GroupDIY

General Discussions => Drawing Board => Topic started by: Spencerleehorton on February 23, 2021, 03:33:04 AM

Title: JTM45 / 50 Build
Post by: Spencerleehorton on February 23, 2021, 03:33:04 AM
Hi All,

Now moving onto the JTM45 build from the success of the fender champ.

First of all I wanted to try and use this line 6 spider MK2 power txf.
I have the following voltages on it:

Winding 1 is 370vac with centre tap
Winding 2 is 60vac
Winding 3 is 20vac with centre tap
Winding 4 is 6.7vac but in 4 leads?
1+3 = 6.71v
1+4 = 6.73v
2+4 = 6.72v
2+3 = 6.71v

My issue seems to be on the 6.3v windings. (And yes I know I could just buy a 5v txf)
By process of elimination, would I be right in guessing that on winding 4, taps 1 and 2 could be the CT on the 6.3v windings?
I need one 5v winding and one 6.3v winding for this txf to work and wanted ideas to get the 5v?
Could I use a high wattage resistor to lower it?

My other option could be if I can split the centre tap of the 20v winding and then parallel the two 10v winds to give me 5v?
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 23, 2021, 04:22:09 AM
I could use a voltage divider of 10k and 6.8k to drop 1.47v, which would get me close but I'm also concern I dont know what wattage these 6.3v windings are?
Is there any tests I can do to establish was they are?
As I need I think at least 2A possibly 3A windings?
Be good to know a few more techniques on this if anyone could share please?
Title: Re: JTM45 Build
Post by: abbey road d enfer on February 23, 2021, 04:36:16 AM
I could use a voltage divider of 10k and 6.8k to drop 1.47v, which would get me close but I'm also concern I dont know what wattage these 6.3v windings are?
You seem to ignore the basics. With 10k in series, tehre will be almost nothing when it is loaded with the rectifier filament. The GZ34 filament behaves like a 2.6 ohm resistor when hot. But when it starts cold, it's like a 0.2 ohm resistor.
That means it would receive about 6mA, which would never start it heating.
If you want to lower the voltage with a resistor, it should be about 0.68 ohm.
However I wouldn't recommend it, because the heater voltage may change when teh tube heats, maybe significantly enough to put it in unsafe operation area.
You must definitely learn about Ohm, Norton, Kirchoff & Thevenin.
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 23, 2021, 06:35:19 AM
Yes my calculation attempt was way off, I am starting to learn it though!!
My second attempt I worked out that I needed 3.15ohms which would give me 2A, which would involve a 0.8ohm 5watt resistor!!
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 23, 2021, 06:44:06 AM
I appreciate it is very frustrating reading my mindless attempts but I am trying to learn this, having been helped in the past I need to like you say get the basics right.
Ohms laws seems to makes sense now but I do need a lot more experience with it all as havent dealt with these particular calculations before.

Would you be able to explain the any possibilities of using this txf for this purpose please? How you would go about it and any calculations would be fantastic.
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 23, 2021, 09:18:04 AM
So from what you have said, if I put a 2.6ohm resistor on one of the 6.3v windings then measure the voltage and see what I've got?
I think I have some 2.2ohm 2W , i definitely have some 7ohm 8watt.
Title: Re: JTM45 Build
Post by: abbey road d enfer on February 23, 2021, 01:47:56 PM
So from what you have said, if I put a 2.6ohm resistor on one of the 6.3v windings then measure the voltage and see what I've got?
I think I have some 2.2ohm 2W , i definitely have some 7ohm 8watt.
Please read again, carefully now.
0.68 ohm.
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 23, 2021, 02:30:40 PM
I'm not talking about within the circuit I'm talking about just testing the transformer.
So are you saying to test the txf just use a 0.68ohm resistor and see what voltage drop I get?
Title: Re: JTM45 Build
Post by: abbey road d enfer on February 23, 2021, 05:15:31 PM
I'm not talking about within the circuit I'm talking about just testing the transformer.
So are you saying to test the txf just use a 0.68ohm resistor and see what voltage drop I get?
With just one resistor and no load there is no voltage drop. The GZ34 must be in circuit.
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 24, 2021, 12:43:36 AM
My aim was to test this txf with a dummy load first, then see what voltage I had.
Just to be clear.

