Supro (Valco) 1688TN Refurbish help (pics)

[ToneJunkie]

Its been a long while since I have been on this forum or done a project but... here it goes.  A buddy asked me to refurbish this amp.  I can't find a schematic so I am in the dark about some of what I should do in this amp.  The things I plan on doing are...
1. Replace the power cable.
2. Replace the electrolytic caps.
3. Replace some broken parts.
4. Return it to the original circuit.
5. Replace the speaker as per my friends request.


I have a couple of questions. 
Question 1: In the picture below are the standard caps like the one with the red arrow electrolytic and should I replace them?  Also the same question for the black cap with the green arrow....
Question 2: In the pic below there are two modern looking caps with the blue arrow.  If I have no schematic is there any way to know if these are just replacements or are a mod...
Question 3: I have searched pretty thoroughly (I think) for a schematic would there be any other schematics that would give me a clue as to what the original circuit should be like? 

There are more pictures to come... and thanks for checking this out :-)

[TIMBO]

Hey Bill, Red and green look like coupling cap (polyester) not e-caps and my be OK but could be leaky. Blue also look like couplers as well and may have replaced old ones.

If you could trace the part of the circuit that would give a better idea of there purpose.
Always fun to unravel a mystery.

[sluckey]

My very first step would be to find the correct schematic. Failing that, I would draw my own. Then I'd evaluate the health of the amp, troubleshoot, fix only what is broken. That would probably include replacing power supply filter caps.

The cap you pointed out with the red arrow is not an electrolytic. That particular cap will likely need to be replaced, but I would do so only after checking it. I can't see enough of the 'green arrow' cap to tell what it is. Hard to say if those 'blue arrow' caps are original. Examine the solder joints to see if they look different that the rest of the amp. Does it really matter anyhow?

[Willabe]

I just read on another site that the Gretsch 6169t is the same amp.

Find that and see if they match up.


            Brad     

[Tone Junkie]

This ones not me Timbo mine has a space between the words confusing.
Bill

[eleventeen]

That those caps are of multiple different manufacture means utterly nothing as far their being original or replacements. There was no "directive" or "design objective" in place at the time these amps were manufactured that "all caps should be by the same manufacturer". The maker simply went into the market to his several suppliers and bought his .02 caps as cheaply as he could and bought his .1 caps as cheaply as he could and if one was this brand and one was another brand it was of utterly zero concern. Some time back, someone posted a gut shot of an amp that had EIGHT different manufacturers of non-electrolytic caps! Maybe the maker of the amp got a super deal on .02's a year or two earlier (for a different model amp) then he ran out of .05's and had to go buy them much later than he bought the .02's.


I would suggest a slightly different though not exclusive approach. Examine the solder joints carefully, with a magnifying glass. If some are of a distinctly different patina than others, the shinier ones are probably replacements. Most coupling caps and small value non-electrolytic caps in an otherwise unmessed-with 50's-60's amp have very long lives are most likely original. It's entirely common to find Fender amps with all their original Ajax blue-molded caps. On the other hand, the black striped "bumble-bee" caps are known to be leakers.


I can understand the desirability of having a schematic and I completely agree. And, I would exhaust every avenue of internet forums and spend plenty of time searching before proceeding without one...but one is not absolutely needed if you just replace one part at a time. This of course depends upon your understanding of tube amps in general and, for example, being able to realize that the little tubes are preamp and tremolo tubes and in the great majority of cases they have about 100K plate resistors and 1-2K cathode resistors, and a 12AX7 has this/these pin(s) as the plate(s) and these two are the cathodes.


It all depends upon how much time you are willing to invest. Drawing a schematic from a circuit in front of you is a pretty tedious job, IMO. And there are no guarantees you won't get something wrong with that.

[sluckey]

Drawing a schematic from a circuit in front of you is a pretty tedious job, IMO. And there are no guarantees you won't get something wrong with that.

It can certainly be challenging but I've never considered it tedious. Doing so will give you a level of knowledge (intimacy) about the amp that will be a benefit to understanding the circuit operation and troubleshooting also.

But if you don't understand basic tube amp circuits then drawing a schematic will likely be very intimidating. And the chance of drawing it wrong increases if you can't look at what you put on paper and question it if something looks odd.

[HotBluePlates]

Question 1: In the picture below are the standard caps like the one with the red arrow electrolytic and should I replace them?  Also the same question for the black cap with the green arrow....

A rule of thumb to use in the future:
- If the cap's value is less than 1uF, it is almost certainly not an electrolytic cap.
- If there is not an obvious marking indicating a + or a - terminal, it is almost certainly not an electrolytic cap.


There are some exceptions to these, but you'll not find them often, and probably never inside an old amp.

[TIMBO]

 

[tubenit]

It is the Gretch 6159 that is similar. 

http://el34world.com/charts/Schematics/files/gretsch/supro_gretch6159.gif

Quite some time ago,  I hid a editable version of the Gretch 6159 here:

http://el34world.com/Forum/index.php?topic=4089.0

This is close enough to guide you when reverse engineering the schematic.  So regarding the Gretch 6159 editable version:

With respect, Tubenit

[tubenit]

This is one of the most impressive amp videos that I have seen on the internet & I am hoping to build one of these some day with  either reverb or an active FX. 

https://www.youtube.com/watch?v=ItoO7l1jYdA

With respect,  Tubenit

[tubenit]

Finding that 1688TN schematic is pretty much unobtainium. 

I  know it would be a real effort to draw out the schematic,  however I sure hope you will take the opportunity to do that.  I've never even seen a real 1688TN in person. 

I am thinking with the editable Gretsch schematic as a guide that you should be able to figure this out.

With respect, Tubenit

[Willabe]

Here, I forgot about this guy, Terry Dobbs. He has been interviewed at least a couple of times in Tone Quest Report mag.

There's a clip of someone demoing a 1688TN on the site.

He will know for sure which different amps made by Valco but for/under different brand names are the same. (I'm not saying tubnits wrong, it will just be more confirmation.) 

http://www.valcoamp.com/


              Brad     

[ToneJunkie]

Thanks for all the replies guys!  I was given the amp all taken apart so I put it together today and it works!  Sweet!  The sound is super trebly and not very loud.  Ill take some time to compare the schematic of the gretch to this and maybe start writing it down.  Here are some more pics.

[ToneJunkie]

Power Amp

[ToneJunkie]

Power amp 2

[eleventeen]

A very impressive sounding amp, wow!  Love to find a schematic!

[shortfuse]

This is one of the most impressive amp videos that I have seen on the internet & I am hoping to build one of these some day with  either reverb or an active FX. 


With respect,  Tubenit

Did you click the link to his home brew combo as well all I can say is wow