Another reel to reel conversion project

Discussion in 'Shock Brother's DIY Amps' started by Nickfl, May 20, 2018.

  1. Nickfl

    Nickfl Tele-Afflicted

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    These seem to be a trend lately. This is the second one I found to turn into a project, this one is a Columbia (Sony) C-690tr. It's a small portable unit with 1 12ax7 and one 12BH7, pushing a 4x6 speaker.

    Here's the schematic:

    Columbia C690-TR schematic-001.jpg

    I only had to move one wire to be able to plug a guitar into the inputs and play through it. It has tons of gain, a pretty decent amount of volume, but a bit of a muffled tone without much treble. It is usable as is but it really seems like it would benefit from some tweaking to make it more guitar friendly...

    Here is a modified schematic with some proposed changes in red:

    Columbia RTR mod schematic.png

    I've changed the input and a few cap and resistor values as well as adding a tone control and a grid stopper to the power tube. Thoughts and suggestions would be much appreciated!
     

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  2. 2 Headed Goat

    2 Headed Goat Friend of Leo's

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    Sweet, but.... pics or it didn't happen ;)
     
  3. ArcticWhite

    ArcticWhite Tele-Holic

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    I'm no expert, but I don't understand that capacitor before the grid of V1. I don't think any small Fender amps have anything between the guitar and grid except the 68k resistors you have on your inputs. Also, the grid leak resistors I've seen have mostly been 1 meg rather than 10 meg.
     
  4. Wally

    Wally Telefied Ad Free Member

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    The original circuit has a true grid leak biased input stage. That cap might best be removed if one is going to go to a cathode biased input stage.
     
  5. Nickfl

    Nickfl Tele-Afflicted

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    V1 is grid leak biased, the large grid leak resistor is there to allow negative voltage to build up on the grid and provide the bias for the tube (note that the cathode is connected directly to ground). This biasing method also requires a coupling cap on the input to maintain that bias voltage at the grid.

    I'm maintaining the grid leak bias on the input, but I'm changing some of the values. It seemed like in the original the coupling cap was too big and the grid leak resistor too small, I changed those and also added grid stoppers to the inputs. I took the input stage values from the 5d2 princeton schematic, which has a grid leak biased input, since its preamp looks vaguely similar to what this amp started out as.
     
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  6. Platefire

    Platefire Friend of Leo's

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    That Fender Tweed type tone stack should brighten it up and give you more tone control. Fender uses 1 Meg vol and tone pot but the 500K might be fine. Platefire
     
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  7. SSL9000J

    SSL9000J Tele-Meister

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    Aren't those little reel-to-reel chassis kind of cramped to work in?
     
  8. Teleterr

    Teleterr Friend of Leo's

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    Cool...Greta, Killer Ant , etc territory. I ve got 2 in the attic. I plugged in to one unmodded using a low Z preamp. IIRC The distortion was pretty gnarlly when hit w a big signal, but clean it sounded good. Which input did you use ? It seems the radio phono in has an ok Z. I think mine just had a mike, so a str8 in guitar is too high Z.
     
  9. Nickfl

    Nickfl Tele-Afflicted

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    I tried both. The mic input started to break up pretty quickly and turned to fuzz about half way up to volume dial, the phono input was more reasonable for a guitar signal, but also had some distortion starting at about noon. Both inputs sounded promising, but both were way too dark (and this was with single coils) which is what led me to look at adding a tone control etc.

    Yes they are! I only needed to move one lead on the record/play switch to be able to plug into it for an initial test, but to get to it, I had to completely disassemble the chassis, remove the tape recording head, and remove the input jacks and volume pot from the chassis. All of this so I could install a jumper between two adjacent terminals...

    Fortunately, most of the resistors and caps are in a box underneath all of the electromechanical guts, which has its own separate access panel. Its all PTP/terminal strip wiring in there, but there is a picture in the manual showing where the key components should be located, so that should be some help when I actually go in there to do the work I have planned.
     
