Johnny B+ Goode (my first amp build)

[JSett]

Hey all. So I decided to progress from repair and maintenance and try my hand at building an amp. A kit seemed the logical choice to start with and I found one called a 'Supa Deluxe' from Modulus Amplification here in the UK. It's a hybrid Blackface and tweed tonestack into a 2x 6L6 uprated Tweed Deluxe power section.

I've spend the last few evenings very methodically building it up. Powered it up this morning - no smoke...that's a good thing. Now, I have a couple of issues and hope someone might be able to throw in some wisdom?

Here is the layout supplied

There was a couple of omissions in the layout. There was an extra orange wire over by the AC feed to the rectifier. I chose to ground that.

When I checked voltages I was getting nothing at all on the heaters. Just a couple of millivolts. I noticed the green/yellow wire on the layout was missing from the transformer...so, no 6.3v center tap? I used a couple of 100r resistors and did an artificial tap to a ground point on the board.

Now I have 3.1v







This I'm still trying to work out...

The schematic is for an 8ohm speaker but I upgraded to the option of a 4/8/16. No wire chart came with the OT. I made an educated guess that the thickest wire was 4, medium 8 and thinnest 16. Assuming black was the ground. Plugging into the 8 (with that black wire soldered) trips the HT fuse.

[Jay]

So I'm basically still a neophyte with this stuff but I think I recognize a couple things I could help with here.

Usually, orange is 16 and green is 4 but you can figure out for sure which is which by doing a turns ratio calculation. Do you have a variac? If so, use some alligator clips to wire up the primary side with a small, safe, AC voltage, like 5V, or whatever amount you can set your variac to hold with some consistency, then measure the voltage on the secondary side to see which taps are smallest to largest. Or you can just email the supplier. But it's a good learning experience to do this if you're up for it. And armed with this info, you'll be able to figure out if some random output transformer you found in a parts bin might work for your needs.

If you're blowing fuses when you ground the output, you have another problem you need to find.

Also, for your heaters, are you measuring to ground or across the wiring at the tube socket? If you measured to ground, 3.1 is half of 6.2 so it's probably correct. If not, you have another issue I imagine.

[jthomas]

Heater voltage is measured from green wire to green wire (i.e., not from green wire to ground). So, if your 3.1 measurement was taken from green wire to ground, it really likely is 6.2 VAC.

Do you have a manufacturer and part number for the OT. If so, it should be possible to double check that.

It might be helpful to use a lamp limiter the next time you fire it up to protect the PT. Your fuse blowing suggests that something is drawing too much current and there may be a short to ground somewhere.

[JSett]

Yeah, I was measuring to ground on the heaters...that explains that then. Pretty obvious now you mention it.

The fuse only blew when I plugged into the 8ohm tap. In the 4ohm the fuse was fine and I was getting sound - but the sound was just hum and AC ripple with an extremely faint guitar signal

The transformers are all Modulus branded ones, which makes things harder. I might JUST wire up the 4ohm tap and see how I get on.

I don't have a Variac yet. It's on the 'to buy' list

Also, the 6L6's are showing 60mA on the BiasRite. I assumed the trimpot in the middle of the board was a bias... It has zero affect

I'm not sure why these two resistors are in series on the schematic, or the turret to nowhere...

*Edit: I've just noticed I've missed a jumper wire. Classic.

[Jay]

Did this kit come with a schematic?

[JSett]

Did this kit come with a schematic?

Nope. Just the layout. A schematic would have been helpful

When I get home from dinner I'll see what adding this missing link will do:

[JSett]

Well, I put in that jumper I forgot, rechecked all my connections and, suffice to say, we have lift-off...



I installed the two spare 470R screen resistors, even though they weren't on the layout...the tubes were running pretty hot.

Can't crank it as it's late here but, so far, it sounds like a very loud BF Princeton with a mids control

[wproffitt]

Congratulations on your progress with this! You certainly started with more knowledge than I did with my build. No wonder you got a sound out of yours sooner than I did!

P.S.- your description of the way it sounds is intriguing. Enjoy!

[JSett]

Congratulations on your progress with this! You certainly started with more knowledge than I did with my build. No wonder you got a sound out of yours sooner than I did!

P.S.- your description of the way it sounds is intriguing. Enjoy!

Thanks! All my 'knowledge' is borrowed and full of gaping holes though.

It's biasing at 60mA per tube still and I actually don't know how to control this trying to work it out now

*EDIT: I think it's the master volume type that controls it somehow. That and apparently cathode biased amps tend to run hot on the tubes (from a quick bit of googling).

*DOUBLE EDIT: Yep, cathode biased means the BiasRite is basically a moot point. 60mA is (apparently) perfectly normal.

Happy to be corrected though on any of the above. The whole point of this is to learn and I'm definitely doing that!

[JSett]

Excuse my ham-fisted playing, it's early and my fingers are a little burnt/sore from the build still

https://www.youtube.com/watch?v=DY06R-r7aec

[ziess]

Yeah you’re right on the bias point. Bang your figures in the cathode bias calf here:

https://www.tedweber.com/bias-calc/

As long as you’re not over max plate dissipation and you’re not seeing any redplating (look for it in a dark room) you’re good.

Tommy.

[JSett]

Yeah you’re right on the bias point. Bang your figures in the cathode bias calf here:

https://www.tedweber.com/bias-calc/

As long as you’re not over max plate dissipation and you’re not seeing any redplating (look for it in a dark room) you’re good.

Tommy.

Thanks! I'll check that later

[Scout]

That's great! I'm dipping my toe into that pool myself, I've been doing some small repairs
but of course the diagnostic skills are the hardest to pick up without actual practice .

Good job!

[jthomas]

Congrats. If your experience is anything like mine, you will find that building amps is addictive.

Regarding bias: As noted above, the issue for amp bias is to not over do the plate dissipation. You can modify the bias of a cathode biased amp by changing the cathode resistor.

As a point of reference, I built a class A, single-ended, 6L6-based amp awhile back (sort of a tweed Fender Harvard... But Class A/single power tube... like a Champ with a tone control). I recently checked the bias while experimenting with several power tube and rectifier combinations. I settled on 53.6 mA plate current with a 5Y3 rectifier, which was at 62% of the maximum plate dissipation for the power tube. Some might say that it is bias is cold, but I can hear no difference between the tone at this plate dissipation and much higher values (e.g., a 5881 and GZ34 rectifier that produced 67.8 mA plate current running at 108% max plate dissipation). You might have a different experience with a different amp.

Running the bias a little cold will lengthen tube life... reportedly sometimes by a lot. Typical recommendations are to bias at 90% of max plate dissipation for single-ended amps and around 70% of plate dissipation for double-ended, AB class amps.

https://www.aikenamps.com/index.php/idl ... 70-percent

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Rock on!

[DeathJag]

Awesome! I’m hoping to get into it as well. I appreciate the journey here and that little dang jumper!