Converting to 6973 tubes.

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Trapland
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Converting to 6973 tubes.

Postby Trapland » Mon 01/23/17 12:54 am

Hi all. I think this may be my first post, greetings.

I picked up a Marshall 1974x and like it well enough, but I would like to voice it more to suit my taste.

2 things I'm not liking: First there's a distortion character that sounds fizzy to me, I'm not sure it's the fizz others talk about, but what I'm hearing seems to overwhelm the notes and harmonics and doesn't really change with the note. I suspect it's just that "el84" distortion character, but I don't really like it.

Second the amp has the most bloated, muddy, unmusical midrange/lower midrange. It's way too much. I have a large selection of speakers, none make a real dent in it. I've rolled dozens of tubes too. Nothing significant. I want to start modding.

I've built amps, and can mod safely. It's my amp and I bought it knowing I would re voice it to suit my needs so I don't need someone to tell me to sell it and get something else, it will be something else. The "bones" of this amp are why i bought it.

Please I am open to suggestions regarding

1) greatly reducing midrange, possibly farther downstream.
2) converting to 6973. I love the sound of these tubes, I love how they break up and I love their overall frequency response. They are tough as nails and production is good.
I know the pin outs are very different, but the voltages LOOK ok to just wire up and go. Anything I really need to change?


Thank you for any ideas.
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Daviedawg
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Re: Converting to 6973 tubes.

Postby Daviedawg » Mon 01/23/17 3:27 am

I don't know about the 6973. But I have tackled the fizz with the Paul Ruby modification. Successfully. Just so you know it is an option.

Regarding the mid range issue. Do you have a schematic? I assume it is very similar to the original trem but there may be component value differences. It may be a case of experimenting with cap values such as the output valves cathode bypass until you hear what you want. I am sure you will get suggestions from the audience here.

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JMPGuitars
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Re: Converting to 6973 tubes.

Postby JMPGuitars » Mon 01/23/17 9:11 am

Is it actually a Marshall 1974X? 1974X is the reissue model. By "picked up" I assume you bought it used? You should check the specific model # and manufacture date if you can.

As DD said, you should start with the schematic that applies to the specific variation you have. I would also suggest including the voltages as it is now to make sure nothing else is wrong.

If everything is as it should be with the amp, what you're describing shouldn't be an issue. If the amp is old enough, your filter caps could need to be replaced. GOOD electrolytics are rated for 2000 hours. That's about 30 minutes or so of playing daily for 10 years. Do the math, a more serious player could have ran up 2000 hours much faster.

Regarding midrange, did you try with a Greenback (or clone)? An accurate schematic of what your amp currently has in it would be needed for any of what you're talking about. If the amp was modded at some point, values may have changed. It would be wise to get a schematic and do your best to verify the values in your circuit before doing too much.

The 6973 pinout is much different than the EL84, and you will likely need to rebias as well to compensate for the tube change. Here's a datasheet including the pinouts: http://tubedata.milbert.com/sheets/049/6/6973.pdf

All that being said, if it is a used 1974X, I would suspect the filter caps age first, and share the voltages using the voltage chart in the download section. Another thing that's common with the 1974X is people tend to upgrade the transformers with great results.
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Trapland
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Re: Converting to 6973 tubes.

Postby Trapland » Mon 01/23/17 1:26 pm

JMPGuitars wrote:Is it actually a Marshall 1974X? 1974X is the reissue model. By "picked up" I assume you bought it used? You should check the specific model # and manufacture date if you can.
I know you don't know me, but isn't it sort of insulting to assume I don't even know what amp I bought?

It is a Marshall 1974x. It was built at the end of 2013. It's broken in but not old. It came dead stock.
JMPGuitars wrote: As DD said, you should start with the schematic that applies to the specific variation you have. I would also suggest including the voltages as it is now to make sure nothing else is wrong.

If everything is as it should be with the amp, what you're describing shouldn't be an issue. If the amp is old enough, your filter caps could need to be replaced. GOOD electrolytics are rated for 2000 hours. That's about 30 minutes or so of playing daily for 10 years. Do the math, a more serious player could have ran up 2000 hours much faster.
Yup. Voltages are fine. Filter caps are fine. I can read a schematic and have for this amp. Values are correct, but don't seem to suit my taste.

