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Marshalls...

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Re: Marshalls...

Postby Sam Spoons » Tue Aug 25, 2020 3:06 pm

Not quite the same spec* but this would be well worth considering if you are up for a bit of self build (I did,** and it was not difficult, just take heed of the warnings about working on valve amps and the high voltages they use). Quite a bit cheaper*** and UK based so you'll save on shipping, duty and 20% VAT which will be added to the Mars kit/amp.

That said, the difference between the kit and the amp chassis built up is only about £40 which seems ridiculously cheap. Even though a pro build would probably be a fair bit quicker it'd probably still cost you at least £120 for somebody to assemble it for you in the UK.

I love mine, it's been the only guitar amp I use since I built it about 4 or 5 years ago

https://www.ampmaker.com/shop/p1800-18w-vintage-plexi-amplifier/

* The mapmaker kit lacks the valve rectifier.

** I actually built the PP-18 which is slightly less complex, it took about 10-12 hours and worked first time.

*** £308 including UK shipping, no VAT as he is no longer VAT registered.
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Re: Marshalls...

Postby Sam Spoons » Tue Aug 25, 2020 3:15 pm

Just to add these 18 watt amps are all based on the WEM Dominator design.
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Re: Marshalls...

Postby ef37a » Tue Aug 25, 2020 3:27 pm

Sam Spoons wrote:Just to add these 18 watt amps are all based on the WEM Dominator design.

Ah! Something I know, I have a Dominator looky-likely, actually a Mersey Super 15 and it is cathode biased with no NFB.

Not that hard to add a negative bias supply and get the anode current down to 20mA* or so. Runs the valves a lot cooler and as said before, beefs up output. NFB is a trickier matter. Get it dead wrong and the amp takes off. Mind you, being valves they will survive the few seconds it takes to kill the power...transistors? POOF! in mu seconds. Even if stable you only want the merest whiff of NFB or you can get squegging on heavy drive.

*Can't remember what 84s bias to, EL34 'we' always set to 25mA per valve. Will look it up.

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Re: Marshalls...

Postby Hewesy » Tue Aug 25, 2020 4:24 pm

ef37a wrote:I am ABSOLUTELY no kind of expert on Marshalls but I would say that THE marsh' tone is EL34s and fixed biased?

That kit is EL84s and cathode bias and that is a Vox AC15 line up. Can you get a schematic out of them before you buy? I seem to recall Marshalls used a bit of negative feedback? Vox did not.

Dave.

It'll be an 1958X clone Dave, supposedly "half" a Bluebreaker but running EL84's not 5881like the BB (which was JTM45 based?).

Essentially the 1958X is Watkins/WEM Dominator circuit/copy but modded, it has come across my radar being more accessible than a BB or even the heads (and it have valve trem for the win) but the EL84'S put me off to as that's basically my Laney.

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Re: Marshalls...

Postby ef37a » Tue Aug 25, 2020 5:44 pm

Never been a fan of the EL84. They run blinking hot and although they seem to last (well, GOOD ones!) much longer than you would think I would much rather a pair of 6V6s. The latter were used in radios and Bell&Howell projectors and seemed last forever!

You could even use a pair of 'detuned' El34s in one of those kits. Strictly speaking the anode loading would be wrong but since when did guitar amp manufacturers fuss about such niceties?

Life is throwing me some strange curves at the moment but in a year or so I MIGHT be able to get back into messing with valve amps at home. Just hope the eye and other 'bits' hold up!

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Re: Marshalls...

Postby SecretSam » Tue Aug 25, 2020 5:50 pm

OK that was me misleading you all by misinterpreting 'British invasion' in the marketing blurb.

Their Marshally thing costs about 30% more, is 30w and harder to build:

"A classic Plexi Amplifier. More than 30watts of power from two KT66’s
GZ34 rectifier tube
This amp kit is virtually a clone of the famous 1960’s amplifier that was widely used by Brit rockers.
This build is more complex than the other kits listed above. If you are uncertain if you’ll manage, book a seat at our next DIY workshop, and we’ll show you how."
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Re: Marshalls...

Postby SecretSam » Tue Aug 25, 2020 5:55 pm

Sam Spoons wrote:Not quite the same spec* but this would be well worth considering if you are up for a bit of self build (I did,** and it was not difficult, just take heed of the warnings about working on valve amps and the high voltages they use). Quite a bit cheaper*** and UK based so you'll save on shipping, duty and 20% VAT which will be added to the Mars kit/amp.

That said, the difference between the kit and the amp chassis built up is only about £40 which seems ridiculously cheap. Even though a pro build would probably be a fair bit quicker it'd probably still cost you at least £120 for somebody to assemble it for you in the UK.

I love mine, it's been the only guitar amp I use since I built it about 4 or 5 years ago

https://www.ampmaker.com/shop/p1800-18w-vintage-plexi-amplifier/

* The mapmaker kit lacks the valve rectifier.

** I actually built the PP-18 which is slightly less complex, it took about 10-12 hours and worked first time.

*** £308 including UK shipping, no VAT as he is no longer VAT registered.

Thanks Lord Spoons. That's a good recommendation.
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Re: Marshalls...

