Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: llama on October 03, 2012, 04:08:13 pm
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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
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They create an artificial center tap to ground in lieu of an actual heater center tap.
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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. :icon_biggrin:
--DL
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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. :icon_biggrin:
--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
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I'm not familiar to why the lower Mu tube for the CF.
More current?
Brad :think1:
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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
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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.(http://):help:
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> 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.
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Ahh yes I forgot about the plate resistance. Thanks PRR
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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!
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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
(http://farm9.staticflickr.com/8322/8057541746_88c2d2df72_b.jpg)
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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
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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.
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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!
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OK, Thanks guys, I'll need to digest this all and will reply. :notworthy: :huh:
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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!
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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. :l2:
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
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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.
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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!!!
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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.
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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!
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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!
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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
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Yes, you want the resistor soldered to the tube pin......it WILL help cut down on micrphonics and even possible RF interference
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OK,
Thanks Will build it that way -- Thanks for your help Silvergun!
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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! :icon_biggrin:
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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 :icon_biggrin:)
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
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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
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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.... :blob8:
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
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How's it going llama, :w2:
You're awfully quiet.....you're making me nervous....
Any time you're working around 400 volts :blob8:.......no news could be bad news.........what's up?
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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?
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How does the Tuesday before Turkey day sound for me to come see your new amp?
Sounds good,,,I'll pencil you in
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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 (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!)
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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.
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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?
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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 (http://www.el34world.com/charts/toolhowto.htm)
for more info about bottom pic
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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 (http://home.comcast.net/~seluckey/amps/index.htm)
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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 :sad:
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 :icon_biggrin: (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 :occasion14:
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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 (http://www.el34world.com/Forum/index.php?topic=14575.0)
Thanks for the opportunity to meet you and see your Laboratory.
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Giving off Gamma Gays!
Gamma GAYS :huh:.......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 :angry:
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OOOOPS- thats the worst Typo I've ever made! :l2:
thanks!
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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 :thumbsup:
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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 (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
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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.
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Yo Llama, are your ears still bleeding? :icon_biggrin:
What's up with the PenTone??....the world (or at least just me) is waiting :offtheair:
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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... :cussing: :sad2:
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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.
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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
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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.
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Let me know if you want to modify the chassis...thats what all these welding and cutting machines are for :thumbsup:
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That's a nice offer, Thanks. I'll let you know!
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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?
No.
Choke goes between B+ PSU nodes A and B. OT then connects to node B but not where you have it. It goes to the OT primary leg shown as A in your drawing.
Brad :icon_biggrin:
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I'm glad I asked. That made no sense to me, but a senior tech where i worked said that was right. Guess I have to stop listening to him!
Thanks!!!!
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Here's a drawing with plate and screen node shown. It's for an octal (8 pin) power tube but it's the same thing just different pin numbers.
You don't have to use a separate B+ screen node for SE amp but you can, it will only help with quiting down any B+ ripple noise.
Depending on what you use for your PSU rectifier, SS or tube you can increase the filter cap values.
Brad :icon_biggrin:
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You don't have to use a separate B+ screen node for SE amp but you can, it will only help with quiting down any B+ ripple noise.
Depending on what you use for your PSU rectifier, SS or tube you can increase the filter cap values.
Thanks Brad!
The pictures sure helped.
If I understand it all correctly, I can use the node B for both the plate and screen, but it may be noisier that using the screen on a later node.
I used my choke differently for my ax84HO build (that this PSU is based on). I put it in front of the first cap in series, and that does a great job filtering noise, plus it dropped my B+ to a better voltage (in my case).
I'm using SS rect., Should I go bigger caps to keep noise down. I recall reading, maybe not, that 22 - 47 uF filter caps do a even job, Is that incorrect?
I appreciate your help. Thank you!
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If I understand it all correctly, I can use the node B for both the plate and screen, but it may be noisier that using the screen on a later node.
Yes.
I'm using SS rect., Should I go bigger caps to keep noise down. I recall reading, maybe not, that 22 - 47 uF filter caps do a even job, Is that incorrect?
There's a limit of how much capacitance you can use at the 1'st cap (A node) if you use a tube rectifier. The larger the caps value the more initial inrush charging current is needed to charge the caps. The inrush current is at least double to triple the normal current needed once the filter caps are fully charged up. The tube rect. can't handle rectifying this extra current and will die sooner or later from stress. Depending on how large the uF value (60, 80, 100uF) is that you use at the 1'st node, it could die the 1'st time you power it up.
IIRC there's a couple of things you can do to get around this?
1. Because you have a choke in series with the B+ PSU leave the 1'st (A node) within the limit on the rect. tubes spec sheet and add any extra uF's needed after the choke at node B. The choke will slow down the inrush by it's nature.
2. Put a stand-by switch after the 1'st filter cap but before the choke. To do this just break the connection at the A node and choke, insert switch. This way only the A node filter cap/s will draw current. Then once the cap/s are charged up and amp is warmed up flip the switch to on/play. Now the A node caps will not need any current to charge up only the caps down stream will need charging and they are after the choke.
