'68 Bass Specs vs '68 Lead Specs vs '70's & Up Lead specs

[Webby14]

Hi Guys,

8 months ago, I completed my first ever 50 Watt JMP Plexi build, using Velvet George's excellent kit instructions, Valve Storm for the majority of the parts, and Classic Tone's Vintage Transformers & Choke.

Come the big switch on day, everything worked straight off, without a hitch!

Now to the main reason for my post!

The initial build was to '68 Bass Amp Specs (with the addition of a Lar Mar PPIMV Mod), but I found that with my stock '97 Les Paul, the sound was a little on the dull side, ie: Not enough gain, and not sparkly enough on the top end.

I then changed to '68 Lead Specs (500Pf Bright Cap), but I found that although the amp sounded great when played solo, when playing with my band, the sound was a little 'congested' in the mid-range, and didn't really cut through the mix.

I then altered the amp to '70's & Up specs (5000pf Bright Cap). Well it certainly cut through the mix then! However, it had so much gain, that even with the the bright volume on 2 or 3, it was still way too distorted. Even changing back to a 500pf bright cap, only tamed things a little.

After reading some advice on this forum, I removed the V2 .68 Cathode Cap, and liked what that did. I also changed to a 100pf Bright Cap. The amount of gain is pretty much bang on now, but I feel that tonally, it's a little too 'honky' in the mid-range, and a little lacking in top end sparkle.

Any suggestions on what board components to experiment further with? I don't want to add any extra gain, just alter the amp's tone stack (although I don't know where that is!), to dial out the mid-range 'honkiness', and add a bit more top end sparkle.

Please bear in mind that, although I come from an engineering background, I have no formal electronics training at all.

My physical dexterity is good, I can follow straightforward instructions, and I know how to use a soldering iron properly.

If you can refer to components as (for example), 'the 500pf cap just to the left of the 100k resistor', etc, that would be good.

I wouldn't know a B+ Voltage from a hole in the ground! Lol!

Many thanks in advance

Steve Webster U.K.

[neikeel]

Hi Steve
No such thing as a 500pF bright cap as stock, so it makes me wonder what you went for when you went 68 lead spec?

There is a misconception that a big cap on the vol pot makes it brighter, not true - a 5000pF cap is a big bypass cap that allows a broad range of frequencies to pass even at low volume pot settings so it makes the amp seem gainier at lower volumes. Both my 68 lead amps (50 and 100) have stock 5000pF caps on vol 1 pot.

There is no doubt that a well set up lead amp will have lots of cut and enough to make itself heard in a band context.

It will depend on what speakers you are using too.

To get gain from a bass amp it will require the amp to be cranked into brighter speakers (probably needing the LarMar +/- attenuator).

Post back the missing details to see if we can help (your preferred EQ settings, or ones you have tried will help too!)

[Webby14]

Hi neikeel,

Thanks for your reply.

What I meant by a 'stock' value, is that a Marshall JMP 50 Plexi Lead Amp, as supplied by Marshall in 1968, would have been supplied with a factory fitted 500pf Bright Cap. The 5600pf value was from the '70's onward. This build was from the Metroamp '68 era, 50 Watt Kit Instructions, which do actually say at the top of page 25, that the stock value is 500pf!

I do understand the way that the bright cap works, and 5000pf + to me (especially on the '70's & Up Specs), just means that there is basically no hope of achieving any kind of clean tone, even with the Bright Channel Volume as low as 1 or 2. Way too gainy sounding for me.

Here's all the background details on my current setup as you have requested, and hopefully with your help (and others if necessary?), I can get the right component layout, etc, to give me what I am looking for.

Current Amp Specs are the same as the '70's & Up' Board Layout on Page 36 of the Metroamp Instructions, apart from the V2 .68 Sozo Cathode Cap, which has been removed. 100pf Bright Cap installed on the Bright Volume Pot.

Guitar: Stock '97 Gibson Les Paul Standard, Volumes and Tones usually full on, bridge pickup 90% of the time.

Amp Settings are: LarMar PPIMV between 4 & 6 (depending on the gig), Presence 10, Bass 5 to 10 (depending on gig), Middle 10, Treble 10, Bright Volume 7 to 8, Normal Volume 0

Speakers: Marshall 1960A 4 x 12" Cab + Marshall 1936 2 x 12" Cab (all containing 25 Watt Celestion Greenbacks).

