Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwinstonwood on July 06, 2021, 08:10:01 pm
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Hey everyone,
The itch to build has returned, and I'm casting around for ideas. It's a lot of fun building a loud, high powered amp, but I just don't get enough opportunities to play one cranked.
I've started reading through this older tubenit & DummyLoad thread that explores using low power tubes in push/pull: https://el34world.com/Forum/index.php?topic=12526.0 and I'm brainstorming about a low power version of the Musing 40 that I built last year with a bunch of guidance from sluckey: http://sluckeyamps.com/misc/AA864_Musings.pdf
This amp has a "wasted" triode in the tube right after the gain pot, and before the PI. So, I'm thinking, why can't that unused triode be wired in parallel with the other one in the same tube, or have the two triodes cascade, or wire it with a switch to have both options?
I haven't even chosen power tubes (6DX8, 6BM8, etc...), or started drawing a schematic yet. I need to do a lot more reading, too. But, I thought I'd post this "concept" and see what kinds of comments I get. :icon_biggrin:
Thanks for any input!
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Admittedly, a weird idea.
I'm going to buy 6V6 iron - which, for me, means 365-ish VDC on the plates and not AB763 DR voltages - and then parallel that dual triode before the PI a little out of balance to thicken things a bit. This way I can use my Weber MiniMASS to push it at home. And, I'll still have the TMB TS and the Gain knob to fiddle with. I can build it in a Princeton Reverb chassis/cabinet. GCD sells a 12" PR cabinet, so I'll have my choice of speakers...
I guess I'll build it here on the forum. Though, I seem to come up with odd "morphed" projects that only appeal to me. :icon_biggrin:
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The Hammond 290AX PR replacement PT is only $70.50 at Hawk. So, I'm going with that.
https://www.hammfg.com/files/parts/pdf/290AX.pdf
True to the original PR PT, the 290AX has no separate bias tap. I've modified the Musing 40 layout replacing the 470 3W bias resistor with a 100K 3W.
Please take a look at these two images and let me know if my changes are correct. One image is the original, and the other shows my mods.
Thanks!
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:thumbsup:
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Thanks!! (and that was supposed to be 470 Ohm, not 470K - I fixed the goof in the post above)
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OK, I don't want to try and reinvent the wheel here, and since I already bought all of the iron for a 6V6 amp I think I'm going with the Hoffman Plexi 6V6, but with his single input/channel "Plexi 50 Hot Switch Gain Mod" preamp.
So, everything to the right of the .022uF cap feeding the PI will be straight Plexi 6V6; everything to the left of that cap will be the Hot Switch circuit.
Plexi 6V6:
https://el34world.com/Hoffman/files/Hoffman_Plexi_6V6-V2.pdf
Plexi 50 HSGM:
https://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf
It would be great to hear from anyone who's built this. Thanks.
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I've built a Plexi 6V6 but with my own board and chassis layout. I didn't do a hot switch and I didn't use Hoffman's stout chassis. I don't care for the tubes on the same side as the pots.
Better do a lot of planning if you're gonna put this in that stout 12" chassis. I'm sure it can be done but you will be the first. The hot switch board will likely not fit the 12" chassis. The hot switch plexi 50 board is 11-1/4" long. Measure twice, cut once. :icon_biggrin:
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Thanks Steve. That's timely advice because I've been looking at the JTM45 chassis - or, similar - so I can use a readily available Marshall head cabinet. Thank you for sharing your experience!
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Still using the Hammond 290AX? I used one for a Princeton Reverb and liked the results with the low voltage secondary taps. Settled on a 5AR4 which gave me a B+ of 377. With a 5U4GB I got 362, 5R4GYB gave 356, 5Y3GT gave 348. 100k dropping resistor was perfect for bias. I wanted lower power so the 100ma rating was fine. If you want big Plexi 6V6 sound you might want more current capacity.
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Thanks ac427v. Yes, I bought the 290AX. I'm hoping to land somewhere around 365 to 375 B+ using the 275-0-275 secondaries. And, I'm also paying attention to Hammond's 100ma rating. With three 12AX7's and two 6V6's I guestimate about 84ma of current draw?
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I put the order in at Hawk Sunday night; the iron arrived this morning. $134.44 for all three parts including shipping seems like a good deal to me, especially with the fast, 3-day shipping. :icon_biggrin: My last pair of trail running shoes cost more than that.
