SLK 75tl rework...

that colorado rocky mountain high.
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JeffreyJ
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SLK 75tl rework...

Post by JeffreyJ »

I figured this was the place for an update.... I have been working on Steve's babies... they are still Telefunken RS241's driving Eimac 75tl's, but now it is stacked supply direct coupled... :D

ok... so while I was under the hood I noticed a few things... one is that I like working on amps like this... really nice iron... nickel.. amorphous.. even in teh power supply... asc caps in teh power supply.. thyratrons for rectification... ancient and wonderful triodes... ahhh... the good life...

but also some questions/comments.... the last power supply capacitor is in the power supply chassis... I am, in general, not a fan of this... if you draw signal loops, it means that we have signal traveling through that umbilical... I was thinking about trying some nice Mundorfs in parallel, but on teh signal side.... so the 20uF might become 25uF, or so.... but close that loop locally.... I havesome reservations as that would seem to open up teh chance for multiple paths and I am never a fan of that, but there is only one way to know what will happpen - ... listening... I suppose I could just as easily parallel teh two 20uF PS caps as the first PS cap that would now be 40uF and then leave that nickel Slagle choke as the last element in the power supply chassis.... then a new ASC in the signal chassis... hmmm... that might be what is best...

and then another opportunity.... steve has an adjustable Vreg for the RS241 filaments... it would be an excellent place to listen for the changes of a final choke as output current reg... I can just twiddle the knob and keep the voltage exactly the same... then compare some M6 versus nickel versus just the Vreg.... I don't think we want the output of that reg in the signal loop... just a guess from previous work...

all of these changes will have to be evaluated on my big horn rig.... the amp isn't full range... which reminds me, I gapped the plate choke for 40H.. brown spacer... so, back to what I was saying... the amp can't be evaulated on my single amp system... so the tri-amped rig... I currently use a mesh 2A3 (modern, obviously) on my mids actively crossed over in front of the amp... so a perfect spot to listen to the amp... I run my mid amp from about 200Hz to whereever my rca compression drivers run out of steam... about 8kHz... so not quite as high as Steve will run it, but a good place for evaluation...

in any case... they are back together.. what are these outputs? 50:1? and the brasss mesh screens are cool.. steve, I think you will like the mods.. I cleaned up a few wire routings.. more direct... fewer "junctions"... more on the changes when we get back from Ijaz's place...

Peace,
Me
dave slagle
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Re: SLK 75tl rework...

Post by dave slagle »

JeffreyJ wrote:Ithe last power supply capacitor is in the power supply chassis... I am, in general, not a fan of this...
nor am i. you need to keep the signal on the signal chassis.
leave that nickel Slagle choke as the last element in the power supply chassis.... then a new ASC in the signal chassis... hmmm... that might be what is best...
that is what i would do. is there room for the ASC on the signal chassis?

it would be an excellent place to listen for the changes of a final choke as output current reg... I can just twiddle the knob and keep the voltage exactly the same... then compare some M6 versus nickel versus just the Vreg....
i agree here too. FWIW, johnny tried some iso chokes on the output of his 75TL re supply and didn't like the result. They were just some M-6 units i had around, and they did get hot sooooo...
I don't think we want the output of that reg in the signal loop... just a guess from previous work...
again i agree... we want the signal to go directly to ground by one single path. A voltage reg just gives the signal a second parallel path.

can the voltage reg be reconfigured as a simple current reg? If those ASC's take up some space on the signal chassis, how much room will there be for chokes?
and the brasss mesh screens are cool.
i love them... but i love the peek-a-boo aspect for the curious to look. From a distance they look solid but as you get closer you realize you can see the transformer through them.

dave
Steve Kaufman
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Post by Steve Kaufman »

Hi guys

I agree here too and then some! Jeffrey, please do what you (and Dave) feel is best.

Thanks

Steve
Johnny
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Post by Johnny »

Hi Guys,

In the spirit of Intact Audio, I think you should listen to the amp with the last cap. of the power supply filter physically mounted on the power supply chassis then move the last cap of the power supply filter to the signal chassis. My bet is you don't hear a difference.
As Dave points out, I tried the chokes on my filament supplies and they sounded worse. However I have separate regulated supplies. Steve's amp doesn't have regulated supplies so maybe it would make sense. Listen to both and let us know what you think.
I see a problem with testing Steve's amp as the "middle" amp in Jeffrey's rig. You won't be able to hear the contribution or lack of contribution Steve's amp makes to the high frequency response. The work I did with reduced-turn transformers from Dave showed me that the high frequency response is a big part of the design and nuance of the amp. I understand there isn't much else you can do but I still think the evaluation is flawed.

Johnny

Johnny
JeffreyJ
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hey Johnny...

Post by JeffreyJ »

I completely and totally agree with everything you said...

I actually hooked steve's amps up to some 12" coax's in the den... full-range... I was quite surprised at the bass emanating... not to mention teh incredible midrange... so I may actuallly use these as the test rig.. or perhaps repeat in two rooms... thank the Great Spirit for clip leads!

