Mojotone JTM45 build issues-RR

[rrozell]

Well, interestingly enough, I replaced the primary fuse, checked the output section for shorts and wiring errors, powered up and the amp works fine! I played through it for about 30-40 minutes with the guitar straight into all inputs and also using my modified Ibanez Tube Driver and also through a Boss Super Overdrive. I can't crank it up too much where I live because I live in a high-rise condo building, but the amp sounds good. There's some hum if I crank up the Presence control all the way, but other than that it's clear and clean!...and loud...The plate voltage came down to 450 VDC on pin 3 of the EL34's, but it's still high. I cannot adjust the bias current down any lower than 71 mA because of this. Does anyone have any recommendations for lowering this voltage? I can get other rectifier tubes, correct? The AC voltage from the primary transformer is 717VAC, and I would prefer not to change the transformer. The screen grids (pin 4) are now 425VDC and Pin 5 is -35.4 VDC. I am a bit of a metal head and want the amp to break up naturally at the lowest volume possible without using a power soak or attenuator. My favorite guitar tones include early Van Halen (A.K.A "brown sound"), George Lynch of Dokken's sound on the "Back for the Attack" LP. Joe Satriani's "Surfin' with the Alien" crunch and anything Jimmy Page ever did. I knew when I bought this kit that I would probably have to modify it away from the stock circuitry. I almost bought a JCM800 kit, but went with this one instead. Any suggestions would be appreciated.
Thanks, Russ

[Chuck H]

Well... EVH just cranked up his Marshall. No way to get overdriven EL34's without overdriving them. Same with Dokken. Joe used the same Marshall and a Keeley mod BOSS DS-1 when I saw him live and his amp is set up for a little power tube overdrive as well. Jimmy also used a four input Marshall. So it seems you have the Marshall part right. The problem would be your high rise condo. I see an attenuator in your future. Keep in mind though that loud has much to do with how your ear percieves tone and how sustain and harmonics respond to your playing. Oh... Jimmy, EVH and Joe all used PAF humbuckers. Joe's were actually a slightly different DiMarzio pickup, but similar. I don't know what pichups Dokken used but I think they were stock Gibson humbuckers (so likely PAF style pickups). Don't know if he was using a stomp box but I'll guess he was. So, overdriven EL34's, Marshall type amp, a PAF in the bridge position and a couple of choice stomp boxes. My favorite boxes to use with a Marshall for these types of tones are the DS-1 (Keeley mod info available on line free) and the older (mid 80's) RAT box (re-issued, about $250 on Ebay). Speakers for all would be Celestion Greenbacks in a 4x12 cab.

[rrozell]

Chuck,
Thanks for your feedback. So those voltages are OK in this Marshall, or will it prematurely wear out the tubes and other components? The tubes they sent me were "matched" or rated for 50mA, but I can't set them that low. Other documentation I found online says to set the EL34 bias current to between 30-40mA. I have a pretty good setup for low volume here at the condo. Either the '57 Champ kit amp I built with a Keeley mod Boss DS-1 and a little delay sings at low volumes, or my Marshall Haze 15 with a Digitech GNX-3000 guitar workstation. The GNX has some pretty cool amp and cabinet modeling along with all kinds of effects (some purists may scoff at digital modeling, but it gets really close to the original sounds and works at any volume level). I agree that there's nothing quite like a loud, sizzling tube amp through a good set of Celestions or similar speaker setup. That's pretty much why I built this kit. I have several places I can take it to crank it up, but Mojotone just sent me the cabinet to mount it in today UPS ground. I placed my order at the end of February and I'm more than a little peeved with them, but I digress...
I read the now famous Guitar Player interview with EVH back in 1978 or 1979 where he said his Marshalls were modded by a gentleman by the name of Soldano and he used a Variac to crank up the AC voltage "...and then watch the tubes melt..." Something like that. Most authorities now say a Variac is not really needed or is just too dangerous to use. I also read an interview with Mr. Satriani published when "Surfin'..." was on the charts where he said he recorded the album with a Marshall and a Chandler tube driver. I went right out an bought a rack mount Chandler tube driver even though I was a poor college student at the time. I still have it, and used it for many years with a Carvin 100 watt amp to get a great sound at lower volume levels. Eric Johnson still uses a similar tube driver for his lead tone. Chandler's out of business now I think. My tube driver's sound has deteriorated over the years and I'm afraid to mess with it too much. I retubed it to no avail. I want to draw up a schematic for it before I do anything to it. I can't find a schematic online.
I'm a huge George Lynch fan. I've seen Lynch Mob twice in the last couple of years. He had several kit amps onstage with him the last time I saw him which looked like Marshall replicas. He had name plates on the amps. One said "Marsha" and the other "Morgan Freidman"...too funny... I have pictures of his stage setup if anyone's interested. I read interviews with him where he said he recorded Dokken albums with a borrowed purple, late 60's Marshall and an original Ibanez Tube Screamer. I bought a reissue of the Tube Screamer and it was way too mid-rangey for me. I did all the mods I could find and it's sort of OK now. The bottom line here is I have lots of overdrive units to test this kit with if I ever get the cabinet to mount the darn thing in!
Yes, PAF or PAF replicas seem to get the nod for most any rock player who is not a Strat player. I have a Jaskson Soloist with a Seymour Duncan PAF replica in the bridge. The sound is good, but I hate trying to keep that damn Floyd Rose in tune. Too many tools needed or I just don't have the patience. I finally bought a PRS Tremonti signature and I love it! It has whatever pickups Mark Tremonti had PRS make for him in it. The bridge is slightly hot like a PAF and the neck is a little mellower for a nice balance when strumming chords. Sorry for the long post...I LOVE this stuff if you couldn't tell!
RR

