Tubes in the JMD-1

[guitarest]

OK I did a search on the JMD part of this site and there are a few posts about individuals thinking abut swapping out the original tubes in the JMD for better or different tubes. Bt the search did not pull up any person who actually did the swap. Has anyone gone through with the tube swap and what were the results?

[Beck-Ola]

I have spent a couple of weeks doing extensive testing of Output Tubes in the JMD. I have not tried any other P.I. tubes as of yet. The reason I did "extensive" testing was because I had three or four sets that sounded good in various ways and I needed to decide what was most important to me. Bearing in mind that these are my ears and expectations, and the individual specimens perhaps vary from others out there. I will be brief and make general impressions rather than detailed analysis. Enjoy. :rocker

I tried out five different sets along with the stock tubes: JJ E34L, JJ KT-77, Groove Tube GT-EL34 (relabeled RFT Siemens), NOS RFT Siemans, and SED Winged "C" EL34, in this order. Briefly, I found the stock Shuaguang EL34 to sound somewhat harsh, thin and metallic.

JJ E34L - Richer and fuller lows and low mids, especially apparent with a Les Paul in the middle position. Energetic high mids and balanced treble. All around full sounding. Ended up #2.

KT-77 - These I expected to win. In this amp, they sounded a lot closer to a 6L6 type. Very solid low end thump and more crisp at the very top too, without as much brashness in the mids as the E34L. Almost high fidelity, maybe even a slightly scooped tone compared to the E34L. Again, leaning toward a 6L6 tone compared to other EL34s. Funny, in my 2204 I run this type and get very rich, complex mids and a less hi-fi sound. Probably a great tube for heavy metal. Ended up my #3.

GT-EL34 (relabeled RFT Siemens) - Bought in 1987 and never used so they are essentially new. Slightly less output than the JJs but a superbly balanced and smooth sound. They have good lows, mids, and highs without exaggeration. Probably the most realistic reproduction of the sound made by the amp. Creamy and rich without harshness or overbearing high mids. Ended up #1 and the ones I am using.

Siemens EL34 - Identical to the GT-34 but labeled Siemens and came in a Siemens box. Mullard type. Similar character to the GT-EL34. Excellent balanced tone across all preamps and guitars. These will be stashed away for the future.

SED Winged "C" EL34 - I had the highest expectations for these, never having tried them before but having heard great things about them, regarding their warmth and fullness. I fully expected to be dazzled and that they might take the top spot. But for me they were disappointing. They reminded me of the stock Shuaguangs as they seemed harsh and aggressive in the wrong places, almost hurting my ears when using the Strat's bridge p/u and sounding severe and thin in the high gain settings. Given that I was prepared to like them it at least verified my ability to tell which ones I prefer and not be swayed by expectations. Maybe they would be somebody else's cup of tea. Still wondering about these. Are my ears that different or do they just not work well with my JMD?

Subjective Rankings:
1st Place - GT-EL34 (Siemans RFT EL34)
2nd Place - JJ E34L
3rd Place - JJ KT-77
4th Place (tie) - Shuaguang EL34B (stock) & SED Winged "C" EL34

Executive Summary: The most important finding is that the output tubes used make a definite overall difference in this amp. It was not easy to decide between the top two (GT-EL34 and E34L). They both had a lot of lusciousness to offer and I went back and forth several times (even recording them to see what the mike was hearing) and finally chose the GT-EL34 for it's evenness and slightly less aggressive high mids. Just slightly smoother and refined but with everything you want in an EL34. But so close that either of them would be fine. Not to shortchange the KT-77 either, which is also an excellent tone that some would probably prefer. But in this amp it seemed just a tiny bit scooped to me compared to the others. You cannot knock it's bold bottom end and high end sizzle though. Although I gave it third place, it's not by much, and owed to my personal preferences. For a more Bassman sound, the KT-77s would be the ticket.

My criteria was to sound equally good in both clean and distorted modes, with both bridge and neck p/u, and with several different guitars (e.g., Deluxe American Ash Strat, Les Paul Standard w/ Classic '57s, Les Paul Deluxe, Carvin TL-60). Anything that hurt my ears was points off. Anything that made me play for an extended amount of time was points for. Extra points awarded for reminding me of classic tones. Hopelessly subjective, I know. Maybe useless to anybody else but I know what I like and these are my findings. For a while I'll stick to the GT-EL34s.

[guitarest]

Damn thats some serious research; felt like I was reading a article in a music magazine. I know musical ears are different as are opinions although with that said I believe we all pretty much have the same ears and likes and dislikes about amps so I feel your ears might be real close to what I might hear. I remember the first time I swapped tubes in the 2203 I went with Groovetube matched Quads and red label 12ax7, I actually liked what the amp sounded like then.