I took what you said about heater filament as if I'm testing without tube that the filament is 2.6ohm, then the load would be 0.68ohms to reduce down to 5v, so 3.2ohms total?
I dont mean to offend but you have not explained how you got to this, could you explain please?
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 24, 2021, 07:31:02 AM
Here is the turret board nearly complete!!

Cant upload any images as the uploads folder is full!!!
Title: Re: JTM45 Build
Post by: abbey road d enfer on February 24, 2021, 04:41:06 PM
My aim was to test this txf with a dummy load first, then see what voltage I had.
Just to be clear.

I took what you said about heater filament as if I'm testing without tube that the filament is 2.6ohm, then the load would be 0.68ohms to reduce down to 5v, so 3.2ohms total?
I dont mean to offend but you have not explained how you got to this, could you explain please?
5V 1.9A gives 2.6 ohms.
Drop 1.3V at 1.9A gives 0.68ohm.
Title: Re: JTM45 Build
Post by: Spencerleehorton on February 25, 2021, 01:22:47 AM
Ok, well it seems I maybe able to move forward with this txf by the use of some diodes on the 6.3v winding to bring it to 5v, I just need to make sure that I have two completely separate 6.3v windings.
So one winding could heat the filaments at 6.3v and the other with two sets of two anti-parallel diodes rated to 5A connected in series with the rectifier filament. The diodes should be placed on a heatsink.
Thanx Milan for this.
Title: Re: JTM45 Build
Post by: abbey road d enfer on February 25, 2021, 06:03:54 AM
Ok, well it seems I maybe able to move forward with this txf by the use of some diodes on the 6.3v winding to bring it to 5v, I just need to make sure that I have two completely separate 6.3v windings.
So one winding could heat the filaments at 6.3v and the other with two sets of two anti-parallel diodes rated to 5A connected in series with the rectifier filament. The diodes should be placed on a heatsink.
Thanx Milan for this.
Simulation shows that with two sets of two diodes, the RMS voltage would be 4.9V; close enough.
You don't necessarily need two separate windings.
The diodes introduce some distortion of the voltage; how significant it is depends very much on the DCR of the xfmr and the sensitivity of the audio circuit.
Note that, even using a separate winding, the waveform would also be distorted, although slightly less.
Title: Re: JTM45 Build
Post by: Spencerleehorton on March 04, 2021, 01:59:14 AM
Trying to post picture of turretboard.
Title: Re: JTM45 Build
Post by: Spencerleehorton on March 08, 2021, 02:28:32 AM
What's your thoughts on using 0.25w pots for this build?
Title: Re: JTM45 Build
Post by: Spencerleehorton on March 11, 2021, 05:47:45 AM
looking at possibly modding this circuit to a dirty shirley but unclear exactly what the changes are?

Anyone have any input please?
Title: Re: JTM45 Build
Post by: Spencerleehorton on March 12, 2021, 01:48:03 AM
This is the schematic I'm going to follow but I'm going to use diode bridge for B+ and will put diode bridge on the rectifier socket like what I did for the champ.
The output valves I have will be EL34s so there also maybe a few changes to components?
Will need to verify these changes.
Title: Re: JTM45 Build
Post by: Spencerleehorton on March 12, 2021, 01:56:08 PM
Its seems I may go for the JTM50 build and put switches on the cathode bias of V1a and V2a.
Will check that this power txf can get 320v ac so after the diode bridge I can get 440v dc.
Title: Re: JTM45 Build
Post by: Spencerleehorton on March 14, 2021, 08:59:40 AM
Just wondered if anyone knows if the cathode bypass capacitors sound better as electrolytic, tantalum, polyester or polypropylene?

Or does it not matter?
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 15, 2021, 02:00:48 PM
Working on the chassis, need to weld tomorrow and add the corner screw holder
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 15, 2021, 02:03:52 PM
Fixed up the router table, made a few adjustments and set to 12mm finger joints.
Glued and will wait until dry then finish off the inside supports and front and back panels.
Will put in a brass mesh on back panel.
Will also need to route out the beading lines.
Then will cover with tolex and beading.
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 15, 2021, 08:29:02 PM
This is true diy,  electronics,  metal work,  and wood work too.

