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I have a pair of dynaco mkIII amps and they are working at this time however they will be needing some improvements soon. One of the input jacks is fussy and shorts out at times by the slightest movement. So since some service is in the near future Im wondering which upgrades should also be considered. Im looking at the driver and cap board upgrades from Triode and/or Curcio. They both seem to offer similiar benifits but Im not sure just which ones to go with. Also considering many of the other basic service parts like new tube sockets, bias control, re-wire, new choke and all that stuff. Im a newbie to this level of diy but these should be fairly easy improvements to do I believe. Thanks
Follow Ups:
I have a pair of Mark IIIs with Curcio quad cap replacement and a Curcio driver board with 2 6cg7 tubes. I really like the sound. Before my updates, I could really tell the high end roll off but with the updates really opened things up. I don't think stock boards are worth buffing out. I haven't heard Uncle Ned's boards but I suspect they are an improvement over the original design.Since you say you are a newbie, get a board and parts from Curcio, Triode Elec or Wellborne (Kennedy's boards still avaliable?), stuff them and get them installed. Google can also help find info.
I have another pair of Mark IIIs that needs updates so I'm looking at other options. I've always followed Douglas's posts for inspiration but haven't decided on a design. Plus I have two more in the wings.
Working on my 5.1 Mark III system ;-),
rick
but you wouldn't believe me. It's too simple. It's also all-pentode, even the phase splitting.But if you want a pair of boards rigged for plug in operation, I may be able to help you...
regards,
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
tell me more please
well, the easy one is pentodes, sort of, cascode is what I have yet to take apart...if you can figure out how a cascode plate curves are created, yuo should have no trouble rolling your own. Or pick op points from Allen 'the cascode Man' Wright...
regards,
Douglas
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
nt
thats what I was just told reciently by wellbourne
Look on Ebay for the drop in replacement curcuit boards. Populate the boards with new parts and away you go ! I would also look into the SDS power supply board. This replaces the problematic quad cap and bias supply with newer, higher rated parts.Probably the best value rebuild option around.
Best,
For about $60 a pair, you won't believe your ears!
Hello ,
There must be far better ways of modifying these amps , what does this mod sound like ? 47k loads on the concertina , driving 0.1uF caps into 100k grid stoppers , with output stage strapped as triodes ? What comes to mind is a subjective rolloff at both the frequency extremes . I have played around with concertinas quite a bit and valves such as 12AT7/12AU7 and 12AX7 just don't cut it , low Ra valves such as 5687 / ECC99 / 6H30 are far superior sonically and with the measured results . Fair enough using a 12AT7 front end (not really a fave of mine) but it's not really a suitable valve for concertina usage , I would imagine a 10kHz square wave would look almost like a sine wave . $60 could be better pressed into service with a 6GK5 or better still 6072 input stage and 12B4A concertina or a 6EM7 rather than a 12AT7 . Better still Allen Wright's PP1C cascode driver stage , superb sounding , stable , with tons of gain .cheers
The least you can do before you write unfounded "facts" against someone's statement is check whether that someone has done his/her homework!In engineering, we do things FOR A REASON. At least I do. Even if it's for a hobby!
I did look into the other dual-triode tubes, in the context of the simplest possible design, using a single 9-pin socket. many were rejected on the basis of low gain in a practical circuit.
Do not "bundle" the 12AT7 with a 12AU7 or a 12AX7. The AU7's mu is too low for this application and the AX7's Rp would indeed cause an undesired rolloff.
I developed several designs, based on a 5687 and 12B4A tubes, but all these require a second socket. I would have proceeded to test them (and I might do it some time) but for the life of me I can't justify it. The sonic qualities of the zero feedback triode front-end are just too good. I can hear the next question: what's his reference... The nay-sayers would probably just go on and on, but here is a sane proposition... Instead of analyzing what's for you is a "paper design", why won't you build it and answer your own question "what does this mod sound like ? "
It would take less time than arguing whether it should or shouldn't sound good. Worst case, in the end you'll have two boards that you can use to try your other proposals rather than "throw them" off your head.
Then you can tell me the rolloff frequency... But fasten your seat belt first... The result might floor you!
But it shouldn't. You're the one that said that you must measure rather than rely on specs, otherwise it's a heresy, right?!
