Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pompeiisneaks on January 25, 2016, 05:47:13 pm
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I've got a first draft at a schematic in easysch for the gibson eh-185. I'm interested in the amp because one of my favorite guitarists, Joshua Homme used it when he played in Them Crooked Vultures.
I noted the common pictures of this amp show it with a treble and bass control, and saw two versions on the schematics page here:
http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_65Q7_PRE.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_65Q7_PRE.pdf)
which seems very much like a clone of the EH-150, but doesn't have a bass tone stack element. Then there is:
http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_6J7_PRE.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_6J7_PRE.pdf)
which does have it, and seems a bit more complex all in all. I was considering basing it off the second, but found that the tubes (6J7) require the anode connector, and don't know how feasible that is to build nowadays... So I kept my schematic based on the earlier one. (If I'm off base on that assumption, please do comment).
I also may end up building this amp as my next build (some day in the future), but am first just trying to sort it out logically. I have a few questions in general about the schematic, as its a bit 'empty' of some data:
1. There is no listed output transformer. Is there a pretty common output transformer for 2 6L6's? Or do you need to fit an OT based upon the entire preamp and power amp stages impedance?
2. There's a named PT, but it seems hard to find, the one I did was pretty pricy, 200$ or so here: http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_gibson.htm (http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_gibson.htm) which specifies it is for a EH-150 and there's a separate one for the EH-185. Could I just use a modern one that can push the right power to the 6L6's and the other tubes? 6SQ7/6N7? I can look that up on the data sheet, and get the total amp required for hte tubes, and see the levels there, that's what you do right? Then I look at normal voltages for these tubes and can look at the expected voltages? (since they're also not noted anywhere on the original schematic?)
3. Could I just add the same bass controls from the second one into the first and get the same effect? Or is the latter schematic above just the more modern version and will likely have a massively different tone than the second type EH-185?
At any rate, here's the .sch as well. Let me know what you see is off/wrong, etc.
Edit: With latest layout and schematic.
Edit: Adding the latest mods as well.
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So trying to do the math, someone can double check me ;) I am guessing a typical fender PT and OT may work here with the 2 6L6's in the amp. From the tubes I get:
3x 6SQ7
Heaters: 0.3A @ 6.3V
Main: 1.1mA
1x 6N7
Heaters: 0.8A @ 6.3V
Main: 7mA
2x 6L6
Heaters: 0.9A @ 6.3V
Main: 145mA
So heaters need:
0.9 + 0.8 + 1.8 = 35mA
and main needs:
3.3 + 7 + 290 = 300.3 mA
If I read the max amps right on the datasheets for each that is.
It seems then that this transformer:
http://www.mojotone.com/transformers/Fender/Power/MT-FENDER-MOJO763.pdf (http://www.mojotone.com/transformers/Fender/Power/MT-FENDER-MOJO763.pdf)
should have excess to cover that need. Does that look right? Is there something better? I see Doug has the one just below it, the 762, but that seems short on current for the amps main power.
Then for output transformer, I see the 6L6 seems to be 6600 ohms per, so 13200 ohms for the OT does that mean that the only ones I see on Dougs page that could cover that and be in the ballpark is the part 022921 that has 14,500 primary?
Or do you treat the 6000 ohms in this case in parallel resistance due to the push-pull and it only needs 3000 ohms? If its 3k it seems that none that Doug carries are 3k, a few 4 and 5k, is 1k impedance mismatch a big deal? I would guess a 1k difference on that kind of winding would be what 3k to 4 ohm 750 winding ratio, then for 4k that ends up as 5.3 ohms... not sure if that's significant?
Or am I missing how you determine the OT?
I've done a lot of reading on this stuff but lost the links and keep looking and getting bupkis...
~Phil
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0.9 + 0.8 + 1.8 = 35mA
double-check, I think your decimal slipped
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2x 6L6
Heaters: 0.9A @ 6.3V
Main: 145mA
When you say "Main" what are you referring to?
The Mojo763 is made for a 4 x 6L6 amp such as a Twin Reverb. Way too big for your needs.
The Mojo762 is made for a 2 x 6L6 amp such as Bassman or Super Reverb. Perfect for your needs.
Doug's 018343 OT is perfect for your needs.
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The Mojo762 is made for a 2 x 6L6 amp such as Bassman or Super Reverb. Perfect for your needs.
Doug's 018343 OT is perfect for your needs.
And if you still want the Mojo762, Doug can get that for you from them at Mojo's quantity discount, even when you buy only 1. :icon_biggrin:
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Here's a little tidbit about schematics from the '50s... Many schematics used "M" to mean 1000 just like the Roman Numeral "M". IOW, a resistor value labeled as 100M is not 100 million. It is 100 thousand. When referring to million they would use "Meg". IOW, 1 Meg is 1 million ohms.
Sometime later it became more standardized, with K = kilo = 1,000 and M = mega = 1,000,000.
So, you may want to fix that on your schematic. While you're in edit mode change the value of R1 to 2M (that's million). And add a 100K plate resistor for V2 and V3.
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If of interest, here a pair of layout for the version that uses the tubes with topcap
(http://i468.photobucket.com/albums/rr48/ghramsey/EH185final.jpg)
(http://i468.photobucket.com/albums/rr48/ghramsey/eh185wo-PT.jpg)
the souce is AX84, this thread (I think the layout are to be revised)
http://ax84.com/bbs/dm.php?thread=457349 (http://ax84.com/bbs/dm.php?thread=457349)
Franco
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0.9 + 0.8 + 1.8 = 35mA
double-check, I think your decimal slipped
Lol it didn't slip it fell right out, thanks 3.5 ;)
~Phil
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2x 6L6
Heaters: 0.9A @ 6.3V
Main: 145mA
When you say "Main" what are you referring to?
The Mojo763 is made for a 4 x 6L6 amp such as a Twin Reverb. Way too big for your needs.
The Mojo762 is made for a 2 x 6L6 amp such as Bassman or Super Reverb. Perfect for your needs.
Doug's 018343 OT is perfect for your needs.
I meant the combination of the anode current and screen current max values per the datasheet. (that's where I got 145, anode is 130 and screen is 14.) Is there a correct term for that or are they called out separately and then added after?
The reason I thought the mojo 753 was needed is because the calculations of current are 300mA and the 752 only lists 250mA. Is it okay to be over current on the transformer design?
~Phil
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Okay, here's the modified version per your comments sluckey. I figured that M seemed really wrong, but I just reproduced it as it came :) That's a great thing to know about the M vs Meg in those old ones.
I also had another question I forgot to ask. I don't think getting a field coil speaker is feasible or worth it nowadays, so I replaced it with a 5H choke, that seemed to me to be common for the 6L6 Push Pull amps, but is there something that can help me decide on a valid choke there.
Per kagliostro's images, it makes me wonder if I should do the newer schematic with the bass and using the top connected tubes. How hard are those to do nowadays? I know the NOS tubes are out there in pretty decent supply, I'm more curious as to how I'll find the tops and how you wire those normally.
Or should I just stick to the older schematic I'm looking at now?
I'm up for the challenge either way, I just don't want to build something that is going to be me manufacturing the parts due to not having them around.
Thanks for all the help.
Edit: remove old schematic. Latest and greatest is in first post.
~Phil
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Unfortunately this kind of ceramic topcap is easy to be find
(http://i.imgur.com/KFj7Mnj.jpg)
but this is good (very good) for topcap plates connections
on the 6J7 tubes on the top there is a grid connection and the cap we really need is this metal topcap shield
(http://www.rfcafe.com/references/radio-craft/images3/metal-tubes-inside-story-radio-craft-october-1935-8.jpg)
(http://i.imgur.com/2KsmewJ.jpg)
that is very difficult to be found (even if not impossible to build, perhaps using one ceramic and a little metal sheet or pipe)
Franco
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I noted the common pictures of this amp show it with a treble and bass control, and saw two versions on the schematics page here:
http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_65Q7_PRE.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_65Q7_PRE.pdf)
which seems very much like a clone of the EH-150, but doesn't have a bass tone stack element. Then there is:
http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_6J7_PRE.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_6J7_PRE.pdf)
which does have it, and seems a bit more complex all in all. I was considering basing it off the second, but found that the tubes (6J7) require the anode connector, and don't know how feasible that is to build nowadays... So I kept my schematic based on the earlier one. ...
