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Old 10th June 2013, 12:16 PM   #5221
male42 is offline male42  Russian Federation
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Originally Posted by M Gregg View Post
Very nice!
How do you cut the covers for the transformers and fix mountings?

M. Gregg
transformer housings are made of stainless steel cups for tea production in India

то mhouston

the circuit is very simple. 6S15P Russian driver transformer 1:1,2
300B transformer. anode voltage is stabilized. fixed bias.
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Old 10th June 2013, 06:17 PM   #5222
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Hi all,

Am a new comer and here is my new amp for daily use: 4D32-PP monoblock
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Old 12th June 2013, 01:37 AM   #5223
kevinkr is offline kevinkr  United States
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Originally Posted by yagoolar View Post
Kevin, those are super-simple! Admire!
Thanks so much!

A lot of good stuff here on this thread, and like many others I find SS amps fatiguing and otherwise not very compelling. Guess I drank heavily of the tube/valve Kool-Aid..
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 13th June 2013, 04:11 PM   #5224
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Default A gift from the past

Decca stereo radiogram srg500 manufactured about 1956.
Original mullard valves seem ok. I decided to make a primarily headphone line source amp for rock and roll valve sound.
I cleared the chassis of all radio components and connections.
I had some cutoffs from stainless steel oven hood extractor ducting.
It was very thin and cut with tin snips then glued with superglue. Inside I put polythene insulation.
This made nice safety covers for all the transformers and choke.
The choke was made from an old E frame mains transformer by splitting the laminations and removing the "I" pieces, then reclamping
as open "E"'s.

The audio design is ECC83 triode stage to EL84 single ended output. I hope the transformers are good quality, they are certainly heavy.
As sold the chassis was not earthed.
I got the circuit from vintage radio on the internet.
I converted the amp power input to a fused euro power socket, with earth connection to the chassis.
Input panel RCA and loudspeaker and power socket connections were to the rear of the unit.
The mains transformer had a bolt stub very handy for clipping the power and speaker cables to the chassis (see pic).
I discovered one toasty EC83 anode load which was replaced (the other channel value was 50% higher than spec probably due to age)
A preliminary switchon showed horrendous hum. (at switchoff, hum fades before amplification does).
I replaced both with 100k.
I noted the original circuit used single wire parallel ac heater supply, with chassis return. This partly accounts for the hum that was acceptable
at that time.
I decided to retain that topology, but use a well smoothed dc. I also added a separate toroidal mains transformer for the heaters.
This not only took the magnetic stress off the HT transformer, but also allows the heaters to warm up before HT is applied.
I suspect the rectifier valve and/or smoothing electrolytics (possibly the EL84 cathode bypass electrolytics too)
I replaced the caps C35,C41,C49 all 32uF 360v (in can). and the bypass C48 25uF 25v.
the first three were replaced by separate radial electrolytics 220uF, 385v.

The original design used single HT supply for both channels (pretty well universal then). It has supply voltages shared at several points, including
a shared grid bias resistor. Since this was to be a headphone amp, I decided to retain this, as a certain amount of channel blending is headphone friendly.
I replaced the EZ81 rectifier valve with a silicon bridge rectifier (600v. piv, 10amp)
Output volts too high to adjust, so I used two diode 1N4007 in full wave in the equivalent to the valve rectifier and CRCLCR C smoothing.
Placed the choke in the smoothing circuit (stage2)
The final R was a mains 40W tungsten filament bulb (about1.4k). This has a nice retro feel and was mounted on the chassis top on vero board.
Attached Images
File Type: jpg P6060004 lo.jpg (873.8 KB, 1226 views)
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File Type: jpg P2230074 lo.jpg (874.0 KB, 567 views)
File Type: jpg P2230071 lo.jpg (801.8 KB, 501 views)
File Type: gif valve-amp-circuit-final.gif (43.5 KB, 369 views)
File Type: gif HT-supply-final.gif (9.8 KB, 310 views)
File Type: gif toroid-heater-supply-final.gif (17.8 KB, 155 views)
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Old 14th June 2013, 07:42 AM   #5225
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Replace EZ81 is not good idea, diodes 4007 is very bad choice. Also use ECC81 or 82 is more better 83.
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Old 14th June 2013, 10:32 AM   #5226
dady is offline dady  Europe
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Originally Posted by male42 View Post
mono 300b lamp SE. the body is made of rosewood
Hi friend, where you got the tranie's cans? Nice aspect at all. How it sound?
Esteban Bikić
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Old 14th June 2013, 04:11 PM   #5227
male42 is offline male42  Russian Federation
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Originally Posted by dady View Post
Hi friend, where you got the tranie's cans? Nice aspect at all. How it sound?
the amplifier in the audio path no resistor or capacitor. sound will only wins. the sound is very accurate, fast, does not distorted. covers bought at the store utensils
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Old 15th June 2013, 06:45 AM   #5228
Riki610 is offline Riki610  Slovakia
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Old 15th June 2013, 06:48 AM   #5229
tinitus is offline tinitus  Europe
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Riki, please post a photo
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Old 15th June 2013, 01:09 PM   #5230
kruesi is offline kruesi  United States
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What a great thread!

I'm listening to this KT-88 beastie I built a few years ago. Since the picture was taken, I've changed the output tubes to Gold Lion. Not so fond of JJ octals but love their miniatures, esp the 12AX7LPS.

The hybrid splitter works really well. U/L, 20dB feedback. Noise is less than -80 dBV, THD less than 0.1%.

I listen about 4 hrs/day.
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File Type: png KT88_ps copy.png (169.4 KB, 839 views)
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