Voltage to reverb transformer

[RC99]

Hi All,

Tried a few one-tube reverb installations, all checks out with schematics but the reverb is very low. I'm wondering...in two of my builds, I had the B+ going to the reverb transformer (red wire) at the first dropping node, where voltage is around 330-350V. Is that not enough?




Driver is a standard Fender replacement 25K:8, tank is a 4AB3C1B.

[Jazz P Bass]

Is there a reason that Fender drove the tank with a both triodes of a 12AT7?
Just asking?
I would put the reverb control on the recovery side.
That way you are driving it hard all the time.

[Leo_Gnardo]

Is there a reason that Fender drove the tank with a both triodes of a 12AT7?
Just asking?

It's a little power amp. Double the power, double the fun. Considering the outboard reverb's tank was run from a 6K6, a single 12AT7 triode is a considerable step down in the power picture. Both triodes, not a bad compromise. (Been satisfying my ears just fine for nearly half a century.)

But I have seen amps that just use 1 triode of a 12AT7 or even 12AX7 (Boogie) and it works OK.

I would put the reverb control on the recovery side.
That way you are driving it hard all the time.

Deafinitely! Also reduces the volume of the inevitable ka-RASH when the amp gets jostled.

RC99, the schemo doesn't show any ground connexion for the drive side of the tank. I hope there is one. If not, that would explain a lack of volume. New crop of tanks have a little circus board at each input & output where you can "program" the connection of the transducer ground to tank box by placing or removing a dot of solder. If you're counting on the ground connexion to the tank output to also carry ground for the drive side, make sure the ground connexion is made. Or if you're like me and don't trust rivets to carry electricity, run a bit of wire instead of trusting those PC jumpers.

[RC99]

The ground/black wire from the transformer is connected/grounded, as are both RCA jacks, if that's what you mean. Are you saying I need to take extra measures to ground the 'input' in the tank? It's a 'MOD' 4AB3C1B. Or did I put one too many grounds?

When you guys mention putting the reverb control on the recovery side, right now the Reverb/mix is coming after pin 6. Is that what you mean, or the dwell?

This is pretty much how I have it laid out in my 5E3x2 (except for obvious speaker outs), which also has the 'Paul C' mod...




The amp does sound great as it is, and after talking with Tubenit (much thanks again!), I realize that this kind of reverb will never be as strong as the kind of circuit that's in a blackface. But as it stands, I really have to rank both dwell and reverb for it to even start to come in. Starting to wonder if there's a problem with the transformer.

[loudthud]

What is the DC resistance on both ends of the tank? Have you tried connecting the output of the transformer to a small speaker? Are you using a 12AT7 or a 12DW7? What is the voltage on pin 3 of the reverb tube?

[RC99]

Thanks for responding, Loud.

What is the DC resistance on both ends of the tank?

Between the 'tip' and 'ring' of the cables on each side....

30.9Ω on the input (white), 224.5Ω on the output (red).

Have you tried connecting the output of the transformer to a small speaker?

Not yet. there is reverb in the actual amp, it's just extremely low until both dwell and mix are cranked, at which point you get a long trail.

Are you using a 12AT7 or a 12DW7?

12AX7 as the one tube. The actual first tube on amp input is a 12AY7, not the AX7 like mentioned in the layout, but the second is an AX7.

What is the voltage on pin 3 of the reverb tube?

2.9V

[Leo_Gnardo]

The ground/black wire from the transformer is connected/grounded, as are both RCA jacks, if that's what you mean. Are you saying I need to take extra measures to ground the 'input' in the tank? It's a 'MOD' 4AB3C1B. Or did I put one too many grounds?

The amp does sound great as it is, and after talking with Tubenit (much thanks again!), I realize that this kind of reverb will never be as strong as the kind of circuit that's in a blackface. But as it stands, I really have to rank both dwell and reverb for it to even start to come in. Starting to wonder if there's a problem with the transformer.

One ground to each of the RCA's that are on the tank, that's enough. (There are some manuf's who "cheat" and run a single wire to the drive side, having the ground wire on the return side handle that function for both input & output. Think old Vox Cambridge, like that.)

"Dwell" is a volume control on the amp that drives the tank. Dial down for mellow moods, dial up & spank the tank for Dick Dale surf reverb. And of course if it's at zero, you're not shakin' the springs. You might want to consider "standing up" your dwell control on a resistor so you will always have some drive, even if the knob is set to zero.

We all encounter Fenders where there's too much 'verb at only 3 or so, and got used to it. If your amp gets to sounding good with the reverb at 7, 8, 9 so be it. As long as you can find a good sonic balance who cares what the number is.

