Reverb Circuit has now killed two reverb drivers. Please help!

[Gaz]

Hi all,

Built a Fender-ish amp with what I thought to be a pretty by the numbers reverb circuit, save the tone control and a few value swaps to make the reverb a bit more wild and stronger than a stock BF/SF Fender circuit.

I played this amp for may a few hours before the reverb went weak. Turns out the primary resistance on the driver was low - about 250 Ohms. New working unit measures about 800 Ohm. Partially shorted?

I had another, replaced it, and chalked it up to being af fluke, but a few hours later, same exact failure. Here's my schematic. The rest is basically BF Fender.

I am doing something stupid here to cause fly-backs to the driver?

Thanks!

[xtian]

Did you measure voltage on the reverb transformer primary?

"same exact failure" ... to clarify, you put in a second transformer that initially measured ~800R primary, and afterward it measures ~250R? That's, like, lottery odds.

[nickb]

Just throwing some ideas out. I see the cathode resistor is 820 as opposed to the more usual 2.2k. This means the DC current thru the transformer will be a lot higher so you might be exceeding it's limit and saturating it. It also means the current in the leakage inductance is higher and that means more energy in any back emf from the transformer. Further, the tank is a highly inductive load and so you'll get energy reflected back from that too. The higher current also means higher tube gm so you could be getting out of the linear region, especially if you made other changes that increased the drive to the 12AT7.

A bad connection between the transformer and the tank could spell disaster.

A 400V MOV across the transformer primary might save the day, and/ or put the 820 back to 2.2k.

[Helmholtz]

I see the cathode resistor is 820 as opposed to the more usual 2.2k.

Common cathode resistors even as low as 680 Ohm have been used in reverb driver stages of SF amps using the standard No. 022921 reverb transformer. (A saturating transformer would supply strongly reduced drive voltage and current to the tank.)
The original Fender 022921 reverb transformer has a primary DCR of 1700 Ohm to 1850 Ohm.

[mozz]

Is it the proper tank for that transformer and connected correctly?

[Helmholtz]

Is it the proper transformer for the drive stage?

[nevetslab]

Is that a typo on the second grid of the 12AU7 driver tube, where it's connected to the source side of R68 56k grid stopper resistor. Is it instead connected to the other grid directly? Just curious.

[Pedro Vecino]

The measurements in an Accutronics 4AB2C1B tank are approximately 10 and 250 Ohms.

[Helmholtz]

The measurements in an Accutronics 4AB2C1B tank are approximately 10 and 250 Ohms.

A tank with an 8 Ohm input transducer as specified in the schem has a DCR of about 1.0 Ohm.

[Pedro Vecino]

A tank with an 8 Ohm input transducer as specified in the schem has a DCR of about 1.0 Ohm.

You are right. I did not press the tips with enough energy

[g1]

As others mentioned, the primary of the 022921 should have resistance in the 1.7K to 2K range.
You mentioned in the first post the new one had about 800 ohms. This makes me question the type and quality of the replacement transformers you are using.

[Gaz]

Did you measure voltage on the reverb transformer primary?

"same exact failure" ... to clarify, you put in a second transformer that initially measured ~800R primary, and afterward it measures ~250R? That's, like, lottery odds.

I did not. You mean measure at the pin 1/6? And yes resistance were not identical, but in the same ballpark.

[Gaz]

Just throwing some ideas out. I see the cathode resistor is 820 as opposed to the more usual 2.2k. This means the DC current thru the transformer will be a lot higher so you might be exceeding it's limit and saturating it. It also means the current in the leakage inductance is higher and that means more energy in any back emf from the transformer. Further, the tank is a highly inductive load and so you'll get energy reflected back from that too. The higher current also means higher tube gm so you could be getting out of the linear region, especially if you made other changes that increased the drive to the 12AT7.

A bad connection between the transformer and the tank could spell disaster.

A 400V MOV across the transformer primary might save the day, and/ or put the 820 back to 2.2k.

I think you may be right on here...

[Gaz]

Common cathode resistors even as low as 680 Ohm have been used in reverb driver stages of SF amps using the standard No. 022921 reverb transformer. (A saturating transformer would supply strongly reduced drive voltage and current to the tank.)
The original Fender 022921 reverb transformer has a primary DCR of 1700 Ohm to 1850 Ohm.

Yes, but I am fully bypassing the 820R as well, so could that be too much potentially?

[Gaz]

Is it the proper tank for that transformer and connected correctly?

Yes.