JTM45 Parts

Littlewyan · Post by **Littlewyan** » Sat May 02, 2015 4:36 am

I'm building a JTM45 and am deciding what parts to use. I've settled on SoZo Vintage Mustards for the coupling caps and a mix of F&T/JJ Cap Cans but can't decide on the mixer bypass, tonestack and bright cap. From what I've read the new silver micas sound a bit smooth and new ceramics sound crap, but I'm struggling to find old ceramics and old silver micas. Where do you find these things?!

I do however have an old oscilloscope which has an old R.B.S. silver mica 500pf cap and a 100pf cap in it but I was looking for 560pf for the mixer and 250pF for both the bright cap and tonestack.

danman · Post by **danman** » Sat May 02, 2015 3:03 pm

Check at "Valvestorm.com". They have several values of the old Lemco silver mica and ceramics.

Littlewyan · Post by **Littlewyan** » Sun May 03, 2015 5:02 am

Thanks mate. Do you think I should bother with carbon comp resistors? From what I've heard they only make a tiny bit of difference in tone in a few places.

danman · Post by **danman** » Sun May 03, 2015 11:02 am

The resistors are really a personal preference type thing I feel. I do use carbon comp in a few spots but I mostly use carbon film in my builds. The longer lead length of the old carbon comps will help you out in several spots where the turrets are spaced quite far apart though. Down in the "technical reference" section you will find a layout for the best positions to use the carbon comps.

Littlewyan · Post by **Littlewyan** » Mon May 04, 2015 4:42 am

Cool, thanks. I have a big bag of carbon comp resistors that I bought for £1 from a radio rally. Might use some of them altho the few I measured had drifted up quite a bit. I didn't find a 100K that measured below 112K!

Haze13 · Post by **Haze13** » Mon May 04, 2015 1:38 pm

Old parts can behave quite different when they'll see a voltage across them. Carbon Comps have best results where there is no voltage drop on them -> like control grid resistors... (68k, 470k...)

Littlewyan · Post by **Littlewyan** » Thu May 07, 2015 6:01 pm

I always thought you needed voltage across CC resistors for them to make a difference.

http://www.geofex.com/article_folders/c ... oncomp.htm

ivan H · Post by **ivan H** » Thu May 07, 2015 7:28 pm

Hi, I was under the same impression. I remember reading on I think it was the "DIY Audio Instruments & Amps" forum, where it was said that carbon comps had the greatest effect when used in places like PI plates where they had a large voltage swing across them. Also, noise (from the cc) is not so much of a problem when used in position like that as the signal to noise ratio is much greater. Cheers

Littlewyan · Post by **Littlewyan** » Fri May 08, 2015 4:33 am

Also if you read that article it states that new carbon comps don't add as much distortion as old carbon comps from yesteryear and the resistance changes depending on the voltage put across them. I do have a lot of old carbon comps that I could try, only certain values for this amp like 100K, 56K and 10K. I think I'll hear more of a tonal change with different valves than I will resistors.

Which reminds me, I'm going to use Gold Lion KT66s, TAD GZ34 and for the pre amp valves I'm leaning towards JJ ECC83 MG valves at the moment. Tried one in my Trainwreck Express in place of a normal JJ ECC83S and it had so much more clarity and sounded a lot sweeter. What do you guys like to use?

Littlewyan · Post by **Littlewyan** » Sat May 09, 2015 5:12 am

Ivan your post disappeared before I could reply?

What is the low end like on this amps? I know they are very bassy amplifiers but can you make it tight by turning the bass down to 0 and the mids up? I've never seen someone play a power chord on the low e on these amps and am interested to find out how it'd sound.

Littlewyan · Post by **Littlewyan** » Thu May 14, 2015 6:41 pm

Where does George get those big Ceramic Caps from? I've searched and searched but can't find any!

Haze13 · Post by **Haze13** » Sat May 16, 2015 3:22 am

When voltage is changing across the PI resistors, the current is changing too => the current that goes trough the cathode resistor is changing also => bias of the tube is changing also, because of the feedback to the control grid trough the resistor from the grid to ground. If the cathode bypass cap isn't large enough, it can't filter out the AC signal (noise) that "rides" on the DC offset voltage that creates an operating point of the tube... It moves, but it should be constant! That's the whole point.
I haven't seen all the photos of the Vintage marshalls boards, but those that I have seen, these CC resistors weren't in the PI.
Of course I've read that there are Marshalls that have CC resistors in these places, but it's not common...
Geofex tells where to use them if you want to hear their effect, the man like Morgan Jones, author of the Valve Amplifiers books, writes about why it's a good practice to use CC only where there is no voltage drop across them.

Littlewyan · Post by **Littlewyan** » Sat May 16, 2015 4:28 am

I've seen plenty of old Marshalls with CC across the board. I suspect Carbon Films were quite pricey at the time and may not have been widely used. I'll build my amp with Carbon Films across the board. I might experiment with Carbon Comp in certain places but if the amp sounds great once built then I'll probably just leave it alone.

So does anyone know where to get the big Ceramics that George uses? I've scoured ebay but can't find them. I'm also interested in what current production pre amp valves are preferred? I was thinking JJ ECC83MG.

Haze13 · Post by **Haze13** » Sat May 16, 2015 5:03 am

For CC resistors, there is a topic where George mods/upgrades a HW Plexi, and he replaces some resistors with CC.
Don't know about caps, sorry

neikeel · Post by **neikeel** » Sat May 16, 2015 3:47 pm

Littlewyan wrote:I've seen plenty of old Marshalls with CC across the board. I suspect Carbon Films were quite pricey at the time and may not have been widely used. I'll build my amp with Carbon Films across the board. I might experiment with Carbon Comp in certain places but if the amp sounds great once built then I'll probably just leave it alone.

So does anyone know where to get the big Ceramics that George uses? I've scoured ebay but can't find them. I'm also interested in what current production pre amp valves are preferred? I was thinking JJ ECC83MG.

The only original Marshalls with CC across the board will be the offsets from 63 which were prototypes made with RS off the shelf stock.
The mid 60s used Piher carbon films with occasional CCs (presumably when stocks ran low), same goes with amps from 66 on that used Iskra carbon films with occasional Piher and occasional CC. Then early 70s (PCB era) the Pihers made a comeback. CCs could appear any where (commonly the 820R cathodes, 10k 1w or 8k2 1w droppers, 470k mixers, PI grids, NFB etc.)

Tonally they only really add on the plates, i.e. 100k on V1 and V2 socket, and NFB. Carbon film are fine elsewhere and from a safety/reliablity point of view I suggest 2w MO on the droppers. Ceramic or WW ar fine on the screens.

Oh and to my ear 500 to 560pF mixer is best and 250-270pF intone stack, 100pF on vol 1 pot (you can use 250pF - never a stock value and 100pF is nicer, IMO of course)
I like mustard coupling caps and trawl ebay for RS and LEMCO micas as my original stash is quite depleted now

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