Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: PharmRock on February 06, 2022, 09:13:03 pm
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For the classic Marshall 18-watt 1974 layout, V1 is the normal channel (single or parallel input), V2 is the PI, and V3 is the tremolo input/oscillator. Is there any particular reason the amp is laid out this way (with the PI sandwiched between the two input tubes)? The board is laid out in the corresponding fashion. It seems to make for some unnecessarily longer wire runs. I know tremolo circuits can be finicky, so maybe there is a good reason that the board/tubes are laid out in this order, compared to the traditional layout of having the PI as V3 (or immediately before the power amp).
I'm building an EF86 version with the EF86 replacing the normal channel, and the tremolo channel staying as it is. I have a draft of a board layout I plan to drill this week, and have the EF86 as V1, Tremolo as V2, and PI as V3. Just wanted to make sure I'm not going down the wrong path.
Link to the traditional layout: https://el34world.com/charts/Schematics/files/Marshall/Marshall_18watt_layout.pdf (https://el34world.com/charts/Schematics/files/Marshall/Marshall_18watt_layout.pdf)
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I think the original was laid out like that. That trem circuit is the only finicky tremolo I've ever built. Lot of complaints and band aid fixes on 18watt.com. I have a couple band aids in mine. What works in one may not work in another. Been a long time since I hung out on the forum. They may have come up with a real fix by now.
http://sluckeyamps.com/18w/18w.pdf
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Thanks for the info. I've downloaded your schematics and layouts already, and planned to merge the Trem channel of the 1974 with the EF86 channel on your Dual Lite. I was able to access the 18watt website recently for a few days (finally remembered old email and password) and downloaded some updated layouts and schematics, then I changed my email address in my account, now my account is deactivated awaiting admin approval. Oh well.
Anyways, I just noticed on one layout I downloaded from 18watt which has the PI in the traditional V3 position. I may go with that and we will see what happens. I did notice you had a couple of extra caps and a resistor in your 1974 schematic added for tremolo circuit stability. Those are in my draft board layout. Also on 18watt there is an improved tremolo circuit (Richie Hall I think), that has a changes...I'm looking to incorporate those as well.
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I think Josh from JMPGuitars.com (administrator 18Watt.com) changed the layout when I asked the same question. I found the PI sitting between the channels (tubewise) gave too much wires crossing and I saw no reason for it to be that way... so the 'modern classic' was born :smiley:
about the tremolo. Steve is right about the termolo being very finicky on those 18 watts. It's pretty hard to build it so it won't thump. But.. my findings on it being more successful:
- find the right tube and take care of the voltages throughout the entire amp
- I use shielded wires on all grids of the preamp tubes and solder one side of the shielding to the back of the pot they're connected to
- I also add a grounding terminal to the tremolo tube mounting and ground the tremolo jack at that point, using a shielded wire, which's shielding is connected from jack sleeve to that ground lug..
Doing these things have made it possible for me to get thump-free tremolo