Grounding and reverb tanks

[yldouright]

I have a Crate KBA60 which has a very weak reverb effect and I noted in this thread that the grounding scheme can greatly influence the reverb effect. In that thread, the example shows how to reduce the reverb effect via grounding changes. If this works in reverse to increase the effect, where is the change more effective (input or output ground)?

[Mick Bailey]

All other things being correct (driver/recovery circuit functioning properly) then if you have an incorrect grounding scheme then fixing this will improve the reverb. That's assuming the transducer impedances are correct for the amp. Just changing the grounding on a pan that's correctly configured to begin with won't improve anything - you're likely to get problems.

If the reverb is weak with your amp, begin by making sure the pan type is correct and the connections are good. Sometimes a few push-pull-rotate cycles with some contact cleaner on the plugs is all that's needed.

[yldouright]

Thanks for your attention Mick Balley, I received this amp as a donor and I believe it is stock. The reason it was seldom used was because its reverb was always weak. I took it apart and cleaned everything, even new heatsink grease and the amp sounds okay save the anemic reverb. It's a 9" two spring tank and I think the impedence is 20/2,000. I can pull the accutronics number if it'll help. I have other tanks from some old Univox amps that were splash and crash on those amps. I'm tempted to switch them out blind but the more prudent thing to do is research first. On the Crate KBA60 there is a small pot on the circuit board very near the output which may control the driver current but without a schematic, I'm hesitant to turn it.

[g1]

I have never ever seen an amp with an internal trimmer for the reverb drive. It is most likely the bias trimmer for the output transistors. Turning it the wrong way could burn out the power amp section.
Please post the accutronics number for the tank in your amp.

Schematics available here: https://supportloudtech.netx.net/lou...category/13064

edit: Crate part number for tank is 79-201-02, accutronics equivalent is 8BB2C1B (or maybe A suffix if it's mounted upside down).

[galaxiex]

I have never ever seen an amp with an internal trimmer for the reverb drive. .......................

Me neither, but I have seen trimmers on the recovery side in some SS amps.

[g1]

That would make more sense. It's hard to get enough drive so usually no reason to cut it back with a trimmer.
In this case the schematic shows a bias trimmer (AP1) for the power amp idle current.

[Randall]

are we sure the input/output jacks are in the right position?

[J M Fahey]

Oh, I have seen reverb circuits with trimmers, both in the drive and the recovery side.
And sometimes I have used them myself.

They are useful when you have weak/limited drive capability and/or poor headroom on the drive side and/or noise (which might be magnetic coupling from PT to transducer and you simply don´t have distance enough inside the head cabinet to separate them more) so to extract that last dB signal-to-noise improvement you set recovery "as loud as you can without feedback or unbearable hum" and then Drive "as loud as you can without clipping drive amp on loud chords" which sounds horrible.

I have seen trimmers in old SS Gibson amps, and famously on Fender Super Champ 12 where reverb tank is driven straight from own speaker out (so you *are* creating a feedback loop on purpose) and then set the recovery gain trimmer so you stop some 2 or 3 dB below ringing or uncontrolled feedback.

Cheesy as Hell but works, I have used a similar trick to add Reverb to amps where there was none, and lack of space or suitable supply voltages to use a conventional reverb circuit.

And so Fender can claim "Tube Driven Reverb" without blushing ... Reverb tank IS driven by a Tube and Transformer (the power tube that is )

[yldouright]

@Randall
Yes, they're connected properly. I double checked by reversing them and the reverb disappeared. The serial number indicates 1BB201A and it measures 206/2830R on the input/output. It's supposed to be a medium decay, no lock, fully isolated and horizontally mounted with open side up.

@g1
I appreciate the link. The amp sounds fine with my Timebender and that pedal usually reveals bad sounding amps (ie: high order harmonics) so I think I should try a new tank. I think using a lower impedence tank (maybe 15/220R input/output) should do the trick. There doesn't seem to be enough voltage to drive the input properly if I'm reading the schematic correctly.

Thanks all for allowing me to tap your fertile minds. What's the diagnosis from the priests of the Oracle?

[g1]

Others have reported the 1BB2C1A as correct for this amp.
However, those resistances seem real odd. DC resistance should measure around 25ohm at input and 200ohm at output end. (150 and 2K2 impedance)
You said it's open side up, so 8BB2C1A should be the one to replace it with.

[yldouright]

@g1
I should have been clearer. I calculated the impedences provided from the measured voltages. Your numbers are closer to the passive ohm measure and that was why I suggested a lower ohm value input tank. A 15R passive reading is likely to excite the springs more than the stock 25R input given the same output value but I may not understand exactly how the tank circuit works so further guidance is welcome. I'm not kidding when I say this combo with max reverb is nowhere near wet but damp. Have a look at what this owner says about it.

[52 Bill]

This is a 30 year old amp, so it is hard to know what has changed from the original build. From the earlier posts, it seem that the tank you have is the correct one for the amp, so what you need to do is to determine if the tank is working correctly and if the amp is working correctly.

Most of the amps that I have seen seem to have a return circuit with lots of gain, so if you shook the reverb tank there is always a loud sproing through the output. Does shaking your tank make this sound?

[g1]

Agree with 52Bill, check that the circuit is working right first. Yes, there are some guys online complaining about the reverb, but if it was that bad, and a design problem, they should all be complaining. The schematic shows test point (5 & 6) and AC voltages you should see under conditions stated in the notes.

If everything checks out, I think you would do better with a higher input impedance tank rather than a lower input impedance. Like maybe an EB or FB type. IC drive is usually limited by current, so a lower impedance input tank will probably just make things worse.
Alternatively, some have reported success by lowering the value of R30 at the drive IC, which will increase the gain of that stage. (RF/Rin, with R30 being Rin, Rf is R31 paralleled with tank input impedance)

[yldouright]

@g1
I think it depends on the amp design and whether it can make current at the given voltage. As a general rule, higher voltages don't mind higher impedences. I would define the voltage rails on this amp as low and that was my rationale for the lower impedence tank. What is the relationship of the input/output ratio to the effect influence? Forgive the lack of photos, I don't really want to take that amp apart right now. I'm busy planning for our Memorial Day celebration and this year is special given the successes Trump has had with the aid of our military. I'm still waiting to see some Bushes, Barry and Hillary behind bars for their complicity in attacking our US Consititution. That document really is the only reason to be a US Citizen and every day those clowns walk around freely should be a day of infamy in the US. Excuse the rant, back on topic

@52 Bill
Bouncing the amp gives the expected cymbol crash sound so yes, the tank seems to be working. It's just not wet enough for my taste. I'm pretty sure this amp came from the original owner and he stopped using it because he wanted more reverb. I guess you guys know I share that opinion. I've fixed a few Crate amps and as a rule, their spring reverb is pretty nice so I don't know why this one is so anemic. Speaking of which, I had a GX-40C a while back that I only half fixed but that discussion is found here for those interested.

[g1]

Ok. Well, I only mentioned the higher impedance tank as in general, solid state reverb circuits use them. Whereas tube circuits with driver transformers use lower impedance tanks. The only thing lower than a B input would be an A, and I've never seen one directly driven by an op amp, they can not provide the current.
Good luck.