The Eagle Supre - Big SE Amp

[RJ Guitars]

Hey guys,

I was watching a Dr. Z amp demo where he switches the amp between the "Speed" and "Comfort" settings. The difference is apparently a change in the plate voltage, I suspect accomplished by switching the power tranny secondary windings. I've had folks ask for this option on our Supre amp and it seems so easy to do that I decided to look into it.

about 5 minutes into the video -- http://www.youtube.com/watch?v=TeXVHhN2I-U

Does anyone have a switch recommendation and maybe some education or experience with this technology? I have some great old Mil Spec switches, but not enough to offer them with the amp parts package. I think it would be useful to identify a switch that doesn't mind the 300+ volts AC.

Cool thing in the video was that he switches the amp in real time... I don't know if that is a good idea but would be a very cool option if it doesn't make smoke and sparks.

Any thoughts?

rj

PS - The Supre power trannies are supposed to here tomorrow... the last item before we can start building some amps...

[M Fowler]

Carling PH-527 DPDT at AES or CED.

250HT on one end and 290HT other end. Center to rectifier.

I switch mine on the fly without any problems.

[RJ Guitars]

For anyone interested, all the parts are in for the Eagle Supre Kit. There is a short list for those that want to build this amp and not that many kits available for the initial effort. Let me know ASAP if you should be on this list and I'll set one aside for you.

I have mine about half finished already and it all has gone together nicely. I'll plan to post up photos and details and have even started a "build guide" that I am putting together as I go.

I just updated the schematic and layout. I did not add the B+ voltage switch into the drawings but I am going to include it in my first Supre build. I did make several component value changes in the schematic and it is looking more "Blackface" all the time.

I received the KT-90 tubes and they are impressive to look at. I'll have to finish the amp to tell you more than that.

rj

[RJ Guitars]

The first eagle Supre is finished and all is well! Everything fit together and it was a pretty easy build. A bit more involved than the Eagle 100 but still a simple outfit.

The Edcor power trannies are typically high quality and quite large... lots of extra iron here.

All the controls work just as planned and the tonestack bypass control is absolutely cool! It's relatively quiet and there doesn't seem to be anything that is begging for redesign.... yet anyway.

I jumped in right away with the KT88 output tube but then went back and tried the EL34 and KT66 as well. Surprising not a huge difference in the sound between them.

The biggest surprise to me was the almost inaudible difference in the voltage switch. I had 427 volts on the output tube plates in one position and 345 volts in the other. I suspect some of the subtleties will become more apparent after I work with it a while but this switch does not have near the sonic impact that the tonestack bypass does.

It's a good sounding amp but not yet an awesome sounding one. It needs more sparkle before I think I'll raise the bar to "awesome". It also has way plenty of bass so I want to do some tone grooming and bring up the highs and the upper mids. Right now it sounds great on the bridge pickup of the strat but I don't like being stuck in that position so I want to bring more top end out of the amp. I would prefer to have the option of dialing down the treble a bit on the guitar if needed.

I deviated from the most recent schematic only slightly in that I put a bigger bypass cap (100uF) on the output tube... maybe not that good of an idea since it is a little bottom heavy right now.

Curious what folks think about the voltage switching non-event and also ideas on a treble/mid high boost option.

rj

[Lonely Raven]

That big tube is great looking. Do you have any clips?

[Zippy]

For anyone interested, all the parts are in for the Eagle Supre Kit. There is a short list for those that want to build this amp and not that many kits available for the initial effort. Let me know ASAP if you should be on this list and I'll set one aside for you.

If I'm not already on the list, I'd like to be.

[RJ Guitars]

Zippy,

You are for sure on my list....

rj

[RJ Guitars]

No sound clips yet but I did play through it a little while tonight. Building amps and playing through them is a rare treat these days... I need to do more of it!

I tried the El34, KT88, KT66, and El90 tubes tonight. I really liked the bite of the EL90 but I think all of the tubes have potential, just want to bring out a bit more of the upper mids and trebles before I am going to believe I am hearing all she has to offer.

