Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on June 06, 2011, 11:45:24 am
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Okay, I'm going back in.
My little SE 6v6 amp with 5879 input and paralleled 12AX7 triodes for 2nd preamp stage, well it has a little unwanted fuzz riding over the notes. Otherwise sounds great. I built it with screw-down terminals, so I'm gonna start there and use solder terminals.
I'm also going to change the paralleled 12AX7 to the input, into a FMV stack, to the 5879 (low gain setup), to the 6V6 output.
Any special considerations concerning the load impedance (or slope resistor, or what have you) of 1) the paralleled triode to the stack and 2) the 5879 to 6V6 output? I see that some like to insert a CF after pentodes, is there that great of a Z mismatch?
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It's all up to you and your individuality & creativity. I'd suggest using the same values as a starting point and get a nice direct comparrison from what you had to judge by. You can always change values around later if tweaking needs to be done.
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Here is the 1st version....
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Here is the new version.
It still may need a little tweaking. I moved the parallel triode stage to the input, & switched the grid of one triode. In the old version, the amp sounded sort of lame with just one triode, so I had to keep the "fat" switch on all the time. Now, both positions provide a usable tone and are nice & chimey.
I reduced the gain of both PA stages and increased the output load on the 5879. I wasn't sure what level the 6V6 likes so I guessed on the 150k grid leak there. It seems to have helped, along with a 4.7k grid stopper. I must have had some blocking distortion before.
Also beefed up the power supply.
On this project I tore almost everything out and started over with new terminal strips, it took me all day and when I finally fired it up-
Nothing was wrong. It worked the first time. Minimal hum, good tone. Color me surprised.
So, I'll post voltages as soon as I'm satisfied this is the end version. They do seem quite a bit higher now, is it the higher value main cap perhaps?
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it has a little unwanted fuzz riding over the notes
Here's what I would personally try to resolve that issue. Just tossing out another idea to consider?
With respect, Tubenit
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Hi tubenit,
I just was curious to say that with the two gain stages going into the CF/V1b w/out any kind of signal attenuation (especially from a pentode too), it seems that there could be some possible blocking distortion when the dials are getting maxed? I'm sure you've used something like this on your TOS or COS but I wonder if there's something about a CF that prevents this by it's nature? (Merlin???)
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some possible blocking distortion when the dials are getting maxed
My guess is that there could or even would be? If I were going to build this, I'd start with the schematic I drew up and then begin experimenting with it to see what was needed?
A 500k volume might be a better choice or a larger grid resistor like 220k bypassed with a 500p going into the pentode?
The fact that it has a volume going into the pentode/CF and a tone stack w/master volume coming out should help control some concerns.
I am personally convinced that the CF following the 5879 helps with smoothness in the tone.
With respect, Tubenit
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I am personally convinced that the CF following the 5879 helps with smoothness in the tone.
Do to your help and conviction w/ this, I have a similar design ready to build w/ my first opportunity. I is paralleling the first two stages w/ a 12ax7 & an EF86 and also cascading the triode into the pentode but was to have a tone stack then going into the remaining triode. But, now all I have to do to switch the triode & tone stack and whallah, I've got the pentode-CF going. :smile:
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If anyone gets this little amp up and running Id love to hear some sound clips. I have an old champ build that could very easily be switched over to something like this.
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I'll try it- the new design still has that blocking distortion- I think. Does it kind of sound like paper being balled up when notes are played? I think the 150k 6V6 grid leak may be sucking a little too much tone out.
Anyhow, the tone is pretty decent with a single triode input stage. So that frees up the other triode for a CF & I'm sure it will help with my issues. I had thought of it before but I REALLY wanted that paralleled input. Time to let it go.
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Whatever the problem was, the CF fixed it. Grossly mismatched 5879->6V6 impedance, probably. Thanks tubenit! I'll work the CF into my TOS at some point.
I didn't follow your schem exactly, a couple of uF/R values were different. The 1M volume pot is a good value. If I want to increase headroom I'll pop in a 5751 or 12AY7, or even 12AU7.
With a 12AX7 this is a real growler when turned up, with very smooth overdrive. Real nice clean tone at middle volume. And every tone setting sounds good- without the CF it was kind of dull with treble down and harsh with treble dimed. Now it sustains forever, even clean, and the attack is hard to describe, chimey I guess is the word. And I get some nice feedback out of my little 8" speaker! Imagine that.
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Whatever the problem was, the CF fixed it. Grossly mismatched 5879->6V6 impedance, probably. Thanks tubenit! I'll work the CF into my TOS at some point.
I didn't follow your schem exactly, a couple of uF/R values were different. The 1M volume pot is a good value. If I want to increase headroom I'll pop in a 5751 or 12AY7, or even 12AU7.
With a 12AX7 this is a real growler when turned up, with very smooth overdrive. Real nice clean tone at middle volume. And every tone setting sounds good- without the CF it was kind of dull with treble down and harsh with treble dimed. Now it sustains forever, even clean, and the attack is hard to describe, chimey I guess is the word. And I get some nice feedback out of my little 8" speaker! Imagine that.
Wow! Very cool. Thanks for reporting back on that. I'm a handful of hrs away from my mini-TOS being completed and I'm excited about it. Hope it works like I'm thinking it will.
With respect, Tubenit
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Yeah, I think so. :worthy1:
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Any chance of getting an updated schematic with the new working values and voltages?
