Mojotone 5E5-A "Pro Amp" Bias Questions

[72Blazer]

Still learning here...

I built this Mojotone Kit and I'm wondering if the Bias is right. 

Question 1:  Is this a fixed bias amp?

Measurements:  Plates are 340v

I istallalled the 1Ohm resistors and I measure 85mA.  This hould be divided by 2 and equal 42.5mA.  Am I correct on this assumption?

If I divide plate current (340v) by .0425 I get 14.45 Watts.  If I got this right thus far, is 14.45 Watts acceptable for a 6L6GC?  The tubes sure do give off a lot of heat.

Thanks in advance.

[sluckey]

This is a fixed bias amp and there is no adjustment.

If you installed a 1Ω resistor on the cathode of each 6L6 (one resistor per tube) and you measure 85mV across one 1Ω resistor, then that tube is idling at 340 x .085 = 28.9 watts. You don't divide by 2. So, 28.9 watts is very hot for a 6L6 that has a max of 30 watts!

[72Blazer]

Ok, Yes I am getting 85mA (correction mV) at each 1 Ohm resistor.

Thanks.  It looks like I confused a little of the cathode bias calculation (watched a video on that) and again not knowing for sure if this was fixed bias or a cathode biased.

What resistor do I need to change to tame this down?

Vr
J.

[72Blazer]

I stumbled across some threads that recommeded (if I recall correctly) adding a bias pot.  Is this the only way to adjust the bias?

[sluckey]

What resistor do I need to change to tame this down?

Decrease that 6.8K just before the diode. Try 4.7K and re-evaluate. You want 19 to 21 watts. 21 watts is 70%. You may need to tweak this resistor up/down until you are happy.

[uki]

This resistor:

[sluckey]

If you want to add a bias pot then simply replace the 56K with a 50K pot in series with a 24K resistor. Look at my bias mod in my Harvard 5F10...

     http://sluckeyamps.com/harvard/harvard_5f10_schem.pdf

[PRR]

> 28.9 watts is very hot for a 6L6 that has a max of 30 watts!

Cathode current includes screen current which is not normally part of Plate Dissipation. For the 6L6 types, I would spitball 5% to screens, so 95% of 28.9 makes 27.5W.

Which is still pretty bold for most 6L6GC (and out of bounds for any 6L6G(m/a/b) still alive). And the Fender original called for 6L6GB, 19 or 23 Watts (I forget).

"Fixed" bias has to be "fixed" by design and verified in production and in maintenance. As said, the 5E5-A has a dropper resistor for the purpose. It maybe should be smaller (grid bias voltage more negative).
http://www.thevintagesound.com/ffg/schem/pro_5e5-a_schem.gif

Or what Sluckey posted while I was walking the dog (really).

[72Blazer]

The best value I could find was @  1.5K.  368v x 75mV = 27.6 Watts

I tried going up and down from the 6.8K.  Up was worse by far.

Is this best I can hope for?

[sluckey]

No. You can do better. But first, measure the AC voltage at the PT bias tap where it connects to the 6.8K resistor. What have you?

[72Blazer]

56.5v AC

[sluckey]

You have a problem somewhere. Measure the resistance of that 56K. Post a hi-rez pic of that end of the board and another one that clearly shows the 1Ω resistors you installed on the tube sockets.

[72Blazer]

The resistor read 55.3K

The pics are hard to see but the 1 Ohm resistor is tied to Pin 1 and 8 with the other end going to chassis ground.

Should I put the 6.8K back in or leave the 1.5K in for now?

[sluckey]

Looks like your bias cap is installed backwards! The positive end must go to ground. Mojo's layout shows it backwards too. The list of shitty mojo drawings just keeps growing.

Turn that cap around and put the 6.8K back in and re-evaluate. Report back.

[sluckey]

Oh yeah, that cap may be damaged. You may need a replacement.

[sluckey]

If you want to make your bias circuit adjustable, look at my Harvard 5F10 amp. The bias circuit is identical to yours except mine uses a selenium rectifier...

     http://sluckeyamps.com/harvard/5F10.jpg

[TitaniumValhalla]

Mojo's layout shows it backwards too. The list of shitty mojo drawings just keeps growing.

It's a shame whoever is running this area at Mojotone does such a bad job. They make great cabinets and for the most part their branded components are good, but these kinds of mistakes with their kits are pretty inexcusable. Do they do zero QC at all? Do they actually build one from their own instructions to see if they work?

[sluckey]

It's pretty easy to overlook small errors like this. Heck, I have to review my drawings numerous times and still find little errors even after posting on the web. But I eventually get them ironed out.

Whoever is doing the drawings for mojo does a nice job, but one itty-bitty mistake like this example is a showstopper for some non-tech people that are likely to buy this kit, especially if they are not familiar with schematics. I've recently seen three mojo kits with little errors in the documents that resulted in people looking for help in this forum. If I was buying this kit I'd report this error to Mojo. I like to believe that they will correct the mistake eventually.