They're actually pretty-different amps in terms of preamp gain.
The 6J7 you're concerned about has a later equivalent: 6SJ7. The "S" means "single-ended" but not in the way you're used to seeing. It means essentially "a 6J7 but with the top cap moved to a base pin". In other words, all connections come out of a "single end" of the tube. So you could just use the 6SJ7 in the schematic for 6J7, and wire as typical for an octal socket.
Catch is, the 6J7/6SJ7 is a pentode, but each are wired as triodes in the schematic we have. And the amplification factor of a 6J7/6SJ7 is 20, on par with a 12AU7 section.
The 6SQ7 is like half a 12AX7 with a couple of diodes (not used here). Circuit values are kept the same in this schematic, but each triode will have an amplification factor of 100 instead of the 20 for the 6J7's.
I would assume one will sound gainier than the other on the mic channel, less so on the instrument channel (because the lower gain 6J7 triode version cascades 2 stages instead of using 1 before the phase inverter, as in the 6SQ7 version).
It would be nice to know which version your icon used...
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just as a point of interest: 6J7 and 6SJ7 are different enough to have their own R-C charts. they are similar but not same.
6J7 uses chart 14, 6SJ7 uses chart 20. 6J7 has about 3/4 gm of the 6SJ7.
see pages 255 and 258 in the link below:
http://www.americanradiohistory.com/Archive-Catalogs/RCA/RCA-Recieving-Tubes-1950.pdf (http://www.americanradiohistory.com/Archive-Catalogs/RCA/RCA-Recieving-Tubes-1950.pdf)
from experience in experimenting with both tubes, i prefer the 6J7: to me it's smoother sounding overall.
--pete
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Yeah, that's the tricky part, I've only seen photos from the front, so I'm not sure which HBP. I think the only days I have is that he uses both an eh-150 from 1939, and an eh-185 from 1938, not sure when they started the second revision of the eh-185.
kagliostro, are you saying the metal call is required to do something akin to shielding it? Maybe I can build the 6j7 version with the 6sj7 like HBP mentioned?
Thanks all!
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just as a point of interest: 6J7 and 6SJ7 are different enough to have their own R-C charts. they are similar but not same.
6J7 uses chart 14, 6SJ7 uses chart 20. 6J7 has about 3/4 gm of the 6SJ7.
see pages 255 and 258 in the link below:
http://www.americanradiohistory.com/Archive-Catalogs/RCA/RCA-Recieving-Tubes-1950.pdf (http://www.americanradiohistory.com/Archive-Catalogs/RCA/RCA-Recieving-Tubes-1950.pdf)
from experience in experimenting with both tubes, i prefer the 6J7: to me it's smoother sounding overall.
--pete
ahh dang, then if I do that, I'd likely want to stick with the 6J7. So if I go that route instead, I'm guessing that I need to find some way of manufacturing those top caps. Are they for shielding? It looks like there is a metal dome with some kind of inner rubberized or plastic section to not short out the actual post when over it? Any ideas? If not, I'll stick to the original eh-150-like version
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http://www.ebay.com/sch/i.html?_nkw=vacuum+tube+plate+caps (http://www.ebay.com/sch/i.html?_nkw=vacuum+tube+plate+caps)
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http://www.ebay.com/sch/i.html?_nkw=vacuum+tube+plate+caps (http://www.ebay.com/sch/i.html?_nkw=vacuum+tube+plate+caps)
Cool, thanks. But don't I also need the metal top part that was shown by kagliostro? Or do those ceramic tops work okay? Sorry I'm confused because if I understood what he mentioned, you need a different kind for the 6J7.
Also, what kind of wire do you use? Just standard wire or is it some kind of shielded cable?
Anyone have any input on the question about the choke?
:)
Thanks.
~Phil
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My opinion is that is preferable to use those small metal shields for the topcap of tubes where the topcap is a grid connection
may be is possible to do without, but is something to be tried same for the grid connection a shielded cable is preferable but ....
To build by themselves those small topcaps shields is surely possible, what is to be discovered is the way to do it with the minimum effort
may be the use of a ceramic or plastic topcap in junction with a piece of aluminium pipe
As suggested, also a version with 6SJ7 tubes can be build, I think it will be in the middle between the 6SQ7 and the 6J7 versions
Franco
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Thanks.
So I think I have just 2 remaining questions on this build that may have been missed.
1. For the field coil speaker, is my 5H choke an adequate replacement? It seems most chokes are 30 to 100 ohms only, but I don't know that a 750 ohm resistor there really matters much, as much as having something for the filtering of the power. Is there some document that helps understand choke selection? I did a lot of digging before I posted here and didn't find much.
2. I asked a question a few back also about the PT. The one I mentioned had up to I think 350mA or 400mA and the circuit design seems to require a little over 300mA but I was told I could use the one that ends in 762 from mojotone. That one only provides 250mA Is that adequate? If I recall from the data, the 6SQ7's tend to use a decent amount of current, thus pushing it over the 250mA that another typical 12AX7 type may not need. I don't know enough about power transformer selection, but I thought you needed to calculate what all the tubes will draw and ensure you have enough?
Edit: I noted that mojotone 762 model supports a 5v tap for the tube rect. so removing that part of the above statement.
~Phil
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I'm just realizing that I'm def confused and potentially wrong on the current requirements for the 6L6's because I have listed 145 mA for those, that's what's putting it near the 300mA level. So if the 250mA PT's are good for a 6L6 maybe I'm just off on that...
DOH, I just reread the datasheet, the value is 145 (or on the RCA one I've got) 152 max mA. FOR BOTH TUBES! It looks like that resolves my question, the 762 tranny would work just fine.
So if anyone has any pointers on the choke? Do I add that one I have and add additional resistance to equal total of 750 ohms (Say the choke I have is a 5o ohm choke, do I get a roughly 700 ohm resistor and place it in series there between caps?)
~Phil
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So if anyone has any pointers on the choke? Do I add that one I have and add additional resistance to equal total of 750 ohms (Say the choke I have is a 5o ohm choke, do I get a roughly 700 ohm resistor and place it in series there between caps?)
only IF you were to use a power transformer with the same specifications as the gibson. on larger amps in those days usually they were around 750VCT to compensate for the field coil and vacuum rectifier losses. the 752 transformer will put the B+ at about 450V with a SSR or 5AR4 and about 425V with a 5U4GB depending on idle load.
do you have any idea what the B+ was on the original EH185? you really should try find out. i'm betting it was around 380-400V in which case the 752 supplied B+ voltage would be out of line with the original design. do you even care? since it is a modern spin of a vintage design.
the sag is part of the tone IF the amp had a field coil. the best way to emulate that would be with a PT with similar ratings of the original and the field coil replaced with a 750 ohm 25W resistor.
--pete
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DummyLoad,
Thanks,
I don't know the original B+ voltages, and I can't seem to find anything documenting it. There is a replacement transformer done by Mercury Magnetics, but I can't seem to see any datasheet/specs for it (maybe they want to keep the sauce secret?).
The rectifier in the schematic is a 5U4.
As for sag, doesn't the rectifier give a decent amount of sag too? I could definitely add a 25W 750 inline with the choke to provide that sag AND provide better power filtering?
Not sure about doing a modern spin per se, just trying to get as close to the tone as possible with the available hardware we have now.
