Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: neddyboy on July 08, 2020, 03:43:44 pm
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This is a follow-on from my 2x12AX7 preamp build. I'd like to build a power amp to go with it using a variation of an old Dynaco power amp schematic (attached). I'm going to jam it all into the same size Bud CU-347 project box that I used for the preamp. I know it's ridiculous, but it sounds like fun.
I have a spare set of Grommes LJ-2 transformers sitting around, intended for use in a PP 6V6 (specs also attached). Is it possible to use these transformers in an EL84 circuit? The Grommes PT puts out 340V B+, but I'd need to drop it to 300V for the EL84s, right? FYI I'd like to use an EZ81 (or similar) rectifier rather than an octal for size reasons. I assume I'd have to drop the voltage before the rectifier tube with a big ol' resistor, but is 40 volts too much to dissipate? And I'm totally out of my depth with the OT -- will it work with EL84s? Any other issues I'm not seeing?
Thank you in advance!
Neddy
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You could use a string of 10V 5W zener diodes from the HV center tap to ground, to knock down the voltage. They’re easy to use, stick them on a terminal strip. Keep them off to the side of the chassis as they throw off some heat. Try 3-4 of them in series; that’ll get you down around 300. Striped end points to ground.
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You can use a 6v6 output transformer for EL84s and vice versa. The only difference will be a bit of a difference in biasing/watts as the primary impedance specs for 6V6 vs EL84 are slightly different (8K vs 6.6K I believe).
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Not enough current capacity in the 6.3 volt winding. Each EL84 wants 760 milliamps, the 12AX7 wants 300 milliamps, and the EZ81 wants 1 amp. Add it all up, and you have 6.3 volt heater current demand of 2.82 amps. The 6.3 volt current supply is limited to 1.4 amps. I wouldn't worry too much about plate voltage. Keep plate dissipation less than 12 watts, and you should be good to go. You can play with the circled dropping resistor value to get your target B+. The output transformer is fine.
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340 volts is no problem for EL84s, l’d skip the zener or resistor to lower voltage. 18watt amps run all day with OT 8k impedance, 360v B+ and EL84’s. As far as the EZ81, why go through the trouble to reduce the socket, maybe use 5Y3.
If you’re PT is an issue, use diodes rather than a tube, voltage will be higher but even 360 is fine for EL84s. If you want lower voltage for tone, use the power resistor or zenner’s off CT like diverted suggested
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Hmm, OK! Dude, I was originally planning on a diode rectifier and have a bridge ready to go. I only thought about going back to a tube to drop some voltage, so SS rectified it is. Amazing that the EZ81 draws that much! One question tho -- this is looking a lot like the power amp section of an 18 watt Marshall, and IIRC the 18 watt circuit doesn't play well with SS rectifiers. Have you heard that?
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Hmm, OK! Dude, I was originally planning on a diode rectifier and have a bridge ready to go. I only thought about going back to a tube to drop some voltage, so SS rectified it is. Amazing that the EZ81 draws that much! One question tho -- this is looking a lot like the power amp section of an 18 watt Marshall, and IIRC the 18 watt circuit doesn't play well with SS rectifiers. Have you heard that?
You're still underwater current wise. Two el84s draw 1.52 amps add in the AX7 and you're at 1.82 amps. The PT 6.3 volt winding is limited to 1.4 amps. Why not build the amp as designed?
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Dang, yeah, I misread your post earlier. I guess it's a no go. Why not build it as shown? Bad reasons! :icon_biggrin: I'm unemployed and trying to build this with parts on hand, and the only chassis I have is a 8"L x 5"W x 3"H project box. I have to admit too that I have an unhealthy fascination with miniaturizing. Anyway, once I get the PT and OT in there, the octal sockets and tube diameter make it very challenging. The 9-pin tubes allow a lot more clearance. I even have a 6.3 volt transformer, but there's be nowhere to put it on this tiny box! I have one more Bud box the size of a large guitar pedal, so I could build a separate power supply box with the PT and the filament transformer, but that seems silly for a guitar amp.