What I really want to know (in 'layman's terms please!), is what a lot of the board components actually do? For instance: When I changed from '68 Bass Amp Specs, to '68 Lead Amp Specs, and then on to '70's & Up Lead Amp Specs, what physical effects did these component changes actually have?

When I changed from a .022 Sozo Cap on the left hand side of the '68 Lead Board, to a .0022 Sozo Cap on the '70's & Up Board, what did that do? Alter the tone or the gain? Both?

Which Component changes alter the gain? Which Component changes alter the tone? Which components constitute the 'Tone Stack?', etc.

Steve

[neikeel]

What I meant by a 'stock' value, is that a Marshall JMP 50 Plexi Lead Amp, as supplied by Marshall in 1968, would have been supplied with a factory fitted 500pf Bright Cap.

Nope, only 100pF (RS labelled red tubular ceramic) on early shared cathode lead amps (or grown circular ceramic Murata on 70s organ amps) or 5000pF RS labelled tubular ceramic or LEMCO brown disc on split cathode lead amps. No stock amps had 500pF - unless you can show me an original amp with red dye or an unadulterated original schematic

The 5600pf value was from the '70's onward. This build was from the Metroamp '68 era, 50 Watt Kit Instructions, which do actually say at the top of page 25, that the stock value is 500pf!

It's wrong, ask George! That schem is a Unicord adapted by Brian Wallace (BKW). 5600 is too big, 5k stock and funnily enough the 1000pF of a 2204 era can work, a 500pF is too spiky (used as a treble peaker on MV amps with a 470k resistor in parallel.

I do understand the way that the bright cap works, and 5000pf + to me (especially on the '70's & Up Specs), just means that there is basically no hope of achieving any kind of clean tone, even with the Bright Channel Volume as low as 1 or 2. Way too gainy sounding for me.

Yes it can be a problem, much more noticeable on clones where everything is fresh, tight and not fully broken in. On a 40 year old vintage amp the 5k cap just works (have a listen to Georges 12 series SL comparison clips on Youtube)

Current Amp Specs are the same as the '70's & Up' Board Layout on Page 36 of the Metroamp Instructions, apart from the V2 .68 Sozo Cathode Cap, which has been removed. 100pf Bright Cap installed on the Bright Volume Pot.
Guitar: Stock '97 Gibson Les Paul Standard, Volumes and Tones usually full on, bridge pickup 90% of the time.
Amp Settings are: LarMar PPIMV between 4 & 6 (depending on the gig), Presence 10, Bass 5 to 10 (depending on gig), Middle 10, Treble 10, Bright Volume 7 to 8, Normal Volume 0
Speakers: Marshall 1960A 4 x 12" Cab + Marshall 1936 2 x 12" Cab (all containing 25 Watt Celestion Greenbacks).

Pretty high presence and treble settings for this amp but I guess it depends on the room, tones you are after.

What I really want to know (in 'layman's terms please!), is what a lot of the board components actually do? For instance: When I changed from '68 Bass Amp Specs, to '68 Lead Amp Specs, and then on to '70's & Up Lead Amp Specs, what physical effects did these component changes actually have? When I changed from a .022 Sozo Cap on the left hand side of the '68 Lead Board, to a .0022 Sozo Cap on the '70's & Up Board, what did that do? Alter the tone or the gain? Both?
Which Component changes alter the gain? Which Component changes alter the tone? Which components constitute the 'Tone Stack?', etc. Steve

The differences between bass and lead are at the front end - shared cathode on V1 for bass has a fuller sound than split for lead but the 0.68uF/2k7 resistor on channel 1 cathode is brighter with more upper mids and will cut through better.
The coupling cap change on V1b (0.022uF bass to 0.0022uF lead) shelves the frequency cut off so you lose a lot of the lows early in the circuit making the amp seem brighter, (as an aside most bright amps will often seem louder and gainier in the room).

