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Edited: Feb 20, 2023

Single-ended pentode (SEP) Spud amp kit

I recently completed this little 6CL6 SEP project. This is the highly regarded kit from Gerald Curtis and is available here: https://spudkit.com

Jerry has clearly taken a lot of care putting this kit together. All parts are high quality. The build is straightforward and the kit comes with exact and easy-to-follow instructions. The Huon Pine (from Tasmania) wood side panels are my own and as you can see are attached with plywood cleats. The finish is just a flat poly coat for protecting metals from tarnishing, as that's what I had in the shed. The quality chassis is beautifully made, quite thick and heavy with a luxuriously textured paint job. This is certainly no pressed steel jobbie. The transformers are by Edcor, with the output transformers having been specially commissioned by Jerry.

His support was outstanding when I needed to debug the power supply due to an error I had made. If anyone is looking to build something simple as a first project, this is it. With Jerry's support, you will not be alone either if you run into a snag. You will be rewarded with an amplifier that will blow your socks off. It really took me by surprise. There is zero hum, even when the volume is turned up to max.

How does it sound? I have compared it directly with my Decware SE84ZS, which runs EL84 or SV83 high-bandwidth triode video tubes. My Decware runs a Russian military version of the EL84, ie. 6P15P-EV. The Spud amp kit comes supplied with GE 6CL6 tubes, also a type of high bandwidth video tube, but these are pentodes. Both amps are simple single-ended designs, both use video tubes, and both put out around 2.3WPC, so a direct comparison is fair enough. However, I really had no expectation that the Spud would be in the same league as the SE84Zs. That it did indeed jump into the ring and whup the Decware was a total surprise. Both amps have gloriously smooth mids and highs, but where the Spud really stood out to be separated was in its bass. There is oodles of bass.

Well, there you go. A simple little project without compromising anything. I love it to bits.

31 comments

31 Comments

bloozestringer

Sep 23, 2023

Sorry about the resurrection. New here and this is my first post but glad to see the spudamp still around. I heard the original 6197 amp this was based on built by Jef Larson (abraxasaudio) back at the Great Plains Audio Fest in 2004-5 or so before you started seeing the kits on then Hawthorne Audio. Jef was notorious for designing amps around less "desirable" tubes although I don't know if he still builds anymore. He helped me design my first and only scratch build (1626 Darling) around that time too. I owned a 12B4 amp he built that was very impressive for its size and I wish I hadn't of sold it.

oztayls

Apr 16, 2023

For the past couple of weeks I've been trying 3 different vintage tube types, cold war era. The first are RCA JAN 6CL6 (Joint Airforce Navy) and the second are RCA FAA 6CL6 (Federal Aviation Authority). Both are excellent, and just as good as the supplied GE 6CL6. I suspect the RCA JAN and FAA are the same tube and just marked differently. The 3rd type are NOS 1958 JRC 6CL6s (Johnson Radio Corporation) which were manufactured by RCA. These are slightly different sounding, I would say a tad warmer, with a silkiness about them.

https://repository.duke.edu/dc/adaccess/R0295

Danny

Mar 09, 2023

Well I just got my kit the other day and it took a good couple hours to build - very very few parts- minus the wood sides- it sounds great- the little guy puts out for 2 1/2 watts - have to sat it's my lowest powered amp I've had the privilege of owning - I added an red led power lite-alps volume pot and did scrape the paint ( not in the instructions) for the star ground.it's as quiet as a church mouse ,, kit is very well put together and very simple- no need for some good coupling caps in there- there are none!! very clean audio path- it's also my 1st SEP - many SET's. I got the cherry wood sides I still need to stain and a new volume knob then she's a rap. Been listening for a few hours today .. I still need to look at her on the scope and do some measurements - but she does sound great.

Danny

Apr 01, 2023

Replying to

Well I just ordered up the parts for Decware Zen ..Iam going to do point to point- should be an easy fast build. I got tired of the volume next to the tubes.. I moved it and put in a metal vent in the old hole.

oztayls

Apr 02, 2023

Replying to

Nice one Danny. The vent will help dissipate any heat faster than convection from the case, so is a great design improvement. I’m looking forward to your Zen kit build!

I built a little 2Tb Rasberry Pi NAS. I don’t know why I waited so long to do this project, It’s brilliant. I just need to add another 2Tb for backups now.

G BRM

May 14

Replying to

Where did you find the vent?

Danny

Feb 27, 2023

Very Very Very intresting - So this beats the Decware Zen? I have been wanting that Decware for a long long time- it has some of the best reviews out there! it's 2.5watts beats just about everything out there that 6P15P-EV tube is supposed to sound awesome. Decware also has the Zen kit also - the waiting list for their amp is about 2yrs. Do you buy their assembled amp or their kit? SE84UFO?Have many tube amps and Skunkie's 300B is by far the best - the Decware Zen has been the next one I was to pursue- So this spud amp is on par with the Zen but better bass? It must sound fantasic - I will look into this for sure.

oztayls

Feb 27, 2023

Replying to

With all due respect to the Decware Zens, there are a few models now. My ZS is a 2008 model, which was a later iteration of the CS. It is in stock form, without the CCE (Christmas Came Early) mod where a wire connecting the suppressor grid to the cathode is replaced by a non-polarised film cap. This mod has been incorporated into all the subsequent Zen amps and no doubt there have been changes made to the transformers along the way as well.

But yes, Jerry's amp leaves the stock SE84ZS for dead. I've had an SE84UFO2 on order since April last year, but based on Decware's build sheet, I'm not expecting it to arrive for at least another year. So, count on a wait time of about 24+ months now for a Decware amp. Maybe their kits go out the door much faster? If you can build one, maybe that's the way to go? Decware kits are PCB based though, not point to point like the Spud. I'd be inclined to go with a Spud, now that I know what they sound like. Jerry's build instructions and support are excellent, but you can also ask him to build it for you, for an extra fee of course.

Skunkie

Feb 20, 2023

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That looks awesome! :) Is it being run as a pure pentode or a UL type design? And given it has no driver tubes, does it need a preamp to drive it?

oztayls

Feb 21, 2023

Replying to

The Spud runs as a pentode without any negative feedback from the output transformers. The 6CL6 has enough gain in this circuit to take a line-level input and amplify it respectably. I haven't had a need to run a preamp on my systems, but Jerry does endorse using one for sources that aren't "hot" enough to get the Spud into clipping.