The Amp Garage

You'll be fine. Play with V1 & V2. V2 is a little more preferable from signal/noise ratio perspective, but just find what you like. 5751s are a good candidate.

1:3 is on the high side for a IT in a guitar amp. Keep in mind that because the PI tube is inductor-loaded it will swing well above the B+, so you run the risk of slamming the power tubes even when the preamp section is very low. IT PIs are usually fed a lower B+ to compensate, usually by taking it ...

Thanks for posting - interesting design!

How does the PI work without the 2nd grid being AC grounded with a cap? Is that an omission, or do I need some learnin'?

I have actually thought about a magnetic tremolo in the past! Magnet on a motor next to a coil. Im not sure how sinusoidal it'd be

A quick experiment tracing curves on a 6L6GC with and without the presence of a 2 x 3 cm neodymium magnet placed 17mm (one toothpaste cap) from the tube envelope, at approximately mid-plate height: I tested with the magnet axis in-line with the electron flow (magnet axis aligned with the flanges), ...

My suggestion for the output transformer would be a quality Hi-Fi one like from the Dynacos (ST35?) - just wire it in Pentode-mode rather than Ultra Linear. There should be some Dynaco equivalent in Europe. Using an OT designed for a Marshall 18W might saturate too early and not have the wide frequ...

My suggestion for the output transformer would be a quality Hi-Fi one like from the Dynacos (ST35?) - just wire it in Pentode-mode rather than Ultra Linear. There should be some Dynaco equivalent in Europe. Using an OT designed for a Marshall 18W might saturate too early and not have the wide freque...

Where are you based?

My guess is something is drawing current and dragging the voltage down. Maybe bad filter caps?

Riffmonster wrote: ↑Mon Mar 29, 2021 5:37 pm Sorry Steve, it has been a while! I finally got some new tubes in and recorded a little bit with a load box today. I am quite happy how the amp sounds now, although I can imagine the tubes still need some time to break in.

JMP1.mp3

That's rock and roll alright!

Nice work. Do let us know how it sounds!

Steve, I had another look at our amps and noticed the 100nf coupling caps on the phase inverter. That is a rather uncommon value for that year, isn't it? Might that be related to the big filter cap values? Good spot I hadn't noticed that! The amp is so super tight though even with these. Actually I...

Also I noticed you have something wired on your output jacks that I don't have on mine. It looks like a wirewound resistor?

Thanks for posting the pics! Looks like you have the exact same OT as me, unfortunately with the exact same spot where the sticker used to be and has since fallen off lol. Will be interesting to see the exact measurement of the laminations. I think these are Dagnalls. I was just curious because they...

Colossal wrote: ↑Thu Feb 04, 2021 4:38 pm Gents,

Thanks for documenting your amps. Always nice to see these old Marshalls. How thick is the lamination stack on your output transformers? 1.5" or 2.0"?

Thanks.

Mine is 1.5"

No problem, its appreciated

I've already noticed a couple of differences from your board shot. Keen to do more digging - especially around the output transformer!