To be more clear about what I COULD do, I could build this amp in a couple afternoons, or a JMP or a JCM, or a 5F4, but I'm new to this circuit and hoped someone could save me the time with overall voicing changes.

And I will try the 6973s. Yes I know I could have built this amp for a fraction of the price of a Marshall, I wanted a Marshall. When I move it along, it will probably be restored to stock, but I bought it for its small size, light weight, low power. There is no other simple vintage style Marshall in this form factor. I want to tune it to my taste and get gigging.
JMPGuitars wrote: Regarding midrange, did you try with a Greenback (or clone)? An accurate schematic of what your amp currently has in it would be needed for any of what you're talking about. If the amp was modded at some point, values may have changed. It would be wise to get a schematic and do your best to verify the values in your circuit before doing too much.
I have a collection of just about every popular Celestion made from 1968 through the early eighties. I also have a 1974cx cabinet. I have tried most of them. Yes the amp and cab comes with the "magical" aged 20 watt greenback. Ewww.
JMPGuitars wrote:
The 6973 pinout is much different than the EL84, and you will likely need to rebias as well to compensate for the tube change. Here's a datasheet including the pinouts: http://tubedata.milbert.com/sheets/049/6/6973.pdf

All that being said, if it is a used 1974X, I would suspect the filter caps age first, and share the voltages using the voltage chart in the download section. Another thing that's common with the 1974X is people tend to upgrade the transformers with great results.
Thanks for those ideas. It's way way way easier to change values in the circuit before tearing apart the whole amp to change transformers, etc. plus it's really easy to restore. Transformer changes are more like swatting a fly with a sledgehammer. I WILL use the sledgehammer if I need to, but let's try the easier stuff first. Circuit changes above the board, change a dozen wires and a cap or resistor and try different tubes.
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Re: Converting to 6973 tubes.

Postby JMPGuitars » Mon 01/23/17 10:49 pm

Trapland wrote: I know you don't know me, but isn't it sort of insulting to assume I don't even know what amp I bought?

It is a Marshall 1974x. It was built at the end of 2013. It's broken in but not old. It came dead stock.
Lol! Dude, read through the site. Or youtube. Or the internet. 8 out of 10 people that say 1974X think the classic 1974 is called 1974X. Don't take it personally, I was just making sure (and you didn't supply any info besides the number). ;)
Trapland wrote: Yup. Voltages are fine. Filter caps are fine. I can read a schematic and have for this amp. Values are correct, but don't seem to suit my taste.

To be more clear about what I COULD do, I could build this amp in a couple afternoons, or a JMP or a JCM, or a 5F4, but I'm new to this circuit and hoped someone could save me the time with overall voicing changes.

And I will try the 6973s. Yes I know I could have built this amp for a fraction of the price of a Marshall, I wanted a Marshall. When I move it along, it will probably be restored to stock, but I bought it for its small size, light weight, low power. There is no other simple vintage style Marshall in this form factor. I want to tune it to my taste and get gigging.
That's cool. We ask for voltages so we can give our opinion on them. Without us seeing them, we can't give a ton of insight into the circuit's behavior. Without a link to your specific schematic, same deal. For people to help, we need info. Otherwise all we can provide is general ideas. Did you compare the voltages to the chart on this website?
Trapland wrote: Thanks for those ideas. It's way way way easier to change values in the circuit before tearing apart the whole amp to change transformers, etc. plus it's really easy to restore. Transformer changes are more like swatting a fly with a sledgehammer. I WILL use the sledgehammer if I need to, but let's try the easier stuff first. Circuit changes above the board, change a dozen wires and a cap or resistor and try different tubes.
Honestly, I would say change just the output transformer first regarding sledgehammering. Then it's only a few wires, and it should have a nice impact on the tone. Heyboer and ClassicTone both sound great and are reasonable. You might find it's still affecting the tone even with the tube change.
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Re: Converting to 6973 tubes.

Postby GlassTornado » Tue 01/24/17 11:27 am

I know you don't know me, but isn't it sort of insulting to assume I don't even know what amp I bought?
With two posts, I can't imagine that anyone does know you. It's not insulting with the amount of "internet smart" folks out there.