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I'm combining Doug's Plexi 6V6 and Plexi 50 Hot Switch layouts. But, I'm going with a different B+ filtering schema. Instead of a 40/20/20/20 can and one 16, I'm using one 32/32 can, two 22's, and one 16. This way I can separate the grounding and put the single caps on the board. I also deleted the 100 Ohm heater CT resistor's.
All I have so far is the board, the power connection and the grounding. There's probably no need to copy the rest of Doug's layout. I did this much to work out the filtering.
PDF attached below...
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It's much easier to understand what you're saying if you post a schematic. Layouts are mostly useless for this kind of discussion.
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My 2cw
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I should know that by now. :icon_biggrin:
Here's what I drew up. All I did was copy Doug's schematics. But, re-drawing a schematic really helps me to see and understand the circuits much better than just looking at it. I have to actually think about each component and connection when I do this, and it all starts to actually make some sense to me. It's a way of studying it that works for me.
PDF: Oops! The first one was wrong. I've uploaded the right file.
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Thanks tubeswell! I'm studying your comments now. I'm not sure what you mean with the galactic ground buss tie? Is that good or bad?
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I'm not sure what you mean with the galactic ground buss tie? Is that good or bad?
Here;
http://www.valvewizard.co.uk/Grounding.html
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You may find this simple hot gain pot useful...
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Thank Willabe. I've never tried a single bus ground before - I've always separated preamp and power amp with their own grounds on opposite sides of the chassis. So, it looks like as long as everything is attached in the proper sequence it works well, like tubeswell was showing me.
That's really interesting sluckey. Now, I'm wondering if I'd even need the switch? If it was "on" all the time (no switch) your Hot Gain pot could turn down the V1-B gain to where it would probably closely resemble the switch in the off position, yes? I'm thinking I would still keep the Gain pot after the tonestack/pre-PI.
Thanks.
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I've never tried a single bus ground before - I've always separated preamp and power amp with their own grounds on opposite sides of the chassis. So, it looks like as long as everything is attached in the proper sequence it works well, like tubeswell was showing me.
Galactic ground refers to separate star ground points chained together to form a galaxy. Keeping each star ground loop as short as within reason and eliminating chassis random grounds.
You make a star for each power supply filter cap node. What ever circuit(s) a single filter cap node feeds, you tie all the grounds from that/those circuits together with that filter caps ground lead. That forms the star. Then you run a wire to the next ground star and so forth.
You can still make the stars for the power amp and preamp and ground the 2 separately. But you don't need to, if you wire up the stars and chain them together right.
You keep the filter cap close to the circuit it powers/feeds, no dog house on the back of the chassis, no can caps with multiple filter cap nodes, because each cap can has only 1 ground, all the caps in the can share the same ground. That lets the current flow round and round in a short loop, without disturbing any other circuit.
Galactic grounding is also a wired ground, not a random chassis ground system. Random grounding has return/ground current flowing through the chassis. Your using the chassis as a wire. Those grounds can cross each other randomly in the chassis, causing problems. You want to get rid of as many chassis grounds as possible.
The noisiest ground is the 1st filter cap ground because it has the most ripple and most current. That's the most important ground to wire up right. PT CT wire directly to that 1st filter caps ground lead, nothing else. Then run a wire to the rest of the power tube grounds, including the OT ground wire.
Merlin goes through all this in that link I gave you.
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More on grounding from Aiken amps;
https://www.aikenamps.com/index.php/grounding
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Thank you Willabe! I found the Merlin passage that illuminated the aha lightbulb: :think1:
"Note that no part of this ground system is yet connected to chassis / earth. The whole audio circuit is still effectively floating inside the chassis. The next section specifies the one-and-only connection to be made between the circuit ground and chassis."
I was stuck on thinking that the individual Stars were each bolted to the chassis. :BangHead:
Now I see how it's done. I'm going to redraw the grounds in my schematic by grouping them into individual stars for each B+ node, "daisy chain" them together and make one chassis connection at the input.
Thanks for your help! It's great to break through the confusion and reach that point of clarity. :icon_biggrin:
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OK, a revised schematic with Merlin's grounding system attached. Which is basically what tubeswell posted.
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And a revised layout where I've attempted to follow Merlin's grounding guide. He does say to ground the speaker jack with the NFP/LTP node if global NFB is used. I'm sure there are errors.
I haven't settled on a chassis. But, the BFPR chassis will work. It actually has exactly the right number of control holes in the front; a place for the cap can; and my 290AX would drop right in. I just want a break from Fender chassis. The other options are the JTM45-type, or an aluminum Hammond 17x7x3 that I'd have to drill and cut. :w2:
It's time to start ordering parts.
