Supro 1622RT No reverb

[Chuck H]

Except for one of the 6973s is red plating.

Moved 6973, red plating moved with tube.

You have a bad power tube or tubes and/or an incorrect bias condition. As John said, this should be corrected first and foremost. This may be causing some hum but it would be independent of the hum increase with the reverb volume control.

Reverb control affects volume of hum, volume pot has no affect on hum. Hear reverb when bumping tank, but no reverb with instrument installed,

Your reverb recovery circuit is working but there is some fault in the circuit that drives the reverb tank. The increased hum with the reverb volume control may be normal as these circuits are typically noisy. If it's excessive hum there may be a grounding fault or an incorrect grounding scheme for the reverb circuit. MOD tanks have nifty little circuit boards on the jacks that allow for easy changes to how the tank chassis and signal are grounded there. Incorrect grounding scheme at the reverb tank jacks will cause hum.

[ca7922303]

You have a bad power tube or tubes and/or an incorrect bias condition. As John said, this should be corrected first and foremost. This may be causing some hum but it would be independent of the hum increase with the reverb volume control.



Your reverb recovery circuit is working but there is some fault in the circuit that drives the reverb tank. The increased hum with the reverb volume control may be normal as these circuits are typically noisy. If it's excessive hum there may be a grounding fault or an incorrect grounding scheme for the reverb circuit. MOD tanks have nifty little circuit boards on the jacks that allow for easy changes to how the tank chassis and signal are grounded there. Incorrect grounding scheme at the reverb tank jacks will cause hum.

I did notice a drop in the amount of hum when touching tip of RCA output cable and other hand on chassis.

[Chuck H]

Yes, a schematic would be helpful. I think they're not readily available at this time though. Supro claims this amp is Class A 25W. Which is absolutely impossible.

You only mention measuring the tank but not whether it's a replacement tank. Is it? Or is this the original reverb tank? If it's original and you haven't changed anything I would expect the ground scheme on the jacks to be correct. If it's a replacement or even a new, but OEM tank there's a chance the ground scheme at the tank jacks needs to be idealized.

If the amp is fixed bias you should check voltage at the 6973 grids. If it's cathode biased you need to analyze that circuit. Do the 6973's each have their own cathode circuit or is it shared? Check resistance for the cathode resistor/s and check the bypass capacitors for shorts. Since the red plating follows the tube chances are good the bias circuit (whatever type) is fine and you just need power tubes. If the amp is fixed bias this should include a bias adjustment. But then... The new version of the 6973 tube may not be as rugged as the vintage tubes. This isn't uncommon. In which case even if the amp is cathode biased it may require adjustment.

In any case the bias circuit should be checked because there's still a chance what you have is one dead tube and the other red plating because of whatever bias condition killed it's neighbor.

This should be dealt with as a priority over any problems with the reverb or hum.

[ca7922303]

Yes, a schematic would be helpful. I think they're not readily available at this time though. Supro claims this amp is Class A 25W. Which is absolutely impossible.

You only mention measuring the tank but not whether it's a replacement tank. Is it? Or is this the original reverb tank? If it's original and you haven't changed anything I would expect the ground scheme on the jacks to be correct. If it's a replacement or even a new, but OEM tank there's a chance the ground scheme at the tank jacks needs to be idealized.

If the amp is fixed bias you should check voltage at the 6973 grids. If it's cathode biased you need to analyze that circuit. Do the 6973's each have their own cathode circuit or is it shared? Check resistance for the cathode resistor/s and check the bypass capacitors for shorts. Since the red plating follows the tube chances are good the bias circuit (whatever type) is fine and you just need power tubes. If the amp is fixed bias this should include a bias adjustment. But then... The new version of the 6973 tube may not be as rugged as the vintage tubes. This isn't uncommon. In which case even if the amp is cathode biased it may require adjustment.

In any case the bias circuit should be checked because there's still a chance what you have is one dead tube and the other red plating because of whatever bias condition killed it's neighbor.

This should be dealt with as a priority over any problems with the reverb or hum.