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Here is the new schem. I'm bad about voltages, I was aiming for @ 250v plate & @80v screen on the 5879, that's all I really paid attention to since the other stuff already worked. Note the changed power rail as well.
I think I have a little parasitic oscillation in there somewhere, probably poor lead dress. It's very slight though.
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I'm Not sure you posted the updated schematic. This "new one" doesn't appear to have the CF like Tubenits does.
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Ooops. I'm on the wrong computer to send a schem. Look for it tomorrow, sorry!
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Let's try that again. This is the latest version, I plan on going in today & placing a bright cap across the volume pot, and a PAB to lift the tonestack 680k(just guessing) off the ground. Also, I think tubenit is correct that a lower value volume pot will work better; at high volume it still has a little ugly distortion riding on top, but only when I hit the strings hard.
In fact, I will also do a large bypassed gridstopper on the 5879 for good measure.
I have not included a master volume so hopefully this will control any blocking distortion generated.
As usual, I'm leaning heavily on tubenit in the design of this thing. :worthy1:
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Right on thats looking more like it. After you make your adjustments let us know how she sounds. Im going to be doing mine as a scratch build using parts from an earlier 5F2A head build so having you work out some of the early tweaks will really help me get a good staring point.
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Since your 5879 has a 56k on the plate, try a 47k or 39k on the cathode of the CF & see if that smooths the tone even more?
I have about 2 hrs left on my mini-TOS to fire it up.
With respect, Tubenit
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Ok so using a previous schematic idea here is what my setup will theoretically provide. (see attatched schematic)
The voltages are what Duncan PSU spit out given 2.7mA to the CF and 5879 and 1.4ma to the first triode. The power tube current was stepped from 35ma-1.5ma on the plates and 2.9ma-35ma on the screens. Im not sure exatly sure what kind of voltage the 5879 will be seeing because im not sure of the current through each branch feeding it(68k and 330k). I know an ef86 typically draws like 1.5ma in examples like these so I used that. Let me know if the suggested voltages look like a decent starting point here.
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If you have the theory correct you'll prob. save yerself all the hassle of trial & error, which is my default method. I'll be making the changes tonite & I'll try to jot down the voltages too. Which reminds me, my voltages will probably still be higher than yours so your results will vary.
Tubenit, I will try a little lower cathode value on my CF in addition to the other stuff.
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Yeah my "theory" work is still only speculation thats why I was hoping someone else who knows the voltages and such could comment. I know the 12A_7 tube in V1 would be ok with the stated voltages its just what the 5879 should be seeing im curious about.
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As far as the 5879 voltages go, I really tried to keep them within max specs per the datasheet. Thus the somewhat big dropping resistor to D on the B+ rail.
I made the changes, had it all tied up then realized, looking at the 6V6, that the side of the plate had this red GLOW. And it must have been overbiased since day 1. So after some T&E I came up with a 220R before the main cap to drop the overall voltage @ 10VDC, and a 740R cathode resistor (270/470 in series). So it's now at max dissipation but not redplating.
I know I sould prob. use a 6L6 with these voltages but I like the 6V6 for this amp.
And I don't know if that is correct power supply design, anyone who believes it may not be, please chime in....
I used a 56k grid stopper to the 5879, bypassed with a 390p cap. Tried a 220k at first but it really sounded thin.
Increased the CF cathode to 47k.
I placed a 1M resistor across the outside terminals of the volume pot, I believe that makes it a 500k for all intents & purposes. And a 68p bright cap.
The "PAB" switch really fattens up the sound, but both settings have a pretty nice tone. I found the Mid resistor (6.8k to ground on the bass pot) used in standard Fender stacks drained the tone right out. Since I like a little mid scoop anyway, I settled on 22k for that value but could easily go higher if necessary.
Finally, I realized that I had no 6V6 screen resistor all this time! So note the added 1.5k screen resistor.
Aren't you guys sposed to call newbies like me out on this stuff?
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You may have it exactly where you want it?
Having said that, when I experimented tone wise, I liked the 5879 to have 140-155v range on it's plates. And closer to the 140-145v range. That sounded warmer to me.
With respect, Tubenit
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I guess in my hap hazard design attemp here im looking to confirm the current draw values for a setup like this. I know a decent rule of thumb value for 12ax7 tube is like 1mA and for the 6V6 the plates should be around 35mA and screens like 3mA. The question im looking to answer is the 5879 CF section. Judging from Firemedics voltages that section is pulling around 6mA if I have my theory correct 2.6mA of that being the 5879 itself. Im just wondering if the numbers im using with the Duncan PSU are anywhere near what they should be.
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You may have it exactly where you want it?
Well, maybe. It was a nightmare last night dealing with the redplating, it just went on & on. I'm tired of messing with it, too many times I've kept tweaking an amp for no good reason when it sounded great to begin with.
Now as I understand it, a little pentode's characteristics are controlled to a large degree by the screen voltage, which is a nice low 74v in this amp. I'm going to leave it alone since the blocking distortion seems to be taken care of, which was the issue I had in the first place.
Inevitably I will go in again & then I'll drop the 5879 voltage, since you are almost always correct in your long-distance assessments of my little projects.
Any thoughts on the 220R B+ dropping resistor, right after the recto? Think it will be stable? It gets warm but not too hot.
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Im just wondering if the numbers im using with the Duncan PSU are anywhere near what they should be.
Any thoughts on the 220R B+ dropping resistor, right after the recto? Think it will be stable? It gets warm but not too hot.