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Nice drawing. Neat and logical flow. I did not look for errors.
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Thanks! :icon_biggrin:
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To prevent inadvertent errors when working from a layout drawing, I include the red B+ connection from the reservoir cap to the OT. And the black wire from the power transformer center tap to the cap can ground.
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Thanks ac427v, I added them in. I'm also finishing up the rest of the chassis components like the PT, power switch, fuse, AC inlet, etc.
One thing I've started doing is to use a terminal strip, or other insulated point, to anchor the input grid stopper to as it comes off of the socket pin. Plus, it's a lot easier to solder (and, unsolder) RG174 to a terminal than to a resistor lead. So, I added that, too. I think sluckey gave me that tip.
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Since this thread title mentions the Musing 40, I have a question about it. Instead of using the fourth unused triode in parallel or cascaded, would it make sense to use it as a DC-coupled cathode follower to drive the tone stack?
In other words, V1-A would be the same input triode, and it would be DC-coupled to V1-B. I'd then use V2-A&B for the other two triodes (see attached schematic). How would that effect the character of the amp?
Thanks.
Here's the schematic that actually represents the original amp as I built it last year:
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The Rubicon bridge is burned. Parts ordered. :icon_biggrin:
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Thanks for the super-fast shipping Doug!
I ordered an aluminum .08" blank, undrilled JTM45 chassis from zachmdhunter; should get here next week.
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Hey all, I need some extra eyes to check my bias circuit. I'm using Hoffman's "variable resistor" circuit with the two 10uF caps.
Below is a detail of my connections and a link to his bias circuit page. I'm pretty sure I have it right, but I want to be 100% certain. :icon_biggrin:
Also, should I go up to a 50K pot? I've used 10K's before and they seemed to work fine.
Thanks!
Scroll to bottom of page: https://el34world.com/charts/Biascircuits.htm
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Layout looks fine. I prefer to use a 50K pot and a 47K resistor. This gives a wider range of adjustment than a 10K pot.
http://sluckeyamps.com/phoenix/phoenix.pdf
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Thanks sluckey. I bought some wrong type caps and need to re-order some stuff anyway, so I'll toss a 50K in the cart while I'm at it.
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I was reading through the Hoffman info pages at work and found these cool pin-out/voltage recording sheets. So, I printed some out. Much better than jotting stuff down on a notepad. There's a sheet for probably every amp tube type:
https://el34world.com/charts/Misc/Tube_Pinouts3.pdf
Just thought I'd share this useful resource.
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I had enough free time to get the chassis drilled and to build and lace the board. This .08" chassis took its toll on my cheap Harbor Freight step bits (no surprise). I could also use a bigger, more powerful drill press. But, I got it done, and it was fun.
So, at this point I basically have a "kit" to assemble.
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That is a good feeling to reach that point. :thumbsup:
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Probably my least favorite step. Double checked for continuity and shorts.
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Nice! :icon_biggrin:
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Thank you.
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I'm finally ready to install the parts on the board. :icon_biggrin:
I'll wire up the pots after that.
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Looking real nice! :icon_biggrin:
Something to maybe think about for next time? I think most guys, including the old Fender amps, fill in the R's/C's on the eyelet/turret board 1st, with fly leads for the tube socket, controls and any thing else.
More free room that way, instead of being constricted in the chassis.
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Thank you Willabe! I did build my 6G3 that way, it worked out fine, and I definitely see what you mean.
But, on a previous build - at sluckey's suggestion, I think? - I followed Hoffman's method of building up the amp in layers. https://el34world.com/projects/Plexi6V6.htm
It allows me to plan things out a little easier and triple check my progress before adding the next layer. Plus, it does make it easier to hook wires onto the turrets. :icon_biggrin:
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It allows me to plan things out a little easier and triple check my progress before adding the next layer.
Plus, it does make it easier to hook wires onto the turrets. :icon_biggrin:
You can do it how ever you want, it's your build. But all that can be done loading the board 1st.
There's no wires to hook up to the turrets, because you install those wires when loading the board and let them 'fly'. You trim them after the board is installed.
Even if you didn't put in the fly leads it would still be easier and faster to load the board 1st.
Sluckey don't you usually load the board 1st on your builds. Unless you only did that on a couple builds then changed your method?
Try installing the empty board 1st in a tweed chassis. :laugh:
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Sluckey don't you usually load the board 1st on your builds. Unless you only did that on a couple builds then changed your method?