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I don't know the original B+ voltages, and I can't seem to find anything documenting it. There is a replacement transformer done by Mercury Magnetics, but I can't seem to see any datasheet/specs for it (maybe they want to keep the sauce secret?). The rectifier in the schematic is a 5U4. As for sag, doesn't the rectifier give a decent amount of sag too? I could definitely add a 25W 750 inline with the choke to provide that sag AND provide better power filtering?
then the MM PT & OT would be the place to start. NOS 6L6G, 5U4G, 6J7, etc.. add the 750R power resistor to emulate the missing field coil.
strapped for cash? - ask MM for the specs and find close match off the shelf.
--pete
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yeah I wouldn't say I'm 'strapped for cash' but always under a budget on my pasttimes ;). Gotta get clearance with the better half. It may be a hard sell for trannies that cost 140$ and 212$ when I can get the fender style ones for 80 to 100 each. I'm just on the fence about 'exact' vs 'modern' like you said as well... I'll decide on it and go from there.
Thanks!
~Phil
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> current max values per the datasheet.
Tube-maker's datasheet is just a suggestion.
You have ALL the data you need to figure what *Gibson* was working it at.
You know there is 106V drop in a 750 Ohm resistor (field) which carries all the current.
You see there is 275V dropped across a 20K bleeder.
Very little math tells you there is 141mA total, 14mA to bleeder, must-be 127mA to amplifier.
(Of course if we keep going, we find 20V across 200r cathode resistor, 100mA P+S in 6L6es, so 27mA to small-tubes, but none of them can possibly suck even 2.75mA because of 100,000r resistors.... take with pinch of salt.)
Knowing the first-cap has 275V+106V= 381V, and about 141mA total, we turn to 5U4 data and find that point. Lies very near the "350Vrms" line.
So 350V AC each side (700V CT).
Why that bleeder? If the 5U4 comes-up before the 6L6, the first cap rises to 350V*1.414= 495V. Which is more than a 450V cap should take steady. It may be fine for a few-second start-up a few times a week, or maybe not. The added 25mA (at 495V) load may hold the peak start surge to 460V, 470V, but something short of 495V. It will also bleed-down to less-fatal voltage in several seconds even if the 6L6es are missing.
The real reason for the 20K may be to up the field coil current.
The power amp: Whether we take the 100mA implied at cathode resistor or the 122mA implied by power supply study, this amp is working at lower current than any of the 6L6 270V conditions I see. I wonder if it is 6K or 8K load, or if Gibson idles it cool and lets it strain at full power at 5K load. In any case it seems to run nearer 14W-18W than the 25W-30W that a self-bias pair of 6L6 can do with more voltage.
The sub-par power rating is roughly confirmed by the field coil demand. IME, field coil power is similar to undistorted sine audio power. (There's no law, but that seems to have been a commercially economical condition.) As FC power is 106V*141mA= 15 Watts, we would suspect audio output in the 15W zone.
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PRR has given us a detailed explanation (Thanks PRR)
on this page you can find a short explanation about the substitution of a field coil
with an inductor + an in series resistor as to build a fake Field Coil Box (useful for radio repairers)
http://plymouthcolony.net/starcity/radios/pages/unispkr.html (http://plymouthcolony.net/starcity/radios/pages/unispkr.html)
Franco
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Your revised schematic is still wrong. V2 and V3 plates should be tied together with a single 100K connected to the junction of R17 and C8. And the junction of R17 and C8 should NOT be connected to ground.
I'd just replace the field coil with a 750Ω or 1K resistor rated for 50W.
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Your revised schematic is still wrong. V2 and V3 plates should be tied together with a single 100K connected to the junction of R17 and C8. And the junction of R17 and C8 should NOT be connected to ground.
I'd just replace the field coil with a 750Ω or 1K resistor rated for 50W.
this should do it.
R7 is removed.
--pete
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6J7 pre version - redrawn with modern component values, eliminated the field coil, and a smattering more legible.
--pete
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Thanks to all! I'll give this a closer look after work. Excellent information that's going to take a few reads to digest :)
Phil
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You know PRR, sometimes I feel like slapping myself when I realize that one of the two schematics I had/showed included voltages (I looked up the tubes first and once I saw that it had tubes with the grid connector on top, I was scared away :D. I never looked at it in close detail enough to see the voltages. (and the transformer ID is the same.)
All the math now adds up.
The other good news is that means the mojotone 762 is a match, its 354-0-354 and has the current to handle it all. Seems like I wouldn't need the 'original' transformer.
Is there any data to back up the idea that the transformers (either or both) have a major impact on tone of the amp? Or is that more related to the tubes, tube biasing and overall design of the entire circuit?
I think some of that question, sadly, is opinion, but I'm now thinking that to get the 'right' tone I likely wouldn't need the original exact style transformer anyway. (I'm trying to justify logically, in my head, why I would go for the extra couple hundred bucks for the MM ones?)
Thanks to all, this really fleshes out the last details.
DummyLoad, I'll take a look at the schematics once I get home, I don't have a schematic viewer on my work mac :)
~Phil
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If you can load on your work Mac the program (I'm not sure you have permission), here you find the program also in Mac version
http://dhost.info/jschem/ (http://dhost.info/jschem/)
Franco
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I can install it, but I tried running it and it apparently wants verison 1.6 of java which is 2 generations old and not considered secure (I'm a java developer :)) so for now I'll wait. I talked with the guy that created that app about getting it up to date and he set me up with the code in github but I've not had time to go back and get it working in java 1.8, but he may have, maybe I need to look that up again.
~Phil
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DummyLoad, looking at your 6J7 schematic, is there a reason that you put in KT88's instead of the 6L6's I've seen in the schematics that were there? Are they nearly identical to the characteristics of the 6L6? Or does it give it a different overall tonal characteristic?
~Phil
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DummyLoad, looking at your 6J7 schematic, is there a reason that you put in KT88's instead of the 6L6's I've seen in the schematics that were there? Are they nearly identical to the characteristics of the 6L6? Or does it give it a different overall tonal characteristic?
~Phil
it's a symbol. change the name. i use it for 6V6, 6L6, kt88, etc.. it was late, i was tired, etc..
rev1a files attached.
--pete
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Pete do you use PC Express to draw the schematics ?
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Someone has info about the correct inductor to be used for the bass control ?
On the layout I've previously posted there is a 20H 1850R inductor but I'm not sure if that is a correct value
Franco
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Pete do you use PC Express to draw the schematics ?
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Someone has info about the correct inductor to be used for the bass control ?
On the layout I've previously posted there is a 20H 1850R inductor but I'm not sure if that is a correct value
Franco
yes. expressSCH. for now. i am looking at design spark. kicad was just too quirky: maybe it's better now, but i doubt i'd warm up to it.
--pete
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Give a try to this
http://darwinne.github.io/FidoCadJ/index.html (http://darwinne.github.io/FidoCadJ/index.html)
(tube library on the program - easy to draw new components - I've some tubes to be added)
Franco
p.s.: If someone knows something about the Bass tone control inductor, welcome
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DummyLoad, looking at your 6J7 schematic, is there a reason that you put in KT88's instead of the 6L6's I've seen in the schematics that were there? Are they nearly identical to the characteristics of the 6L6? Or does it give it a different overall tonal characteristic?
~Phil
it's a symbol. change the name. i use it for 6V6, 6L6, kt88, etc.. it was late, i was tired, etc..
rev1a files attached.
--pete
Thanks, and no worries, I'm so new I just wanted to make sure I understood it.
I ask a lot of questions, and I appreciate the answers!
~Phil
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I think that tone control has major problems as valued.
The 500K pots hardly do anything until you get near the ends.
I empirically derived this variant.
As drawn here, the bottom of the pot is "more".
Oddly enough, the pots should be Linear.