Yes for both. If my math & thinking are correct 10v dropped from the 220r shows 0.045A going through it.
For minimal power rating of resistor: 401-391=10v, 10²/220=0.45W x 2 (for safety) = 0.91W rating to use. You "should" be okay using a 1 watter but if your 10 watter is warm stay w/ the less heated part. It should barely feel warm being this large.
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Firemedic --
What are the specs for the PT you are using in this amp? thx.
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It's the 125P1B from Doug's store.
I turned it up last nite.....
You know how some amps make you really sound like you know how to play guitar?
The 5879 chime is unmistakable, even with treble turned way down. What a beautiful tone. It makes a big difference, having something driving that pentode vs dry guitar signal.
It still needs minor, minor tweaks, I'm hearing just a trace of blocking distortion when it's turned up & the tonestack is lifted from ground (PAB). It's barely there though. Prob. just a 330k or so instead of 680k lift, will do it. As it is there is a HUGE gain increase with the lift. If that fails I'll try a 250k volume pot.
I'll also try a 33k mid resistor, per tubenit's schem; the stack sounds great but could use a touch more midrange.
It was hard to put the guitar down, it's a pretty nice feeling when that happens.
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Glad to here you've almost got things where you want them. Let me know how the tonestack sounds with the 33k mid since thats going to be my starting point I think.
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Go ahead & build it, the midrange resistor is about the easiest thing to change out. 33k is a pretty good bet IMO.
Hopefully you won't have to drop your B+ voltage like I did, I still feel weird about that big 220R even though it's most likely fine.
I'm going to see how it sounds w/ 5751 & 12AY7, just for jollies. It may fix the minor distortion problem until I open it up again.
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Yeah I need to make an order for parts before I can really get rolling so it really doesnt hurt to get my ducks in a row. I plan to throw in a few extra resistor and cap values to have something to experiment with when I do order. I think I have a good handle on my B+rail for a starting point but thats where a lot of the extra resistors will come in handy.
Speaking of the B+ rail im curious why both you and tubenit decided to feed the 5879 CF stage from the last filter stage on the rail instead of the one before it? Was that just because you didnt feel like switching all the dropping resistors around or did you want to use the smoothest DC for the CF? Im in the process of working out some of the different B+ possabilities now and I can get pretty caparable plate voltages across the board flipping the B+ feeds and using different dropping values. I'm just not sure if there is a benefit to using that final stage of filtering on the 5879 CF or not.
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The pentodes like lower voltage than 12A_7s, at least according to the datasheets. Especially the screens. That is, to get the "pentode sound" according to Merlin. So far my in my builds this has proven to be true.
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Yeah I've noticed that and from what i've read about the 5879 so far around 150 on the plate and like 75 on the screen is the sweet spot. My setup will be a custom build so i dont need to worry about working around a prebuilt circuit. Right now using the Duncan amps PSU simulator I can get voltages that look like this.
(http://img851.imageshack.us/img851/7863/5879sepsu.png) (http://imageshack.us/photo/my-images/851/5879sepsu.png/)
This is using a 32mA current to the 6V6 plates 3mA to the screen. The 5879 CF section is assuming a 5.6ma draw and the first preamp stage the usual 1.2mA for the 12ax7. Assuming a 2.2mA current to the plate of the 5879 and a 68k resistor would put 145V on the plate and a 0.44mA screen draw across 470k puts 75V on the screen. Im not 100% on the CF triode but I estimated a 2mA draw. The firstat satage would use an 82k putting 194V on its plate or 100k for 173V. The 100k 173V is reallt close to the old champ rated voltage of 150V.
Now all of this is speculation but I think my numbers make sense. Im really just hoping someone will cut me off im way off the mark.
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Ok so heres a quick Hoffman style layout for what im planning. This should be a simple layout to modify values. Do me a favor and check if I missed anything.
(http://img832.imageshack.us/img832/5679/5879champlayout.png) (http://imageshack.us/photo/my-images/832/5879champlayout.png/)
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A couple things-
I would get some smaller value caps to experiment for the treble bleed snubber across the 5879 plate resistor- esp without a bypass cap on the V2 grid you might kill your treble. This is not a High Gain design & you may not need to bleed off the highs at all.
The 5879 datasheet suggests connecting p6 & p2 to ground for additional internal shielding, probably not much difference.
That's all I see at 1st pass, have you ordered the parts? Keep in mind, you are almost certainly going to have to tweak some values to get this sounding like you want it....
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No parts have been ordered yet. What sections of the circuit did you find yourself tweaking with the most and ill graba few values above and below when i place the order. Thanks for the pin 2&6 info for the 5879 I hadnt checked the datasheet yet but was curious what was done with those pins. I noticed you had a treble bleed on the 5879 grid on your build so I intended to grab a few values for the grid resistor. If you can make value suggestions I'd greatly appreciate it.
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I can't see the schem (no jschm program loaded) on my fancy electronic device but you have power going to v1 pin 6 but the signal isn't going anywhere? Wait I see, it appears you need a wire connecting tag board 22n/630v to 82k/2w. You may only need a 2n2 cap instead here.
The grid resistor 68k is pretty high, consider 22k, 10k, or you may not need anything here? Anything less the full up on gain pot gives you a grid stopper value.
Interesting descision on panel/pot layout order? It seems you are missing a filter stage. With a single ended amp, you'd be wise to follow prior examples using 4 filter caps w/ a resevoir>resistor>then 2nd cap for node A>resistor>3rd cap/node B>resistor>4th cap/node C, etc... check other single ended amp's power sections to see what I'm talking about.
