Supro 1624T (1965) cab dimentions...

[knockbill]

Hi Guys,,,
This is my 1st post, other than introductions... I found two NOS 6973 tubes, and they led me to a Supro 1624T build... I found a few schematics, but conflicting cab sizes,,, the new issue amps seem to be 23x16x9 or so,,, but pics of originals look to be more square than rectangular... Anyone know the size of an original 1965 version? Did they originally have angled fronts like the newer releases?
Thanks...
Regards, John

 

[EFK]

Yes, the earlier (pre 1962-ish) amps were much smaller, originally with the "step back," then the short lived tv front with double width upper panel and slightly larger size (late 1960-late 1961), the finally the larger cab w/ the slant front. The original step back cabs are very difficult to work in different speakers, as they were apparently designed around the cheap Rola alnicos (much like a Jensen P12Q) and it is very difficult to fit other speakers without interfering with the tubes. The later cabs offer a lot more room, although in many of them (every Oahu version I've had, most Airlines and most Supros) I find the location of the output transformer both bizarre and highly frustrating.

 

[knockbill]

Thanks for replying,,, I'll build the cabinet with the modern dimensions, and slanted front, and the OPT on the chassis...
Regards, John

 

[knockbill]

I got my cabinet built,, bent a chassis and got it filled and running... it would be nice to find a schematic with voltages listed to confirm my readings,,, I've searched thru most of Valco's brands, but none of teh schems list running voltages...
Anyone have a link or info to this info?
I took pics of the build process so far, but can't seem to load them off the camera or computer folders...
Thanks...

 

[Muzzy]

Would love to see it.

 

[knockbill]

I'd like to share it!! can't figure out pictures here,,,, I started a thread on the Audiokarma tube forum...

 

[Vibroluxman]

Upload photos to pbotobucket or another sharing site. Click the image button here and copy & paste in the direct link to the image.

 

[gkoelling]

@Leonc
might know the voltages you're looking for. Also, you should be able to copy and paste your photos.

 

[knockbill]

Tried copy and paste, couldn't get em loaded... Not really a software guy... Pics are on AK tube forum, if you want to see them...
Thanks for the info and interest...

 

[EFK]

What voltages are you looking for? The output *should* be in the 320-ish vDC range on the plates and about a volt or two lower on the screens, but when plugged into modern wall VAC, you can see the plate voltage spike up into the 340-350 vDC range which is too hot.

Grid leak input (true 1624T, not 6424) will only be @ 80-90 vDC on the input 12AX7 plate, the cathode biased side (trem channel) is also set up so as to be somewhat comparable in the 100 vDC range (plate) which helps with the cathode shake when the trem is activated.

The paraphase will be around 170 to 190 vDC on the plates *with no signal* although I generally measure them all over the place as the somewhat low quality carbon comp 1/2W resistors are usually pretty noticeably drifted in value. Sometimes as low as 160-ish, sometimes up to @ 210 or 220 or so; all depends on how the resistors have drifted, both plate and cathode.

This is all for the versions 1 & 2 1624T w/ the shared paraphase cathode R and grid leak input. Version 3 (split paraphase cathodes) and version 4/6424 will be different although the output voltages always stayed pretty much the same as the PT was consistent throughout all 4 versions.

 

[knockbill]

Appreciate the info!!!
I have 333.4 and 333.5 Vp on the 6973s,,, screens are 330VDC,,,12AX7 Vp is 97.3,, PI 193VDC, so its in the ballpark... Used a 6SF5 for Tremolo,, and used the .01 cap for the tone control, as the .005 didn't have any range... Also used a ground bus to eliminate noise, and tied the heaters to the output cathode... Amp came out came out dead quiet... Used a Motorola PT that speced out almost the same as the replacement Triode is selling...
Thanks again for taking the time to post these numbers... Tried again to post pics, but its beyond me! Check it out on AK..

 

[EFK]

The only thing I find surprising is that you found the .005 for the tone control too small in capacitance, because frankly I tend to prefer dropping back to around .0033 or even .0022 to get better cut and to allow some range (with .005, I typically have it at 3 o'clock to max depending upon where the volume pot is set). The .01 was used for the 1615/1690/1695 series of amps, but I generally have that maxed out all the time.