Don't waste further time on unfounded statements - just do it! (TM - Nike...)
Don't trust my reference amps? Put this one against yours in a competent system with fair component matching (for this class of an amp). Only then can you voice a **specific** opinion regarding the sonic qualities of **this** amp.
You didn't hear it. I did!!!
Hello ,
What are the measured results then ? I'll make a guess , 15Hz-20kHz before even taking the output transformer into consideration . The coupling caps are too small for a start and the concertina loads are too high .'Only then can you voice a **specific** opinion regarding the sonic qualities of **this** amp.'
'But fasten your seat belt first... The result might floor you!'
...sorry but I find cascaded diff pairs and differential cascodes far superior . If I was to build one of these things the result may make me want to pull the thing to bits after 5 minutes . I'm not interested in building this amp or anything like it anymore . From previous experience I know the concertina stage is the stumbling block and less than adequate for the job it is intended for in this case . I challenge you to provide proof I'm wrong here ! Anecdotal evidence is all you have so far , so get your scope out and try to prove me wrong ! Oh , and stop all this 'naysayer' crap , you're starting to sound like cheap-jack. Thanks !
cheers
> I'll make a guess , 15Hz-20kHzI'm not interested in your guess and neither are other innocent readers that might mistaken you for an "authority" on the subject.
Try 12Hz to 80KHz at 10 watts into 8 ohm, which I measured on the particular amp only a few weeks ago with a real scope and signal generator. Not only fresh in my memory, but also written on real paper.> The coupling caps are too small for a start
Too small? Your own "guess" was 15 Hz on the low side, what's too small in that?!
By the way, the math fully agrees with you and with the measured bass rolloff. 47K plate load plus 100K grid/bias load in series with 0.1uF yields a rolloff at 11 Hz. I measured 12Hz!> Anecdotal evidence is all you have so far , so get your scope out and try to prove me wrong !
Sorry to disappoint you.
The scope was out and all my quotes are from REAL MEASUREMNENTS.
Perhaps you should get your scope out and be more careful to not mislead people with blank statements about something you don't know for sure and apparently someone else does.
There is no anecdote or guess in my statements - all HARD EVIDENCE. So, I'm asking again - where are YOUR REAL MEASUREMENTS?If you neglect to measure, at least offer a theoretical analysis of some sort. If you can't do that, then offer us circuit simulation results. Takes only a few minutes to simulate my circuit on HSPICE (which I did!).
I have nothing against your cascode diff amps but I have a lot against your blank statements with nothing behind them regading my circuit. If you had measured it or listened to it and then debate facts, I might have paid more attention. So far all I hear is "your guess"!
Do you have any clue how many engineering projects fail because someone guesses instead of doing the job right?!
Sorry, not interested!
.
...FB
Hello ,
Temper , temper . Remember deep breathes and count to then then breathe out slowly . 12Hz is still piss-poor LF response and will impact into the audio band but if you're happy with it , up to you and your ego . It has been a pleasure disscussing such a great design with such huge claimed bandwidth , it must be the work of a true genius , surely a manufacturer will incorporate it into their designs . Never realised just how fantastic a 12AT7 running at 2mA per section could sound . All my experience into concertina stages is completely wrong and I had better get my ears syringed .Happy trolling !
ps I would still like to see the 'HARD EVIDENCE' which contradicts with my own findings . You ask me for evidence , I'll have to find my old notebooks which have the results in . You have the amp already so perhaps to provide convincing (instead of anecdotal) proof , any chance of a picture of a scope trace of 500kHz sinewaves or 10kHz squarewaves ? Thanks !
In the coarse of a constructive discussion I will accept all sorts of critique and suggestions, but that's not what you are conducting here.
You just continue to post misleading general opinions as "facts".
You may have general experience with this type of circuit but at some point it's time to stop waiving your hands and be more specific. While I built this circuit and listened to the modified amp and posted specific data, you continue with unfounded "facts" - anecdotes...
Since all that "blurry" talk makes me look like someone who has no clue what he's doing and the (proven) mod as incompetent, what could I do but set things straight?I just wanted to clarify that issue to other readers.
It is legitimate for you to say "it's my experience" or "it's my opinion", but going on and on with blank claims and dismissing all my data, just because it suits your pre-conceived general opinion?
