Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: moosetone on April 28, 2024, 08:08:05 pm
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Hi All,
I'm planning my next build; a Martin 112 / DeArmond R15 look-alike of sorts. In many ways its like a 5E3, yet different at the same time.
Mercury Magnetics has cloned the PT and OT for this amp, and they are on order.
The schematic is below, and also attached as a PDF. I've drawn up the layout in DIYLC as the amp was originally built - for those who don't know, they used a goop-encrusted "inverter pac," which I have sussed out from the schematic. Who knows, maybe I'll goop it to, just for kicks :icon_biggrin:
I'd appreciate anyones eyes on it for anything that looks amiss, and if can we confirm that the cap between eyelets 6 and 8 on the inverter pack is really 1u/200V as shown on the schematic, I'd appreciate it!
Dan
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I'd appreciate anyones eyes on it for anything that looks amiss, and if can we confirm that the cap between eyelets 6 and 8 on the inverter pack is really 1u/200V as shown on the schematic, I'd appreciate it!
Typically that cap would be the same value as the cap between pins 1 and 4 of the pack. IOW, should be .1µ. I've never seen a 1µF used in that position.
Your layout would be neater if you put those two boards together. There's no magic in keeping the pack separate.
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Thanks Sluckey.
Ultimately, I'll probably do as you suggest regarding mounting all the components on a single board. I like to suss things out as built first, as a mind exercise.
Dan
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I started down the path of building one myself but still haven't gotten there. I have a marked-up 5E3 schematic somewhere that notes the differences between the 5E3 & R15, which were not really all that many.
Guessing you have seen the Leon C vids of his clone? (trem-version)
&t=239s
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An infamous yet notorious amp. I could have bought one of these (or the 112T tremolo version?) in 1969 for a couple hundred bucks. The few listings for these on Reverb and elsewhere are asking kilobucks.
FWIW, I've been looking at the schematic for the 112T. I've been assuming that the 112 is the same, minus V6 and the other tremolo components. (Anyone know otherwise?) Look at the last post in https://www.heritageownersclub.com/forums/topic/23418-dearmond-amplifiers/ (https://www.heritageownersclub.com/forums/topic/23418-dearmond-amplifiers/), among other places.
I see .047mF cap's in the Inverter Pac. I also see 12AX7 in V1, not 12AY7.
Let me stop there - I see other differences, but they may be attributable to trem vs. non-trem. (The Inverter Pac on my schematics reads EP-9097-1.) And there are two other Pacs - Input and Coupler.
That said, both schematics show 350K and 650K pot's, which others have said is another part of the R15/112 notoriety. The 350K Volume pot's load V1 a bit "harder". I see from your layout that you plan to improvise the 650K Tone pot by paralleling a 1.8M resistor with a 1M pot. Do you have a source for 350K pots, or will you do something similar there?
Finally, how do we feel about having no grid-stopper resistors on the 6V6s?
Is there a head-scratching emoji?
Ed
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Thanks NewYorker (BTW I am also a New Yorker - way up on the St. Lawrence River).
I found 350K Reverse Audio pots at Amplified Parts (https://www.amplifiedparts.com/products/potentiometer-omeg-reverse-audio-20mm-panel-mount-350k). They can be made also with a 500K pot with 1M2 resistor in parallel.
Adding the grid stoppers is easy enough.
Dan
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Dan:
Actually, I'm in eastern PA. The "NewYorker" reference comes from a late-1800s vintage Martin guitar that I have, which has the "New York" stamp.
I've thought about building a "non-tremolo" version of the 112T/R15T amp, starting with a 5E3 kit and substituting transformers and other components. Now it appears that the 112/R15 differs in terms of tube complement, coupling cap's, and some other components. Also, the 112T/R15T appears to have some voicing differences between the normal and tremolo channels.
May I ask where and how you obtained the 112 schematic?
The last time I checked, it looked like the only 350K pot option was the 350K/70K-tap pot from the brown-era 6G16 Vibroverb. Then those went away. Now I see them again, along with the OMEG pot that you've found.
Ed
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May I ask where and how you obtained the 112 schematic?
It's included on the CD that accompanies the Aspen Pittman book.
Dan
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Ah!
By chance does the CD also include a schematic for the 112T/R15T? It would be interesting to look at something other than the low-rent scan that is out there on the web.
It would be an interesting compare-and-contrast exercise, at least for me.
Ed
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By chance does the CD also include a schematic for the 112T/R15T?
It does not.
It would be interesting to look at something other than the low-rent scan that is out there on the web.
Post the schematic you have. Maybe we can help improve the quality.
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Here is the version of the 112T schematic that I found somewhere online, in both jpg and PDF.
Dan
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That schematic looks very readable to me.
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Yeah, that's what I've been looking at for a few years. It's been easier since my cataract surgery.
Interesting that there are several differences between the 112T/R15T and 112/R15 schematics, aside from the tremolo circuitry.
Ya gotta love this stuff.
At least the 112 schematic spells the name of my hometown correctly.
Ed
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There's an old issue of Tone Quest Report Magazine where they bought either an old Martin 112 or DeArmond R15 that they did an article and review of. I don't remember if it had trem or not.
They brought it in to their tech to go through it, I forget his name, but he's very experienced. I know Ed Chambley went over to his shop a few times to talk with him. He was impressed with his knowledge.
They felt that a big part of why those amps sound so good and different than 5E3 Deluxes is the over sized iron set.
