Tube Depot JTM45+ mods, KT88s, split/shared cathodes, safety, and transformer questions

[asd123asd234sdrf]

Hi all,

I recently posted a question about which amp/kit to get, and I think I've narrowed it down to the Tube Depot JTM45+. Initially I thought I wanted a true JTM45 replica, and was turned off by this kit. Then I realized it might be exactly what I want, as I am more chasing a Zeppelin-type tone, and not the normal JTM45 tone. I've been going through the schematic, and it seems to have more in common with later Plexis or maybe the JTM50, which is likely the tone I want anyway.

That said, I have some questions about the circuit as provided by Tube Depot. I plan on building it bone stock initially, then playing with it, as half the fun for me is building something myself and then being able to mod it. I also will be building the head version and cranking it into a Captor X for headphone use. I know this limits me somewhat, but thems the breaks in an apartment.

1. What is the consensus on the ClassicTone transformers that come with the kit, and is there any need/benefit to looking into upgrading them?

2a. If I wanted to run KT88s, what type of transformers would I be looking for? I email Tube Depot, and they said I would definitely need beefier ones.

2b. What other mods would be needed to run KT88s? I'm guessing some resistor changes to get them to bias properly?

3. The Tube Depot circuit has a master volume before the PI. Is it worth migrating to a PPIMV, and if so, which type?

4. From my limited understanding, the original JTM45 had a shared cathode between V1a and V1b. The Tube Depot circuit has a split cathode. Is the split cathode the configuration I want, given my tonal goals, and is it possible/worth putting it on a switch?

5. Are there any existing safety issues in the way the Tube Depot amp is laid out? I've seen people mention that some layouts had issues (I'm thinking the MetroAmp JTM45?), but I don't have the expertise to identify them.

6. Does anyone have any mods they've done to their JTM45+ that they really liked?

7. Is reverb or an effects loop a realistic option for this circuit?

Thanks for reading my wall of text, and for in advance for any answers or input!

 

[playloud]

I'll shoot:

Hi all,

I recently posted a question about which amp/kit to get, and I think I've narrowed it down to the Tube Depot JTM45+. Initially I thought I wanted a true JTM45 replica, and was turned off by this kit. Then I realized it might be exactly what I want, as I am more chasing a Zeppelin-type tone, and not the normal JTM45 tone. I've been going through the schematic, and it seems to have more in common with later Plexis or maybe the JTM50, which is likely the tone I want anyway.

The switchable rectifier will be your friend here. A tube rectifier is a nice sound, but I predict you're going to want the SS rectifier to get Zeppelin tones (it will tighten things up).

Also interesting to use a JTM45 PT (i.e. 345-0-345 HT winding), as you'll have higher B+ than a later JTM50/JMP50-spec PT. That will get you closer to a 100W sound, I believe - and therefore JP.

That said, I have some questions about the circuit as provided by Tube Depot. I plan on building it bone stock initially, then playing with it, as half the fun for me is building something myself and then being able to mod it. I also will be building the head version and cranking it into a Captor X for headphone use. I know this limits me somewhat, but thems the breaks in an apartment.

This sounds like a great setup! I agree, being able to mod it yourself is part of the fun.

1. What is the consensus on the ClassicTone transformers that come with the kit, and is there any need/benefit to looking into upgrading them?

Are you sure the transformers are ClassicTone? They've been out of business for a few years now and the PT/OT pictured appear to be Heyboer (based on stickers). The choke does indeed appear to be CT - but I wouldn't be surprised if you end up getting a Heyboer/Hammond in practice, unless they have a stash of old chokes sitting around.

2a. If I wanted to run KT88s, what type of transformers would I be looking for? I email Tube Depot, and they said I would definitely need beefier ones.

2b. What other mods would be needed to run KT88s? I'm guessing some resistor changes to get them to bias properly?

The supplied transformers should be fine. The schematic claims that the 6.3V heater winding is spec'ed for 5A (standard), and KT88 heater requirements are only marginally greater (1.6A) than EL34 (1.5A). Since 2*1.6 + 3*0.3 (ECC83s) = 4.1A, you'll be cushdy (as they say in the UK).

Btw, Marshall (well "Park") used 50W iron with KT88s themselves! (Cf. Park 75)

The only thing you might need to play around with is R25 in that schematic, to achieve proper bias with KT88s (note that KT88s have a max plate dissipation of 35W). I see that this is already mentioned in the schematic for other tube types (6L6 etc.) I would suggest having extra 56k and 68k resistors on hand.

3. The Tube Depot circuit has a master volume before the PI. Is it worth migrating to a PPIMV, and if so, which type?

I would ditch the MV personally. You already have the Captor X, which is a better solution.

4. From my limited understanding, the original JTM45 had a shared cathode between V1a and V1b. The Tube Depot circuit has a split cathode. Is the split cathode the configuration I want, given my tonal goals, and is it possible/worth putting it on a switch?

