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In Reply to: Dukane 1A475C still in house posted by Interstage Tranny on March 12, 2007 at 17:11:42:
Intended or not, I found the tone of your response to my question somewhat condescending and preachy. Thus my response. I try not to be argumentative but I was also hoping for something more substantial than what I got.While it's generally accepted that specs and measurements don't always correlate well with the listening experience, I've found that most amps that sound good also measure "good". I therefore place some value on the technical aspects of an amp. You seem to not value them...so be it.
On paper the Dukane amps have much going against them for high quality music ranging from the small OPT to the use of global corrective feedback over four stages to domestically unfriendly aesthetics and physical layout to limited ability to match common speaker loads.
Obviously you have actual experience with these amps and I do not. I would have appreciated concrete tips and suggestions on how to get the best out of them such as what OPT sec tap to use for speakers of X impedance. Or what if anything to do about the snubber networks on the primary of the OPT. Or what brand of 6CD6GA works well. Or what parts should be replaced or upgraded. Or... Instead you dismiss my technical concerns and proceed to praise the amps with vague and difficult to relate to descriptions of the sound. IOW, "trust me" on a subjective basis when I don't really "know" the reviewer.
As for how I buy new equipment; yes I bring a familiar CD and not test equipment in a retail setting. I listen attentively to the equipment under consideration; I do not audition it. Used or vintage equipment (where my primary equipment interest now lies) is an entirely different situation. For this stuff, test equipment is an absolute necessity. Nothing, even from reliable sources goes into the chain w/o verification of proper operation and comparison to mfgrs specs.
And then you end you second post with this "Maybe you should listen to your system more. Go to some acoustical music concerts and train your ear to how music sounds live." I guess it just provides further proof of the wisdom of that old proverb about what happens when one assumes...you regarding my "experience" and me about the status of your 475s.
BTW, as a result of this discussion, I'm going to fire up and verify operation of one of the 475s. If all is OK I'm going to do some objective measurements and then AB it with a SS amp of decent quality on a pair of difficult-to-drive moving coil monitors. I don't have great expectations but who knows?
Follow Ups:
Hi Steve,
Let us see if we can come to some common good thoughts. There was, and is, no condescending statements or thoughts on my end. You simply inferred this. I did not try to speak down to you. Now, let's see if I can help you and earn some trust.
The loudspeaker taps on these amps are only the 25V and Bal. connections. If I recall, both of those taps have less than 1 ohm DCR to the Common tap, making it a balanced, Center-Tapped winding, with the mode switch set to Position 1 or 3. (Setting the mode switch to Position 2 ungrounds the 25V/Bal taps leaving them floating, and usually requiring only 16 Ohm speakers). Using either the 25V or Bal. connection and the Common connection, most 8 Ohm speakers are easily accommodated. Even using the 25V and Bal connections for the speaker leads, can accommodate many 8 Ohmers.
This amp does have some trouble with some 4 Ohm speakers, with some glare in the highs, sometimes. Experimentation will decide for you.
Tubes are easy to prefer, but not so easy to find. 7867s have black plates. So, any 6CD6GAs with black plates, will mimic the designers intentions best. Since these amps are PPParallel, tube matching is not that critical. The amp is very stable with 8 or 16 Ohm speakers. These beasts run cool all the time, even in continuous useage.
They have two direct coupled stages, which helps making them sound very fast with transients. The global feedback is actually less than many typical feedback type amps.
From your reply, it appears that you do not like feedback. I don't blame you, either. Feedback can be a good thing and it can be a bad thing. Like you, I first use an amp as the design engineers intended, stock. The only upgrading I have done with these amps is changing the coupling caps and adding power supply capacitance to beef up the bass output. They have a unique character. With some electrostats, they are the king, I kid you not. With inefficient, sealed woofers, they can cook. I used them in the '80s, with Advents that had AR9 woofers. 1812 Overtures had neighbors calling the police on me. I briefly used them with Altec 755As, but they were too glarey. 755As can often sound too hot up top. My older, speaker guru buddy had a pair of WE728Bs on bass and 755As for treble, and ran them with the Dukes for quite a while. That was an intense sound.
But, the 755A alone, full range, is not their best match. Neither is the Norelco 8 inchers. Let us know what you think...
Concerning the concert going, that was a simple suggestion, no snide intentions meant. Everyone should learn to use acoustical instruments and voices as their reference, don't you think ?
At the time I used these most, my reference amps were Brook 22A, 2A3 PPtriode monoblock integrated amps, very upgraded. Then, my world changed when I began using Push-Pull Triode amps, with no feedback loops, and interstage transformers to split the phase. I still prefer preamps with feedback RIAA equalization, though.