I will do an A>B with teh cap... no idea if we will hear it or not... but I wouldn't do anything blindly to Steve's babies...

ok.. now filament chokes... I heard HUGE improvement on my 75tl's when using a switch mode voltage regulated supply... not much difference on current regulated 71A's.... and steve's is a voltage reg... so I give it better than 50/50.... after all, we will be changing the signal path...

I listened to them some today... but nothing critical yet.... just sweet tunes...

[back from Ijaz's place... weather was horrible... the trip to Ming's took just under 25 minutes saturday morning... but sunday when I was leaving town, the same exit was over four hours! long days on the road... I am beat... I saw over a hundred cars littering the medians and shoulders... lots inverted.. scary...]


Peace,
Me
JeffreyJ
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the 75tl's....

Post by JeffreyJ »

hello, all...

I just wanted to pop in and say that I ran Steve's amps for at least a couple of hours every day this week... I think I have a good handle on the sound, and everything should be broken in...

compared to the 112A -> 75tl that I took to RMAF, it is only slightly different.. I will attribute the differences to the front end tube, but of course, there are many minor differences....

Steve's has more beauty in the treble... upper midrange... it sounds really nice... the micro dynamics might be slightly better.. you know, leading and trailing edge transients... the 112A is definitely warmer through the midbass... Steve's won't play as loud, but it shouldn't... mine is still much more power... and the very bottom end is also better on my amp, but Steve's was built to have nothing below 50Hz... I will measure it to see where the rolloff starts.. I would guess it starts at nearly 100.... but, amazingly, it sounds wonderful full-range on my speakers... because of that, I will do my critical listening/parts swapping on the JBL 2435 le Cleac'h / 515 Onken / jbl 2405 system.... it is a very wide bandwidth system...

more soon!

Peace,
Me
Steve Kaufman
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Post by Steve Kaufman »

Thanks, Jeffrey.

The amps were designed to go down to 100Hz flat and then tail off. There is still significant info at 50 to 60 Hz. It was designed that way because we didn't want to sacrifice any openness in the mids and highs.

I look forward to hearing how they do in the "Big Rig."

Steve
JeffreyJ
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Post by JeffreyJ »

hey fellas...

today I did listen to them in the big rig... I started with the 304tl's, as they were already hooked up... then the SLK amps... then my 75tl's... it was torture... :wink:

ok, ok.... seriously... here's the thing... the 304's were big and bold... teh onken's kinda like the extra juice, too... but not nearly as exact... still, a *wonderful* sound.. did I say big?

and then Steve's... like a breath of fresh air... the treble is so beautiful you can hardly stand it.... just delicate.. and round and so nice.... and yet very quick... no slouch.. at all... just so clean and clear...

and mine was very similar to Steve's... the differences were that mine tended to be more even-handed.. the focus of my amp was the midrange.. while Steve's focused a region slightly above mine... mine couldn't touch his in the delicacy department... and mine was just slightly warmer in the lower mids... of course you had to listen through the low bass being rolled off on Steve's amps... mine were just 1% more "fuzzy".. I purposely chose a tube that was leaning toward warm to complement the 75tl beauty..... it worked... (but I also secretly want to try that LP2... the phono is sounding incredible)

so.. the speakers were definitely flavored with my amps... therefore, of course the combo was even-handed... I bet with Steve's Azuras and Goto tweeters, the sweet treble will bring tears... in fact, I guarantee that it will... it sounds amazing...

wednesday I will try a few mods.... but unless they are dramatic, I likely won't change much more than what I have so far... "if it ain't broke..."

Peace,
Me
Steve Kaufman
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Post by Steve Kaufman »

Hi Jeffrey

Thanks for all the torture, I mean diligent listening. When John reads the post he'll have chest pain! This should be really interesting. My amps only function down to 160Hz. Below that the sound is handled by the small solid state NuForce amps. Me thinks my amps will thrive in that environment. I really appreciate all your effort and look forward to hearing the fruits of your labor.

Steve
Johnny
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Post by Johnny »

Hi Guys,

Sounds like you've got something special there. Congratulations!!!


John
JeffreyJ
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Post by JeffreyJ »

ok... so here's what I learned today... well, let's hope I learned more than this, but this is what I learned about Steve's amp today...


I listened to mine... it was hooked up.. then his.. chose some acoustic music... bluegrass, actually... so that I would focus on the mids and up.. mandolin, fiddle, guitar...

my amp still is 1% fuzzier than Steve's... but Steve's was nice and round in the upper mids... maybe a touch too much for me, as you might have guessed... but just one touch too far... still wonderful.. no issues at all.. but being picky, just one small touch...

I was able to get a 1.8uF Mundorf film and foil to connect directly from the B+ of the 75tl to the cathode... just the leads used.. no clip leads or anything.... and then I listened.. quite a change... much more even sounding.. better transients... seemingly more extended top end...