[rrozell]

Well I did a little research on my own and it appears that plate voltage in the 450-500 VDC range is not really a problem by itself for an old Marshall amp. The schematic with this kit I bought said to use a 47K resistor in series with the bias trimpot for an EL34 circuit and an 82K resistor for a KT66 circuit, so I used a 47k ohm resistor. I found a schematic for a Marshall JTM45 reissue amp and it uses a 22k trimpot in series with a 68k resistor for 5881 tubes. Interpolating from Ohm's Law, I felt I needed more resistance in the trimpot circuit. I installed 82K in place of 47K and it put me right where I needed to be. Once again the Mojotone diagrams are called into question. Silly me for following directions. Live and learn. I just played throguh the amp at low volumes. Plate voltage=477VDC and Bias current=25mA for about 12 Watts. It sounded pretty good. I'll adjust it for the tone I want from here, which is tube overdrive ASAP, so I guess I'll increase the current to push the wattage closer to 20 or something like that.
Cheers
RR

[Chuck H]

You got it. Bias is about balancing the voltage with the current. So, if the tube has a max current of 50mA then you can can extrapolate a minimum plate voltage that achieves desired output without exceeding the 50mA. Same with max voltage. You can figure the current (which at 470Vp will not even be close to 50mA) to idealize for the voltage. There are no absolutes, just ranges. Marshalls, even of the same model, varied some on voltage. Some old metal face amps have been reported well over 500Vp. Some plexi's as low as 430Vp. Later JMP master models were between 380Vp and 420Vp. For a four input plexi I think 470Vp is just fine. In fact my 1991 1959 re issue has 470Vp. Supposedly this is an early one with the better sounding transformers. I don't know about that, but it could also be the voltage influencing the tone that gives this model it's reputation.

On the bias circuit, it's more common to adjust the dropping resistor for desired range than adjusting the series resistor. But your not dealing with any extreme resistance and either changes the divider ratio so I'd say your good now. Typical would be to set the bias to 75% dissapation at idle and adjust by ear from there.

Let me know how it sounds when you finally get to crank it up.

[rrozell]

Chuck,
Even though I don't yet have the cabinet, I took it to my friend's basement where I'm storing my old Carvin amp. It sounded good even though it didn't break up that much at high volumes...at least to my taste. Your earlier point is well taken that I need an OD unit or something else to please my metal head ears. I took my Keeler mod DS-1, my modified Ibanez TS-808 Tube Screamer re-issue and one of my modified Boss SD-1 units. The SD-1 unit sounded the best with this amp, although I wasn't using the best speakers or guitar that I have. I had to pull the documentation to remember what I did to the SD-1. Replaced the caps with better quality polyethylene or poly styrene caps, stuck an RC4558 op amp in there, upped the gain and added circuitry to switch between silicon and germanium diode signal clipping. The SD-1 is a great little stomp box for the money ($40). The circuitry is very close to a Tube Screamer, it's much less money and the mods are pretty much the same. I'm really starting to think the Tube Screamer is a myth. I've never been able to get anything close to the sound of the artists mentioned earlier in this thread with any amp or guitar at any volume. I'll test with my Chandler tube driver tonight after I push the output tubes a little bit more.
Cheers, RR

[Chuck H]

Well, you should be able to get some pretty generous distortion with a plexi circuit cranked up. Not the modern uber crunch two bricks being cracked together tone but think EVH. I think he was actually ODing his input a little with his wireless gain cranked up but that's just a theory. Still, close to that. Even Angus only runs his JTM45 around 5 or 6. Malcom uses similar settings with a single coil and gets good crunch.

Getting the bias right will be important. After that if your not getting some prety big crunch there may be something else miswired.

[rrozell]

Cool. I finally got the cabinet, but I'm too tired towork on it. Tomorrow.