Regarding capacitors, you're probably already past this part,
but film is better in the signal path if possible.  Though you might need to go electrolytic if the cathode bypass cap is really large.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 16, 2021, 01:06:24 AM
Yeah I've read that film sounds less harsh than electrolyics, will experiment, as this is exactly what it's all about.
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 16, 2021, 01:48:37 AM
If you really want to go down the rabbit hole you can try different types of film, Mallory 150, Orange Drops,  Mojotone Dijon,  Sozo mustard caps, are some common guitar amp choices.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 16, 2021, 07:39:59 AM
i was mostly using polyproperlene for the caps and mica for the smaller ones, these seem to fit into what people prefer tonally. where i was going to experiment was the cathode bypass caps, as ushally used are electrolytic but i have read that film in these places is tonally better?
Title: Re: JTM45 / 50 Build
Post by: Heikki on March 16, 2021, 07:57:22 AM
i was mostly using polyproperlene for the caps and mica for the smaller ones, these seem to fit into what people prefer tonally. where i was going to experiment was the cathode bypass caps, as ushally used are electrolytic but i have read that film in these places is tonally better?

Any nonlinearities any type of cap might have won't matter in a guitar amp. Distortion from tubes will always be much higher than any distortion caused by capacitors. If you plan to sell the amp then use whatever snake oil caps gets you the best price.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 16, 2021, 10:13:46 AM
Started on the front and back panels, it's all very rough but it's starting to look like it's an amp head now
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 16, 2021, 10:14:13 AM
From the back
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 16, 2021, 10:17:44 AM
Testing this power txf I have which has a centre tapped 190vac, so its 2 x 190vac or 370v ac it tests.
My only worry here is that after the diode bridge I'll be up in the 500v dc area rather than 440v dc!!!
The other windings are 60v, 20v and 2 x 6.3v.
I did want to try this txf but now I'm thinking it'll be a problem?

Any thoughts on what I can do?
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 16, 2021, 01:08:29 PM
Chassis welded now!!
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 16, 2021, 02:24:15 PM
For a plexi type 0.68u cathode bypass cap go with film.  But the jtm45 schematic I've seen used 330u. You will have a hard time finding film in that size.  It will also be very physically large and expensive. So you probably need to go electrolytic in that spot.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 16, 2021, 02:46:27 PM
yep spot on John, any ideas on what i should do with getting my B+ to 440v dc?

With my power transformer i can get 370v AC but after diode bridge that would give me 518v!!

Whats a good way to comfortably reduce it 50-60v?
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 16, 2021, 07:15:16 PM
You could string together some high power zener diodes, but  that would still be a decent amount of heat and wasted power to dissipate.  I would suggest just getting a different more appropriate transformer if that's possible.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 17, 2021, 12:41:44 AM
Yeah ok, dumping 80v ac is a bit difficult and tbh I'm not even sure what rating these windings are so could lead to having some hum issues, probably best to just get the proper power txf as you say.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 17, 2021, 04:16:06 PM
this is the third time ive tried to put this post up!!

ive made my own dual cap in the V2 pin 6 position, its only 350v is this going to enough or should i have at least 450v here?
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 17, 2021, 07:12:44 PM
Could you post a schematic? Would make it easier to identify things like V2.

For an output tube plate a 350V cap will almost certainly be too  low.  Possibly acceptable for a preamp tube.

Guitar amps will often use a half wave rectifier with a center tapped power transformer.  You might see something like 700VCT or 350-0-350 speced for a Marshall. In that case peak voltage after rectifier could be viewed as 0.7x of the 700 or 1.4x of the 350, either way around 490V. In practice the loaded transformer will be less than this,  around 1.25 or 1.3x, so you might get 450V with the aforementioned transformer. That's with diodes,  a tube rectifier will drop it a bit more. Note a tube rectifier is half wave,  as are two diodes. A four diode bridge is not as common in guitar tube amps.