Choke is 5H to 10H. I like ~~7H best. 2K DCR limits the boost, you want less. Going below 500 gains nothing. This IS a small SE OT primary. If you can find table-radio 5K 25mA, that may be a good start.
The 0.003u and 0.5u caps appear to shave the bottom octave of guitar. I have used full-bass values. If you use a small cheap tight speaker, or only play soprano, you may prefer the stock values.
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PRR, Thanks I'll look into those mods, I'm thinking I'm going for the newer, less complex 6SQ7 version first, and they I may just try for the other with the choke for the bass controls.
I've got a layout for the build now too. Let me know if I'm missing anything obvious here.
Edit: latest layout and schematic in first post
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Thanks for sharing the .vsd version of the layout :thumbsup:
Franco
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The ground connection for your volume control is on the wrong lug. As drawn, the pot will operate backwards.
Just a personal choice... I like to select all turrets, then right click and chose 'bring to front'. This keeps the cross hairs visible. I print and tape the board layout to the actual board for a drill guide and the cross hairs serve as an accurate location for center punching prior to drilling.
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Ahh yup, fixed and I did the turret trick. It looks a lot nicer now too, thanks!
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For those curious, I'm starting to buy the parts to put this one together.
I'm going to have to take it slow, I try to keep my builds under a budget, and the parts are going to cost enough to eat up about 3 months of budget :) I got the board, and most of the components this month (caps, resistors, tube sockets), I got the chassis last month after my Vox parts (Glad that one's working lol).
Next month will be a transformer and some of the other sundries, and in May I'll finish off with the other transformer and tubes (they'll all be NOS so I don't want to buy them, find out they don't work and not be able to return them).
As with the vox I'm going to do a video series on the build. The videos were fun, hopefully for others as well heh.
~Phil
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So I just was reviewing my layout and noted I'd missed the second volume pot for the guitar input heh. Added.
Edit: latest schematic and layout are in the first post.
~Phil
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Not sure how I missed this thread. I am very interested in how this turns out for you. I am amazed how something this old can have such a bite. I am a big fan of the player as well-hence my user. Maybe down the road you can try the Checkmate 25. Ha.
I'll be watching. :thumbsup:
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Thanks! I also dig Johnny Marr a ton too :)
I've just updated the my layout, I got the first wave of the parts, and found I had put all of the caps as 0.01 when only the first was 0.01uF the other three are 0.1 uF. Here's the updated version. I don't know if I've tweaked the schematic much, but I'll attach that too as my 'latest'.
Edit: latest schematic and layout are in the first post.
~Phil
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Doing more review, (as I don't want to be caught like I did last time and miss something on the translation) and I found another error on the layout. Attaching latest layout and schematic.
Edit: latest schematic and layout are in the first post.
~Phil
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I was wondering when you were gonna fix that. :icon_biggrin:
I had meant to mention that when I first saw it but got busy doing springtime stuff. So, I'll mention a couple other things I saw on your layout. You have 3 20 watt power resistors on the board, but you used a symbol that's sized for a 5 watt resistor. I don't know of any 20W that will fit in that space. But even if they would fit, I would not put a 20W resistor on the board. Those resistors need plenty of air space for cooling.
Those input jacks with the 100K resistors are wired wrong on your layout. As you have them either unused jack will short out the other jack. Compare your schematic to your layout. You'll see what I mean.
Visio Pro does a very nice looking schematic, much better than expressSCH. Why don't you give it a try. The shapes are under "Electrical Engineering".
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Oh thanks! I'll check that out. Yeah I've realized that, now that I have the actual resistors in hand, they'll be going off the side (the two 1.5k have screw holes so I can mount them to the chassis for heat dissipation.) The other big 20k I'll mount to the B node capacitor and have it sitting a good half an inch above it for room to breathe. Oh sheesh, the jacks should be like my vox AC30 for the dual jack setup right? Sorry I'll fix that too. I'll upload an updated one later.
I'll have to play with the visio shapes for schematics too, thanks.
~Phil
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I think this is right for the jacks... I've moved the 20k 20W resistor to page 2 with the caps, and moved the 1.5k off the board near it for reference.
Does that look good?
Edit: latest schematic and layout are in the first post.
~Phil
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Input jacks still wrong.
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Oh yeah I see, d'oh! Here's try three? :)
Edit: latest schematic and layout are in the first post.
~Phil
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Strike three! You have the middle jack tip strapped directly to ground.
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GAH!!!! lol okay this is right now, (Or at least I hope, if not, I need to drink more this st paddy's day and just give up for 24 hrs lol)
Edit: latest schematic and layout are in the first post.
~Phil
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Nope. The attached layout matches your schematic. Be aware that your schematic will result in only 1/2 of the instrument signal reaching the volume pot. You can snip the jumper between the ground lug and switch lug on both jacks to deliver all the instrument signal to the volume pot. The original schematic does not use a switching jack.
If you prefer to have a typical HI/LO jack input like a typical Fender, look at Doug's Library of Information.
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Trying again...
Thanks Steve, sorry I'm getting it now, I was trying to do the fender jacks, but even that wasn't quite right because I don't have the 1M to ground. I'd rather stick with the schematic, I want to see what it sounded like as best I can.
Edit: latest schematic and layout are in the first post.
~Phil
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I'd rather stick with the schematic, I want to see what it sounded like as best I can.
Then remove the jumper between the ground lug and switch lug on both jacks or use Switchcraft 11 jacks.
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There's a discussion about this amp over at TAG that you may find interesting or make some contacts for additional info...
http://ampgarage.com/forum/viewtopic.php?t=29052 (http://ampgarage.com/forum/viewtopic.php?t=29052)
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Thanks, gotcha, so the big issue is that I'm using a drawing of a 12A switchcraft type instead of 11. On the 11 there's no 's' switch post. I'll update that. (I'm at work now, I'll get it later). Sadly I can't load that link I get a security warning that the site has malware in chrome. I'd rather not risk it at work, I can load it from a Linux host later maybe?
~Phil
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There's a discussion about this amp over at TAG that you may find interesting or make some contacts for additional info...
http://ampgarage.com/forum/viewtopic.php?t=29052 (http://ampgarage.com/forum/viewtopic.php?t=29052)
that site's been blacklisted as a malware source.
--pete
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So I've removed the link from ground to switch. Here we go.
Edit: latest schematic and layout are in the first post.
~Phil
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The resistor that connects to the 6L6 cathodes should be 10 watts.
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Oh, I knew that and ordered one, but your'e right, the actual resistor on the diagram looks like a normal 1watt or less type. Fixed, thanks :)
Edit: latest schematic and layout are in the first post.
~Phil
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Just a suggestion... rather than posting a new layout in every reply and ending up with dozens of layouts scattered across multiple pages, just post a single layout in the first message and replace it with the most current revision. Then we will never get confused about which layout to use and the forum server will remain clean and lean.
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Oh excellent idea, I should have thought of that
Edit: and done :)
~Phil
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Just a council, if you use trasparency (near 20% will be OK) on the board, the route of the wires that are under the board is readable
and so is easier to read the layout
Franco
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Oh very cool, thanks!
~Phil
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There's a discussion about this amp over at TAG that you may find interesting or make some contacts for additional info...
http://ampgarage.com/forum/viewtopic.php?t=29052 (http://ampgarage.com/forum/viewtopic.php?t=29052)
that site's been blacklisted as a malware source.
--pete
Guys, I would never post a link to a malicious website. TAG has been on Google's warning list since June of 2015 when they were hacked. The site admin removed the malicious code but for unknown reasons has not coordinated with Google to be removed from the list. That's a shame. Traffic is way down on TAG because of the Google warning.
I'm not advocating ignore Google warnings. Just saying that TAG is safe, There is no danger on the TAG site.
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I don't disagree, or doubt you, but I was attempting to follow that link from work (and I am at work again) and they're going to not be happy if I even try. Once home, and if I can remember to look it up, I will.
