KT66 JTM45 / JMP50 hybrid?

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ezietsman
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KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Tue Aug 04, 2015 11:28 am

Hi All,

I'm currently planning my next amp. My plan is to start with a JTM45 as the base design but because I want some more options, I'd like to build in a switch to flip it to 1987 mode (kinda like some of the boutique amps have). I'm also planning to have switchable SS and Valve rectifier on a different switch. Now I've figured out that such a 'mode' switch would need to flip between split and shared cathode on V1 and the different slope resistor/cap values in the tonestack. I'd also really need to switch the negative feedback between a lower and higher value for JMP and JTM 'modes'.

I really like the sounds George gets with his GPM45s but I also like some of the slightly higher gain sounds too (If you listen to ZZ Top bootlegs from the early 1970s you now what I'm talking about). I don't like to use boost and drive pedals and just ride the guitar's volume knob for clean and dirty (I have an 18 watt and a 5E3 that I built and currently use). I'll be getting a Weber MASS 100, because where I am I can't even play my 18 watter at more than 1/4 power at gigs.

Has someone done a JTM45 with the option to switch to something more like a 1987 style V1 cathode / tonestack / NFB / SS rectifier? Would this be a worthwhile feature or should I just build a standard JTM45 then tweak it until it sounds the way I want it?

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Re: KT66 JTM45 / JMP50 hybrid?

Post by Pressed_Rat » Mon Sep 07, 2015 6:31 am

There is a schematic on Yahoo's JTM45 Group - Craigtone 45. It has all the swtiches you mention except tone stack, which shouldn't be much trouble.

https://groups.yahoo.com/neo/groups/JTM ... s/CT45.pdf

A big difference between the 45 and the 1987 is the power supply filtering. There is an extra node in the 1987 that isn't there in the 45. If you're going to do this, I would use the 1987 PS filtering, dropping the dual filter arrangement when using a tube rectifier.

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Sat Sep 19, 2015 7:36 am

Thanks a lot, I'll have a look.

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ivan H » Sun Sep 20, 2015 6:49 pm

Hi, I have an amp that has switchable rectifier/main filter using one of the 3 pole double throw switches from Valvestorm. U need to use a bias supply dropping resistor across one pole of the switch so the bias is the same in both modes. (IIRC, mine is 15k). Switches between tube rec/32uf filter & SS rec/64uf filter with less than 1mA difference in bias current between the 2 modes. Worth doing in my opinion. Cheers

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Mon Sep 21, 2015 2:44 pm

ivan H wrote:Hi, I have an amp that has switchable rectifier/main filter using one of the 3 pole double throw switches from Valvestorm. U need to use a bias supply dropping resistor across one pole of the switch so the bias is the same in both modes. (IIRC, mine is 15k). Switches between tube rec/32uf filter & SS rec/64uf filter with less than 1mA difference in bias current between the 2 modes. Worth doing in my opinion. Cheers
Hi Thanks for this. How did you wire the diodes and the tube rectifier? Did you put the switch on the B+ side or the AC side? I'm assuming B+ since you're switching the extra filtering in and out too?

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ivan H » Tue Sep 22, 2015 5:22 am

Hi, no, its on the AC side. 2 poles of the 3 pole switch are used for this. The 3rd pole is used to switch in the extra 32uf, which is 1/2 of a dual cap can. The AC is fed the the center terminals of 2 poles of the switch. The diodes are connected to one throw of these two poles, with the other ends being connected to the extra 32uf cap. This cap has a 2 watt metal oxide resistor to ground as due to the bias supply resistor (which I'll get to next) there is always voltage on it. I cant remember the resistance value off the top of my head but could look if u really need. The bias supply dropping resistor is connected from the center terminal of one of the poles on the switch being fed AC, its other end goes to the pole with the diodes (SS rectifier) on it. Ur bais supply is taken from this point. This way, when switched to SS rec mode that bias supply dropping resistor is being bridged, or shorted by the switch so is not in circuit, thus ur bias circuit gets the full supply voltage. When u switch to tube rec mode, the bias circuit is supplied thru that now un-bridged resistor. This is how 1/2 of the SS rec puts voltage on that cap. Probably doesn't need the bleed resistor but I stuck it in anyway. I imagine u can sort the rest of the connections on the switch, or just yell out if u need help with. Cheers

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Tue Sep 22, 2015 5:39 am

Ah ok. I'll make a drawing at some point. Still a long wait for my parts. I'll be installing bleeder resistors on all the filter caps anyway, I don't like discharging the amp manually.