David Wilson, publisher/founder, RIP, loved the amp. IIRC, they sent the transformers over to Mercury Magnetic's and had them cloned. That's what you bought from them. I seem to recall they even did a small run of that amp with that iron, that's why they cloned them?
All their back issues are available at the Tone Quest web site.
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ToneQuest had Jeff Bakos build a run of "Clarksdale" amps, around 2006. Here is an old Reverb listing for one of them:
https://reverb.com/item/2581653-2006-tonequest-tq-clarksdale-dearmond-r-15-boutique-tube-guitar-amplifier (https://reverb.com/item/2581653-2006-tonequest-tq-clarksdale-dearmond-r-15-boutique-tube-guitar-amplifier)
This example would be a 112/R15 clone.
Another turned up in Walter Becker's estate.
Ed
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ToneQuest had Jeff Bakos build a run of "Clarksdale" amps, around 2006.
Yeah, that's it. :icon_biggrin:
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"There's an old issue of Tone Quest Report Magazine..."
Had that issue saved.
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Had that issue saved.
Any chance you have the issue with the article on the DeArmond amp that they bought?
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Any chance you have the issue with the article on the DeArmond amp that they bought?
I don't think i have that issue. But I'll look & post it if I do.
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Any chance you have the issue with the article on the DeArmond amp that they bought?
I don't think i have that issue. But I'll look & post it if I do.
Thank you. :icon_biggrin:
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Maybe Eagle-Eye Steve can make out the component values in the attached picture. To me it looks like the coupling cap's between the PI and 6V6s in this Clarksdale may be asymmetrical, like in the schematic.
Ed
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Here is a gut shot from Serial Number MM9204 (expired Reverb listing).
Apropos to my question about the 1uF versus 0.1uF cap in my first post, notice the size difference of the two caps (arrows in image) in the inverter pac.
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Well, there ya go!
Apparently it worked, intended or not.
Smarter people than I will have to comment on how or why it worked. Was it a mistake? Was it intentional, to compensate for some characteristic of the PI?
Or, why ask why?
Ed
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Here is another expired Reverb gut shot from SN MM9264 showing the same difference in cap size.
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Here is another piece I gleaned off the interwebs...
I reluctantly admit I forked out 25 bucks to get the Tone Quest Report, after hours of tracing down which issue it was in. Let's just say it was underwhelming. I'd post it, but its still covered by copyright and all that.
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Dan:
After 60+ years, parts of this trail have gone cold. I think we know more about 5E3s because there were way more of them.
There is another 112 near me. (Just confirmed that.) I might be able to convince the owners to let me get a gut shot, or for them to get a gut shot. Then again, I think we've established a pattern by now.
Try building yours with the asymmetry, and find out what it does. I'm pretty sure it won't catch fire.
Coincidentally, I found the same article in your latest post last night, while looking for history on Erie Resistor.
FWIW, I've seen these resistor / capacitor "packs" elsewhere. A few years ago I recapped a Hallicrafters S-120 multi-band radio. (Basically a multi-band version of the All-American-Four-Tube AM radio.) There were a few packs in there. I didn't study them closely, and I don't have the radio anymore. In any event, the concept found use elsewhere.
Ed
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OK, the 112 that I know about is basically functioning as a display/conversation piece, and I've been told that it doesn't work, or it does work. And, getting a gut shot would require taking the upper-rear panel off. Well, yeah. I'll let it go for now.
One more thought - build yours with 0.1mf cap's in both positions, then clip a 1.0mf cap across one of these (total of 1.1mf), and find out what happens.
This has been a learning experience for me. The 112T was more different from the 112 than I had thought.
I'm gonna have to try this myself.
Good luck.
Ed
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... That said, both schematics show 350K and 650K pot's, which others have said is another part of the R15/112 notoriety. The 350K Volume pot's load V1 a bit "harder". ...
The 350kΩ Volume pots load the preceding gain stage more, but all that does is cause them to deliver less amplification. IOW, the 5E3 Deluxe gets higher gain from the same tubes & same parts because of the "lighter AC loading" of 1MΩ Volume pots.
Apropos to my question about the 1uF versus 0.1uF cap in my first post, notice the size difference of the two caps (arrows in image) in the inverter pac.
Well, there ya go!
Apparently it worked, intended or not.
... Was it intentional, to compensate for some characteristic of the PI?
0.1µF results in a bass roll-off that is sub-sonic. 1µF just goes even lower, in a way that's not useful.
It means the cathode-output of the split-load inverter will be even slower to recover from grid current of the overdriven output tube, and the subsequent blocking distortion. So it's unlikely to be a helpful feature in a guitar amp.
... Was it a mistake? Was it intentional ...
These encapsulated packs were available from certain suppliers, and only had a limited range of parts & configurations. You got whatever the pack-manufacturer made, and didn't so much specify your own custom design (that would be uneconomical for the pack-manufacturer). Therefore, I strongly suspect an off-the-shelf pack was used, and dissimilar coupling caps were a part of its design for whatever reason.
If the output tubes are never overdriven, and if there's no negative feedback around the power section then the user would never know the difference between 1µF and 0.1µF.
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For those interested in the Martin/DeArmond amps, the current issue of Vintage Guitar (June 2024) has an article about the DeArmond R5T (single ended with Tremolo). There was an article about the Martin 112T in the May 2014 issue, but I don't have it.
Dan
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Just a quick update - I've been assembling the amp, and found some rather important mistakes in the layouts I originally posted. Hence, I have removed them, and will upload corrected layouts when all is working.
Dan