5879 Preamp With Ax84 P1 SE

[llama]

Hi everyone,

I have consulted (briefly) with Firemedic on his 5879 SE Redo amp and have decided to build a copy with a EL84 Power section.  This is my second build, my first being the venerable AX84 HO.  I have the a second PT and OT that I used for the ax84 HO and would like to experiment with the 5870 since I have 10 NOS sitting in my shop.

I figured, as a noob will, that i can just marry the pre to the power section.

I've attached  what I've come up with.  The B+ voltage is what I got from my first build using the same R values, but with 47uF caps in the power supply (minus the choke).
Note:--the 47uf caps are what i have in my HO not what I'm planning on using in this build - sorryr for any confusion. --: end note

I humbly ask you for your assistance and guidance in my first custom-ish build.

BTW  What are the 100R resistors on the heater wires (brought over from Firemedic's dwg) used for?

-Scot

[phsyconoodler]

They create an artificial center tap to ground in lieu of an actual heater center tap.

[DummyLoad]

suggest using a 12DW7 in place of 12AX7 and 12AU7. 12DW7 is 1/2 12AX7 and 1/2 12AU7. four bottle amp becomes three bottle amp...

BTW, doug sells 12DW7.   

--DL

[llama]

suggest using a 12DW7 in place of 12AX7 and 12AU7. 12DW7 is 1/2 12AX7 and 1/2 12AU7. four bottle amp becomes three bottle amp...

BTW, doug sells 12DW7.   

--DL

What if i were to use a 12ax7 or a 12a_7 for both?  I'm not familiar to why the lower Mu tube for the CF.  -sorry

[Willabe]

I'm not familiar to why the lower Mu tube for the CF.

More current?


                Brad     

[tubeswell]

I'm not familiar to why the lower Mu tube for the CF. 

The mu of the tube doesn't matter for a CF application, because there is only unity gain with a CF in any event (no matter what the mu of the tube is). As Willabe says, the critical thing about the CF is its ability to drive more current through a load. Higher current tubes make better CF stages. A 12AU7 triode is great with 10k-20k cathode load - heaps of current

[llama]

Well, I guess that's important!  I do have a 7247 tube, and several 12at7's (which is a poor compromise).  It looks like the pinouts are the same.

Thanks for enlightening me!

Still not sure about the rest of it. 
I exported my dwg as a gif so you all can see it without any external apps.  Hope that makes it easier.

[PRR]

> The mu of the tube doesn't matter for a CF application

In a way it does.

The cathode resistance (1/Gm), *at the same current*, is about the same for all the small tubes. Roughly 1,000 ohms at 1mA.

The plate resistance is Mu times higher. So higher for high-Mu tubes.

The maximum current at a given voltage is limited by plate resistance.

So a low-Mu tube can pass more current at a given voltage.

llama's plan shows perhaps +250V at node D, and 171V at CF cathode. 250-170= 80V across the tube. The 56K cathode resistor must be flowing 3mA. The tube must flow like a 80V/3mA= 27K resistor.

12AX7 gets down to 40K. It needs 150V across it to flow 3mA. It won't work here.

12AU7 gets down to 6K. It will flow 3mA with 35V across it; or 3mA at 80V with a nice -2V grid bias. It will work.

[tubeswell]

Ahh yes I forgot about the plate resistance.  Thanks PRR

[llama]

Yes, Thank you PRR, as well as Tubeswell, phsyconoodler, DummyLoad and Willabe.

PRR,  I'm sorry, but I failed to take Firemedic's voltages off from his DWG so you might have been going off of false info on your previous post.  I have removed all but the B+ voltage, which is base on a the same PT /ss rec as this one.  But your point is well received; I found a 12wd7 in my stash last night.

Tubeswell,  You said "A 12AU7 triode is great with 10k-20k cathode load - heaps of current" - should I change my R14 to a smaller load, from 56k?

overall, I'd like to have a bright sounding amp that can purr and growl with that pentode flavor.

Any other suggestions?  God knows I'm open to any and all-- Thanks so much!

[tubeswell]

A 10k to 20k load will allow more tube current in a 12AU7, and providing current is the aim of a CF stage.