Most of my wires pass through holes in the board so I always make the wire connections to the board first leaving the wires long. I usually test fit the board several times during this process just to be sure I don't put a 10" wire where only a 2" wire is needed. After all wires are attached, I fit the board in place and trim each wire to it's final length. Then I put the board back on the bench, strip/tin the ends. This is much easier outside the chassis. Remember, I use teflon wire which can be a bitch to strip in tight quarters.
What I do next varies.
For most of my builds I loaded all components while the board is still outside the chassis. I think having the board out of the chassis makes this easier, especially for a tweed style chassis (which I hate). Then I permanently install the board and make all wire connections.
However, on two of my builds, L'il Maggie (http://sluckeyamps.com/lil_maggie/lil_maggie.htm) and Phoenix (http://sluckeyamps.com/phoenix/phoenix.htm), I deviated from the above procedure. I still connected all wires to the board first. But then I permanently installed the board, made all the wire connections, and then finally loaded the components on the board. The advantage of this method is that the component leads stay straight and neat and don't inadvertently get bent while soldering all the wires.
Either of these methods work fine for me. And there may be slight variations from one build to another. If I was using solid core wire with push back insulation, I'm sure I would use a different method.
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To be totally honest I've been cheap when it comes to buying wire. I've always skimped on the amounts I buy. I wasted so much wire with my 6G3 build because I either attached wires to the board that were way too long, or just a tad two short. I was new to amp building - I still am.
I know the solution: I simply need to buy 100' spools, and not 48" lengths.
Also, one of the things about amp building that I enjoy the most is creating what I'll call "working" layout drawing that tends to morph as I progress with the build (weird, I know). I've already changed several things in my layout for this amp, and I'll probably change some more. It's a work in progress; I'm figuring things out and moving things around as I go. And I want to be able to change some component values if I want or need to. If I built a second, identical one I'd know exactly how long to cut each wire.
But, I have to say this. This is only the fifth tube guitar amp I've built (I built my first one in 5/20). And this has been, hands down, the most enjoyable and stress-free build yet. Here's what I've learned so far. The placement and location of parts is very important for a quiet and stable amp. The lead dress probably just as important. Certain component values much less so. Coupling caps, cathode bypass caps, NFB resistors, etc., can all be tweaked to taste. I'll more than likely have to adjust my B+ dropping resistors and maybe a bias resistor, too. I'm thinking about trying to learn Duncan's PSUD2 app to estimate the dropping resistor values I'll need, because I don't know what they need to be at this point.
By installing the components last, I can wait until the end to decide on that stuff. I'm still researching various Marshall amps. They used a lot of different component values and I want to keep my options open. In the mean time I can solder up everything that I won't be changing. :icon_biggrin:
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> I have to say this. This is only the fifth tube guitar amp I've built (I built my first one in 5/20). And this has been, hands down, the most enjoyable and stress-free build yet. Here's what I've learned so far.
You should write a book.
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You should write a book.
Thanks PRR :laugh:
I don't think this is right, but I'll try to figure out how to use this app. :dontknow:
If it's close, I'll need to lower the 10K resistors.
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Well, your simulation prompts a question: Does the simulation as drawn take into account the current draw through various components? As it is drawn, the circuit consists of several RC filters and a final 6.6K resistive load. Is this sufficient to model the amp?
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Thanks acheld. No, I doubt it represents my amp. :w2:
I will read up on how to use the app tonight and see what progress I can make, like how to add current draw, for example, which I predict is somewhere between 76ma and "some higher number" (3 12AX7's and 2 6V6GT's). :dontknow:
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First, thanks everyone who's taken the time to advise me with this project. Willabe, I hope I didn't come across as being argumentative. I really value your input. But, I've found a process that seems to suit the way I want to enjoy building these things. :icon_biggrin:
Unfortunately, I'm having trouble with Duncan's PSUD2 app. I'm missing something key it seems.
Here's a config that I've set next to Hoffman's 6V6 Plexi B+ voltages. My voltages in PSUD2 are darn close, but I just don't think I'm using the app the right way. I tried to put an 85ma load on the end. Any input would be great.
Thanks everyone!
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I tried to put an 85ma load on the end. Any input would be great.
That's not right. The bulk of that 85mA load only goes as far as node A (plates). Then the rest of the load is distributed across the remaining nodes. The final node probably only supplies 1mA or 2mA current.