This gets back to the debate about which Marshall JP used. Shared cathode is "bass spec" (including Super Bass) and split is "lead spec" (i.e. 1987/1959 amps after '68).

Try for yourself and see which you prefer! (Btw, you may also want to switch R2 for 820R if you prefer split.)

5. Are there any existing safety issues in the way the Tube Depot amp is laid out? I've seen people mention that some layouts had issues (I'm thinking the MetroAmp JTM45?), but I don't have the expertise to identify them.

It has a dedicated grounding point and live > fuse > switch mains wiring. The switchable rectifier arrangements looks like it should do the job, and bias supply is unaffected by standby switch (good).

The grounding doesn't look ideal, but then it wasn't on the originals either. Shielding cables on inputs look unnecessary to me (it's not a particularly high gain amp), as do back surge diodes on power tube anodes. The way they've used pin 6 on the power tubes is unusual, but should work (perhaps others have thoughts/experience with this?)

The switchable tonestack will give you options, and the 22nF PI coupling caps will reduce mud (although I would also experiment with stock 0.1uf values).

Those are just my initial thoughts, and others may have more informed opinions.

7. Is reverb or an effects loop a realistic option for this circuit?

Since you have the Captor X, I'd give this a miss too (but then I'm not a big FX user).

 

[_Steve]

That schematic is definitely more JTM50 than JTM45. I think its a good choice for you!

Echoing what @playloud said about the rectifier - I also really prefer the feel of solid state vs tube. You could consider building as per the schematic and then using one of these instead of a switch: https://www.tubedepot.com/products/solid-state-rectifier . Be careful of the bump in B+ however (but you might also prefer that too).

I've built a couple of variations from that era now and my preferred approach to the tone stack is to have BOTH the slope resistor and treble cap switched on one Pull-switch Potentiometer. I like it on the Mid pot as that makes the most sense to me. You're basically switching between Fender and Marshall spec tone stacks.

If you go down the KT88 route my advice would be to make sure you still have a way to also run EL34s (or KT77s which raawwwk in these). There's always the chance you might not like the KT88s as much as you think you will. IE don't commit yourself in solder

Good luck, and post updates!

 

[asd123asd234sdrf]

Thanks @playloud ! I really appreciate the detailed response, and you really helped answer a lot of my questions. However, we all know no good deed goes unpunished, so I have a few more for you based on your reply

Are you sure the transformers are ClassicTone?

I am only going by the provided BOM. Presumably they will be another brand. Assuming they are Heyboer or Hammond, are those decent enough? And do you happen to know how difficult it is to swap them in a completed amp? I'm not necessarily shooting for the most high end iron I can get, but at some point I'll probably want to upgrade everything as much as possible. The DIY/mod bug bites lol.

you might need to play around with is R25 in that schematic

R25 seems to control the bias of the power tubes. Would it need to go up or down in value? You mention keeping some spare 56k and 68k on hand, is that because R25 could burn up, or just because those would be the likely values for biasing?

I would ditch the MV personally.

I hadn't thought about this, but I totally could. If I build the amp stock, and then just turn the MV all the way up, would that be electrically equivalent to removing it? I assume removing it is simply removing the pot and connecting the wiper of VR3 directly toC12, but I'm not at all sure of that.

you may also want to switch R2 for 820R if you prefer split.

What would be the intention behind this, more gain? And is the intention with split cathode not to have 2 different values and therefore different tones?

The grounding doesn't look ideal, but then it wasn't on the originals either.

Is it possible/advisable to improve the grounding without completely changing the layout?

Thanks again for your reply and your time!!

 

[asd123asd234sdrf]

Thanks for your thoughts @_Steve ! Good call on not committing to the KT88s in solder, I hadn't considered that but I will going forward. I will also give KT77s a try as well!

I also hadn't seen the SS tube drop-in thing before, I will definitely be giving that a try. What exactly does being careful of the bump in B+ mean in this case? I get that it will increase the voltage on the plates, presumably making the tubes work harder and therefore more likely to die sooner?

Thanks again!!

 

[_Steve]

Thanks for your thoughts @_Steve ! Good call on not committing to the KT88s in solder, I hadn't considered that but I will going forward. I will also give KT77s a try as well!

I also hadn't seen the SS tube drop-in thing before, I will definitely be giving that a try. What exactly does being careful of the bump in B+ mean in this case? I get that it will increase the voltage on the plates, presumably making the tubes work harder and therefore more likely to die sooner?

Thanks again!!

Its close to exceeding the limits of the filter capacitors. And now looking closer, I see on the schematic they have kinda cut corners by only having single 500V caps in the plates and screens nodes. Marshall, and others use 2 caps in series for each of the first 2 nodes to increase the rating. The schematic notes 480VDC at those nodes - ideally we really need to know if that's with the tube or solid state rectifier. If it were me I'd use 2x100uF (or for my taste 2x64uF) in both those nodes.