I took a few pictures as I built it so I could eventually make up a build guide. The first thing I did was start adding parts to the chassis. You have to think ahead a little bit but basically you need to put the transformers on before almost everything else.

Also, these are really beautiful little chassis, but they are pretty soft and it's easy to scratch them so you need to have a padded surface and avoid scratching the surfaces.

I added a switch to allow the B+ to be switched between the 2 windings. It's not on the drawing but you can see it's pretty easy to do if you have the inclination.

rj

[RJ Guitars]

I brought this amp into my office and out from under the florescent lights in the shop. That usually reduces the hum in any amp... but not in this case. I hadn't really had this in a totally quiet room before and I wasn't pleased with excess hum once I had it in quiet surroundings.

1) I had tried to partially bypass the cathode on the first gain stage and reduce some of the excess low end that the amp is producing. I accomplished this by using a 2.2uF bypass capacitor. In Merlin's book he suggests this may lead to excess hum and it's best to use a 22uF bypass cap and fully bypass in order to get rid of as much AC hum in that first gain stage as possible. I replaced this with a 22uF 25v bypass capacitor and the hum dropped a bunch... lesson learned for me!

2) I had gone with a 1K and 10K resistor (in that order) on the dropping string in the power supply. I can't remember where I read it but something I read recently stated that if you get these resistor values too low you may get some hum... I changed them to 22K and 10K (backwards from the 5E1 spec) which is something similar to what is used in most Trainwreck circuits, but has one less node.

3) Then I replaced the first filter cap (33uF) with a 47uF and again the hum went down. It's not quite as good as my push/pull Rocket builds, but not too shabby at all. I am not sure if I should expect any less or not? Any thoughts?


I am pleased with the hum issues at this point so i want to see if I can coax just a little more sparkle out of the amp. It's got a good pure tone to it, it just seems a little bland and I'd like to add some shimmer, sparkle, harmonic richness, whatever flavor it is, I'd like more of it.

One other interesting note - As I worked on the hum issues I was using a 6V6GT tube. Compared to all the big glass I had been using up to this point, this seemed to add some welcome brightness to the tone. Also I found that a 6V6 in this amp is surprisingly loud! I think it must be the big iron.

Curious what the wisdom of the tube gurus has to say toward tweaking this to the next level.

rj

[surfsup]

Are your grounds the same as your last pic still? Looks like three stars:

One for PT, IEC
One for rail caps, lower left corner of board
One for preamp and PTk, lower rt corner

Would it be a valid effort to move the PT RCk ground to the Ptube's filter cap? Then maybe ground the rail to the same place aa the PTransformer, but perhaps not with the IEC.

I did exactly that with my SE and it solved all my hum issues. I tried grounding the heaters to the PTk and a separate elevation circuit (46v) and neither helped.

The heaters around your PT seem to form a loop as well. On my amp people kept telling me to not do that especially on SE.

Was the eagle 100 similar with the heaters? Did you try moving the heater run "loop" wires a bit further away while the amp was on to see if it helped?

[Zippy]

I am pleased with the hum issues at this point so i want to see if I can coax just a little more sparkle out of the amp. It's got a good pure tone to it, it just seems a little bland and I'd like to add some shimmer, sparkle, harmonic richness, whatever flavor it is, I'd like more of it.

One other interesting note - As I worked on the hum issues I was using a 6V6GT tube. Compared to all the big glass I had been using up to this point, this seemed to add some welcome brightness to the tone. Also I found that a 6V6 in this amp is surprisingly loud! I think it must be the big iron.

RJ - I applaude your commitment to sorting out this amp prior to shipping - even if it is a group build exploration.

Seems I recall Cliff commenting recently on 6V6 tonal characterisitics - that tone spectrum seems to vary with era of manufacture.

Regarding sparkle, shimmer, etc.: I was going to build mine with a Marshallesque bias on the first stage (vs the "Fender" style). And given my preference for all things tweed, I was going to use a 5E3-style tone knob that sucks less signal - resulting more shimmer as well as gain.