Emailed supro support about a schematic. Meanwhile have readings on all 7 tube sockets, power on, no tubes installed(some are going to be difficult so until I can come up with an adapter or a way to test all of them, I left all tubes out)

6973 x2 #1(426vdc)
#2(3 mv)
#3(3mv)
#4(-02mv)
#5(-0.2mv)
#6(-0.2mv)
#7(0mv)
#8(0mv)
#9(426vdc)

12AX7 next to 12AT7
#1(420vdc)
#2(413vdc)
#3(0mv)
#4(-0mv)
#5(-0mv)
#6(420vdc)
#7(413vdc)
#8(0mv)
#9(-0mv)

12AT7
#1(413vdc)
#2(0mv)
#3(0mv)
#4(-0mv)
#5(-0mv)
#6(414vdc)
#7(0mv)
#8(0mv)
#9(-0mv)

3-12AX7s side by side on preamp board
From left to right
12AX7#1(414vdc), #2(0), #3(0), #4(-0), #5(-0), #6(414vdc), #7(-0), #8(-0), #9(-0)

12AX7
#1(414vdc), #2(0), #3(0), #4(-0), #5(-0), #6(4mvdc), #7(2mv), #8(-0mv), #9(-0mv).

12AX7
#1(390vdc), #2(-0(, #3(0), #4(-0), #5(-0), #6(414vdc), #7(390vdc), #8(1.5mv), #9(-10 mv).

[Chuck H]

Well there's not much to be gleaned from measurements without tubes in, other than being able to see that voltage IS being applied to the tube sockets, And...

The amp would appear to be cathode biased OR there is no voltage from the bias circuit.

The second to last tube listed (12ax7) appears to have an unused triode OR there is no voltage from the plate supply for that circuit. Could this be part of the reverb driver? (no schematic)

The third tube listed (12ax7) has HV on the grid of both triodes. Which seems odd to me. Maybe a pair of direct coupled cathode followers after the PI?

[g1]

Here is the tube layout drawing.
Install the tubes except the power tubes.
Measure again the voltages on the Reverb Driver tube (12AT7).

[ca7922303]

Here is the tube layout drawing.
Install the tubes except the power tubes.
Measure again the voltages on the Reverb Driver tube (12AT7).

Appreciate that. Before I saw this, I ordered match set of 6973s and swapped a 12AX7 for the 12AT7 and now there is reverb. I'm attaching a photo of socket showing why I couldn't get readings with tube installed. I'd like to go ahead and get readings, but still thinking about how to get probes on back pins.

[g1]

In that case, either the 12AT7 or the solder connections on it's socket are the problem. Sometimes with bad solder on the socket, just the wiggling from pulling a tube out & back in will temporarily 'fix' the connection. Probably the 12AT7 was just bad though.

If you still want to do those measurements, take a piece of component lead that you snip off after installing a resistor. Wrap one end around the tip of your meter probe. That should give you a thin enough probe tip extension so it should be able to fit in those slots cut in the socket.

[Kevina]

No schematics; even their distributors are not given them????????

[ca7922303]

In that case, either the 12AT7 or the solder connections on it's socket are the problem. Sometimes with bad solder on the socket, just the wiggling from pulling a tube out & back in will temporarily 'fix' the connection. Probably the 12AT7 was just bad though.

If you still want to do those measurements, take a piece of component lead that you snip off after installing a resistor. Wrap one end around the tip of your meter probe. That should give you a thin enough probe tip extension so it should be able to fit in those slots cut in the socket.

12AT7 installed:
#1(286v)
#2(167v)
#3(0)
#4(-0)
#5(-0)
#6(286v)
#7(167v)
#8(170v)
#9(-0)

12AX7 installed in 12AT7 socket:
#1(289v)
#2(169v)
#6(286v)
#7(168v)
#8(170v)

#3,4,5,9 same as 12AT7.

[g1]

So do you have reverb still with one but not the other? Or neither?

[ca7922303]

So do you have reverb still with one but not the other? Or neither?

Deoxit, now both work. Thanks again.

[g1]

Then I think you probably weren't getting good meter contact with some of those voltage readings (pins 3 & 8). But if it works, that's good enough. Maybe just oxidized socket pins.
If the reverb conks out again, that tube socket probably needs resoldering.

[Chuck H]

Do we still have the red plating tube issue???

[ca7922303]

Do we still have the red plating tube issue???

Yes, still red plating one(1) 6973, have a set on order, will post power tube(installed) readings shortly.