Next the tone stack 250pF/56k in the bass again favours the mid range tones whereas the 500pF/33k selects higher upper ranges.
The 0.68 on V2a (the one that I think you removed) gives a perceived boost in the upper mids.
Moving along to the phase inverter when you removed the 0.1uF output couplers and changed them for 0.022uF this will have made the amp a little quicker and more direct in response by losing some of the flabby lows that you can get with 0.1uF output couplers, although they seem to work ok in early lead (shared v1 cathode amps).
The NFB resistor for a 68 lead would be a 47k hooked up to the 8ohm tap of the OT, keeps things reasonably tight but also lets the power section sing. Early amps used 27k on 16ohm tap which is more NFB that can keep things nice and tight, particularly if you are maxing out a shared cathode amp stopping it flub. Least NFB is 100k on 4ohm tap that came in during 71, looking for more gain with looseness.
All these changes affect an amp in its mid ranges of volume and tone control. I have a 67 JTM (all the early specs with 100pF on volt) that when properly cranked has as much gain as the 68 Lead (all the later specs of split cathode etc with 5000pF cap on v1)
Oh and the unused volume turned up can give some fullness, or you can jump ch1 to ch2?

[Webby14]

Hi Neil,

Thanks for your reply.

Firstly, let's put this bright cap business to bed! At the top of page 25 of George's instructions, it clearly states that 500pf is the stock bright cap value ...... so there! If that's wrong, then it’s George’s fault for printing spurious info! Lol!

Seriously though …… I do sincerely appreciate your input but, like my late 'well meaning' friend Jim (who was a nice bloke, and a technical whiz), you start off O.K. but then go in to a load of technical jargon that is way over my head!

'Split cathode'? 'Shared cathode'? I'm sorry, but I don't really know what any of that means!

Let’s try a different approach. Maybe it would be better if I simplified things, by explaining my reasons for choosing to build a '68 era 50 Watt Plexi in the first place? Then we can take it from there.

The best sound I ever got, was a ’71 Les Paul Deluxe Goldtop into a 1966 JTM45 Head, with KT66 Output Valves, that I bought second hand in ’73. It wasn’t perfect though…..

The Pro’s were: Beautiful rich clean tones with just the right amount of top end ‘shimmer’, just the right amount of gain for my tastes with the bright volume up around 6 or 7.

The Con’s were: Bottom end a bit flubby, especially at higher volume levels, and the amp just ‘ran out of steam’ at bigger venues. After about 7 on the volume, it didn’t get any louder, just really ‘mushy’ and indistinct.

I decided that the logical way to go, would be to build a ’68 era JMP50 Plexi to Bass Specs, as the Bass Circuit Board and the JTM45’s Board are very similar. The end result however, was a bit flat and lifeless. A tighter bottom end (S/S Rectifier?), but not quite enough gain or brightness.

Using the Metroamp board layout diagrams as reference points

Moving from left to right on the JTM45 board, the first difference I can see between the two amps, is that bottom turrets 8 & 9 have 270K Resistors instead of 470K, then on bottom turret 14, the single radial 16uf Cap instead of the dual radial 33uf Cap and the 10K Resistor between the two positive terminals. On bottom turret 19, you have a 27K NFB Resistor instead of 47K. On bottom turret 20, you have a single 8.2K resistor, instead of an 8.2K & 10K in series. In between the two 10uf Radial caps, you have a 68K resistor, instead of 47K. Finally, on the far right of the board, you have a 220K resistor instead of 150K.

In a nutshell, what I am looking for, is the sound of a JTM45, without the flubby bottom end, and a power amp stage with more headroom.

The ’68 & ‘70’s Lead Specs are proving a bit too ‘brash and unruly’ for my tastes.

Should I go back to the ’68 Bass Amp Specs, as a starting point?

If so, what components/value changes would you advise, to get into JTM45 tone/gain territory?

Thanks for your patience!

Steve

[neikeel]

H
Using the Metroamp board layout diagrams as reference points

In a nutshell, what I am looking for, is the sound of a JTM45, without the flubby bottom end, and a power amp stage with more headroom.

The ’68 & ‘70’s Lead Specs are proving a bit too ‘brash and unruly’ for my tastes.

Should I go back to the ’68 Bass Amp Specs, as a starting point? Steve

The 270k you refer to are the mixer resistors where the signal from each channel comes together before passing into V2a (second gain stage). 470k tends to give more grind, most people prefer it in a 50w.

Yes go for Bass spec and use a 100pF 'bright cap' on V1 volume pot. Adds a little sparkle at medium volumes. IMO the sweetest 50w for all round use is the JTM50 (similar) but it does have a GZ34 valve rectifier which adds a little sag and compression as you turn the wick up - I like it but you need to decide for yourself. One of my babies:

[Webby14]

Hi Neil,

Thanks for your excellent, and simplified (Lol!) advice.

