adjust it check pins 5 at both tube sockets with the tubes out and make sure you have
-dcv on both pins in the range you need before you put the power tubes back in.


            Brad      

[tfrost]

I measured -55vdc at pin 5 of V5 and V6.  

That's why it's running cold, too much -bias. Bring it up to ~-32 or so and see what you get for a current reading then. Make sure you watch the output tubes as you do this, if the tubes plates start to turn red shut down the amp. Plates turning red means the tube is drawing too much current.

Figure out why your -bias is high and after you adjust it check pins 5 at both tube sockets with the tubes out and make sure you have
-dcv on both pins in the range you need before you put the power tubes back in.


            Brad      

Can I do that by simply replacing the the 3.3k resistor with a larger value?

Thanks for all the help so far guys!

OK scratch that question, I answered it myself and that answer is yes....  I did the adjustable pot trick to find a resistor value.  As close as I could come to the -32vdc neg bias  was with a 10k resistor in place of the 3.3k(its all I had that was close, my next lower was 5.6k "not enough" and next higher value I had laying around was 47k "too much")  This 10k resistor put me at -40vdc.  I am going to install power tubes and see what I get!

Edit #3 ;)  Ok that looks like it did it.  V5 is now reading 41.4 and V6 is now at 45... 45 is a bit higher that the 70% but close enough for me.  I will run it hot I guess and replace tubes as needed!  Should be ok though I think.....

Thanks again fella's!!!

[tfrost]

Ok so a sound check resulted in near perfect operation.  Amp still sounds great.  However, I noticed that I am getting a loud and noticeable rattle in one of my 6l6GC power tubes.  It is as if it is rattling internally...  Anyone ever run into that issue?  I suppose I am going to need new tubes now?

I noticed it before also, particularly on certain notes.  I have owned a lot of tubes and never had this happen?

After some internet searching I might have to do some more troubleshooting.  However I have taken a cloth in one hand and applied some slight pressure on the V6 tube and the rattle goes away.  Definitely sounds like it is internal in the tube...

[HotBluePlates]

However, I noticed that I am getting a loud and noticeable rattle in one of my 6l6GC power tubes.  It is as if it is rattling internally...  Anyone ever run into that issue?  I suppose I am going to need new tubes now?

Does it come through the speaker?

Microphonic output tubes manifests as a rattle (rather than a whistle or howl). It generally doesn't hut anything as long as the noise is acoustic and not coming through the speaker.

If you can apply pressure to the tube and stop the rattle, are you certain there's not something loose on the amp? Is the glass tube envelope loose on the 6L6 base? If the glass is loose, it can be glued with a high temperature RTV sealant.

[tfrost]

It is definitely NOT coming through the speaker...  I checked all the socket connections, tight and sound.   Mostly on certain notes, I'm sure due to the frequency of certain notes causing a rattle.  With the amp turned off, I can tap the tube and here rattling inside it that sounds just like what I'm hearing when I play.  Can't hear it at all if you crank the volume up ;)  This sucker is way louder than it was before with cold bias.   

[tfrost]

OK...  so after a week or so of playing the amp I have some concerns and am not quite sure where to start?  First off the amp sounds incredible so no issues there.  But if I crank the amp, and I mean crank it, I start getting an electrical odor that seems to be coming from the power transformer.  As soon as I started smelling it I turned the amp off.  This does not seem to occur when I have the amp at reasonable volume levels. 

Any ideas?  Or are there any readings I should post before asking such questions to assist in finding if there is an issue?

I have built a couple amps before, and I also work in a construction trade that deals with electrical circuits and transformers and I KNOW this smell, and it usually is not good!  Usually it is because something is shorted to something or to ground that should not be, but typically that makes them go poof quickly.  And isn't that what my fuse is for?  My fuse has not blown, it is a 3 amp fuse.

With that said I am an amateur amp builder!

[HotBluePlates]

When that happens, are you looking in the back of the amp at the tubes? Are the plates glowing red (not the heaters, where you might see a spot of orange at the top of the tube, where the heater sticks out from the cathode sleeve)?

