yes and yes.
6922 plate remains 75V as prescribed.
I did not change the 1 1/2 6922s for each channel.
Just REMOVED the first 6FQ7 and reconfigured the 2nd as common cathode.
Note that in the orig circuit the 6922 with plate = 75V is directly coupled to the grid of the 2nd (driver 6FQ7 anyway)
6H30 plates are now 280V (B+ still 455V)
I can send you a spice file. |
I think you mean cross couplers? So insted of using 2)6FQ7 per channel you are using one 6922 and a 6H30 as the driver. The input tube remains a 6922. What are you setting the plate deferential voltages to? I believe with the 6FQ7, now a 6922, it was 75V matching from plate to plate. What percent THD are you getting? |
No worries hifigeek1 I removed the 2 6FQ7s (V4V5) which are the capacitatively coupled bottom tube of the 4 triode criss cross diffamp driver subcircuit so the net filament draw of this new config is 1 pair of 6H30 vs 2 pairs of 6FQ7 ie 30% lower than before.
The 4 6FQ7s previously shared the same 6.3V AC filament winding.
My amp now runs with 2 less tubes and 4 less coupling caps than stock,
6922 phase-split->cap->6922 gain->6H30->cap->KT88. |
6H30's draw more heater current... |
Ampex53: I dont have a thread on modding the D115ii.
The schematics are floating around the Internet.
The docs Kalvin sent me are the schematics (same) and parts list.
A 1987 build would be a D115ii.
Hope the fuse holder is repaired by now.
Biasing is a nightmare but since bias V is regulated you can put in 1 row of KT88s and bias them then add the other row. You MUST use DVM probes that have smaller exposed tips otherwise you risk shorting out things.
More importantly is the diffamp balances.
The gain stage 75V plate voltage and the 250V driver plate voltages must match (the 250V is hard to match you need to have 6FQ7s that are very symmetrical within 1 envelop)
Mods that I have done:
* All coupling caps to 0.33uF Russian K40y9 PIO
* Triode mode (pull the screen grid resistors and connect plate to screen grid with 100Ohm resistors @ each KT88(need change out a few resistors @ the bias regulator circuit to raise regulated ference bias voltage to close to 50V otherwise you smoke the KT88s)
* a very involved mod which involves REMOVING 1 pair of the 6FQ7 (V4V5) and substituting the V6V7 driver 6FQ7 with 6H30. THis involves tying the 2 6kohm (schematic says 5k) R27R28 together to form a common cathode R of 3kOhm and reducing the Plate R and pot to 13k plate resistance. (this is NOT for the faint of heart, I came up with those values running many rounds of spice simulation)
I am extremely happy with the results. |
The D-70 is truly an old friend.... |
In the mid 80s, I used to put the D-70 on Quad 63s (with custom stands) for my clients.
Sometimes used Entec subs, sometimes not.
To this day, some of the most lifelike and stunningly involving music reproduction I was ever privileged to hear (within its dynamic envelope, of course).
I suppose the D70 was colored to some extent, but, IMO, it remains one of the most listenable amps ARC ever made.
And I go back with ARC to be early 70s, and worked there from '76-'77. |
Johnsonwu ... G'day ... I have just scored a D-115 MK2 for a second system ... I didn't pay very much & haven't seen one since the 80's ... I use Beast OTL's I built for my main system but love the "classic" Tx coupled amps. And I'm looking for an SP-8 to partner it. I use a slightly modded CAT MK3 as my main pre.
I noticed in your thread you were doing some modifications to the 115 .... is there a thread that discusses mods for these amps? I am also just checking it over at the moment & looking for a schematic to figure out what adjustments can be made ... particularly how to adjust bias.
ARC doesn't have an email to request a service manual .... intact I really don't know much about them so I'm not sure they'd sell me one?? any clues?? Would you know where I can score one? quite willing to pay.
