Ampeg V-4 weak reverb.

[drewl]

Plenty of signal coming off the plate of the 6CG7 driver tube, like 15v AC with volume up half way, but signal on other side of cap to tank is like gone.

Reverb recovery is fine.

Tried other tanks, driving an external tank from a tacked in plate cap, all the same.

As soon as you connect a tank, reverb drive signal is just knocked way down.

Any ideas? Thanks.

[nosaj]

Plenty of signal coming off the plate of the 6CG7 driver tube, like 15v AC with volume up half way, but signal on other side of cap to tank is like gone.

Reverb recovery is fine.

Tried other tanks, driving an external tank from a tacked in plate cap, all the same.

As soon as you connect a tank, reverb drive signal is just knocked way down.

Any ideas? Thanks.

What about the cable any issues with it? Maybe shorting to ground when connected?
nosaj

[drewl]

Thou must think me the fool!

Just kidding, cables are fine, no shorts, no opens, good contact.
Voltages for drive and return tubes good.

Unfortunately I can't swap in the new EH 6CG7 to rule out a weak original as it's too fat to fit in the pull down shield.

[nosaj]

Thou must think me the fool!

Just kidding, cables are fine, no shorts, no opens, good contact.
Voltages for drive and return tubes good.

Unfortunately I can't swap in the new EH 6CG7 to rule out a weak original as it's too fat to fit in the pull down shield.

You need one ? I think I can dig up some . Pay for shipping via Paypal If interested I'll hunt in the morning.
nosaj

[g1]

It's the stock tank with the spring lock?

[nosaj]

It's the stock tank with the spring lock?

That would be crazy....
nosaj

[g1]

I'm not asking if it's locked. Just an easy way to spot a non-stock tank.

[g1]

I'm also hesitant to consider 15VAC plate healthy when there's 270V worth of headroom.

[SoulFetish]

Plenty of signal coming off the plate of the 6CG7 driver tube, like 15v AC with volume up half way, but signal on other side of cap to tank is like gone.

Reverb recovery is fine.

Tried other tanks, driving an external tank from a tacked in plate cap, all the same.

As soon as you connect a tank, reverb drive signal is just knocked way down.

Any ideas? Thanks.

Yes, if the reverb tank had the wrong (lower) input impedance than the correct one, then the loading of the low input impedance would collapse the signal voltage. Also, keep in mind that this type of reverb drive might exhibit vastly different drive voltages depending on the drive frequency.
The 2nd digit in the reverb tank part # should be "F" for the stock Ampeg V-4 reverb, and no lower than "E" if you have to use another to test it.
Also, if the reverb tank is old, then there are a few things which can degrade the performance and cause problems even if the tank is has the proper input impedance.
What's the tank # of the one currently installed?

edit: also, what's the DC resistance of the tank input (disconnected)?
– and the DC resistance measured from the output side of the capacitor to ground?

[drewl]

Amp has the original correct tank with Ampeg number stamped on it with Accutronics correct part number.

Spring lock works, learned that lesson years ago!

Not enough drive voltage?

Might take up your offer Nosaj, thanks.

[drewl]

Just to clarify:

Cables are good.
Tank DC resistance around 130 ohms in and out, input grounded output not, both shielded cables have shield grounded.

With volume cranked it puts up to 50 vac to tank which is knocked down to .5v with tank connected.

[dmeek]

Did you check the 10K plate resistor on the 6CG7?

[drewl]

Aha!
Something to do with the coupling cap or cable connection spot on board.
Footswitch connects there, but that reads fine.

Using my meter leads connected between plate and tank with a cap across meter reverb signal is much better.

[drewl]

And that was it, bad plate cap