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  10. bigguy12321

    bigguy12321 Tele-Meister

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    Looks like a great bedroom/living room type amp. I have done a few of these but I bend some 18ga. into a C and load everything into that point to point style. It's neater for me.

    I have my favorites but any all tube reel to reel will work.

    a
     
  11. Teleterr

    Teleterr Friend of Leo's

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    At second glance it seems the dark tone is all Z mismatch. The one is like having a 50 K in parrallel w the guitars V pot and the other is like having a 1.05 Meg pot, but turned down to 50. I think substituting a 10 meg R for the 50 would be a fix. I m not anyway near the level of most posters to amp threads, but IME this seems whats up...I think you might have to disconect the heads as well.
    Have you tried a low, or at least lower than guitar Z preamp or effect into the phono since it has a grid stopper ?
     
  12. Wally

    Wally Telefied Ad Free Member

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    Here is a little silvertone tape unit that needs to be put back into service as something.....
    Tape recorder, soldano 009.jpg
     

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  13. Nickfl

    Nickfl Tele-Afflicted

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    Last night I got around to doing the work described in my first post and fired it up this afternoon to test it out. The verdict is partial success, the tone control and alterations to the input work nicely, much more appropriate for a guitar frequency range.

    It seems like it will need some more tweaking though as it has a massive amount of gain, too much really. It really starts to fuzz out and fall apart about half way up the volume control. I am thinking about changing the plate and cathode resisters on the middle two gain stages to something closer to stock Fender values and also adding grid stopper resistors while I am at it. I may also need to put in a voltage divider somewhere to cut the signal level down in one or two places. The other alternative would be to cut out one of the gain stages entirely...
     
  14. Nickfl

    Nickfl Tele-Afflicted

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    Right after my earlier post, I decided to try bypassing the first 12BH7 triode instead of swapping out components. All I had to do was swap out a couple of leads and it was a huge improvement. Much more headroom and usable sound.

    Here are some pictures of the (mostly) finished project:

    IMG_20180617_214428.jpg

    The knob on the right is the new tone control, I am planning to use the original knob from the tape player, but it was a D-shaft knob and I need to drill it out to use with the new pot. Note the 1/4" to 1/8" mono phone cord, I found this in a bag full of odds and ends parts, I have no idea where I got it, but it worked out to be a perfect match for the jacks built into the amp.

    IMG_20180617_214623.jpg




    IMG_20180617_214653.jpg

    I like the convenient little power cord compartment, its just big enough to fit the new three prong cord I installed.

    So at this point it is sounding pretty good. Its surprisingly loud through the cheesy little 4x6 speaker, not quite as loud as a tweed champ. Its fun to play, but I think the speaker is definitely holding it back. I'm thinking about rigging up a speaker out of some sort, maybe using a switching jack so that it can easily be used with a "real" speaker. I also need to go back in and glue down all the loose mechanical elements of the old tape system with locktite or something to kill off some rattle.
     
  15. CharleyPatton1929

    CharleyPatton1929 NEW MEMBER!

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    Hello! I have been following and reading this post for quite some time now and am wondering if you could help me out? I have the exact same model and would also like to convert it into a small working amp just like you. I unfortunately only have a very novice ability to read schematics. I see that you said you soldered a jumper wire to two adjacent terminals and I am curious to know what exact changes you made to make it functional. So far I think where you soldered was the unit labeled as M4 on the original schematic. I made an account to post this so I am not very familiar with this forum but any help would be massively appreciated, thanks!!!
     
  16. Nickfl

    Nickfl Tele-Afflicted

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    It's been a while since I did this project, so my memory of it may not be exact, but I'll do my best. Yes, the switch labeled M4 is where you need to swap a lead to make the inputs functional. If I recall correctly, you need to jumper between 17 and 18 and clip the lead at 16. Then you should be able to plug into the input jacks and play through it, but as you read, I found it needed some other alterations to get the best out of it.
     
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