For more values to sub, you may want something different than the 820 on 5879 like 1k or 1.2k? You may want different values on 5879 like 56k/270k or 220k combo (the higher here the more headroom available for pentode). Get a few small caps for treble bleeders on your vol pot - like 100pf, 250pf, and 500pf values to tailor more clarity on low settings, especially if using humbucking pu's.
Hope this helps,
Keo
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Oh, yeah, I really like a choke on my PS's, BTW. I used a small fender choke from Doug on mine. Nice quiet build, for a SE.
I have procured Merlin's preamp book, thus I have this idea that (among other things I only half understand) the 5879 grid can have a fairly high resistance on it before affecting the tone too much. I tried a bypassed 220k which WAS too much, then settled on a 56k bypassed with a 390p cap.
Note that it is a bypassed resistor (high-pass) rather than bled to ground or B+ (low-pass) Keo is probably right but I'm happy with the sound.
As far as tweaking, I tweaked EVERYTHING on this build. Resistors are cheap, get a bunch of different values & don't forget 1 & 2 watters for your plates/PS. Plus various 10Ws for your power tube bias. Ditto caps. Your calculations are awesome, & you're likely right on the money with yer values but your ear is not a calculator.
Find a local electronics supply store besides Radio Snack (good for fuses, jacks & solder in a pinch), you will be going there a few times for common value stuff you forgot to order; xicon caps are real common & work fine. They usually carry silver mica caps too. Mallorys, ODs, not so much. You don't want to be stuck, one little resistor away from firing up your creation.
I agree w/ Keo that your layout is unusual, consider giving the 12AX7's filter cap a little more room. Also take another look at the bass pot connections, somehow that doesn't quite look right.
PS I'm real flattered that you chose my topology -really tubenit's- on this project, I'm new at this too.
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So you guys are suggesting I do a PSU setup like this?
Cap - Choke - Cap to OT - Resistor - Cap to Screen - Resistor - Cap to 5879 CF - Resistor - Cap to 12ax7
I can get my hands on a hammond small fender choke replacement pretty easy so Maybey Ill adjust for that. As far as the layout goes how much would you say a hoffman style layout would really make in a single ended amp like this?
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Cap - Choke - Cap to OT - Resistor - Cap to Screen - Resistor - Cap to 5879 CF - Resistor - Cap to 12ax7
Either a choke or power resistor - not a real big deal in a small amp but it's better to not follow Fender's Champ style power rail by going stright to the OT w/ the first resevior cap node on a single ended design.
As far as the layout goes how much would you say a hoffman style layout would really make in a single ended amp like this?
How much what? "Difference" would it make? As compared to what? This needs clarifying.
BTW, what will make a big difference as far as noise/humm is concerned is where & how you run and center tap your heater wires, your grounding plan, and lead dress. I don't think these were mentioned?
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In terms of "Difference" I meant compared to a typical non hoffman design.
Grounding I intend to split the Power section grounds and preamp grounds to seperate chassis points. For example the resevoir cap the OT cap and screens filter cap would be grounded at one point along with the power tube cathode and the other grounds would be somewhere on the other side of the chassis. The HT center tap would join at the power section ground and the heaters would be atrificially centertapped with 100R resistors at the positive side of the powertube cathode to provide a DC level. The pre grounds I would likely join somewhere near the input and the power ground would be near the power transformer.
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sounds like you'll be fine
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Ditto. Plug in your iron, let's see what happens.....
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Hey Firemedic how would you describe the tone of your 5879 SE build? Is it a Chimey distortion or more creamy sounding?
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Ok so I think I have the layout set up as clean as I can make it let me know what you think. I didnt draw in a few basic connections but I believe the meat nad potatoes are all there. If this one looks cool Ill fire an order off monday for parts. Thanks again for all the help and inspiration on this one.
(http://img849.imageshack.us/img849/1885/5879layout.png) (http://imageshack.us/photo/my-images/849/5879layout.png/)
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At a quick glance it all looks good except remove the 3k3 grid stopper to the 6v6 from the board and attach it directly to the 6v6 pin socket w/ a lead back to the board.
As for chime, I have a 5879 pp using 6bm8s and it's very chimey w/ Weber blue speaker. I noticed that you have a 33k mid resistor. This will give you more mids and help to get more smooth and full tone. Smooth and creamy seems to have a direct relationship w/ the signal containing a lot of mids and high mids - aka Tweed-type of tone. A certain type of speaker as well as humbuck pu's also helps too if that's your goal for "smooth & creamy".
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I've got a question for my own interest. I notice on the 5879 you have 2 and 6 connected externally, and then going to ground. Is that the way it should be done? If so, I've made (one more) mistake with mine.
Thanks,
John
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no, you're fine - these are "no connection" see data sheet
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Also move the 33k gridstopper to V1p2 (most important), and the 68k gridstopper to V2p1. Solder them right to the pins. Other than that it looks like you're square.
I find it hard to quantify in words the tone I find pleasing. Mine is quite chimey, and I like the very rich clean tone I've gotten here; I may really be liking the partial, half-wave distortion that is a characteristic of well-but-not-over-driven tubes (and old-school PI circuits). I do not have an o-scope so it's hard to say. I'm trying lower-mu tubes in V1 because I don't really want distortion per se.
