new jtm45 style build.....

Get support and show off your MetroAmp JTM 45 kit builds.

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rockgod212
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new jtm45 style build.....

Post by rockgod212 » Sun Jan 25, 2015 7:34 pm

hello all,
I built a weber jtm45 kit, I modified a lot of areas to my liking. the tranny's are the weber's except for the choke which is a classic tone 3h. jtm45 style turret board and a filter cap turret board mounted on tranny standoffs, mojo tone couplers, metro carbon film resistors, cliff jacks all around, alpha pots, marshall style knobs, a decent impedance switch, stock ceramic pre sockets and phenolic power/ recto sockets, etc. im using the supplied shungang kt66's at the moment and the supplied shungang 12ax7's, I have other pre amp tubes that will make their way into the amp eventually.

now I just play tested the amp and I know these amps are really clean sounding to begin with, but I would like a bit more dirt or grit to it. I know rolling some pre amp tubes will do some of what I want, but what about lowering my preamp voltages down a bit, my voltages are as follows:

this power tranny has a higher plate voltage, so with a el chepo sovtek 5ar4 recto
480vdc on the plates down to about 474vdc biased to about 40Ma, screens around 474vdc

V1- pin 1 and 3 around 232vdc
V2- pin 1- 201vdc
pin 3- 344 vdc
V3- pin 1-276 vdc
pin 6- 259vdc

any help or info would be great with this build.
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Re: new jtm45 style build.....

Post by rockgod212 » Mon Jan 26, 2015 9:21 pm

well I subbed out the 8.2k dropper for a 18k dropper and lowered my voltages back down to where I would like them. guess I just have to crank this amp to get it to clip some, as I get no dirt once so ever. it's really clean sounding.

I don't know, maybe I will stick some el34's in it one day for a black flag kind of vibe. :mrgreen:
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Re: new jtm45 style build.....

Post by danman » Mon Jan 26, 2015 10:22 pm

I was gonna suggest that you could play around with the dropping resistor values to lower the voltage in the preamp. It won't make a drastic change but will help slightly. You could also add a bypass across V2 cathode for a little extra gain. You can try values anywhere from .33uf-.68uf to see what you like.

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Re: new jtm45 style build.....

Post by danman » Mon Jan 26, 2015 10:26 pm

I also find that after playing a new amp for several hours, to burn it in, that it starts to sound much sweeter. Try your new setup with the 18k for a few days and then go from there.

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Re: new jtm45 style build.....

Post by rockgod212 » Mon Jan 26, 2015 10:56 pm

ok thanx for the info.........yeah Im happy with where the voltages are now, im running these now

v1- 1-202
6-180
v2- 1-171
6-290
v3- 1-235
6-219

I also added a master volume before treble pot, I kinda like it there. im using these old vox Buckingham oxford bulldog speakers, so I don't want to blow them. I will get a proper attenuator later on.

im using the 330u shared cathode, so I might lower the cathode to the 250u or even split it later on. I still have to adjust the brite cap, using a 100p right now. sounds good though, with the added master I can get a slight breakup now.
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Re: new jtm45 style build.....

Post by neikeel » Tue Jan 27, 2015 9:11 am

I found OT very influential on these
I had my first clone with MM 16 OT, very clean virtually no grind.
Replaced it with one of Georges Heyboer -103 clones bang there it was.
I used Mullard pres, GEC KT66s and Mullard GZ34.

The amp sttod up very well next to my original late 65. As is typical a bit louder and held together better until cranked then the same was pretty similar. I also use mustards throughout but a mixture of cc and cf (as in the kits) in this amp but usually prefer Pihers.

They are simple circuits and components make a difference, IMO.
Neil

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Re: new jtm45 style build.....

Post by rockgod212 » Tue Jan 27, 2015 1:52 pm

thanx neil, it will probably get a different OT at some point, I think I found my issue- bad batch of shungy tubes, i had a hard time biasing them and now they red plated, so i just ordered some valve arts.
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Re: new jtm45 style build.....

