The 8233 (E55L) does make a nice spud amp. Pete Millett has some data in his pentode comparison, High Gm driver pentodes
G2/G3 tied to plate, 180V, Ip about 55ma nice grid bias so you have enough overhead to work. I haven’t tried it down at 140V where Millett tests.
Matt
G2/G3 tied to plate, 180V, Ip about 55ma nice grid bias so you have enough overhead to work. I haven’t tried it down at 140V where Millett tests.
Matt
Just finished my Ultimate spud. I went with a E55L in the end.
Configuration is;
-Grid choke using a telema encapsulated power transformer, configured for a 2x step up
-E55L configured as a parafeed. All pins have ferrite beads and the g1-a connection is 1K. resistor bias is 39R unbypassed
-plate load is a IXYc10M90 with a B+ of 250V, output transformer is a Triad toroidal power transformer 240V:6V. Negative output of transformer is referenced to an 8uf vintage PIO cap and the negative of this is sent to the top of the cathode resistor.
-There is a 820K resistor from the anode to the top of the input transformer to introduce a small amount of plate to grid feedback (DC current is small enough that it doesn't appear to saturate the transformer).
It worked first time which is a miracle in itself. The feedback tightened up the bottom end a little. It sounds remarkable good, in fact its really not much different to my bigger PP amps.Haven't yet done any detailed listening yet.
Will lash up a schematic later and post it up.
Shoog
Configuration is;
-Grid choke using a telema encapsulated power transformer, configured for a 2x step up
-E55L configured as a parafeed. All pins have ferrite beads and the g1-a connection is 1K. resistor bias is 39R unbypassed
-plate load is a IXYc10M90 with a B+ of 250V, output transformer is a Triad toroidal power transformer 240V:6V. Negative output of transformer is referenced to an 8uf vintage PIO cap and the negative of this is sent to the top of the cathode resistor.
-There is a 820K resistor from the anode to the top of the input transformer to introduce a small amount of plate to grid feedback (DC current is small enough that it doesn't appear to saturate the transformer).
It worked first time which is a miracle in itself. The feedback tightened up the bottom end a little. It sounds remarkable good, in fact its really not much different to my bigger PP amps.Haven't yet done any detailed listening yet.
Will lash up a schematic later and post it up.
Shoog
So here is the schematic.
I made a few alterations to the amp as described initially.
-It wasn't placing enough voltage on the plate's of the E55L's so I increased the cathode resistor to 49R (anything from 49-63R would do just fine. which placed the anode at about 130V.
- I decided to see if I could replace the 1K screen resistor with a ferrite bead - I can and it remains nicely stable.
- I wasn't happy with the rather bloated sound of the amp with the 830K feedback resistor, so decided to replace it with a 470K and add the cap in to remove DC from the input transformer. Sounds a lot tighter now which is very good.
It has the right amount of gain for my system and is capable of driving my very sensitive vintage speakers into distortion.
Overall it fits the description of been the Ultimate Spud amp IMO
Shoog
I made a few alterations to the amp as described initially.
-It wasn't placing enough voltage on the plate's of the E55L's so I increased the cathode resistor to 49R (anything from 49-63R would do just fine. which placed the anode at about 130V.
- I decided to see if I could replace the 1K screen resistor with a ferrite bead - I can and it remains nicely stable.
- I wasn't happy with the rather bloated sound of the amp with the 830K feedback resistor, so decided to replace it with a 470K and add the cap in to remove DC from the input transformer. Sounds a lot tighter now which is very good.
It has the right amount of gain for my system and is capable of driving my very sensitive vintage speakers into distortion.
Overall it fits the description of been the Ultimate Spud amp IMO
Shoog
Attachments
Yes it is. Its a toroidal mains transformer configured as a choke with a 1:2 step up ratio. Works remarkably well all things considered.
the secondary is unconnected.
Shoog
Thats how it works.
The transformer probably has about 100-150H inductance, which is very low when compared to some grid chokes with inductances out at 1000's H. Resonances is the main issue to consider here - and interwinding capacitance. However I am running this from a transformer coupled preamp with an output impedence down at 10's of ohms and no coupling cap. As such it seems to work just fine and isn't doing anything particularly nasty to the output that i can tell. The plate to grid feedback might be helping. Other people may feel far more comfortable using a conventional grid leak resistor which should work similarly well. I may even try it myself to get a comparison.