3. Install SS diodes in series with each tube rect. plate. IIRC they do the work of rectifying the AC to DC and protect the rect. tube from wall voltage line spikes? I don't recall if they allow the rect. tube to pass more current at start up? Tubeswell knows.
4. Do all 3 of the above.
I added how I'd wire the ground for the circuit with a single connection to the chassis. The other chassis connection would be at a different area on the chassis for the power cord green ground wire.
Here's another drawing of it with what I've listed.
Brad :icon_biggrin:
Edit; They add the SS diodes as a back up if 1 of the tube rect. goes out. The SS will keep working.
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I pulled my PT last night to investigate the "deadness" of it. Turns out that it was only mostly dead (thanks to Billy Crystal's line).
One of the HT secondary leads was severed about a 1/2 inch outside of the end bell port. I pulled the endbell off and had 2 inches of wire to work with and soldered a new lead and shrink wrapped the junction. I did that same for both the the Primary connections, as they were very short and giving intermittent readings. Both the Primary and secondary DC resistances are reading within spec, now.
Just a small setback, but what a great lesson, in how to test, troubleshoot and repair a transformer that someone would have left for dead.
I haven't reinstalled it to see if it "works", so no victory dance yet, just in case there is another issue! It is, after all, a 51 year old transformer. It's ugly inside, See pic below.
I will dispense with the crimp connectors and solder it all!
But, I'm hoping for speaker tests by the weekend.....
Willabe, I'll make that change to my circuit as well, Thanks for your help.
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One of the HT secondary leads was severed about a 1/2 inch outside of the end bell port. I pulled the endbell off and had 2 inches of wire to work with and soldered a new lead and shrink wrapped the junction. I did that same for both the the Primary connections, as they were very short and giving intermittent readings.
Good job!
Brad :thumbsup:
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Just a small setback, but what a great lesson, in how to test, troubleshoot and repair a transformer that someone would have left for dead.
Way to go L,,,,,thats great news and a commendable job of troubleshooting
:happy1:
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And now the Ubiquitous Good News / Bad News scenario.....
The good news is that I fixed the tranny and she's pushing 529Vac between the Secondaries. :icon_biggrin:
The bad news is that I have a short somewhere towards the input side of the standby switch. :w2:
-I'm using the Paul Ruby Startup guide. Now with the PT is in order, I move onto soldering the PT to the rectifier (SS).
-I'm also using a Lightbulb limiter in series with the amp.
-When the unit is on Standby, i see 6.8Vac across the heater wires, 300V -dc after the rec, and the lightbulb is glowing dimly. When I go to full on, the bulb gets bright and i only measure 12V dc at the rec. -Sounds shorted to me.
-Just to be clear - the tubes are not installed yet.
- I suppose I could unsolder the path half way toward the input, and see what that tells me.
-Anything that I'm missing here?
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I would suspect filter caps first. Disconnect them one at a time and see if the short dissapears. You don't need to power up to do this. Just connect your ohm meter between the rectifier output and ground with the STBY switch in operate mode.
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I would suspect filter caps first.
Are you still trying to save a buck and use those NOS caps? :huh: :rolleyes: :embarrassed: :angry:
Bring it here and I'll show you how to figure it out........whatever you do, DON'T DIE :wink:
I'd hate to have to explain to your wife that you were trying to use 10+ year old caps, in a brand new amp design
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Are you still trying to save a buck and use those NOS caps? :huh: :rolleyes: :embarrassed: :angry:
Nice!
No the the filter caps and all electrolytics in this build are new. The NOS's i've used are ceramic.or poly.
I might have already performed the test that Steve mentioned, but will do so as he described.
how low of a resistance reading should raise a flag. I tend to look at the value of the resistive components and see if that is close to what I'm reading.
Divide and conquer is what I'll try next - thanks for your concern SG!
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Here's a quickly done visual explanation of what Sluckey is telling you to do.....please forgive the slop
I tried to write things out as simply as possible, so please excuse me if it's too simple,,,,but I'm not sure what you know
how low of a resistance reading should raise a flag. I tend to look at the value of the resistive components and see if that is close to what I'm reading.
An ideal capacitor would have infinite resistance
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Holy cow SilverGun! That was AWESOME!!! I loved you use of colors. Very Festive!
Thank you. Your picture made more sense out of it for me!
I never did have to do that though.....I found the culprit, once I started asking intelligent* questions. The fist question i asked, was why is B+ to ground 100R? Then i saw the 100R bleed resistor next to C1..... Somehow I installed a 100R resistor instead of the 220K for the bleeding. My amp was a hemophiliac.! It bled out all if it's current!!!! Good thing I'm not building humanoids.
Once I switched it out for the correct value the Voltages were all OK - but very low.
