18 watt original merged with 18 modern classic

[petedub]

Hello, I have some simple questions regarding my 18watt build, I wonder if someone would be so kind as to help me. I should add that I prefer late 60s rock to heavy metal in my playing style and I use a 60s Strat with Alnico 5 pups. main amp at the moment is a 22 deluxe reverb clone that took me 6 months to build. I want to use the 18watt for very small bar gigs. I also own a PRRI and a supersonic 22. I am helped by an old mate who is a valve radio enthusiast and electronics whizz.

I'm rebuilding my 18 watt amp, previous build was on a tag strip (including smoothing caps) with a SS rectifier.

My new build is on a turret board and I'm installing a EZ81 rectifier and a 32/32 can cap. I have the correct chassis. My turret board is identical to the Mark Huss revised April 27 2004.

I'm trying to get this build as robust and quiet as I can manage.

I have been looking at the modern classic layout/schematic from JMP dated 04/25/2020 and I wonder if I should include the 10k resistors on the inputs. On channel 2 I think they replace the 68k resistors and allow for a shielded input wire to almost reach the valve, this looks like a good option and I'm currently mounting some tiny bits of tag strip next to the valve base for this purpose. However on channel one there doesn't seem to be any 68k resistors on the original diagram/layout.

Other questions - I have a 150ohm 6.5 watt resistor and a 47uf capacitor for the cathodes on the EL84s, does this look ok, the modern classic has higher values.

also - does it make more sense to use the left hand side of V3 for the input (looking from inside) or follow the original layout.

also - I have a 2.2k 5W resistor and a 1.5k and need to decide which is best in the power supply.

also - 470ohm resistor goes straight from v5 to cap can at the moment, is this a bad idea - likely to cause problems? (see photo).

also - I've studied the modern grounding schemes which worked well in my deluxe - am I right that the final filter cap is not part of the power supply ground but included in the preamp ground rail.

I have attached some photos of my amp as it stands, it still has some wires from the prev build attached to the valve bases and the 68k resistors on v3 so it needs a tidy up and some new wires. The turret board is only half done and not soldered yet.

I'm not a pro builder so any advice would be more than welcome. (this is the 3rd rebuild of my 18watt and I'm getting better with each build)

[Bieworm]

Hi

The easy and most obvious way is to build the modern classic according to the docs available. When you deviate from the standard or merge you need to know exactly what you're doing. It is pretty useful to understand the schematic and layout i.e. the way circuits work and what is what. It's also useful to start saying things like 'cathode pin of V1b goes to cathode resistor/cap ' rather than pin 8 goes to this turret. Believe me.. it's crucial to understand circuits and avoids wiring and connecting the wrong parts.
The reason for switching V2 and 3 on the Modern Classic vs the standard classic is to idealise the wire runs and crossings.

About the grid stoppers.. it really is not that important what value you use. The higher the value , the darker the sound will become. We use the 10k because it works and has no tone muffling properties.

In a standard 18W schematic the cathode cap value on the EL84 is indeed 47uf. The reason for using the 2000uf is purely to have a better bass response. I mostly use 1000uf for real estate reasons, but then again.. my amps always carry more circuit than standard builds
The 150 Ohm cathode resistor is not arbitrary. You need to determine the resistor value according to the bias setting. Ideally you bias the EL84's around 85% dissipation. You should get yourself some other values to set the proper bias. (150, 180, 200, 220, ..)

The last filter cap is indeed for the preamp and should be grounded at the preamp ground. Ideally all preamp ground points should be connected to the ground side of that cap and a wire from that point to the preamp ground (I usually connect this to the input jack ground and run a wire from there to a dedicated grounding terminal on the chassis near that input jack) In the case of this amp there is no other preamp filter cap, but you may come across other circuits that have multiple filter caps in the preamp. That's why you should consider the filter cap of the PI the last filter cap in the power section. All filter caps after this one are preamp related and thus grounds must be separated form that point.

There is no harm in PTP wiring the 470 ohm screen resistors directly to the filter cap. But I would suggest that you cover the bare wire with heat shrink or silicon tubing. That's a dangerous wire to accidentally touch. You might run into a situation where the orifice of the filter cap lug is too small to connect all necessary wires.

NOTE: it's better practice to pre tin the wires. There are wires in your pics that look like they weren't tinned before bending them around the turrets. It is also a good habit to seal the cut off wire isolation with heat shrink. This avoids the isolation from melting open and looks a lot better too.

[yello]

The reason for using the 2000uf is purely to have a better bass response. I mostly use 1000uf for real estate reasons.

I do this, and love how it sounds. I've tried 2200uf, but not 1000uf. Those are the two values I typically see in relation to 18w stuff - has anyone deviated slightly to try 1500uf, 2000uf, 2500uf, or even 3300uf, etc.? How did we arrive at 2200uf and 1000uf specifically?

I have some unused 1500uf caps sitting in my drawer...

[petedub]

Thank you Mr Bieworm, this is a great answer, and I am grateful to you for taking the time, I read these forums a lot and I realise some of my questions have been asked and discussed before.

I always pre tin my wires although the solder I'm using seems a bit reluctant to work properly - I've ordered some Kester 63/37 and I'm hoping this will work better. I've read a great forum entry on wiring to turrets and I think I will do better by heating the turret rather than just the wire.

I will wait till its finished and working to determine the cathode resistor value on pin3 V4 and V5, as for the capacitor in the same place, if I'm honest I always find it hard to discern a difference in sound when I tweak component values so was looking for what people prefer generally and your answer of 1000uf seems generally acceptable to folks here.

I will go with the 10k grid stoppers on V3 pin7 and that should be fine.

I will look into the heat shrink idea too, I'm trying to make this look nice and tidy as well as sounding good, while I realise that looking tidy inside won't necessarily make my playing any less sloppy My early builds were spaghetti monsters all PTP just trying to get some sound and they were very noisy - this is an area I am paying attention to as once it starts to look untidy it's like a slippery slope to a **** build. Thanks again to yourself and everyone here for sharing your expertise in such a friendly unselfish way.

[JMPGuitars]

Watch the soldering videos linked through my signature to improve your soldering technique for both turrets and tinning.

Thanks,
Josh