New JTM/JMP50 build w/switchable rectifier

Get support and show off your MetroAmp 50 Watt kit builds.

Moderator: VelvetGeorge

shakti
Senior Member
Posts: 2008
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: New JTM/JMP50 build w/switchable rectifier

Post by shakti » Mon Apr 23, 2012 2:27 am

I went with 470k mixer resistors instead of the 270k ones. 270k was a feature of the JTM45 lineage, and was used in the early '67 EL34 amps, i.e. the black flag JTM50 and JTM100s. I started off with that value in this amp, but changing to 470k was a much bigger difference than I expected. The amp is smoother and fatter with 470ks, which I think works better for the classic Marshall EL34 crunch.

Incidentally, it turns out I did have UF diodes in my black flag JTM100, but I changed to 470k mix resistors in that one as well. I'm comparing that amp to a benchmark '67 black flag 100-watter that belongs to a friend of mine, which does have 470ks. This resistor swap did take it one notch closer for sure! It seems to sustain more, and the tone is fuller. 270ks do have more "snap" if that's what you're after though.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

shakti
Senior Member
Posts: 2008
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: New JTM/JMP50 build w/switchable rectifier

Post by shakti » Mon May 14, 2012 3:56 am

After a bit of tweaking, I've ended up with more or less late '67 specs for this amp. 470k mixers, 27k @ 8 ohm negative feedback. Amps from that period came with both tube and solid state rectifier, but were otherwise identical: single voltage dropper between screens and PI, same filtering throughout. I may try out a 16+32uF cap for the shared screens/PI cap as that would be the authentic specs for that period amp, but it sounds really good now. Perhaps a touch stiff in solid state mode - I tend to prefer the softer feel of lower filtering with the early/bass preamp, so 64uF mains plus 32uF screens and PI may be a bit too much.

One slight problem though: I get a hum when turning up channel 2 volume with nothing plugged in. The hum goes away when I plug the guitar into channel 2. I've tried reflowing the grounds connections for channel 2, but no change. Could it be a faulty potentiometer?

Speaking of which...I suspect more and more that some of the difference between vintage amps and replicas lies in the potentiometers. Has anyone had a chance to measure the pots in several vintage Marshalls to see if there's a pattern? The difference when switching to 470k resistors was very evident, so I can only imagine that pots with widely varying values (due to drift or due to tolerances), acting as voltage dividers and deciding impedance/load between various parts of the circuit, could greatly influence the overall tone of the amp?
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

Reeltarded
Senior Member
Posts: 512
Joined: Sat Aug 20, 2011 4:24 am
Just the numbers in order: 7

Re: New JTM/JMP50 build w/switchable rectifier

Post by Reeltarded » Mon May 14, 2012 6:41 am

Yes, I have bunches of pots that all measure around 5-10% over that are pulls. I have a few volume controls that are 20-30% over, for sure. I am with you on the mixers. I don't care for the 270k, but they are kind of nice for the kind of playing I would never do.. I like it hot and thumpy.

Thanks for posting your amp stuff, very interesting read. I saw a couple other ways that guys handled tube/ss rec since I started reading this. There seems to be many ways to skin that cat.
not kicking the dead horse

User avatar
neikeel
Senior Member
Posts: 7063
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: New JTM/JMP50 build w/switchable rectifier

Post by neikeel » Mon May 14, 2012 7:09 am

shakti wrote:One slight problem though: I get a hum when turning up channel 2 volume with nothing plugged in. The hum goes away when I plug the guitar into channel 2. I've tried reflowing the grounds connections for channel 2, but no change. Could it be a faulty potentiometer?
More likely the earth tab of one of the switching jacks - it is open circuit without ground reference on that channel which is addressed by putting a jack in.
BTW here is my latest toy, started off looking for a mid 60s JTM45 but this turned up - much more interesting - and relevant to the current discussion. I could have started new thread but I did that at PP. The resizing issue from Photobucket is a pain here - kind of puts me off :|

http://s76.photobucket.com/albums/j2/ne ... g%20JTM50/

This has 270k mixers, 27k/16ohms NFB.
I believe the original mains filtering was 32uF but I used refomed 32/16 for 48uF.
When I first set it up I used a spare Russian (ie expendable) GZ34 that put 380v on plates which browned the sound a little with great 3-D sound, lots of drive and surprisingly high gain for a shared cathode 50watter. I put in a NOS yellow label Mullard in to get the volts up to 410+ and still pretty amazing tones. main thing is how well it cleans up with the guitar pot.

