I came across this thing recently and thought it would be a cool project to fix up and maybe come up with some mods. It's a really solid box - all tube, nary a transistor in sight! In one box I can get stereo spring reverb (really, with 2 separate tanks!), and maybe just use it as a stereo tube saturation/distortion too. Mine has super weak signal so I figure a bunch of the old Suzuki oil-filled caps are bad. Got some replacements in the mail.
As I've been digging into it I realize that there may actually be a third effect it produces: some sort of stereo field widening scheme. Take a look at this schematic I've annotated:
Notice that the Blue and Pink nets go between channels, doing some sort of cross-feedback thing combining each dry signal with the opposite wet signal. It's further complicated by the negative feedback from the reverb tank drive output back to the cathode of the first stage. I'm trying to figure out exactly what is going on here:
1. The Green and Yellow nets are negative feedback, right? Just like a Champ 5F1 - even has the same 22k resistor! Just wanna make sure I'm understanding this right.
2. What's up with the caps parallel to the attenuation resistors on the Blue and Pink cross-feedback nets? C13 and C14. I guess we want the full audio signal to carry, but only an attenuated DC voltage?
3. I'm having trouble understanding how the Blue and Pink cross-feedback lines interact with the negative feedback lines and the input signal on the first triode V1A/V2A. Which direction is the feedback going? Is the V3B/V4B cathode driving or being driven? I would think being driven because there's no other way for a dry signal to reach the outputs, but what causes this to be true?
4. If the Blue and Pink lines really are driving the V3B/V4B cathodes, that would mean we're putting an out-of-phase signal from the opposite dry channel on each wet output stage's cathode - positive feedback-style, but not the same signal, just a presumably "similar" signal if it's normal stereo music. What are the implications of this? Do we get weird stereo phase-cancellation stuff? Is this the origin of the stereo-widening effect? What are the effects on mono-compatibility of the output?
I'd appreciate any help y'all can offer! I'm pretty handy with electronics, but not any sort of expert on tubes or too complicated of analog stuff. I'm planning on installing a switch to optionally disconnect those cross-feedback lines to get wet-only outputs, but I'd like to keep the option as part of the unique sound of the unit. I just want to understand what's going on!
Thanks!
As I've been digging into it I realize that there may actually be a third effect it produces: some sort of stereo field widening scheme. Take a look at this schematic I've annotated:
Notice that the Blue and Pink nets go between channels, doing some sort of cross-feedback thing combining each dry signal with the opposite wet signal. It's further complicated by the negative feedback from the reverb tank drive output back to the cathode of the first stage. I'm trying to figure out exactly what is going on here:
1. The Green and Yellow nets are negative feedback, right? Just like a Champ 5F1 - even has the same 22k resistor! Just wanna make sure I'm understanding this right.
2. What's up with the caps parallel to the attenuation resistors on the Blue and Pink cross-feedback nets? C13 and C14. I guess we want the full audio signal to carry, but only an attenuated DC voltage?
3. I'm having trouble understanding how the Blue and Pink cross-feedback lines interact with the negative feedback lines and the input signal on the first triode V1A/V2A. Which direction is the feedback going? Is the V3B/V4B cathode driving or being driven? I would think being driven because there's no other way for a dry signal to reach the outputs, but what causes this to be true?
4. If the Blue and Pink lines really are driving the V3B/V4B cathodes, that would mean we're putting an out-of-phase signal from the opposite dry channel on each wet output stage's cathode - positive feedback-style, but not the same signal, just a presumably "similar" signal if it's normal stereo music. What are the implications of this? Do we get weird stereo phase-cancellation stuff? Is this the origin of the stereo-widening effect? What are the effects on mono-compatibility of the output?
I'd appreciate any help y'all can offer! I'm pretty handy with electronics, but not any sort of expert on tubes or too complicated of analog stuff. I'm planning on installing a switch to optionally disconnect those cross-feedback lines to get wet-only outputs, but I'd like to keep the option as part of the unique sound of the unit. I just want to understand what's going on!
Thanks!
The feedback from the driving coils appears standard distortion reducing. The cross coupled one probably resulted from an experiment which produced a pleasing sound, so they made it a feature. You'd make it even more of a feature with your cut-off switch.
If I owned this, I'd make it work correctly maintaining its stock form as much as possible, perhaps replacing tubes. Then I'd sell it on ebay to some collector in Japan for a healthy sum. Quite a Gem of a unit as-is; I've certainly never even seen a tube version of this; it's historic.
I imagine it would also serve well as a "lampizator" stage to get tube sound in an otherwise solid state system. Along the lines of an Aune T1 or a Jolida JD1301, which stick a tube in the signal path specifically for such a purpose. It's a nice piece - be careful with its value as a stock, unmolested unit, if any.
If I owned this, I'd make it work correctly maintaining its stock form as much as possible, perhaps replacing tubes. Then I'd sell it on ebay to some collector in Japan for a healthy sum. Quite a Gem of a unit as-is; I've certainly never even seen a tube version of this; it's historic.
I imagine it would also serve well as a "lampizator" stage to get tube sound in an otherwise solid state system. Along the lines of an Aune T1 or a Jolida JD1301, which stick a tube in the signal path specifically for such a purpose. It's a nice piece - be careful with its value as a stock, unmolested unit, if any.
Thanks for the response. So does no one actually understand feedback, and it's trial and error, even for the engineers who designed this thing?
I'm not too worried about keeping it pristine. Looks like they sell on eBay for maybe $200 in good condition. I paid $50 for this one - honestly I probably overpaid considering it's broken. Didn't get to test it first.
Yeah I'm hoping to use it for exactly that in addition to actual reverb. Just turn the verb down and use it for a bit of tube-ness when I want it.
I've seen that article, and it is helpful. Unfortunately it doesn't answer the questions I outlined and glosses over the fact that the cross-feedback seems to be out of phase.
Is cross-posting from Reddit against the rules here?
Last edited: