JTM 45/100 build not as loud as original??????

[JTM100]

I built a clone of my real '66 JTM 45/100 using the basic Metro kit and original mustards. The clone sounds great!!! But, compared to my original, it is not as loud. I am running both amps on standard wall current with the same tubes. The clone, compared to the orginal, sounds like a really robust 50w in terms of volume. I have checked all my voltages on the voltage tables that George provides and they are all correct. I have swapped tubes.....no change. I have had 2 techs check it out and they couldnt find anything wrong. One tech said that the amp is putting out closer to 75-80w instead of 100w. Which sounds about right based on the volume difference. I originally used the Metro 45/100 PT and the M e r r en 1202-84 OT . I swapped that O/T out with the Metro 1202-84...which i actually think I like better tone wise...but the volume of the amp is still the same. My filter caps are all F&T 32 uf with a 16 uf on the board. B+ is right at 500v on the clone. Does anyone have any ideas why there is such a noticeable difference in volume between the 2? Could it be the PT?

[neikeel]

If all else the same - yes

Even an original -43 PT with 530- 560v is only an 80w amp

I found my 45/100 got louder as it bedded in and I like it for gigs because it only needs slight attenuation, i find it very versatile

Mine uses Mules and GECs, Georges trannies http://forum.metroamp.com/viewtopic.php ... ics#p59812

IIRC mine is 530v on plates (before George dropped the B+ to give modern KT66 an easier time)

[JTM100]

So this is normal then and perhaps my particular original is an unusually loud one?

[Ted B]

This doesn't sound right to me.

For a start, I recommend checking and comparing the working voltages throughout both amps, and note any significant disparities. If you're only seeing 500V B+ at the plates with the Metro PT, that's down about 10% from the originals, but shouldn't cause that much of a disparity so long as that figure remains relatively constant throughout the amp. It might boil down to a single duff component that no one has (yet) identified.

[neikeel]

A KT66 45/100 is not as loud as an EL34 45/100 which is not as loud as a 12 series or a '69 on spec amp, that is a given (I have examples of the above.
In fact I worked on a beautiful sounding '67 which was also relatively quiet for a 100watter (I thought something might be wrong with it but when I quizzed the owner it had always been so).

Some of it is simply perceived loudness due to the frequencies that are boosted in later amps (with a 12 series or early metal panel the big 5nF bright cap exaggerates this as the volume comes in with a bang around 2.5 on the but doues not get louder gjust more harmonics and general filth as you head to 10) etc, but 80w of clean output from a JTM45/100 is probably about right.

The later Metro PTs did have the B+ dropped deliberately (can't remember when).

Ted B may well be right, and yes you should go through the amp carefully but you say a number of techs (presumably competent with the right equipment etc), but do not expect huge wall peeling volume from a KT66 45/100 like you find on an early 70s metal panel.

Btw you should stick your original on a 'scope and see what it puts out, clean and maxed out - might be interesting

[JTM100]

Ok, I discovered the reason. The B+ of my original on wall current is 560v. The B + of my clone on wall current is 500v. If I run my original at 110v on a variac ( the way I run all my other vintage Marshall's) the B+ drops to 505v and the volume levels between the amps are now identical. It is that extra 60v of B+ on the P/T of the original that is causing the difference. The lower B+ does save on the modern KT66's but it does change the sound. I am debating as to whether or not to replace the P/T with one that will do 560v. I know tha MM and M e r r e n both make one. Any reccomendations as to which one would sound better if I were to go this route?

[Ted B]

If I run my original at 110v on a variac ( the way I run all my other vintage Marshall's) the B+ drops to 505v and the volume levels between the amps are now identical. It is that extra 60v of B+ on the P/T of the original that is causing the difference. The lower B+ does save on the modern KT66's but it does change the sound. ...

Well, there you have it.

Evidently, that 10% deficit (which occurs throughout the amp) is enough to make the difference after all. I've never used a clone PT with reduced voltage, so you know more about the actual sonic difference than do I. I wouldn't have guessed the difference to be so vivid, but ...