Interestingly this piece suffered the same fate as yours ... I got it from West Australia & it travelled to Cape York ... which is a remote area of Far Nth QLD on the East Coast - I'm about 680kms NW of Cairns on the Gulf coast - so the guy I bought it off packed it in quite a good box but used styrene foam as the buffer!! of course its weight was way too much & I have bent rack mount handles, broken fuse holder, fascia damaged but salvageable ... and a few marks and scratches. It took a month to get here via Darwin! anyway, if you can refer me to any sites or net areas that can give me the info to at least bias this unit ... I'd be much appreciated. I paid $500 for it & dates on the O/put Tx's indicate a 1987 build. It also has 8 x GEC KT-88 which test at around 90% ... I'm subbing KT-90's which I prefer. My email is Cinerama55@yahoo.com
cheers .... Ampex53 |
Johnsonwu, I actually have not owned the D70mk2 for more than about 18 months. However, I am pretty enamored of this amp and would recommend it to anyone thinking of a great mid powered tube amp. BTW, I have a feeling that Dick Olsher is into good sound like the rest of us, and if it comes from triode or pentode isn't the main issue. Just so long as the gear can portray music. That is my way of looking at gear, I'm not attached to triode or pentode or tubes vs. ss or whatever, IF the piece under question sounds great, that's what I'm looking for. Isn't that what we are all seeking? |
@DaveyF: great to hear u are in love with your D70 all these years... much like me --> my Counterpoint NP220 Premium Gold. I am surprised that Olsher didnt mention anything about converting it into triode. I was under the impression that he's all for triode glow. |
Johnsonwu, good to see that you like your modded D115.
I feel this way about my D70Mk2 and IMHO it's one of the best sounding
ARC amps. I have a feeling that this is why Dick Olsher bought one too!
A stone cold classic! |
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@Hifigeek1: After pulling 8 screen grid Rs and tying pins 3-4 on all 8 power tube sockets and 2 resistors in the bias regulator circuitry (was tuned for pentode mode at 40V I need 49 or so) I got myself an all tube CL60 :)
Whether running it as a triod D70 (half tubes) or D115 this is by far the absolute most delicate and musical sounding Audio Research Power amp I have ever owned.
The Classic series never got close to this level of transparency and involvement. |
I was reading the new Absolute Sound today and noticed that Dick Olsher just acquired a D70Mk2.. He comments that the D70mk2 has "glorious midrange" in a review of the CJ mono amps. I totally agree with him in this statement. |
Yes. I was thinking it was a D-70. D-115 will get you 60Wrms.. |
D115... Becomes a tubefied CL60... |
You realize you will only get about 35-40Wrms across 8 ohms in triode configuration... |
Checked all the regulated B+s and they are quiet and stable. No more oscillations.
All diff amp balances measure AOK.
Gain is too high for my liking so in a sense it's indeed not quiet enough.
Maybe a triode mod will give me the extra benefit of lowering the gain.
The sonic signature is not lightweight and refined enough, lots of power though. I guess its what full pentode/tetrode mode is all about. |
My guess is it's not quiet enough |
Point well taken, esp with coupling caps where I often choose not to parallel a tiny 0.01 for the sake of coherence. Would rather invest more and get a better single cap.
For B+ bypass my experience is YMMV. I noticed that blackgate lytics will sound hashy and smear if bypassed with a small film whereas others may benefit.
the D-115 B+s are all lytic + Rel2.0uF + Wonder 0.01 with regulated B+ to the screen rel2.0uF + wonder 0.01.
I also notice that the B+ to the 6922 gain stages have nothing at regulator (TIP41) output at all (!).
I think I am ready to move on to triode mode. Just realized the screen grid supply is actually the 6550 pass element's output. This will be useless if I convert to triode, kind of a waste.
Wonder why they use the solid state buffered regulated output for the small signal B+ and not directly from tube reg. |
Bypass with the same manufacturers caps as the coupling caps.. |
@Hifigeek1: Never mind, the relcap 2.0uF bypassing the cathode (regulated B+) output of the regulator 6550C has deteriorated enough that when I paralleled in another 1uF all is good. Time to replace them bypass Relcaps too I guess. |
@Hifigeek1: I was trying to do some final checks on the regulated voltages to the first stage B+ and regulated screen grids and I found an ultrasonic oscillation of about 20mV peak to peak. My mistake for not turning the horizontal dial of my scope all the way to look at ultrasonics. Thought the 20mV was just hash or the probe.
This was traced all the way back to the plate of the 12AT7, and then back to pin 6 of the TL071 (U1) which was sending 100mV of sawtooth.