Odor from the PT, huh? When you shut the amp off, does the PT feel HOT? Not just warm, but almost instant-burn?

Do you hear any arcing noises from the PT?

Can you play with the back panel off, and watch the PT to see if you see anything odd when the odor happens? Is there anything obviously-burned (darkened resistors, melted wire)?

I might be used to the smell of old dust in amps, and "tubes burning"... I expect some smell when you have hot output and rectifier tubes. But burning resistors, etc do make a different smell. A careful visual inspection will probably turn up any actual fault.

[tfrost]

When that happens, are you looking in the back of the amp at the tubes? Are the plates glowing red (not the heaters, where you might see a spot of orange at the top of the tube, where the heater sticks out from the cathode sleeve)?

Yes, All appears normal.  Although the tubes get quite hot, however nothing out of the ordinary.

Odor from the PT, huh? When you shut the amp off, does the PT feel HOT? Not just warm, but almost instant-burn?
 No, I checked that right away because thats what I thought too.  But cool to the touch, so I felt relieved when I touched it.  Also checked the choke and output transformer for heat and nothing there either.

Do you hear any arcing noises from the PT? No

Can you play with the back panel off, and watch the PT to see if you see anything odd when the odor happens? Is there anything obviously-burned (darkened resistors, melted wire)?

 Yes, played with back panel off and no indications of anything unusual visually speaking.  Double and triple checked my wiring, and also inspected for fries resistors, proper diode orientation.  I also thought of checking the speakers.  They are brand new Weber P12Q Clones the 12A125A's at 30 watts each.  After a google search I found somewhere that people have smelled this coming from speakers?  I have never heard of this before.  I'm not convinced about that yet?

I might be used to the smell of old dust in amps, and "tubes burning"... I expect some smell when you have hot output and rectifier tubes. But burning resistors, etc do make a different smell. A careful visual inspection will probably turn up any actual fault.  I know this smell also, the amp is a brand new build of which I did the entire cab as well.  No dust yet ;)  Unless my nose is tricking me I am pretty sure its that tell-tale fried electrical odor, and when I move my nose in the direction of the power transformer the smells gets stronger.  All the test readings were right on, except at first when I had cold bias, but I fixed that problem.  Still 490 B+ though, seems high.  Could that have anything to do with it?  Again, only when the amp is cranked and I am pushing it hard.  Although I have not tested yet to see if it does this with out a guitar plugged in?  Meaning I noticed while really ripping it up with the amp full up.  I never just let the amp sit idle (not playing) while it was cranked up.  I will try that tomorrow and see what happens.  Just did another quick search and it seems that others have had similar problems and it was a bad PT out the gate....?  Maybe?  Its brand new from Mojo....

Thanks for the response HotBluesPlate!!!

[HotBluePlates]

Your B+ is still 490v? What kind of power transformer did you use? What kind of rectifier are you using? 490v seems way too high for an amp that's supposed to be at about 415vdc.

If you smell something, but there is zero visible damage or malfunctioning, I'd be inclined to think everything is okay.

[tubeswell]

... what are some advantages of fixed bias? ...

One advantage: Peak tube current can be significantly more than double idle current, allowing the output stage to operate deeper into class AB (more output power). Additionally, higher B+ can be used with "deeper AB" and fixed bias allows reducing idle current to stay within tube dissipation limits...

Also in fixed bias, you get 'more bang for your voltage buck' that you do in cathode bias, because the plate-to-cathode voltage is greater in fixed bias.

[tfrost]

Your B+ is still 490v? What kind of power transformer did you use? What kind of rectifier are you using? 490v seems way too high for an amp that's supposed to be at about 415vdc.

Ok just double check it (the B+) and it is exactly 484vdc.  The transformer is from the stock low power tweed twin kit from mojotone.  I guess I am not sure what brand that would be.  I have used a lot of heyboers in the past, but I don't think the mojo kit has or comes with heyboer trannies?

If you smell something, but there is zero visible damage or malfunctioning, I'd be inclined to think everything is okay.

