Since my favourite reissue Plexi has been through so many experimentations and modifications throughout the years, including a full power scaling install at one point, it was in a pretty messy state and needed a serious overhaul. After tidying up the power stage wiring, neatly twisting the wires to avoid any excess noise (and also create free space inside the amp), I decided this amp would really benefit from a brand new circuit board – especially since the old one had been sawn in half at one point!

After reviewing pricing and options, I decided to go with a board made by BrianH from the Metroamp forums. The price of $60 was attractive, especially since Brian doesn’t charge extra for custom boards, which was particularly important in this case. The 1959SLP board mountings don’t match either the original plexi amps or the Metroamp kits, so boards made for those two types of amp would not fit mine.

Brian was particularly helpful and patient in getting the measurements just right with me for this board to fit the 1959slp, and came up with some innovative ways of getting the spacings between components to “look vintage”, ie. very closely resemble the orientation of parts on a vintage plexi Marshall. We also had to figure out how to incorporate turrets and space for the effects loop resistor, and an elegant solution was eventually found.

The board that arrived was of high quality; rock solid, perfectly drilled and staked with split top turrets and to my exacting specifications. Moreover, the colour and feel of it was great and looked extremely close to the look of a vintage Marshall circuit board, except for the changes I’d made for mounting locations, and a space to put the large ceramic effects loop resistor.

Overall, I was really pleased with it, and it makes my amp look so much tidier!

- Bainzy

Adding capacitors in parallel

When you add capacitors in parallel (i.e. connected together twice, at both ends), the new value you end up with is simply the value of the first capacitor plus the value of the second capacitor. For example, a pair of 250pf capacitors in parallel would give a value of 500pf. A 500pf and a 250pf capacitor in parallel would give a working value of 750pf.

Therefore it’s fair to say that C1 + C2 = Ct, where C1 is the first capacitor’s value, C2 the second’s, and Ct is the resulting capacitance value.

Adding capacitors in series

Here’s where it gets a little harder to work out. In series (i.e. connected one after another), the resulting capacitance is found by this formula:

1 + 1 +  1 +   ….  = 1
C1     C2      C3                 Ct

so you work out the total capacitance by using fractions. Let’s try this with the examples above (250pf and 250pf, 500pf and 750pf):

1 + 1 = 1
250    250     125

1 + 1 = 1
500    750       300

Therefore a pair of 250pf caps in series gives you 125pf capacitance, and a 500pf then 750pf capacitor in series would give you 300pf in capacitance.
Adding resistors in parallel

Now that you’ve learnt how to add capacitor values, you can easily do it now with resistors – it’s just the same but backwards. You use the same formula for adding resistors in parallel as you would for adding capacitors in series, and you also use the same formula for adding resistors in series as you would for adding capacitors in parallel.

Therefore, the total resistance when adding resistors in parallel is simply:

1 + 1 +  1 +   ….  = 1
R1      R2      R3                Rt

where R1, R2, R3 etc is the first resistor, second resistor, third resistor etc, and Rt is the total resistance gained.
Adding resistors in series

Like adding capacitors in parallel, this is just simple addition again:

R1 + R2 + R3 + … + Rn  = Rt

And like all the other examples you can add the first resistor (R1), second resistor (R2), etc, all the way up to as many resistors as you like (Rn being the last resistor), and then of course again Rt is your total resistance.

Generally speaking, pretty much any NOS or used tube made around 20 years ago or more will sound better than any current production tubes that you can buy. The reasons for this are varied, but most are due to the fact that tubes just aren’t in demand anymore.

Manufacturers can’t afford to make the metals used in construction of the tube as pure, and the vacuum in the tubes isn’t as tight because tighter vacuums cost more time and therefore money. That’s why you will often find complaints today of certain manufacturers’ tubes being unreliable (eg. power tubes ‘red plating’ soon after they’ve been installed). Certain chemicals can’t be used for tube production because they are toxic, and the ones that are still allowed to be used today tend to be only legal in countries where workers have less rights, and they don’t cost as much to employ either (eg. China, countries formerly in the Soviet Union). That’s why old factories in those countries are mostly the only ones still making tubes today, as opposed to old factories in England or USA.

Why go for old tubes?

• Old tubes were made so well that they last much longer than current production tubes. Preamp tubes have been known to last around 30 years, or around 20,000 hours. That’s practically a lifetime for a tube amp.
• In most cases, vintage tubes sound better than their modern counterparts. The quality in manufacture of these tubes has a large effect on the sound, particularly in preamp valves. You really have to try some vintage tubes instead of modern production ones to experience just how better they sound, they make your amp come to life. The more gain your amp uses the less noticeable this effect may be, as in some high gain amps, transistors or diodes are used to get more gain instead of tubes, so the tubes will be less important.
• Get them while you can – since these tubes are obviously no longer being made, once they’re gone they’re gone – and prices are always going up.

What should I buy?

• Different manufactures produced tubes which sound different – a tube made in a different plant or by a different manufacturer will probably sound different.
• Many tubes were relabelled, so they may not necessarily sound like you’d expect by looking at the manufacturer name. The best way to know who made your tube or where it was made is to learn how to recognise the internal construction of tubes, and learn how to interpret the codes printed or moulded on the tube. For example, today from eBay I received a Tungsram 12AX7 that I won for £3, yet the valve said “Made In England”. Tungsram 12AX7′s were made in Hungary, and by looking at the location of manufacture, the date codes, the sound of the tube and the internal construction, I know that this was a tube actually manufactured in Britain by Mullard in their Blackburn factory.