Regarding the big-iron hum in the PT - How does this compare with other PTs from Edcor? Aren't these smaller than the iron you've been using in your Rocket builds? What would happen if you used a Rocket PT for this build (just to compare mechanical hum levels)?

I'm looking forward to wring out these amps with some P-90s.

All the best to you and yours (including that lil' dog-like pet),

George

[M Fowler]

rj and zippy,

This looks like a great amp and having dual HT 345v/427v on the plate option is a great way to make this a more versitile sweet little amp. More voltage then a 6V6 problem can handle though.

Adding a sag resistor between the rectifier and 47uf main filter cap might be worth it as well.

I don't see the grounding points being a problem. In single ended amp your going to have some hum and probably going to improve going to DC filament or elevated filament.

Tube choice here is going to be a big factor in sparkle, NOS pre and power amp tubes. Or at least some good rated tubes.

my 2 cents for today.

Mark

[Cliff Schecht]

I've been meaning to post my SE schematics but lately I've been very busy. I'll try to post them this weekend, they're both based off essentially the same topology with different voicings for their intended applications. I sold one of the amps (my KT88 build, one of the few that I've sold!) and sorta wish I hadn't but I have the schematic kicking around and my friend (who lives in NY) can always open it up and send me more details. I also have some cool little 6BM8 based amps which can kick out a LOT of sound for not a lot of size.

The one thread that runs in every SE amp I build however is 3 triode gain-stages as opposed to 2, cathode follower optional. I won't go back to building standard Champ's because I love the sound of three gain stages so much in a SE format, but I usually make that third gain stage optional via a switch. But shit, when you flip that switch up it's easy to go into Marshall plexi high gain territory with a really wide open, harmonically rich sound. Also FWIW I usually don't run negative feedback on my SE amps, it neuters them too much IMO. I like that sparkle that a SE amps offer and negative feedback takes this away. If the amp is too bright, get some darker speakers!

RJ the few tricks that I use to control hum in SE amps are careful grounding, elevating the 6.3V CT or faux CT to the power stage cathode and isolating everything from the chassis but the input jack and IEC ground. Even with gobs of gain, this really seems to do the trick. As you noticed the input capacitance has a direct effect on hum as well, I usually never go below 47uF and in bigger amps with higher voltage I might use 100uF-220uF to keep things extra quiet. Remember these caps are what determine how much ripple is removed from the supply line which is extra-important because SE amps don't naturally cancel hum as push-pull amps do.

[Cliff Schecht]

rj and zippy,

This looks like a great amp and having dual HT 345v/427v on the plate option is a great way to make this a more versitile sweet little amp. More voltage then a 6V6 problem can handle though.

Adding a sag resistor between the rectifier and 47uf main filter cap might be worth it as well.

I don't see the grounding points being a problem. In single ended amp your going to have some hum and probaly going to improve going to DC filament or elevated filament.

Tube choice here is going to be a big factor in sparkle, NOS pre and power amp tubes. Or at least some good rated tubes.

my 2 cents for today.

Mark

I agree with this but you don't have to go NOS power tubes to get sparkle. I really like JJ output tubes (EL34, E34L and KT88 specifically) for SE applications, they get a lot of sparkle IME and don't break the bank. TBH the KT88 had the most unique tone of any output tube I've heard, had a character that was described by my friend as "smokey". Very complex and rich sounding, and I was using one of those cheapo 20W transformers from musicalpowersupplies.com. I think what you are looking for is a relatively low plate impedance and a transformer that doesn't saturate quickly. I used a high plate voltage so KT88 plate current was low enough to not saturate the cheapo 20W transformer badly so if anything, I lucked out a bit there. You guys are smart going with Edcor, they do make some great iron!

[M Fowler]

I've built a few SE amps without a bunch of noise and one I built is high gain with no noise. Not sure where to go with all this adding DC or elevating filament issue.

Visio broiler is also called the Swampthing amp.

Mark