For example: a 12AU7 with a 250V supply on the plate and a 10k load will result in a cathode voltage of about 57V, so you want to aim for a bias voltage at the grid to be about 7 or 8V below that (i.e. a grid voltage of about 50V). If your driver tube plate voltage is higher than that, then you could either use a resistive divider to knock down the DC coupling voltage to the CF stage, or you could use AC coupling (i.e.: a coupling cap) and use a fixed bias or cathode bias method to bias the CF stage. With AC coupling, cathode biasing is better IMHO because you don't have to worry about figuring out the bias voltage. These are based on some examples from Merlin Blencowe's 1st book and provide about 6mA of tube crrent

[DummyLoad]

proper CF design dictates that cathode should be sitting at approx 1/2 B+ for maximum symmetrical output swing for lowest distortion. Vo symmetry is also affected by the AC impedance of the load, in this case the tone stack.

with guitar amps, the caveat is that we may NOT want fully symmetrical output swing and want 1/2 of the wave clipped prematurely.

--DL

[tubeswell]

The symmetry/clipping also depends on how big the input signal is. If the cathode is sitting at 58VDc at idle, then you can expect that you could put a ~116V P-P signal on the grid without causing any clipping of the CF. If you want slightly less current and a bigger potential signal try a 20k load and cathode bias with a 1k bias resistor, or a 27k (or 33k?) load etc. Bear in mind that the more current you can get the CF to deliver, the better the signal bandwidth will hold up through any following load (like a TMB tone-stack for example). However I guess the audible difference between driving the tone stack with 3mA vs 6mA may be a moot point, so the best thing would be to try it and see I guess.

[darryl]

One thought about the circuit - the negative feedback loop includes both the tonestack and the master volume. This might work, but it is not good practice, as the results can be very unpredictable.

The tonestack introduces varying degrees of phase shift, depending on settings, so the amplifier may become unstable at some control positions, as the negative feedback actually becomes positive.

Having the master volume inside the NFB loop is less of a problem, but the amount of NFB will vary with the position of the master. Intuitively this is not a problem - you normally turn the master down for more distortion, so the reduced negative feedback could be seen as an advantage!

[llama]

OK, Thanks guys,  I'll need to digest this all and will reply.

[llama]

Gentlemen,

I have digested your suggestions and info.  I'm also reading Merlin's Book on preamps - great stuff!

I have decided to build the amp as designed by Firemedic (perhaps minus the NFB) and the straight connection to the P1 power section (as drawn above). I'm designing the layout now with the idea of ease of changing values and circuits as a learning tool.  This is the best way for me to learn the black arts of ampcraft.   So then I will be able to make the changes you all have provided.

Next step is to gather components. 

I have received a boat load of NOS capacitors for free (over 100!) , of all different styles and values.  I have a few questions:

I'm assuming any polarized (+/-) cap is electrolytic and may be useless, but test OK on my DMM (Capacitance setting).  is this true?

Many of the caps have a voltage rating of 200V or below, can I use any of them in any location in the circuit?  I know that the cathode bypass can use a low Voltage value- but anywhere else?

Thanks!

[SoundmasterG]

Any electrolytics could possibly be reformed but there isn't too much point in bothering. Save them and safely wire up a huge transformer with a voltage quintoupler and long lead wires, run the wires to the street, and get an electrically isolated switch....you can use them as firecrackers. 

The electrolytics may test ok on the DMM...and actually lots of caps may test ok on a DMM, but a DMM can be inaccurate when not checking a cap closer to its actual operating voltage. You can get a Sprague Tel-Ohmike meter that tests caps for lots of things including ESR, and you can use those to reform old caps if you really want to. They will test a cap up to 600V.

You can connect caps in series with resistors in parallel with each cap to get higher voltage ratings, but that is often impractical. Aside from cathode bypass caps, you ca't use them many places except a tone stack if you stick a .1uF higher voltage cap in front of them. If many of the caps are old paper caps they may leak too much DC to be useable. You would have to check them for leakage.

greg

[sluckey]

BTW  What are the 100R resistors on the heater wires (brought over from Firemedic's dwg) used for?

There's a problem with your schematic concerning the 100Ω artificial center tap resistors AND elevating the heaters to the EL84 cathode voltage. The way you have it drawn you have both 100Ω resistors in parallel with each other but also in parallel with the 130Ω cathode resistor. That means you effectively have a 36Ω cathode resistor! The EL84 wont be happy until the 100Ω resistors pop.

Here's the correct way to elevate the heaters when using an artificial center tap.

[llama]

Thank you Slucky!!!

I will make the change ASAP.  That is defintely  the blurred area where I married to two sections.  I took a shot at it, but missed in this case.