I'm thinking you are correct.  I just turned it on, cranked all of the knobs to 12 with no guitar plugged in and just let it sit while I watched it for anything unusual.  Nothing... and no odor... But I did discover something interesting while doing that.  I decided just to tap on tubes, all of them to see what would happen.  V2 has a very definite hum as soon as I touch it, and if you take hold of it between your fingers and just slightly move it around, you still hear that hum and it fluctuates slightly with the movement.  It would sound exactly the same as if you had your guitar cord plugged into the amp and then stuck your finger on the end of the cord that should go into the guitar.  That same exact sound.  Like a ground sound.  If that makes sense?

Anyway, I think I will chalk it up to being crazy!  I don't know, I will continue to play the amp and see how it responds each chance I get to crank her up.  I will be sure to bring my back up amp to gigs.

I appreciate all your guys help!!!

[tfrost]

... what are some advantages of fixed bias? ...

One advantage: Peak tube current can be significantly more than double idle current, allowing the output stage to operate deeper into class AB (more output power). Additionally, higher B+ can be used with "deeper AB" and fixed bias allows reducing idle current to stay within tube dissipation limits...

Also in fixed bias, you get 'more bang for your voltage buck' that you do in cathode bias, because the plate-to-cathode voltage is greater in fixed bias.

I wonder if all these years I have been mis-understanding what fixed bias vs. cathode bias actually is?  I am going to post some pics of this build, give me an hour or so...

[tfrost]

Okay here we go...  Posting some pics of the amp.  Let start by saying this was a really fun build.  I purchased the mojokit minus speaker and cabinet.  I went to home depot and bought a length of 12" wide by 3/4" thick pine and built the cab and then applied the tweed and lacquered it all myself.  It was my first time doing it and there are some mistakes, its not historically accurate and all that, but its unique to me.  Enjoy!  and if you see anything out of place let me know ;)  Next up is Tweed Deluxe build!!!  If anyone wants more close ups of this amp to assist them in their build just let me know and I would be happy oblige.

[HotBluePlates]

The 5E8-A is fixed bias.

Later Fender amps like the Deluxe Reverb or Twin Reverb had adjustable fixed bias (a pot and resistor in place of a single resistor in the 5E8-A's bias supply).

If the amp has a resistor in the range of several-hundred ohms per tube between cathode and ground, that's cathode bias.

... what are some advantages of fixed bias? ...

When I wrote that, it was "... advantages compared to cathode bias." Maybe you're thinking about "fixed" fixed-bias... meaning non-adjustable.

Start backwards: think about cathode bias, then non-adjustable fixed bias, then adjustable fixed bias.

For all of these, the amp designer selected output tubes, a transformer and a set supply voltage to allow the output tubes to create an a.c. power output to be passed to the speaker. The supply voltage, tubes and load imply voltage and current swings at the output tube plates to deliver the designed output power.

We don't think of output tubes having a specific amount of gain, but they do: a large plate voltage swing is caused by a smaller grid voltage swing. The designer would work from the plate voltage swing dictated by the other considerations I mentioned, divide by the gain of the output tube and arrive at a needed grid voltage swing to cause the plate voltage swing to occur. Bias voltage is generally equal to the peak of this needed grid voltage swing (or slightly bigger).

So you'll see the bias voltage is a product of other design considerations; there is a right bias voltage for maximum output power, all other things equal.

The amp designer, knowing the needed bias voltage from these other factors, would divide the needed bias voltage by cathode current for the output tube(s), and arrive at the needed cathode resistor value for cathode bias.

For non-adjustable fixed-bias, the designer would simply create a bias supply with the correct output voltage. It's not self-adjusting like cathode bias, but stays rock-solid, and is essential for maximum output power when working deeper into class AB or class B.

Adjustable fixed bias is functionally the same as non-adjustable fixed bias, except that it allows for variations in output tubes. A tube data sheet gives values of a "bogey tube" but real tubes have many characteristics that can vary inside of pretty wide tolerances. Tube characteristics can also change over the tube's life. If all tubes matched the "bogey tube" an adjustment would never be needed.

Let's say the above a different way: If you had a bogey tube and adjusted the fixed bias voltage away from the design value, you'd get more distortion, less output power, or both.