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:laugh:
be creative;
http://sluckeyamps.com/smoky/smoky.htm
3lb coffee can and your box :icon_biggrin:
I built a wooden box under a metal box, worked well
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Dang, yeah, I misread your post earlier. I guess it's a no go. Why not build it as shown? Bad reasons! :icon_biggrin: I'm unemployed and trying to build this with parts on hand, and the only chassis I have is a 8"L x 5"W x 3"H project box. I have to admit too that I have an unhealthy fascination with miniaturizing. Anyway, once I get the PT and OT in there, the octal sockets and tube diameter make it very challenging. The 9-pin tubes allow a lot more clearance. I even have a 6.3 volt transformer, but there's be nowhere to put it on this tiny box! I have one more Bud box the size of a large guitar pedal, so I could build a separate power supply box with the PT and the filament transformer, but that seems silly for a guitar amp.
Been there done that on more than one occasion. I was a victim of corporate "right sizing" on 4 occasions. I hope that you have better fortunes soon. With the box that you have, I believe that you may have enough room to build a "tweed" style box. Input and controls mounted outside on one 3 inch side with the tubes mounted to the opposite 3 inch side. Transformers mounted outside on the 3x5 with the circuit card mounted inside. Mount the completed chassis in a mini head box. It would be a tight squeeze. But, I bet you can do it. It's worth scribbling out some layout diagrams to try for size.
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:l2:
"right sizing"
I got sold once and down right :cussing: the last time
Tubes are good therapy :icon_biggrin:
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You guys are inspiring me to heights of "creativity" aka silliness. FYI 66strat, this build is a discrete power amp with (probably) no controls, just a power switch, a fuse, and a pilot light. It's meant to pair with the preamp I built recently. I'm going for a component-style setup with 2 separate chassis for no good reason other than having the parts already and being bored. Maybe I will build a separate power supply on that last project box!
That raises a new question: I'd likely make my own umbilical out of transformer-lead type wire. Do any of you guys know of any issues with running HV and heater parallel in an umbilical? Ah jeez, would I have to twist everything for the length of the umbilical?
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Shooter, I built a spring reverb unit using a Trader Joe's peppermint bark tin as a chassis. I'd show you a picture, but it never worked well and became the donor for the preamp project. I love that purple guy too!
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You know, there's nothing etched in stone that says that you must build using a metal chassis. :wink:
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About heater consumption it was told that EL84 requires more current than 6V6
I'm wondering which will be your B+ with a 680V AC CT transformer
(https://i.imgur.com/woQQZsl.jpg)
340V AC x 1.4 (Solid State rectifier) will result in 476V DC B+
assuming you solve the heater consumption problem (an additional heater transformer) you can drop B+ a bit using a Vacuum Tube Rectifier, but I think you'll obtain around 440V ...... it will be better you use 6P14P-EV tubes (OK till 500V) instead of EL84 tubes (OK till 300V)
K
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OK, I decided to bail on the EL84 substitution. Attached is my attempt to arrange the components for a full 6SL7/6V6/5Y3 circuit on my chassis without using a separate power supply chassis. Pretty tight, but it seems possible. Clockwise from top right, 5Y3, 6V6, 6V6, 6SL7. I chose to put the OT closer to the PT so the tubes could be farther away. I may have the option of putting the OT inside the box, like it was on the Grommes. I plan on installing a steel bulkhead inside the box to isolate the power section. Given that, would the OT be better shielded inside the box or outside? Any other comments/suggestions on placement would be most appreciated!
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maybe move the OT to the back of the chassis
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Thanks Shooter! You think mounting the OT outside the box on the back would be quieter than inside the box with a bulkhead? I'm resistant to the idea, as the OT & PT at the back might affect the balance the box and make it likely to tip over backwards. These things will sit on top of a speaker cabinet, and I'd like to keep them there! Do you think the benefit would outweigh the potential problem?
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If the amp tips over you're playing to loud :icon_biggrin:
the OT is noisy, you want it away from the preamp AND "decoupled" from the PT
putting it inside changes planes with PT but puts it even closer to the preamp tubes.
If you're like most, down the road it'll become something else, so where you mount it now will gain you knowledge for the future :icon_biggrin:
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...I'm wondering which will be your B+ with a 680V AC CT transformer
(https://i.imgur.com/woQQZsl.jpg)
340V AC x 1.4 (Solid State rectifier) will result in 476V DC B+ ...
476vdc with no load, which is important for filter cap ratings.