Then you tell me "Anecdotal evidence is all you have so far", refering to my actual measurements? Why?!
If you submit repeated hard accusations you better have irrefutable evidence, Counsellor!You can review my simulations (below). Please don't post that it's all wrong without serious data!
You want scope pics? You'll dispute anything I say anyway, so it's best that you take them yourself (with my specific circuit please).
After you review that data against my claims, I believe an apology would seem quite appropriate.
Hello ,
It's amazing just how misleading these simulations can be . Quite frankly I don't believe the graphics provided (anyone could do this in MSPaint) . Please supply some shots of some scope traces . What about the distortion ? Surely you can provide a shot of a (non-simulated) spectrum analyser sweep , can't you ? I'll believe you then ;) I'm very interested in 12AT7's , after all I have 700 of them . If your circuit performs as specified , I'd love to press a few into service . Not really a fave of mine at the I'm afraid . It was the same with E88CC's until I tried Allen Wrights circuits . Convince me or stop all this bull
cheers
How predictable...
Since we don't have any baseline rules, what's exactly the point you're trying to make in asking for more data?
Hello ,
There is no need to get defensive and no I'm not prepared to build one . The bench is already taken up with a PP GM70 amp , no more room ! Scope traces perhaps will settle this ? I thought you had access to distortion measuring equipment also ? You have a digital camera and you say you have a scope . Off you go , I'm easily pleased . Looking forward to seeing the pictures :)cheers
Given your ever argumentative suspicion, how will I ever be able to prove to you that the pics are not taken directly from my signal generator?
As we both know, anybody that has a scope and a digi cam can do that, especially if he's profficient in MSPaint... :-)
(You got a chuckle out of me on that one...)
It's a futile exercise. Just take my word for it...
Never mind , I'll leave you to your simulations . Just make sure you use a scope and sig gen to take your measurements next time .
I built that one, a 12AX7, a 12AZ7, a 12AY7, a 12AU7, a 12AV7, a 5965 and found all them to turn square waves into triangles with a .25 uF coupler to a 100k grid load( with no grid ) by 10k cps. this is with 25, 30 and 40k splitter section loads. It was a bit better with 25k, but not much. I left that design on the shed floor and moved on to 5687 and 12A4 diff amps...i did not listen to them, but found the stock 6AN8 to sound OK and it turned square waves into triangles by 7k...Hell, some folks like 45's in SE, no accounting for taste.
live long and prosper,
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
I measured too...
Try my bias points with my circuit, not with anemic tubes... I found practically unlimited bandwidth from the driver side. The real limitation comes from the output stage and my hunch would be the output transformer. 12 Hz to 80 KHz 3 dB bandwidth from input to 8 ohm output is what I measured. Incidentally, the 12 Hz directly corresponds to the time constant of the phase splitter and that 0.1 cap. The theory does work :-)Two comments:
While all those dual-triodes work for low level signals, only a few would swing the full dynamic range required for full output, hence my choice of a 12AT7.
A diff amp design will develop symmetrical (phase-inverted) drive to the final stage under feedback only if the open-loop gain is extremely high (basic FB theory leads to this standard "opamp" concept).
This required that I use at least two gain stages (for around 5% asymmetry, which I consider acceptable) which raised the amount of NFB beyond a "good taste" (that's MY taste!) and you can hear it!
First things first, I measured only the front end stages, loaded lighter than they would have been had the power valves grids been involved. No loop nfb, and only the gain and P-S stage was being put through its paces. NFB cleans the whole amp up nicely and to the limits of what the OPT is capable of. So flippin' what...I found no measurable un-balance with CCS cathode load diff amps. i don't give them any NFB input and I only would publish test results which I had duplicated with good tubes, known good and NOS, and incidently some weak sounding used did still measure quite well. Good matching makes it very difficult to determine which is which at a first glance. Two identical traces...180-degrees OoP
Split load inverter is only going to have a maximum output of 25% of it's B+, and likely less when it is part of a direct coupled circuit. Between a fifth and sixth is allIcould tweak out of mine, and at published Dynaco OP points, it is more like a sixth. which for a UL rigged outut valve is just enough.
