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Old 11th Nov 2022, 2:04 am   #21
joebog1
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Default Re: Dynaco SCA-35 kit amp.

The iron in the OPT's isnt "ordinary " iron. Its early grain oriented and I "think " it is also Silicon. Dont quote me on that. Somewhere in the back of my head its called Super silcor.

Mains transformers can run at about 75 or 80 C without too much problem. Yours is new so I cannot see any dramas.

I have mentioned before about the thickness of laminations. Generally the thinner the better, especially in the sizes we are discussing here. In Australia .35 / 14 thou is the norm, but there are some cheaper offereings. If you make a new mains transformer, I would be using a somewhat bigger core and I would use .35 mm. But thats just me.

Good luck on a good kit.

Joe
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Old 11th Nov 2022, 2:12 am   #22
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Default Re: Dynaco SCA-35 kit amp.

I wouldn't rush into converting it to fixed grid voltage bias. It is definitely the way to go if your prime goal is to extract the maximum amount of output power from a given valve.

But good old cathode bias leads to better stability of quiescent currents which has several benefits.

If you want more power, start with a different amplifier design with a bigger output transformer and bigger output valves. EL84 are being flogged to death to get the powers mentioned. Go for EL34s KT66 or even 807s and you can have something cruising unruffled at those powers. Things are more stable, valves last longer and all sorts of other good things happen.

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Old 16th Jan 2023, 1:01 pm   #23
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Default Dynaco SCA35 kit build finished

Rather than revive the original post made in November last year on the Dynaco SCA 35 kit amp I decided to make a new post on my experience building the kit, the mains Tx rewind from 110V to 240V turned out ok and has been the subject of a separate post.

It's been many many years since I've built a kit amp - the last was back in the early 70's, and I found following a set of instructions rather than a circuit is quite foreign to me and probably took a lot longer than it should have, all up I estimate a complete week-end to assemble the amp, a bit longer than the Dynaco ad I recall seeing ' all you need is a few hours'.

I was surprised to see not one length of coax - the assembly manual shows all signal wires as twisted pairs, my kit is supplied with ribbon cable, the manual states that some hum will be present and to adjust the hum pots for minimum hum, the assembly manual looked to be a B&W scan of a photocopy, some of the illustrations showing wire positioning were not that clear and would have been much easier to follow if they were in colour.
Most of the solder terminals had oxidised over time and had to be cleaned before soldering, one of the hum balance pots had suffered some transport damage and I had to replace it with a near equivalent I had on hand (2k in place of 1K).

The fuse holder refused to be tightened and I ultimately replaced it - the retaining 'nut' is thin brass with a serrated edge to give a pliers something to grip, the issue is that the nut thread is a tight fit on the fuse holder body and to grip and turn with pliers distorts the nut enough so it grips the fuse holder tighter and refuses to turn.
The pot shafts had frozen solid, and refused to turn, a tiny squirt of CRC freed up all except the treble which does turn but is a bit stiff, the selector switch had also frozen, it looks quite delicate so I stripped it down, gave the wafers a quick dunk in liquid silver cleaner, rinse off in water, a good soak of contact cleaner and finally a blast of compressed air to blow it dry, the shaft came free after a squirt of CRC then working it back and forward number of times.

Not trusting the low level input wiring to twisted pairs I used shielded cable, for everything else I followed the layout in the Dynaco instructions, as the assembly manual states I do have some hum, it is very low level and with the volume at minimum one needs to put an ear right up next to the speaker cone to hear it. I carried out a quick frequency response check and it looked flat enough from 20Hz to 20000Hz quite pleasing, I checked the output power at approx 1Khz - into 8 ohms I measured 12V at clipping, one channel driven, a nice clean sinewave with no signs of crossover distortion.

The input selector switch has tape playback, phono (3 options on the rear panel, low level, high level or ceramic), tuner, spare(Aux?) and tape record. The spare and tuner inputs are not line level as I discovered during the power test but need something in the order 3 volts for full output, the other inputs (tape and phono) have the appropriate response curve for the type of input used, the phono hi and lo level inputs are 4mV and 2.4mV for full output.