Second intelligent question: Why is my voltage reading 188 at B+ for a 280V CT PT? -- Then I saw the light....it was dim. My Light bulb limiter was dropping the voltage in series. Thank you Ohms Law! I felt comfortable to remove the limiter and my voltages at B+ are now 316Vdc. That's better. I know that's a bit high for the tubes, but it was what I was expecting.
I took all the measurements as per the Ruby guide and played guitar :m11 for the rest of the night, while the family slept.
It sounded good. More to come!
*Note: The use if the word intelligent does not indemnify me as one who is. Just used as how someone who has a clue might think.
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I have edited my schematic to show the changes and mods I've made up to this point, from the original.
I hope you can see the voltages at each node, plate and cathode.
right now, the amp how nice growl at medium gain settings. if I turn gain and master Volume up i get a flubbing sound that i guess is an oscillation. kind of a rapid thub thub thub sound, not too loud, but definitely there.
also the tone stack makes no change in sound. (assuming CF load resistor (more on that in a later post))
could more trained eyes tell me if my voltages seem OK and.
BTW I am using a 7189 instead of a EL84 because it has a 400V plate limit vs the 300V in the EL84. But i hear that 316V on the plate of an EL84 is usually OK!
Thanks for all of your help through this
-Scot
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You have B+ nodes C and D fliped at their tubes.
Brad :icon_biggrin:
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You have B+ nodes C and D fliped at their tubes.
Brad :icon_biggrin:
Hi Brad, Thanks,
That's the way Firemedic had it. I thought it was for the fact that the pentode likes lower voltages. I realize it is more noise for the input.
If you don't agree, then I could change it around, but that's how it's wired!
-Scot :dontknow:
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That's the way Firemedic had it. I thought it was for the fact that the pentode likes lower voltages. I realize it is more noise for the input.
OK, that makes sense. Thought it was just a typo.
Brad :icon_biggrin:
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How did it go last night? :w2:
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I didn't have much time to do a whole lot, but I was able to remove the CF cathode resistor (56K) and add a 10k in series with a 100K pot.
I played through it while adjusting the pot and sound hardly changed (except for it quieted at the lowest settings and lit got much louder at the highest settings. 91K sounded loud and clear (plenty of distortion-for me).
The tone stack was completely useless at all settings.
I followed PRR's advice from a much earlier reply on this topic. :worthy1:
Beginning of quote----------------------------------------------------------------------"
Re: 5879 Preamp With Ax84 P1 SE
« Reply #7 on: October 04, 2012, 08:37:10 pm »
Reply with quoteQuote
> 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. "
End of Quote----------------------------------------------------------------------
I used his method for calculating the best cathode resistor, and came up with 21K (+243V at node D, and 153V at CF cathode. 243-153= 90V across the tube. The 56K cathode resistor must be flowing 4.3mA. The tube must flow like a 90V/4.3mA= 21K resistor.) Obviously I just copied PRR's text and used my values - don't know what I'm doing here!
So I set the pot to measure 21K across the pot and 10k resistor, and no change in sound, nor tone stack...
I'm bewildered!...actually, I'm too NooB to be bewildered....More confounded, but thanks for asking, Silvergun!
-Scot
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As Promised.
I recorded the 5879 amp for your critique, Sorry that I only used my iphone, and the video points at my ceiling (but the mic is at the bottom).
The guitar is a Epi LP with A2 pickups, and the speaker is an Emenace pulled from a 96 Fender HRD.
The vid lives on my Facebook fan page (eh..:angel)
https://www.facebook.com/LLamaTone (https://www.facebook.com/LLamaTone)
I plan on doing better recoding once the amp is in good shape to travel to the studio.
-Scot :angel
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Thanks for the update....
Keep up the good work,,,the amp sounds great and of course I like it best with the gain cranked!! :grin:
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Fellas, I glimmer of understanding has illuminated my brow. I get it now. the Tone stack is a voltage and frequency divider.?.!
I never added the PAB? switch from Firemedic's original drawing. I figured that I'd leave it out and then see what it does.
1. I have added a rotary switch with different R values to be in parallel with the existing 330K to ground (straight wire, 100R, 4.7K, 470K, Open). I did start with a potentiometer before, but thought this was more elegant.
-I couldn't give it too much time to nail down my observations but her's what I recall -the wire and the 100R sound vintage-like - less volume, softer sounds, the 4.7K has a great sweep for both treble and base knobs & maybe more mids. The 470k has limited sweep for both knobs, but has a louder, rapier sound, and open (no paralleled resistor to the 330K) sounds loud, and in your face!
Is this type of switching used much for voicing changes? are there better locations?
2. I ended up soldering in 92k of resistance for the Cathode follower Cathode load (say that 3 times fast). the sound was richer to my ears. I need to reread the Merlin chapter on CF's. My tone stack seems to work fine at that and any load i tried with my 100K pot. Am I creating much current amplification with such a high load? Would i be better off switching to a normal gain stage (low gain)?
I know I've been riding the coat tales of Silvergun, but The mad scientist thing is really cool...
Thanks for all your help!