The NFB-less triode V-A and direct coupled split-load did not ever measure well, usually not even close to acceptable. More like POOR, sometimes *REALLY* poorly.
regards,
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
> First things first, I measured only the front end stages...
> I found no measurable un-balance with CCS cathode load diff amps. i don't give them any NFB inputI just realized what you're saying.
You are correct on one point. Without feedback, a differential stage with CCS on the cathode is perfectly balanced.
Your mistake is to assume that the balance is maintained when you close the feedback loop.
Measure with feedback... Imperfection redefined. Been there, done that!A single diff-stage just won't cut it! Try adding loop gain, but then you have to accept lots of NFB. Not that ideal!
You're not alone here... Some commercial "upgrades" assume the EXACT SAME FAULTY ASSUMPTION OF PERFECT BALANCE...
from you> > > Your mistake is to assume that the balance is maintained when you close the feedback loop.what sort of feedback loop are you talking about? I guarantee, I have not been using it. My NFB loop when needed is balanced and does not involve teh grid circuit of the diff amp.
regards,
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
> My NFB loop when needed is balancedI'm not a fan of balanced circuits in general and balanced feedback is even more complex. Haven't tried it.
Theoretical analysis of a differential feedback amplifier shows sensitivity to variations in OPEN-LOOP GAIN in the multiple paths.
Only when that gain goes to infinity can we claim absolute balance and only in ideal zero-delay conditions. This inevitably means high level of NFB, which I try to avoid (based on my sonic evaluations).
I'd love to hear about your balanced solution if it's simpler and achieves a well-balanced drive to the final stage.
well, since both infinite gain and zero delay are not attributes of the system in question, the amount of NFB need not follow your prediction either. Of course it can...but, it does not need to in what I have come up with.
regards,
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
> I found no measurable un-balance with CCS cathode load diff ampsThat's very interesting, because I SPICEd this so meticulously and even the theory says no go. It was my first try, just that it didn't do the job and I do not want to risk saturating the PP OPT.
I'd consider interstage tranny to correct that imbalance, but that's a whole new story.> Split load inverter is only going to have a maximum output of 25%
That's in the ballpark. Triode-strapped 6550 only needs 50V peak-to-peak for full power, easily achievable with 480V supply. I myself had doubts about the direct connection, until I realized that the unity-gain splitter circuit is based on high local feedback, which stabilizes also the DC operating point. As long as you bring the tube to a decent operating point by design, it will adjust (based on tolerances) to still operate close to that point. The major sensitivity is to variations in plate voltage of the first stage.
In fact, the no-signal bias of the splitter is around -2V. The grid and cathode "move together" in the presence of signal, thus maintaining the bias. An extremely safe design!> The NFB-less triode V-A and direct coupled split-load did not ever measure well
My estimate is less than 2% THD at 10 watts (the amp is capable of much more power), mostly coming from the output stage. As I said, many dual-triode tubes would run this amp to a few watts output without much of a problem, but few can avoid clipping at large dynamic swings (close to full power). You must be careful what you measure. This is a DIY project, so you better "qualify" a tube for the application. I found mid-60's Sylvania 12AT7 very good and my friend tried United Electronics 801's successfully. Some tubes sound horrible, apparently not meeting their large signal specs.
I should have used the distortion analyzer while I still had the amp here. Sine wave was not visibly squished and crossover was smooth. It didn't sound distorted, but the same can be said for an SE 2A3, which measures miserably...
In any case, it sounded very fast and produced realistic tone and good dynamics. Bass was tight and lively and cymbals sounded totaly clear and undistorted to my ears. I've heard recently so many outrageouly expensive tube amps that won't hold a candle to this modified MkIII. Naturally you won't use it to power Wilson Sophias in an auditorium, but it would be extremely musical and tuneful on Sonus Faber Amatis in a large living room, for example.
And you won't feel like the Amatis deserve a better amp!
we're talking about 2 different things, you, the whole amp ( I think ) and I am only refering to the front end, loaded a little bit lighter than it would be in service. plus a 1M/40 pF probe circuit, alos balanced. They all turned square waves to something like a saw tooth by 10k, not to mention being imbalanced.Add a loop NFB and it can get fairly clean at the amp's output.
regards.