So the input choke's success is likely to be very dependent on the type of preamplifier driving it.
Its been running for a few days now - and I am finding it very likable. Its relaxed but with tight bass. I really am finding it difficult to tell much difference between my Class A PP DC coupled amps.
I think the main thing which can be taken from this - so far - is that ferrite beads work very well to stop these high gm pentode's/triodes from bursting into spontaneous oscillation and becoming AM transmitters which is a great result.
Shoog
Congrats. A good spud SET is as close to wire with a gain as possible IMHO. I've got a couple EL55's that I am considering using to rebuild my WE417A spud amp.
With parafeed I imagine you get a lot less power loss from the transformer? This can really add up with a SPUD.
Very creative stuff. Thanks for sharing.
With parafeed I imagine you get a lot less power loss from the transformer? This can really add up with a SPUD.
Very creative stuff. Thanks for sharing.
The input transformer arrangement is only really going to be satisfactory if you have a preamp with an Output transformer since otherwise there isn't going to be enough inductance and there is a strong likelihood of resonances. If you have one - I have no hesitation in saying it works wonderfully.
The parafeed allows extremely good bandwidth from the transformer, both top and bottom - which to my mind is the main limitation of conventional SE transformers. It certainly sounds as extended as my PP amps.
I was concerned that the parafeed cap would degrade the sound, but placing it in the negative leg of the OT and returned to the cathode seems to largely take it out of the mix. It certainly doesn't bring any syrupy qualities which you might expect from large vintage PIO caps.
I was working on ironing out a few wrinkles from my last PP project - so haven't listened much to it recently - but it matches the performance of the PP amp in every way and adds more detail. It may very well become my main amp.
I have just sprayed it, and if anyone is interested I can post up a few photo's.
Shoog
Here are a few pictures. Looks better in the flesh than the photo's imply. I will add some form of top cap for the big exposed capacitor terminals and I hope to add a wooden front panel.
I had a friend over yesterday to do a comparison between it and my latest PP amp. The PP amp was considered to have a fuller sound without giving up much detail. The Spud was leaner overall with a better top end. Overall the PP amp had it by a very small margin.
Since then I decided to reduce the feedback by changing the 470K resistor for a 640K resistor to see if the sound would fill out a little.
I had a friend over yesterday to do a comparison between it and my latest PP amp. The PP amp was considered to have a fuller sound without giving up much detail. The Spud was leaner overall with a better top end. Overall the PP amp had it by a very small margin.
Since then I decided to reduce the feedback by changing the 470K resistor for a 640K resistor to see if the sound would fill out a little.
Attachments
Really like your project even ordered a pair of EL55's, but surprised at the lean. I've never considered SET lean, always the opposite. My guess the budget signal transformers? Just opportunity to consider.
I think you might be surprised at how much deeper and fuller and tighter the bass can be from a good PP amp.
I am coming to the conclusion, after listening to this amp for a few weeks now, that I prefer my PP amp. The Spud does everything asked of it very well - but its top end is slightly less extended and the bass isn't as good. It might be that I could get a bit more extension from changing out the input transformers - but I have good reason's to suspect that it wouldn't make any difference since I use a similar approach in my PP preamp.
Shoog
Hi there! Because the name of the thread is " Ultimate spud amp - any ideas?" please allow me to give you some ideas guys... ☺ This is a multifunctional input transformer that I have designed lately for SPUD apps but not limited... It drives directly from CD/DVD/smartphone/PC/DAT tubes like EL84/6BQ5/6P14P, 6L6GC/6P3S, EL34, etc, etc ... both in SE or PP configuration! ... this being the toughest challenge, to keep the high specs both for SE and PP and also to be able to deal with balanced/unbalanced sources... I have a full set of measurements if someone is interested. Cheers!
Attachments
Looked at my schematics file and noted the "Chinook" or "Baby Amp II" which uses a 6FD7 per channel. 6EM7 is another great option for a SPUD and would work in that circuit too.
Link:
DIY Audio Projects Forum • SET - Single Ended Tube Amps
A search on 6EM7 will bring up other options.
Cheers,
Ian
Link:
DIY Audio Projects Forum • SET - Single Ended Tube Amps
A search on 6EM7 will bring up other options.
Cheers,
Ian
Last edited: