Tube Amp Power Trans voltage question....

[Tonerocker]

Greetings, Can anyone tell me the results of this, tone wise. How does
lowering or raising the amount of voltage a power trans feeds to the amp affect the character & tone of the tube amp. And swapping out a 50 watt trans with a 100 watt trans ?
I recently aquirred a 1972 Marshall JMP model #1987 50 watt head. I have found out the power trans had been replaced with a PT normally designed for a fender twin reverb or showman amp. This trans is sold from NSC [NEW SENSOR CORP.] They told me the high voltage secondary is 690v center tapped at 310 ma with a 50 volt bias tap . filament winding 6.3v at 6 amps. With this being done, how did this affect the original character of the amp. With my BIAS KING bias meter it has 455 volts to the plates biased at-40ma with EL34.
What I want to do now at this point is do want ever I need to do to make this Marshall amp have the best possible tone that will suite MY TASTE. I feel need to most likely change the PT but have some quetions as to what I
need to look for in a replacement trans. So I'm asking for help from anyone
that can guide me in the right direction. It seems that there is a big following to mod later JMP 50'S to JTM 45 plexi amps. I know they used
KT66 tubes as opposed to EL34. Why would it not be a simple tube change
and rebias? And why is there such an attraction to revert back to those
EARLIER AMPS TONE? How are they different?
MERCURY MAGNETICS seems to have a wide variety of PT and OPT available for just this model alone, so I'm trying to get a better understanding of all this in order to make some choices.
Thanks to all that can shed some light on this.
MAY TUBE AMPS NEVER DIE !
Tonerocker

[Tom Phillips]

I think that several lively discussions can be started because of your questions.

Meaning no disrespect to the advice you will get here, I would suggest that you check out the Marshall forum at http://vintageamps.com/plexiboard/vi...62d4a6008b0d73 and read-read-read the posts. (There are 138,000 posts there) I'm sure that you will find several that will answer some of your questions and help you narrow down the focus of the remainders. Plus you will get a lot of other Marshall information. As always you will need to learn to sort the BS from the good info.

IMHO a vintage Marshall "50W" PT is more like a "100W" Fender PT. Therefore, direct comparisons between the various PTs cannot be made using the descriptions that parts suppliers put in their catalogs. You will also find vast differences in opinion as to what plate voltage sounds best. I have heard people tout the merits of everything from 370V to 580V in a Marshall.

Regards,
Tom

[Tonerocker]

Thank you Tom for the web site forum , I will check it out. I do hope I spark
some interest from my questions asked. I am looking forward in gaining as much knowledge as I can about my amp and tube electronics as possible. I have learned over the last eight or nine years how valuable it is to learn about the how & why of the way tubes work in guitar amps is to a player of tube amps. The tube amp it self is an intrument on its own merit which the more the musician understands it , the more he/she is able make it do what they want it to do & take care of it properly, THANKS

[Tonerocker]

Thank you Tom for the web site forum , I will check it out. I do hope I spark
some interest from my questions asked. I am looking forward in gaining as much knowledge as I can about my amp and tube electronics as possible. I have learned over the last eight or nine years how valuable it is to learn about the how & why of the way tubes work in guitar amps is to a player of tube amps. The tube amp it self is an intrument on its own merit which the more the musician understands it , the more he/she is able make it do what they want it to do & take care of it properly, THANKS

[Satamax]

Tonerocker.

Lowering the voltage on a marshall is sometimes reffered as brown sould or EVH brown sound. I think the tubes get more of a workout at lower voltages with more amps drawn, than in the 450/500vdc zone.And they get more workout above this, but it's not that easy to get the voltages higher in the 7/800vdc range, and then you need separate screen supply etc.

There's a few sites which have answers to questions you might have.

http://www.geofex.com/tubeampfaq/taffram.htm

http://www.geofex.com/ampdbug/ampdebug.htm

http://www.aikenamps.com/TechInfo_2.htm

http://www.tone-lizard.com/

[Steve Conner]

Hi Tonerocker, all,

Glad you're interested in the "other" part of the instrument This is a fairly complicated subject with a lot of different opinions involved, but this is what I know:

Early JTM45 type Marshalls used EL34 tubes, and a very high B+ voltage, around 500-550V. The tube screens were fed from this same voltage, which is way over what the datasheet for an EL34 recommends. This led to the screens running too hot, and a lot of tube failures (most famously in Hendrix's Superleads, afaik)

However, these amps also have an awesome reputation for tone, and Marshall recently reissued them with the same high B+ voltage as the originals, to cash in on this. From the rumours I've heard, they may well have the same appetite for tubes as the originals, too.

Later Marshalls use a lower voltage, around 450V, IIRC, so the replacement PT fitted to your amp is roughly correct. This is easier on the tubes than an early plexi, but it may not sound so good: who knows. My own personal feeling is that the best sounding amp in the world is useless if it blows out halfway through a set.

If you want to try high voltage, you can use larger screen resistors and suchlike to increase tube life, or go to sturdier tubes like 6550s. Also, new production EL34s may be stronger than the originals.

steve

[bob p]

Steve, do you have any experience with the KT77? i know that they're optimized for UL operation, and IIRC they're rated at higher voltages, which might suggest that they have more robust screens than the garden variety EL34. i'm wondering if this translates into more robustness in a high voltage Marshall application.

[Steve Conner]

I've never used KT77s, but as far as I know, they are beam tetrodes. The screen current and dissipation is a lot lower in a beam tetrode. This should make them a lot more tolerant of high screen voltages than EL34s which are regular pentodes.

[Ray Ivers]

Bob,

Just FWIW... 5 out of the 6 original KT77's are still going strong in my '81 Marshall 2000 amp, running until very recently at @ 700Vplate/700Vscreen. I think this amp got very little use, though.

Ray

[Tonerocker]

Forgive my ignorance & I'm new to this forum. But what does IIRC stand
for? Do you Have any thought to any of my questions at the top this tread?
TONEROCKER

[Tom Phillips]

... what does IIRC stand
for?...

"If I Recall Correctly"

[bob p]

that's standard internet g33k-sp34k, IYKWIM. (if ya know what i mean). LOL.

but seriously, Tom is right that you're asking some very open-ended questions, and giving straight, satisfactory answers will be hard enough, let alone giving you answers that will produce tone to satisfy "YOUR TASTE." I would read those links that Tom and Satamax posted, there's a lot of good information there. if you have specific questions, they'd be a lot easier to answer.

regarding MM iron -- don't fall for the hype. resist the temptation to buy expensive boutique iron until you really know its what you want. imho, with the recent predatory price increases, MM is skinning people alive who are willing to pay for tone.

[Tonerocker]

Thank you Bob & all of you for your advice & info ! I was kind of suspicious of the MM prices as they are over twice the price of some others. But I don't want to settle for lesser quality either. This is where input from [ YOU WHO ARE IN THE KNOW ] is so valuable.
I'm still at the stage where I'm trying to determine if I can keep the PT thats in the amp & still have a JMP 50 that will sound as what most factory stock model #1987 should sound like!
Mine is year 1972 so it is near the end of the PTP era so I feel lucky to have found it. It is my first experience ever playing an old Marshall and it
seems to have much more detail & sensitivity to the tone than any other
amp in its class I've ever played.
I feel it is a little too much on the bright side in the treble channel which
could be expressed as harsh, ear fatiguing, piercing but I know that can be fixed. Don't know if the PT contributes to that?
So I have set off on a journey to learn as much as I can about these old amps.
Again Thanks to All Your Input,
tonerocker