I happen to like some mid-scoop, common to the oft-abused FMV tone stack. That said, having tried a standard 6.8k (anemic), 10k (thin) and 22k (getting warmer) midrange resistor I'm willing to bet the 33k will do the trick on this particular build. For me. For more-than-average mids I would feel confident with a 56k-82k in there too without making the midrange overwhelming. My PAB switch substitutes a 680k to ground, which IS too much.
But we are getting ahead of ourselves, this conversation is for later, The Tweaking Phase.
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I've always wondered about this. Base pinout shows NC for pin 2, 6.
But look at page 1, bottom right, installations and applications, recommended pin 2, 6 be grounded. Hmmm....... :think1:
Brad :dontknow:
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Phew, thanks. I did look at the data sheet, but after my debacle with the way I read the relays on the layout, I was worried!
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It took me a whole day to figure out how my little TOS relay was supposed to be wired. A whole day.
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Well don't feel bad. I think I fried my relays; at least, they no longer make that cheerful little clicky sound. I bet Doug just loves newbies... double the parts! :l2:
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I've always wondered about this. Base pinout shows NC for pin 2, 6.
But look at page 1, bottom right, installations and applications, recommended pin 2, 6 be grounded. Hmmm....... :think1:
Brad :dontknow:
Good eye Brad. I'm more familiar w/ ef86 and you have to be careful w/ tube connections that say NC or IS or other stuff. I wish they would be labeled a bit better than say IS (internal screen or shield) for example. Sometimes I think there is an internal connection that shouldn't be connected to anything from the outside? I believe I done that before by tying a grid stopper or screen resistor one time and the resulting sparks the occured upon fire-up. So it always makes me apprehensive doing so w/ out knowing for sure.
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I'm more familiar w/ ef86 and you have to be careful w/ tube connections that say NC or IS or other stuff. I wish they would be labeled a bit better than say IS (internal screen or shield) for example.
Yes, it is confusing the way they are labeled from one type to another. I've seen different sheets show different things for different small signal pentodes. IIRC some have the shield connected internally to the cathode, some don't.
I seem to recall, sometimes even the same tube number/type, but from a different maker will have a different pinout --- for the shield --- as far as IC or NC.
I guess you could use a DMM and check the tube pins for continuity.
Thanks, Brad :icon_biggrin:
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Ok so things have gotten a little more discussion over the weekend. In light of some of the comments here are a few new questions.
Not sure if you guys settled on it but do pin2 and 6 need to be grounded or not on the 5879?
joko - You mentioned the creamy tone typically comming from mids and that thats typically the "tweed" tone. That would be what im looking for with this amp and was wondering if there would be any benefit to using a "tweed" tonestack instead of the TMB?
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Not sure if it "needs" to be, but I did ground mine with no ill effects.
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I have never grounded 2 & 6 on my 5879 amps when I built them.
I tried it once (after I had played a build for a while) since someone suggested it and found it made zero difference & the amp was already quiet.
With respect, Tubenit
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joko - You mentioned the creamy tone typically comming from mids and that thats typically the "tweed" tone. That would be what im looking for with this amp and was wondering if there would be any benefit to using a "tweed" tonestack instead of the TMB?
There's many ways to increase or maintain mids. Your 33k can be increased even more say by a switch for several settings of mids. Or you can simply put a switch btwn the 33k & ground which is called a "ground lift". You'll notice also an increase in signal strength which is nice. Or you wire a tone stack bypass w/ a switch. Or don't use a tone stack? It just depends what you want. Just remember that when lifting/bypassing or having no tone stack that at moderate to high power levels, it's very easy to have too much bass which muddies your tone. So you have to lower coupling cap and/or cathode bypass cap values depending what route you take.
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I finally got rid of the little fuzzy noise that made me start this thread in the first place.
Stupid silver mica tone cap.....I (errr, Tubenit) redesigned & rebuilt the entire amp because of the noise that thing made.
I assumed that since I was getting this fuzz riding the notes, with every tweek, then there must be a fairly narrow range of useful resistor & cap values (i.e. grid current, output impedances, etc.) I could use with this topology.
But I was wrong- It's wide open as far as tailoring the sound to your liking. It sounds great w/ 12AX7, 5751, and 12AY7.
Neil, is yours built yet? I know you weren't looking for more options but you should know the possibilities you have to work with....
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Yeah nothings been built yet i've been dabbling with another idea along the same lines. Any new information would go a long way though.
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Well good luck, there's only about 750 million custom schematics here to choose from.
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:cussing:
There is the faintest little white fuzz, it starts after the note starts to decay. It's not nearly as bad as the other noise was but it is there, at pretty much all settings, even low volume. What could THAT be?
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:dontknow: only wild guesses w/out more info.
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Another revision.
I messed with a couple values & rewired the PA again, to accommodate a 12DW7. The triodes needed to be reversed to make the AU triode do the CF job. New tone caps, & I was vewy vewy careful with the leads & ironing.
Whole different amp. With lots of high-end, so when treble is dialed back, even all the way off, there is still considerable brightness. Guitar tone control is the only real treble cut. Maybe I wired it wrong- but I like it.
It's got a ballsy sound, real in-your-face. Esp. with the PAB. The chime is still there.
I'm waiting on a backordered 50V 50W zener from mouser, when it comes I intend to drop the voltage, it's still too high for my liking....
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I took out the 47pF cap paralleled with the 100k V2 gridstopper. Now I have functional tone control, looks like that did it. Sounds sweet.