But the PT also has winding resistance, and the rectifier has voltage drop. The PT in my Vibro Champ has a loaded winding voltage of 340-0-340vac, but after the 5Y3 the voltage at the 1st filter cap is 388vdc. In this case, the B+ winding resistance is 378Ω, which lowers the B+ voltage right along with rectifier internal resistance.
If it weren't for other constraints, the OP could add a resistor between the B+ winding & each rectifier plate to help knock down charging current & B+ voltage. It would probably be wise to add a solid-state diode between the resistor & the actual tube rectifier plate, to guard against future rectifier failures.
A lot of modern replacement transformers seem to have much lower winding resistance than the actual vintage parts, which plays a factor in the higher B+ output we keep getting.
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HotBluePlates, I've decided to be conservative and build the 6SL7/6V6 circuit. I'd have to buy almost nothing, and if I use a diode rectifier I can fit it all onto the chassis. Would I see higher voltages with diodes than with a tube? I don't know anything about diode vs tube past the basics. My idea was to use a diode bridge and a sag resistor (rather than a tube) as they'd be easier to squeeze into the chassis next to the PT, but I'm not sure if this is feasible. Thoughts?
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here's a good read on reciters, he also covers the rest of the PS
http://www.valvewizard.co.uk/bridge.html
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I'm still exploring some wackier options on this, and I have a couple more arcane (for me) questions. Rest assured, I do try to track this stuff down before I ask you guys.
First, if I were to build a separate power supply box including the caps, rectifier bridge, and a choke, I could then send B+ voltage through a hard-wired umbilical. IIRC it's not good to run AC filament voltage alongside the DC. Do you guys know anything about this stuff? I could A. install a 6.3v filament transformer in the power amp box, or B. rectify the 6.3v AC from the PT before sending it down the umbilical. My vast parts bin includes a Hammond filament transformer, so no added expense. Any thoughts or OMG-don't-do-thats?
Second, and related, could I place both the filament transformer AND the OT in/on the box shown above without adding a lot of noise? I've done some reading and I think I get the basics of transformer alignment, but my situation doesn't come up in anything I've seen! My preference, mostly for aesthetic reasons, would be to mount one transformer inside the box like the layout attached. I've shown the filament transformer mounted inside the box aligned vertically on the the side wall, and the OT on top. Advisable?
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if I were to build a separate power supply box including the caps, rectifier bridge, and a choke
My Magnatone M10-A amp does just that. There is no problem having B+ and filaments in the same cable.
http://sluckeyamps.com/magnatone/magnatone.htm
Here's another similar project. PS and PA on one chassis, preamp on another chassis with connecting cable that carries B+, filaments, and signal. No problems.
http://sluckeyamps.com/vox/vox.htm
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Thanks Sluckey!
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OK, checking back in! I decided to build a separate PS box using a Classic Tone 5E3 PT I had. It's about the same as the Grommes PT, but with slightly higher voltages. I'm planning on 6 conductors in the umbilical: 3 @ high voltage, 2 @ 6.3v, and ground. The 5E3 circuit runs high voltage to the OT direct from the rectifier, which would require another conductor in the umbilical -- why couldn't I just tap into the high voltage from the other end, within the power amp chassis?
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You only have 3 filter caps so you only need 3 high voltage wires. Think about it. :think1:
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Oh, I do. Sometimes I don't trust my thinks though! Follow-on question: as you know, I used the Tube Pedal schematic to build my preamp. I'm planning on using this with my next project, a 5E3 power amp in a tiny box. Can I assume that the only tube I'll need in the chassis with the output tubes would be a 12AX7 to act as a phase inverter?
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yup, unless you use a transformer PI, but then it'd be a Gibson :icon_biggrin:
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Thanks shooter! I've built two tweed Deluxes in normal chassis, but this project is challenging my weak understanding of theory, AKA actually knowing how stuff works. Watch this space for more questions...
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Next step, the power amp! I'm putting it into another 5" x 8" x 2" chassis. I have 8" of space to use for the OT, the 2 power tubes, and the driver/PI, and I'm unsure about placement. The OT will be on top at one end, oriented across the 5" width. I'll put a bulkhead inside, between the bottom of the OT and the tubes. Here's the question: should all the tubes be as far as possible from the OT, even if it means the driver/PI is close to the the power tubes?