 

[asd123asd234sdrf]

Its close to exceeding the limits of the filter capacitors. And now looking closer, I see on the schematic they have kinda cut corners by only having single 500V caps in the plates and screens nodes. Marshall, and others use 2 caps in series for each of the first 2 nodes to increase the rating. The schematic notes 480VDC at those nodes - ideally we really need to know if that's with the tube or solid state rectifier. If it were me I'd use 2x100uF (or for my taste 2x64uF) in both those nodes.

Thanks @_Steve! So how might I go about adding another cap in series with the existing ones? I think we're talking about C21a/b and C22a/b right? Those are the big can caps, so I don't think I can just slap one in there without drilling holes and dealing with mounting it. I guess I could look for higher rated can caps? Though Tube Depot at least doesn't have any greater than 500V.

 

[playloud]

I am only going by the provided BOM. Presumably they will be another brand. Assuming they are Heyboer or Hammond, are those decent enough? And do you happen to know how difficult it is to swap them in a completed amp? I'm not necessarily shooting for the most high end iron I can get, but at some point I'll probably want to upgrade everything as much as possible. The DIY/mod bug bites lol.

I would ask before ordering so you know for sure. Heyboer have a great reputation - they offer a wide range of iron at good prices, and seem to be fairly true to overall specs - although I think I'm one of the few that has had a bad experience with them (won't bore you with the details).

If you want a truer replica (layer wound etc.), I would look to Soursound or Merren (if you can get hold of him).

R25 seems to control the bias of the power tubes. Would it need to go up or down in value? You mention keeping some spare 56k and 68k on hand, is that because R25 could burn up, or just because those would be the likely values for biasing?

It's just to get the bias value right. Nothing more frustrating than turning the trim pot and finding you can't reach the ideal bias current!

I hadn't thought about this, but I totally could. If I build the amp stock, and then just turn the MV all the way up, would that be electrically equivalent to removing it? I assume removing it is simply removing the pot and connecting the wiper of VR3 directly toC12, but I'm not at all sure of that.


What would be the intention behind this, more gain? And is the intention with split cathode not to have 2 different values and therefore different tones?

More bass response (as was done in '68). You'll still have significantly different tones between 820R/330uF and 820R/0.68uF.

Is it possible/advisable to improve the grounding without completely changing the layout?

Thanks again for your reply and your time!!

No problem! Yep, it's easy. There are a couple of popular methods:

1. A single 0V reference (in addition to safety earth), which is theoretically elegant. See this Modulus JTM45 layout for an example: https://www.marshallforum.com/threads/jtm50-build.130877/post-2348046
2. "Larry grounding", which has a few variations (let me know if you want details) and has been used on many quiet builds in practice.

 

[playloud]

Its close to exceeding the limits of the filter capacitors. And now looking closer, I see on the schematic they have kinda cut corners by only having single 500V caps in the plates and screens nodes. Marshall, and others use 2 caps in series for each of the first 2 nodes to increase the rating. The schematic notes 480VDC at those nodes - ideally we really need to know if that's with the tube or solid state rectifier. If it were me I'd use 2x100uF (or for my taste 2x64uF) in both those nodes.

482VDC sounds about right for SS, since sqrt(2)*345 ~ 488 (GZ34 would be lower).

Honestly, I think you'd be fine with single 500V cans. If your mains voltage exceeds the rated voltage (which looks to be 120V), then you could exceed 500V, but any decent modern cap should have some wiggle room.

Cans in series was always a 100W thing, anyway. JTM45 uses two halves of a single can to supply each of mains and screens (stock value would be 32+32uf I believe, but they've bumped it up to 50+50uf here, i.e. 50uf each).

Note: if using the GZ34, I'd be wary about going much higher than 50uf (although that should be ample).

Thanks @_Steve! So how might I go about adding another cap in series with the existing ones? I think we're talking about C21a/b and C22a/b right? Those are the big can caps, so I don't think I can just slap one in there without drilling holes and dealing with mounting it. I guess I could look for higher rated can caps? Though Tube Depot at least doesn't have any greater than 500V.

You'd tie both halves of each 50+50uf can together, for 100uf total (sections are in parallel), then put these in series for 50uf overall (caps in series obey the reciprocal rule: 1/C = 1/C_1 + 1/C_2; if C_1=C_2, then you get C = C_1/2). The overall voltage handling capability is roughly double the handling of each individual cap, so 1000V in this case.

Note I really don't think this is necessary though (see above).

 

[_Steve]

Thanks @_Steve! So how might I go about adding another cap in series with the existing ones? I think we're talking about C21a/b and C22a/b right? Those are the big can caps, so I don't think I can just slap one in there without drilling holes and dealing with mounting it. I guess I could look for higher rated can caps? Though Tube Depot at least doesn't have any greater than 500V.

What @playloud says is correct - you're probably fine