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Okay, jjasilli has turned me on to using some NFB in this thing to finally lose this fizziness once & for all. I'd really like a small global NFB loop from the OT secondary but given the unusual topology on this thing (see schem above) where would I insert it? i.e. is there anywhere on a CF I can insert it? How about the 5879 cathode, removing the cathode bypass cap?
Update:
So far my best option looks like the AA764 Vibrochamp style NFB insertion (thanks JJ), however a pentode/CF followed by a tone stack is a very different animal than a simple triode stage. I'm not smart enough to figure out the various phase shift issues presented by this topology. Y'all think it will still work?
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Well it did work, in fact the split cathode alone on the 5879 seemed to stabilize the circuit more than anything; the global NFB insertion didn't make a huge difference but did help. The tone is smoother now, less harsh overall & especially during the attack. Thanks jjasilli!
I still would like to lower the voltage. I doubled the B+ resistor feeding the 5879/CF but it had almost no effect on the voltage. I gave in & am using a 5881 which sucked the overall B+ down @20v- not quite enough. The Zener arrives tomorrow.
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:smiley:
Where do you want to lower voltage? If everywhere, maybe use a bucking tranny. If only to the preamp, you can try a voltage divider; i.e. not just a series dropping resistor but a shunt resistor also. E.g. 22K 1W series / 100K 2W shunt should knock 20% off your B+, downstream from the power tube screen supply.
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I need the entire B+ dropped. What's a bucking transformer?
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IF I were in your shoes, I'd remove the choke and add a 3.3k/2w between node A & B & 10k/2w between B & C. I think your 6V6 plate voltages are just fine personally.
Maybe you could move the choke prior to node A? I'm not smart enough to know if that would work or not but maybe Sluckey could chime in on that?
With respect, Tubenit
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IF I were in your shoes, I'd remove the choke and add a 3.3k/2w between node A & B & 10k/2w between B & C. I think your 6V6 plate voltages are just fine personally.
Maybe you could move the choke prior to node A? I'm not smart enough to know if that would work or not but maybe Sluckey could chime in on that?
With respect, Tubenit
Fender Champ 5E1 and many other single-ended circuits have the reservoir cap followed by the choke before the power rail node feeding the plate. Push-pull design rejects noise at plates. Single-ended doesn't. Also, you'd need a BIG choke to filter plate supply for two 6L6 or EL34 power tubes but not so much for a single 6V6. The reservoir cap doesn't have to be really big. Playing around with Duncan's PSU, the second cap value (plate/OT supply) had a much bigger affect on ripple.
It is possible to have a choke follow directly off of your rectifier (IOW no reservoir cap before the choke), but getting the specs right for that choke are much more complicated. It's called a "choke input filter" IIRC.
Are you using an NOS 5Y3GT rectifier tube? If not, you'll drop about 25 volts just switching to a real 5Y3.
HTH,
Chip
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Bucking tranny: checkout this thread: http://www.el34world.com/Forum/index.php?topic=11981.0 (http://www.el34world.com/Forum/index.php?topic=11981.0) post if you need more info.
Choke: subjective thing: I love 'em!
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Are you using an NOS 5Y3GT rectifier tube? If not, you'll drop about 25 volts just switching to a real 5Y3.
A 5Y3 recto only supplies 125mA of current. Not enough for a pair of 6L6 or El34 pp in AB1
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A 5Y3 recto only supplies 125mA of current. Not enough for a pair of 6L6 or El34 pp in AB1
Silvertone 1483 has 6L6's and uses a 5Y3GT.
http://www.el34world.com/charts/Schematics/files/silvertone/silvertone1483.pdf (http://www.el34world.com/charts/Schematics/files/silvertone/silvertone1483.pdf)
So in some cases, it apparently can be done. I've seen some other amps with 6L6's & 5Y3GT also but not very many & it would be easy to exceed the 5Y3GT limit just as jojokeo has suggested.
With respect, Tubenit
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Maybe post a schematic with voltages of the amp as it is now.
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It's closed up again. Voltages-from memory:
@380V on the plates & screens,
@220v 5879 plate,
@85v 5879 screen,
@210v CF cathode
and some normal voltages on V1.
The 5879 plate & screen are higher than I want, I doubled the B+ dropping resistor- almost no change.
The CF heater-cathode voltage is too high, even for a 'U7, and I don't want to raise the CF heaters. I'm just lazy that way.
Yes it is an NOS 5Y3, Providing max drop. SE amp, not PP, so current draw is OK.
Please note there is a big 220R 10W dropping resistor after the recto, which gets HOT. It only drops about 10v and I want it out of there.
All these minor issues could be resolved w/ a 10-15% lower B+.
I like the IDEA of the bucking tranny but
1- this amp is tiny, there's no more room.
2- it drops the heaters too.
3- The Reverse-polarity 45v 50W Zener arrives today.
I really appreciate the input from wiser minds. It's good to have options (that I wouldn't have thought of on my own) to compare.
I too love the choke. What a difference in noise level.
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Assuming tubenit's schematic from P1 of this thread is reasonably accurate to your build:
* Maybe move the choke to before the 1st, pre-filter stage. A choke input filter will drop B+ voltage. It will also cause voltage regulation -- less sag. This may not be an issue in an SE amp. Maybe worth a try. N.B.: the Henries of the input choke must match the specs of the rectifier tube. If an issue, I think it may be OK to isolate the tube rectifier from the input choke with a small value power resistor, say 10Ω (probably at least 3W).
* If your 10W resistor gets too hot, go to 25W.