I think you are wasting time on PSUD2 unless you are having fun. Reading this is a lot more useful to building and understanding power supplies...
http://www.valvewizard.co.uk/smoothing.html
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Thanks, I agree sluckey. It does seem like a waste of time! I'll sit back and and re-read Merlin until it clicks.
In the end I suspect I'll be taking actual voltage readings at the nodes and swapping out resistors until I get them close. I've done it that way before. But, I do want to more or less aim for the voltages on Hoffman's 6V6 Plexi schematic - specifically his C, D, and E numbers.
And, Merlin's Preamp book has been my poolside reading material this summer. :laugh:
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Well, one last bit of noodling with a 2ma load...
This one has an 8.2K, a 4.7K, and a 2.2K. Pretty close.
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I am interested in modeling, but have no experience. Sluckey is correct.
While the power supply is modeled, the loads represented at nodes A,B,C,D,E are not. That's the part that has always confounded me.
The current draw for each node is fairly easy to guesstimate, but what I have trouble knowing are the conditions of that current draw (the resistance and inductance), eg the power loss in each node.
LOL, maybe I need to go back to school. . . or read Kuehnel's book on modeling, which is sitting on my desk.
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acheld, I hear you. I also have Kuehnel's Circuit Simulation book siting around here somewhere... unread.
And, I have no idea what I'm doing with software. :dontknow:
The soldering is almost done. I just need to finish wiring up the Hot switch, which means I need to make a shielded cable and probably remove the switch up out of the chassis to get to it. No problem.
Then, I'll go over every component and connection a few times to find mistakes.
As far as the B+ is concerned, I'm just going to change dropping resistors until I get what I want.
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Double checking a build;
https://el34world.com/Forum/index.php?topic=17701.0
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Thanks Willabe. I'll print out some layouts.
And, I just found error #1. I left off the 33K "slope" resistor.
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PSDU is for the MAIN power node. Like a radio transmitter.
It does not cater to the long chain of filtered loads we use in guitar amps. It does not have a way to attach loads all over.
This is not a problem because we can readily hand-estimate voltage drops and ripple reduction. Practice! (Also plagiarize.)
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Thanks PRR! And, some time back you did show me how to calculate voltage drops and current simply from the info on classic Fender schematics. I need to work out a few of them to get a better hang of it. :icon_biggrin:
But... PSUD2 did let me insert current taps after each node. However, I don't know if it's actually modeling what I want to see:
(the graph is only showing the preamp nodes)
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OK. But you are not running it long enough to estimate final values (you won't play much in 1 second after turn-on). And you are hiding the number-table which is often the best way to read ripple.
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10 second run time
10K, 10K, and 4.7K dropping resistors
Is RMS the column I should be looking at?
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I've been over it and over it. I think the soldering is complete.
I went with 20K - 10K - 10K dropping resistors.
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Looks real nice. :icon_biggrin:
But the 3rd (green) wire/safety ground for the power cord should have it's own dedicated chassis bolt.
And for the quietest operation, the PT B+ CT, red/yellow wire, should go directly to the B+ OT CT/screen cap cans ground lug, and leave the black wire going from the cap can ground lug to your chassis ground bolt. (It should be without the screen cap, but your wiring is next best thing.) And it should be twisted in with the 2 B+ red wires on the way to the cap can.
Both are in here;
http://www.valvewizard.co.uk/Grounding.pdf (http://www.valvewizard.co.uk/Grounding.pdf)
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Thanks Willabe. I thought I had followed Merlin's scheme. I copied the diagram in the attached image. But, I'll go back over it. :icon_biggrin:
OK, after a test power up with the dim bulb tester I plugged it straight into the wall.
No Tubes Installed.
Filaments: 6.9VAC
Bias range on Pins 5: -49.1VDC to -83.3VDC and -49.3VDC to -83.5VDC
Obviously, I need to change bias circuit resistors to bring this down to a 6V6 range. I have a 100K coming off of the PT secondary into the diode right now. Maybe I should try a 220K?
But, this is all I have time for right now. I have to go earn a living. :sad2:
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Willabe, I'm not 100% clear on your post. But, this is what I did:
PT CT, Cap Can Negative, Bias Circuit, and 6V6 Cathodes all connected to one star. This is how I understood/misunderstood Merlin's diagram.
Merlin says to try grounding the Filament CT at different places to find what's best; and, the IEC Earth ground gets its own bolt.
Please take a look at my layout detail below and let me know what should be moved around.
Thanks!
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I would just try it as is. If you have hum, then deal with it.
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Thanks sluckey.
