@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. |
I have some 1mfd@600V teflon caps if you are interested... |
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 :) ) |
These caps are huge and made by the company that makes them for ARC. |
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? |
@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. |
@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: 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. |
Bypass with the same manufacturers caps as the coupling caps.. |
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. |
My guess is it's not quiet enough |
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. |
You realize you will only get about 35-40Wrms across 8 ohms in triode configuration... |
D115... Becomes a tubefied CL60... |
Yes. I was thinking it was a D-70. D-115 will get you 60Wrms.. |
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. |
@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. |
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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! |
@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, 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? |
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 |
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. |
The D-70 is truly an old friend.... |
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. |
6H30's draw more heater current... |
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. |
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? |
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. |