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
I wouldn't get too stuck on square wave response, despite the popularity in the "free press". My amp has zero feedback, so the front-end response is just that - and it shows a tremendous rise fall time.
The front-end by itself extends to well over 500 KHz, while the whole amp from input to 8 ohm output (I measured at 10 watts) extends to 80 KHz - without any global feedback.
The front-end produces a near-perfect 10 KHz square wave. The amp as a whole shows some smoothing of rise and fall, as one might expect. But do you really care when it extends beyond the last RIAA mandated pole (that very few implement anyway...), not to mention the roloff of your super tweeters at 40 KHz (typical) and your average ear at 13 KHz (as I found in a group test!)...
quoted> > > The front-end produces a near-perfect 10 KHz square wave.into what load? try it through equal coupling caps and a grid resistor. Add to this a scope probe and it will probably look like a power valve grid, or at least enough to make some judgements.
Don't worry, not hung up on the square wave response, just a measurement ref. and test method. WIth loads other than the 'scope probe, response sucked on my test rig. B+ in the 420V range, plate V of the amplifier stage about a fourth...
Clearly we are doing something different, else our results should match more closely.
regards,
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
> try it through equal coupling caps and a grid resistor.Load was 100K bias resistor and a standard probe.
The difference is a 12AT7 at 4 mA vs. a 12AX7...My friend built a second amp and measured 175 VDC on the voltage amp plate and 130 VDC plate-to-cathode on the splitter, just intended by design.
Start with a 500 volt supply, a 12BY7 for the voltage amp, and direct couple it to a 7233 running with 6k loads...I'll bet it will run to past a hundred kHz before it thinks about dropping a volt out of 40.
regards,
Douglas
!
Beware the man who behaves deceitfully towards others, for there is nothing else to save you from his deceiving you
I need to look closer at this but I would like to use a 6sc7 instead of the 12at7. Old guitar amps use this octal and I would hope I could use this twin triode for audio.Do you have some ideas on updating the schematic to use the 6sc7 octal? I have a pair of Mark IIIs waiting updates.
I'd be interested in hearing what you and others think.
Hello ,
Sorry , the 6SC7 is unsuitable for concertinas driving triodes , Ra too high . For a single valve , you need something like a dissimilar triode such as 6EM7 or 6GL7 that has mu 68 , Ra 45k and mu 7 Ra 1k respectively . I used a 6EM7 with NimH battery bias and a CCS top load for the input stage with 10k loads for the 6EM7 beefy section . I can't remember the gain and clean P-P swing I got from this setup but it was more than adequate . The valves I really like for the concertina stage are the 12B4A and 6BL7 (paralleled) and should be low Ra , medium mu for optimum performance . Triode strapped EL84 also work well . For the input stage 6072 may be a suitable candidate for a 'clean' sound but may not have enough gain if you don't use a pre-amp . The decisions regarding valve choce are mainly dictated by the amount of HT available . You need to have approx HT/3 on the anode of the input valve . The input valve needs to be biased in such a way as to meet this requirement and have enough headroom to drive the grids of the output valves to full power . Hope this helps :)cheers
Thanks. This does help. I don't have a 6em7 but I do have a TungSol 6sn7gt. I'll check the specs on this (gain of 20?, Ra 7.7k). I think this is more like a 8 pin 6cg7 and each triode is pretty close to each other. I may not work either.Thanks again,
rick
The popular 6SN7 and 6SL7 are not acceptable. The only octals I found acceptable for this circuit are dissimilar tubes, like the 6EM7 that was already suggested. The popular 6DN7 and 6EW7 are a "no go"...
You may expand the variety of tubes that will work, if your source (or preamp) puts out a signal much larger than 1V RMS. The considerations then are further complicated by the high bias required at the input stage to avoid clipping.
Zero feedback with a single dual-triode for the MkIII is an act of fine balance...
I'd recommend that you stick to the 12AT7 circuit. It works well - without a question!
Thanks for the note. Preamp will either be a modd'd Lafayette KT600 (12ax7) or a Curcio PAS3 (12at7) line stage (or a Hafler 101 or a Mac c26 or ...).
Either preamp will drive this amp with 100K input impedance.