I thought I'd offer this update... It's now 4:30am and I can't drag myself away from the Home Theater system. I've been listening to digital MusicChoice or MTV Hits, which is a much higher quality signal then the usual music video channels.I finally started my migration to building the tri-amped setup, starting with finally firing up the Dukan and NTS amps. I'll sum up my feelings by saying I feel compelled to visit the Klipsch appreciation thread on AudioKarma where I avoided the "must be tubes" debate, to add my $0.02 to support "must be tubes"; at least that's so for the La Scala given tonights experience.
I had been using the La Scalas 1st with the built in Onkyo 939's amps. Wore that 5-ch chip-amp out. Bought a Butler 5 channel, which is a 'typical' hybrid, with a tube front-end and bi-polar back-end. Nice step up; but lacked 'brilliance', punch, and some clarity/depth. Was kinda analytical and dry; and harsh-ish if the signal wasn't perfect. Did a smashing job with reproducing my new favorite of the Cream DVD @ Royal Albert Hall as it is a great recording and clean. But still, something's not 'perfect'.
So first, I setup the Dukanes 1A475 amps (I was distressed to realize one is a 'C' and one was a 'B'! The 'C' has more gain). Youza. Can you say punch and bass?! I was -really- surprised. Highs were noticably subdued. The Cream DVD was even 'realer' then usual and less fatiguing. This amp I understand is from the early 60s; and just a 'PA' amp? Well, I could agree with the latter given the 'liveliness' of the reproduction; but still very much 'HiFi' too!. But most notable compared to the amp I was coming from was how 'dampened' yet rich the sound was. It was incredibly less congested then the modrun highly rated Butler amp I had been using with the La Scalas. However no doubt, much 'crunchier' sounding too. But in a good, nea, great way.
I then finally setup the Bogen made National Theater Supply 1125 125watt 807 output tube amps... (this, being the amp made in the late 40's with 6 x-formers I discussed in a different but similar thread some liek Interstage Tranny have participated in... the eBay seller said 5; I found the 6th hiding under the chassis
) Since I was bringing the amps up slowly, I did one at a time, using one Dukane and one of these. This amp must have a low impedance input, as I had to boost the signal +13db from the Dukane. The tonal balance was very different though too, and I couldn't really listen with one and one amp mixed. This amp was definitely lighter in the bass. When I switched the other channel to the 807 amp too, the Dukane's deficincy on the top end really became apparent. More amazing still though was how much better the La Scales responded throughout the whole frequency spectrum to this tube amp (that's not to say it wasn't the same with the Dukane; I'm saying both mate so much better then the Butler). And how much better the treble was! (I'm using a JBL 2402 driver with a mini-smith horn tweeter instead of the stock tweeter) Clear and crisp. And how much more cohesive the upper mid to treble was. Shocked I say! And though bass wasn't as strong as the Dukane, this was a much tighter cleaner bass. Interestingly, I had to flip the 2 M&K Subs phase 180 degrees then how I ran them with both the Butler, and with the Dukane. [!!!] It's also tough to say, but there might be a bit more congestion then the 1A475 amps though; perhaps though only because the Dukanes are so 'dampened' sounding in the presence and above. But the tone is just so pleasing I can't deny it. The upper harmonics of Jack Bruce's bass guitar came through better with this amp then any of the others I've used.
I could definitely live with 4 more of these 807 output tube amps, for use with both bass and midrange on all front 3 speakers. Not being likely, and where I was 1st planning on using the 807 amp for bass up to 500 and Dukane between 500 and 7K (or there abouts), I think I'm going to do it the other way around now! Dukane for bass, and Bogen/NTS for midrange.
I'll add, the NTS amps have had some caps replaced. The Dukanes don't show any evidence of being updated. So that's definitely called for. The PS filter caps are necessary on both IMO. But I am content in the knowledge that I have two great examples to build ontop of.
Coming also is a couple of Bogen MO-100 amps; one a mess; one in working order. Super simple circuit; easy to support and upgrade. Should work great for bass on the center; mid may be that rare Arc M-60C ... maybe. Arc techs tell me it's not very reliable and to stick it on a shelf and not use it... :( So I might use another bogen ... what I really want is another 807 output tube amp like the NTS! I know a guy who might be up for building me one... or four. Figuring out the circuit without deconstruction will be tough I think though.
...This type of info is more encouraging. Thanks.A point of clarification: I don't have anything against corrective feedback as such if properly applied and not used to compensate for marginal open loop performance. I do become concerned when it is applied globally across more than say three stages. In the 475, technically, five stages are involved although the fourth stage is a split load phase inverter with gain <1. The amp also employes the traditional phase compensation network at the output of the second stage so combined with the output snubbers and direct coupling for two of the stages, maybe stability is OK. Hope to find out soon enough.
