My RH84 Build - first amp after a 25 year break -:) - diyAudio
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Old 8th June 2011, 04:30 PM   #1
prelius is offline prelius  United States
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Default My RH84 Build - first amp after a 25 year break -:)

Hi folks,
Just wanted to introduce myself...

After almost a 25 year break, I am back into the amp building hobby again with a RH84 SE amp as my first project. Pics attached...
Since this was my first amp in awhile, I set a few requirements/limitations that guided me in this buildout, like using electronics parts on hands, as well as metal/wood for the chassis and enclosure.
I used a power tranny from a busted radio chassis (I think it was a Philco 41-190). Power supply designed using PSUD with parts I had on hands: 10H/100mA choke and a bunch of 33uf/450v caps... Tranny put out a bit more voltage that I needed (351V) so had to add voltage dropping resistor.
Coupling caps are generic poly ones, but quality turned out to be quite good, and I increased their size to .47uf.
Tubes are the only "high-end" components in this amp. All are original Telefunken 12AT7 and two EL84s. I also tried a pair of Russian 6P14P-K, and they sounded decent as well.
OPTs were the only components I spent the money on... They are Edcor XSE10-8-5K..
Enclosure is made out of birch plywood, veneered with mahogany and lacquered. Chassis was made out of scrap aluminum I found at work, and its dimensions dictated the layout, parts placement, etc. Transformer cage is made out of vent cover I found in the recycling dumpster.
Given the selection of parts the amp sounds good, there is slight buzzzing heard if you are less than a foot away from the speakers (smoothing caps are only 33uf...) Even with my crappy speakers (taken from Dell 42" LCD TV) the sound is loud enough in a 12x12 room... Overall acoustical qualities of the amps cannot be discussed at this time, obviously, as I need a decent set of speakers to hear the amp. If someone has an inexpensive set of Advent BabyII (there is sentimental attachment to them) in metro Boston area, please let me know.
It took about a week worth of nights to complete, plus one night to convert hand-drawn chassis and cabinet sketches into CAD files (I am using ViaCAD on a Mac). Given the goals of the project (see where my soldering, mechanical and other skills are), I am almost satisfied with the outcome...

Off starting to think about more ambitious project. I have a few rooms in my house, and if I count the bathrooms as well.... Hah- hah!!!

Cheers... Paul
Attached Images
File Type: jpg RH84-V1-Front.jpg (414.0 KB, 401 views)
File Type: jpg RH84-V1-Zoom.jpg (367.5 KB, 367 views)
File Type: jpg RH84-V1-Under.jpg (351.1 KB, 357 views)
File Type: jpg RH84-V1-Iso.jpg (385.5 KB, 358 views)
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Old 8th June 2011, 06:05 PM   #2
frugal-phile(tm)
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Kudos. A really nice frugal-phile(tm) build. I really like the jeweled indicator lamp.

dave
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Old 8th June 2011, 06:52 PM   #3
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Quote:
A really nice frugal-phile(tm) build.
.....big second

IMHO you've spent the money just where it's most effective.

There are lots of reasons for hum but I would start with checking out the heater circuit as it might be an easy fix. If the heater transformer has a centre tap then ground that somewhere close to the transformer. If not then a pseudo tap can be made out of a pair of resistors. ( about 150ohms )
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Old 9th June 2011, 01:12 AM   #4
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A very cool piece. Nice style, great colors..
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Old 9th June 2011, 01:58 AM   #5
prelius is offline prelius  United States
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Thank you for the kind words, gents...
I've seen pseudo tap in quite a few schematics: Fisher amps, some Maganavox and others... Just added it to my amp, and what-do-you-know, the buzz is almost gone! I can still hear a tiny spec of if I get real close to the speaker, but I don't care about that. Thank you, Soonerorlater, for the suggestion.
I decided I will not make any updates to it, other than tying down the wires and may be trying .22uf caps (per schematics).

And I think I found my next project: RH84 with pentode driver.

Cheers, Paul
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