But, sluckey, if you're still there! I changed the bias resistor to a 220K (that's what's on Hoffman's schematic) and my negative bias voltage range is now -47.2VDC to -25.58VDC. Is that an OK range for 6V6's? This is with no tubes in the amp.
Thanks!
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perfect
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cool
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I don't like having the power ground connected to that pot buss bar. If you decide you don't like your ground scheme, this is how I would do it...
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sluckey, actually the buss bar is floating; it's not connected to the pots. It captures the star grounds and runs them all to the input jack - the only ground point (except for the power earth and the filament CT grounds). This is how I interpreted Merlin's method.
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sluckey, actually the buss bar is floating;
Yes, I know that. But the Vol, Gain, Mid, and Presence pots are connected to the buss. I was suggesting take the power grounds off the buss. Then cut off the excess buss bar that extends past the presence pot. This is easy to do. But I still think you should try the amp just as you have it before making ***ANY*** changes to the grounding.
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Thank you sluckey. I got confused. :icon_biggrin:
That sounds like a good plan. Who knows, it might be quiet enough as it is.
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10 second run time
Why start from zero time? How fast can you start playing?
If you start from several seconds, nothing happens different in second 10 than second 1. In fact one cycle tells all. That's 20mS at 50Hz line frequency. (Altho I have 60Hz on tap here, I use 50 for calcs because any mental-math is easier, and to be conservative.)
Is RMS the column I should be looking at?
No. Here, "Diff" is peak-to-peak ripple.
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Wow, so the choke/capacitor (LC filter) reduces ripple to about 1/100th.
In the world of guitar amps is is 0.27V ripple considered good, average, or noisy?
Thanks for the help with this.
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fwiw;
I shoot for <50mV ripple on the 1st preamp tube (V1)
I shoot for <500mV up to the PA tap
In SE I try for <2V at the PA tubes, <4V on PP
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Thanks shooter. So, the sim would indicate 270ma ripple on the 6V6 plates. If I get what your saying, that's not bad?
When I get home I'll see what it predicts for V1.
Speaking of V1, I saved some expense on this amp with the blank chassis and parts I had on hand, so I bought a TAD 7025WA for V1, and a TAD 12AX7A-C for V2. These are supposed to be quiet tubes. And, since this is the highest gain amp I've built I figured it might make a difference. :dontknow:
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Make it work, then make it different :icon_biggrin:
I'll wait for full power meter readings, but 270 is real good at the power tubes
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So, the sim would indicate 270ma ripple on the 6V6 plates.
No. You have the current loads in the wrong place. The ***BIG*** load (plates) is on the left side of choke. There will be several volts of ripple on the plates. Not uncommon to see 10V ripple.
But that's not a problem in a push/pull circuit with balanced tubes because the P/P action cancels the ripple. Big problem for SE amps though.
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No. You have the current loads in the wrong place. The ***BIG*** load (plates) is on the left side of choke.
Of course, the first node connects to the OPT and then to the plates. It's been a long day.
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Hey, you got a lot done. Why ain't you playing guitar?
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I'm at work. :sad2: It's not an amp-friendly place. :icon_biggrin:
But, now I'm wondering if I should go with the F&T 50uF/50uF instead of the 32/32? Well, I'll wait and see how it sounds.
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Hmm... a 50uF/50uF can would squeeze the ripple down from 22V to about 13V.
It's an easy part to swap if I think I need it.
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Hmm... a 50uF/50uF can would squeeze the ripple down from 22V to about 13V.
It's an easy part to swap if I think I need it.
You still have the model set up wrong. I1 (plates) should be about 50mA. I2 (screens) should be about 10mA. These are more realistic numbers for your fixed bias P/P 6V6.
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Dang!! :BangHead:
OK, well that brings the ripple down to about 15V on the plates with the 32/32 cap, and 138ma on the screens.
With a 50/50 it's 10V plate ripple and about 62ma on the screens.
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10%-15% ripple On The Plates is no big deal. Plate is a very high impedance so most of the ripple is across the tube, not across the load winding. (Why we like pentodes.)
Screens are much more sensitive.
Which is why you GOTTA put them right, in the sim and in the amp.
Frankly I think you have about 3 pounds more filtering than you need. COMPARE to other amps known to be satisfactory. With modern high-uFd caps, plate-chokes are unknown and screen chokes getting rare.
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Alright, I'm moving slow here because this always gets my heart rate up.
All tubes installed.
Plate to cathode voltages: 379.5VDC and 381.0VDC.