I don't know what their gain spec is and whether they can deliver more than 1-2 V RMS, so I will not recommend that you look at the low gain alternative tubes at this point. It gets a bit more involved and practically is a new design, even though you keep the same basic circuit. You'll have to make sure you have enough bias to not clip at the higher input and then adjust the input stage so it biases to the point where the direct-coupling to the splitter does not "choke" the splitter tube.
Unless you go back and forth on this "exercise" to reach an acceptable final circuit, I'd say you better just duplicate the 12AT7 circuit.
thanks for posting it.Regards
Use metal-film resistors of the "garden variety" but try decent caps. Sprague "Vitamin-Q" are great but even "orange drops" produce very good sound. Just don't use a Solen, please...
Post your impressions when you get to it!
Hi,I still have my MKIII's I acquired in the late 70's. They did a credible job driving a pair of Acoustat electrostatic panels.
I completely morphed mine into something different , and you can get carried away on theses things, regulation of the B+ and bias..solid state rectification with computer grade caps, bias level and balance controls , teflon sockets etc..so mine is not a MKIII anymore.
The problem with the MKIII is the underated can cap..for the power supply. They tend to fail. Replacements are available from AES ( reliability issues on these?) and maybe Vibroworld can make something up...otherwise you're into a curcio or Triode electronics board for the capacitor bank. The choke is not usually a problem...but tube sockets can cause you all sorts of grief in biasing..I'd change them all. Virtues of the new board topologies can be debated but there is someone who sells new boards in the original circuit configuration...maybe someone can chime in with his website or you can try a search for dynaco boards. Bias components should be changed and input sockets as you mentioned. So I'd keep it "stock" and move forward slowly on all the upgrades.
That all said I never thought the MKIII's ever had the magic of a ST70. Mine have been sitting on the shelf for maybe 20yrs! However, to buy an equivalent type mono tube amp today with probably inferior iron would probably cost you $3-4K a pair....so they are still worth the effort in restoration.
I don't believe there is a definitive answer.I've got a pair of MKlll's and have often wondered which would sound better/be best to do.
Once, I got up an order for Uncle Ned, a couple of years ago - and was severly struck with a case of the shorts _grin_
Then about a year later, late one night I got out a credit card, was going to order some stuff from Curcio - but am so ADHD, I wasn't able to figure out _exactly_ what to order, due to site design and layout.
You've got the support of the Dyanco Doctor forum if you go with Curcio.
OTOH, Uncle Ned seems to be very good to deal with, good prices and will not try to sell you something you don't really need- my chokes looked pretty rough, but IIRC, Ned told me that in all likehood they were fine and rarely died.
Plus I like Neds individual tube bias, which I don't remember the lower price Curcio stuff having.
I honestly don't know the best approach - and if someone else has posted it, I didn't see it or don't recall.
HTH
Regards
Ken L
Ken,
The Curcio boards have the ability to bias each output tube.I think both Uncle Ned's and Joe's boards will be an improvement over stock boards.
rick
First thing first, replace the caps by the new stock ones from AES at $30 each whilst stock lasts.Secondly, reckon on the price of those upgrade boards, I won't pay 100% of original equipment price for upgrading. I have seen many Dyanco III with upgrades for sales on Ebay and guess what?
chris,I don't know the MKIIIs- and I've only scratched the surface of the arcane world of the PAS3 and ST70, but I think Triode Electronics and our friend Uncle Ned could make some suggestions and provide the pieces for your rebuild.
There are so many options for Dynaco, I've decided to take a very cautious modification approach. I have never heard an ST70 and plan to rebuild mine as stock first. once I determine there are deficiencies that I can solve, I'll make changes. I can see right away I will be replacing the qaud capacitor, bias and the RCA jacks need renewing anyway, so these can be a nicer kind. I have a replacement new stock main board that may or may not go in right away with modern lower tolerance compoents. There's also that ratty thin power cord!
But, as I say, my first round will be stock. Really, the sound of the old Dynaco is supposed to be good when everything is in order.
I do have several NOS 7199s, but also a few EF86s in case the main boeard goes to another type!
You may wish to look into the special Dynaco site at this site:
http://www.audioasylum.com/forums/dynaco/bbs.html
- and there are many helpful chaps with piles of experience with these amplifiers and every sort of modification.
Cheers,
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