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Next step
the PS tests out good without a load :icon_biggrin:
I've put power tubes < a tubes width from the OT no problem. the Pi isn't the sensitive type like V1, but should keep it's distance from the OT
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I'll be testing the power supply tomorrow, fingers crossed. I had to wait for a fuse holder to arrive. I will keep you posted!
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I brought up the power supply on a variac, and the unmarked probably-a-5Y3 rectifier was indeed a 5Y3. The voltages I got are A: 482, B: 486, and C: 450. Please bear in mind that I replaced the 5k dropping resistor with a Fender-correct Hammond choke. The voltages seem too high even for unloaded. Opinions? How would I address this if it is an issue? An extra resistor before the choke?
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Don't do anything until you have the proper load connected.
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Will do!
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Hello all! Since I posted last, I finally got the 500v aircraft connector for the umbilical, got the umbilical built and tested. I'm bummed that I ran out of room for a pilot light on the PS chassis. I even tried an LED, and there's just no room. I'll rely on the pilot light on the power amp chassis, and they'll never be disconnected. I finished the power amp section today and got the voltages measured. They're weird: A: 20v B: 335 C: 350. That 20v on A was up in the range of the other two before I connected it to the OT. Why would it drop like that? It's a 60-year-old transformer, but it worked when I stuck it on a shelf!
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That 20v on A was up in the range of the other two before I connected it to the OT.
you mean hooking it to the PA - with tubes? If so you might have a BIG problem with the PA sucking all the current
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They're weird: A: 20v B: 335 C: 350. That 20v on A was up in the range of the other two before I connected it to the OT. Why would it drop like that? It's a 60-year-old transformer, but it worked when I stuck it on a shelf!
If your node A is connected to the OT I would expect the other nodes to be reading low also. You must not have your nodes labeled like we normally see them labeled.
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Does your parts bin include a fuse for the HT?
Either there’s a bad connection or, as noted, a lot of current being drawn.
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Yes, there's a fuse in the PS. My skill level demands fuses!
I tested the PS unloaded and all was well, so the error must be in the PA. I also suspect a bad connection, or should I say, a swapped connection between the 3 nodes. I'll be re-re-checking them today. Sluckey, I'm referring to them as A B and C based on Weber's 5E3 schematic. I didn't even know they were called nodes until now! How should I be labelling them?
Shooter, what would be "sucking all the current"? This is completely new to me. IIRC the voltage on A without tubes installed was correct. When I brought it up on the variac with tubes and speaker, nothing catastrophic happened -- no smoke, no blown fuses, no hot transformers -- but when it got up to 115v I did see 2 pops inside one of the 6V6s before I shut it down. Shorts maybe? Last, although I did have the preamp connected and a signal going in, I got no audio at all through the speaker.
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Shorts maybe?
that's what sucks current, Ohm says so :icon_biggrin:
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Sluckey, I'm referring to them as A B and C based on Weber's 5E3 schematic.
That's the commonly accepted way to label the nodes. However, it is impossible to only have 20v at node A and still have 300V at the other nodes. I suspect the OT is not connected per your schematic. You most likely have the OT connected to Node C in order for the other two nodes to still have about 300V present. It would be very easy to miss-wire since you are using an umbilical cord. Should also be an easy fix.
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Sluckey, you are the Dr. House of amplifiers. I'm very embarrassed to say that I did swap the A and C high voltage wires. I screwed myself by trying to be too organized. Trying to deal with A-B-C-Orange-Yellow-White-Pin-1-Pin-2-Pin-3 broke my brain!
I definitely don't understand why swapping the two HV leads would cause such a power imbalance tho. The 3 nodes aren't that different in voltages. Anyway, that's corrected. With tubes installed, I got A=386, B=386, and C=328, and no shorts. Still high, but at least less weird.
FYI the 22k dropping resistor got so hot it changed its value! Not really, but it did change red-red-yellow to black-black-grey. It still measures correct but I'll replace it. Any thoughts on the voltage measurements? No voltage drop across the choke?
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Sluckey, you are the Dr. House of amplifiers.
No, I'm the Ice House man! :icon_biggrin:
FYI the 22k dropping resistor got so hot it changed its value! Not really, but it did change red-red-yellow to black-black-grey. It still measures correct but I'll replace it. Any thoughts on the voltage measurements? No voltage drop across the choke?
That's what happens when you try to shove 100mA through a resistor that's only expecting 5mA.