* A bucking tranny is another solution, as stated before, to drop all B+. For $9 Radio Shack gives you multi-tap tranny which can be made switchable. If 120VAC into your PT yields about 345VAC out (loaded): 345/120 = a factor of 2.9. With a bucking tranny: the 6V CT will drop 120VAC > 114VAC X 2.9 = 330VAC X 1.1 forward conversion factor for 5Y3 = 363VDC plates & screens. That's about a 17V drop from what you have; which should yield about 203VDC on the 5879. The 12V tap on the bucking tranny will double that drop. That should get you about 185VDC on the plate of the 5879.
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Um, is there anything technically wrong w/ using the zener though? I mean does it suck the tone out or something? I'm planning to use it on the PT CT, but everyone seems kind of resistant to the idea.
"For $9 Radio Shack gives you multi-tap tranny which can be made switchable."
Radio Shack? Has something I can use?
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The zener is a good way to drop voltage w/ out an effect on tone. No worries there.
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The zener is a good way to drop voltage w/ out an effect on tone. No worries there.
I used a 50 volt zener on the center tap of a Tweed Princeton (sort of) and it worked like a champ! Sorry, I love really bad puns.
Cheers,
Chip
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Princeton. . .worked like a champ! :laugh:
Nothing operationally wrong with zeners. Except some users claim bad experiences with failure.
Re bucking trannies, read the thread I posted above + especially the site in there by Keene on "vintage power" at www.geofex.com (http://www.geofex.com)
Zeners provide B+ voltage regulation. I'm prejudiced against that in a guitar amp.
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Got the zener in the mail today. From the mounting hardware included, it looks like it's supposed to be completely (electrically) isolated from ground? Is that right?
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Please specify the specs of your zener - what specific item do you have?
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It's an NTE 5273AK. Cathode-to-case, or reverse, polarity.
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Got the zener in the mail today. From the mounting hardware included, it looks like it's supposed to be completely (electrically) isolated from ground? Is that right?
No, you drill a hole through the chassis and bolt the zener's cathode down. The chassis serves as your heat sink and that's important in this application. Make sure that the center tap of the PT and the negative lead of the reservoir cap (very first in power rail) are the ONLY things grounded on the anode (loop) of the zener. They will be at -45 volts with respect to the chassis and the rest of your grounds. The highest current in your amp cycles through the PT secondary and the reservoir cap, so they should always be tied directly together. It reduces noise IME. Everything else in the amp should be "grounded" to the chassis as usual. Also, clean the metal and scour it with sandpaper or something around the hole for mounting the zener on both sides of the chassis. You want the best electrical connection possible here.
That's one expensive little critter unless you found better prices than I did!
http://www.alliedelec.com/Images/Products/Datasheets/BM/NTE_ELECTRONICS/235-0139.PDF (http://www.alliedelec.com/Images/Products/Datasheets/BM/NTE_ELECTRONICS/235-0139.PDF)
Cheers,
Chip
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There is no PT center tap; this is an SE amp. I don't where to put a reverse zener in an SE circuit. :BangHead:
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There is no PT center tap; this is an SE amp. I don't where to put a reverse zener in an SE circuit.
You're thinking of an OT/output push-pull tranny vs a SE OT tranny. The PT/power tranny B+ secondary is the center tap for this.
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The attached schematic may help.
Chip
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Hey guys, I went ahead last nite & installed it, prior to reading your posts. The reservoir cap is still tied direct to chassis & I will change that ASAP. Otherwise it's right.
It was $18 at Mouser, not too bad really. I like how the notes declare "not recommended for new designs". It gets warm but not hot.
I like where the voltages are now. I got rid of the hot B+ dropping resistor.
I don't have my voltage sheet in front of me, will post tomorrow.
6V6 biases a little cold at 33mA- plate voltage 330, 5881 at 70mA-plate voltage 275, just barely less than 100%. W/ existing bias resistors.
The 5879/CF was still high, @212v on CF cathode, 210v on 5879 plate, and 73v screen. After experimenting w/ voltage shunt from the D node, I settled on 27k to ground, which gave me 5879 plate at 165v, screen 63v, cathode 1.8v, and CF cathode at 167v. Much better.
So I'm happy with the voltages now. The fam was asleep so I won't know how it sounds till later today.
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There is no PT center tap; this is an SE amp. I don't where to put a reverse zener in an SE circuit.
You're thinking of an OT/output push-pull tranny vs a SE OT tranny. The PT/power tranny B+ secondary is the center tap for this.
Yes, whoops, sorry: late night brain dysfunction! :BangHead:
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I went in to move the 1st filter cap ground to the zener, when I noticed/remembered that that is also the node feeding the PT. i.e. no reservoir cap.
Should I still connect that cap to the zener?
Will it throw off the power tube bias?
Or should I connect the cap AND the cathode resistor?
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The reverse zener goes between the PT CT and ground (not in the B+ rail).
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I was referring to Fresh Start's suggestion to also connect the ground of the reservoir cap to the zener. No B+ involved except thru the cap.
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Please look at the schematic - "G" is the connection I was referring to.
Chip
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I see that the reservoir cap negative side is connected to the zener. But your princeton has nothing else connected to that cap. My question is, since my OT tap is connected at my first cap, maybe I shouldn't connect the negative side of said cap, to the zener?
Because I don't really understand what the zener is doing. Just that it's doing it.
