Converting an old radio - Tremolo help?

[johnnyreece]

Good afternoon, folks!

I recently picked up a free console radio from the mid-30's from a friend. They plugged it in, and the power cord melted...so...assuming the transformers/speakers check out (not a certainty...), I'm going to try to bring this back as a guitar amp. I'm going to try to keep the original power tube (6F6G) so that I don't have to think to hard about the OT and what other tube *might* match. The radio has a field coil speaker, and the radio is rated at a whopping 3W output. With what I have, I'm looking at maybe a Vox AC4-ish amp with tremolo. I've attached the schematic for that, as well as a 6F6 amp schematic I found from D-Lab electronics that I want to be my base (with minor changes: Field coil speaker as a choke, 5Y3 rectifier, probably using the AC4 power supply). Wanting to use all octal in this, I'm thinking of using a 6SL7 for tremolo. I've attached Sluckey's 6SL7 tremolo, as it seems like it might work (but, I don't know much about tremolo). Would I be able to insert that at the cathode of the 6SJ7, as the Vox does with the EF86? Any help would be appreciated!

[johnnyreece]

Well, it was a slow day at work, so I slapped together a layout. It is not yet verified! Hopefully, it illustrates what I'm going for better than words. If anyone has any suggestions, I'm all for it!

[SoulFetish]

You have a schematic for the Radio?
Does this unit have a power transformer? If the cord melted, that might be cooked anyway. But what ever you decide to do, you want to make sure not to overload the PT.
But, you can get a new one for a small project like that for so cheap it might be worth sending that one from the 30's out to pasture and start fresh anyway.

[johnnyreece]

Yeah, I'm hoping it didn't fry. It does have a PT. I'm using fewer tubes than the original (had 5 to begin with). I haven't looked at current requirements on the tubes, but again, since I'm going with fewer, I'm *guessing* it's okay. Here's the schematic. Not much to work with, guitar-wise.

[johnnyreece]

So, I've checked out the PT. The unloaded secondary numbers look good. The bulb doesn't come on with the limiter. However, both 5V and 6.3V taps are quite high...I'm getting upper teens on both. Is it cooked, or is that possibly normal with unloaded trannies?

[Stevem]

When windings are tested out unloaded there should not be more then a 20% increase in voltage seen even in a really cheaply made PT!

Voltagedas high as you are reading are due to a short in that PT on its primary side.

[Mac Dillard]

This might help check the transformer https://www.premierguitar.com/articles/ ... r-tester-1

[johnnyreece]

The rectifier socket was still wired up. Popped in a 5Y3, and it dropped to under 5v. It’s a Stancor 6011. Thanks for the link. I’ll check it out!

[Phil_S]

According the the Stancor catalog of 1948:
350-0-350 @ 70mA
5VCT @ 3.0A
6.3V @ 2.5A

[johnnyreece]

Believe it or not, the neon lamp transformer test worked. I suppose I'll move on to construction and see what happens...Thanks!

[johnnyreece]

Okay, so I can make the planning stage of anything take forever...something struck me as funny about the original schematic, and it is the field coil setup...it's not like how I've seen it before. The original shows it being in line with the secondary CT to ground. And, this explains why the voltage chart in the schematic shows much lower voltages than expected, right? As such, I'll need to revise my layout.

Also, after really looking at it, it basically looks like the AC-4 but with 'Merican tubes. I'm thinking of trying to use a 6V6 here, and ditching the 6F6. I'm hoping the OT is "close enough for rock and roll". Probably would have a change of cathode resistor, but other than that, would it work?

Finally...I'm hoping someone with Tremolo expertise will chime in and let me know if this will do the trick as illustrated, as I've hacked and slashed schematics to get this thing...well...pre-designed?

[thetragichero]

you can measure the ot primary impedance feeding the primary an ac voltage (a standalone filament transformer would be great for this. i just hook up to mains because the power strip screwed to my bench leaves a switch in a handy spot... i wouldn't suggest using 120v mains voltage but it works)
measure ac voltage on ot secondary, divide the input voltage by the secondary voltage. square both sides. that's your turns ratio. multiply by whatever speaker load you're desiring and you'll get the primary impedance. take a look at 6v6 data sheet or (as i do) check out the specs of some replacement output transformers (i usually use the classic tone site) and see how your transformer compares to known circuits

[Phil_S]

You could always buy a filament transformer. They go for $10+/- on ebay.

[RWood]

I do alot of these. This one's a princeton.

Princeton.jpg

[johnnyreece]

Cool! I don't understand why we don't see more of these. Usually, all the expensive stuff (Iron, chassis, speaker, and cabinet) is there and can be had for almost nothing. I rehabbed one console to actually work again, so I don't need more. I always wanted one of these as an amp, and now I'm finally (sloooooooowly) getting around to it.