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I definitely don't understand why swapping the two HV leads would cause such a power imbalance tho.
surf up some "understanding series circuits" a good read on ohm's law might also help
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I am suitably chastened! :icon_biggrin: I love doing this stuff, but every time I try to learn the fundamentals of electronics my mind turns to mush. I have a mental blind spot with electronics theory, and it doesn't help that it's so boring. But I will try again.
I have to say though, this project (and you guys) have taught me a huge amount. Breaking down an amp into its component parts forced me to think more about structure rather than a simple execution to a preset plan. As I mentioned before, this is my first P2P project, and the first for me without using a layout.
What damage may I have caused by swapping A & C? I'll test the tubes, and will replace the 22k resistor. How can I address the voltage measurements?
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I have a mental blind spot with electronics theory, and it doesn't help that it's so boring
I'm the opposite, love electronics because it's mostly logical
humans on the other hand are mostly boring :icon_biggrin:
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Yup! I suppose I'm relatively technical, considering all the software, code, etc. I used to deal with, but I always had to force myself to pay attention to the technical details. I was definitely an art major, not an engineering major.
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What damage may I have caused by swapping A & C? I'll test the tubes, and will replace the 22k resistor. How can I address the voltage measurements?
Barbecued resistor is probably the only casualty.
Measure voltage at all B+ nodes. Measure voltage on every tube pin.
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OK, will do. FYI before I saw your post, I plugged in some tubes a fired it up (on the variac). It passed signal, sounded pretty dang good, but not as loud as I expected. I know it was rash, but it seemed safe enough.
I have 2 extra gain stages with two preamp tubes plus half the PI, but it's not as loud as a normal Deluxe. The 6V6s tubes are an unmatched pair, but that's never bothered any of my amps very much, at least for testing.
I'll check the pins and make sure the tubes aren't damaged from the internal short and report back.
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It passed signal, sounded pretty dang good
once you fix the gray R you should be able to go to the wall
and
Measure voltage at all B+ nodes. Measure voltage on every tube pin.
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I measured all the voltages this AM. The 12AX7 was higher than normal, but proportionally correct. Voltages as follows:
1= 183v
2=7mv
3=2.2v
4&5=3.34
6=270v
7=20.8v
8=63v
9=1v
The 6V6 voltages were not normal, like 5-10% of expected. I suspected the tubes, and indeed, they're really really bad. They were old and didn't have long to live anyway, and the internal shorts must have killed them. On the last round of testing, the tubes got insanely hot. Could the fried tubes explain the low B+?
To be clear, I was measuring voltage to ground with tubes installed, the preamp plugged in with volume to zero, and a speaker plugged in.
At this point, the only pair I have left are a pair of black plate RCAs, so I'm going to get a new set that I'm willing to test with. I'm sad -- the fried pair were also RCAs, just old and nearly worn out. Still, hard to toss 'em in the bin! Anyway, I'll post when I have some new tubes.
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what value cathode R are you using on the 6V6?
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240ohms. Couldn't find a 250 locally. I got all excited because I thought I could drop in a pair of 6K6s that I have, but they require rewiring. I have an offer in on an affordable pair of Sylvanias -- we'll see.
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The 6V6 voltages were not normal, like 5-10% of expected.
What were the actual measured voltages?
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I obviously misspoke with my 5-10% comment. They're low, but not 5% low. Except for pin 4 on 6V6 #2 at 167mv! Honestly, it's such a simple circuit that I don't get where that extra voltage is going unless it's because of the failed tubes.
6V6 1
2=3.3v
3=138v
4=152v
5=.2mv
7=.2v
8=1.23v
6V6 2
2=3.3
3=somewhere in the range of the other tube. Voltage wouldn't settle, and the tube starting making a whining noise. Not through the speaker, the tube itself. Anyway, I got nervous and stopped.
4=167mv (!!@%#?)
5=0
7=.17v
8=.8v
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Except for pin 4 on 6V6 #2 at 167mv! Honestly, it's such a simple circuit that I don't get where that extra voltage is going unless it's because of the failed tubes.
Easy to prove. Just pull the tube. What voltage is on the pin 4 now?
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Tomorrow! My afternoon has been dominated by the chore list.
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Where is your schematic?