I very pleased that you contacted me about the issue-- Thanks A bunch!!!

[llama]

You can connect caps in series with resistors in parallel with each cap to get higher voltage ratings, but that is often impractical. Aside from cathode bypass caps, you ca't use them many places except a tone stack if you stick a .1uF higher voltage cap in front of them. If many of the caps are old paper caps they may leak too much DC to be useable. You would have to check them for leakage.

greg

Thanks greg,

I feared that the electrics would be useless.  What a shame- I found some really nice Spragues!!!

I forgot to mention that a majority of these Caps were produced for Military use and are all mil-spec- a few are individually foil pouched in the 80's .  These Caps have a label with they say "Fixed, cer"  Which, of course is ceramic.  Well one set stated "Fixed pla"

I used the old band saw and opened one of these, and found foil separated by this plastic cassette tape-like film.  I'm assuming pla is plastic which is mylar.  These are the ones I though were oil/paper.  I have a shite load of these at some good voltages.

Are these caps useful?  I'm assuming that the mylar will not break down as paper in oil or Electrolitics.

[llama]

Hi.  I have been expanding my mind with the layout of this amp.  It's taken me a bit, as this is my first attempt to do one, but the lessons were huge!

Could you take a look to see if any glaring mistakes and omissions were made on my part.  I'm way too close to this to see anything obvious, although i will continue to proof it out.

I just received my components from Doug and am ready to make the walls rumble!

Any advice is welcome!

BTW the layout was dictated by the unique chassis.  It's from an old Roberts/Akai Monoblock amp for a R2R recorder.

Thanks!

[SILVERGUN]

I would plan on moving your input resistor (grid-stopper) R23 off of that terminal strip, and instead, use a shileded cable from the input jack directly to that resistor, wired directly to pin 7 on V1
It will greatly reduce noise going into that very critical 1st stage.
Connect the shield to ground at one end only (input end) and cut the other end back and insulate / heat shrink so that it doesn't come into contact with any other tube pins.
It's probably not a bad idea for R9 and R20 as well...

I personally would add a series diode on each leg of your secondary, just for security....if 1 diode failed, the other would still protect "block"

I don't have time to look too close, but that stuff stood out to me.

Have fun and stop by here when you get a chance.....we can blow some of those nos caps up!

[llama]

HI Silvergun (the name is making sense to me...),

OK so your saying that it is quieter to have it go Input|Shielded cable|gridstopper|tube,  than having it go input|gridstopper|Sheilded cable|tube?  I know i wasn't showing any shielded cable in my drawing, I wasn't sure how to in Express SCH.

Does the order matter?  - asked in all sincerity!

I'm looking forward in meeting with you. - I'll contact you soon!
-Scot

[SILVERGUN]

Yes, you want the resistor soldered to the tube pin......it WILL help cut down on micrphonics and even possible RF interference

[llama]

OK,

Thanks Will build it that way -- Thanks for your help Silvergun!

[llama]

Updated the Layout as per Silvergun's advice and other tweaks,and fixes.

-Properly grounded the OT,  showed use of 2 resistors to make up R22, and adjusted the spacing between posts to model reality

I have the components and I plan to start the build maybe today!  Unless......

I hope another set of eyes may flush out any fatal mistakes.

Thanks again to all of you! 

[SILVERGUN]

I wish I had more time to help you....I spend weeks working out the bugs in my layouts, and it makes me crazy...

I'm just going to remind you of a couple of things you already know:
1) Triple check layout against schematic, and then double check it again (that's my method )
2) Make EVERY solder joint count....and use your heatsink everytime no matter how much of a pain in the A it is....clean tip often
3) Twist your heater wires tight and keep them away from signal wires....also keep other AC wires twisted tight and away from DC wiring

[llama]

Silvergun, I've been following you post and I believe that you are going crazy!  This system is being build for experimenting as well, but I don't have a tone to chase at this juncture!

Thanks again for the advice - What do you mean by heat sink?  Like alligator clips to draw heat away from components?

-Scot

[SILVERGUN]

Silvergun, I've been following you post and I believe that you are going crazy!

I would like to consider myself a mad scientist at this point, but thanks for the compliment..
I will continue on this path of experimentation until something really good or really bad happens....
I have just enough information to be dangerous

Thanks again for the advice - What do you mean by heat sink?  Like alligator clips to draw heat away from components?