RFT ECC83 with Mullard box and insert

What do “xxxx”‘s tubes sound like?

Here’s a rough and by no means extensive guide on what 12AX7/ECC83′s by the following manufacturers tend to sound like:

• Mullard ECC83 – The most popular preamp tube for a Marshall. Used by Marshall in the 1960′s in their amps, this tube is chock full of mids, and has a sort of “3D quality” to it’s sound. Most examples are British made.
• Sylvania 12AX7WA – This is an American made tube reputedly used by Eddie Van Halen. It’s got less ‘upper mids’ than the Mullard, and has more of a ‘cutting’ midrange. Lots of these tubes have been released onto the market as NOS U.S. Military surplus, and can be found at great prices on eBay and the like.
• RFT ECC83 – East German made tube, quite a ‘dark’ sounding tube, with it’s midrange focussed on lower mid frequencies. A very ‘ballsy’ sounding tube, great for metal, hard rock and the perfect tube for a JCM800 in my opinion.
• Telefunken ECC83 – Regarded by audiophiles as the best tube of this type, it’s best used in audio/hi-fi applications but some guitarists like it in their amps too. They favour the tube because it sounds good but doesn’t colour the sound too much, and sounds bright and present. NOS examples of these are very expensive compared to modern produced tubes.
• Valvo ECC83 – Very similar in construction to the Mullard, both companies were owned by Phillips. These tubes were made in Holland. Consequently the sound properties are similar, the Valvo has a similar midrange emphasis and almost as good as the Mullard for a considerably smaller price. Many Valvo tubes are actually relabelled Mullards.

How much will they cost?

Not all vintage tubes cost a lot of money compared to modern production. You can get slightly used tubes from eBay or garage sales for less than you’d pay for modern tubes. I’ve never spent more than £7 on any vintage tubes on eBay, but I’ve managed to get lots of them that sound far better than modern produced, new tubes.

Getting most types of tube on eBay for cheap is easy, you just have to be patient and know what to look for. Slightly used ones are fantastic, as old tubes last so long, the difference between them and NOS tubes is insignificant. I think they actually sound better after being “burned in” a bit.

You can probably get NOS Sylvania 12ax7′s for the same as modern tubes at the moment, the market is flooded with them. Those are a great tube to grab now.

The really expensive NOS tubes are EL34′s/KT66′s/KT77′s/KT88′s, as they don’t last as long as preamp tubes and are harder to come across. Fortunately, they don’t affect tone anywhere near as much as preamp tubes, and the modern equivalents aren’t that much behind in terms of sound. For power tubes, reliability is the big problem with the modern ones. Most companies putting out EL34′s today don’t spend time with them as thoroughly as they should after making them, before shipping them out to distributors.

Copyright © Richard Baines 2006

There ARE small tone altering and/or gain altering mods.
I do have pic’s so I CAN physically see that the amp has a split cathode arrangement.
…the cap on v2(330uf) cathode.
…its a 2.7k/.68 and the resistor looks like the stock part (but I don’t think it is) but the cap is one of those yellow square type from the 70′ Marshall’s. From the circuit card It looks like it came stock with this arrangement. The post that is pressed in looks stock too which tells me more than anyone could. In the pics the serial # is 12301.

His treble cap was a round shaped ceramic that was a 250pF that said MURATA … With 56K on the tone circuit feed..

One of the 470K mixer resistors was bypassed with a round hollow tubular MURATA cap 500pF… If memory serves me right on the value, or it was a 330pF..I will check my notes….

The real kicker, his phase-inverter “get-rid-of-the-FIZZIES” cap was a 100pF instead of the normal 47pF ….and this my friends is how the “brown sound” with that added compression happens..

Oh, BTW those 820 ohm resistors were carbon-comp and drifted in value up about 1.1K and make the amp much more gainy and warmer, since these re-bias the 12AX7 valves in a bit more non-linear region..

I am pretty sure the feedback resistor was a 47K …I will have to check my notes…

The filter cap in the center of the board was a dual 16uF grey RS cap…

The screen filtering was 2 DALY 32uF light-blue caps in series… The voltage doubler were 2 100uF DALY royal-blue caps…

Rear cap on top of chassis was a royal-blue HUNTS 32uF or 16uF…need to check notes..

The value of the coupling cap between V1 and V2a….022uF.

At least that was what it was in 1980…

1.The first 820 ohm resistor (carbon type) measured a little over 1K. It was bypassed with a blue 330uF resistor. can

2.His treble cap was a 250pF Murata flat ceramic one. The cap across his 470K was a Murata hollow round 330pF. cap

3.His second stage 820 ohm (which also measured a little over 1K, was also bypassed with the exact same type 330uF blue coloured cap that was on the first cathode resistor.

4.The filtering caps for the middle of the board were grey coloured RS caps that had dual 16uF values.

5.The screens were 2 blue caps in series (DALY 32uF’s).

6.The voltage doubler were two big blue caps (100uF DALY’s). The one on the outside was a blue Daly – and was a single 32uF.

7.The feedback resistor was a 47K of unknown origin.

8.His power transformer was the smaller one of that era. His OPT was also the smaller one with 1.5″ stack.

The amp was either left stock into a load resistor or, a big Ohmite (or other) power resistor was placed somewhere in the circuit to cut the power of only the output stage meaning Sylvania 6CA7′s were the only valves to hold up to this.

If this is true – doesn’t this kinda dispel the whole ‘special’ 67 slp 100 myth …as almost all amps of that year had pretty much identical configurations (with the small exception of the 330uf cap on the second preamp stage?

Dankuwell ! (dutch for many thanks)
Plaap