I will go back to Bass Specs and add the 100pf bright cap like you suggest, and I will give you an update if it all goes well.

Just a couple of things to clarify before I sign off:

1).The NFB wire is currently connected to the positive of the output jack (as per the Metroamp instructions). Am I right in my understanding therefore, that the amount of NFB depends on the setting of the impedance selector (currently set to match an 8 ohm speaker load)?

2). If I find the amp still needs a little more brightness or gain, should I try a 100K NFB resistor? NFB wire on different tap maybe? Bigger bright cap? What do you think?

Anyway, I'll let you know how I get on.

Thanks again, and bye for now

Steve

[neikeel]

1).The NFB wire is currently connected to the positive of the output jack (as per the Metroamp instructions). Am I right in my understanding therefore, that the amount of NFB depends on the setting of the impedance selector (currently set to match an 8 ohm speaker load)?

Yes, correct, If your NFB resistor is hooked up to the speaker jack then your amp will see whatever your output selector is switched to (not necessarily the load cab as some people run a selector:cab mismatch)

If I find the amp still needs a little more brightness or gain, should I try a 100K NFB resistor? NFB wire on different tap maybe? Bigger bright cap? What do you think?

Sure if you want looser feel with bit more apparent gain try 100k on 8 or 4ohm selector taps (I like slightly more NFB but aim to work the power section harder (i.e. turn up a little and attenuate)
A bigger bright cap will just decrease your clean headroom, IMO 100pF is about right for a shared cathode amp.

[Webby14]

Hi Neil,

Thanks!

I'll keep you posted.

Regards

Steve

[Webby14]

Hi Neil,

I haven't got around to changing my '68 50 Watter back to Bass Amp specs yet, but I have in the meantime, viewed a board layout for a JTM 50 'Black Flag' Amp Head (which I think you have mentioned as one of your personal favourites.

As you mentioned, the board components are very similar.

Maybe you could indulge me, and fill me in on what the differences in some of the components/values have, on the sound of JTM 50 over the '68 Bass Amp?

'68 Bass: V1 Cathode Cap = 330uf JTM 50 = 220uf
'68 Bass: NFB Resistor = 47K JTM 50 = 27K
'68 Bass: 8.2k + 10K 2 Watt Resistors in series (to right of NFB Resistor) JTM 50: Single 8.2K 2 Watt Resistor
'68 Bass: 47K Resistor (in between 2 x 10uf Caps) JTM 50: 56K Resistor
'68 Bass: 150K (Bias Supply Dropping Resistor?) JTM 50: 220K Resistor

Given that the JTM 50 has a Valve Rectifier, as well as explaining how these differences affect the sound, can you tell me which components/values, can be safely 'experimented with', and which I should leave well alone!

I guess that the 47K/56K Resistors between the 2 x 10uf Caps, and the 150K/220K Bias Supply Dropping Resistors are probably the one's to leave alone?

Regards

Steve

[neikeel]

Hi Neil
.........fill me in on what the differences in some of the components/values have, on the sound of JTM 50 over the '68 Bass Amp?

My comments in red

'68 Bass: V1 Cathode Cap = 330uf JTM 50 = 220uf (I personally suggest a 25uF)
'68 Bass: NFB Resistor = 47K JTM 50 = 27K (yes the JTM is on 16ohm tap, but experiment yourself)
'68 Bass: 8.2k + 10K 2 Watt Resistors in series (to right of NFB Resistor) JTM 50: Single 8.2K 2 Watt Resistor (these are the B+ supply resistors, 8k2 gives slightly higher preamp voltages, 10+8k2 is a bit browner)
'68 Bass: 47K Resistor (in between 2 x 10uf Caps) JTM 50: 56K Resistor (these are the bis resistor - change to whatever works in your amp)
'68 Bass: 150K (Bias Supply Dropping Resistor?) JTM 50: 220K Resistor (actually the 150k is wrong for a clone, the JTM45 with Drake 1202-55PT uses a 180k but all the JTM50 and JMP50 using the Drake 1202-118 PT use a 220k)

Given that the JTM 50 has a Valve Rectifier, as well as explaining how these differences affect the sound, can you tell me which components/values, can be safely 'experimented with', and which I should leave well alone!