-LLama
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1. I have added a rotary switch with different R values to be in parallel with the existing 330K to ground (straight wire, 100R, 4.7K, 470K, Open). I did start with a potentiometer before, but thought this was more elegant.
-I couldn't give it too much time to nail down my observations but her's what I recall -the wire and the 100R sound vintage-like - less volume, softer sounds, the 4.7K has a great sweep for both treble and base knobs & maybe more mids. The 470k has limited sweep for both knobs, but has a louder, rapier sound, and open (no paralleled resistor to the 330K) sounds loud, and in your face!
Is this type of switching used much for voicing changes? are there better locations?
That's a cool idea, and as long as the results are noticeable enough to deserve a dedicated switch, that looks like a great way to achieve a few different flavors.........better locations?,,,,only if your experimentation deems them to be better
If you're going to keep the switch you can eliminate the parralel 330K (just so I don't have to do the math on your newly paralleled resistor values)
2. I ended up soldering in 92k of resistance for the Cathode follower Cathode load (say that 3 times fast). the sound was richer to my ears. I need to reread the Merlin chapter on CF's. My tone stack seems to work fine at that and any load i tried with my 100K pot. Am I creating much current amplification with such a high load?
Where is that reflected in your schematic? (56K-R14?).......that value is going to affect the overall distortion character of the amp, and I'm hoping one of the big boys here will help you figure it out...it's my understandeing that lowering that value can help warm up a fuzzy sounding distortion...?(merlin)
One thing to keep in mind....If the amp sounds great,,,it may be done!.....if that CF circuit was designed around the 56K value, it might be in your best interest to keep it there and look elsewhere for enhancements (play it safe with the bias point).........dont forget about the point i hit where I was "boosting" the signal at every possible location, and it always sounded "better", BUT I also could've just increased the Volume and gotten a better sound (pushed more signal)...make sense?
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The 330k resistor location on bottom of the stack serves the midrange part of the frequency. With nothing (bare wire) you get the least amount of mids with the famous deep notch in low mids famously used by metal heads. The greater the value, the more mids and increase in signal strength. That's why it's called a "boost" too because it's also called a tone stack "lift". Meaning when the stack is not grounded it is lifted as if it was out of the circuit and it gives the boost because it's not loading down the signal. This gives the "tweed" effect of the entire full signal w/ all the mids included. I've made amps w/ the mid pot containing a switch to switch it out of circuit and w/ a DPDT switch to give varying degrees of mid range content and frequency response. It's up to you whatever you like.
Generally the CF stage has a matching resistor to the plate's but Merlin suggests 'lowering' this value for smoother operation/grind like what SG said.
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The 330k resistor location on bottom of the stack serves the midrange part of the frequency
So consider just putting a mid pot in there, if it makes more sense to you than the wiring and extra resistors for the switch
The switch could be viewed as a cool little "trick", and maybe incorpoated as a 2 position slide switch (vintage - boost) OR (good Llama - bad Llama)
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Thank you JojoKeo,
It took me a bit to adsorb your comments, plus a little interpretational help from Silvergun. I see what your saying. I could use a 500K pot instead of switching resistors as well. Actually, I tried it last night, and I'm amazed by the sweeping change the mids make. Thank you.
I decided to move on with the switch since it took me a while to find a switch that would fit in the chassis. I'm using a Jamco military rotary switch that is used in aircraft, ships and tanks..and for free....why not! It's all in the name of mad science! I have 4 positions so I plan on going with a 100R, 5K, 50K and 330k, or somewhere in the neighborhood.
I plan on removing the 330k from the board and adding it to the switch so it all is in series. no more parallel resistors. I'll have a bit of time to get the R values figured out tomorrow.
...and now I know the the Tweed effect is all about...full mids and more signal -- thanks for your help JojoKeo.
BTW, Silvergun here's what I'm talking about- Mine has 4 positions but I found some with up to 12 (now that's overkill!) and I need to save a bit of room for one more knob for Merlin's Pentode Blender thing. But I'm sure that your right!
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Boy that's too nice of a switch to use for midrange controls. If it was me, I'd use a 25k or 50k pushpull pot or one w/ a SPST switch.
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The issue is that I'd have to actually pay for a Pot with a pushpull. My company has military grade components. My amp is filled with them!
I though that i should use a 500K pot as a resistor (lug 3 not used) because of what you said about the high resistance. I guess the spst switch would cover the 330K then the pot would cover the lower end? I think i see said the blind grasshopper!
BTW let me know if your interested in any components, my company does not yet sell to the consumer market, nor does it market these components i find so useful in amps. I was going to contact Doug about selling off some neat things....eventually :icon_biggrin:.
Thanks again JojoKeo
:worthy1:
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lucky you, you get nice components for a great "price"! That would be expensive normally. I don't know the power ratings but it looks pretty robust, you could use it as an impedance selector switch maybe where they're 12 to 15 bucks ea., the power supply area to adjust voltages, fixed power scaling, etc...