FWIW, I've owned a couple of original JTM100s and serviced several others. I'd run them on the 240V tap with a 1:2 step-up (a smart idea), and bias the Valve Art (Shuguang) KT66s around 17-18w at idle. I don't recall having any issues. YMMV.

[Roe]

the marstran 1204-43 pt is great, offering both 560v and 490v.

[JTM100]

Thanks...I will check into the Mars-Tran also. I actually have a MM M3-67P P/T that is not in use. They say that the B+ is pretty high up there on that tranny. I was just wondering if there was any real sonic difference between the MM and the other 2 on the market. If i decide to swap out the P/T, I would prefer to only have to do it once!!

Ted,

That is what thru me off too because the bulk of my experience with Marshall's has been with the real ones and not clones. I have owned a few JTM 45/100's, EL34 JTM100's, JMP's, etc....so I have always been comparing the real things to each other mainly. I knew about the JTM 45/100's not being as loud as the EL34 Marshall's in general but I didnt know that the B+ on my '66 was so high. I had always assumed that it was around 500v or so. I took it out of the headbox for visual reference when I built the clone but I didnt fire it up and take any readings. I am running the new Gold Lion KT66's and so far I really like them. I have a step up transformer. I may try running it on 240.

[Ted B]

I have a step up transformer. I may try running it on 240.

That's usually a safer bet with these old amps. Oftentimes the '110V' tap is inaccurate as it is, in addition to the fact that power in the U.S. is usually closer to 125V - not good.

Also, I've proven to several doubters on multiple occasions that some old amps (especially Vox) actually sound better when run on the 240V tap, even when the B+ voltages are roughly equal. I'm not sure quite why that is, but it is what it is. Go figure.

[Roe]

...I am running the new Gold Lion KT66's and so far I really like them. I have a step up transformer. I may try running it on 240.

are you running gold lions at 560v and shuguangs at 500v? it seems that the lions handle high screens voltages better than shuguangs. metro, germino, and TAD all lovered the voltages on their 45/100s in order to prolong tube life

[neikeel]

I installed Gold Lions in a '66 45/100 recently, plate voltage was over was over 540v and sounded pretty good to me, and would happily use them again.

I always use 240/250vac setting but then I am the UK

Tone at 530v plates is fine with the GECs. I also inly use 470R on the screens on my amp.

Glad you confirmed why it is quieter

[JTM100]

...I am running the new Gold Lion KT66's and so far I really like them. I have a step up transformer. I may try running it on 240.

are you running gold lions at 560v and shuguangs at 500v? it seems that the lions handle high screens voltages better than shuguangs. metro, germino, and TAD all lovered the voltages on their 45/100s in order to prolong tube life

I used to run the Valve Art's but now I am running the Gold Lion's in both amps. I have apparently been running them for a while now at 560v in my original and so far so good. I think they are a little more rugged than the VA's and I like the sound better also.

[JTM100]

I have a step up transformer. I may try running it on 240.

That's usually a safer bet with these old amps. Oftentimes the '110V' tap is inaccurate as it is, in addition to the fact that power in the U.S. is usually closer to 125V - not good.

Also, I've proven to several doubters on multiple occasions that some old amps (especially Vox) actually sound better when run on the 240V tap, even when the B+ voltages are roughly equal. I'm not sure quite why that is, but it is what it is. Go figure.

With the exception of my JTM 45/100, that is precisley why I run my old Marshall's , VOXES, and HIWATT's on a variac set to read 110v on my meter. That 125v wall current has caused instabilities in some of my amps, especially a '69 SLP I have, in the past. The B+ would skyrocket and the amp would become unstable. When I got the B+ down to 475-500v the releiability issues went away. The 45/100 actually sounds better at the higher voltage and it never gave me any issues so I never really ran it on the variac too much. You really have me curious about 240v now. I have a '63 AC30. Would I just need to select the 220-240v tap on the selector and go into the step up transformer?

[Ted B]

You really have me curious about 240v now. I have a '63 AC30. Would I just need to select the 220-240v tap on the selector and go into the step up transformer?

Correct.

Listen for a subtle raggedy character in the sound that disappears when the amp is used on the 240 setting.