Reverb Tanks: An Orientation Experiment

[nickb]

I recently had a combo where I thought the tray had been mounted upside-down - open side up - and no cardboard top. With the cables stuffed in the back of the amp the bag sagged against the springs. so I thought I'd mount it open-side down and put a stiff cardboard lid on the tray. Bad move - no reverb. When I pulled the tray out I could see the whole suspended innards flopped downwards just enough to make contact. Never seen this before and mounting position has never been on my list when replacing a reverb tray.

That's very interesting and a new consideration. Was it a 9.75" or a 16.25" spring job? I see the shortest sag about 5mm and the longer about 10mm. I wonder if the springs age and sag? A different slightly issue though. The explanation I have, see attached pages 5 & 6, on the orientation is quite clear about the magnets.

[nickb]

It probably is a long term consideration. Just because it doesn't matter right now doesn't mean 10 years down the road it won't. See this and scroll down to "Mounting Considerations".
https://www.tubesandmore.com/tech_co...d_and_compared

Dang! I attached the same article before I read your post - sorry!

On the tanks I have there is no difference in the placement of the internal floating tray between types but I think you raise an interesting and different point. Do reverb tanks 'wear out' due to spring sag? I never seen it and I can't say I ever looked either, but Mike Bailey maybe did.

[Chuck H]

The recommended drive is 6.5mA... for 3.5AT for the 150 ohms coil.

The specs for that tank indicate an input impedance of 160. But 150/160, that's a nit pick. So let's ignore that. But what is "3.5AT" ?

...and my signal generator only goes to 6Vpp.

AH HA! Did you measure the voltage from your signal generator with the test in progress? Most signal generators just can't provide much current. When driving a load the voltage may drop. I learned this when using my sig gen to test salvage transformers. I was getting all kinds of erroneous calculations before I started checking the voltage from the sig gen under load.

More to my point would be that regardless of the MOD/Belton/Accutronics specs., most good sounding reverb circuits drive the pan a lot harder than that and it's common. I even wonder if a reverb wouldn't be too noisy once recovered and re-amped with a tank driven at that level. It works with an amp down low, but with a guitar signal at some 25 watts a re-amped reverb signal that could keep up from a tank driven at only .2W would probably introduce a lot of noise IMHE. Anyway...

If you examine known good sounding reverbs you'll see a lot more drive to the tank. The classic 6G15 uses a 6K6 power pentode! The Fender BF design puts about a watt into the tank. I'm sure the spec. has something to do with liability or some such. But in the end it's up to the builder I suppose.

This is offered for your consideration. I'm not trying to be contrary for it's own sake at all.

[Mick Bailey]

It's a 16.25" tray in a Tone King combo - about 6 years old. I don't think the springs sag - maybe the tank was altered. I never investigated - just put it back to how it was.

[nickb]

I can only go by what is in the article, but somewhat agree with you as to how they can compensate for such tiny differences. Like I said, I think it is a long term consideration. I restore old Hammond organ and PA amps for guitar and have many of the old Gibbs tanks, made before Accutronics even existed and do observe the springs sagging due to hanging there for 50 years. I have many of the same style tank and there are several differences in how much reverb signal is present when changing out the tanks. Whether this is due to old transducers or spring sagging, I don't know. I DO know that there is a lot of difference in reverb signal between tanks of the same design.

This is a useful observation - we seem to be getting more evidence that the springs do age. Adjustment of the mounting tray height would compensate for this. But let me say again, it is a different, but related, issue.

Also you make a great point about the differences between tanks be much greater than the difference between orientations. +1

[nickb]

The specs for that tank indicate an input impedance of 160. But 150/160, that's a nit pick. So let's ignore that. But what is "3.5AT" ?



AH HA! Did you measure the voltage from your signal generator with the test in progress? Most signal generators just can't provide much current. When driving a load the voltage may drop. I learned this when using my sig gen to test salvage transformers. I was getting all kinds of erroneous calculations before I started checking the voltage from the sig gen under load.

More to my point would be that regardless of the MOD/Belton/Accutronics specs., most good sounding reverb circuits drive the pan a lot harder than that and it's common. I even wonder if a reverb wouldn't be too noisy once recovered and re-amped with a tank driven at that level. It works with an amp down low, but with a guitar signal at some 25 watts a re-amped reverb signal that could keep up from a tank driven at only .2W would probably introduce a lot of noise IMHE. Anyway...

If you examine known good sounding reverbs you'll see a lot more drive to the tank. The classic 6G15 uses a 6K6 power pentode! The Fender BF design puts about a watt into the tank. I'm sure the spec. has something to do with liability or some such. But in the end it's up to the builder I suppose.

This is offered for your consideration. I'm not trying to be contrary for it's own sake at all.

I don't mind contrary at all - it's important to a full discussion - so thanks

The signal at the the tank was 5.85Vpp. "AT" is amp-turns.

I just tried driving with to 6 to 24Vpp is 6V increments using a power amp. The 2nd harmonic steadily increases by about 20dB and above 6Vpp the 3rd becomes significant too. At 24Vpp the 2nd harmonic distortion is around 3%.

Above 27Vpp the longitudinal wave amplitude is so large than turns of the springs hit each other - you can clearly see the standing wave in the springs, hear them rattle and of course the output is terrible.

At no time did I see any clipping type effect due to saturation. There is a huge air gap to I guess that is no surprise, on reflection.

At no time could I see any transverse motion in the springs so there is not an issue with springs hitting anything.

The change in 2nd harmonic between up and down orientations remained about 2-3dB in all drive cases.

Here are my conclusions:
1) Orientation is of no practical significance as other distortions are dominant
2) The maximum drive current is about tens times the tank spec. I guess the spec is chosen for low distortion.
3) The springs age and sag.

[Chuck H]

Hey! Thanks for playing along. So you pushed the tank to over three watts with only 3% distortion?!? I might not have unless it was a tank I didn't mind losing, but someone had to do it Fun, and great info.

Can I assume that there was no noted change in signal output from the tank in any orientation?

[nickb]

Hey! Thanks for playing along. So you pushed the tank to over three watts with only 3% distortion?!? I might not have unless it was a tank I didn't mind losing, but someone had to do it Fun, and great info.

Can I assume that there was no noted change in signal output from the tank in any orientation?

There was no change in the output other than the 2-3dB in second harmonic and I only tested open face up vs down.