One last thing that should'a been first. Dummy check (I have to do this myself) - you don't have the input & output swapped by any chance? That will keep a good tank down.

[RC99]

One ground to each of the RCA's that are on the tank, that's enough. (There are some manuf's who "cheat" and run a single wire to the drive side, having the ground wire on the return side handle that function for both input & output. Think old Vox Cambridge, like that.)

It's like in the layout I posted, I have the transformers black/ground wire running to a ground on one jack, connected to it on the other jack, and then that's connected to a 'master'/star ground running to the main ground

"Dwell" is a volume control on the amp that drives the tank. Dial down for mellow moods, dial up & spank the tank for Dick Dale surf reverb. And of course if it's at zero, you're not shakin' the springs. You might want to consider "standing up" your dwell control on a resistor so you will always have some drive, even if the knob is set to zero.

You mean just using a 1M resistor to ground instead of the dwell pot?

We all encounter Fenders where there's too much 'verb at only 3 or so, and got used to it. If your amp gets to sounding good with the reverb at 7, 8, 9 so be it. As long as you can find a good sonic balance who cares what the number is.

Yeah, lots of blackface verbs can have just too much dwell. This one even with the dwell up high, you have to crank the 'mix' up to about 7-8 to actually get some reverb, and even though the mix is still rather tame, the dwell/trail is pretty long by then. On this, it's hard to hear the reverb until the dwell is really far up, and it's always rather 'distant'.

One last thing that should'a been first. Dummy check (I have to do this myself) - you don't have the input & output swapped by any chance? That will keep a good tank down.

Yep, first thing I tried...several times back and forth. You hear no verb at all when they're reversed. Thanks.

[Leo_Gnardo]

30.9 on the input (white), 224.5 on the output (red)

a-HENH! There you are! Most reverb transformers sold currently (all I have seen...) drive a reverb tank input that is "nominally" 8 ohms but measures about 1 ohm with your meter. Somethin' funny about that tank. I thought I recognized the part number as the
"old standard" but your measurement is telling me no, that's not the case. Find yourself a tank that measures more like what we're expecting & I'll bet you have all the reverb you'll ever want.

Apologies to loudthud for jumping in. And thanks for asking the right question!

[RC99]

a-HENH! There you are! Most reverb transformers sold currently (all I have seen...) drive a reverb tank input that is "nominally" 8 ohms but measures about 1 ohm with your meter. Somethin' funny about that tank. I thought I recognized the part number as the
"old standard" but your measurement is telling me no, that's not the case. Find yourself a tank that measures more like what we're expecting & I'll bet you have all the reverb you'll ever want.

This is the one I got....

http://www.tubesandmore.com/products/P-RMOD-4AB3C1B

Apologies to loudthud for jumping in. And thanks for asking the right question!

I'm also wondering if I just measured wrong somehow. Between the 'tip' and 'sleeve' of the RCA cables on each side. I have other tanks that I'll try, but for some reason I have a feeling it's the transformer.

[RC99]

Tried two other tanks that measured correctly on my MM (a bit over 1Ω input, 225Ω output), but still have to crank both to really hear it.


And...now there's some sizzle/distortion coming in when the dwell is cranked up, so I'm going to have to backtrack some more and see how things are.

[loudthud]

According to the AES catalog the 4AB3C1B should be 8 ohms in to 2250 ohms out. So there's something wrong there.

Most of the one tube reverb circuits I've seen want you to use a 12DW7 so you can get some decent drive from the AU7 side. A 12AT7 might be good enough if biased properly. You want to get the current up to about 7mA which puts the cathode resistor at about 560 ohms or a little higher.

[RC99]

According to the AES catalog the 4AB3C1B should be 8 ohms in to 2250 ohms out. So there's something wrong there.

Most of the one tube reverb circuits I've seen want you to use a 12DW7 so you can get some decent drive from the AU7 side. A 12AT7 might be good enough if biased properly. You want to get the current up to about 7mA which puts the cathode resistor at about 560 ohms or a little higher.

Thanks Loud....


...so something like this?

[loudthud]

The 560 just goes on the drive side, the receive side doesn't need that much current. Try a 1.5K on the receive side, it may take a slight adjustment to find the sweet spot but it will be close.

[RC99]

Yeah....so it turns out the tank that was mistakenly sent to me is a 4BB3C1B (150 ohm input impedance), not a 4AB3C1B. they screwed up. Other older tanks that I have and measure correctly are a bit better but still rather weak. I'm getting some sizzle/distortion as I turn up dwell now, so that might have something to do with it.