Thanks for the information, it is sad that they can't be bothered to work with google to get the all clear.
~Phil
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I have some 3K CC resistors if you are interested.
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OO I may be, I'd love to keep this closer to stock, everything now is only 3.3k, I'll pm.
~Phil
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I have some 3K CC resistors if you are interested.
Got them today, Steve, thanks so much :bravo1:
Phil
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Okay,
Amp is all built and works great, but I've got the instrument jacks wired wrong obviously because only the 'mic' input works. I got switchcrafth 11 type jacks since you said that jumpering the two S to G is not needed. I thought I read that 11 would work in my case, but I may have done that wrong. Here's what goes on.
If I measure tip of either to the other end where it connects to the pot, it reads 100k just like it should, if I connect tip to tip I get 200k like that should. I get no continuity to ground on either tip connection. I plug a sine into either and I can see it going down the wire TO the resistor, but on the other side of the resistor, its completely gone.
If I input into say instrument 1, and then jumper from instrument 2 to mic, it works.
What am I doing wrong?
I'll go get a pic of it too, to see if that helps.
Edit: Pics: https://goo.gl/photos/uLDupe6vkZehhWyt6
~Phil
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NVM, found it, nothing to do with the jacks. It was the connection of the output from the volume pot to the input of the grid that somehow was touching the pot and grounding out. I desoldered it and the ground loop disappeared, resoldered it and it was fine, so it was just that.
I'll have a video up probably tomorrow night with the actual build details and final output of the amp. Sounds quite cool, except that the 'mic' input is way sensitive, I can't get it much over maybe 3 or 4 before it starts overdriving quite a bit, and getting it to like 7 makes the distortion sound horrible ;)
The instrument jacks get a bit distorted/od at about 7 or 8 but still really sound good.
~Phil
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Your layout shows a 1K resistor connected to pin 6 of V2 and V3. Your schematic shows 100K which is correct. What did you actually put on the board?
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It's 100k, thanks for catching that on the layout, I'll fix it.
Something else kinda weird. I get no output for a while, sometimes only, and if I start probing around with my oscilloscope probe, it suddenly outputs sound and starts working. Why would it be silent until i do something like that?
I think it turns on every time I probe the output of the capacitor that is going into the treble pot... Makes no sense. Other times when I turn it on, it just works. Last time it was dead silent for almost a minute until I grabbed the scope probe and touched it and then boom, sound.
~Phil
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Bad connection comes to mind.
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--pete
Guys, I would never post a link to a malicious website. TAG has been on Google's warning list since June of 2015 when they were hacked. The site admin removed the malicious code but for unknown reasons has not coordinated with Google to be removed from the list. That's a shame. Traffic is way down on TAG because of the Google warning.
I'm not advocating ignore Google warnings. Just saying that TAG is safe, There is no danger on the TAG site.
Absolutely true. I got that malware warning and then decided to investigate and can confirm what he says is true, The site is clean and no problems for me for quite a while now.
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Bad connection comes to mind.
It is possible, but where would it be? works perfectly from there on out.
I have one other thing I noted that may be related... the way the phase of the two inputs works seems like they're out of phase. If I input the guitar into one of the two instrument inputs and then jumper the other over to the mic input, the volumes are decreased some, until you get the mic level high enough. Also, though, when I connect the scope probe to one or the other side of the PI, they seem opposite of one another if I just do one or the other. Seems odd, though, because the mic has two tubes which should mean its back in phase by the time it outputs at the second tube right?
~Phil
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If you want to compare phase relationships of the signal as it passes through the amp you'll have to trigger the scope from the same source. Don't set the scope to trigger on the same signal you are looking at. IOW, if you are injecting a signal from a generator, then externally trigger your scope with that signal. Or, if you have a dual trace scope, connect the sig gen to channel two and trigger the scope on channel 2. Then use channel 1 to check the signal as it passes through the amp. You'll be able to see every phase inversion this way.
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Yeah I have a 4 channel scope, and I was using inputs 1 and 2. 1 is the input signal, and 2 I was putting at the PI on one side. If the volume was at max on the instrument input but 0 on the Mic it was in phase with the input, but if I raised the volume of the mic, it lowered the volume for a bit until the mic input seems to overpower it and then the phase is reversed.
Or so it seems to me anyway. If I put instrument at 0 and mic up a bit, the phase is def reversed. I'm not sure why, when the mic goes through two tubes but the input only goes through one.
Oh wait...
That makes sense after all. The mic input is in phase when it enters the PI but the instrument is opposite phase as it has only gone through one stage. so I guess it means I need to not jumper them. The mic input is probably just too hot for a guitar at this point, maybe I need to put a 68k or 33k inline on that as well? Then I'd want to use either the one or the other at that point.
~Phil
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That makes sense after all. The mic input is in phase when it enters the PI but the instrument is opposite phase as it has only gone through one stage. so I guess it means I need to not jumper them.
Ah, now I understand. You're correct. When you jumper the instrument input to the mic input the signals will be opposite phase when they are recombined at the common plates of V2 and V3 and will cancel. Since the mic path has more gain, that signal will tend to swamp the instrument signal.
Since the mic channel is too hot for guitar, you could use a split load on the plate of V1. IE, replace the 100K plate resistor with two series 47K resistors and connect the coupling cap to the junction of the two resistors rather than directly to the plate. The two resistors don't have to be the same value but their total resistance should be close to 100K.
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Sounds like a good idea. I'll give that a try.
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I did it, (I only had a 47 and 55k around but I figure its pretty close). That seems to have tamed it a bit, would it also be good to put a resistor inline between the input jack and the grid like on all others I've used? Attenuate it a tad?
Also, I still can't find the source of that odd behavior. I'm going to attach a image of the board and circle in red (with arrows pointing to them if they're not obvious :D ) showing the locations I can touch that seem to 'kickstart' the circuit. If I probe anywhere else, it stays silent. I've reflowed all the connections in the area.
One other oddity, I found. After the circuit is 'up' the areas near the blue output wires to the 6l6's have continuity to ground. But ONLY after that, before it happens there's nothing. IS that supposed to happen? (Doesn't seem right)
One final question...
I looked up the 6l6 datasheet and the grid and anode voltages are supposed to be different for a self(cathode) biased amp, but in this amp they're not. Both sit at 278 V.
I checked the cathode voltage at the 250 (245 measured) resistor and I get 25.5 V idle, which at 235 ohms gives me 104 mA which seems to be okay, but I'm not sure that's a perfect comparison since the datasheet is for a 400V Anode, 300V screen. (or 400V anode 250V screen). Or is that all fine? Doesn't seem like its having too much issues on the power amp sid.
It seems like something between my preamp section and the PI is 'stuck' and gets a kick in the pants if I touch any of the red circled areas shown above, and then starts outputting to the Power Amp side.
~Phil
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BTW, Here are some voltages. The first set is pre 'kickstart' when there is no output. The second set will be post.
(V1-V3 preamps, V4, PI, V5/V6 6l6's
Before:
V1: Pin 2, 12.8mV, Pin 3 1.09V, Pin 6 138V
V2: Pin 2, 120mV, Pin 3 .77v, Pin 6 94.3V
V3: Pin 2, 94.3 mV, Pin 3 .7V Pin 6 94.3V
V4: Pin 3 279V, Pin 6 107V, pin 8 1.96V,
V5: Pin 3 311V Pin 4 313V Pin 5 35mV Pin 8 25.5v
V6: Pin 3 311V Pin 4 314V Pin 5 53mV Pin 8 25.5v
Then after:
V1: pin 2 13.9 mV Pin 3 1.0v Pin 6 125V
V2: pin 2 25 mV Pin 3 .7v Pin 6 85V
V3: pin 2? Pin 3 .7v Pin 6 85V
V4: pin 3 148V Pin 4? Pin 6 118V pin 8 2.5v
V5: Pin 3 290V Pin 4 293V Pin 5 50mV Pin 8 30V
V6: Pin 3 287V Pin 4 292V Pin 5? Pin 8 30V
the ? are all low mV ranges that kept coming and going so I didn't feel confident calling it 300mV no 10 no 0 no 30 no .... (if you get the idea, it was almost like a cap discharging and then charging).
