Jansen 8/40 reverb tank specs?

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Jansen 8/40 reverb tank specs?

Post by RectifiedAmps »

Would anyone happen to have a multimeter and a 60's era Jansen with spring reverb? I have an 8-40 on the bench with a bad spring tank (long tank, 2 spring). The DC resistance of the tank output is around 160ohms, so it'd be equivalent to an Accutronics 4xBxx1B tank, but I'm wondering what the input DC resistance might be, so I can work out the AC impedance. Very likely a 4AB model but I thought I'd check before I pull the trigger on a new tank order. Thanks.

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Re: Jansen 8/40 reverb tank specs?

Post by JHorner »

Phone up Jansen and ask.

/joking

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Re: Jansen 8/40 reverb tank specs?

Post by Anvil Amps Alan »

If it is driven via a transformer by a valve I would use 4EB3 C1B.
I use those in my AB763 builds.
The driver transformers usually have an output impedance of a few Ohms.
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Re: Jansen 8/40 reverb tank specs?

Post by RectifiedAmps »

Anvil Amps Alan wrote:If it is driven via a transformer by a valve I would use 4EB3 C1B.
I use those in my AB763 builds.
The driver transformers usually have an output impedance of a few Ohms.
Ok, thanks for the tip. But aren't 4AB tanks more common for tube/transformer-driven circuits, like BF & SF Fenders? 8ohm input impedance would be a better match than the 600ohms of a 4EB tank, wouldn't it?

I think I have a spare 4EB tank around so I'll give that a try anyways and see how it sounds.

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Re: Jansen 8/40 reverb tank specs?

Post by Anvil Amps Alan »

Just checked a 4EB3C1B - (DC Ohms) 70 in 210 out.
Don't know where I put my codes for tanks list.
That tank sounds really lush on the AB763 and plenty of reverb effect available at the pot.
The mismatch is of less importance than with speaker output loads.
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Re: Jansen 8/40 reverb tank specs?

Post by RectifiedAmps »

I did a bit of poking around last night and it looks like Jansen used a push-pull reverb transformer - each half of the 12AX7 driver connected to an end, centre-tap to the B+. Another 12AX7 acts as a phase splitter. Pretty cool, albeit unnecessary idea.

The 4EB tank I have works fine with the circuit. It's not quite as lush as I'd expected but that may be due to other aging components in the circuit or perhaps by design.

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Re: Jansen 8/40 reverb tank specs?

Post by tubeswell »

If its a push-pull reverb driver, it'll zoom along if you use a 12AU7 for the driver stage (as long as you bias it for about -8V to -10V) and assuming the plate-to-plate load reflected through the transformer is somewhere between 10k and 20k with somewhere in between being a happy medium)
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Re: Jansen 8/40 reverb tank specs?

Post by RectifiedAmps »

tubeswell wrote:If its a push-pull reverb driver, it'll zoom along if you use a 12AU7 for the driver stage (as long as you bias it for about -8V to -10V) and assuming the plate-to-plate load reflected through the transformer is somewhere between 10k and 20k with somewhere in between being a happy medium)
Hmm - this is interesting because it had a 12AU7 in the previous position serving as a phase splitter to drive each half of the 12AX7, which drove the reverb transformer. I wonder if someone swapped them around at some point? Jansen did like to use 12AU7s as PI's in their power amp sections, but there doesn't seem to be much point here- your idea of it driving the transformer seems like a much better idea.

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Re: Jansen 8/40 reverb tank specs?

Post by tubeswell »

There are several 1W guitar amps out there that use 12AU7 for an output stage.

The effective plate resistance is lower than for a 12AT7, so it has less 'loss of gain' (when you hook it up to a load) than an AT, but with higher power handling/current sourcing and really good bandwidth - for extra dimension in the reverb driver side signal. The lower amplification factor means the reverb send is a bit tamer, but that's not always a bad thing in some amps. YMMV
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Re: Jansen 8/40 reverb tank specs?

Post by Anvil Amps Alan »

12BH7 is another option for the driver. The Hammond P40 organ amp used this with a push pull stage to drive their very long "necklace" dangly spring reverbs. I have dragged 3 Watts out of a 12BH7 when configured similar to a Firefly amp.
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