It sounds like you already have your heart set on 6973 tubes.
Convert it. Bias it.
JMPguitars gave you the data sheet with the pin out.

Once you've done that, maybe then you can tackle the mid-range thing.
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Re: Converting to 6973 tubes.

Postby CurtissRobin » Tue 01/24/17 1:52 pm

Please I am open to suggestions regarding

1) greatly reducing midrange, possibly farther downstream.
2) converting to 6973. I love the sound of these tubes, I love how they break up and I love their overall frequency response. They are tough as nails and production is good.
I know the pin outs are very different, but the voltages LOOK ok to just wire up and go. Anything I really need to change?
Welcome aboard. It's unusual that your amp emphasizes the mids. The 18 watt is most often accused of being over treble-y, sometimes ice picky, depending on the speaker(s). In any case, the top three influences on the sound of the 18 watt amp are (1) the circuit, (2) the speaker(s) and (3) the OT. Many builders would argue that 2 and 3 should be swapped. You've tried various speakers, so that's answered. Swapping the OT can be pushed down the road, though I've not read anyone praising the reissue OT. It's not bad, consensus says it's just OK. I doubt the OT inordinately emphasizes mids anyway. That takes us to voicing the amp.

There's been volumes of discussion on this forum about voicing and I strongly recommend browsing at length. If you have spare tubes you can swap in and out, that'll often help. The cap values at the tone control will be the most likely tweaking site.

This (http://18watt.com/storage/18watt%20Sche ... 3%20-3.pdf) is our current reference schematic, arrived at in the course of comparing originals, reissues, mods et al. I'd start any project on your amp by comparing it against this diagram, particularly part values. Where you see differences, let us know so we can understand and help. Also in the downloads section is a table form you can use to record voltages for posting. Simply stating that "Voltages are fine." tells us nothing we can use to analyze the amp. A build with 360V on the EL84 plates is fine but it sounds really different from a build with 340V or 345V, these being the B+ sweet spot for this amp.

As to swapping to a different output tube, rock on! I have an 18w with 6V6 power and several other types have been used by other builders. Rewiring the sockets will be easy given the experience you say you have. You'll need to tweak values for the plate and cathode resistors and maybe cathode and coupling caps but that's a lot of the fun of building and modding.
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Trapland
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Re: Converting to 6973 tubes.

Postby Trapland » Tue 01/24/17 11:28 pm

@CurtissRobin Thank you! I found Marshall using 470 microfarads on the el84s cathodes of my amp. That particular schematic helped as it showed a range. I made a big change and it worked miracles on the muddy lower midrange. 10 minutes to pull the chassis, tone test a cap change and stuff it back in the box. Best 10 minutes I've spent with this amp, it made me finally want to play it.

Sounds so good I almost like el84s.....naw just kidding, gotta have the 6973s.. :)

I registered here weeks ago and have been browsing since but I'll confess, i didn't look very closely at the schematics here. There's so little description of what the schematics are about, i just thought they were all individual peoples favorite custom builds.

I mostly signed up here because I saw some web browser reference to using 6973 in an 18 watt in this site. I could not read the site without registering. I still intend to try the 6973, but I can't find anyone that has done it. Unfortunate, as I could have used some guidance on setup.

I always have known the pinouts are different. I have really low plate voltages and the 6973 handle more than el84 so maybe the cathode resistor is all I need to change? Maybe an old Supro schematic will help, but I had hoped to find reports ON THIS amp here.
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Re: Converting to 6973 tubes.

Postby CurtissRobin » Wed 01/25/17 12:23 am

I have really low plate voltages and the 6973 handle more than el84 so maybe the cathode resistor is all I need to change? Maybe an old Supro schematic will help, but I had hoped to find reports ON THIS amp here.
I just looked up the 6973 in my notes. Its characteristics are similar to the 6V6 with even higher plate voltage capability so it should be a great substitution. I sure like my 6V6 18 watt. That is, I like it when I can get it back from the friend who gigs with it all the time. He owes me big time!