I'm leaving this for another day now but am curious if you think I should replace the 3 0.22 caps surrounding the input pins of the TL071 opamp.
Much appreciate any guidance. |
@HifiGeek1: Audio Research seems to source caps from The Reliable Cap company (Audio + Rel) and TRT (which some think is sourced from ASC). I will pass on big sized caps for now. Even the 0.47 I have I couldnt find anything to fit them in.
@DaveyF: For coupling caps I would say V-cap Teflon TFTF or Mundorf Silver Gold Oil. Tried/have them on many friends'gear as well as my own. Never tried the Duelunds, no I didnt win the megamillions lotto. Jensen ceramic caps, maybe, too expensive still. |
Hifikeek1 and Johnsonwu, IF you had an unlimited budget for cap replacement, which caps would you use in the D70 or the D115? and why? |
These caps are huge and made by the company that makes them for ARC. |
I have a batch of 0.1, 0.22, and 0.47 FT3s laying around.
Cant use them anywhere other than my point to point wired projects. Too big.
All my Vcaps are deployed in various pcs of existing gear.
Are yours branded ones or generic Russian? Definitely not the Psvane Audio Teflon leaded caps right?
Tried to send you a PM but cant get around this user interface merry go round (pun somewhat intended relative to the Moncrief merry go round :) ) |
I have some 1mfd@600V teflon caps if you are interested... |
@DaveyF: It's AOK... to each his own.
I am writing this echoing my opinion which concur with that of many others, esp on the more DIY oriented forums.
Perhaps too many people who posted similar opinions have metal dome or ribbon tweeters that need some balance.
BTW I didnt come up with the idea of replacing the Rel+Wonder combo with Inifinicaps. It was a suggestion by none other than Leonard @ Audio Research. |
JohnsonWu, I just don't agree with your assessment of the sound that the older ARC amps exhibit. My D70Mk2 has wowed all of the a'phile group that has heard it in my system. So much so, that one of the group traded a BAT VK75SE for a pair of M100's ( with the same wonder caps) and never looked back after he heard my ARC/CAT set up.
I'm sure you are aware that ceramic tweeters will exhibit glare and hardness with aplomb. The same can be said, to a slightly lesser extent, for the beryllium tweeters in your Utopia's. A hard dome can be bright and exhibit glare IF any of the upstream components are at all bright...speaker cables, preamp, front end, amp etc.
I believe and i think many others are beginning to believe also, that a silk dome or a ribbon is far less likely to exhibit this behavior than a hard ceramic or metal dome. Which, if I'm not mistaken is one of the reasons that Dave Wilson chose a silk dome for his top of the line speaker, the Alexandria XLF. I guess when it comes to this discussion, YMMV. |
@Smoffat: The reason why I sold my CL120 (and CL150 of course) was the solid state input and gain stages.
I believe in the Counterpoint (Michael Elliot) philosophy of getting the small signal stage as tubey and transparent as possible and use whatever as the output stage as it has the least influence to the sound.
The 10uF caps had to be output caps (from pre to power) or from phono to pre. Upgrading those to any exotic brand like Duelund or Mundorf or VCap would be cost/size prohibitive.
The coupling caps within each device had to be 2uF or under I imagine. But until I get to see the schematic I cant comment.
Anyway, upgrading the small signal coupling caps would have a more noticeable benefit than the output caps.
|
Johnsonwu, i am following this thread with interest. I have has several ARC amps and preamps modded by GNSC )Steve Huntley) over the last 10 years.
I still own a pair of Classic 120's which i hope to put head to head with a REF150 soon. I also have a REF3 LE and REF Phono 2 both modded by Steve.
The only thing that Steve did not replace in the critical signal path are the interstage coupling caps (4 in each unit). He felt that the stock caps were good enough and need not be replaced. What ARC use in their SE upgrades for the REF5 and REF2's are teflon 150 vdc 10.0uf =/- 10%.
What similar caps would you recommend as replacement (Vcaps?). I doubt that ARC would release those proprietary caps for installation in the field. Any recommendations much appreciated. Thanks again. |
@Daveyf: I meant even after replacing the Rel+Wonders with the class B caps like Auricaps there is still glare remaining. Not so this time, I am using oil caps, not polyprop.