So you definitely need high rated caps at the early positions.  Marshall will often use 500V can caps. Some builders will use two 300V caps in series with resistor dividers across them.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 18, 2021, 01:03:06 AM
Yes sorry here is the schematic I'm trying to work off.
Could I put two 350v caps with resistors and resistor dividers after the diode bridge to handle the 520v dc

On this schematic its half wave, but I will be doing full wave bridge rectifier.
Is this the simplest solution?
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 18, 2021, 03:47:37 AM
So hopefully my best bet is to have 2 x 47uf 450v with 2 x resistor dividers after the diode bridge and change that other dual cap for 2 x 33uf 450v?
Then I should be good right?
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 18, 2021, 03:49:29 AM
My other option I thought of was to take out the power txf and output txf from this egnator tweaker 40 amp and see what they are like, then if they are ok use them and if it has a choke in there use that and use the normal config?
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 18, 2021, 04:37:54 AM
I'm working on the fact that there is 125ma current and this wont change.
So adjusting the caps and resistor dividers up until there is current draw to pull the voltage down?
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 18, 2021, 05:09:51 AM
Just to be clear in terminology,  what you had originally posted is a layout, not a schematic

The resistor values are not critical,  just make sure they have sufficient power handling.  220k is fine,  470k is fine.

I'm not sure about the 20k, are you not using a choke?  Most of the current in an amp will go to the power tube plates.  Not that much to the screens and preamp tubes.

Attached is a jtm45 schematic.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 18, 2021, 05:51:12 AM
The only thing I'm worried about is if its 520v dc after the voltage divider it would still be over the 500v rating of the dual cap 500v which is before the choke, that's why I did the second drawing.

So if I understand this correctly putting in the voltage divider in place of the dual cap 32uf 500v before the choke?

Would I then have to adjust anything else?

Is there a drawing or image which I could see which is the correct way?
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 18, 2021, 06:06:14 AM
Look up the SLO100 schematic and you can see one way to do it.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 18, 2021, 04:01:31 PM
This is where I've got to so far in drawing the power section out.
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 18, 2021, 04:37:11 PM
Looks fine. Some notes,  which you already might be planning on.  You also want to fuse the live.  The earth ground should be it's own dedicated chassis connection.  The 220k should be at least 0.5W, really all tube amp resistors should be 0.5W or more.

You will have high plate and screen voltages,  so you need to pick power tubes that can handle it. The preamp tubes will be high voltage as well. Overall this will end up a rather clean amp with headroom.  Cleaner than a typical jtm45.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 19, 2021, 01:00:23 AM
Yes sorry, mains fuse and HT fuse will be used.
Yes to earth connection at IEC socket on chassis.
I'm using all 2w resistors apart from the ones needed to be higher.
I'm using EL34 power tubes, I think they are good up to 800v.
I'm interested in how it will become cleaner than normal jtm45, as I was hoping this would distort nicely.
Could you explain why it would be cleaner?
Is it because of less sag?
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 19, 2021, 01:18:04 AM
I've done some calculations, dont know if they are correct but they do seem consistent to other documented voltages.
B+1 = 518v to CT of output txf
B+2 = 516v to EL34 screens
B+3 = 396v to V3
B+4 = 356v to V2
B+5 = 336v to V1
Title: Re: JTM45 / 50 Build
Post by: john12ax7 on March 19, 2021, 03:24:07 AM
Some of the 6L6 variants are limited to 500V plate voltage or less.  EL34 will be ok,  but be mindful of the screen voltage which still might be limited to 450V, so you need appropriate resistors feeding the screens.

Generally a lower B+ voltage will have earlier breakup.  A typical jtm45 might use 400V, so 500V will have later breakup.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 19, 2021, 03:33:51 AM
I might reconsider and just get a proper txf as I did want this to break up.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 19, 2021, 09:02:32 AM
I think I've located a fairly local company who can wind me a new mains txf, output txf and choke for a good price.
Title: Re: JTM45 / 50 Build
Post by: Walrus on March 19, 2021, 10:01:33 AM
I think I've located a fairly local company who can wind me a new mains txf, output txf and choke for a good price.
Sounds interesting! Can you name names?  :)
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 19, 2021, 10:49:03 AM
Demeter windings in Essex, Keith is the owner and his son Russel.
Give them a buzz.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 19, 2021, 11:05:03 AM
I've gone for a 2 x 345v ct @ 200ma, 6.3v ct @ 6A, 5v @ 3A mains txf, 3H choke and output txf with 4, 8 and 16ohm secondaries.
Title: Re: JTM45 / 50 Build
Post by: Squeaky on March 22, 2021, 11:36:42 PM
I have built one of these amps (from an JTM50 schematic that was a GZ34 valve rectified version). I wouldn't call it a clean amp, it is a pretty classic Marshall sounding amp and rocks if you like that sound. It is probably a bit crunchier that a JTM45 if that makes any sense. Nothing like a Hiwatt though. I think the JTM50 has really high V1 gain (big value bypass cap)? The one I built was compared to an original and the sound of my amp was pretty spot on so I was happy with that.

