Simple 5842-45 SET Amp Design Idea - diyAudio
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Old 10th July 2007, 07:17 AM   #1
eeyore is offline eeyore  Australia
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Default Simple 5842-45 SET Amp Design Idea

Here is my attempt to design a simple to build SET amp that uses 5842 as the front end, and a 45 to produce a 1 to 2W amp. Hopefully this does not require the use of a preamp. Please provide your thoughts and corrections to any of my assumptions/calculations.

Input Stage
- 5842 with 20mA CCS loading.
- Grid bias set at around -1.5V.
- Grid swing between -0.5V and -2.5V (equals swing of 1V)
- Plate voltage swing with the above settings 95V to 180V. (so a plate supply voltage of say 200V)
- Approximate plate swing of 41V.
- Approximate gain of 40 (rounding down).
- No idea to cathode resistor or capacitor value yet.

Coupling
- 0.22uF with 470K RC coupled.
- f-3dB at 1.5 Hz

Output Stage
- 45 plate load of 5k
- Plate voltage of 250V (from data sheet)
- Grid bias -50V (from data sheet)
- Max plate current 34 mA (from data sheet)
- No idea to cathode resistor or capacitor value yet.

From the above design, I would say that it is a conservative design, that wouldn't drive the 45 into clipping, but should be able to amplify most sources that can output 1V p-p. Schematic is coming (provided this is all good)

This will be used with 90 dB + speakers (most likely 93-96dB).

Thoughts? Suggestions? Comments?
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Old 10th July 2007, 08:21 AM   #2
dejanm is offline dejanm  Austria
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Considerable improvement to your design would be the usage of interstage transformer instead of capacitor. It costs more though ...

5842 from Raytheon (NOS) provides exceptional imaging and is considered as a very good choice. Costs about $20 on DIYSupply ... Be aware though that the life of this tube is a bit shorter than of modern tubes ... they say: only 1000 hours. If you are not happy with this, I heard several good sounding 2A3/45 amps using 6SN7/6SL6 in the input stage - that might be also an alternative to 5842.

Good output transformer is crucial.

What about supply ? Tube or SS ? Which one you prefer ?

Regards
Dejan
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Old 10th July 2007, 08:31 AM   #3
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Hi,
I did something very similar. the only diference is I used 6C45pi instead of 5842, the plate is choke loaded insted of CCS, to ~160Vp, battery bias (1.2~1.4V) on the cathode.

0.47uf PP coupling cap.

310V B+, at i.e. 260V plate, 52V on the cathode, forgo the cathode capacitor of the 45, AC filament.

45 is a wonderful tube on the mid and highs, hope you enjoy it as much as I did.

I drop any cap with any chance I get...

My PS is simple 0.1uf/20H/47uf, dual mono. and regulate the driver's B+.

It's a bit on the heavy side with al the choke... but simple construction.

As for your cathode resistors,
for 5842, you wanted current at 20mA, bias at 1.5V so cathode resistor of this operating point will be 1.5V/20mA=75ohm.
and for 45,
@34ma, -50Vbias, therefore, you need Rc=50/34ma=1470ohm


Hope this help


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Old 10th July 2007, 08:41 AM   #4
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The 5842 works well with LED bias. With a 20mA CCS and a red led (1.8~2V), you'll end up w/ about 160V on the plate which I have found to be a good spot. Also, try the 6688 triode strapped -- electrically very similar, sounds at least as good, and costs less.
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Old 10th July 2007, 08:42 AM   #5
arnoldc is offline arnoldc  Philippines
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I've used 5842/417A in two of my amps-

1. choke load with 45 - http://azcruz.brinkster.net/diy/AK-450.htm
2. diyAudio CCS load with 10Y/50/300B - http://azcruz.brinkster.net/diy/AK-300.htm

pick your poison. I like them both, and use LED bias (was bypassed cathode resistor) now.

Not having a preamp depends on the source and efficiency of your speakers.

ps.

My 45 amplifier drove an Avant Garde Duo directly from the output of a Clearaudio Balanced Reference phonostage.
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Old 10th July 2007, 08:56 AM   #6
jnb is offline jnb  Australia
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It sounds great.

Do you think you'd have enough gain? I'd hardly begrudge 6dB for quiet sources, 6dB headroom for the driver/input stage, and 6 more dB for good measure.

YMMV, naturally, and this amp will probably sound best without another stage. I think I'd use fixed bias in each stage (an LED for the 417) as a way of keeping the gain up. The CCS on the first stage is a good idea. Try 15mA and 150V.
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Old 10th July 2007, 09:09 AM   #7
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Hi,
From my limited experience, I found that any power amp (2 stage) will have immediate improvement in staging, image defination improve with a pre-amp stage. but some how the "feel" or emotion bit is sort of watered down... I wonder if anyone has the same experience.