Regarding age of the 475, the date in title block of the schematic I have is blurry but looks like 2-25-61 or 64. I tend to think the 61 date is correct because I recall seeing a rack of four of these amps in the auditorium of the Jr. High I attended which was built in 61 IIRC.
Steve O, regarding "BTW, as a result of this discussion, I'm going to fire up and verify operation of one of the 475s. If all is OK I'm going to do some objective measurements and then AB it with a SS amp of decent quality on a pair of difficult-to-drive moving coil monitors. I don't have great expectations but who knows?"...Did you ever get to doing this? I'd love to hear your thoughts. I personally feel this amp will do better as a bass amp actually then a midrange amp in my personal case, in my migration towards tri-amping my horns.
Hello there, I'm glad that info was helpful. Since this amp can sometimes sound hot up top, with certain systems, I would not remove the snubber or compensation networks. Try the amps stock, first, at least. With a light Variac warm-up, they usually work, even if stored for long time periods. They were overbuilt.
Some of the versions I have seen use some questionable grade coupling caps. Those can certainly be upgraded. I would beef up the first power supply cap, which is after the input filter choke. That will give the bass some more speed and punch ability.
Speaking of smaller output transformers, sometimes the smaller ones have better high frequency extension, than their larger brethren. This might be because the smaller coils tend to yield less interwinding capacitances and leakage inductances, thus less high filtering. Whatever the reasons, sometimes the smaller ones are good ones. In this amp, the PPParallel tubes have a plate to plate load that is low, which is another justification for a smallish output, not needing high primary inductance. Let us know what you hear and Good Luck bringing these heavyweight brutes back to life...
Regarding mention of "with a light Variac warm-up, they usually work"... I'm hoping to learn under what conditions should that be done? Only if the amp hasn't been used for a long time, or are there other situations warranting this? (I asked this in my Bogen thread so please ignore here if it was answered there... thanx much everyone for all this info)
Interstage Tranny, it may have been a circuitous route getting here, but this is definitely more of what I'd hoped for. Fantastic insight into these amps. I thank you. (I actually was thinking when deciding to buy these I may eventually "fiddle" with the feedback, floating the speaker ground (-) by a 1 ohm resistor and taking feedback off between that and the (-) of the speaker, to create a constant-current amp.)I look forward to using them to drive midrange horns without a passive x-over network. Based on what I've read here and in the post linked to, I believe with the OPT taps as they are and how that will reflect the lower impedance of the speaker back to the power tube stage, and I'm surmising the the feedback loop, this amp may exhibit 'impedance follower', a.k.a. 'constant current' characteristics naturally (without my "fiddling" necessary).
I also base this theory off a comment made in the thread in the Tube DIY forum linked to by Steve O of "I would think the OPT is too small for good bass, however, it beat the pants off of both of my ARC d90 and d70mk11 amps, no contest there." I think what was happening there was the bass response was following the impedance curve, with the voltage varying with frequency and more power provided at high impedance of the speaker's resonance and much much less the few octaves above it. I'll actually be measuring that with my Terrasonde ATB-3 by feeding a fixed voltage frequency sweep into the amp and measuring the input sweep's output voltage being delivered to the speaker as influenced by the complex impedance. I suspect it will -NOT- be a fixed voltage to the speaker but rather to vary as the impedance varies, following the impedance. Anyway, that's just conjecture at the moment. I'll know for sure after i get the amps and do some measurements.
As mentioned with my desire to experiment with constant-current amplification to single drivers (specifically, ones that exhibit a frequency response as somewhat an inverse of the impedance curve) this is actually a desirable behavior to me (I won't be using passive crossovers). It's a highly undesirable behavior from an amp when passive crossovers are used however, as the power delivered is not controlled by the driver alone but by the driver and crossover together, making a 'false' emphasis usually in the midrange where crossovers combined with the speaker often produce in impedance hump. Additionally with the amp's active impedance being relatively "high" on the 25V or 70V taps when a signal is fed being, and with that in series with the x-over circuit, it's no longer negligible compared to the speaker's impedance and it ends up essentially 'moving' the crossover point. (this, from what I've read regarding difference between typical super low-impedance amps and high-impedance constant-current amps ; I haven't measured this myself.)
On a different note, I had first thought these were amps made for theaters but learned here they were ['budget'] sound reinforcement PA amps. Do you know approximately in what years these were built?
Also, so then no one has every had any exposure to the Bogen / National Theater Supply amp I asked about in a thread posting before this one? (titled [mis-spelled] "Hoping for a History Leason #1: Bogen / NTS-1125") While this thread's been incredibly enlightening and all I'd hoped for educationally, discussion on that other amp I just purchased in that thread hasn't gotten any love... ;)
-steve
(p.s. I just read this back; woops! thanx for reading this marathon message of mine)
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