D-Lab 6F6 amp advice

[Blooze]

I've built a few things on turret board and one hi-fi amp on terminal strips (lots of room), but want to build this little 6F6 amp that D-lab/Blackcat describe on their YouTube channel. It will be in a 9.5" x 5" x 2" hammond chassis to fit the old empty radio cabinet I scavenged. I've come up with a couple of layouts, but not sure if one is "better" than the other, or if makes any difference. I've attached a schematic and the layouts for your suggestions. Obviously the wiring isn't exactly to location (heaters are showing twisted or any of that). The chassis dimensions, the transformer outlines, terminal strips, and sockets are to scale. Mainly looking for an optimal layout if there is one.

[sluckey]

Are the tone and gain pots laid out that way so they will line up with some holes in your radio cab?

[Blooze]

Are the tone and gain pots laid out that way so they will line up with some holes in your radio cab?

Yes. They will be mounted on the face of the chassis. And come out the front of the cabinet where the original volume and dial adjustment knobs were.

[sluckey]

Show me several pics of your radio cab. That will help determine an optimized layout.

A couple suggestions to utilize that small chassis. Drop the SS/Tube switch and use one or the other. I would just use SS for that single ended amp. Also drop the impedance selector and just use one or two speaker jacks.

Take a look at my radio conversion. The pics may give you some ideas about a better layout for your project.

     https://sluckeyamps.com/RCA/RCA.htm

[Blooze]

I don't have any pics on my person (at work), but I believe it is this model Crosley. I'll take some when I get home. I know that a 10" x 5" is as large as will fit into the cabinet.

https://www.worthpoint.com/worthopedia/crosley-121-table-model-tube-radio-170461221

Might be a 58TA as well. Both are similar.

[Blooze]

It’s a 58TA. Original chassis size is 9x3.5x1.5.

[bmccowan]

For additional clues on layout, you might look at the Fender 5C1 Champ layout. It is basically the same amp, but without the tone control. Using shielded cable in and out of the pots should help, but I'd consider dropping the tone control altogether. I built a Valco version of the 5C1 and discovered the tone control on my guitars.
You could still put a pot and knob in that position for looks, and pretend that its doing something.

[Blooze]

For additional clues on layout, you might look at the Fender 5C1 Champ layout. It is basically the same amp, but without the tone control. Using shielded cable in and out of the pots should help, but I'd consider dropping the tone control altogether. I built a Valco version of the 5C1 and discovered the tone control on my guitars.
You could still put a pot and knob in that position for looks, and pretend that its doing something.

Thanks for the direction. I could put a rotary power switch there since the mains will be coming in on that side.

[tdvt]

I have been buying concentric pots for situations like that &/or adding a control without drilling more holes. They are not the larger 24mm size we are used to, but work fine. Also a bit pricey compared to standard pots but they solve a problem.

The only real rub is sorting out the knobs for the 2 shaft sizes.

As far as R value, I haven't taken any apart yet & yet swapped out the halves for dissimilar values (Fender sells a 250K/500K for the Jazz bass, 24mm too) but I am assuming it could be done.

Think that the 5F2 Princeton uses 1M for both volume & tone.


https://www.ebay.com/itm/373006274610?itmmeta=01HZCZDZ7PCH16WVK3HAWYN8XK

[Blooze]

I do have the original CTS rotary power switch/volume pot from the radio, but I’m not sure I’d trust it.

It didn’t come with knobs so I’m free to find some anyway. A vintage matching pair would look nice though.

[bmccowan]

Some cool knobs here:

http://renovatedradios.com/productlist.php?category=92

[tdvt]

Some cool knobs here:

http://renovatedradios.com/productlist.php?category=92

Those are some cool knobs, I will have to peruse.

I would love to find a "back knob" (in radio speak) to use along with brown/blonde-style Fender knobs for use with the concentric stacked pots.

[tubeswell]

6F6 Plate Pmax = 11W (g2 = 3.75W)

https://frank.pocnet.net/sheets/127/6/6F6.pdf

With 250v B+, you can afford to bias hotter than your schematic shows.

If you biased to 11W, the optimal load resistance for center-bias Class A (single-ended) would be 5k7 I.e., 250/(11/250).
 