[dude]

You should add the bias pot as in Sluckey’s Harvard schematic, do you have two 1 ohm R’s on pin’s 8 to ground?  You take a Mv reading across each 1 ohm R, look for about 42Mv each, should be within 1 to 5 Mv’s if matched 6L6s. With the bias pot you should be able to get from 30 to 60 Mv or close.

[PRR]

Home page. Resources. Documentation?
https://www.mojotone.com/documentation

404 page not found
The page you are looking for does not exist

[sluckey]

This is the proper link to the layout...

     https://www.mojotone.com/Amp_Kits/Tweed/Pro_5E5A_WD.pdf

[tubeswell]

This is the proper link to the layout...

     https://www.mojotone.com/Amp_Kits/Tweed/Pro_5E5A_WD.pdf

Holy crap! 8-\


And here they are trying to make money out of it!

[72Blazer]

Ok,

As Sluckey instructed, I connected positive side of Bias cap to ground and reinstalled the 6.8k resistor.

I've let the amp bake in for 20 minutes now.

6L6 readings:

I now have 412v on pin 3 and 24mV on the 1 Ohm resistor.  414v x .024mV = 9.936 Watts.

I have -44v on Pin 5

Lastly, I measured the bias cap (while I had it out of the circuit) and it read 70uf (supposed to be 100uf 160v cap)

Should I increase the value of the 6.8K resistor?

[sluckey]

Should I increase the value of the 6.8K resistor?

Yes. But following this path you will likely have to change that resistor again sometime down the road. The simple bias mod I suggested will make life easier.

[dwinstonwood]

I now have 412v on pin 3 and 24mV on the 1 Ohm resistor.  414v x .024mV = 9.936 Watts.

So, that's sort of like a 6V6 amp... kind of like the 5E4-A Super:

https://el34world.com/charts/Schematics/files/Fender/Fender_super_5e4a_schem.pdf

 

[72Blazer]

I just need a solenium rectifier-thats it?  Where can I find one?

Thanks for all the help

[sluckey]

I just need a solenium rectifier-thats it?  Where can I find one?

You're kidding, right?

You need to replace that 56K resistor with a 50K pot and a 24K resistor. That's all.

It's possible that your 6L6s may be damaged/weakened due to improper bias. You now have -44v bias and 412v on the plates, but only 24mA current. I would expect more current.

[72Blazer]

No I was not kidding.  Forgot you mentioned the 50k pot early on in the thread.  Went back and re-read that.  When I looked at your drawing I saw there was no diode/saw that solenium rectifier-anyway...

When I first corrected the bias cap orientation/re-installed the 6.8k resistor-i was continuously monitoring the plates and the current.  I watched plate voltage rise while the current slowly dropped from 46mA down to where it finally settled to around 24mA over the course of about 20min.  Just curious if I was seeing the tubes weakening even more?  They probably were weakened severely prior to that though.  Not a good time to be cooking tubes...

What's really got me scratching my head after the fact is how good the amp sounded while it was improperly wired.  I mean it really sounded good.  Unfortunately It had been played for several hours before I came up on the forum.

Lastly, while it was improperly wired-it still had negative voltage on pin 5 of 6L6s.  -25v to be exact.  I did not think that possible with the bias cap reversed.  Sux learning the hard way.

Vr
J

[glass54]

You could try these  Work quite well at a reasonable cost.

[thetragichero]

similar to ^^^^^^ i just picked up a bunch of these to meet a minimum order threshold, although the above does help support this forum

and while there may be reasons to still use selenium rectifiers that i'm not aware of, sand (silicon) is considerably cheaper and more reliable these days not to mess with the old stuff for geetar amps. i keep a decent stock of 1n4007 (and 1n4937) on hand so that they're never something i *need* to order, just when i'm restocking. round it out with various zeners, big current (10a) diodes for solid state amps, bridge rectifiers (20/30a ones can be found dirt cheap), and the ubiquitous 1n4148/914 and you've got mostly all you need in terms of diodes (besides really odd ones like 1n3754 on ancient kustom amps that are rare as hens teeth)

[dude]

http://sluckeyamps.com/harvard/harvard_5f10_schem.pdf
Go to the link above, picture 3, the schematic.
See where on the schematic at the bias supply it says -38 volts in blue?
You're going to take that 56K out and put in the 50K pot, trim pot best as it's small.
One leg goes to the diode and + end of the capacitor that you had backward and now the right way, (Mojo error.)
Connect a jumper to the middle leg and the point above as in the schematic.
The other pot leg put the 24K R and the end goes to ground.
A trim pot is small and can be held tight when you solder the leads.
Check the Mv readings across each 1 ohm R you installed across pin 8 and ground on the 6L6's, you want about 42/50 Mv's depending on plate voltage. Take the plate voltage of the 6L6 plates, pin 3 to ground on a 6L6, (be careful high voltage meter leads can slip) go to    https://www.tedweber.com/bias-calc/  Class AB, tube type 6L6GC, put in the plate voltage you measured, then hit calculate. Set bias mV to this setting for a safe 70%.


Note as you change the Mv's the plate voltage changes, just do these steps above a few times.


