Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BobL on February 26, 2021, 10:01:17 pm
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I have two questions, but I guess I'll put them in separate threads for people searching for answers later...
1. I just built up this amp based off of a Benson Monarch, and it sounds *really* good.
2. What are the steps for measuring the amp itself to figure out the appropriate reverb tank? The thread I found on this topic kind of confused me vs what I'm seeing.
With amp off, I measured from the tip of the 'send' to ground... and... I get nothing.
I measure from the tip of the 'return' to ground and I get 29K ohms...
I think I'm doing something wrong... finding lots of info on measuring tanks themselves, or to convert the resistance to the appropriate impedance for the tank, but having a hard time finding info on how to measure.
Here's the schematic:
http://www.boblefevremusic.com/temp/Monarch/BENSON%20MONARCH%20REVERB.jpg
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It seems that there is not a grid leak resistor in the reverb driver.
/Leevi
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> measuring the amp itself to figure out the appropriate reverb
There's not much to go on.
By inspection, this is a naked cathode follower. You want the HIGHEST input impedance tank you can buy. 2K?
The output is naked grid so again a high impedance. 5K?
> not a grid leak resistor in the reverb driver.
It is direct-coupled to a preamp plate at nearly +200V DC.
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not a grid leak resistor in the reverb driver
It is direct-coupled to a preamp plate
Yes Indeed!
/Leevi
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I've found this link helpful for figgurin` out what the accutronics naming convention means
I've used the tank from a hammond l100 organ (gibbs 4l/4fb1b1c) for a similar capacitor coupled 12dw7 reverb and it seemed to work well. obviously you'll have to figure out where and how the reverb tank will mount to get the proper one
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There is no reverb driver transformer !
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There is no reverb driver transformer !
As per some Ampegs
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_reverb_rocket_12r.pdf
Hence the requirement for the excitation coil to be high impedance.
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There is no reverb driver transformer !
As per some Ampegs
https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_reverb_rocket_12r.pdf
Hence the requirement for the excitation coil to be high impedance.
I always learn here, thank you !
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Thanks all.
Looking at the Mod models (I think that's my preferred brand, and what Benson is putting in their reverb units), the choices for high impedance inputs would be 1925ohms and 1475ohms.
Am I understanding correctly that the impedance of the reverb tank doesn't really have any considerations in terms of damage to the tubes that a mismatch on impedance for speakers would have, and it's more a consideration on sound? I think I'd prefer a 3 spring tank, and those appear to all be 1925 ohms in, 2575 out... would I need an insulated input and grounded output for this? That's how I'm reading the schematic...
Would an option be to just order a tank and see how it works, and then adjust down if it doesn't sound good? They aren't particularly expensive...
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The higher the impedance of the excitation / drive and pick up coils, the better, as that will make the most of the limited send power and recovery gain.
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Here is an article: "Finding the right tank for an old amp."
http://dougcircuits.com/reverbtanks.html
Hopefully this will help.
Looking at your schematic, the driver end of the tank is not connected to ground. Maybe that is why you get an open when you measured from the tip of the 'send' to ground.
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Here is an article: "Finding the right tank for an old amp."
http://dougcircuits.com/reverbtanks.html (http://dougcircuits.com/reverbtanks.html)
Hopefully this will help.
Looking at your schematic, the driver end of the tank is not connected to ground. Maybe that is why you get an open when you measured from the tip of the 'send' to ground.
Trying to wrap my head around this, but I don't believe that I have a plate resistor on the driver side of the reverb tube, and that's what he's using to calculate... I'm also confused because his example has the signal coming off of the plate and into the tank, but on my schematic, I show the tank being connected to the cathode... so I'm lost.
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Don't fret about 1500 vs 2000. (They may even be the same coil measured different.)
An 8 ohm coil will suck so bad you might not hear it.
Most of the grounding options are unimportant in DIY because you can snip-add jumpers and screw to wood.
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> I don't believe that I have a plate resistor on the driver side ....his example has the signal coming off of the plate and into the tank, but on my schematic, I show the tank being connected to the cathode...
Just mentally swap things.
However that link's method is incomplete and over-simplified. We do not want a "match" but a "mis-match". Which may be difficult. And the way this Benson happened, we don't want "the optimum fit" but a "what can we do for a buck?" fit.
However: tube is high impedance, we want hi-Z tank.
We know that Hammond and Fender devised much more elaborate reverbs, for some reason. So we may suspect that this 1-buck in-built will not equal the big external units like Mr Dale played. Which went to 100% or more. But many other players just want a 5% taste of reverb. Does not have to be "perfect".
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> I don't believe that I have a plate resistor on the driver side ....his example has the signal coming off of the plate and into the tank, but on my schematic, I show the tank being connected to the cathode...
Just mentally swap things.
However that link's method is incomplete and over-simplified. We do not want a "match" but a "mis-match". Which may be difficult. And the way this Benson happened, we don't want "the optimum fit" but a "what can we do for a buck?" fit.
However: tube is high impedance, we want hi-Z tank.
We know that Hammond and Fender devised much more elaborate reverbs, for some reason. So we may suspect that this 1-buck in-built will not equal the big external units like Mr Dale played. Which went to 100% or more. But many other players just want a 5% taste of reverb. Does not have to be "perfect".
Cool, appreciated. I am for sure in the '5 - 10% reverb' camp. I'll get the high impedance MOD and see how it goes.