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lucky you, you get nice components for a great "price"! That would be expensive normally. I don't know the power ratings but it looks pretty robust, you could use it as an impedance selector switch maybe where they're 12 to 15 bucks ea., the power supply area to adjust voltages, fixed power scaling, etc...
Now Your talking! I know the one I have has a 1 amp rating, Some smaller one are a 1/4 amp. Dunno the Voltage ratings, but I assume I'm good. I think that they go for about $50 to Uncle.
Tell me more about power scaling... :angel
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if I turn gain and master Volume up i get a flubbing sound that i guess is an oscillation. kind of a rapid thub thub thub sound, not too loud, but definitely there.
Did you ever fix this "flubbing"? It's really motorboating...
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Did you ever fix this "flubbing"? It's really motorboating...
Hi Blueplates, no I still have the issue at higher signal levels and bass levels. I haven't yet looked into it but now I'm nearing the finish line with this one, and would love to get it right.
Btw just so we all are aware, last night installed Merlin's pentode morpher from the end of chapter 3 in his preamp book. Sounds neat. So my schematic is not up to date.
Thank you for inquiring,
Scot
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Okay, then...
Two things to try:
1. You have a 1k resistor between your output tube plate/screen node and the following node. 1k and 16uF is 3dB down at ~8Hz, which seems reasonably close to how fast the "flubbing" was. Since you're only pulling a couple small tubes' current through that resistor, make it 10k and see if the noise goes away.
2. If it doesn, add a resistor and cap between R2 and C2, and shift your preamp power supply to the left on the schematic. Essentially, you're just adding another R-C filter section to your power supply, duplicating R2 and C3. This new node will feed your screen and keep it separate from the plate node for the outputt ube. Again, due to low current demand, the "old R2" should be bumped up to ~10k, while the "new R2" might be something like 4.7k.
So your power supply, working from the rectifier to the preamp will look like:
20uF -> choke -> 20uF (plate node) -> 4.7k -> 20uF (screen node) -> 10k -> 20uF (5879 node) -> 10k -> 12AX7 node.
But we'll try the simple fix first before adding a power supply node.
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So your power supply, working from the rectifier to the preamp will look like:
20uF -> choke -> 20uF (plate node) -> 4.7k -> 20uF (screen node) -> 10k -> 20uF (5879 node) -> 10k -> 12AX7 node.
Good suggestions HBP but I think you may have forgot one last capacitor after the last 10k to 12ax7 node. Should be: ...->20uF (5879 node) -> 10k -> 10uF -> (12AX7 node).
The morph control just makes a transition into triode mode w/ a pentode tube rather than only being a pentode-only/triode-only option. It lessens gain doing this which should help lessen oscillations / motorboating. Do you notice this to be the case llama? After many pentode amps I don't include a triode mode option anymore, why use/design for a pentode tube if you're going to run in triode mode? Doesn't make any practical sense, should just have made or remake using a 12ax7 then. What I do like and prefer is using a pentode w/ variabe gain control. The "Squishy" control is also pretty lame/weak and a waste of time and faceplate area but I digress...
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HBP,
Thanks for your input. I will swap out the 1k resistor with the 10K as soon as i'm able, then will repost. I found your second option very enlightneing, as I can understand how the motorboating may come about from my the plate and the screen supply coming from the same node. (but of course, I will try the resistor first).
JojoKeo,
I definitely like reading your input and opinions. Keep them coming. This is an experiment in understanding and tone. I can see why you dispense with the gagetry and go for the proven. A pentode gain control would be a much simpler and useful. But at least I can see where your option comes from. I'm like my son, I can try to save him the time and pain in his journeys based on my experiences, but he feels the need to try it himself and learn from the pain.
Which brings me to my latest lesson learned. After dicking around with it and adding and removing elements, the amp sounds fizzy and the bass is flubby, especially with my single coil (lace sensor) Strat- yuck. Now I'm going to have to go back to basics. Plus since the chassis in non standard, I cannot have a super clean build. I had a series of shorts yesterday, from moving soldered components around so i could solder in a tight spot.
I must have damaged a component or two in my zeal to get it up and running before an event.... :BangHead:
I will test and learn and will be better for it,... eventually. Thanks guys!
Thanks guys.
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So now this is the point where you can't let yourself get down.....you WILL figure it out and it WILL be another AH-HA moment, and it's what everyone here has gone through.....
Be patient and work from input jack wiring, straight through the circuit, and follow along with a highlighter on a printed out schematic to eliminate what you have tested, and what you must.....don't take anything for granted
On my wreck, I plugged it one day and it was GONE,,,so I remembered having tugged on the input jack with a cable plugged in , and sure enough, I got lucky,,, and that was it.......1M resistor to ground buss (that I had been messing around with previously) came loose off of the input jack......soldered it back in and wham,,,,back to melting faces
GOOD LUCK...........Llamatone is for winners.......winners dont quit............not all winners are Llamas, but ALL Llamas ARE winners.....little known fact
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GOOD LUCK...........Llamatone is for winners.......winners dont quit............not all winners are Llamas, but ALL Llamas ARE winners.....little known fact
Thanks for the cheer leading, Silvergun! You are a good guy, I don't care what the other guys say about you...