With your research I was considering JJ E34L but now I am more sure of it. So with the newer or different tubes from JJ was a rebias required? I do not know allot about these things.

Thanks again for the very informative read.

[surfnorthwest]

Interesting post. First thanks for taking the time to post that information. I am surprised you did not care for the Winged C's but the JMD is a different amp then the one I buy those tubes for (Metro) I also recently tried some JJ KT77 tubes in a amp I had and was left unimpressed by them. Your test is what it is and glad you found something you really liked. I am not going to mess with mine for a few more months as it really is doing everything I ask of it stock.

When I go to re-tube and bias all my amps in January I will take a look at some different EL34s also.

[Beck-Ola]

With your research I was considering JJ E34L but now I am more sure of it. So with the newer or different tubes from JJ was a rebias required? I do not know allot about these things.

I would advise checking the bias when changing output tubes, especially in this case where they are of different manufacturers. The pair of E34Ls tended to idle a few mA warmer than the stock tubes. Nothing that would have caused harm but the life expectancy of the output tubes would have been less without adjustment.

In the JMD it is a very simple process. The most difficult part is removing the back panel and chassis (9 screws). Unlike most amps of yore, Marshall has kindly installed measurement points and mini-pots for adjustment. I set them to idle in this amp at around 38.5 mA each. (about 70% of maximum dissipation which is generally accepted).

Just FYI, the stock tubes as they came were idling at 63% and 68% respectively so they needed balancing anyway. It's not unusual for them to drift a little as they "burn in". In the interests of science I tried different settings from much warmer than usual to frigidly cold and there wasn't a great deal of difference. Nothing to obsess over. Don't want to be like this guy.

damnbias.gif (21.15 KiB) Viewed 5069 times

[bluesbrother]

Interesting post. First thanks for taking the time to post that information. I am surprised you did not care for the Winged C's but the JMD is a different amp then the one I buy those tubes for (Metro) I also recently tried some JJ KT77 tubes in a amp I had and was left unimpressed by them. Your test is what it is and glad you found something you really liked. I am not going to mess with mine for a few more months as it really is doing everything I ask of it stock.

When I go to re-tube and bias all my amps in January I will take a look at some different EL34s also.

Very cool post indeed as i never knew that KT77 tubes could be used in this amp.
That is something i really would like to hear .

[Hellhound]

Thank you very much Beck ola! I found the jmd to be a little metallic sounding as well. After I read your post I decided to swap the standard valves for JJ EL34's. WOW! What a difference. Everything you said +1 :party I biased to 38 each side (I have the 50 watt combo) and it is now so much more warmer. Les Paul, middle position, heaven. You were spot on all the way. Excellent post chap.
:rocker

[Makkon]

Mind if I ask , What meter probes do you guys use for biasing ?
Insulated gator clips ? Or is there something better ?

Thanks
In advance

[Beck-Ola]

Mind if I ask , What meter probes do you guys use for biasing ?
Insulated gator clips ? Or is there something better ?

I use insulated alligator clips that snap onto the regular probe to measure the bias. Just the regular probe for measuring plate voltage. I always clip the black lead to the chassis for ground to start.

[Makkon]

Thanks Beck ,appreciate your input.

[spikei]

i cant find any jjel34s tubes, are they the same as the jjel34 or the jjel34L confused.

[Beck-Ola]

JJ has both EL34 and E34L types and they are in the same family with slight differences. EL34s is just the plural of EL34.

[spikei]

Hi glad you cleared that up, thanks. EL34's it is then.

-- Sat Sep 04, 2010 10:17 am --

Just put a pair of jjel34 valves in my jdm1 50w combo and it has made a big difference, a lot warmer sounding and more rip with the pick, set the bias at 38 mv (was at 44mv with old tubes). good post and great advice.
just wanted to add that after removing the back cover and the chassis i can tell you the build quality off this amp is top notch, and now the sound is as well.
cheers.

[Makkon]

Installed JJ E34L's last weekend............ Agree with the other posts
Much warmer sounding , more alive to my ears anyways. :-)
Glad I did it.

Thanks for the tips and opinions.

[Mpesta]

All,

I really appreciate the work on this thread. Has Marshall released anything on how to bias the JMD? In other manufacturer's forums they typically don't recommend biasing using those measurement points, they usually want you to use a socket probe or measure across some component on the board.

Thanks,
Mike

-- Tue Mar 29, 2011 7:15 pm --

All,

I found my answer in another thread.

Sorry about that,
Mike