I set the negative bias voltage to -20V and -18.5V respectively. This is on the cool side - around 60% - which is ok for now.
I did disconnect the NFB. When I first turned on the amp with it connected the speaker made a pop and I could just barely discern a very high frequency ringing. I guess I'll have to swap the OPT wires.
I'll get relaxed and plug in a guitar.
BTW, I'm not hearing any hum, but I haven't turned up the volume control, either.
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Willabe, I'm not 100% clear on your post. But, this is what I did:
PT CT, Cap Can Negative, Bias Circuit, and 6V6 Cathodes all connected to one star.
Different people wire things differently and still have a very quite amp. I'm just going by Merlin, Kevin O'Connor, PRR and others say on this. It works for me, you can wire it up as you think best. What Sluckey said will work fine.
What you have now is very good, probably wont notice any difference in noise by moving this 1 wire. But for me, I do this to help eliminate a possible problem. If my build has noise, then I know it's probably not from this ground star. And I look elsewhere. And since it's a new build and your in there, I would do this.
The 1st B+ filter cap has to deal with the most ripple and filter the most current in the amp. This is the noisiest B+ node in the power supply. That's why Merlin and many other are detailed in how to wire it up.
Merlin could have been a little clearer in his drawing on this. What you missed is (see attachment below) he shows 2 wires coming from the rectifier, 1+ / 1- going to the 1st filter cap. The rectifier - /negative wire going to the filter cap negative lead IS the CT of the PT. (If it was a SS FWB it would be the FWB's ground.) So he shows the rectifier - lead going directly to the filter can - lead. There's NOTHING else connected there. That way all the current flowing through that filter cap can go round and round without disturbing any other B+ filter node. Then he runs a separate wire from the 1st filter cap - lead/rectifier - lead over to the tubes K, bias star.
In the pic below, the red arrow points to your CT red/yellow wire and the purple arrow points to your black wire that goes from your cap can ground over to your ground buss.
So I'm saying leave the black wire as you have it now. Move the red/yellow CT wire from the ground buss back over to the cap cans ground lug. This way you will have it wired up so that 1st B+ filter node can go round and round in a loop all by it's self. Your black wire from the cap can to the ground buss isolates all that current flowing in that loop from the rest of the ground buss.
Merlin says to try grounding the Filament CT at different places to find what's best; and, the IEC Earth ground gets its own bolt.
Yes, but he doesn't' say to try grounding the heaters with the IEC ground.
Nothing else should be wired to the IEC ground bolt.
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10%-15% ripple On The Plates is no big deal. Plate is a very high impedance so most of the ripple is across the tube, not across the load winding. (Why we like pentodes.)
Screens are much more sensitive.
This is why Merlin, KOC and others don't like to use cap cans. ^^^^^^
Can't separate the grounds on that double cap can feeding both the PT CT, which feeds the power tube plates and the screens. So now you could have that 1st filter caps current/ripple modulate the screens ground.
Yes many, many amps have used cap cans and many, many amps don't have a wired ground, they have a random chassis ground and don't have drastic noise problems, but why take the chance? I figure any noise you I eliminate or reduce is a step in the right direction. That's why I wire grounds up like Melin and KOC recommend. It's not that much more work.
High gain amps seem to be more prone to grounding wiring schemes, but the theory still stands.
Although, I think you'll be fine.
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Thanks Willabe! You cleared that up. I will move the PT CT from the star over to the can's negative lug.
But, I wired something else up wrong. I'm not getting any sound at all. Just dead silence. I don't have time to track it down today, but I'm off tomorrow. I suspect it might be the mini switch that I goofed up. I might just bypass the switch and wire the hot on all the time. I already own enough clean, lower gain amps, anyway.
But, first I'll trace through everything a fourth time.
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I moved the PT CT and I ditched the mini toggle.
Maybe some wiser eyes can glance at this and spot my error(s). Sorry the text is upside-down, but it's easier for me to look at the pots when they're right-side-up like this.
Thanks! I'll have all day tomorrow to work on it and pull my hair out.
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I was worrying that I might have damaged my OPT, but I'm measuring about 204 Ohms on each primary to center, and "0" Ohms across the two secondaries. From what I've been able to research that's normal, I think.
Maybe there's a grid that's not referenced to ground, or a cathode, or bungled pot wiring. I hope I find whatever it is. Anyway, after I go over all the wiring again I'll take voltage readings on all of the tubes. Maybe that will show something.
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I didn't have time yesterday to make a version of the schematic without the hot switch. Here it is.