It turns out that my amp just did not like the JJ El84 i switched to from the 7189, but then I went with a 6005 in the seven pin spot - sounds really nice in most of it's settings.
I'm figuring that since This is an experiment, that not all of the settings will be winners, but many of them sound really nice with both my strat and LP.
I still have a strange chassis grounding issue that i can tap on the amp to get working at times - like the old Tvs _not so hard though. and the 5879 is really sensitive to vibrations and osculations.
HBP, I replaced the 1K resistor with a 10K and the motorboating still occurs, and now I have a high frequency noice that I can describe as the tubes screaming. all of these noises occur at high gain, high volume.
_I guess it's time to think about the extra RC node - After the holidays!
I asked my web expert how to take vids with good sound quality - I hope to do that sometime this week.
Thank you for everyone who helped and those also who read and followed my plight!!
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Hi all. THe holidays are over, and I'm digging into my amp again.
Below are my updated schematic and layout (not exactly a matching set, yet) reflecting changes I've maetiond here, as well as HotBlue Plate's suggestion for the extra PS node.
I Plan on adding the new node very soon, and finalizing values soon.
-LLama
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Hey...welcome back :occasion14:
We've missed you around the disfunctional round table...
Here's that link I was telling you about:
http://www.geofex.com/article_folders/vintvolt/vintvolt.htm (http://www.geofex.com/article_folders/vintvolt/vintvolt.htm)
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Hi Llama,
After looking at the schematic I have concerns regarding how you drew that 5879 going into the 12au7. The pot is doing "double duty" a) for the signal voltage divider going into the 12au but also b) the .1 capacitor for the 5879 - which affects the respsonse of the 5879. Is that truely what you want going there instead of being seperately controlled?
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Hi Llama,
After looking at the schematic I have concerns regarding how you drew that 5879 going into the 12au7. The pot is doing "double duty" a) for the signal voltage divider going into the 12au but also b) the .1 capacitor for the 5879 - which affects the respsonse of the 5879. Is that truely what you want going there instead of being seperately controlled?
Now your making me think above my understanding level.... I believe the Morphable Merlin Pentode control does two things. It affects the Screen Compression through the screen grid voltage amount and used as a triode/pentode control in either end of the dial.
I'm pretty sure that I wired and drew it according to the Merlin pic (attached below), But I've been wrong before!
Since Merlin usually escapes my full understanding, here is an except from his book. (I hope this is not in violation of this site or of Merlin's Copyright)
But overall I like the effect with a single knob - Thanks again JojoKeo!!
----------------------------------------------
Triode/pentode ‘morph’ control:
Taking this even further it is not hard to combine both pentode/triode switching and variable screen bypass in a single control, if we appreciate that for triode mode the screen only has to be connected to the anode as far as AC is concerned. With the addition of one capacitor we obtain the circuit in fig. 3.29. We now find that with the control fully clockwise, the screen bypass capacitor, Cg2, is connected directly to ground and we have normal pentode operation. P1 is large enough that is does not heavily load the pentode.
When turned fully anticlockwise we see that C1 and Cg2 appear in series and effectively connect the screen directly to the anode, as far as AC is concerned, resulting in triode operation. (C1 is included simply to keep DC off the pot’.)
The actual DC voltage on the screen remains unchanged, and this causes the ordinary triode characteristics [fig. 3.6] to be shifted to the right by an amount equal to Va-Vg2;6 we effectively create a unique set of triode characteristics depending on the screen voltage chosen!
At intermediate settings we obtain the screen-compression effect described earlier and ultra-linear operation, all through the setting of one potentiometer! Assuming we want the same variation in effects at all frequencies, then C1 and Cg2 must be large enough to pass all useful frequencies between anode and screen.
C1 must be large enough that the reactance of the series combination of C1 and Cg2 is less than Ra∥ra(triode) at the lowest desired frequency. Therefore C1 should be at least five times larger than Cg2, if possible, for triode operation down to around 100Hz. The frequency response of the circuit in fig. 3.29 is shown in fig. 3.30, and it is interesting to note the stage has the lowest gain at an intermediate (ultra-linear) setting of P1.
EXCERPT FROM
Cover Art
Designing Tube Preamps for Guitar and Bass
Merlin Blencowe
Category: Engineering
Blencowe, Merlin. “Designing Tube Preamps for Guitar and Bass.” Lulu.com, 2012-02-17. iBooks.
This material may be protected by copyright.
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add a resistor and cap between R2 and C2, and shift your preamp power supply to the left on the schematic. Essentially, you're just adding another R-C filter section to your power supply, duplicating R2 and C3. This new node will feed your screen and keep it separate from the plate node for the outputtube. Again, due to low current demand, the "old R2" should be bumped up to ~10k, while the "new R2" might be something like 4.7k.