Funnily enough (as Shatki knows) I have almost exactly the same amp 'in embryo' but I was going to go with 470k mixers dual 32 in preamp, which I may still do although it is tempting to build same specs as this amp to see how Brian's trannies compare!
Neil

shakti
Senior Member
Posts: 2008
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: New JTM/JMP50 build w/switchable rectifier

Post by shakti » Mon May 14, 2012 9:25 am

neikeel wrote:
shakti wrote:One slight problem though: I get a hum when turning up channel 2 volume with nothing plugged in. The hum goes away when I plug the guitar into channel 2. I've tried reflowing the grounds connections for channel 2, but no change. Could it be a faulty potentiometer?
More likely the earth tab of one of the switching jacks - it is open circuit without ground reference on that channel which is addressed by putting a jack in.
Thanks for the tip, I'll check into it. I think I actually have some of those Chinese Cliff jacks in this amp, if that makes a difference...

I did see the cool Black Flag JTM50 on block end chassis you got, nice score! I can only imagine it sounds great! If you get a chance though, could you measure the pots in your vintage amps Neil? Would be very interesting to see if there's a pattern or even how much variability there is. With new production ones (at least with Alphas) there's a lot of variability with the 1M ones, but I don't tend to see as much spread with the 25k and 250k ones.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

shakti
Senior Member
Posts: 2008
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: New JTM/JMP50 build w/switchable rectifier

Post by shakti » Mon May 14, 2012 9:27 am

Reeltarded wrote:Yes, I have bunches of pots that all measure around 5-10% over that are pulls. I have a few volume controls that are 20-30% over, for sure. I am with you on the mixers. I don't care for the 270k, but they are kind of nice for the kind of playing I would never do.. I like it hot and thumpy.

Thanks for posting your amp stuff, very interesting read. I saw a couple other ways that guys handled tube/ss rec since I started reading this. There seems to be many ways to skin that cat.
All I can say is the tube/SS switch works great, and could be used as a standby switch if you want to.

As mentioned, I see 1M Alpha pots that vary between 0.9M to 1.3M. Roe has even seen some that were 700k-ish, IIRC! I imagine that treble and mid pots with varying tolerances would possibly influence the voice of the amp quite a bit...but it would be nice to see if there's a pattern with Plexis.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

hdahs
Senior Member
Posts: 111
Joined: Tue Mar 29, 2011 1:56 pm
Just the numbers in order: 7

Re: New JTM/JMP50 build w/switchable rectifier

Post by hdahs » Mon May 14, 2012 11:32 am

I wonder if that is why Greg Germino uses a mix of PEC and alpha/cts pots in his builds. Not sure whether it is tone, tolerance or both.

Very nice build by the way. :)

shakti
Senior Member
Posts: 2008
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: New JTM/JMP50 build w/switchable rectifier

Post by shakti » Wed Jun 20, 2012 7:21 am

Yeah, I've wondered about the same...

I've been tweaking this build lately. Decided to go with a 3H choke from M e r r e n, which is a Dagnall C1999 type consistent with a '68 style amp. The difference between this and the 352-114 wasn't huge, but a little rounder and more compressed with the 3H one. I also changed the front panel to a JMP one and put in Castelco black bat switches, so it now looks very much like a mid/late '67 tube rectified JMP50.

Whats' more interesting though is another tweak I've done with the voltage/power supply. The Marstran PT has two sets of secondaries, for either 420V or 370V plate voltage (with diode rectification). 420V is typical for a 1202-118 power transformer, as used from '67 and up through the early 70s. There was a stand-up model with a different type number introduced in 1969, but I believe it had the same voltage until sometime in the 70s, when the voltage was dropped to 370ish.

I was not 100% happy with the amp in solid state mode. I am shooting for a Duane Allman-type tone in this mode, with a little more filtering for a more solid feel. I had it set up with filtering like a '68-69ish amp. However, the late '67 and on amps had two voltage drop resistors between screens and phase inverter (8.2k+10k) vs only 8.2k for the earlier tube rectified and first solid state rectified ones. This gives a little lower preamp voltages and a slightly browner, more compressed and overdriven sound which goes well with the more solid filtering. However, with a single voltage dropper, the more solid filtering can sound a little stiff and clean, which was the case with my amp. I was struggling to get the very harmonically rich, warm sound that's typical of Duane Allman. It doesn't help that the wall voltage often shoots up to 243-4V, so I'd end up with a plate voltage closer to 440V in diode rectification mode.

