Supro 1622RT schematic

[lowell]

Hey all, I've searched for this schematic but couldn't find it... may be user error as sometimes my searching skills kinda stink! I emailed Supro as well but figured I'd ask here as there is never a guarantee they'll respond.

This amp is low volume and distorted output. Tried new tubes and another speaker just for a quick test... didn't help. These are really tough to troubleshoot by eye!!

[Kevina]

Two chances, little hope and no hope: Little hope has just left the studio.

Two of my buddies work for one of their distributors. They tell me that even distributors are permitted service data; any faulty units must be returned to the US for service.


But, the service data must be out there.

When the manufacturer had the amplifier certified for European electrical safety, the test lab would not have touched it without the data, and that data will be included in the test report. This report just may be published on the web. Try searching for phrases like EN60065 Supro 1622RT etc.

Kevin

[Enzo]

Isn't this a 50 year old amplifier? Or is this some latter day resurrection of the brand name?

[vintagekiki]

https://www.jhs.co.uk/news/2016/09/26/supro-tremo-verb-1622rt-manual/

[lowell]

Enzo this is a reissue... Full on PCB. Very hard to trace and is double sided board with only one side of board visible without disassembly.

Power tubes, 6793 tubes, are redplating, even a brand new set of EH. I am getting OPEN from grids to ground and this is a cathode biased amp, so there is a bad connection there, just having a hard time locating it. I've yet to find the grid leak resistors. I will though. The grid leak for these tubes is supposed to be 1M and yes I have my meter set for 6M.

[The Dude]

If it's cathode biased, there should be an obvious larger wattage cathode resistor (or 2) near the output tubes. Measure the resistance from cathodes to ground. What do you get? Maybe something around a couple hundred ohms- give or take? You might have a shorted cathode cap. That would more than likely cause red plating. I'd be checking the cathode circuit before I worried about the grids.

[lowell]

Yep it's 200ohm Rk, and Ck is good too as I have 18v on the cathodes when amp is in play mode. I found that these tubes pins 3/6 both are G1, so discovered that only Pin6 is used in the circuit. I do indeed have 220k grid leak from tube socket pin to ground. I retentioned both sockets to make sure the tubes are making good contact. I can only assume they are.

Still seriously red plating with all those measurements.

I get 75 ohms from CT to each end of primaries, and 4.5v across each primary when on and in play mode. Something is causing these tubes to draw massive amounts of current. That's 60ma just sitting there, on. ?? Baffled.

[lowell]

Also, no voltage at all on the grids, so it's not leaky coupling caps from the PI.

[The Dude]

I'd pull the output tubes to save them from burning up and measure the voltage at the grids of the tubes. Maybe you have a leaky or shorted PI coupling cap.

Edit: Sorry, you made that last post while I was typing.

[lowell]

I beat you to it!

[The Dude]

What does your B+ measure?

[lowell]

345v
I've seen this issue in Blues Jr's before. It was parasitic oscillation. On those amps, if I remove the PI, the problem goes away. The removal of the PI interrupts the oscillation. I tried that here, no effect. Still 4.5v across each OT primary.

Being that this is a 2-sided board, my gut is telling me there is conduction between traces where there shouldn't be.

What's your gut telling you Dude? I mean the China-bots that made this board are NOT the issue man, besides, China-bot is not the preferred nomenclature Dude, it's Asian-bot.

[Helmholtz]

Yep it's 200ohm Rk, and Ck is good too as I have 18v on the cathodes when amp is in play mode. I found that these tubes pins 3/6 both are G1, so discovered that only Pin6 is used in the circuit. I do indeed have 220k grid leak from tube socket pin to ground. I retentioned both sockets to make sure the tubes are making good contact. I can only assume they are.

Still seriously red plating with all those measurements.

I get 75 ohms from CT to each end of primaries, and 4.5v across each primary when on and in play mode. Something is causing these tubes to draw massive amounts of current. That's 60ma just sitting there, on. ?? Baffled.

Ohm's law tells me that you can't have 18V across a 200R cathode resistor with 2x 60mA plate currents. Re-measure resistance from cathodes to ground. I expect a lower value than 200R.

BTW, what is a 6793 tube? Datasheet, please.

[lowell]

Correct, that is 45ma per tube... still insanely hot for these tubes which are 12W rating. The discrepancy with your calculation and my other one on the OT primaries is most likely due to tolerance in my meter's measurement of the primary resistance and or voltage across the primary.

[lowell]

https://www.tubesandmore.com/sites/d...les/6973eh.pdf

apologies, 6973