I am glad to see that you decided to use a fit for purpose power transformer. The power transformer that I used had three different HT taps so you had a choice which voltage you wanted to go for. It was a 50W Marshall type power transformer that tried to cover off on a range of the different HT secondary voltages that had been employed over the years. The company unfortunately shut down recently. I wish I could still play this amp (too loud for me now and I don't gig anymore). It sits around gathering dust.

The only problem I had, which I believe might also have been true to character of the originals, was some feedback/microphony problems when the presence setting was up pretty high. I fixed that using a sheet of aluminium, as a shield, attached to the cabinet (inside) underneath the chassis. The fact that I used a massive cap dangling from the pot with long leads probably didn't help. I think you also need to watch the run of the feedback wire.

As you can see I like using WMF capacitors. These are film/foil polyester capacitors. The same sort of caps would have been found in the originals (probably Mullards).

Looks like I used a veritable lolly scramble of different resistor types. I had a thing for 1W CC for a while. Doesn't seem to be much rhyme or reason to it.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 23, 2021, 04:24:21 AM
Thanks for sharing your build, ive just cut the chassis and hopefully getting a price on these transformers from Demeter windings, im hopefully going to start with this JTM 50 plexi 68 lead style build but may experiment with another turret board and mod up to something like a chupa!!
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 23, 2021, 04:27:37 AM
Here's where I'm at
Title: Re: JTM45 / 50 Build
Post by: Squeaky on March 23, 2021, 06:35:09 PM
Great to see you are even building the chassis. I wish I had the wherewithal to build chassis. However if I did, I'd probably have many more projects lying around! I read earlier in this thread consideration of whether to use large film coupling caps for cathode bypass. I bought this capacitor years ago for that purpose. It is only 20uF, but 80mm x 40mm x 50mm in dimension. Close enough in value to Vox amps though. More of a gimmick than anything else really, who knows what is inside the steel case.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 24, 2021, 02:09:38 AM
It's a paper in oil cap, I have lots of 1uf versions from Russia.
Title: Re: JTM45 / 50 Build
Post by: Squeaky on March 24, 2021, 04:45:38 AM
To be clearer, I guess I was wondering whether it was one cap inside or a few in parallel.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 24, 2021, 12:13:13 PM
I've got no idea what's in them, might cut one of mine open and see!!

Meanwhile, back to the build!!
Have made my own dual cap sockets and mounted them, the iec is installed along with the octal sockets and one 9 pin socket, have to order some more.
Have offered up the turret board to see where I need to mount it.
Pots I've put in along with fuse holders.
Head cabinet I routed edges and gave it a sand, will order tolex at end of the month and hopefully next month this should be all finished and operational!!
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 28, 2021, 03:38:27 AM
Looks a bit better without that big pink dual cap on the turret board.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 29, 2021, 05:25:37 AM
Demeter windings price for all three transformers is £187.20 inc vat and delivery.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 29, 2021, 02:57:38 PM
Starting the wiring now.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 31, 2021, 02:44:52 AM
Starting to look like an amp
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 31, 2021, 02:45:42 AM
Need to start thinking about how to do the front panels
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on March 31, 2021, 02:47:14 AM
Inside
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on April 01, 2021, 02:38:25 AM
Have built the fx loop, I may just try the passive version first of all, then after install this one.
Title: Re: JTM45 / 50 Build
Post by: Spencerleehorton on April 06, 2021, 10:13:38 AM
Few more bits and pieces came in the post today but gonna have to wait 2 - 4 weeks for txf.
Hopefully can get everything ready for them.
I'm interested in buying a variac, they seem to have some on ebay and alibaba, it's a red one with a digital readout, anyone had any experience with these?