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Old 12th September 2004, 10:06 PM   #1
BHD is offline BHD  United States
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Default 300B stereo SE amp and PP2A3 monoblocks

I've decided to stop threadjacking the PLC thread and start my own...

I'm in the early breadboarding stage of building a stereo 300B SE amp using the following iron, already purchased:

One Electron UBT-3 output transformers (3K input impedance)

One Electron PRC-1 choke wired in parallel (5.5H, 32ohms, 230mA)

NOS UTC R-5 power transformer (385-0-385 @ 180mA, 5VCT @ 4A, 2.5 VCT @ 6A, 6.3VCT @ 5A, and finally a 5V winding @ 4A) The 2.5VCT winding won't be used, and neither will the 5V filament tap. I have two 5VCT hammond filament X-formers for the 300B's.

BTW, if anyone knows of a source for another R-5, I'd love to turn this into a monoblock project...

I'm going to be using valve rectification, type to be determined. Are there any decent ones currently being manufactured? The NOS ones are getting ridicuously spendy. I'm also curious about the possibility of using a solid state FW bridge rectifier followed by a damper diode instead of the standard FW 5U4GB, GZ37 or GZ34. Opinions? PS caps haven't been determined.

I'm planning on trying copper foil in oil coupling caps in the final build, but I'm using poly/foil orange drops for the experimenting. I haven't completely decided on resistors, but I'm thinking of tantalums for the signal path, since there won't be many and I'm curious as to what the fuss is about. I'm also considering non-inductive wirewounds.

I have bunches of used and new Sylvania 6SN7 GTA and GTB tubes and while I will try the JE labs circuit, I also want suggestions for alternates. I've seen some of the posts by KYW (really good stuff) and others, but would welcome anyone elses input.

The project I'm planning after this one is a PP2A3 amp, and really want to avoid the typical Williamson input/driver circuit. Not to slag on the Williamson (my WM-4's sound great on my Tannoy 15DMTII's), but I want to try something different. Ideas and schematics would be greatly appreciated.

The reason I'm doing this is that there isn't any way for me to hear these types of amps here in flyover land, dealers don't sell them and I'm really curious. Plus it'll keep me busy over this upcoming Minnesota winter...

Thanks in advance for any input!
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Old 12th September 2004, 11:18 PM   #2
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Hi,

Quote:
I'm also curious about the possibility of using a solid state FW bridge rectifier followed by a damper diode instead of the standard FW 5U4GB, GZ37 or GZ34. Opinions? PS caps haven't been determined.
You could also consider a hybrid FWB using a pair of SS diodes and a FW tube rectifier.
Alternatively you use a FWB using damper diodes a la 6D22S.

Rectifiers currently in production_you're never really sure about this_ are the 5U4GB, 5AR4, EZ81/6CA4, 5U8 (special Russian type using a special socket), 6D22S...maybe others.

Quote:
The project I'm planning after this one is a PP2A3 amp, and really want to avoid the typical Williamson input/driver circuit.
O.K....Here's the one we talked about before, with a paraphase input.
The basic circuit is kept simple, the PS and heater circuit is deliberately kept simple for "economical" reasons. A bit of a cost cutting exercise if you like.
Of course you're free to change that if you like.

Cheers,
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Old 12th September 2004, 11:21 PM   #3
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............And the PSU as it is built.

Circuit is by Triode Dick, I just adapted it a little.

Cheers,
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Old 12th September 2004, 11:50 PM   #4
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frank, i have that amp on breadboard, but before completing it i was trying to get a DC regulated filament for my other 2A3.

isn't it that the calculation for the DC out is ACV x 1.44 - diode voltage drop?

i have a 3.3V AC that passes through a bridge configured 1N5402 and 4,700uF cap. but under load, it is only 1.6VDC for the 2A3 filament.

now, you schematic has lower ACV for the filament. care to tell me how that works?
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Old 13th September 2004, 12:13 AM   #5
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Hi,

Quote:
now, you schematic has lower ACV for the filament. care to tell me how that works?
If we do the math using 1.8 VAC FWB rectified into a capacitive load we arrive at 2.592 VDC.
The bridge loses about 0.5VDC but I suspect my xformer is about 25% too high on current (not just the heater winding).
When measured I arrive at a little short of 2.3 VDC which is still fine for the 2A3s.
In fact I do have a little advantage here as my bias is more stable and the tubes will last longer this way as well.

Quote:
i have a 3.3V AC that passes through a bridge configured 1N5402 and 4,700uF cap. but under load, it is only 1.6VDC for the 2A3 filament.
3.3 VAC * 1.44 = 4.752 VDC - (2 * 0.6 VDC) = 3.552 VDC
Where you're losing half of the voltage I really don't know.
Did you measure the voltage without the load attached to the supply?

Cheers,
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Old 13th September 2004, 12:19 AM   #6
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Quote:
Originally posted by fdegrove
Did you measure the voltage without the load attached to the supply?

Cheers,
hi frank, without load it measures 3.6VDC

what bridge rectifier did you use? i'm ordering BR81 from RS Components.

will i have higher voltage drop with 4 x 1N5402 compared to 1 x BR81? sorry for the newbie questions

ps. i just saw the BR81 specs, it says 1.1VDC voltage drop per element at 4A DC. what do you think of the "per element" spec?
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Old 13th September 2004, 12:40 AM   #7
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Hi,

Quote:
hi frank, without load it measures 3.6VDC
Could you check what the voltage is when only the heater is hooked up to the 2A3 and nothing else?

Obviously something is not hooked up as it should be, I just don't know what.

Sorry to be of so little help, this puzzles me but someone else may see it straight away...I hope.

Cheers,
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Old 13th September 2004, 12:42 AM   #8
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hi frank, that is the only load at the moment - one single filament and everything else is not connected to the circuit (wires up in the air )
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Old 13th September 2004, 12:57 AM   #9
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BHD, i made a JE Labs 300B myself, the deluxe circuit - it is here in derek walton's world 300B builders - http://indigo.ie/~walton/world300b.html see the Arnold Cruz entry.

5U4-GB are cheap in the Philippines, from the right source.

this amp will have its heaters changed to DC once i get my Altec open baffles tomorrow right now the hum level is 7mV which does not bother me on my current Infinity Kappa.
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Old 13th September 2004, 01:01 AM   #10
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Hi,

Quote:
hi frank, that is the only load at the moment - one single filament and everything else is not connected to the circuit (wires up in the air )
I suspect you're measuring 1.6 VDC at either pin with respect to chassis ground...If you put the pins of your DVM across the heater pins you should measure 3.2VDC.

Quote:
what bridge rectifier did you use? i'm ordering BR81 from RS Components.
I used a Philips BR150 or something similar.

Quote:
what do you think of the "per element" spec?
In theory that should mean per output. The only datasheet I found so far says "low voltage drop"...Great...

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