But with the 250 B+ and 28mA tube current you have, you’re getting about 23mA plate current (and 5mA g2 current), so the plate is idling at 229v x 0.023A (5.3W)  - I.e., 1/2-power. Moreover, in this condition it wants to see 250/(5.3/250) = 11.8k for center-bias. But all this adds up to lower than possible output power (which is fine if you want lower power). But as it is, you only have 7k load resistance, so the output power will be lower still. Just so you know what to expect. YMMV

[Blooze]

Some cool knobs here:

http://renovatedradios.com/productlist.php?category=92

That’s a cool site!  I could have swore I had the knobs, but my rooms such a mess they could be anyone. Probably put them someplace where I wouldn’t forget them.

I’ll need ones that have the extended part off the back that go through the cabinet. There’s some Zenith ones on there that might work.

[Blooze]

6F6 Plate Pmax = 11W (g2 = 3.75W)

https://frank.pocnet.net/sheets/127/6/6F6.pdf

With 250v B+, you can afford to bias hotter than your schematic shows.

If you biased to 11W, the optimal load resistance for center-bias Class A (single-ended) would be 5k7 I.e., 250/(11/250).
 
But with the 250 B+ and 28mA tube current you have, you’re getting about 23mA plate current (and 5mA g2 current), so the plate is idling at 229v x 0.023A (5.3W)  - I.e., 1/2-power. Moreover, in this condition it wants to see 250/(5.3/250) = 11.8k for center-bias. But all this adds up to lower than possible output power (which is fine if you want lower power). But as it is, you only have 7k load resistance, so the output power will be lower still. Just so you know what to expect. YMMV

Yep. At most it’s about 3W, which is fine.

[sluckey]

I’ll need ones that have the extended part off the back that go through the cabinet. There’s some Zenith ones on there that might work.

I made some shaft extensions for my radio project that allow a greater choice of knobs. Lowes has the 1/4" aluminum round stock and the 1/4" rod couplers. Look at the link in reply #3. Scroll to the bottom. This might work for you too.

[Blooze]

Bringing this up for another question since I'm in process of wiring up. Besides the fact I have so far cut the rectifer 6V leads and the OPT ground wire too short and had to splice the cut ends back on it's going well 

I was wondering about pin 1 of both the 6SJ7 and the 6F6. I have metal shell tubes for these and it seem like you'd want pin 1 of both of those tied to ground? On the 6SJ7 tie pin 3 and 1 together then to ground. Any merit in this?

[bmccowan]

Bringing this up for another question since I'm in process of wiring up. Besides the fact I have so far cut the rectifer 6V leads and the OPT ground wire too short and had to splice the cut ends back on it's going well 

I was wondering about pin 1 of both the 6SJ7 and the 6F6. I have metal shell tubes for these and it seem like you'd want pin 1 of both of those tied to ground? On the 6SJ7 tie pin 3 and 1 together then to ground. Any merit in this?

Cut a wire too short - bummer, I have done that. I also do a lot of carpentry: measure twice, cut once is the old adage.
Yes to grounding pin 1 on those tubes - the metal shell acts as a shield. Pin 3 on the 6SJ7 also goes to ground and can be tied to pin 1. Some folks tie one or both 6SJ7 Pin 1 & 3 of those to pin 5 (cathode) to elevate the voltage a bit to lower noise. I think its a try it and see sort of thing. I did that with a 6SH7 in one build. If it made a difference it was small.
I'm curious to see how this project turns out.

[Blooze]

Cut a wire too short - bummer, I have done that. I also do a lot of carpentry: measure twice, cut once is the old adage.

Not just one wire. Three for crying out loud. The filament wires was an easy mix up, but for some reason I took the OPT ground wire to the impedance switch common. Which of course made the wire 3/4” too short to get to the output jack.

I’m a measure twice, cut once, build it three times sort of person because I was obviously measuring the wrong thing. I’ve got a couple of 1x10 cabs I’m in process of finishing up. So far no issues.

Thanks for confirming the pin 1 grounding. I figure the easiest way on the 6F6 is to put a solder tab on one of the socket bolts.

[bmccowan]

I’m a measure twice, cut once, build it three times sort of person because I was obviously measuring the wrong thing.

That's funny. I am a bit Dyslexic so sometimes I go to the wrong side of the inch mark on a tape measure, and the board winds up exactly one inch too short! 

pin 1 grounding. I figure the easiest way on the 6F6 is to put a solder tab on one of the socket bolts.

A lot of older amps soldered those shield pins right to a tab on the tube socket. So functionally the same thing if you get a solid connection.