But we often don't worry about absolute maximum clean power in a guitar amp, unless an amp sounds like 10w when it should be 100w. However, if you keep the above in mind, you can understand why Fender had non-adjustable fixed bias in the bigger 50's amps, moved to adjustable fixed bias in the bigger amps by the mid-60's (the Princeton still had non-adjustable fixed bias), and eventually to bias-balance in the late-60's and 70's. The point was to help accommodate varied tubes, but not stray far from the as-designed operating point.

[tfrost]

Well put...  I think I understand now!  Thank you sir!

[Willabe]

Looks real nice!

You have a heater CT (green/yellow) so take out the 100R from the heater wires/pilot light to ground. 


              Brad        

[tfrost]

Looks real nice!

You have a heater CT (green/yellow) so take out the 100R from the heater wires/pilot light to ground. 


              Brad        

Thanks so I don't need those in there then!?!?

[HotBluePlates]

Thanks so I don't need those in there then!?!?

Right.

If your power transformer has a heater center-tap, use it. If it doesn't, you create an artificial center-tap with the 100Ω resistors.

[Willabe]

Your B+ is still 490v? What kind of power transformer did you use? What kind of rectifier are you using? 490v seems way too high for an amp that's supposed to be at about 415vdc.

I think something's wrong here. 5E8 schemo says 375dcv at the plates, 5E8A schemo says 400dcv at the plates.

I get 17.9 mvdc at V6 and 16.4 mvdc at V5.     V5 is now reading 41.4 and V6 is now at 45.

Even after you had more than doubled the current draw on the output tubes, your plate voltage only dropped 6dcv and is 484dcv.

I'd call Mojo and tell them the numbers on the PT they sent you, I bet it's wrong for that amp kit.

I bet it's the high power tweed twin PT and that PT would made for 4x6L6 plate current draw and that might be why when you doubled the current draw on 2x6L6 the PT didn't feel it and drop any plate voltage.

It would also not have enough 5vac heater current for 2x5U4 = 6amps. High power tweed twin used 1x5AR4/GZ34 = 2 amps. Maybe why you smell something. Although that would not account for the PT being cool to the touch.



            Brad            

[Willabe]

That amp is choke fed, what B+dcv do you get before the choke and after the choke?


                     Brad     

[sluckey]

That amp is choke fed, what B+dcv do you get before the choke and after the choke?

Bingo! That's why the B+ is so high.

Mojo's schematic clearly shows a choke input supply just like the original. However, their layout erroneously shows the choke wired the more usual way, between node A and node B. Wire the choke IAW the schematic and the voltages will fall in line.

This is not the first major mistake I've seen recently with the Mojo kits.

EDIT... Scratch all that! The layout is correct. Mojo just forgot the first input filter cap on their schematic. Minor error. I should have looked at the original schematic first.

[tubeswell]

Diagram showing relative differences plate-to-cathode voltages. (Assumed hypothetical voltages - actual B+ as measured in real life may differ etc - the point is that Fixed-bias has more gain relatively speaking than Cathode-bias given the same B+ supply voltage)

[Willabe]

Mojo's schematic clearly shows a choke input supply just like the original. However, their layout erroneously shows the choke wired the more usual way, between node A and node B.

On the Mojo low powered tweed twin, 5E8A? The wiring drawing looks right.


             Brad         

[sluckey]

On the Mojo low powered tweed twin, 5E8A? The wiring drawing looks right.

On second look, that's correct. Their layout agrees with the original schematic/layout. The error is on the Mojo schematic. I corrected my post above.

However, "if" you omit that first filter cap as Mojo did on their schematic, the B+ would drop considerably since you would now have a true choke input filter. But that would not be like the original. Sorry for the confusion.

[tfrost]

On the Mojo low powered tweed twin, 5E8A? The wiring drawing looks right.

On second look, that's correct. Their layout agrees with the original schematic/layout. The error is on the Mojo schematic. I corrected my post above.

However, "if" you omit that first filter cap as Mojo did on their schematic, the B+ would drop considerably since you would now have a true choke input filter. But that would not be like the original. Sorry for the confusion.

