5C8 Twin Triad 6516 Transformer Replacement.

[RichardM]

Does anyone know the output voltages and current of the Triad 6516 PT used in the 5C8 twin?

What would be a suitable new replacement?

This is for a new build not a restore.

Thanks guys.

Richard.

 

[5F6Animal]

The Classictone 40-18073 should work fine. It's a 5F6A Tweed Bassman power transformer.

373VDC @ 150ma w/40uF cap
3 amp 5v heater for the 5U4 rectifier too.

http://www.classictone.net/40-18073.html

You need to measure the cutout and mounting holes before ordering anything.

What's wrong with the old one?

 

[RichardM]

Thanks,much needed.

Nout wrong with the old one, I just don't have it.

I'm doing a new build.

 

[RichardM]

My progress so far.

 

[5F6Animal]

I see, that's an oddball circuit to copy. People usually build the 5E8A or 5F8A twin circuits. I use one on my 5F6A and it sits at 435vdc/60% bias with a GZ34. A 5U4GB will get you into the 370vdc range, just like the voltages on the old Fender layout.

As long as the cut out on the chassis is the right size, that bassman transformer will work perfectly.

Buy a handful of 10 watt resistors for the power tube cathode bias. Values between 150-300 ohms. The stock 250 ohm will result in a very cold bias for 5881 tubes. 6L6GC's might not even fire up.

Up the filter caps to 22uF@500v. The schematic calls for lower voltage caps. It's always nice to have a cushion, especially if you plan on using the standby switch.

 

[RichardM]

Thanks heaps for that, can't find a lot of info out there on the 5C8.

 

[Wally]

This was Fender's first amp to have a more complex tone stack, isn't it? I have never seen one of these.

 

[RichardM]

It does have bass and treble controls, so maybe.

I have not heard one in person, but those I know who have, say it has a "To die for" tone.

 

[Watto]

Buy a handful of 10 watt resistors for the power tube cathode bias. Values between 150-300 ohms. The stock 250 ohm will result in a very cold bias for 5881 tubes. 6L6GC's might not even fire up.

I don't totally agree with this 5F6Animal

using the cathode Bias calculator on this site https://robrobinette.com/Tube_Bias_Calculator.htm and taking the voltages off the 5C8 Layout from Fender + my own readings from 2 5C8's I made using Merc Mag transformers

It says 24v on the cathode resistor/cap @ 250 ohms 25v/25uf , with 2 x 6L6GC tubes - the calculator says it'll be 15 watts output per 6L6GC at 52% plate dissipation . Yes a bit cold. The optimum plate dissipation for tube amps is 60% for slightly cold and 70% for slightly hot. Taking Fenders schematic plate-cathode voltage of 346v it says a 250 ohm resistor is best for 5881's at 68.3%, then you have to take into account each tube draws different currents , voltages etc etc , the variables are many, I recommend 200ohm for 6L6GC's - 250 ohms for 5881's,But I've always seemed to end up with 250 ohms on cathode biased Fender circuits when I finish with the math , and do the ear test.

Having said all that this really refers to the Vintage tubes, the modern day 5881's 6L6GcC's etc etc are pretty much the same internals with different labels , Not the big difference they use to be

The only issue with the original Cathode cap is that the 24v is too close to the 25v rating, so by using a 25uf/50v cap there will be no problems

Cheers
Watto

 

[Watto]

Nice correct value 250k resistors there for those early tweed circuits RichardM

 

[RichardM]

Thanks mate.

 

[RichardM]

Now I just need to save my $5.00's for the PT.

 

[Wally]

I ran the stock cathode value in the 5B5 Pro I resurrected...250ohms. The 5B5 is very similar to the 5C8 but for that 6J5 and the more complex tone stack...same PI and output section...same PT, iirc...6516. The plate dissipation was right at 53%. The sonics were not anything close to 'cold'. Cold biasing to me would be down in the 40% and lower range...ymmv. The Pro sounded fantastic....big warm cleans, smooth overdrive when pushed. Wonderful amp. I thought about decreasing that bias resistance...but why mess with perfection.

 

[danlad]

After popping 6L6s into an overdissipating 6V6 cathode bias output stage I totally agree. After all this time thinking I needed to always hammer the output to get decent guitar sounds it was a revelation to realise setting it as for old style 19w or 23w 6L6s or 5881s with cathode bias sounds really good!

 

[RichardM]

Transformers are here!!

 

[RichardM]

A bit more progress.

Before:

After:

More progress.

Had to move the rectifier tube cause I didn't allow for how big it is, I couldn't plug it in 'cause the PT was in the way.

I wanted 165mm wide chassis material but could only find 150mm. :mad

50mm x 25mm angle attached at both ends and 6mm inserts fitted.

Here you can see how much I had to move the rectifier.

Now to mount the tag-board.

 

[FenderLover]

Cool! I've been getting interested in a circuit with grid leak bias input stages lately, and this would be a wonderful circuit to play with. Elliott Sound Products states that there is no good reason to use grid leak bias and should be avoided. Read reviews of the 5C1 or Gibson GA-20 and you would think they are the amplifiers that God has in his living room. The moral it that you gotta try it you don't know, right?

 

[Wally]

Fender Lover, I really like the 5B5 and 5C5 Pro circuits. I have never heard one of these grid leak bias Twins. They have a more elaborate tone control circuit than do the Pros....and one additional gain stage. INTeresting amp.
The project is looking good, Richard. Kudos...

 

[RichardM]

Getting ready to power it up as we speak.

 

[RichardM]

It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works! It works!