Mojotone JTM45 build issues-RR

[Chuck H]

I've had Groove Tubes run hot enough to brown the paint markings a bit and they performed and sounded fine in the same amp with the same bias for over two years. I think Groove Tubes has that special paint set up to brown too easily so they can sell more tubes. If they still work and sound fine, they are. As for the Sylvania, same thing basically. A tube can usually tolerate a red plate condition for a while without failure. So, if it's still working and sounding good, it is.

As a general rule always start out biased 60% to 75% of a tubes max diss spec when biasing for AB1. Example: Max diss=25W, 25WX.75=18.75W, Vp is 450 so, 450VX40mA=18W, 40mA per tube is a good place to start at 450Vp. Now... If you had, say, 350Vp on a tube with the same max diss spec then... 350VX54mA=18.9W, so at 350Vp that 54mA marking would be accurate. In truth the 54mA inducation on the tube is probably a number that has to do with matching parameters used by whomever matched the tubes and has nothing whatever to do with your application other than to indicate that all the tubes you bought are within a certain range under certain test conditions, and presumably therefore "matched".

Now, 464VpX35mA=16.2W So your biased now at 65% diss at idle. If it sounds good, it is good. Nothing to stop you from biasing hotter if it sounds good EXCEPT if the tubes misbehave (red plates, cutting out, etc.) Whatever bias gives you the tone you like and doesn't cause the tubes to misbehave is correct. Hotter bias will usually wear out tubes faster than a cooler bias. ALSO, when experimenting with tubes of different make, brand, design, age or rating ALWAYS adjust the bias specifically for the tubes in use. Every tube has a different ideal bias condition to achieve the ideal dissapation at idle. And always use an oven mit when changing hot tubes (OUCH). That's it.

P.S. It sounds like once properly adjusted your new build is working properly with minimal corrections. Nice work.

[rrozell]

Chuck,
Thanks for everything. You really know your stuff. Is there something else I can do to thank you?
How'd you know the paint on my Groove tubes went brown? I never said that. Hey, they're working and they're staying in there for now.
I've learned a lot and I now have a vintage Marshall...sort of...I like to tweak things to try and get the "perfect" tone, but I have to learn to give up on that.
There's no such thing. I've read too many interviews with rock stars who are getting paid by equipment manufacturers to sell their stuff. Satriani has the "Satchurator", Steve Vai has his own guitar, amp and fuzzbox, etc...I did buy a PRS signature guitar, but only after I played $6000 vintage reissue Les Pauls and numerous other guitars and found that I had a hard time...a really hard time putting the PRS down. I'll let you know how this kit tests out after I bang on it a while.
You're a gentleman and a scholar. I'm gonna go play.
RR

[Chuck H]

How'd you know the paint on my Groove tubes went brown? I never said that.

Educated guess.

I like to tweak things to try and get the "perfect" tone, but I have to learn to give up on that. There's no such thing.

Sure there is... Well, kind of. Good players find the best of what an amp has to offer as much as hearing the properties in an amp that can help bring out the best that they have to offer. Remember that tone starts in the hands. If you think that making your particular amp more/less bright or boosting gain or midrange will make the amp do something for your playing that you want then tweak away. The magic happens when you find your own sound. Almost never when you try to have someone elses. And if you listen close to players that seem to have great tone, listen to pick attack, mutes, accuracy of right /left hand sync, and figure in all the polishing that a studio has to offer, you start to realize that good tone is mostly technique and being able to take advantage of what a particular amp and guitar have to offer. I'll bet Eric Clapton could make fine music on a Peavy Bandit and people would hammer Ebay to buy them.

[diagrammatiks]

that's not how that works. with series parallel connections on resistors...

[rrozell]

Absolutely. Most of the great players would sound good on a Montgomery Ward 3 watt solid state amp and a plastic guitar. In fact, many of them started out on something similar. I just love to build and experiment with stuff. I find it strangely therapeutic. I'm back to 42mA and 456 VDC plate voltage (19.1 Watts). LOL. This time I'm done. Really. I promise. I popped the secondary fuse during that process and nearly gave myself a heart attack. Thought I really "blew something up". Actually I did. a cheapo DMM I have smoked and died in the process. I've attached some pics and a pic of my Tube Driver that I have to restoe at some point. The JTM sounds really, really good at the 19 Watts its at now.
Cheers,
RR

[Chuck H]

Purdy. Looks like nice work on the lead dress. It also looks like heavier than average lead wire. Much harder to work with. I'm finding myself working with thinner lead wire than vintage amps used just because modern jacks and switches have such small holes. If you have to share a contact point on a jack or switch it becomes art work intense trying to make a neat looking joint. The big wire is not at all needed for most of what we do but we keep using it because it's typical of the construction techniques we use.

[rrozell]

Thanks, man. I used the cloth covered wire that came with the kit except for some copper stranded green wire I used for some of the grounds. Nice, big terminals on most of these connections and I'm pretty handy with a soldering iron and solder sucker. I read that some people have seen the cloth catch fire, but something else bad had to be going on before that would happen. Now that I better understand biasing, I'm going back to recheck some of my previous projects. A '57 Champ kit, a late '90s Fender Blue Junior I totally redid with the kits online and my Carvin and Valco amps. I'll be careful! Any recommendations on EL84, 6V6 tubes and 6973's?
Cheers
RR

[Chuck H]

It's been a long time since I bought any 6V6's. I bought a pair of EH 6V6's about seven years ago and they sound good to me. I use plain ol Sovtek EL84's in my current amps and I think they sound good. Most are using the Russian 6N14 types for EL84's with good reports. JJ's have a good rep. I've never used 6973's. They look like a 7189 with a different pinout. If the amp in question doesn't have more than 400Vp you could probably rewire the sockets for EL84's and forget about hunting down expensive tubes.