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Fri Oct 23, 2015 4:44 am

Would there be a major danger if I use a 50µF cap for mains filtering with the GZ34? I'm thinking of doing the filtering like 50 - choke - 50 - 32 - 32 instead. That way I'll have something a bit tighter than a JTM45 but with the lead / bass tonestack switching as well as GZ34 / SS switch. Which would make it closer to 1986 / 1987 with KT66s than a JTM45.

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Re: KT66 JTM45 / JMP50 hybrid?

Post by neikeel » Fri Oct 23, 2015 8:45 am

50uF with GZ34 is fine (64 probably highest you can go - with a Mullard at least)
Neil

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Fri Oct 23, 2015 11:47 am

OK sweet. So my plan now is as follows:

Up the filtering to 50, 50, 32, 30
Split cathode on V1
.68µF on V2a
3PDT for 56k / 250pF -> 33k / 500pF AND 22nF + 2.2nF -> 2.2nF coupling on bright channel
3PDT for SS / GZ34 + bias correction
22nF coupling caps elsewhere
KT66
NFB to taste, will start with 47k on 8Ω tap.

With this I can change it into a stock JTM45 later if I feel I need to, without taking the board out.

Obviously not a clone but it should be distinctly Marshally :-)

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Re: KT66 JTM45 / JMP50 hybrid?

Post by neikeel » Sun Oct 25, 2015 11:01 am

I might suggest 0.039 or 0.047uF PI output couplers (you can get these and 0.056uF in 400v mustards quite cheap).

Variable NFB is also very effective (a pot in series with the resistor).
Neil

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Mon Oct 26, 2015 4:10 am

neikeel wrote:I might suggest 0.039 or 0.047uF PI output couplers (you can get these and 0.056uF in 400v mustards quite cheap).

Variable NFB is also very effective (a pot in series with the resistor).
My friend has a 6V6 plexi and he has switchable split/shared, switchable 0.68µF on V2a and a pot for NFB. When I tried it I foudnd that I liked the feedback generally higher (weirdly). So one option is two use the standby switch (that I won't use) to wire in an extra resistor so I can flip between high and low NFB. I'm still deciding about this but will do so soon because I need to make the faceplates to match.

I specificaly ordered all yellow caps (for consistent looks) but I only got 22nF and 100nF for the couplers, so it'll have to be the one or the other at first. I'm nowhere near a country that has these things in stock and it was quite a process to get all the parts here in the first place. Our postoffice cannot be trusted to not lose shipments so I'm a bit weary to order stuff that doesn't get here via fedex or similar. I see the 1986 uses 0.1µF in the output and the 1987 uses .022µF. I've got these values so I can see which I like better.

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Thu Nov 05, 2015 1:34 am

So after some digging I realised I basically want a 1968 spec 50 watter. So my design has now morphed into the following:

Filtering: 100 (50 on valve rectifier) - 3H - 32 - 32 - 32 - 32
22nF coupling caps throughout
Switchable SS / GZ34 on 3pdt switch
Switchable Split/Shared cathode and Lead/Bass tonestack on 3pdt switch
27k / 68k NFB @ 16Ω on spst switch
Probably 0.68µF bypass on V2a
Probably 500pF bright cap.
KT66 Tubes.

Do I need to change anything on the 50 watt circuit, besides the bias circuit, in order to use the KT66? I have a Classictone JTM45 PT and a 8k OT. I'm assuming that besides the 470R Screen resistors and the bias I can leave the rest as is?

Now, since I ordered mostly the JTM45 kit I didn't get the dual 32µF axial capacitor so I have to use 2 separate ones, which means my turretboard is a little bit cramped. I have a long enough piece though so I'll just lengthen it by the required amount to fit both big filter caps in the middle.

I've made faceplate templates which I'll have lasercut soon.

Image

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Re: KT66 JTM45 / JMP50 hybrid?

Post by mananaja » Tue Nov 10, 2015 2:46 am

So after some digging I realised I basically want a 1968 spec 50 watter.
gclub โปรโมชั่น

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Re: KT66 JTM45 / JMP50 hybrid?

Post by ezietsman » Fri Nov 20, 2015 1:22 am

Got the faceplates. They fit my chassis perfectly. I started painting them but now of course I need to try and match the colour somehow. Also, the plexi is very smooth so the paint isn't going on evenly, easily. It looks like this so far.

Image

and I'm trying to get it to look like this 1968 Marshall I have:

Image

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