My initial tests were done with a mobile phone as a music source and while it did work the results weren't great, the phone didn't have enough output to drive either of the spare or tuner inputs to more than a few watts and the tape and phono inputs didn't sound great due to their respective eq curves and were easily distorted.
Today I lugged my Yamaha turntable out to the workshop, only to find that my son had raided my vinyl collection and didn't leave me with a great deal left to try, The Angels (good Oz rock band) and Bad Company were good enough for a quick test. I don't have high end speakers only a pair of Realistic Minimus 7's, but the listening test with vinyl was very pleasing, a very nice sound, still a little bit of work to do but so far I'm happy.
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Old 17th Jan 2023, 8:54 am   #24
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Default Re: Dynaco SCA35 kit build finished

Very good job!

Jac
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Old 17th Jan 2023, 10:32 am   #25
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Default Re: Dynaco SCA35 kit build finished

Hello,

That’s a cracking build.

I must admit it’s nice to build a kit, as I suppose, it reminds me of making Tamiya RC Cars and suchlike.

I can quite easily believe some of the terminals had oxidized... I used some period NOS parts in a couple of recent amplifier builds plus the build I’m currently doing, and I also found the terminals on these parts needed cleaning, not by an awful lot but, enough to necessitate a clean and re-tin.

I wonder, apart from cost, the using twisted pairs or ribbon cable in the kit instead of co-axial cable was because of the required skill level to successfully use co-axial cable? I suppose they have to assume the wirer and assembler has good soldering skills, but may not have the required experience with coaxial cable? I suppose the coaxial cable sets could be pre-prepared by the kit supplier, but that would add to the cost. I suppose I could have to look at one of my Heathkit amplifier manuals to see what Heathkit did for coaxial cables... That's if they used it.

As I said I’ve recently built a couple of amplifiers from a kit of parts. I currently have a build on the bench at the moment. These aren’t a full kit of part as I am using quite a few of my salvaged and NOS parts and transformers. I followed a classic design from the 1960’s, which is relatively well documented, along with the basic instruction/layout provided by the supplier of the kit of parts, however, I found I was using some artistic license when wiring it up – especially in the power supply and earth layout.

Ah, Bad Company, cracking band, and Ideal test source

Ps. Its surprising how long it takes to do the mechanical assembly!

Terry

Last edited by Valvepower; 17th Jan 2023 at 10:46 am. Reason: Triping Errot
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Old 17th Jan 2023, 10:51 am   #26
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Default Re: Dynaco SCA35 kit build finished

I have been following your travails because I am trying to breath new life into a Pioneer SM R150, which like your Dynaco is an integrated PP amp with inputs like 'tape', and with a US transformer.

I have looked at the tone stack and phono section of the SCA35 as a possible replacement for the one I have. I am looking to fit a 'Direct' option so I can connect Aux input to the last gain stage via a volume control. Do you have any ideas of mods to update the design to something more relevant to our line level inputs these days, phono notwithstanding?
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Old 19th Jan 2023, 3:14 am   #27
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Default Re: Dynaco SCA35 kit build finished

A chap called Gilespie makes and sells the Dynaco pre-amp and power amp pcbs so it may be possible to fit his preamp board to your Pioneer, a google search should find him - he posts on the audiokarma forum, but if your Pioneer amp is in half decent condition and has an auxiliary input then I'd be inclined to leave it as is and investigate adding something external to achieve your aims, I'm assuming you are wanting to provide some appropriate input for a media or CD player, leaving your amp stock would probably help to retain its $ value. Some years back I had a Pioneer integrated valve amp with dual tuners, I parted it out so I could use the transformers for a project, and now regret it.

The Dynaco phono and tape inputs have eq circuits that use a type of passive integrated circuit called a PEC module which tailors the frequency response of the phono and tape inputs, so they sound 'right'. I haven't traced out the tuner and spare input circuits but I believe they are designed to have a flat response, I should mention that the 3 odd volts input needed for full output (mentioned in my first post) is a peak to peak figure - I had forgotten that the level meter on my signal gen shows peak to peak so the tuner and spare inputs are probably close to line level inputs. The tone controls also use a PEC module - no doubt to keep the component count down and make it easier to build. If you absolutely must mod your amp, the input switching circuit looks easier to follow than the Dynaco circuit, for my own benefit I redrew the Dynaco circuit as the copy I have is not real clear, others might find it useful.
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Old 19th Jan 2023, 11:32 am   #28
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Default Re: Dynaco SCA-35 kit amp.

Hi retailer, thanks for the information. I did persist with my amp, aiming for originality, but it got to the point where I would have to do major surgery just to get to components for diagnosis, that were crammed on switches, so I am aiming for a happy medium. The unit is tatty and is not particularly sought after, so I think I will add value if I can keep the basic operation the same, but work on reducing hiss and hum and improving sound quality.
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