Postby ef37a » Tue Aug 25, 2020 6:25 pm

Found some data on fixed biasing 84 and, Ooo! Interesting
https://frank.pocnet.net/sheets/084/e/EL84.pdf

It seems two valves will deliver 17W for 300V on anodes at 4% THD which is naff all for guitar amps and you should be able to push that well over 20W.
The very good news is that -14.7V on the grids gives an Ia of just 7.5mA and thus a Pa of around 2.5W meaning even the minging modern valves should last a long time.

I wonder why more amp makers have not done this (they don't do it for 6V6 either and they should deliver similar results) ? Maybe fixed B 84s sound BAAAAD!

Hmm? I have read that there are a few guitar amps that produce a decent and loud overdrive sound but cannot do 'clean' at any volume. A very coldly biased amp mayhap?

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Re: Marshalls...

Postby Hewesy » Tue Aug 25, 2020 6:35 pm

Interesting, I've a 5w Kustom head which cannot do clean at all.

1x 12AX7 and 1x EL84 IIRC, nice amp but not a "daily" for me.

I did fit a 12AT7 which calmed the drive but still not actually clean.

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Re: Marshalls...

Postby Sam Spoons » Tue Aug 25, 2020 6:43 pm

My 18 watt doesn't really do 'clean' in the Fender Twin sense but it does do a decent 'clean with just a little edge' which works well enough for my needs and is reasonably loud driving a V30.
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Re: Marshalls...

Postby ef37a » Tue Aug 25, 2020 8:38 pm

Hewesy wrote:Interesting, I've a 5w Kustom head which cannot do clean at all.

1x 12AX7 and 1x EL84 IIRC, nice amp but not a "daily" for me.

I did fit a 12AT7 which calmed the drive but still not actually clean.

Hewesy

Well it orta! I suspect they designed it to give a distorted sound because, 1) that is what they think everyone wants and 2) distorted sound always give the impression of greater volume.
OR! Maybe they just don't know how to design a linear valve amp.

Not hard. There was a very famous Mullard design, the 3-3 that produced 3 watts at hi fi distortion levels(ok yes, it used an EF86 not an AX7 but could be done) The EL84 and ones section of the triode could be given a bit of NFB and the result deliver 4 or 5 watts at lowish distortion* and that would easily be loud enough through a 100dB/w/mtr speaker in a pub sans drummer.

After all, there have been millions of radios and TV sound stages made with a single 12W valve output and these were quite clean and loud enough to cause social rifts! Even though the speakers were at least 10dB less sensitive.

*And for guitar you are not trying to deliver clean tones at 40Hz which is where transformers show their problems unless VERY expensive.
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Re: Marshalls...

Postby Folderol » Tue Aug 25, 2020 8:45 pm

ECL86 comes to mind. A pair make a complete amp with push-pull output.
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Re: Marshalls...

Postby ef37a » Tue Aug 25, 2020 9:00 pm

Folderol wrote:ECL86 comes to mind. A pair make a complete amp with push-pull output.

Cracking valve Will! The ECL82 was good as well, slightly less power. The other numbers, the 83s and 84 not so reliable. The ECL83 in particular was very compact and ran very hot.

I cannot understand the present rave for single ended amps? Ok, if you want to market something really cheap and bit nasty, a double T and 12W pentode, VC and top cut pot and you have a 'practice' amp but for more than 5 watts or so, go push pull. Cost? Nah, the OP traff can be much smaller and cheaper. Standing current is lower so the power transformer can be a tad smaller. Smoothing caps can have lower values because the OP stage 'hum bucks' I dare say also that a pair of 6V6s cost about the same as a single EL34 and less than a 6L6 or KT66, and MUCH less than a KT88!

But then I came late to the industry and found it is waist deep in snake oil, hype and marketing bllx.

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Re: Marshalls...

Postby SecretSam » Wed Aug 26, 2020 8:48 am

Hm. How's this for a lockdown project, Dave: design a circuit for the amp you think should be out there, (double-ended valves etc etc) and sell the drawings plus idiot-proof instructions as a Kindle book on the interweb for twenty quid a time.

Would it sell ? Well, Wampler's book on modding pedals is currently US$ 69.95 in paperback.

I always thought a KT88 was a Japanese motorcycle, so am always slightly awed by your posts.
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Re: Marshalls...

Postby ef37a » Wed Aug 26, 2020 2:08 pm

Heh! Oh if only I had the energy!

I assume as well that if you SELL detailed designs you need to submit a prototype for safety and RF testing? That is a long and expensive process. I have also written 'Working Instructions' for VERY simple assemblies to 9001 specc' and BELIEVE me! NO such thing as idiot proof instructions!

And Oh! The KT88 is a lovely valve (well, was) Two of them can produce an easy 30 watts at what was state of art distortion levels and even pushing to 50W the sound quality can be very good. Get serious and 4 can deliver 200W but you need about 700V on the anodes anf the modern ones are not up to the job...Well, maybe selected, high grade/hi fi types are but they are just not cost effective for the guitar amp market.

If'n I Was to do anything I think I would dabble with some of the techniques in Merlin Blencowes Tube pre amps book? There are many ways to have very versatile biasing circuits that could produce a variety of harmonic effects that I don't think any major amp maker has explored? Very little has been done with Pentodes for example such as the EF86 and cheaper ones such as the EF80, EF812, ECF80.

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