Ok so with that said, would it be worth removing that first filter cap to see if I drop into a more reasonable B+ range?

By the way, thanks for all the responses guys, what a great resource!  I very much appreciate it!

[sluckey]

Ok so with that said, would it be worth removing that first filter cap to see if I drop into a more reasonable B+ range?

You could try that, but I'd rather pursue other options. First, I'd verify that Mojo sent the correct PT. Measure the AC voltage on pins 4 and 6 of the rectifier tubes. How much?

Another option would be to use two NOS 5Y3s. They will carry a 2 x 6L6 amp and also drop the B+.

[tfrost]

Ok so with that said, would it be worth removing that first filter cap to see if I drop into a more reasonable B+ range?

You could try that, but I'd rather pursue other options. First, I'd verify that Mojo sent the correct PT. Measure the AC voltage on pins 4 and 6 of the rectifier tubes. How much?

Another option would be to use two NOS 5Y3s. They will carry a 2 x 6L6 amp and also drop the B+.

Thank you, will do that first...  For the record, I have all NOS glass in it now, but the rec's are 5U4GB's....  Could that be an issue using the 5U4GB's vs. 5U4G?

[sluckey]

Could that be an issue using the 5U4GB's vs. 5U4G?

No problem. I mentioned the NOS 5Y3 because they have a sizeable voltage drop.

How much voltage did you measure?

[tfrost]

Could that be an issue using the 5U4GB's vs. 5U4G?

No problem. I mentioned the NOS 5Y3 because they have a sizeable voltage drop.

How much voltage did you measure?

I see....  About as far as I have gotten is making sure the correct transformers are in place.  According to the stickers on the trannies themselves all matches with Mojo's parts list.  Which now I remember checking it before I put the kit together.  But now its off for shopping with my wife and daughter!  Yee Haw!  Be back later hehehe.... 

Thanks again guys!

[tfrost]

Ok so with that said, would it be worth removing that first filter cap to see if I drop into a more reasonable B+ range?

You could try that, but I'd rather pursue other options. First, I'd verify that Mojo sent the correct PT. Measure the AC voltage on pins 4 and 6 of the rectifier tubes. How much?

Another option would be to use two NOS 5Y3s. They will carry a 2 x 6L6 amp and also drop the B+.

OK...  So my measurements were as follows

Pins 4-6 of the rectifier tubes all measured exactly 350vAC

My B+ continues to be 484vDC

Measured at the positive side of the filter caps (I think this is how I check "through" the choke?) moving from the end of the board toward the center I got 420vDC on the first cap, 408vDC on the second and third caps, then through the resistor on the 4th cap I got 307vDC. 

How does that all sound? 

[Willabe]

Pins 4-6 of the rectifier tubes all measured exactly 350vAC. My B+ continues to be 484vDC

Measured at the positive side of the filter caps (I think this is how I check "through" the choke?) moving from the end of the board toward the center I got 420vDC on the first cap, 408vDC on the second and third caps, then through the resistor on the 4th cap I got 307vDC.

Now that's with all the tubes in (PT loaded) right?

If so, now I could be wrong, but I think that's kind of high for the B+acv winding.

Your choke readings sound right dropping 12dcv. Fender 5E8A schemo lists a dcv drop of 10dcv, very close. And yes you checked the choke voltage right.

Fender schemos that I looked at do not have PT B+acv listed for 5D8, 5E8 and 5E8A, 2x6L6 (low power tweed twins). Fender does list PT B+acv for 5F8 and 5F8A at 300-0-300 which are 4x6L6 (high powered tweed twins).

Fender tweed bassman 2x6L6 schemo lists PT B+acv for 5F6 and 5F6A as 325-0-325 but they have a 83 (5F6) rectifier tube and a GZ34/5AR4 (5F6A) instead of 2x5U4GB. But with 2 5U4GB rectifier tubes you should get less B+ voltage drop.


          Brad          

[tfrost]

Yes...  with all tubes. 