So your power supply, working from the rectifier to the preamp will look like:
20uF -> choke -> 20uF (plate node) -> 4.7k -> 20uF (screen node) -> 10k -> 20uF (5879 node) -> 10k -> 12AX7 node.
Hi Hot Blue Plates. I performed your advice and added a new node as you and JojoKeo described. And that sure fixed the motor boating issue I had. I returned the Screen resister back to 1K also. Thank you Thank you Thank you!!
I also reduced the Fist Coupling cap from 0.22 to 0.1. That brought the bass to a fine, fine level. Still enough to shake the walls with a wicked growl, but much more tight. Dropped D with a Les Paul in high gain sounds Deep and tight - very modern. But with a Strat, I have a nice bassy blues bottom end with plenty of vintage feel, especially on the Neck. Pure Silky drive. It's like getting kicked in the balls while wearing silk boxers! Kick me again! :huh:
The only ugly noise left is a high pitched drone that appears when all of the knobs at dimed. The frequency lowers when I roll off the gain a bit and stops after a 1/5 turn of the Gain Pot. My guitar signal has no affect on it.
With a scope app I can see two frequency peaks; one near 3200 and lesser at 6300 KHz (all knobs on 10).
Soo Close to Perfect (my description of it). Thank you to everyone who responded to my queries. I am in indebted to you all!!!
Tube Amps Rock!!!
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Here is a 5879 with triode-pentode morph control from a circuit I have built. Works fine. (The input is where I had the vol control - so to make this schematic technically correct, it'd need a grid leak resistor there)
(http://farm9.staticflickr.com/8373/8380309019_37f8b80fb8_z.jpg)
The only thing I would review is the morph control pot - it doesn't need to be 1M (not enough useable turning range of the pot). 500k or even 250k would probably suffice.
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"I believe the Morphable Merlin Pentode control does two things. It affects the Screen Compression through the screen grid voltage amount and used as a triode/pentode control in either end of the dial.
I'm pretty sure that I wired and drew it according to the Merlin pic (attached below), But I've been wrong before!"
You don't have it drawn like it's supposed to be. If you drawing is correct then you are doing it incorrectly. The morph control only affects tube compression btwn the .1uF cap & Ground. If it was to involve "morphing btwn pentode/triode modes" then your drawing doesn't do that either. Lastly, your volume pot should be by itself and not mixed with these other two controls as you'll never be able to control them independantly.
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OK Thank JoJoKeo,
But I'm a little confused. are we all seeing my updated schematic? :w2:
:worthy1:
***Note * Saw that my CF section was disconnected from the B+ node. Redrew line. Hope this was not a point of contention.
I replaced the image
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sorry, comparing w/ tubeswell's schem I see it now...
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sorry, comparing w/ tubeswell's schem I see it now...
.....Whew! I was getting worried that I F#%&ed up something royally... Thank you for even reading my posts!
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No, you're fine Llama! What messed me up was the morph control combining with the cathode follower use. I was going to give you another thing to experiment with but in seeing the tube & it's use, I think it's okay the way it is? But - as with all CF's for grins, see how much gain you add/remove by lifting the bypass cap on the 5859. It can clean up the tube even more & also differently than the morph control does. It's only a little switch if you like the option and want to add it? Sometimes the triode mode doesn't do enough and this would help give a bit more clean tone/less CF gain for a little smoother response too.
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If you want to use it with a CF, try hooking the morph control pot input to the CF cathode (better load impedance for the pentode). I've done this and it works good.
(http://farm9.staticflickr.com/8192/8381168687_a3a6c43e81_z.jpg)
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I just got my copy of Merlin's preamp book and I am still on chapter 1, a long way from morph controls, but I notice a similarity to the "squish" control on the AX84 Blues Preamp, which I have built. It has one less capacitor and the pot is wired differently. http://ax84.com/static/corepreamps/Blues/AX84_Blues_Preamp_Schematic.pdf. (http://ax84.com/static/corepreamps/Blues/AX84_Blues_Preamp_Schematic.pdf.)
As you turn it up it gets louder, more aggressive sounding, and a lot noisier. I don't know that I would describe it as a compression effect, but then, I have never used a compressor, and I'm not sure I would recognize a compressed signal. I pretty much keep it half way up.
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WOW! Page 3...
From Advice I received from Jazbo8 on the ax84 forum (he originally sent me here when I was looking for a 5879 build- Thanks Jaz!), I chopped-sticked my build last night with the knobs dimed to find the source of the High pitched wailing. I scared my wife half to death when I moved the wire coming from my Bass pot to the mid switch. It shook the house. It got way worse, with all the settings giving osculations. I finally rerouted the wire and now the noise is gone all together!
I was going to closer 'er up, but now with this new information from JojoKeo and Tubeswell, I'm going to have keep digging in.
Jojo, I guess I can get away with a mini switch since the voltage is less than 2V. That would be nice to try. I did try to 10uF bypass, but I don't recall hearing too much of a difference from the 2.2uF ( but the 10 was and old NOS...so...), Never heard it lifted. Looking forward to trying it out!
