Old National amp

[Valvehead]

Im working on a 63? National Glenwood. Im aware there is no scem for this amp , but theres a Valco thats close , but has no reverb. And thats my issue - reverb.

Heres what ive checked -
all resistors , coupling caps and electrolytics at the reverb tube ( 6973 ) , and right before / after it .
i have not checked all the disc caps .

chop-sticked everything
swapped tubes
cleaned / checked reverb pot ( about 300k)

cleaned/ tensioned the tank jacks.
tank readings are 1.3 ohm at input jack
168 ohms output jack
springs are fine...its a Gibbs tank

if i smack the tank i can hear the verb.....but its simply not in the signal when playing . cables test good--removing and touching rca cables when on , input has no noise , output has loud buzz

swapped the 6eu7 tubes around that are before/ after the 6973 tube.

here are DC voltages at the 6973 =
pin 1 n/c
p2 = n/c
P3 = -0.7mv
P4 = 000.5mv
P5 = 000.5mv
P6 = n/c
P7 = 15mv
p8= 400V
P9 = 400V

also cleaned / jumpered the Reverb 1/4 fsw jack


any ideas fellas ???? does that tank input measurement of 1.3 ohm sound way low ?

[Valvehead]

well...it wont let me post the scem (pdf) ?

its a Valco 510-80

[The Dude]

Something seems funky with those voltages. If I'm looking right, pin 9 is plate, and I don't see anywhere close to sufficient plate voltage for a typical reverb circuit- maybe it's not typical? Either an error, or you are indeed missing plate voltage. The Valco schematic I found doesn't have reverb, so I can't say more without seeing a schematic of what you have in front of you.

[Valvehead]

Something seems funky with those voltages. If I'm looking right, pin 9 is plate, and I don't see anywhere close to sufficient plate voltage for a typical reverb circuit- maybe it's not typical? Either an error, or you are indeed missing plate voltage. The Valco schematic I found doesn't have reverb, so I can't say more without seeing a schematic of what you have in front of you.

SORRY---- i posted the wrong tube

Its edited now

[glebert]

For the old amps that drove the reverb tank like a speaker I don't think 1.3 ohm DCR is too low. Usually the AC impedance is 8-10x the DCR. So when you factor in a few tenths for your probes that is probably an 8 ohm input tank. https://www.studiosoundelectronics.c...erbdcchart.htm

Valco also made Gretsch amps, did you look at something like a 6162? It also has 6973 power tubes, the preamp tubes are 12ax7 but those are kind of like a pin swapped 12ax7. http://www.prowessamplifiers.com/sch...tsch_6162.html

[Valvehead]

For the old amps that drove the reverb tank like a speaker I don't think 1.3 ohm DCR is too low. Usually the AC impedance is 8-10x the DCR. So when you factor in a few tenths for your probes that is probably an 8 ohm input tank. https://www.studiosoundelectronics.c...erbdcchart.htm

Valco also made Gretsch amps, did you look at something like a 6162? It also has 6973 power tubes, the preamp tubes are 12ax7 but those are kind of like a pin swapped 12ax7. http://www.prowessamplifiers.com/sch...tsch_6162.html

this National has 2 , 6L6 power tubes .

[The Dude]

Is the tube in the amp an actual 6973 or a 6CZ5? In a proper 6973, pins 1 and 8 are both screen connections. In a 6CZ5, only pin 1 is connected to the screen. So, if someone put a substitute tube in the unit, it would have no screen voltage. You could just move the wire from pin 8 to pin 1 as a check to see if that's the case. That way either tube would work in the amp.

[TomCarlos]

Valco 510-80 schematic, just in case - https://schematicheaven.net/bargainbin/valco_510-80.pdf

[Valvehead]

Is the tube in the amp an actual 6973 or a 6CZ5? In a proper 6973 pins one and 8 are both screen connections. In a 6CZ5, only pin 1 is connected to the screen. So, if someone put a substitute tube in the unit, it would have no screen voltage. You could just move the wire from pin 8 to pin 1 as a check to see if that's the case.

You got it-----it is a 6cz5 , but the seller said this is a version of 6973. Ill try swapping the wire

[The Dude]

The 6CZ5 is a valid substitute for the 6973, except for the G2 pin thing.

[Jazz P Bass]

Here is the Valco reverb circuit.

[g1]

It's a power tube, so it should either have a negative voltage at the grid (pin3 or 6), or a voltage at the cathode (pin7). I don't see either in your listed voltages.
Does it have a cathode resistor connected to pin 7 ?

[The Dude]

I think the tube is not conducting because there's no screen voltage to pin 1 of the tube. Hence, no cathode voltage.

[Valvehead]

The Dude.................is a damn genius.

I moved that wire and now i have huge , lush reverb ............THANKS MAN !!!!

[The Dude]

Glad you got it!

Just an FYI and clarification from Mr. Obvious:

A 6973 will still work in the socket as wired. If you ever change tubes, no rewire is necessary because it's screen grid is connected to both pins 1&8. I'm not sure why they didn't wire it that way from the get-go.