Post by rockgod212 » Tue Jan 27, 2015 2:05 pm

hey neil,
how much does the recto tube impact the tone of the amp, right now I have a sovtek ar4 recto, would the tone be better off with the mullard reissue gz34?

I also have dual primaries on the weber power tranny, right no its on the higher voltage tap which puts out 480vdc after recto, would it be better to use the lower voltage tap? the lower tap should put the voltage around 420vdc or so.

I guess I should be looking for a 8k primary OT, if I change all of this I will change the tranny as well, so info needed.
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Re: new jtm45 style build.....

Post by rockgod212 » Tue Jan 27, 2015 3:25 pm

well, since I have a classic tone jtm45 power tranny here, I got the wrong one for my 50 watt el34 build. I will go ahead and switch out my power tranny and I will order the correct classic tone 8k OT for this amp. the amp just needs to be right or its not worth doing at all. stay tuned. :mrgreen:
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Re: new jtm45 style build.....

Post by neikeel » Tue Jan 27, 2015 4:57 pm

I think that a JTM45 with KT66s sounds best with a B+ of 440-450v. The classic tone with 420 probably suits a JTM50 better with EL34s (just personal choice, but I know others do agree).

The Metro is an 8k OT and sounds good, but so do the Marstran and ******, down to supply, price and service you get form the supplier...............

I have used a Sovtek for testing and reforming caps but until I dropped it :oops: ) it gave 15vdc less than my NOS BVA Mullards (aim for £50 on ebay - they last forever too). Which is why my Metro PT had no issues with oomph but I can imagine a poor quality rectifier make an amp sound weak.
Neil

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Re: new jtm45 style build.....

Post by rockgod212 » Thu Jan 29, 2015 7:19 pm

well after some filing I got the new power tranny in and wired up, my 8k OT is on its way,
as are my VA kt66's and a mullard re-issue gz34 recto. time to build.

rockgod212 wrote:well, since I have a classic tone jtm45 power tranny here, I got the wrong one for my 50 watt el34 build. I will go ahead and switch out my power tranny and I will order the correct classic tone 8k OT for this amp. the amp just needs to be right or its not worth doing at all. stay tuned. :mrgreen:
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Re: new jtm45 style build.....

Post by rockgod212 » Thu Feb 05, 2015 11:52 pm

ok, I got the new classic tone transformers wired in, here's where I'm at right now.

im running 476Vdc biased to hottest tube of 39mA, so the plates are at 464Vdc biased. pre-amp voltages are still a bit high at 230Vdc pin 1/3 of V1, so I might drop those down a bit later on, as im out of dropper resistors.

here's something strange though the 40-18033 classic tone power trans i used is spec'd at 345Vac on the red leads going to the recto, but I get 372Vac. I used a mullard RI gz34, could this be the cause of this or the xtra 3volts wall voltage? I don't know.

from what im hearing so far, it sounds real good. these classic tone transformers sound so much better.

only other thing is I sometimes get pops from the standby, any good way of fixing that? :mrgreen:
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Re: new jtm45 style build.....

Post by neikeel » Fri Feb 06, 2015 4:01 am

rockgod212 wrote:
only other thing is I sometimes get pops from the standby, any good way of fixing that? :mrgreen:
4k7 or 5k6 grid stoppers on the output valves (like the later 100w) and a 0.05uF 1000v cap 'snubber' from the apex of rectifier B+ to ground before the standby switch (tuck it under the mains input) as per original schem:

Image
http://marstran.com/JTM45.gif (the GI is too expanded!)
Neil

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Re: new jtm45 style build.....

Post by rockgod212 » Fri Feb 06, 2015 11:44 am

ok thanx neil, I did use 1.5k swampers on power tubes, I will try the .047 cap trick.
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Re: new jtm45 style build.....

Post by rockgod212 » Fri Feb 06, 2015 6:50 pm

hey neil,
I have a different problem, I seem to have some motorboating going on, I can tap on the mains cap and i get some oscillation as well, could this be my problem? I kinda skimped on filter caps, I got the cheap ones, im ordering some F&T's now. should I up the mains at all?
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