Yes, exactly,,,don't solder without one, especially your caps which do not like heat.....they really do work

HEAT + CAPACITOR = DAMAGED CAPACITOR

[SILVERGUN]

How's it going llama,
You're awfully quiet.....you're making me nervous....
Any time you're working around 400 volts .......no news could be bad news.........what's up?

[llama]

Sorry,

Everything is fine.  I am just about ready to begin the actual build.  My mental building is just about finished, so the questions are mostly answered until I turn it on.

But life is bumping the the hobby out of the way for a few days.

I hope to have the heaters wired up by the weekend, then i can proceed!

Thanks for the concern.

How does the Tuesday before Turkey day sound for me to come see your new amp?

[SILVERGUN]

How does the Tuesday before Turkey day sound for me to come see your new amp?

Sounds good,,,I'll pencil you in

[llama]

Well, after a Whirlwind of a Weekend of Wedding activities  (not mine)  I was able to run and solder the heaters.  I think they look good, but I getting surprising resistance readings.  here is a pic https://www.facebook.com/photo.php?fbid=470153676370968&set=a.461869500532719.133116.433849353334734&type=3&theater

With no tubes installed,  The resistance between one secondary point is low and close to equal with any other secondary point (measures at several heater tube pins.  So i read .2R between the black wires at the 1st and 4th tubes, abd .3R betwenn the black and the green wires anywhere.  I see no shorts between the tube pins.  Can the DC resistance be so low within the Heater secondary?

I know its a stepdown transformer for the heater winding so would that mean the imedance will be lower too?  I gues it makes sence, but such a low value makes me wonder.

BTW the white 7-pin socket on the left is for a 6005 tube run in parallel with the EL84 - for swapping out between the two (never at the same time!)

[sluckey]

So i read .2R between the black wires at the 1st and 4th tubes, abd .3R betwenn the black and the green wires anywhere.  I see no shorts between the tube pins.  Can the DC resistance be so low within the Heater secondary?

It's very normal for a filament winding to read near zero ohms dc resistance.

[llama]

Thanks Sluckey.

That's a relief!  Now I cam move forward in the build.  My next step is to wire the power section, up to the rectifier, and test the heaters.  Then I'm onto populating the circuit with the components.

One more question.  How do the experienced builders here populate the circuit?  Do you fill in and solder the resistors and caps first, then run the wires?  Or start at one end (the input) and move along the circuit? of something else?

I ask because I was pondering the build procedure for ease of component swapping and experimentation.  I know that once the build is complete, some components/wires will be below others.

Any opinions?

[SILVERGUN]

I wouldn't call myself an experienced builder,,,,but after much research, this method worked best for me:
1) Tie common turrets together with buss wire and solder (check EL34's build pages)
2) Attach colored wires (to identify different paths in the circuit) by wrapping around turrets (they take more heat than the components, so do them first)
3) Install components
4) Use the holes in the top of the turrets to run control wires last (again EL34 method)

See dougs page here: http://www.el34world.com/charts/toolhowto.htm
for more info about bottom pic

[SILVERGUN]

Also just as a GREAT reference....see Sluckey's webpage/site...he has done an incredible job of documenting his work for all of us to learn from...
I am a smarter person just because I visited this site:
http://home.comcast.net/~seluckey/amps/index.htm

[SILVERGUN]

Llama came to visit my shop last night and these were the results:
1) He may be the nicest guy I have ever met.........I thought I was a nice guy,,,,BUT, now I know I have some work to do
2) His little LlamaTone amp is a very ingenious design that made me morph into "blues guy" immediately.......the touch sensitivity of that amp really stood out to me, and it was very cleanly built AND dead quiet....very nice,,,,I might have to build one
3) He tried my amp, and if there was ever any doubt, we now know for sure: he's not really a "high gain" guy...........in fact I think my amp probably expanded his definition of high gain (or scared the crap out him), AND he may have permanent hearing damage as a result........he did seem to appreciate that there was a certain something to my design......the look on his face was priceless as I pushed all 44 watts (momentarily) out of a nearby 4x12 cabinet.......he commented that the amp did seem to "fit" ME (he's right)
4) We shared some design ideas and I showed him how to potentially electrocute himself,,,,,and he assured me that he won't be following my example..............we'll see
5) I'm really looking forward to the completion of the PenTone amp, and may have to make an effort to go see his shop and hear how that amp tuns out
6) He's an extremely intelligent guy who is VERY deserving of all the help you guys can give him........he is doing his homework and really trying to understend the theory behind the "magic"
7) I now want a LlamaTone t-shirt (black- XL -meter logo in blue and grey,,,,,i'm just sayin)

Good Luck Buddy, it was a pleasure to meet you

[llama]

Silvergun,

You Crack me up!  Your amp was Awesome Set on Fire and Giving off Gamma Rays!  I may now have cancer from standing near it!  I really dig your ear and how you can discern the slightest frequency nuance.  I wish you great luck in chasing your tone!

