New build: Supro 1624T in a Marshall Lead 12 donor amp

[klooon]

Time for a new build. I will post the progress here and, again, your support and comments are welcome!!

The idea is to use an old Marshall Lead 12 solid state amp. Rip the electronics out and convert it to a nice tube amp. It will be a bit crowded as it has a small chassis, so I have to make some compromises in terms of space and functionality...

Inspired by a few guys LeonC (Valclone), Eric (DG amps) and Snicksound, I like to take my best shot of the various 1624T versions.
As the basis for my schematic and layout, I will use these
1) http://www.dirtygirlamplification.co...iles/1624T.pdf
2) http://www.snicksound.com/wp-content...Of-24-v0-1.gif (draft)

In order to save space and also because my experience with tube rectifiers (especially 5Y3 type) is that they start to rattle fairly quickly in my combos, I am intending to use a big sag resistor to create some sag and voltage drop. Further, I will use 6V6s as power tubes instead of the 6973, due to the better availability and as LeonC showed in his youtube videos: a great sound.

First: transformer choices...
For the PT I am thinking of this one (less than 40 euros):
https://www.tube-town.net/ttstore/Tr...6VA::4908.html
which is a 56VA with 300V secondary@100mA and 6.3V@2A. Q: Should be enough for a pair of 6V6's and three preamp tubes?? My Princeton has a Hammond transformer with 81mA and, although getting a bit hot from time to time, it works perfectly well.

For the output transformer, I found [link] that it is similar to a Tweed Deluxe OT, so ~8kOhm primary . As it is a pain to import parts/transformers to Europe, I am stuck to the "European supply chain". Hammonds are readily available, so I think a 1750E would do.
Out of curiosity, does somebody have experience with this one? Tube-Town Fender Style 25W OT 8.5k

Regards and cheers,
Mark - The Netherlands

[mozz]

2 supro builds in my future, don't know which models yet, would like to hear how the 12ax7 grid leak bias works out.

[olddawg]

I like to find dead SS Marshall Amps and use the cabinets. Much cheaper than buying a cab. I doubt space will be a huge issue. Parts are much smaller than the dinosaur stuff. You can fit power supply electrolytic on a tag board instead of giant can caps. If you have to .... you don't necessarily have to mount the OT on the chassis.

[klooon]

2 supro builds in my future, don't know which models yet, would like to hear how the 12ax7 grid leak bias works out.

For me one of the reasons to build this amp - I am also curious! To be continued.

[klooon]

I like to find dead SS Marshall Amps and use the cabinets. Much cheaper than buying a cab. I doubt space will be a huge issue. Parts are much smaller than the dinosaur stuff. You can fit power supply electrolytic on a tag board instead of giant can caps. If you have to .... you don't necessarily have to mount the OT on the chassis.

Let's see how it works out. I already made a micro-Bassman (Rob Robinette) in a Marshall SS Lead 12. Fitted perfectly (only 3 noval tubes). The toroidal PT fits nicely underneath the chassis, no need to remove a huge square space and thus more space for the eyelet board (I hope it doesn't interfere...). I will fit the power supply electrolytic caps throughout the board to follow the grounding scheme in which you put the filter cap close to the supply/grounding node.

[klooon]

First: transformer choices...
For the PT I am thinking of this one (less than 40 euros):
https://www.tube-town.net/ttstore/Tr...6VA::4908.html
which is a 56VA with 300V secondary@100mA and 6.3V@2A. Q: Should be enough for a pair of 6V6's and three preamp tubes?? My Princeton has a Hammond transformer with 81mA and, although getting a bit hot from time to time, it works perfectly well.

I put some more thought in the PT choice...
With this 56VA toroidal transformer of 300V@100mA and 6.3V@2A after (SS) rectification is about 420VDC (300*1.4). To bring this down to 320VDC with a sag/dropping resistor is maybe too much. You need a very large resistor, with a huge drop and it may get too "spongy" (not sure).
For my micro-Bassman build [link build report], I used a 50VA toroidal transformer rated at 250V@120mA and 6.3V@3A. After SS rectification I measured 350VDC (corresponding nicely to 1.4*250). This could be a good choice together with a smaller sag resistor (100R-200R) to knock down the B+ a bit to the required 320VDC [link]

[klooon]

Work in progress... First version of the layout.


Comments are welcome!

[mozz]

I wouldn't put grid stoppers on V1 or output tubes unless they are needed. Too many small changes and it won't sound like a Supro anymore. But i am from the school of screen 470's.

[klooon]

I wouldn't put grid stoppers on V1 or output tubes unless they are needed. Too many small changes and it won't sound like a Supro anymore. But i am from the school of screen 470's.

Thanks! I will leave them out for now. Also the 4k7's? Any other things?

[klooon]

some progress...

[mozz]

How come when I use those Neutrik style jacks, they always strip? I have the transformers and 6973's from a Thomas organ amp , and a deluxe sized chassis for my build, may start on it next month.

[klooon]

How come when I use those Neutrik style jacks, they always strip?

Maybe too much heat applied to the jacks? I have good results so far with these Neutrik/Rean/Cliff jacks. I had quite some failures with the Switchcraft jacks. The switching failed after a while.

I have the transformers and 6973's from a Thomas organ amp , and a deluxe sized chassis for my build, may start on it next month.

Great. Looking forward to see your amp!!

[klooon]

The amp is finished!





and the layout (version 2) with notes and measured voltages:

[The Dude]

Nice tidy build! How does it sound?

[klooon]

Then the sound...

The first channel (grid leak preamp) works great... nice bold sound with great clean overtones. I bypassed/did not use the grid stoppers (470R, 4k7, 2k2) and so far it seems fine, but more experimentation is needed.

The second channel is NOT so great. The tremolo is fine, but needed some adjustment to get lower speeds (0.01uF to 0.02uF, see layout v2). BUT the volume is much and much lower and distortion is AWFUL... Tremolo pedal/switch on/off did not help anything, BUT when I physically disconnected the tremolo circuit (disconnect wire to V1 cathode pin 8), then it was great!! I changed the 2k7 cathode resistor to 1k2 (2k7 parallelled by a 2k2) and it is a bit better, but still not okay... So what is happening here?? The tremolo circuit, also disengaged (by switch) is apparently interfering a lot... Any explanation and ideas for improvement???