SR-101 Pioneer Spring Reverb

[JKowalski]

Skip down to below the schematic for my question if you don't feel like reading all the background info!

Okay - so I bought this thing on eBay listed as broken for $10 + $15 shipping from this really really nice guy - he sent me a broken Altec 1589B preamp along with it for free without even telling me about it!

I'm a beginner in audio electronics, I have built a number of pedals and hundreds of low voltage circuitry of all sorts. I decided I want to build a tube amp in the near future, and then I found the Sr-101 on eBay - it seemed like a perfect thing to get. Cheap, introduction to tube electronics, 60's vintage effect!!!, and it's been noted for a great reverb, and amazing tube sound with reverb bypassed.

Annnyways. I got it, and obsessed over it for a day - first thing I noticed was a broken connection to one of the reverb tanks. I fixed it immediately. Then I tried it out - the reverb didnt seem to work on either channel. So I took it apart and spent a day or so checking internal connections, analyzing the schematic, etc. Then I realized that the schematic I found online has one huge difference from the one in the manual for the SR-101.

Since it is a stereo reverb unit for stereo systems, it is meant to be hooked up with a L/R input, going to L/R outputs. Wellll, apparently they decided to make it so the reverb sound from ONE input mixes with the OTHER signals clean output, and the reverb from the OTHER signal goes to the FIRST channel clean output. So it was working perfectly all along. And I wasted an entire day to realize it XD

The schematic is below - the highlighted wires are the ones I was talking about. On the schematic I found online (before I knew i was getting the manual with the unit) the red wire goes to the same final triode as the clean signal does. On the actual physical unit & official schematic, however, they go criss cross like you can see...

[ATTACH]4151[/ATTACH]


So here's my problem. I wanted to get rid of the current (IN 1)->(OUT 1 + REV 2) and (IN 2)->(OUT 2 + REV 1) configuration and change it so theres two mono amplification/reverb stages to make it easier to use for mono instruments. I thought that I could switch the two highlighted wires and get two seperately driven/modulated signal paths. But I tried it and it didn't work. At all. Both channels would no longer output either the clean signal or the reverb.

Can someone enlighten me as to why this doesnt work? And perhaps point me in the right direction towards rewiring it for dual mono input/output?

Lastly, a minor thing: all voltages are close to the schematic +- 5V except the plate voltages for the two different pentodes. One is 127V, the other 140V... Is this just insignificant large resistor tolerances or is this something I should be concerned about?

EDIT: I noticed they had the tolerances shown as 10% marked (i assume that means the delta symbols) and 20% unmarked. So it probably is tolerances (20%!!)

Thanks alot!

And I HAVE to mention, this unit sounds absolutely DELICIOUS. It really brings out the mids and can make ANY amp sound utterly amazing. The reverb is verryy nice, but it needs quite a bit of space between it and the amp to prevent feedback. I'm pretty much in love with this thing.

[Marc]

JK,
did you disconnect (lift) the feedback loop entirely or simply switch the feed sides L/R?
The Fisher spacexpander reverb does something similar but on uses one tank to blend the channels so it couldn't be dual mono.....
Marc

[JKowalski]

Thanks for the reply,

I switched the two L/R completely. I assumed that would simply result in the clean signal and that same signal's reverb mixing at the final tube instead of mixing with the other channels reverb. I did not leave anything disconnected.

[JKowalski]

Okay - This thing has driven me insane for the past few days.

I redid my L/R modification on a whim - and guess what - it worked perfectly. What gives? I am pretty much 100% sure I did it the same exact way... Theres only two wires, and theyre pretty noticable, and I could see where I resoldered the connections. But whatever, it works, it just didnt feel like it the first time I tried it.

But then, one of my channel's reverb stopped working randomly, while I had it hooked up and was testing it. I spent all of TODAY checking wiring, resoldering bad looking connections, etc... Looked at all the relevant points on my scope... And it all seemed to check out fine, but the reverb STILL wouldn't mix at the last triode?? So I gave up on it, came back to it later and plugged it in again on a whim... AND IT WORKED! WHAT GIVES??

This thing has caused me quite a bit of of grief for not having anything wrong with it! : |

Anyways - Just wanted to wrap it up.