The fact that the 1974X is a reissue is supposed to make things easier but what we've learned over the years is that the original schematic diagrams don't entirely match the original amplifiers. The factory either didn't bother proofreading the drawings or they were busy tweaking during production and didn't update as they went. Likely enough, both are true. Anyway, where they drove a stake on the reissue design is in your hands, now. On our Downloads page check out the first heading, "Classic Marshall 18W" because in there there are diagrams made by tracing out a couple actual production amps from the original run in the '60s. Colin's is one and Ian's is the other. Actually, Ian is the source of the schematic I linked earlier.

Changing your power stage cathode cap is one of our most common tweaks. A lot of us went all the way to 1000uF to get what we were after. There were reports of happy places at 50 to 100uF and at 1000uF but muddiness around 250-330uF. Curious how the middle ground isn't always a happy place. That's kind of what you found, isn't it?

There are a few other tube types people have tried and recommended such as 6CM5, 6AQ5 and 6N14N.
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Re: Converting to 6973 tubes.

Postby Trapland » Wed 01/25/17 1:43 am

It looks like I can just rewire the sockets and change the cathode resistor as a starting point. I assume 250 ohm is a starting range but I did see a schematic call for 250k. I hope that was a typo.

Anything I may have missed that must be done for tube safety or is it all seasoning after that?
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Re: Converting to 6973 tubes.

Postby Daviedawg » Wed 01/25/17 3:24 am

Good news Trapland. I have an even bigger cathode cap in my 18 watt which is why my original response suggested you look at the schematic with that in mind. It is a simple option but my purpose instead was to get better bass response to suit my Celestion V10s. So even the 18 watt can be modded to suit circumstances.

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Re: Converting to 6973 tubes.

Postby geoff 1965 » Wed 01/25/17 7:06 am

hello trapland,
you say you have really low plate voltages,so why swap to tubes that run higher plate voltages? won't you find it hard to find the B+ sweetspot?
Curtiss says the el84 sweetspot is around 340-345 and most people think around 360 for the 6V6.what is your B+?
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Re: Converting to 6973 tubes.

Postby Trapland » Wed 01/25/17 12:15 pm

geoff 1965 wrote:hello trapland,
you say you have really low plate voltages,so why swap to tubes that run higher plate voltages? won't you find it hard to find the B+ sweetspot?
Curtiss says the el84 sweetspot is around 340-345 and most people think around 360 for the 6V6.what is your B+?

Ah yes. The sweet spot for EL84 perhaps. Or maybe for el84 in this design. Capable of higher voltage and requiring it are very different. But I am of the opinion that lots of voltages (in the PA) can sound good, maybe different, but good. Off the top of my head, 2 of my favorite Marshalls both running Siemens el34 run 340 and 485 VDC on the plates.

My plate voltage with no signal is 303, lol. Having said that, I have no intention of running el84s, but if y'all want to do the "fer instances" assuming el84, it could be informative.
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Re: Converting to 6973 tubes.

Postby geoff 1965 » Wed 01/25/17 9:08 pm

this will be interesting! so keep posting your progress so we can see,does the 6973 have another code? i can't find any reference in my tube data manual but it is the 1970 edition!
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Re: Converting to 6973 tubes.

Postby Trapland » Thu 01/26/17 4:07 pm

geoff 1965 wrote:this will be interesting! so keep posting your progress so we can see,does the 6973 have another code? i can't find any reference in my tube data manual but it is the 1970 edition!
I'm curious too. I think someone posted a link to the datasheet farther up this thread. I think they were first marketed in 1959 which was late to the party, so they weren't likely a consideration in England when the 18watt was in production.

Probably the most famous use of the 6973 tube in guitar amps was Supro from about 1960 till the end (1964?). And the reissues of course. It's interesting that the Supro model 1624 has so many similarities to the 18 watt Marshall.

I know there's guys here that would rather have fun fixing this EL 84 amps inherent fizz problem. I love studying all that they've done and commend them for it.

To my way of thinking, right or wrong it's an el84 problem. I've never cared for the tone of overdriven el84s in any amp. Since there's so much evidence that using different power tubes solves it in this amp design I would rather do that.

I've never built or converted using 6973, so I'm greatful for the 18watt.com depths of experience. Maybe I'll have something fun so share when it's done. I just had a back procedure, so I may need to heal a bit before I dig in.
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