Tubeswell, In looking at your DWG, I think I only need to move the input of the 470nF cap from pin 8 of the 5879, to pin 3 of the CF. Is that correct? Is the 1000pF cap on the output part of the morph? I go into the Tone stack at that point.
What does works good mean in "I've done this and it works good"? I guess I'll find out for myself. :icon_biggrin:
Thanks again guys.
pullshocks, I looked at the blues preamp, and looks different to me, but I'm not qualified to answer your question. I'm sure your find your answer here.
-LLama
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Tubeswell, In looking at your DWG, I think I only need to move the input of the 470nF cap from pin 8 of the 5879, to pin 3 of the CF. Is that correct?
Yup
Is the 1000pF cap on the output part of the morph? I go into the Tone stack at that point.
The 1000pF is for AC coupling to whatever comes next. If your tone stack provides AC coupling, then go with that instead. I used 1000pF as the coupling cap because its sufficient to pass just about all the frequencies you need for guitar with that 12AU7 CF.
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Jojo, I guess I can get away with a mini switch since the voltage is less than 2V. That would be nice to try. I did try to 10uF bypass, but I don't recall hearing too much of a difference from the 2.2uF ( but the 10 was and old NOS...so...), Never heard it lifted. Looking forward to trying it out!
You won't hear a whole lot between the 2.2uF & 10uF but you will with lifting it completely then switching one of them in. You will get a big more bass response with higher cap value is all. Depending on mood, volume, and style of playing - I like it w/out the cap at all but then I'm not quite using it in the position you are on most of my amps. Mine are the first tube of the preamp. You could use a 2-way or 3-way switch and get creative here. :icon_biggrin:
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Jojo,
I lifted the Bypass cap with a SPST mini switch and did hear the difference between the lifted and 2.2uF (and then the 10uF).
The lift quieted the amp a bit and took off some edge, but only when the Morph knob was at or near 100%. In my case the morph actually made a bigger difference when turned to 25% to 0% rotation.
I ended up restoring the 2.2uF cap to it's original soldered spot. Since I now have a switch on the front of my amp, I decided to lift the Power tube Bypass cap, instead. That made a huge difference at all knob settings. The switch cuts some bass an maybe compresses the sound for a more rhythm setting. This is turning out to be a more versatile amp than I had hoped for.
Thank you for getting me to try something new.!
I'm trying to get to a buddy's studio for some real sound recording, My Chinese condenser mic does not cut the mustard with my amps.
BTW, Jojo, What is your Profile picture of? Is that you and Buckethead? What are you doing with your hand? :w2: The pic is small on my laptop!
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Hey guys. Sorry to revive this post, but I have a question about the pentode grit.
I tend to have guitars with lower output pickups (strat and LP with AlNiCo2 pickups). With those guitars, the amp sings and has a beautiful deep drive that out of this world, but If i play any higher output humbucker on it (even a LP with PAF's), the tone gets very gritty and looses a lot of what makes it sound good. I guess that the circuit needs to be tweaked to accommodate a wider range of guitars.
My mind tells me to look at the pentode Cathode load and bypass, but then think of attenuating the signal going into the pentode.
I love the sound of the amp with the lower output guitars, I'd hate to loose that sound. Any suggestions?
i hope you can see the attached schem.
Thanks - LLama
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put a switch on the cathode cap or make it variable, I much prefer this to the triode / pseudo triode or whatever other manipulations are out there
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Jojo, what do you think about an input trim pot ?.....
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Not a fan of loading the guitar like that, I'd rather:
1) lower gain of V1a via:
a) decrease load resistor, increase voltage, and/or utilize split load resistors
b) remove or switch bypass cap
c) create voltage divider on output
d) lower output load resistance/grid leak (which is in parallel w/ plate resistance) such as simply using a 50k (or 100k) gain pot then eliminate the treble bleed cap (wouldn't be necessary any longer).
2) bail the CF set-up, create voltage divider on the output of the pentode (two 220k) AND still switch or make variable the bypass cap
3) utilize NFB on pentode
4) make batter use of using a single 12a_7 for V1a and V1b thereby eliminating the extra (12au7) tube OR if you have two more triodes to play with???
I forget if there's something paralleled or not? But it's not shown on the schem so it appears two stages are being wasted? Personally I like the pentode first in V1 then a normal CF stage driving the tone stack/vol > output tube. This is plenty of gain and then I'd run two inputs for the typical Hi/Lo set-up and plug the HB's intothe Lo if wanted.
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:worthy1: :worthy1: :worthy1: JojoKeo,
Thanks for all of the input! I'm likely to go with the lowering of gain at v1a. to start. This amp is all buttoned up, But I'm likely to build another one soon enough. I especially like the two input idea. I believe the idea for having the pentode as V2 comes from Merlin, but I'm sure your experience counts for a lot. I appreciate all of your help with this build!
:worthy1: Silvergun,
Thank you for throwing gasoline on the fire of knowledge!!
Thanks guys!