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Voltage is relative. Zero volts at "ground" is a convenience. The zener provides a lower (negative) DC voltage reference for the PT ground, with the effect of lowering the PT's high voltage secondary lower relative to everything else.
HV "285" volts DC
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Chassis - "ground" at "zero" volts DC
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Zener anode / PT secondary center tap at -45 volts DC
Given a supply voltage applied to the PT primary, you can't do anything to change the voltage difference between the HV end and the center tap end of the PT secondary. It's still 330 volts (a guessimate based on schematic voltages). The zener shifts that voltage range relative to everything else in the amp.
285 - (-45) = 330
I suggest adding an isolated reservoir cap to the power rail if you can. If you can't, you'll have to experiment (unless someone else has a better idea). The PT center tap, the filter cap for the OT tap (supplying power tube plate), the filter cap for the power tube screens and the power tube cathode "should" all be grounded in the same place. That minimizes the loops current needs to travel through. However, you want the power tube cathode grounded at "zero" (chassis), not at -45 (zener anode). Otherwise, your power tube bias will be right back where it was before. (Took me a few minutes to remember what went wrong when I first implemented the zener on the PT center tap. It was my first amp build...) I don't know whether you will get lowest noise with the screen grid filter cap grounded on the chassis or on the zener's anode. I'd try the chassis, tied to the cathode, first.
I sure hope that helps more than it confuses things,
Chip
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Okay, that's what I thought. :think1:
The chassis is from a tiny, tiny little SS amp I converted to tubes & there's just no more room for another cap.
It's remarkably hum-free though, & I like the voltages, so I'll leave it as is. Thanks guys!
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Hello,
This has been a very interesting thread, and I would like to build one for m'self. I have a donor, an RCA tape recorder, that I've gutted and am about to build back up. It already has a shocked-mounted, shielded 5879, a 12AX7 and a 6AQ5 power tube. Rectifier is a 6X4. I love the idea of the triode >pentode > CF preamp as described by Tubenit and built by Firemedic
My concern is that the RCA's PT, at 200-0-200, won't get me quite enough voltage for the preamp tubes. Using the tube recto, the schem shows 210vdc on the power tube screen and 200 on the plate, loaded. I am okay with lower voltages on the power tube.
Any ideas on how I should approach the B+ for the two preamp tubes?
Thanks
RWood
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Have you thought about a solid state rectifier instead of the 6X4? That should give you around 200v x 1.4 = 280v on the plates. The Gibson Scout using 6AQ5 has 305 volts on the plates. That would allow more room to work with on the preamp tubes.
With respect, Tubenit
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Funny you guys should exhume this thread: I found the source of my noise problem. It was a bad pickup selector switch on my guitar. I guess the noise was canceling out or something in my P-P amps, making it harder to diagnose.
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Thanks; I will try ss rectifier. I assume I would just use low value dropping resistors in the B+ in order to retain as much voltage for the preamp tubes.
Btw, in the RCA there was a 470Ω after the first cap, before the tap to the OT, so that's why the voltage was lower than expected even for a 6X4 tube. I like using a 'poor man's choke' there, too but usually a much lower value like 47Ω, just to get that extra stage of filtering in a SE amp.
I will reply back with my progress.
Firemedic - glad you found your noise.....and great threads deserve to be brought back to life!
RWood
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Hi,
Sorry for the reply on this old Post, But I'm hugely interested in building this as my first 5879 amp (second amp overall)
The Chassis I want to use, as well as the PT/OT are all for an el84 amp, I cannot fit an octal socket into this (its an old Roberts R2R mono block chassis).
1. as the schem shows, how is having the 5870 as the second stage tube different in tone/sound/ vs stage 1?
2. Would using an EL 84 power section (ala ax84 P1/HO) make this build less desirable?
I've been through the ax84 forum, where I was guided into using the HOSO Pre with the HO power, but this schem seems simpler for my experiment. Any thoughts. Thanks, I'm new to this forum, but have read through many of your posts and am very impressed with what many of you have accomplished, and level of expertise :worthy1:.
Thank you.
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1. I changed the 5879 to the 2nd stage because according to Merlin, who seems to know what he's talking about, the very tiny grid voltage you get from a guitar pickup makes an input pentode sound like a triode. Or words to that effect. So I switched the order of the gain stages to take tonal advantage of having a pentode in the first place.
2. EL84's and 6V6s are electrically equivalent tubes for the most part.
3. You should know I scrapped the whole thing & made a slightly modified Vibrochamp anyway.
I rebuilt the amp several times because I was convinced the 5879 was sensitive & noisy. When I finally figured out where the crappy noise was coming from, it turned out to be the cheap tone cap in my guitar! Thus endeth the lesson.
llama, you are now the 5879 guy, congratulations! Please post w/ results, I'm sure you will build something cool. You're discovering that there are a lot of ways to make these circuits and they're mostly all good. I strongly suggest getting Designing Tube Preamplifiers for Guitar and Bass by Merlin Blencowe, or at least checking out his website (the name escapes me at the moment).
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Can we hope in some samples ?
Thanks
K
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Thanks FireMedic, And also for your reply to my message.
I've been digesting Merlin's Web site (valvewizard.co.uk) Much of it is above my head, but I'm growing taller.
I will build the Redo with the el84 and gladly post sound samples , now that the "studio" is back up in my basement.
It is a slow process since my life is owned by my family :BangHead: , but I will avail! and will gladly take over the reigns of 5879 guy - as long as it pays well....
More to come. is there a quality layout for this one anywhere?