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Good morning! I used the attached Weber 5E3 schematic for the PS and PA. I measured the 6V6 pins without the tubes, and saw the following. The high voltage is still too high (I think!?) even for unloaded, but please correct me if I'm wrong. I just ordered a pair of Sylvanias on eBay, so I'll be able to test with tubes in a week.
6V6 1
2=3.5v
3=523v
4=523v
5=bouncing around in the low MV range
7=3.5v
8=0
6V6 2
2=3.5v
3=516v
4=523v
5=.3v
7=3.5v
8=0
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8=0
should be in the 15-25vdc range
suspect an open
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8=0
should be in the 15-25vdc range
suspect an open
No tubes ---> No current drawn ---> No voltage across cathode resistor
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Don't sweat the high voltage with no tubes.
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I did one last sweep through my storeroom for 6V6s and found a pair of EHs that I was willing to risk. I just put them in and fired it up. Good & loud, and no red plating! The PT stays cool, but it does vibrate more than any I've used. Huh. I'll be measuring voltages later today and will let you know. Thanks!
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Both Pin 3s are at 372v, both Pin 4s are at 325v, and Pins 8 are at 27v. I think the upper limit on 6V6s is 370v, yes? Do I even need to lower these voltages?
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Fender wasn't afraid to run 6V6s at 415V.
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Wow! I'd imagine these EHX 6V6s are pretty robust too. Well, the last part is arriving today, the jewels for the pilot lights. Once I get all the boxes pretty, I'll post photos.
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Both Pin 3s are at 372v, both Pin 4s are at 325v, and Pins 8 are at 27v. I think the upper limit on 6V6s is 370v, yes? Do I even need to lower these voltages?
Those 6V6GTs can tolerate higher plate voltages, but excessive current not so much. Based upon the numbers posted, the plates are dissipating ~ 18 watts. Design Max rating for the tube is 14 watts.
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Thanks 66strat! You're also a JT in real life? Anyway, how would I address that? I see online that folks recommend a zener diode before each plate to lower the voltage.
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Use a real 5Y3. Use a 500Ω cathode resistor. Post your new voltages
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You know, there's nothing etched in stone that says that you must build using a metal chassis. :wink:
Hey sorry to hijack, but Im building a wooden amp (actually a prototyping station) as well and wondering how you did your grounding on this? Are you taking everything back to the B+ side of the rectifier and then patched across to earth ground, or something different?
- Steve
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Hey Sluckey, wow I'd forgotten I was originally going to use a diode bridge! I actually switched back to a 5Y3 pretty early in the PS planning. It's a vintage US tube, Hytron maybe? You'd think I had a silicon rectifier in there tho, with those voltages. I will try the 500k resistor.
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Steve, the stuff I just built are in aluminum Hammond project boxes, not wood, but my grounding scheme was pretty straightforward. I'll also point out that I'm likely the wrong person to ask! Anyway, the power amp chassis and the PS chassis are linked via an umbilical. Each chassis has a star ground, and the PA is grounded to the PS via the umbilical. Honestly, I'm not sure what you mean about taking it back to the B+ side of the rectifier. As Sluckey and Shooter will tell you, my technical skills leave a lot to be desired!
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I will try the 500k resistor.
It's not a 500K resistor. It's a 500Ω (That's five hundred ohms). Gonna need to be 5 or 10 watts.
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Sorry, yes, slip of the index finger. 500 ohms.
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Thanks 66strat! You're also a JT in real life? Anyway, how would I address that? I see online that folks recommend a zener diode before each plate to lower the voltage.
Yes, I am a JT in real life. There are several ways that you could use to address the problem. Sluckey's recommendation of a 500 ohm cathode resistor should put you in the ballpark. Alternatively one could add a zener diode between the PT secondary center tap and ground, add a 250 to 300 ohm resistor between the center tap and ground, or add a 250 to 300 ohm resistor in series between each PT secondary and the rectifier plates.
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Thanks JT. I'm going to start with the cathode resistor, then try the center tap resistor if that doesn't do it. Is 5 watts enough on the center tap? These are either/or options, yes?
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A 5 watt resistor will suffice. The zener diode and center-tap resistor are either/or options. The center-tap resistor could be used in conjunction with a higher cathode resistor value to lower the dissipation, or could be used in lieu of a higher value cathode resistor. It would be a matter of tweaking the resistor values. The center-tap resistor will lower the B+ voltage. A 250 ohm resistor should put you in the ballpark.