Good luck, don't electrocute yourself  

[dwinstonwood]

You're going to take that 56K out and put in the 50K pot, trim pot best as it's small.
One leg goes to the diode and + end of the capacitor that you had backward and now the right way, (Mojo error.)
Connect a jumper to the middle leg and the point above as in the schematic.
The other pot leg put the 24K R and the end goes to ground.

Hey, dude, I believe you meant to say the "-" end of the cap. 

[sluckey]

See how easy it is to miss that little detail. At least when you're just writing about it you ain't burning tubes!   

[dude]

See how easy it is to miss that little detail. At least when you're just writing about it you ain't burning tubes!   

Sh!t, me bad, “pop goes the weasel or should l say tubes”. Sorry about that, this is why l don’t give too much advice.    But l can’t believe l put a + instead of -.       

[PRR]

....when you're just writing about it you ain't burning tubes!   

If you aint burning you aint learning.

Big balls of fire, or even a $76 sack of new tubes, is good teaching.

[glass54]

Sh!t, me bad, “pop goes the weasel or should l say tubes”. Sorry about that, this is why l don’t give too much advice.    But l can’t believe l put a + instead of -. 
dude, we understand 
2 years plus of covid and now "an insane little czar" All contribute to anxiety, related to amplifier parts supply and logistics.
I call it "environmental stress"
Kind regards
Mirek

[sluckey]

If you aint burning you aint learning.

I did my share of burning back in the day. Thankfully tubes were cheap and very available. Have you shopped for a matched set of Putin's 6L6s today?

[dwinstonwood]

My hobby budget is too meager to learn by blowing things up. So far, the worst I've done was to power up an amp with my cap discharging tool still attached. The inline 3W resistor popped like a firecracker, scared the heck out of me and filled my work area with smoke. But, I guess I did learn from a mistake. With things the way they are now, I don't even want to sacrifice any of the SV83's I have left over from an old Decware amp. As I understand it, they're like EL84's with a lower screen voltage/PD rating.

[dude]

There’s still a lot of guitar tubes out there.  Russia’s out, China had a fire issue at their  factory but still JJ’s are being made and shipped, big back order but still available.
Plus used NOS American made tubes are available on ebay and swap meets.
I personally wouldn’t have connect the bias voltage directly to ground, l just was trying to help the guy as he seemed lost. He probably would have picked it up, well hopefully.

[72Blazer]

Gents,

Big thanks to all for the high learning curve as we used to say in the Air Force.  BTW, high learning curve means-"you screwed up and you learned a little".

I want EVERYONE to know I called Mojotone today and told them about their error-that you pros from Dover (Sluckey) discovered.  No names mentioned btw.  Anyway, without hesitation they verified-admitted the error-and are sending me the bias cap and 2 new 6L6GC tubes free of charge.  When I named the initial thread-we did not know of their error so it was never intended for their name to be mentioned.  It's all good now.

Dude thanks for the idiot proof instructions on Sluckey's design-I think I got it??!!  lol   And copy the correction regarding NEG vs POS end.

As I re-read all these posts-I thank you all again for sharing your knowledge and experiences.

Long live rock!
Vr
J.

[sluckey]

Mojo is pretty quick. The layout has already been corrected.   
Wish I could remember the other drawing errors.

     https://www.mojotone.com/Amp_Kits/Tweed/Pro_5E5A_WD.pdf

[tubeswell]

If you aint burning you aint learning.

I did my share of burning back in the day. Thankfully tubes were cheap and very available. Have you shopped for a matched set of Putin's 6L6s today?

EHX is going have to make a profit out of pedals https://www.guitarworld.com/news/electro-harmonix-russia-tube-exports-ban

[thetragichero]

without hesitation they verified-admitted the error-and are sending me the bias cap and 2 new 6L6GC tubes free of charge

now that's what i call customer service/standing behind your product. i've only bought oddball parts from them i couldn't get elsewhere (pink tolex, fender-style chokes after classictone closed, etc) but something like this would make me want to give them more business.
too often companies (and especially low folks on the totem pole with absolutely nothing to gain) will dig their heels in in these situations and you hate to see it... i have a mentor who was handed an extra $100 bill by a bank teller and when he, being an honest fellow, brought it to their attention he was met with a "we don't make those kind of mistakes"....

[72Blazer]

The free replacemnt tubes and the cap arrived this evening.  I'll report back once I install these sometime tomorrow.

[72Blazer]

Replaced the cap and tubes.  After swapping in and out several resistors values and wound up settling on a 12k resistor in place of the 6.8k.  I have 382v and .047mA for 18watts on the 6L6GCs.  Amp sounds good and quiet as well.

[sluckey]

I have 382v and .047mA for 18watts on the 6L6GCs.

That would be .047A or 47mA.

[72Blazer]

My mistake. 47mA.  I must have been thinking the math...382v x .047 = 17.95 Watts.  I think if I had an 11k resistor that would been the best.  I did order the micro 50k pot to try later.

[HotBluePlates]

....  I think if I had an 11k resistor that would been the best.  I did order the micro 50k pot to try later.

Once you have the pot in-hand, you may need to go back to the 6.8kΩ resistor.

That's because with your current arrangement you will need the pot turned almost all the way to one side the get the idle current down where you want it.