DIY Audio Projects Forum

1930 171-A SE amp
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Author:  mwhouston [ 27 Mar 2013, 16:43 ]
Post subject:  Re: 1930 171-A SE amp

One thing this amp appears to do extremely well is throw a huge and sharp sound stage. I am using the Fostex FE206 speakers and I have them heavily toed in and set back. This tilts the Fostex up points the cones directly into my ear. I sit above the center of the speakers normally but with them tilted back there is so much more music and soundstage sharpens up. Very delightful.

I tried Intermezzo with the 90lb Osborn 3-awys. It was awful, just distortion and muddy. Though the Osborns are efficient they are 6 omhs. This placed too much load on the tubes which just couldn't drive them. With VoXDeuX the sound was OK just not much volume. This amp needs something like the Fostex to achieve good volume and easy driving. Perfect match.

Author:  mwhouston [ 28 Mar 2013, 01:04 ]
Post subject:  Re: 1930 171-A SE amp

Looking at the image you will see there has been a serious upgrade to the tranni brd. The two HT trannies I had in place just couldn't cope and I never got any where near a reasonable HT. Not only that the last part of the HT filtering was C-R-C. Because of such a low HT to start with this resistor stayed 1ohm. Useless for filtering. Now this limiting resistor is 1K and drops 37V across it.

Not hard to workout the total current for the amp is 37mA. 40MA would have been max so this is a nice safe "max". The 1K limiting R and 100uf 400V cap also provide the final stage of filtering. I increased the bias to -36V. The HT will generate 185V but I would need better than -40V bias to work with this. I though it better to bring the HT to 170V+ and make the bias -36V.

The amp appears to have much better drive and greater detail and much better bass. I would now say from a purely sonics point of views it may challenge the Silver Dragon 300B. While I had the amp apart I corrected the out of phase speaker terminal and brought the bias voltage to a test point near the standby switch. Now I'm very pleased.

I have had one faulty tube from two matched pairs. I now have in-place the last tube (part of a second matched pair). All looks and sounds good. Now trying to get the eBay seller to replace the faulty tube.

Author:  RossD [ 28 Mar 2013, 08:34 ]
Post subject:  Re: 1930 171-A SE amp

Keep up the good work.

I like the updates and your frequent opinions on how things sound. It gives people with less experience, like me, some extra guidance.

Author:  Soundbrigade [ 28 Mar 2013, 09:49 ]
Post subject:  Re: 1930 171-A SE amp

I am sort of stuck with my unfinished projects but will finish (I hope) a preamp this weekend. Else I am very intrigued by simple designs; checked the schematic - SO SIMPLE! So far I haven't fallen into the IM transformer hole but I have the GU-72's to find a driver to.

When it comes to design I am working at a plant where we can process steel sheets but it's somewhat difficult to work with wood and get good result. I one belonged to a happy gang that met one evening every week to work with wood and we had access to machinery. I can still join the troup but that will cost me some and I haven't really got the time or energy.

Author:  mwhouston [ 28 Mar 2013, 18:09 ]
Post subject:  Re: 1930 171-A SE amp

RossD: thanks for the nice comments. I have been trying different Fostex speaker arrangements. Currently with the boxes toed in to meet 1M behind my head and tilted back and being driven by Intermezzo is producing some very exciting listening. The Fostex take a while together use to but once you spend some time with them they are shockingly intoxicating.

You are all invited over for a listen (haha). Had another big listen last night and I'm amazed how loud 1\2W can sound through sensitive speakers. Cassandra Wilson with just dobro guitar caused the hair on my neck to stand. Simple combos of two or three instruments and voice Are about the best I have heard anywhere. The bigger trannies and higher HT have really made a huge difference to this amp.

Author:  mwhouston [ 29 Mar 2013, 06:34 ]
Post subject:  Re: 1930 171-A SE amp

I made comments about the HT voltage. This was measured at the plate not at the OPT primary.

I have the original owner and financier of the Silver Dragon over for a listen to Intermezzo. He is critical but loves amps which produce perfect voice. I will let you know his comments. So far you have only read my babblings.

Author:  mwhouston [ 29 Mar 2013, 21:50 ]
Post subject:  Re: 1930 171-A SE amp

I thought some may like view the PS. Here two step-down trannies are used. One steps 240V down to 15V. Then this 15V is feed into the secondary of another tranni to produce about 240V in the primary. This works well and provides good mains filtering and isolation. An old "80" rectifier tube is used for half way rectification and the un-smoothed DC feed to the filter brd. a C-L-C-R-C network smooths the DC. The 1K resistor drops the HT from over 185V to 170+ at the plates. Total current draw is 37mA.

A dual 6.3V 2A X 2A Hammond filament tranni provides 6.3V to the filter brd. to be rectified by a 4A diode bride and filtered. The filtered DC is feed to two 5V regulators one each for each filament of the two 71-A tubes. The other 6.3V winding has a small dropping resistor to provide 5VAC to the "80" rec. tube used in 1/2 wave configuration.
Intermezzo PS.jpg

Author:  mwhouston [ 05 Apr 2013, 23:57 ]
Post subject:  Re: 1930 171-A SE amp

I had bought two pairs of 71-As. One pair supposedly NOS and NIB and the other just a matched pair. One tube arrived DOA. The grid was shorted to the filament. I put in two tubes which looked best from a distance but noticed that the RCA logo stamped on top of the tube of one was different from the other. Over a few nights listening I nights I notices the right channel louder than the other by about 3db. When I rotated the tubes the higher level followed the tube.

I swapped the quieter tube for the one remaining UX-171-A I have and the balance is now perfect. Solo voice (radio announcer) comes from the dead center of the sound stage. I also notice that both the top RCA logos look exactly the same. The tubes just show a different side of themselves with this combo. Well Mark it's not always about the looks.

I'm having audio buff friends over tomorrow for a listen to Intermezzo. I'm going to play music from my iPad beamed to the amp via a Belkin tiny blue tooth receiver. 1920's tubes hooked-up via blue tooth. There is just something weird about that.

While listening to National Classical radio (ABC CLassic FM) via the IPad streamed to Intermezzo I noticed it sounded harsh and irritating. I swap from 48kbs to 96kbs and this is much better. The Fostex were resonating to the poor treble quality.

Author:  mwhouston [ 06 Apr 2013, 00:59 ]
Post subject:  Re: 1930 171-A SE amp

Some low-rez shots of the setup.The Blue tooth receiver sits on the BoZSE preamp.

Author:  mwhouston [ 06 Apr 2013, 21:45 ]
Post subject:  Re: 1930 171-A SE amp

My gut told me that the 4 X 9V bias batteries for this amp will last a long time. I installed a bias test point on top of the amp so it was easy, today, to check the bias voltage. When the batteries were newly installed the bias read -37V. Today they still read -37V. I think the bias batteries could last many months. There is no on\off switch for the bias batteries. But if you look at the schematic it is obvious there is no need for one.

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