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I just replaced the 240 ohm with a 470ohm on the cathode, and the voltages have risen. 6V6 1 is now 420v on pin 3, 411v on pin 4, and 31v on pin 8. 6V6 2 is 3=410, 4=406, and 8=31v. The replacement resistor is a new Dale and tests correct. I'm somewhat suspicious of my DMM tho -- it's been cutting in and out and acting weird. The measurements are inconsistent even tho the batteries are new.
I also realized that I likely can't do the center tap resistor as the interior of the PS is filled to the brim. I MAY be able to run a series of 2watt resistors, but there's just no room for a cement block.
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math says ~ 12W a tube so you're making heat :icon_biggrin:
a good meter protects good tubes :laugh:
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I just replaced the 240 ohm with a 470ohm on the cathode, and the voltages have risen. 6V6 1 is now 420v on pin 3, 411v on pin 4, and 31v on pin 8. 6V6 2 is 3=410, 4=406, and 8=31v. The replacement resistor is a new Dale and tests correct. I'm somewhat suspicious of my DMM tho -- it's been cutting in and out and acting weird. The measurements are inconsistent even tho the batteries are new.
I also realized that I likely can't do the center tap resistor as the interior of the PS is filled to the brim. I MAY be able to run a series of 2watt resistors, but there's just no room for a cement block.
The 470 ohm puts the plate dissipation ~ 12 watts, good to go IMO. :thumbsup: How does it sound?
edit:
What shooter said. :icon_biggrin:
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It sounds great! I don't have an appropriate cabinet for it (yet), so I'm using my Blues Deluxe with a Warehouse ceramic 12 as a cab. It's not as loud as I expected, but that's most likely my perception as my everyday amp is an Ampeg VT-40. Very nice break up with a Tele. My only mild complaint is that the bass is a bit "bloomy", but I suspect a more appropriate speaker & cabinet will change that a lot. I have one '60s Jensen C10Q and I may look for another one for an open back 2x10.
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It sounds great! I don't have an appropriate cabinet for it (yet), so I'm using my Blues Deluxe with a Warehouse ceramic 12 as a cab. It's not as loud as I expected, but that's most likely my perception as my everyday amp is an Ampeg VT-40. Very nice break up with a Tele. My only mild complaint is that the bass is a bit "bloomy", but I suspect a more appropriate speaker & cabinet will change that a lot. I have one '60s Jensen C10Q and I may look for another one for an open back 2x10.
That's the nature of a 5E3. The interactive volume and tone controls offer the means to lessen the boominess somewhat. If it's still to boomy, swapping out the .1 uf coupling caps for .047 uf in the preamp and .047 or .022 in the power amp.
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If scaling down in size was important I'm surprised Ned didn't think of using 6AQ5's instead? Might be too late (stepping back into the shadows now)?
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Thanks jojokeo. Heh, yeah, too late! I chose 6V6s for no better reason than I already had a few 6V6s and sockets left over from an earlier build. Early on, I was thinking about EL84s, but I doubt minis would have saved that much room since most of the size difference is outside the chassis anyway.
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Well, the project is nearly complete. The three components are finished and running well, and I finally trust the power amp enough to put vintage tubes in it! I've attached some pics of both the chassis (plural) and the 5E3-style cabinet I built for 'em. Yes, that's Marshall grille cloth. The PA and the PS will sit in the bottom of the cab, and the preamp will sit on top.
I also wanted to thank everyone who contributed. I feel like I passed some sort of knowledge threshold on this project, and it's due to the patience of this forum. See you next time!
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:thumbsup:
Now it's annoy the neighbors time :icon_biggrin:
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Not 'til I can afford a speaker! :laugh:
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hey man i only read half of the posts so maybe this was brought up later. you can get a cheap 6.3 volt transformer that handles 1 to 2.5 a for like $10-$15. i used one to run a 6ax5 in a build, about the size of a doorbell tranny. the last one i bought i paid $10 each and they handle 1.4a @6.3v. you can use the main tranny for either preamp filaments or power filaments or vice versa.
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Hey, thanks for the suggestion! I actually solved the filament issue by using the correct tubes for the PT. I have a Hammond 6.3v that I keep thinking I might use for something!