[tfrost]

Well, in that last post of mine I was going to say this " I am beginning to suspect its a Power transformer issue" but then decided against it and erased it...  Then I searched! And found this!  Seems obvious but maybe you guys make a little more sense of it?  This thread gets into the mojo kit power transformer and seems to be what I am experiencing! 

http://www.tdpri.com/forum/amp-central-station/216249-low-power-tweed-twin-5e8a-build-hot-off-press-2.html

[Willabe]

Here was Mojo's response:

"I can assure you that the 5E8-A kit has been sold for many years with the MOJO752 and I've never had any problems, failures, or complaints. The current ratings on the Mojo transformers is not a precise science. Heyboer determines the transformer current ratings based on the size of the wire they use to wind them. When they provide me with the ratings they always err on the side of caution which means that the maximum current handling is always more than the published specification.

While the MOJO753 or MOJO753EX would definitely power the 5E8-A circuit it would also be much more iron than is needed and it would unnecessarily increase the cost of the amp kit. Additionally, the Mojo 5E8-A chassis is not cutout to accommodate the MOJO753 or MOJO753EX transformer."

The Mojo representative went on to guarantee that the PT would be replaced if it should fail on me.

Bob, despite Mojo's stand on the matter, your comments have haunted me. I contacted Heyboer and spoke with Phil directly. He felt the transformer was "not the wrong thing", saying "these can be run up to 130 C (250 F)". Even so, I told him I wasn't comfortable pushing it that hard. So...he's winding me a custom tranny with a doubled 5V winding that can handle 6A all day long. He's stacking it with M-6 laminations. He said "the grain oriented M-6 will loose heat faster and is also more efficient for working field." He said it will spec as follows: 120 x 355-50-0-355 no load @ 150mA, 6.3 VCT @ 6 Amps and 5V @ 6 Amps. Basically this is a beefed up 8087 Fender PT. This is not a production model.

PT B+acv winding is specked at 355-0-355 NO LOAD. Yours is 350-0-350 LOADED. That's the difference that's causing your B+ to be at 484dcv.

Yes I see it's a costume wind PT but it seems it was the 5acv wind they beefed up to 6A. So I'm thinking Mojo's low power 5E8A tweed twin PT, which seems like it is made by Heyboer, would also be 350-0-350 LOADED.


           Brad            

[tfrost]

PT B+acv winding is specked at 355-0-355 NO LOAD. Yours is 350-0-350 LOADED. That's the difference that's causing your B+ to be at 484dcv.

Yes I see it's a costume wind PT but it seems it was the 5acv wind they beefed up to 6A. So I'm thinking Mojo's low power 5E8A tweed twin PT, which seems like it is made by Heyboer, would also be 350-0-350 LOADED.


           Brad            
[/quote]

So whats the cure?

[sluckey]

My B+ continues to be 484vDC

Exactly where are you measuring that 484VDC?

I got 420vDC on the first cap, 408vDC on the second and third caps, then through the resistor on the 4th cap I got 307vDC.

These voltages look typical to me. I'm not sure you have a voltage problem.

Measure the DC voltage on pin 3 of a 6L6 tube. How much?

Measure the DC voltage on pin 4 of a 6L6 tube. How much?

[tfrost]

My B+ continues to be 484vDC

Exactly where are you measuring that 484VDC?  Pin 8 of the rectifier tubes and that side of the standby switch

I got 420vDC on the first cap, 408vDC on the second and third caps, then through the resistor on the 4th cap I got 307vDC.

These voltages look typical to me. I'm not sure you have a voltage problem.

Measure the DC voltage on pin 3 of a 6L6 tube. How much? With power ON 406vDC

Measure the DC voltage on pin 4 of a 6L6 tube. How much? With power ON 408vDC

Thank you for the continued support!

[sluckey]

Exactly where are you measuring that 484VDC?  Pin 8 of the rectifier tubes and that side of the standby switch. And I bet the stby switch is open. If so, that's fine.

Measure the DC voltage on pin 3 of a 6L6 tube. How much? With power ON 406vDC ...Very good.

Measure the DC voltage on pin 4 of a 6L6 tube. How much? With power ON 408vDC ...Very good.

As for the voltages on the filter caps, look at the attached pic and compare to the voltages on the original. It's a perfect match. I'd say all is good. You're done.