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Old 22nd September 2005, 03:13 PM   #1
mr mojo is offline mr mojo  United States
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Default New amp project-some ideas of my own and looking for input from others...

Well folks, the PP amp is done and sounds fantastic. I really thought-even though others warned it would not be the case-that I wouldn't build another amp for quite a while.

Guess those folks were right! I've got the itch and wanted to run a few ideas past you and see what you think.

At this point I know I can build a SET amp no problem. For now I'm focusing on the Angela Instruments 300B. Anything significantly better to look at?

The main thing for this project for me is the power supply. The plan is to veer off the Angela circuit a bit and design my own.

Here's the start:

Another power xfrmr from Heyboer. I'm hoping they can pot it and install terminals on the bottom, if not I'll look elsewhere. It's gonna be a stereo amp on a single chassis so I'll have mains in, HV CT secondary, 6.3ct and 5v filaments-so far pretty standard.

Here's where it gets a little unique:

I'm thinking about using dual 83 mercury vapor rectifiers. 450-550 working plate voltage, 15v voltage drop, 225ma working current draw and 3 amp heaters. Max peak current is 1 amp and max peak plate is @1000v.

Useing a pair in the power supply and I've got 450ma working current, allowing for beefier filter caps in the supply(this time around the newer dry 'lytics good for 100uf and 750v.), with less voltage drop and better regulation.

Plus a pair of 83s just seems to be a pretty cool solution for rectification.

What are your thoughts so far?

Best,
mr mojo
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Old 24th September 2005, 02:44 PM   #2
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Mercury (Hg) vapor rectifiers may sound as good as some people say. I don't know. However, sooner or later you WILL have a highly toxic problem to cope with. Xenon filled diodes and TV damper tubes are quite reasonable, SAFER, alternatives. If you do go the Hg vapor route, be sure you have a supply of either powdered Zinc or powdered Sulfur on hand to sequester any Hg that gets out of a broken bottle.

With a custom power trafo, think about using a hybrid bridge rectifier and holding the end to end voltage of the rectifier winding down. It's another safety thing. 600 VAC is more than 2X as dangerous as 300 VAC is.

I too have a hankering to do a 300B SET, but TIGHT money says NO. FWIW, what I have in mind are the 5 KOhm primary Plitron O/P trafos. Dennis Boyle's combination bias, and higher voltage/lower current operating point than is frequently used would be employed. I think that would mate well with 5K primary O/P "iron". 6BM8 based mu followers are being used to drive 300Bs. I'm thinking about a 5965 section joined to a Gary Pimm style CCS load. GP's CCS has a built in mu follower O/P that would work well into the smallish grid leak resistor needed when a bias supply is used.
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Old 24th September 2005, 03:58 PM   #3
jsn is offline jsn  United States
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Default look for old potted transformers

You might want to have a look on eBay and any electronic surplus stores you can find for power transformers. Usually you cen get something very similar to what you are after for far less money than a custom trans. You might even be able to pick up a couple potted power chokes too.

The Angela model 91 is supposed to sound great, though Ihave never actually heard it. I appreciate your resistance of currently popular designs - instead going for push-pull and pentode drivers. Bravo.

Judging solely from the amazing build quality of your push-pull amp, maybe you are ready to try a design of your own? I'm sure many folks here would be willing to help.

As for Hg rectifiers, I say go for it - as long as you don't have any tubes in your amp that are sensitive to RF oscillation. I just built a 6c45 amp with gas voltage regs in it that is a bit of a hassle.

jsn

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Old 25th September 2005, 01:44 PM   #4
mr mojo is offline mr mojo  United States
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Eli, I appreciate your input on the rectifier idea. I hadn't really thought about what might happen if a tube broke. I'm still pretty green at tube stuff, but I haven't had one break yet. I've blown a few rectifiers and power tubes but no breaks-is it possible in any reasonable situation or just the exception that proves the rule?

Quote:
Originally posted by Eli Duttman
With a custom power trafo, think about using a hybrid bridge rectifier and holding the end to end voltage of the rectifier winding down. It's another safety thing. 600 VAC is more than 2X as dangerous as 300 VAC is.
Could you expand on this a bit?


jsn,

I take it Boozehound labs is your site? If so, I'm a regular. First time I came across the site I not only learned alot but thought:

"Why can't I find a group of guys like this to hang out with? It's like Jackass for smart kids."

And in case you're wondering I did in fact openly weep. Whablam indeed.

If t-shirts are available I'm ordering two; one for everyday use and another to keep in reserve-you know for Bar Mitzvahs, weddings, mug shots; that kind of thing.

Thanks for the kind words about my amp as well-sure means a lot to me. I do kind of like to go against the flow-guess that's where the thought of using 83s for rectifiers came from.

When you meantion VR tubes, how did you use yours and what caused the hassles?

Thanks guys,
mr mojo
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Old 25th September 2005, 02:07 PM   #5
amperex is offline amperex  United States
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I do not prefer a MV rectifier when vacuum rectifiers are available. MV can create RF noise issues as mentioned in the older Radio Amateur Handbooks (Ham Radio).

The rectifier I use in a few amps that provide consistant & excellent sonics in a few different DIY preamps & amps are Sylvania 5R4GYB with smooth black plates. The micas are a rectangle type shape (not round) that partly contacts the glass plus plates are spaced approximately 1/8" from each other in the center. Another Sylvania 5R4 design exists with the plates spaced about 1/2" apart in center & produce slightly less detailed sonics.

Another great rectifier group are the Mullard EZ80, EZ81 & GZ34. These rectifiers produce much less voltage drop than the 5R4 tube.

I have no idea why a rectifier tube makes a difference in sonics, but Tung-Sol, GE & RCA 5R4 fell short of the Sylvania every time- go figure.
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Old 25th September 2005, 02:44 PM   #6
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It's a matter of when, not if; sooner or later, for whatever reason, a tube's envelope will get broken. If that tube contains Hg, you've got a toxic waste control problem. Damper diodes sound GOOD, exhibit low forward voltage drop, and are not toxic if they break.

I'm not sure about all of the factors. However, I am certain that danger grows exponentially as the voltage rises. Things get truly "hairy" when the B+ is over 1 KV., as is frequently the case when using 211s and 845s.
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Old 25th September 2005, 03:29 PM   #7
SY is offline SY  United States
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I second amperex's comments- MVs are toxic and usually noisy. I used 866s in my old ham rig and had one hell of a time getting them quiet. The advantages are not overwhelming, and the performance is inferior to a high speed silicon diode. This is DC, after all.
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Old 25th September 2005, 03:49 PM   #8
kmtang is offline kmtang  Canada
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Default Stereo 300B Power Supply

I built my 300B SE amp based on JE Lab's design for more than two years. I noticed that the power supply having large impact on the sound quality.

Here's my idea for a stereo 300B power supply.

400Vac CT transformer => 5U4G rectifier => 47uF oil capacitor => 5H 500mA choke => 100uF Blackgate => separate 10H 300mA and 100uF Blackgate for L and R channels 300B.

The Bypass capacitor for the 6SN7 would be 47uF Mundorf TubeCap.

This would provide excellent hum ripple free power supply.

Don't forget to lift the filament potential for the 6SN7 to approx 80Vdc in order to avoid it picking up hum from it's filament.


Johnny
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Old 25th September 2005, 05:19 PM   #9
jsn is offline jsn  United States
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Mr Mojo,

Wow dude, thanks for the insanely nice words. I'm glad you dig the site. Check it now for T Shirts. I have 2 designs and all sizes right now.

Whablam indeed!

jsn

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