[rrozell]

Chuck,
Sorry for the delay. I finally got to crank this amp in my friend's basement. It breaks up nicely at about 3/4 to 4/5 max volume, although not to the degree I would like. It's that late 60's early 70's classic rock tone, which is more than I could ask for. I think an attenuator or power soak may be in my future.
The co-owner of Mojotone found and read this thread and told me he wishes I would have asked them for help before going on here because my posts are injurious to his business (which was not my intent). I did ask them for assistance an got an email back with links to purchase assembly instructions from 3rd party individuals. I personally don't think it's right to sell amp "kits" without detailed assembly instructions unless that fact is up front and center when and where the purchase is made. I feel like it's important for other builders to know what they're getting into. Had I known what I know now, I might very well have made a different purchasing decision, but then maybe not. I would be interested in what others think constitutes an "amplifier kit". If I buy a model airplane "kit", I have every expectation that I will get a set of instructions even if not directions on how to do a professional job of building a model airplane replica.
RR

[Jazz P Bass]

Back in hallicon days of Heathkit, it was not expected to have a detailed "connect iten A to item B".
The targeted purchaser usually had an interest in electronics.
As well a a background.
This is proved out by the test procedures that Heathkit supplied.
I todays kit world you have an awful lot of assemblers that do not have a clue what they are doing.
They cannot identify components, do not "know" what the components "do".
To me this is a recipe for disaster (spelled DEATH).

[rrozell]

JPB,
I completely agree with your comments. These kits are not for some 16 year old or working musician trying to save a few bucks on an amplifier, particularly those without knowledge of basic electronics. My point is that this should be up front and center at the point of purchase. It took me 4 weeks to get my electronics and a full 8 weeks to get the cabinet, and it came with defective components, which I do not fault the supplier for (stuff happens). I do fault them for making me wait 4 weeks to get the replacement output tube, which was also defective (stuff happens again), so they could save a couple of bucks in shipping costs. To their credit, Mojotone changed their website after I complained about the length of time it took for them to ship me the kit. However, you still have to click on the "shipping latency" link to see the policy and it's still unclear how long, on average, it will take to get the kit after you order it.
They are giving me a credit for the bad parts they shipped, but I still have not seen it on my credit card statement, and they told me the parts tested good. I had the tubes tested and documented the results before I sent them back, so the owner is either calling me an idiot or a liar. Why would I make that stuff up? That's a lot of trouble to go through to try and scam him out of an $15-20 EL34, and I sent them back the bad components and paid for the shipping to do that.
If one goes to the mojotone site, Mojotone.com - The Best source for Vintage Amp Parts, Cabinets, Guitar Parts, Bass Parts, and Pickup Parts, and follows the links to the JTM45 head kit there is no where on any of those pages that states it does not come with detailed instructions. I know because I looked before I ordered it. Now go to this one:
http: //www.tubedepot.com/diy-jtm45.html.
Right above the "order" button, in BOLD text, it states that there is no detailed instructions and that kit is intended for advanced builders. I bought a '57 champ kit from Tube Depot a couple of years ago, it came with complete instructions, and an inexperienced builder could most definitely successfully build that kit without much outside help. That particular Tube Depot kit is THAT good.
To be fair, if saving money is the priority and support and time are lesser issues, then buy the Mojotone JTM45 kit! It's a really accurate representation of the original Marshall. I think they are really more tuned to be a distributor. Others buy from them, mark it up, add some better instructions or some other perks and sell it for a proft. That's what businesses do. I'm a businessman too, and it's ALWAYS good business to inform the customers of the facts ASAP, even if it might be difficult or costly to do in the short term.
Thanks for your feedback. Duly noted!
RR

[diagrammatiks]

the mojotone kits aren't cheap at all.

thousand bucks for a kit is actually really expensive.

[rrozell]

diag,
Agreed. They're not inexpensive, and that's why I presumed erroneously that I would get more than a box of parts and a poorly drawn drawing . Compared to a Marshall JTM45 reissue which sells for about $1750 plus tax and shipping, or a vintage 1960's era JTM45 which go for thousands of dollars, it's a deal. I originally paid $845, but after all of the problems I had, and buying parts and paying for expedited shipping to get the electronics part of the kit shipped to me quicker, I'm in this thing for well over $1200. Not a deal, but a learning experience. That's why I'm posting this stuff so others can get better information than I had.
Rhetorical question: "How much is building and owning a replica of the amp that was the foundation of classic rock and blues sounds worth to you?"
RR

[diagrammatiks]

well i know your question is rhetorical but if I really wanted a replica in every sense of the word...I'd be willing to do what it takes I suppose.

Mustard caps are absurdly expensive right now though.

[rrozell]

Here's a couple from Plexi-palace...1964 JTM45 for $16,500:
Vintage Marshall Amplifiers
I'll pay for the mustard caps instead, but that's just me!
RR