The B+ reads:
A 408V
B 313V
C 271V
D 164V
Now maybe this is just somethig I'm not getting, but why is the second anode of the PI only at 107 V on the exact same rail as the other which is then at 279V? They almost balance out when it's 'running' (148 Vs 118)
Also I noted as I put my Voltage meter on the treble pot, input side, the voltage was negative about (Edit, this was actually wrong:-20V it's actually about -170 when I first hit it, and then it drops fast) but slowly went down to a point where it 'turned on' maybe -2 v or so? Does that mean something is pulling the voltage negative there that shouldn't be and is causing this imbalance, and until it gets nearer to ground potential, it won't output? It seems the problem is the PI or something leading into or out of it?
~Phil
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Figured it out!!!
Layout error :)
ON the schematic, the output of the treble ties into the output of the 750 pF cap, then THAT goes to the grid of the first side of the PI. I was skipping it entirely and going from output of treble straight into the grid, skipping the 750 pF completely. I've fixed it, and it's working! :)
~Phil
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Okay so after a bit of playing with it, I do have something odd going on. I think it is an oscillation issue. I have the instrument inputs at low volume and they sound great, when I get up to maybe 7 or so, it starts in with a high pitched squeal/whistle. The Mic channel seems fine at first too but sometimes ends up with an insane amount of hum buzz at times as well, but it also is intermittent. Is this a ground loop issue, or something else seriously wrong? Could it be interaction between the two channels after the mods I made to make the mic input a bit more calm?
~Phil
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At this point it could be one or more of several things, layout, bad connection, wiring error, tubes, etc. Show us some pics. Your backward chassis layout could be a major factor. I would pull V1 and V2 and concentrate on getting the instrument channel right first.
I doubt the V1 plate resistor mod is a factor if you did it logically and neatly.
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Ok thanks,
I'll try that and get pics. I think I did try that and I had clean tone completely at that point, but now I don't remember (I really need to write notes as I troubleshoot, it would make things a lot easier for me later heh). I can also try just the mic input and remove that V3 and see what taht gives as well.
~Phil
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Here are some pics.
I've had some serious issues troubleshooting because it got significantly worse. I started tapping around with the chopstick but the sound was so loud I could only handle it in short bursts. Then I found that when I touched the grid input from one of the tubes it went either way worse. I traced that back to the pot and touched the pot and it disappeared and it was working fine. I touched that center pole on the pot again and it would go nuts. I pried it a touch and it seems to have stopped completely. Does that mean the pot is bad, or do I just likely ahve a bad solder joint there? it seems okay for now. The mic input also has a decent amount of buzz/hum, maybe the ground on that stage is a bit weak? The other two inputs seem really clean if I just turn the mic input down to 0.
Here are the pics of the guts:
https://goo.gl/photos/REerjstcWZ88ySXq5
~Phil
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Sounds like a bad pot or a bad connection on the vol pot wiper or ground terminals. V3 grid relies on a good path to ground through the vol pot. Touch up all solder joints and spray clean the pot. Use electronics spray such as Caig Deoxit. Don't use WD-40.
I thought you were going to pull V1 and V2 until the instrument channel is fixed??? Doing so will prevent the mic channel from being a factor in the instrument channel problems. Once the instrument channel is working well, then plug in V1 and V2. Any problems that crop up should then be associated with the mic channel only. If you are satisfied that the instrument channel is working as expected, then ignore this paragraph.
You need to use a switching jack for the mic input and it needs to be wired IAW the original schematic. This will help keep the high gain mic input quiet when nothing is plugged in. The mic input jack on your *.sch schematic is drawn wrong.
Your pics are very good. But, we need at least one high rez pic that shows the ENTIRE completed chassis wiring. The pics you posted don't even have the rectifier tube socket connected.
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Thought I explained that, sorry. I did that and got the instrument nice and clean before I found this. Sorry I had swapped the left power tube location with the rectifier and the noise was worth. I'll get some now that I reconnected it all up.
How can you tell a switched input from a non switched input on a schematic?
Phil
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I'm still confused. Are you saying the instrument channel is fixed and the only problem you have is the mic channel?
A switch jack has a circuit symbol that is different from a non-switched jack. This pic shows Switchcraft 'type 12A' switched and 'type 11' non-switched jacks.
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Oh gotcha, I see that now, so I do need to get a switched one for the mic input. Okay. I'll have to swap that out.
Second thing, though, is that I did it again today where I have just the V3 in and I can get intermittent good performance out of it. If you look at the same link I've added photos from today and put them to the front. (With the system working). https://goo.gl/photos/REerjstcWZ88ySXq5 (https://goo.gl/photos/REerjstcWZ88ySXq5)
I'm also going to attach a link to an audio sample I took of it: https://www.dropbox.com/s/hc8ql1qwexplneh/guitarEdit.mp3?dl=0 (https://www.dropbox.com/s/hc8ql1qwexplneh/guitarEdit.mp3?dl=0) If you listen at the first, you can hear a bit of 'buzzing' in the sound, but if the volume is low (which I do for a bit after that), it's very clean. Then I turn it back up and it starts buzzing, snapping, popping etc, and there's a very high pitched almost inaudible squeal going on.
I also Still have this weird issue where the center post of the volume pot for the instrument input, no matter how many times I try to resolder it etc, seems to go berserk until I push it to just the right position, and then it goes mostly clean like this except for the high whistle/squeal at high volume. When it was freaking out, I get a huge hum and something akin to radio freq noise. If I move the anode wires and ground wires around it changes the pitch etc, which leads me to think this is oscillation caused b y the wiring. (The mic input seems to not have this issue, just a heavy grounding noise when I have the volume up, but maybe because its amplifiying it so much more than the instrument input. When I was playing with the scope, I got maybe 20V output from the instrument at max volume, but the mic gets probably 4x that after we dropped it down with the split anode load and I added a 100k resistor in series on the input.
Let me know what you think of the pics and of the sound.
~Phil
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Replace the vol pot. If you don't have a spare, steal the pot from the mic channel. I really couldn't tell much from the sound sample using my cheap computer speakers.
I do have a concern about your pics. Your speaker jack sits right above your mic preamp tube. That's the worst possible place for the speaker jack. Unscrew the jack from the chassis and just let the jack dangle off the end of the chassis, far away from anything. And get that coil of OT extra leads far away from the input jacks too.
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Will do, I'll report back when complete.
Phil
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Okay did all of the above and it seemed better, but to test I put the output jack back where it was and it didn't change the sound/tone at all.
I found there were definitely two culprits.
1. the pot is bad, it keeps going intermittently connected on the wiper. It also seemed to make the weird high pitched noise. When I replaced the pot with a spare one from the old Vox, it went way better but had a nasty hum, I kept messing with the ground a bit and got that to go too but I think if I probe the bus with a chop stick it still pops occasionally so somewhere else i need to fix a poor solder connection to ground. I know the pot was bad also because I had it out of the chassis and as I'd connect it to the wiper and either side it would go from reading fine to suddenly 0L (dead open, no conductivity) to like 3M or 4M etc, like only partially any connection was happening, and then if I'd put a little pressure on it, the resistance would come in until I released the pressure and then it would go back out again.
2. I realized the bulk of the noise may ahve also come from my grounded wire from pot to the grid of the tube. It was really doing weird things, so I just replaced it and bam, the rest of the problems went away.
Thanks for the help!