I am going to listen like that for a while tomorrow.. make sure that it doesn't sound like a cap.... I do have some of the silver/oil and silver/gold mundorfs around...


the point is that this is definitely a sensitive spot... so it should be noted.. perhaps it is the extra DCR of the umbilical that makes the impedance of the loop through the last power supply cap much higher than this film cap... I dunno... just a guess... but you can certainly hear the difference easily...


gotta get some sleep...

Peace,
Me
Steve Kaufman
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Post by Steve Kaufman »

Hi Jeffrey

What effect did the caps have specifically on the lower end? It would be interesting to hear the effects of the different Mundorfs. I have used both the silver and gold versions and they are pretty good. I never made a direct comparison in a given location, however.

Thanks

Steve
JeffreyJ
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Mundorfs...

Post by JeffreyJ »

I listened to Steve's amps with a few different cap combinations yesterday.. I eventually settled on a tin foil cap (mundorf) across the RS241 power supply and a silver foil and oil (mundorf) across the 75tl... too much oil and it gets opaque... I personally tend to like the Mundorf Supreme (tin foil) better than the others... I tried the silver/gold in the amp.. both places... and it is just too dead for my tastes...

I wish that I had some Jensen or Ausdio notes here... I tried a local ASC... but it was 15uF, so that wasn't really apples to apples.. the others were all 1.8 or 2.2.... it was great, but just not quite as transparent as the others...

Steve, are those GE caps Spearinol? I think I have a pair of them...

I am almost done.. one or two more days to just make sure I didn't miss anything in the sound... and then they are off!

Peace,
Me
Steve Kaufman
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Post by Steve Kaufman »

Hi Jeffrey

The 10 uF caps are actually made by Industrial Condenser Corp. I have a pair of GE 4 uF caps in my preamp. I don't know if they are Spearinol. The common point is that they are in big old ugly gray rectangular cans. The GEs I had in my 45 amps sounded better that all the others. Why don't you try the GEs and see how they sound?

Steve
JeffreyJ
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trying to document....

Post by JeffreyJ »

hello fellas... just trying to document the current status of Steve's amps... the great thing about Dave's forum being set up this way is that we can easily search and have thoughts and findings easily accessible...

so Steve's amps went back home and dropped one resistor connection in transit... Grant and John found it pretty quickly and then the gang started some music... sounded bad... no smiles... everyone goes home relatively sober... :(

Steve plays them for two or three days... calls and says that they now sound great... likely because they sat in my garage for a week being packed and shipped... so time to go over to John's to compare direct coupling (steve's) and transformer coupling (john's)... the amps sound bad again... dry, almost wrong.. steve is at a loss... so they yank the last power supply caps that I put in the signal chassis.... "a little better, but still bad".... in the ensuing investigation of the amplifier, John bumps the 75tl with his magic groove-a-licious rump... :wink: now the amp sounds wonderful... :lol: must have been a bad connection in the tube socket.... could have been plate cap or grid cap...

amp goes home with steve.. happy guy... until a filament regulator for the RS241 blows... ouch... no tunes for Steve... :cry: it was one of those kits that you buy... the guys come to the rescue with a few bench-top supplies... but they aren't quiet enough... so he is now on batteries.. which sound fine, but are a bit of a pain... so we need to build him a small rectified supply...

oh.. and most importantly... the amps go to John's for another head to head... and the gang says that they really can't tell a difference between the direct coupling and the transformer coupling! which is a wow.... :shock: especially since John's amps we know have an LC tank resonance to help keep that bump in his trunk... I think the stacked supply direct coupled tightens the bass and really gives it some control... but this is also a huge testament to Dave's nickel 1:1's... transparent, for sure...

ok guys.. let me know if I have missed any of the details or the order...



so why am I finally posting this?... I am supposed to be outside working on Ming's horns right now... but after ten straight days well over 100 (supposed to be 105 today) and 90% humidity, I just can't do it yet again today... a fan blowing 100+ degree thick, heavy air is really insulting rather than refreshing... ugh... :twisted:

Peace,
Me
Steve Kaufman
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Post by Steve Kaufman »

Hi Jeffrey

That was surely a nail biter until the last paragraph.

I do have more bass control at my house, but my amps go down lower than John's. In his setting both versions of the amps sounded great. John plans to switch to NuForce amps for his amps for the bass cabinets and he will remove the frequency limiting circuit. He may still not go a deep as mine because of the way the iron was tuned originally.

I would like to put in something for filament regulation. The battery thing is getting old. Although the noise is very low, the charging is a bit of a pain. Also, the filament voltage varies between 4.2 and 3.6V as the battery drains. It would be nice to turn the amps on and not worry about it if the low noise can be maintained. I have some thermistors so the filaments come up gently.

On those hot hot days when Ming's horns look less than appetizing you could always solder up the LC phono stage. With the circuit board it shouldn't take too long. Did Dave redo the iron for the one that went to Europe for the shoot out? My output trannies are different from the ones Steve used originally in that mine are amorphous core.

Fall is just around the corner.

Steve
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