I've been over my wiring many times, I've compared it to Hoffman's hot switch schematic and layout. I can't find my mistake.
I'm going to take voltage readings on all the tubes next.
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spend a couple minutes verifying continuity on Input jack and speaker jack, AND associated grounds
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Schematic looks OK. I bet the amp is not wired exactly like the schematic.
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Schematic looks OK. I bet the amp is not wired exactly like the schematic.
Yep!!! It wasn't until I started taking voltage readings that the mistake stood up and slapped me in the face.
I had left out the ground connection for the V2 cathodes (see attached image)! :BangHead:
I soldered in that wire and WOW! This thing has some gain. :icon_biggrin: It's also very quiet; I'm not hearing any hum or hiss.
>> I'm editing this to say that with the gain/master (whichever it is) up past midway there is a slight bit of hiss, but not enough to hear when playing. No hum at all. It's a quiet amp. So, thanks y'all for all the help with the grounding scheme!
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Here are my voltages.
V1:
pin 1 - 154.5
pin 6 - 157.5
pin 3 - 1.45
pin 8 - 1.38
V2 (cathode follower):
pin 1 - 129.4
pin 6 - 233.4
pin 3 - 0.857
pin 8 - 130.7
V3 (LTP):
pin 1 - 183.1
pin 6 - 176.8
pin 3 - 30.17
pin 8 - 30.17
V4 (6V6GT):
pin 3 - 379.3
pin 4 - 380.6
Plate to Cathode - 379.1
V5 (6V6GT):
pin 3 - 378.9
pin 4 - 381.4
Plate to Cathode - 379.5
Bias is set to an average of 21mA across the 1 Ohm resistors for about 65% dissipation.
I'll post a final schematic and layout (with a board template) later today. Thanks again Willabe, sluckey, PRR, shooter, and others for guiding me through this! Maybe the thread title should be changed to "Hoffman 6V6 Plexi with Hot Switch" to reflect the actual build?
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Here's my finished schematic with my voltages, and the layout.
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Looks great!
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Thank you!
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I had some laserable acrylic left over from my AC15, so I guess this one's going to have a blue faceplate, too. Here's a mockup drawing.
I've been playing it through the Celestion Creamback that's mounted in my AC15 cabinet. I haven't played it enough to really dial it in yet. But, it's a cool amp! :icon_biggrin:
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So, I'm getting ready to order a Marshal head cabinet for this build (I have to budget and save for stuff).
It occurred to me that I should have used an OPT with multiple speaker Ohm secondary taps to allow for various speaker cabs other than 8 Ohm.
My current Deluxe Reverb OPT reflects 6,600 Ohms to the primary with an 8 Ohm speaker load.
It sounds really good as is, but I'm wondering if I might get less clean and less headroom if I went with an 8,500 Ohm 5E3 OPT (with multiple speaker taps) instead.
Contrary to logic and the interests of most gigging guitarists, I actually want the amp to distort sooner at the same volume.
Would the 5E3 OPT make a noticable difference at the same volume level in that regard?
Thanks. All advice and suggestions welcome.
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Hi dwinstonwood. I had that exact same OT, it was a 6.6k 20 watt, with only the 8ohms tap on my Princeton reverb. I replaced it with a 18 watt 8.4k OT with multiple taps and even though I still am using the 8ohm the amp has more grunt now.
Good luck...Glenn
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Thanks Glenn! That's what I was suspecting. Good to know. I'll look at one of those 18w OPT's. You can never have too many transformers. :icon_biggrin:
It looks like the Hammond 1750PA is a good, inexpensive choice:
https://www.hammfg.com/part/1750PA
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Yes, that’s the one I used... I still have one left that I might use on a 6y6 Express I’m interested in.
How do you like the 6l6 bassman, you and Sluckey designed?
I’m interested in that too. Does it break up before the cops come to the door?
Thanks...
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Glenn, the Musing 40, as I called it, has beautiful clean tones, but it's really way too loud for the townhouse I live in. I can get it into overdrive with the gain pot, but just for a few minutes at a time. I couldn't get away with playing it that loud for extended periods.
Now, I'm using a WGS 12C/S which is a warm speaker with a very smooth top end. High notes are really clear, but without being so bright that it hurts (I ended up leaving off the Bright switch). The filtering (coupling cap choices) seems very balanced through the whole frequency range. It would probably be a good Jazz amp. I think it would really shine with a big archtop.
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Thanks for the explanation, cheers.