Guys,

His amp gave off a intricate brown texture that Eddie would have drooled over.  We tried the 7247 (12DW7) tube from my AX84HO in his Input tube.  The 12au7 side of the tube was his second stage. the lower MU (20?) and special "U"-mojo that is has, seemed to tame some of the lower gain settings to add a second trick to this young pony.  Follow Silvergun's post about it here: http://www.el34world.com/Forum/index.php?topic=14575.0


Thanks for the opportunity to meet you and see your Laboratory.

[SILVERGUN]

Giving off Gamma Gays! 

Gamma GAYS .......whoa Buddy, your gonna have to fix that typo?

You must have really taken it the wrong way when I asked if you wanted to try my equipment......GEEZ......you're not THAT nice of a guy

[llama]

OOOOPS- thats the worst Typo I've ever made!

thanks!

[SILVERGUN]

Hey Buddy,,,,don't miss this....
I'm really thinking about building one of these soon
No reverb,,,no tremolo,,,,one channel,,,,,sweet

Sluckey makes it a no brainer,,,,look at the layout on page 2

[SILVERGUN]

Sorry I keep hijacking your thread, but I cant send an attachment in PM;
Here's another simple one not to miss......this guy is another top notch nice guy:
http://www.el34world.com/Forum/index.php?topic=14692.0

Listen to his description of this amp.....I want to build one just to hear it side by side with the PenTone

[tubeswell]

Updated the Layout as per Silvergun's advice and other tweaks,and fixes.

FWIW, with SE amps, a CLC filter works better for smoothing the ripple current if its between the 1st (reservoir) filter cap and the OT plate supply. The schematic/layout of your amp to-date shows the CLC filter between the plate and the screen (which won't do much to alleviate ripple hum).

Also, in case you're wondering, given that its a single EL84 (or equivalent) output stage, a 50mA DC-rated choke should work fine in a 'whole-of-supply' CLC filter for that amp.

[SILVERGUN]

Yo Llama, are your ears still bleeding?

What's up with the PenTone??....the world (or at least just me) is waiting

[llama]

Hey SG,

I finished wiring up the entire amp, and am reading through Paul Ruby's start up guide.  But I have hit a wall,  I looks like my PT is kaput!

Which is a real problem as it fits perfectly into the cramped chassis.  the PT and the Chassis were produced together in 1961.  I'm going to have trouble finding a replacement....so sad!

I get no continuity between the secondary HT leads.  On lead to ground is fine, but there must be a break in the other.

The odd thing is, that I had the heaters glowing and the light on for a short time at one point.  I never had connected my secondary HT's the the circuit.  but now I get no voltage out, so I must figure this one out...

[llama]

FWIW, with SE amps, a CLC filter works better for smoothing the ripple current if its between the 1st (reservoir) filter cap and the OT plate supply. The schematic/layout of your amp to-date shows the CLC filter between the plate and the screen (which won't do much to alleviate ripple hum).

Also, in case you're wondering, given that its a single EL84 (or equivalent) output stage, a 50mA DC-rated choke should work fine in a 'whole-of-supply' CLC filter for that amp.

Hi Tubeswell,

Thank you for your input.  So, for best smoothing on an SE, I should connect the Inductor's second lead to the OT where the signal is coming in (plate connection), and then connect the second filter cap to the first directly , where lead on of the inductor taps off?

I included a jpg to help me convey my understanding.

[SILVERGUN]

 I looks like my PT is kaput!

Very sorry to hear that.....I knew I was worried about you for a reason..
Good luck buddy

Are you sure you're reading the correct wires?.....not all PT's share the same color scheme for wiring

[llama]

Yeah,  I had a guy with much more experience check it out.  I'll tear it apart tonight to bee if I can salvage it.  I'll keep you posted.

[SILVERGUN]

Let me know if you want to modify the chassis...thats what all these welding and cutting machines are for