JMP50 with GZ34

[Littlewyan]

I originally built a JTM50 with a GZ34, however it just didn't do it for me tonewise. So after much playing around I basically converted it to 68 lead spec. So split cathode (820R/.68uF), .0022uF Bright Channel Coupling Cap, 5000pF Bright Cap (ceramic multilayer), .68uF on V2 820R, 33k/500pF Tonestack, .022uF PI Coupling Caps and 47K Feedback Resistor on 8Ohm Tap. Anyway it sounds ok but not as good as I think it should be. There doesn't seem to be very much clarity. I do have lower filtering than a normal 68 and obviously I am using a GZ34.

Filtering is: 32uF Mains, 32uF Screens, 32uF PI and 32uF+32uF for the Pre Amp.

Do you think I just need to increase the filtering to perhaps 50uF Mains and 50uF Screens? Or just stick a Sovtek SSR in place of the GZ34? I'm thinking the Mains filtering just isn't enough, but as its a dual cap which the screens shares I will have to up the screens as well.

[neikeel]

32uF on this amp is probably too low (a stock JTM45 has that), my 68 JMP 50 (lead) has 100uF mains, 50 screens, 50 PI and dual 32 on the board, my 67 (JTM50 with GZ34 ) has 48uF mains (it's a 32/16 can) with 32 screens, 32 PI and single 32 on board. With both amps cranked they are remarkably similar in tone, at lower volumes the 68 has a bit more cut but not quite as fat.

Normally the can next to the GZ34 is the mains can (dual) and the can inside the chassis is a dual can for screens and PI, sounds like you have slightly 'customised' arrangement.

You should not go much higher than a dual 32uF tied with a GZ34.

[Littlewyan]

I made the amp the same way as a Metro. The Dual Cap Can inside is solely for the PI. I was thinking upping the mains and screens to 50uF may be enough as the 100W 68 Marshalls only had 50uF Mains. In fact even the reissue 1959SLP and 1959HW both only have 50uF on the mains. Odd considering the 100W amps obviously draw a LOT more current and therefore must require a larger reservoir cap. Unless there is something about using 2 Caps in series that I'm missing which help the 100W amps.

I did think about using a 32uF+32uF for the Mains alone and using my current 32uF+32uF in place of the internal cap can and modifying it so the screens and PI use it. However I was unsure about current production GZ34s standing up to 64uF. I don't use Mullard GZ34s am afraid .

I was basing my tone on this amp:
https://www.youtube.com/watch?v=I49eCSgM1us

Obviously it won't be exactly the same due to different speakers, guitar, valves and room etc. But it definitely has a LOT more bite than my amp and its the same spec except its obviously 100W, not 50W, has a solid state recto and higher filtering.

[neikeel]

The differnce between the 100 and 50 (I believe) is because of the rectification. A 100w uses a bridge type for full wave rectivication and the 50w just the diodes in parallel, so you need mor filtration for the ripple control - of course I might be wrong but I understand that tp be why.
The 100w are under filtered in many ways, particularly early ones (they ghost quite a bit when pushed) but that is also part of their charm!

[Littlewyan]

Ah ha, that would make sense. I did wonder how those 100W amps survived such low filtering!!

So do you think its the Power supply sag that is my issue? I do have a 22uF Cap I could put in parallel with the 32uF as a test but I think it may only be rated for 450V. I'll have a look later and see what I've got. The plan I thought of earlier was to buy a 50uF+50uF Cap can, install it so the mains has 50uF and Screens has 50uF, if that doesn't do it, change it so the mains has 100uF, then every node after has 32uF and use a Sovtek SSR in the Rectifier socket.

[neikeel]

Ah ha, that would make sense. I did wonder how those 100W amps survived such low filtering!!

So do you think its the Power supply sag that is my issue? I do have a 22uF Cap I could put in parallel with the 32uF as a test but I think it may only be rated for 450V. I'll have a look later and see what I've got. The plan I thought of earlier was to buy a 50uF+50uF Cap can, install it so the mains has 50uF and Screens has 50uF, if that doesn't do it, change it so the mains has 100uF, then every node after has 32uF and use a Sovtek SSR in the Rectifier socket.

I would go 50 mains, 50 PI and 32 screens and 32/32 preamp

[Littlewyan]

Always got to be different haven't you Neil .

I do have a 22uF and a 47uF Cap BUT they are only rated for 450V and my HT is roughly 460V. I'm sure I used these as a test in this amp though. Whoops!

I should stop listening to John Segeborn's clips. Getting very jealous. I'll get a 50uF+50uF on order.

[Littlewyan]

Just a thought Neil. If I wanted to try solid state rectifier, I could just solder diodes across the pins on the socket couldn't I. I'm sure its not the GZ34 though as I've heard people switch between solid state and GZ34 recto and there wasn't a massive difference.

[neikeel]

Just a thought Neil. If I wanted to try solid state rectifier, I could just solder diodes across the pins on the socket couldn't I. I'm sure its not the GZ34 though as I've heard people switch between solid state and GZ34 recto and there wasn't a massive difference.

Leave the GZ34 in but solder a couple of diodes on the socket (between 6+7 and between 5+4) see this diagram:http://ceriatone.com/images/layoutPic/m ... iatone.jpg 1N4007 should be fine.

The GZ34 only sags when really pushed (then you are alos getting OT saturation and PT sag too!)

This was discussed beforem more as a failsafe than acting as a rectifier.http://forum.metroamp.com/viewtopic.php ... es#p424329

If going SS you could use two UF5408s soldered to the socket as you suggest between pin 6 + 8 and 5 + 8.

[Littlewyan]

Kevin O'Conner said it wasn't a good idea to install the diodes between pins 4+5 and 6+7, but then again many people have done it. I don't think I'll notice a lot of difference then if I remove the GZ34 as like you say it doesn't really sag unless pushed. I think my best bet will be to raise the filtering as we discussed. Then once I've got it tweaked how I like I think I'll install the diodes in as a failsafe.

I'm going to mess with the amp tomorrow night hopefully. I will report back!

[Littlewyan]

I bought a 50uF + 50uF Cap. I'm going to wire the amp up so it has 50uF Mains and 50uF Screens to see how the sag and tone changes. Although I may end up just removing the GZ34 and soldering diodes across the socket. I will then wire up the amp for 100uF Mains, 32uF Screens, 32uF PI and 32uF + 32uF Pre amp.

[Littlewyan]

Just to give you an update. I stuck the 50uF+50uF in place of the 32uF+32uF so the mains now has 50uF and the screens has 50uF. Straight away there is a massive improvement. Now when I turn everything up to 10 van halen style the amp sounds good! Before I think it was ghosting pretty bad as with everything on 10 the tone was awful and didn't sound in tune.

Anyway I plan on soldering diodes across the GZ34 socket (obv removing the GZ34) and rewiring the amp so the mains has 100uF and the screens has 32uF.

[neikeel]

I would try 50 mains, 32 screens, 50 PI and 32/32 preamp and leave the GZ34. I have done all this before, but you must decide with your ears and guitar.

[Littlewyan]

Yeah I do want to have a play with the filtering a bit more. Its one of those things that I've never experimented with.

[SawGani]

As per my experience 32uF on this amp is probably too low. The 68 JMP 50 have 100uF mains, 50 screens, 50 PI and dual 32 on the board, my 67 (JTM50 with GZ34 ) has 48uF mains (it's a 32/16 can) with 32 screens, 32 PI and single 32 on board.
With both amps cranked they are remarkably similar in tone, at lower volumes the 68 has a bit more cut but not quite as fat.
You should not go much higher than a dual 32uF tied with a GZ34.