Do you have 500 pF bright caps on the volume pots?

 

[knockbill]

Yes,,, .005 on both VC pots,,, I built the amp with both the hi and low inputs, so the high jack provides all teh highs I want, and the .01 on the tone pot allows more low range... Are you Eric of Dirty Girl amps? I noticed the 1624T DG schem shows .01 on version one (6V6) and .005 (6973)... I tried the .005 first, and figured a little more capacitance would act as a low pass filter... Its easy you swap out, if it proves to be too much, down the road...
This my first Supro/Valco build, so I'm new to teh circuits,,, made a couple Champs last year...

 

[EFK]

Wait a minute. The 'bright caps' on the volume pots should be 500 pF, not .005. This is for a 1624T. The bigger 1690/1695 amps had .001 on the volume pots as bright caps, 1615 accordion amp had none. Then for the tone control, the earliest 6V6 amps (at least those I've seen, I know Leon had one or two and maybe he can weigh in) had a .01 tone cap, then for the 6973 amps Valco dropped it back to .005. I like my tone caps to be as small as possible because even with the tone pot maxed, you're bleeding off some highs at the input to the paraphase and the smaller the cap, the less you cut into the mids and the further up the treble band the cap is affecting. On the amps which have a 500 pF to ground off the grid leak signal before the volume pot, I find I pretty much always just leave the tone pot maxed out. Or I yank that cap (500 pf to ground after the .005 grid leak coupling cap and before the volume) which I think is stupid and shouldn't be there anyway, I only see it in some amps, not others, even in the same year. When you say high and low inputs, do you mean you wired them up as stock i.e. a treble input with a high pass filter and a standard/normal/low input which is actually just a normal input? Yes, I drew the schematic you referenced. I tried to cover all the variations I've seen in a number of these amps. And I'm still seeing them! I had one a while back with all 220K resistors in place of all the standard 270K in the paraphase - must have run out that week!

 

[knockbill]

Sorry, I misspoke... I have 500pF on the VC pots,,, and replaced the .005uF Tone cap (as drawn on the 4 schems I have) with the .01... I didn't try the 500pF on input 1 (referred to as "seen intermittently" on your schem)...
Its pretty hard to expect to see matching parts on 50+ YO stuff... a musician/tech could have easily replaced parts on an amp when it was pretty new, and it would have looked original by now...
Thanks for the sanity check!

 

[knockbill]

I find the Volume pot has to be raised to compensate for moving the Tone pot towards full Bass,,, Is that normal, or a result of using the .01 Tone cap?
Finally got the cabinet stain mixed and brushed on,,, giving it a couple days to set before a clear coat of some kind and final assembly...

 

[EFK]

Yes, this is typical and the larger the tone cap (.01 vs. 005. vs. .003 or whatever you choose to use) the more signal in the upper mids is being bled off before hitting the paraphase which I turn reduces both the perceived volume as well as the amount of breakup (the 1624T is primarily preamp and paraphase breakup until around 2-3 o'clock on an audio pot). Because of the way the shared tone pot is set up, no matter what size cap is used, some degree of signal will be bled off. You can hear it simply by lifting the tone cap ground with the amp volume turned up a bit - you'll hear a very noticeable difference in background "hiss volume" and the larger the tone cap, the more dramatic the difference.

 

[knockbill]

I noticed that the more the Tone pot was moved towards Bass, the Volume would drop off some,, and it seemed to be more apparent with the .01 tone cap... Since that is the way the amp was designed, I'll experiment with .0068 and .0082, and find a happy medium... I will also try the lower values like you first stated, (.0022,, .0033) just to see the results...
Thanks for the info...

 

[knockbill]

Got the cabinet finished, and amp installed,,, it sounds pretty good, the tremolo is surprisingly strong...
Posted some pics in the build thread in AK tube forum,,, sorry I can't post em here...

http://audiokarma.org/forums/index.php?threads/supro-1624t-clone-winter-project.755937/

 

[knockbill]

Finally got back to the Supro... reduced the tone caps down to .004, and totally agree with you... the highs are much better, and the range seems wider also... I still want to try .003 and .0022,,, but I'll need to add them to the next Mouser order,,, The next size down I have on hand is .001...
Thanks again for the tips...