~Phil
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Btw here's the view of it from the outside now:
https://goo.gl/photos/FhvWnswzuqDzLsuBA
You can see the hole for the missing pot and the temp pot sticking out on the bottom, I'll need to get a new pot, and I think I'll get isolating washers for the inputs and replace the mic input with the switched one as mentioned and she'll be done! ;)
~Phil
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I've been following this thread with great interest for awhile now. I've been wanting to try and build a 185 since playing an original one a friend got lucky enough to buy pretty cheap on Craigslist. The thing is amazing.
Since then I've bought a couple old 16mm projector amps. They use very close to the same tube compliment so I'm hoping to be able to salvage a lot of what I need from these two including the transformer.
They both came with their field coil speakers. One's kinda shot but the other works fine and sounds pretty good when i play through it's amp.
i also picked up a reconed vintage Gibson 10 inch FC so hopefully with all that and the great info on this thread I might be able to make a go of it.
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Oh soooo cool! Definitely keep me posted. I found some interesting news. There's a guy on facebook that told me he's got a '39 EH-185 and it uses the 6SQ7's like mine, instead of the 6J7 like you've got there, but it would definitely be cool to see that build too. Keep me posted! I'll have some audio samples in the next video I think, I end up usually with an hour or more of footage on each build and try to break the videos down to 10 min segments so they don't get too boring :)
~Phil
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I'll def keep ya posted! And so you ARE the guy posting the 185 videos on YouTube??
I'm subscribed if so. Actually found those before even seeing this thread.
And I'm gonna open up my friends 185. I'll post some pics if it's interesting enough.
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Yup that's me, I've also done a video on my Vox AC100/2 build from scratch there as well. The forum members have saved me time and again when I get stuck lol :) I've finished it and have an audio demo coming soon. I also am starting an 5F6A bassman build I'm putting up there. Let me know if you think I need to add/remove things etc.
~Phil
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Well one more thing I learned today. The damn thing still had this odd squeal from time to time, and it seems variable. At one point today I noted that if I touched the preamp tube with my finger the noise disappeared... I then remembered that pin 1 on the 6SQ7's is connected to the metal chassis of the tube. I tried clipping an aligator clip on that and the chassis and boom, sound gone. I've since soldered a grounding wire between all three pin 1's of the 6SQ7's and then to ground and its completely noiseless. I thought I may have had one tube microphonic, but they somewhat ALL were until I did that ;)
~Phil
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I've got a first draft at a schematic in easysch for the gibson eh-185. I'm interested in the amp because one of my favorite guitarists, Joshua Homme used it when he played in Them Crooked Vultures.
I noted the common pictures of this amp show it with a treble and bass control, and saw two versions on the schematics page here:
http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_65Q7_PRE.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_65Q7_PRE.pdf)
which seems very much like a clone of the EH-150, but doesn't have a bass tone stack element. Then there is:
http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_6J7_PRE.pdf (http://el34world.com/charts/Schematics/files/gibson/Gibson_EH-185_6J7_PRE.pdf)
which does have it, and seems a bit more complex all in all. I was considering basing it off the second, but found that the tubes (6J7) require the anode connector, and don't know how feasible that is to build nowadays... So I kept my schematic based on the earlier one. (If I'm off base on that assumption, please do comment).
I also may end up building this amp as my next build (some day in the future), but am first just trying to sort it out logically. I have a few questions in general about the schematic, as its a bit 'empty' of some data:
1. There is no listed output transformer. Is there a pretty common output transformer for 2 6L6's? Or do you need to fit an OT based upon the entire preamp and power amp stages impedance?
2. There's a named PT, but it seems hard to find, the one I did was pretty pricy, 200$ or so here: http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_gibson.htm (http://www.mercurymagnetics.com/pages/catalog/manufacturers/MM_gibson.htm) which specifies it is for a EH-150 and there's a separate one for the EH-185. Could I just use a modern one that can push the right power to the 6L6's and the other tubes? 6SQ7/6N7? I can look that up on the data sheet, and get the total amp required for hte tubes, and see the levels there, that's what you do right? Then I look at normal voltages for these tubes and can look at the expected voltages? (since they're also not noted anywhere on the original schematic?)
3. Could I just add the same bass controls from the second one into the first and get the same effect? Or is the latter schematic above just the more modern version and will likely have a massively different tone than the second type EH-185?
At any rate, here's the .sch as well. Let me know what you see is off/wrong, etc.
Edit: With latest layout and schematic.
Edit: Adding the latest mods as well.
Hello,
I hope this helps with some of the confusion.
Gibson made the EH-185 from 1939-1942.
The first 60 built they actually still had the EH-150 stamped on the faceplate but they were 185's.
The early 185's had separate bass and treble controls and then shortly after they switched to the single tone control.
I have one of the early EH-185's and it is working great and sounds awesome. I also recently picked up a great playing condition Gibson ES-150 (1942) guitar which was the first successfully produced electric guitar and is what would have been sold with the guitar.
I tried to post a few pics but said the file was too large sorry but hope this helps a little.
Cheers,
Mark
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I've been following this thread with great interest for awhile now. I've been wanting to try and build a 185 since playing an original one a friend got lucky enough to buy pretty cheap on Craigslist. The thing is amazing.
Since then I've bought a couple old 16mm projector amps. They use very close to the same tube compliment so I'm hoping to be able to salvage a lot of what I need from these two including the transformer.
They both came with their field coil speakers. One's kinda shot but the other works fine and sounds pretty good when i play through it's amp.
i also picked up a reconed vintage Gibson 10 inch FC so hopefully with all that and the great info on this thread I might be able to make a go of it.
The EH-185 had a 12" field coil speaker made I believe by Jensen.
Cheers,
Mark
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Cool, I'd love to see them, you could upload them somewhere like imgur or google photos and link that page here?
~Phil
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Cool, I'd love to see them, you could upload them somewhere like imgur or google photos and link that page here?
~Phil
I hope this works
Cheers,
Mark
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Okay I uploaded the pics by texting them to myself and that downsized the files so a I could then upload them.
You can see where the faceplate still shows EH-150 because the first 60 they made they were just thinking it would be the new model 150 before they decided to call it the EH185.
Cheers,
Mark
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Outstanding, thanks! I'll bet it sounds incredible.
~Phil
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I just started looking at EH-185 schematics.
The very high resistances between the 6L6 plates and ground are not typical in more modern amps...I know these are quite old (late 30's?).
Any idea what they are for...stability technique prior to, or in addition to grid stoppers?
Thanks
Murray
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The very high resistances between the 6L6 plates and ground are not typical in more modern amps...I know these are quite old (late 30's?).
Hmmm. I have very high resistance between plates and ground in all my amps. Can you be more specific?
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Do you mean the 20K bleed resistor in the power supply? That's probably there to guarantee at least a minimum current through the field coil under all conditions
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Some schematics online show resistors from 6L6 plates to ground with very high values...like 500 M each, and a smaller value (250 M?) I'd have to download schematic again (will do) to clarify what/where & component designators. PS bleeder makes sense and would swamp the hundreds of megohms, but I'm looking at a schematic only - no measurements :0)
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I've never seen an EH185 schematic with any resistor connected from 6L6 plate to ground. I think you are mistaken about that. But if you have seen such a schematic please post it and we'll get in put into Hoffman's schematic library.
During the era when those schematics were drawn "M" was used to denote thousand, not million. Remember Roman numeral "M"? If something was supposed to be million, it would usually be denoted as MEG.
Around 1950 maybe a bit earlier, the symbol M was replaced with K (Kilo, 1000). There was an overlap time before K was adopted by everyone, so you would see both symbols, sometimes even on the same schematic. So, that 500M is really 500,000 or 500K.
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EH185 schematic
:laugh:
It's a Gibson, they put all manner of things they didn't mean on schematics :icon_biggrin:
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I was initially going to say someone handwrote the M's incorrectly and they have been written on every copy since (graffiti syndrome...too focused on the spray can nozzle or marker tip to notice one's errors).