See above.
This https://www.youtube.com/watch?v=6tvY57FpRY0 scotsman shows how a good JTM50 sounds, listen to all of it as he covers most of what it does. IMO better than a JTM45 which is sweet in the studio but lacks clean headroom in a band context. I AB'd my 67 JTM vs my 68 JMP with both cranked and they are surprisingly similar at volume. The 68 has more natural treble and cuts a little more and with the big bright cap the gain comes in sooner on the dial so helps you start to rock out at lower volumes. I have not got a 68-70 bass/organ spec 50w at the moment but assuming good components and tidy build one has the best of both worlds without the expense of a valve rectifier.

[Webby14]

Hi Neil,

Thanks for that.

I have put my comments in blue

'68 Bass: V1 Cathode Cap = 330uf JTM 50 = 220uf (I personally suggest a 25uF) You did mean 25uf, not 250uf? What audible effect will that have?
'68 Bass: NFB Resistor = 47K JTM 50 = 27K (yes the JTM is on 16ohm tap, but experiment yourself) O.K.
'68 Bass: 8.2k + 10K 2 Watt Resistors in series (to right of NFB Resistor) JTM 50: Single 8.2K 2 Watt Resistor (these are the B+ supply resistors, 8k2 gives slightly higher preamp voltages, 10+8k2 is a bit browner) 8K2 a bit cleaner sounding then? Preamp valves O.K. with slightly higher voltage?
'68 Bass: 47K Resistor (in between 2 x 10uf Caps) JTM 50: 56K Resistor (these are the bias resistors - change to whatever works in your amp) How would I know whether or not 47K or 56K would work best with my amp? What would the audible differences be, and would the 56K be safe to use with the '68 Bass design?
'68 Bass: 150K (Bias Supply Dropping Resistor?) JTM 50: 220K Resistor (actually the 150k is wrong for a clone, the JTM45 with Drake 1202-55PT uses a 180k but all the JTM50 and JMP50 using the Drake 1202-118 PT use a 220k) Did George get that wrong then? His 50 Watt Build instructions clearly show a 150K resistor, and the build is based on using a Drake 1202-118 PT. 220K Resistor safe to use there then? What audible effect will it have?

Webby14 wrote:
Given that the JTM 50 has a Valve Rectifier, as well as explaining how these differences affect the sound, can you tell me which components/values, can be safely 'experimented with', and which I should leave well alone!


See above.
This https://www.youtube.com/watch?v=6tvY57FpRY0 scotsman shows how a good JTM50 sounds, listen to all of it as he covers most of what it does. IMO better than a JTM45 which is sweet in the studio but lacks clean headroom in a band context. I AB'd my 67 JTM vs my 68 JMP with both cranked and they are surprisingly similar at volume. The 68 has more natural treble and cuts a little more and with the big bright cap the gain comes in sooner on the dial so helps you start to rock out at lower volumes. I have not got a 68-70 bass/organ spec 50w at the moment but assuming good components and tidy build one has the best of both worlds without the expense of a valve rectifier. Sounds great! The only difference between you and me re: bright cap value is, that I don't want the gain to really start coming in until about 6 on the bright channel volume.

Thanks again.

Steve

[neikeel]

25uF will be less likely to flub out.
I like the 8k2 only but is less 'brown'.
The bias adjustment resistor is what its is when you set the bias, it is there so that your bias pot is in the range, only audible difference will be if you cannot get your bias in the right area.
George does some good stuff but not infallible, he may have been working with different PT clones, I am only telling you what is correct for an original. It will affect the other bias resistor value we mentioned above.
The bright cap will be what works for you. My 67 and 68 are both original vintage ones. I run a 100pF on my switchable 1987/2204 clone on the plexi channel but 1000pF on the 2204 channel.

[Webby14]

O.K. Got it!

Thanks Neil

Steve

[Webby14]

Hi Neil,

I decided to go for JTM 50 Board Specs (with the exception of the 56K Bias Resistor, and the 220K Bias Supply Dropping Resistor). Bias Range is bang on.

Substituted all the necessary components, checked bias, fired everything up.

Bass Channel fine (all tone controls working), but no Lead channel! Completely silent.

Have checked continuity from Lead Volume Pot, through all the relevant Board Connections, and on to preamp pins. Checked continuity through to board, from Input Jacks. All check out fine!

It's like 'open circuit', wire/connection missing, or some such thing. Any ideas?

Steve