Novar Spud :: A $200 sweep tube Spud with PCB

[tomchr]

The 6LR8 does make nice music .

That it does! Especially when triode strapped.

To really hear the advantage of Schade feedback; omit C3 and D1 on the driver's cathode.

Hmmm... Hadn't thought of that. I'll give that a whirl.

D1 is actually not populated by default. That's why it says "DNP" = Do Not Populate on the schematic. I included it as an option. I actually get quite good performance with a green LED in that spot. Then you'll need R9 = 0 Ω and R10 = DNP.

~Tom

 

[Tubelab_com]

I thought that this was the proper design.

 

[tomchr]

Ya know... I might build my headphone amp like that. I want to use it at work...
One of those Heiniken mini-kegs would work as well.

An externally hosted image should be here but it was not working when we last tested it.

~Tom

 

[roline]

6LU8's makes a killer PP amp..... I did a PCB concept for Triode, UL or Pentode mode, I did Pentode mode for the first one... Great with 6.6K Iron!. I used a 6CG3 diode for slow B+ start in the power supply filter chain.

 

[nikon f]

When will the board be available?

 

[tomchr]

I have opened up for preorders if you want to be among the early adopters. I expect the board to be available by the end of this month.

You can preorder on my website: Neurochrome :: Audio - Novar Spud.

I'm nearly done with the evaluation of my prototype. So far the board looks solid. I expect to submit it to the board fab on Monday or Tuesday. The turnaround time is two weeks (including shipping to my door). Once I have the boards in hand, expect it to take three days for shipping within the US and 1~3 weeks for international shipping.

~Tom

 

[vinylkid58]

One of those Heiniken mini-kegs would work as well.

As long as you don't have to drink it first.

jeff

 

[tomchr]

Oh... I can think of worse beer than Heineken. Of course, five liters in one sitting may be a bit much...

~Tom

 

[tomchr]

To really hear the advantage of Schade feedback; omit C3 and D1 on the driver's cathode.

Tried that. Wouldn't recommend it.

So... I played around with the feedback resistor. I performed a sanity check with values of R3 both above and below 150 kOhm. R3 = 150 kOhm continues to provide the best performance.

I did find that the THD at higher frequencies was quite a bit higher than at midband, so I decided to investigate this further. It turned out that the transient response at the anode of the driver stage was less than stellar as the feedback from the output stage added significant overshoot. It made me wonder if the output stage was actually operating at the edge of stability.

To tame the transient response, I added a small (22~33 pF) cap in parallel with R3. That worked like a charm. This cap does limit the bandwidth of the amp to about 30 kHz (-3 dB), but its transient response is nice and symmetric without tendency to overshoot.

I need to run the circuit through my measurement suite, but I think this is it folks! Ready for prime time!

~Tom

 

[tomchr]

Primetime has come...

Wow. What a difference a sliver of mica makes. Adding a 'speedup' cap to the feedback really tightened up the transient response. Metallic instruments are now rendered with much more detail. Smooth...

The amp sounds good and measures well. The board will be submitted for fabrication by Monday morning. It should arrive on my doorstep by the 29th or 30th of September.

I will probably order a fairly small batch of boards as there doesn't seem to be as much excitement about it as I had anticipated. If you want to get your paws on one of these boards, I suggest getting your order in early.
The board will come with a full set of schematics, bill-of-materials, circuit description, getting started guide, and include the latest tweaks and optimizations.

Order from my website: Neurochrome :: Audio - Novar Spud.

~Tom

 

[tomchr]

The boards were ordered this morning. They should be available late next week (9/27ish).
My website is open for orders...

~Tom

 

[larryldspkr]

Interesting project, could this be switchable between triode and ultra linear mode?

Larry

 

[tomchr]

Yep. That's the idea. By moving a wire jumper, you can select between UL and Triode mode.

I suppose, you could also hook up a DC supply to the terminal block (in UL mode) and get true Pentode operation if desired.

~Tom

 

[tomchr]

According to FedEx, the boards will be on my doorstep by Wednesday (9/25).

If you want to get in on the first batch of boards, please preorder through my website.

~Tom

 

[dovla]

Commercial??

please preorder through my website.

~Tom

Tom, no hard feelings, but doesn't this belong to commercial section?

 

[tomchr]

I publish the schematics for all my circuits and nothing prevents you from building your own without using my boards. The PCBs just make it a lot easier.

My mode of operation is to build circuits that are useful to me and offer excess PCBs up for sale. It's a self-sustaining hobby more than a business. I'm sorry if I step on your toes by offering my boards up for sale. That's not my intent.

~Tom

 

[dovla]

Nothing wrong with what you're doing and nothing at all to do with my toes, it's just that the hobby part belongs here and the sales part doesn't.
Quite recently there was a fellow who wasn't even offering anything for sale and yet his threads were promptly exiled from tubes forum to commercial section (to be 'reinstated' later once 'proven innocent') just because his pictures were of the quality found in commercial catalogs.

 

[tomchr]

I must say that I really enjoy the sound of the triode strapped 6LR8. It's not quite up there with the 300B, obviously, but neither is the build cost.

I've attached the measurements of amplitude response and THD vs power, frequency for the 6LR8 in triode mode. It's curious that the THD increases a bit at the higher frequencies. This may be an effect of the Schade feedback as I haven't seen it for other feedback types. The higher THD at the low frequencies is due to the output transformer. No surprises here. The Edcor CXSE series is rated for operation down to 50 Hz. I measure to 20 Hz.

~Tom

 

[jackinnj]

I publish the schematics for all my circuits and nothing prevents you from building your own without using my boards. The PCBs just make it a lot easier.

My mode of operation is to build circuits that are useful to me and offer excess PCBs up for sale. It's a self-sustaining hobby more than a business. I'm sorry if I step on your toes by offering my boards up for sale. That's not my intent.

~Tom

I did the same thing with SY's "Impasse" and "His Majesty's Noise", Dick Marsh's headphone amp -- thus the boards I use were free for me, and I made a couple score of people happy by charging cost+shipping and fees.

 

[Discochicken]

I'm in

Hi Tom. This looks like an inexpensive, fun project. I'm in, sent the funds to you via paypal earlier today. Shipping to Australia can take some time, so I'm wondering if there's any chance you can send me a copy of the BOM? It'd be nice to get the digikey order under way.
I'll probably go with the James and Antek transformers as per your recommendation. One question: do the James OPT have UL taps? I don't see it mentioned in the specs.
Thankyou for making this project avaliable to less experienced DIY'ers such as myself.
Best regards, Peter.