My 2 main pairs of speakers are Avalon Eidolons (too big for my 14 x 18ish room) and a highly modified pair of Utopia Divas. The Eidolons with their Accuton ceramic tweeter tend to exacerbate the glare and hardness more so than the Utopia.
I use the 4 Ohm taps.
I never have a glare problem with my Phi70s (well after all its an expensive amp with all 300Bs and Infinicap-S, they MUST not have glare) or my Counterpoint NPs (Vcap TFTF).
With the stock caps, and if you are not driving the amp hard, you wont hear a lot of details in the recordings.
Thats my main complaint of the Rel+Wonder combo... low volume and its muddy, high volume and it pounces at you.
Anything oil or Teflon will convey the microdetails better and be a lot more linear when going from soft to loud. I never got a chance to discuss with Leonard about teflon caps but figure why he recommended Infinicap and later on Audio Research went from Rel+Wonder to Dynamicap and now Teflon (in some of their amps), not sure about the TRT Stealth which should be a modern version of Dynamicaps. |
JohnsonWu, you say that every time you have used vintage ARC amps, even after replacing the caps, that they still exhibit smear and glare but at higher headroom.
In my system and with the original caps in the ARC, I am not exhibiting this symptom. As I asked before, could it be something else in your system that is the cause of what you are hearing? How sensitive are your speakers and what tap are you using on the amps to connect them to? I have a small dedicated room, so I am not driving the amp that hard ( which could be why I am not hearing your symptom ;))), what size is your room? |
Johnsonwu,
Thanks for your input. I think the current ARC cap is the TRT Stealth cap. No outside reviews that I've been able to find. I do know that Aesthetix also uses these in their Eclipse series. From what I've read, I'm not sure these are teflon - they sound simular to the Clarity MR's if you read the write up. If you look closely at ARCDB for the Ref 110, there's a side photo that shows the gold & red Stealth cap with a small silver cap which looks to bypass the Stealth. I would venture a guess that the small silver cap is teflon.
Another interesting cap is the duelund Alexander's. In the smaller size range, they are not nearly as expensive as the teflon caps. |
My experience with stock vintage Audio Research gear is that all of them distort/smears quite early with complex passages esp. chorale and loud piano chords.
I also have a problem with constantly trying to adjust the volume when listening to music. To low and I miss resolution, too loud and the glare is mighty annoying.
Not sure if the Relcap/Wondercaps are not aging well which contribute to that or not.
Everytime after I re-cap these amps with any new caps I often find they get more headroom before they start smearing and exhibit glare.
I have tried Infinicap, Dynamicap, Obligato, and Auricap on my previously owned Audio Research amps with varying levels of success. Each has their strengths and weaknesses. All of them, while better than the Rel/Wonder combo, still exhibit glare IMO.
My old Counterpoints (except for the NPs) all use Wondercap. All have benefitted from changing to oil or Teflon caps.
However, I have not tried the state of the art caps like Mundorf SGO or Vcap or even Jensens on my previous Audio Research Power amps ($$$, number of caps involved) like i have on other gear but based on my experience with the k40y9s on other gear and on this D115Mkii I am confident that this amp can even be further improved with the class AA+ caps. (Audio Research uses Teflon nowadays, wonder who makes them?)
Also regarding the size of the caps, look at what other manufacturers use these days for similar circuits. Very few of them use 1uF or larger anymore. Preamp output caps are a different story.
This is only the first step in modding this unit. I am still trying to find a way to install fast recovery diodes and am considering upgrading the high voltage bypass as well. Not now but later. |
Thanks for the tip the V-cap's website. Do you think that the mid high glare you refer to is from the Rel/Wondercaps?? |
@Hifigeek1: Those huge lytics are Sprague and Mallory and hard to find in-stock @ Digikey or Allied. The cap plugs are impossible to buy in qty < 1000. |
@Sbrown: Check VHAudio V-cap's web page on sizing coupling caps. The -3dB rolloff point is lower than audible. Plus there is hardly over 2 inches of traces from plate to grid for each coupling stage.
Also even in the 90s when I used to call Leonard at Audio Research to get suggestions on how to upgrade the Classic 120s he's recommend 0.47 or 0.68uF Infinicaps (Dynamicaps werent available yet) to replace the paralleled 1uF Rel 0.01 Wondercaps.