Therefore I tried it at 370V instead. I didn't expect to like it too much, as I typically like a little higher plate voltage, but in this case it was a perfect fit! Super rich, creamy, warm overdrive with lots of overtones, but still cleans up nicely. I got the best Allman-style sound I've ever had!! I guess the single voltage dropper and high wall voltage may help so it doesn't get too soft. I got about 385V plate voltage in this mode. A pleasant side effect was a slight volume drop as well.

However, I still like the higher voltage (420V) with tube rectifier, so I am going to rewire the switch like this: run the 420V secondaries directly to the tube rectifier, and the 370 ones directly to the diodes. B+ from tube rectifier and diodes respectively to the switch, and from the pole of that switch to the HT fuse. So I can select either 420V/tube mode, or 370V/solid state mode. The next pole of the switch will be used to adjust the bias (using one half of the 420V secondaries for raw bias feed, and adding in series resistance in solid state mode), and finally the last pole, like before, will be used to switch in the additional mains filtering in solid state mode.

Will post pictures!
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

User avatar
neikeel
Senior Member
Posts: 7063
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: New JTM/JMP50 build w/switchable rectifier

Post by neikeel » Mon Jul 30, 2012 5:40 pm

shakti wrote:Yeah, I've wondered about the same...

I've been tweaking this build lately. Decided to go with a 3H choke from M e r r e n, which is a Dagnall C1999 type consistent with a '68 style amp. The difference between this and the 352-114 wasn't huge, but a little rounder and more compressed with the 3H one. I also changed the front panel to a JMP one and put in Castelco black bat switches, so it now looks very much like a mid/late '67 tube rectified JMP50.

Whats' more interesting though is another tweak I've done with the voltage/power supply. The Marstran PT has two sets of secondaries, for either 420V or 370V plate voltage (with diode rectification). 420V is typical for a 1202-118 power transformer, as used from '67 and up through the early 70s. There was a stand-up model with a different type number introduced in 1969, but I believe it had the same voltage until sometime in the 70s, when the voltage was dropped to 370ish.

I was not 100% happy with the amp in solid state mode. I am shooting for a Duane Allman-type tone in this mode, with a little more filtering for a more solid feel. I had it set up with filtering like a '68-69ish amp. However, the late '67 and on amps had two voltage drop resistors between screens and phase inverter (8.2k+10k) vs only 8.2k for the earlier tube rectified and first solid state rectified ones. This gives a little lower preamp voltages and a slightly browner, more compressed and overdriven sound which goes well with the more solid filtering. However, with a single voltage dropper, the more solid filtering can sound a little stiff and clean, which was the case with my amp. I was struggling to get the very harmonically rich, warm sound that's typical of Duane Allman. It doesn't help that the wall voltage often shoots up to 243-4V, so I'd end up with a plate voltage closer to 440V in diode rectification mode.

Therefore I tried it at 370V instead. I didn't expect to like it too much, as I typically like a little higher plate voltage, but in this case it was a perfect fit! Super rich, creamy, warm overdrive with lots of overtones, but still cleans up nicely. I got the best Allman-style sound I've ever had!! I guess the single voltage dropper and high wall voltage may help so it doesn't get too soft. I got about 385V plate voltage in this mode. A pleasant side effect was a slight volume drop as well.

However, I still like the higher voltage (420V) with tube rectifier, so I am going to rewire the switch like this: run the 420V secondaries directly to the tube rectifier, and the 370 ones directly to the diodes. B+ from tube rectifier and diodes respectively to the switch, and from the pole of that switch to the HT fuse. So I can select either 420V/tube mode, or 370V/solid state mode. The next pole of the switch will be used to adjust the bias (using one half of the 420V secondaries for raw bias feed, and adding in series resistance in solid state mode), and finally the last pole, like before, will be used to switch in the additional mains filtering in solid state mode.