I then interpreted the M to be a tolerance of +/- 20%, believable for that era. There might be annotation on the schematic clarifying all values are in k unless otherwise stated, but I'm on my phone & that's just a lousy way to check carefully.
I see M's all over the schematic so at least they were consistent. With all due respect, I don't like the M changing to k historical theory, unless that is just referring to Gibson when they replaced the woodworkers in the documentation dept. with draftsmen.
Changing ,000 to k I'd buy...
Sure, there were uuF's before pF, but Mm =k =1E6*1E-3=1k is as weird as writing in Roman numerals. Oh krap, M = 1000 in Roman numerals...I was joking, but this is turning into a joke that isn't funny (like at work).
I'll just call them k then, but that gives me little confidence in (scope TBD).
Thank you.
Today is Monday isn't it...
And I NOW see sluckey posted the Roman numerals explanation.
Browsing on a phone sux.
This must date back to burning poplar bark to make resistors.
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Uncle Doug went through a 1946 Gibson BR-1 with a similar power amp topology.. the high current/low voltage and it worked pretty well it appears. Also had the big ole resistor to chassis ground. schematic around the 19:50 mark.
https://www.youtube.com/watch?v=cPnPN8Z_BH8&list=UUuR4hQTXkG_KxozLxwPzEjQ&index=3&t=1462s (https://www.youtube.com/watch?v=cPnPN8Z_BH8&list=UUuR4hQTXkG_KxozLxwPzEjQ&index=3&t=1462s)
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With all due respect, I don't like the M changing to k historical theory, unless that is just referring to Gibson when they replaced the woodworkers in the documentation dept. with draftsmen
M is still used in many engineering arenas. MBTU is thousand BTU not million.
The MM is used widely in the oil and gas business; it stands for "a thousand thousands". The original symbol for thousand was M, based on the Latin word "mille". Often the lower case is used. So, for example, a million Btu is mmBtu.
https://www.quora.com/What-should-be-the-abbreviation-for-million-M-or-MM (https://www.quora.com/What-should-be-the-abbreviation-for-million-M-or-MM)
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Also had the big ole resistor to chassis ground. schematic around the 19:50 mark.
That's true. But the resistor is not connected from 6L6 plate to ground. BIG DIFFERENCE.
It was quite common back in the days of field coil speakers to see a big, high current bleeder resistor such as u. doug shows.
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You will find in the electronics world that M=1,000,000, m=.000001.
https://www.quora.com/What-should-be-the-abbreviation-for-million-M-or-MM
Whoever wrote that has never become familiar with electronics or computer jargon. G is a very common symbol for 1,000,000,000 (billion). It's used everyday by even non-tech people. Ie, my dad just bought me a 256Gbyte thumb drive. I'm stoked! Or my new Motorola cable modem, the MG7550 is a dual-band Wi-Fi router, and supports clients that operate on both the 2.4GHz and 5GHz bands.
So, Bill Gates may never use a "G" when he writes a check, but I bet he knows what "G" stands for. Most people have no clue and don't care. :icon_biggrin:
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m=.000001.
I missed something, I use m for milli, .001 u for micro .000001
and I think we were both using GHZ before the cool kids :icon_biggrin:
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And I have an MG. Its a 1960 MGA 1600. It even starts occasionally.
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m=.000001.
I missed something, I use m for milli, .001 u for micro .000001
and I think we were both using GHZ before the cool kids :icon_biggrin:
Oops! You got me! :embarrassed:
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I see M's all over the schematic so at least they were consistent. With all due respect, I don't like the M changing to k historical theory, unless that is just referring to Gibson when they replaced the woodworkers in the documentation dept. with draftsmen.
old US manufacturer generated schematic notations 30s-50s:
m = milli.
M = Mille (america shortened to Mill-Ohms).
mm = milli-milli Farad = micro Farad.
MM = Mille Mille = Meg Ohm.
fender used the same plate to plate NFB scheme in the AB165 that gibson used in the EH-185 ver.1
--pete
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I'll attach the Gibby with power tube plate-grid NFB resistors.
Pre-WWII plans "often" used "M" for 1,000. Post-war, "K" became common. The change took a couple of decades.
I remember when there was no "pico", only "micro-micro", strangely spelled "MM".
The 6N7 cathode resistor surly must be "1 K". Small plate resistors are 100K just like so many other amplifiers. (Anyway a cheap composition "100Meg" would be real rare and real unreliable.) Pots are 500k. B+ drop resistors 10K.
The 750K 6L6 plate-driver resistors tend to give NFB on the OT primary, which is nearly as good as taking from OT secondary, and avoids bring signal back from the jack; also uncertainty about OT polarity). They actually give very little NFB at nominal load; this is to damp speaker bass resonance where load impedance (and gain) could be very high. You could play with none or with 330K to taste.
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I'll attach the Gibby with power tube plate-grid NFB resistors.
Pre-WWII plans "often" used "M" for 1,000. Post-war, "K" became common. The change took a couple of decades.
I remember when there was no "pico", only "micro-micro", strangely spelled "MM".
The 6N7 cathode resistor surly must be "1 K". Small plate resistors are 100K just like so many other amplifiers. (Anyway a cheap composition "100Meg" would be real rare and real unreliable.) Pots are 500k. B+ drop resistors 10K.
The 750K 6L6 plate-driver resistors
i believe that those plate to grid are 250K - not 750K
--pete
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those plate to grid are 250K - not 750K
That's what I see too. That schematic is in Doug's library. Still no resistors from plate to ground.
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Is someone able to school me/us on that tone stack in the 6J7 version? I understand the treble pot, but the bass pot filters to ground through an unlabeled inductor? Was that somehow common back in the day?
Thanks
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I like 250k better now.
If you understand how the treble pot cap shorts highs to ground, then it is clear how the bass pot choke (the opposite kind of reactance) shorts lows to ground. It is labeled GQ13, which is probably a factory code.
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Thanks PRR. I had not noted the GQ13 as the label for the inductor. Did a search and it does seem to be a factory code. Later I may look for a spec, but most of what I see are folks calculating a likely value of 1.5 - 10 H.
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For full deep bass working it wants to be 5H-10H.
Also try increasing the 10K in the cathode to 22k or 47k. You get less gain, but more symmetric curves.
Choke location may be critical because it is a potent hum pick-up. (A gyrator wouldn't pick-up hum but was much too costly for the time, and even now is awkward because of high signal voltage.)
I think we see why this type plan was not popular.
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Yes - I read elsewhere that the choke could be a hummer - the wrong kind.
But it was more for my learnin' that I asked about the choke.
I do have sitting in my project pile a Stromberg Carlson Muzak amp. It is an all 6SJ7 preamp (5 of them - two are paired up for the PI) I have rebuilt several old Gibson, Valco, and Magna amps with one or two 6SJ7s in the preamp, but not 5! I was thinking of loosely following an old Dan Electro schematic, or an Electromuse, but this thread got me thinking. If I follow the Gibby route, I would use a more conventional tone circuit.
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I would like to see a schematic of that amp with 5 6SJ7's.
Do you know the model?
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Hi Brewdude,
It's a Muzak 908B made by Stromberg Carlson.
I pulled it from the shelf to make sure of the model # and it has 4 rather than 5 6SJ7 tubes. (faulty memory)
The only schematic I have is very aged - brown - glued to the base cover. I will try to take a photo of it later.
I will likely redraw the schematic in order to track circuit changes, and will share when I do.
Tube lineup is 4 6SJ7 - 2 6V6 - 5Y3.
It looks like it had an easy life - probably elevator music in a Macy's, eh?
There are a couple of Youtube vids of this model, and an expired Reverb listing for a 908A that I have seen.
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Thanks. I’ll have to check into that amp.