Larger caps were used in the 70s and 80s to add a thicker sonic signature to mask the mid high glare. |
KT-120's are not recommended by the factory for use in a D-115mkII. I would however use caps furnished by ARC as well as the red cap plugs that go on top of the cap . |
Johnsonwu,
Are you saying you'd replace the 1.0uF + 0.15uF caps with the 0.22 to 0.33 values? Seems like a big change.
Scott |
Sovtek and Svetlana tubes are definitely good value for the money. I have owned no less than 8 Audio Research amps and I have only 1 tube blow out on me since I never run the bias rich. 50-55mA instead of pushing 65.
There are other ways to get richer sound and snappy bass than just biasing up the power tubes (eg. choice of coupling caps and making sure the diff amp tubes are fresh, and adjusted for symmetry with the trimpots)
I still remember fondly the conversations with Leonard and how he got me started with Inifinicap for my old Audio Research amps.
If cost were no object for this project, (hopefully sometime later), I'd be using V-cap teflon 0.22 - 0.33uF for the coupling caps (yes they are *that* good). Anything larger simply wont fit.
My next choice would be Jenseon oils but after having a few oil leak incidents I'd rather stick to the cheaper (but metal casing so use extreme caution) k40y9. Auricaps and Dynamicaps will fit easily, a few notches above the Relcaps, but not enough of an improvement to justify all the work esp on this amp IMO. Too many caps to replace.
All the above have leads which will not fit into the solder holes, you will need to do a 90 degree bend of about 2-3mm of the leads and solder them parallel to the pad. Also be EXTRA gentle and careful not to damage the traces when you are extracting the old caps.
The large can capacitors are always Sprague or Mallory or Cornell Dublier. Easy to swap out if you can find them in stock at Allied or Digikey or Mouser. Buying from Audio Research would be fail-safe of course. |
I was asking about the kt120 more from a increased tube life perspective as I knew that I wouldn't get a power increase without mod's to the amp. I don't have any issues with power with 115 - it seems to be able to drive about anything I hook up to it. ARC did recomend the Sovtek 6550WE's which seem fairly easy to find.
I have no issues with a non factory mod. I am considering recapping it but haven't made up my mind on which cap(s) to go with. Any suggetsions?? |
@sbrown if you are just after the sound of the kt120 and not the marginal increase in power you can use just 4 tubes in your 115. That's how I sometimes run my old vt130 anyway. Make sure you are using one tube per phase per channel. Filament supply is parallel so you will never be stressing the unit with half the power tubes. |
Sbrown, I just replaced the 6550 in the power supply of my CD-8. ARC sold me a 6550 WE. No idea what it is, but if ARC sells it, I'm not worried. |
Sbrown, Thanks for sharing that info on the KT120's. I was wondering about this and was going to call Arc about it. You just saved me the call, LOL.
Too bad, as the KT120 is a very good tube. With decent 6550's getting a little hard to find, I'm glad I just re-tubed with SED's. |
I wouldn't put 8 kt120s into it. Consider how much more current the filaments will draw. 8 kt88s in tetrode mode should be able to power the hungriest of hogs. The apparent lack of power is from the stock caps. If you are not comfortable with DIY or non factory mods, ask Calvin at audio research if they offer recapping service for the coupling caps like they did i(infinicap and dynamicap) in the 90s. The relcap plus wonder cap combo is not conducive to good sound at all. |
Here's an FYI. I own a D115MkII and called ARC about putting KT120's in the amp. They said don't do it as the heater in the KT120 draws alot more current than does the 6550s. He said that this will kill the power transfer over time. I would think that the D70 would also be in this camp. |
all measured OK. Thanks HifiGeek1 for the pointers.
Took me 2 hours to carefully pull all the wondercaps and Relcap coupling caps out, and another 2 hours almost to install k40y9s into the amp (size and metal casing does NOT help at all).
The foggy when low volume and glare at high volume problem is mostly gone.
I get a lot more natural and airy sound with strings.
More to do but at least this IMO is going absolutely the right direction. |
That's the one. Also make sure you measure 10 ohms at ground to common tap at output transformer. |