Will post pictures!
Update please!
Neil

shakti
Senior Member
Posts: 2008
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: New JTM/JMP50 build w/switchable rectifier

Post by shakti » Tue Jul 31, 2012 12:35 am

Thanks for pushing me! I've done the mod, will have to get photos. Works really well!
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

ivan H
Senior Member
Posts: 150
Joined: Wed Oct 10, 2012 10:01 pm
Just the numbers in order: 13492

Post by ivan H » Mon Jan 28, 2013 10:47 pm

Hi all, I'm thinking of following Shakti's idea of using the 3PDT switch (available from Valve Storm) to switch between tube rectifier/32uf filter or SS rectifier/64uf filter, as outline at the beginning of this thread. I do have a question tho,,, when in tube rectifier mode, one half of the full wave SS rectifier is still being fed AC voltage thru the bias voltage dropping resistor that is wired between the AC feed & diode side of 1 pole on the switch. Is this of concern in any way? Could I wire the output of the SS rectifier to the 32uf half of the dual cap that is only switched in when in SS mode to get round this, or would this not be a good idea? Any input much appreciated. Cheers

ivan H
Senior Member
Posts: 150
Joined: Wed Oct 10, 2012 10:01 pm
Just the numbers in order: 13492

Post by ivan H » Thu Apr 04, 2013 5:58 pm

Hi, so I added the switchable rectifier with a small difference. I attached the output side of the diodes (uf4007's, 2 in series on each half of) directly to the half of the dual 50uf/500V cap that is swithed in while in SS rec mode. Upon testing without tubes installed I found that as I had thought, when the switch is in either tube rec mode or the center stanby position, the half of the cap that the diodes are connected to has 487V on it from 1 half of the full wave rectifier being fed 350VAC (ish) thru the bias dropping resistor that is wired across the switch. I figure that if the diodes were connected to the same half of the dual cap as the GZ34, the B+ when in tube rec mode would be slightly (20V or so) higher than it should be & the "sag" effect would be somewhat diminished. I have installed a 340k 2 watt bleed resistor (2 x 680k 1 watt in parallel) from the half of the dual cap that the diodes are connected to to earth. Whether this will help or not I'm not sure. So now, when I power the amp up, I do so with it in SS rec mode for a few minutes to charge all filters, then flip the switch thru standby to tube rec mode so as to avoid the filter inrush current strain on the JJ GZ34 , so as to prolong its life. The amp has a Marstran 1202-55 PT & a Heyboer C1999 Choke & 784-139 OT with Winged C EL34's. Bias in SS mode is 37mA/463V & in tube rec mode the bias supply runs thru a 15k resistor wired across the switch. Bias is 39mA/440V, so both modes are biased close to 70%. The 50uf cap that the diodes are connected to has 385V on it when the 3Pole switch is in tube rec mode or the center standby position now that it has the 340k 2 watt resistance tying it to ground. Thank you Shakti for the great idea. Works very well. Cheers

User avatar
neikeel
Senior Member
Posts: 7063
Joined: Tue Dec 06, 2005 8:31 am
Location: Suffolk, England

Re: New JTM/JMP50 build w/switchable rectifier

Post by neikeel » Wed Jun 24, 2015 12:04 pm

Bump for Thorlief to post pictures :whistle:
Neil

shakti
Senior Member
Posts: 2008
Joined: Tue Apr 05, 2005 9:06 am
Just the numbers in order: 7
Location: Ramnes, Norway

Re: New JTM/JMP50 build w/switchable rectifier

Post by shakti » Wed Jun 24, 2015 4:56 pm

Jeez, it's only been three years and already you're complaining? That's what happens with the youth brought up on the Internet, they want EVERYTHING RIGHT HERE RIGHT NOW!

Actually I thought I had posted photos but it may have been just of the initial arrangement. I'll see if I can get to it. All I can say is it works brilliantly and offers two subtly yet noticeably different flavours.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

User avatar
JimiJames
Senior Member
Posts: 3537
Joined: Fri Dec 23, 2005 6:32 pm
Just the numbers in order: 13492
Location: Chicago
Contact:

Re: New JTM/JMP50 build w/switchable rectifier

Post by JimiJames » Wed Jun 24, 2015 10:23 pm

m a n, I bet it does function as you say. 8)
I think the kool thing is about the feel. In Fender's; my BF Super was modded with a SS rectifier. (The tube is still there, dormant in place.) I have played others they way I play my own, and I can definitely feel the difference when comping chords and/or pulling off some aggressive leads.
In fairness though, I re-baffled to a 10x15 changing the Ω's from the traditinal 4x10's.
I don't know if that plays a part on the voltages. As does in changing rectifier tubes drawing different currents. The feel of the amp is most definitely affected by the rec and filtering. That's really kool to me as I would want to dial that in on a JTM.
:thumbsup:
RIP Mark Abrahamian-rockstah -classmate/roommate
RIP Ben Wise -StuntDouble- comrade-in-arms

__________________________________________
Build'sClip'sVid's

Post Reply