[rrozell]

Some inherent crunch would be cool. I didn't get a chance to work on it the last couple of days, but the cabinet did finally come. I want to have the whole thing done by the weekend. EHV never did say what Mr. Soldano did to his amps other than beefing up the parts to handle the Variac voltages. If you look at the Soldano amps now, they have numerous input stages and like 6 12AX7's instead of the three in the JTM45. Maybe my Tube Driver will push it over the edge into super saturated distortion.
Cheers,
RR

[rrozell]

I got the 1 ohm resistors permanently soldered on for the output tubes, and tried to go back to the tubes they sent me for this kit which were marked 54mA. They sent me a replacement tube with the cabinet for the defective one they first sent me and it was defective as well! It wouldn't heat up and glow at all, so I had to abandon that idea. I put a used set of Groove Tube EL34's back in and set them for 43.7 mA. I had to change the resistor in series with the trimpot again to get that much current. I used 62K. It seems like the 10K trimpot sent with this kit doesn't have enough variability. The Marshall schematic I found has a 20K trimpot and a 68K resistor. The plate voltage, pin 3 on EL34s, came down to 451VDC, pins 4 439VDC and pins 5 -40.4 VDC. A test drive at relatively low volumes had it sounding pretty good, particularly with my modified SD-1 and the Chandler Tube Driver. This build's finally shaping up and now it's a matter of personal taste and testing it at higher volumes. I'll keep you posted.
Thanks! RR

[Chuck H]

+++

[rrozell]

Chuck,
I got gutsy and pushed the output tubes even more. In retrospect probably not wise until I finished testing the amp at higher volumes. Got it to 52mA bias current. Plate voltage was 454 VDC. I played it at low volumes for quite a while and it sounded OK overdriven with previously mentioned overdrive boxes, but I was hearing some crackling noises. I mounted the jtm45 in the cabinet and took it to our warehouse at work today and cranked it up. Lots of crackling noises which seemed to get worse at higher volumes, but didn't totally go away at lower volumes. Same noises no matter which input I was plugged into. I'm using a 2X12 Celestion G12m-70 cabinet. The crackling noise almost sounds like a blown speaker, but the cabinet is fine cranked through my Marshall Haze amp and at 70 watts per speaker they are more than beefy enough for this amp.
I swapped out all of the tubes and the output is weak again, the crackling went away (as far as I could tell) and one of the EL34's was glowing red hot. Yikes! All of the metal inside the tubes was glowing red. I killed power immediately and pulled those EL34's out. They are a really old set of Sylvania tubes that came with my Carvin amp in 1979, but they were working when I pulled them out of the Carvin years ago. I had them in the JTM when the EL34's Mojo sent me didn't work. Might be what melted the rectifier socket, but I always thought bad tubes would just be weak or noisy and pull less current, not more.
Right now I'm pulling the amp back out of the cabinet, checking for cold solder joints, backing off the bias current, and trying a whole new set of tubes. What is the number rating that they mark the output tubes with mean? It looks like a current number like 27, 34 or 52 mA. Is that the current they were set to when they tested for matching purposes? I.E. a "matched set". Or is it a maximum or recommended current for biasing them? The J&J EL34s sent with this kit had "54" marked on them. The Groove Tubes I have been using have a sticker on them that says "56" or "S-6" or "5-6". It's hard to read. I bought another set of Mullard EL34's and they are marked "27".
Cheers,
RR

[Chuck H]

That 54mA indicated on the tubes is not a blanket figure for how much current to bias the tube at. Get that right out of your mind before you blow something up. There are a couple of things that figure may mean to the guy who tested the tube, but it is NOT a suggestion of ideal current. The reason is that the best current is entirely subject to voltage. The tubes dissapation in watts is the target, not current or voltage alone. Current and voltage must always be considered together, not exclusive of each other, ever.

Rught now you have THAT pair of tubes idling at 100% of max dissapation. That's waaayyy to high for that amp. That would be class A, Your amp is AB1. Other design considerations must be made before biasing an amp class A. Other tubes you plug into the amp will be drawing different current since different tubes behave differently. Perhaps dissapating over 100% just idling. You NEED to back the current down. About 40mA would be correct for your voltage.

That crackle you hear is probably due to the drive from the PI being much too high for the bias conditions you have now. The glowing tube/s are due to the over current frying the tubes.

[rrozell]

Oh yeah, before I mounted the amp in the cabinet and took it for volume/loudness testing, I recorded volategs on the 3 12AX7/ECC83 tubes.
All three have 6.8 VAC heater voltages between pin 9 and pins 4 and 5 which are jumpered together.

V1 (closest to inputs)
pins 3 and 8 (jumpered together)- 1.7 VDC
pin 6- 213 VDC

V2
Pin 1 (100k across p1 to p6)- 180 VDC
pin 3-1.13. VDC
pin 6- 327 VDC
pin7- 180 VDC
Pin8- 181 VDC

V3
Pin 1- 245 VDC
Pin 2- 26.4 VDC
Pin 3 (jumpered to pin 8)- 43.4 VDC
Pin 6- 245 VDC

RR

[rrozell]

Chuck,
I did back it down to about 35mA at 464VDC and it's behaving much better with the same set of tubes in it. If the guy testing the tubes they sent me was doing it at 54mA as they were marked, then maybe that's why 2 of the 3 they sent me were bad? Having them marked with a 54 on them definitely confused me. I thought that's why there was not much breakup because I had them set much lower than that initially. Live and learn. Now I know why everyone says using a Variac is too dangerous. That Sylvania tube looked like it was going to melt. The Groove Tubes look a little worse for wear as well.
Thanks, RR