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Old 10th July 2007, 09:28 AM   #8
eeyore is offline eeyore  Australia
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Thanks for the replies. I guess I should describe why I designed this amp. I wanted to put together a simple SE amp that used reasonable common and available parts. Wanted to use an output valve that was readily available NOS and new manufacturer, that wasn't too expensive (always relative). What I wanted also was to bring in a few newer ideas that have been around the mainstream for some years. I wanted to update and modernise the very well documented and built SE amps like the JE Labs SE, etc, etc... I also wanted to keep the costs relatively low and simple construction so that new comers (like myself) can build this amp easily and also learn about tube audio and be able to test and improve this over the years. The amp has to be integrated, so that no preamp was required. There are plenty of schematics and designs that have the 6SN7/6SL7/etc., and I didn't want to duplicate what was already done.

Power Supply
How about a bridge rectification with diodes and a 5U4G that is capacitor loaded, followed by two stages of LC filtering. Using some cheaper Hammond chokes, I think two 7H with 47uF or whatever you have capacitors could work nicely. I have a Elna Cerafine 220+220uF, which will do fine. I was thinking of having a 10uF Solen after the 5U4G, and then one 45uF Solen for each channel before the B+. Something simple and with good performance (at least modelled in PSU2D).

What about using a bleeder resistor to assist in discharging the the caps after power down. What is the best place to put it, and what value/ratings?

Output Stage
Is there a cheap way of implementing a 'parafeed' or parallel feed topology? I was thinking of using James transformers. From what I have read, they are good value.

Coupling
I love the idea of a transformer coupling of direct coupled design. Any suggestions to designs which are cheap (transformer) or safe (direct coupled). Remember this is designed for a beginner.

Input Stage
Will investigate the LED idea, sounds good, and might make the chassis more interesting. So go to find some LED datasheets!

Will get around to a schematic soon...

Thanks!
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Old 10th July 2007, 09:45 AM   #9
dejanm is offline dejanm  Austria
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If your objectives for this project are: simplicity and low-cost, then probably it would be better to use capacitor as interstage. I believe that transformer on this place has to be of a high quality and I am not sure whether you can find them cheap. I really do not know ... I was using Lundahl here and it is definitely not cheap.

Where you can achieve cost reduction is a power supply. Instead of using tube you may use SS diodes and only one CLC filtering stage. Be aware that Elna is also not very cheap (and Elna Cerafine electrolytics are not produced any more) - so probably you will have to search here for a cheaper but good alternative. Maybe something from JJ or Nichicon ... I am using Jensen but they are also quite expencive.

The art of building an amp with these objectives is to find the right measure between used parts, design decisions and compromises that are made and to achive still very good sounding amp. I guess it is possible to do it, but you have to investigate a lot of time in trying and hearing different implemented options. It could be though very interesting journey ..

Regards
Dejan
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Old 10th July 2007, 04:14 PM   #10
kevinkr is offline kevinkr  United States
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I designed and built a somewhat similar amplifier recently for headphone use.

The driver stage uses a Raytheon 5842 with a red led and 1N4148 in series for bias of about 2.6V - no cathode bypass caps needed. Loading is via a magnequest 100H (dcr 910 ohms) choke and the plate voltage is 160 -165V depending on the particular tube's transconductance.

The output stage is a fixed bias 71A dht with dc heating. Here you can simply substitute a 45 with either fixed or cathode bias, and you can use ac heating on the 45 without serious hum issues..

I generally prefer fixed bias because it eliminates an expensive cathode bypass cap, but at the expense of a little more supply complexity. Cathode bias of course is safe whereas in the event of a mistake fixed bias missapplied or non-existant will result in a roached output tube.

I used a Russian made surplus teflon coupling cap.

Getting a good choke is less expensive than a good interstage and will give you more linear voltage swing range than a ccs for a given supply voltage. In simulations the choke version was more linear, I did not build the ccs version so I can't compare, but I will say that linearity was very close to the prediction, even at 80Vpp swing the distortion was down better than 60dB, and at saner levels it was close enough to the tester residuals to be unmeasurable.

The 5842 is a very quiet tube as well so the amplifier if carefully layed out with good power supplies should be very quiet. I find elevating the filament supply on the 5842 to about +30V prevents any hum being coupled to the cathode circuit.

My first SE design was a 45 driven by a 6SL7 in srpp, it rewarded me with great sound and finally sold me on SE over PP - to that point I was skeptical to say the least. It is also the only amplifier I've kept from that time frame. The article I wrote about that design is still on my site. It also featured fully tube regulated supplies, somewhat novel for the time.
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