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 Post subject: Re: 1930 171-A SE amp
PostPosted: 22 Jun 2013, 15:39 
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mwhouston wrote:
Looking at some past notes for 5V input I was getting 13V out. I didn't expect this amp to have huge gain so these figures look about right. But negative gain? never.
Any suggestions where to look first? {from the "Show us pictures of your system" thread}

Mark;

A thought occurred to me. Are you using battery biasing? The bias has to be pure DC and the AC impedance to ground must be close to zero. One of the effects of battery aging is increased ESR even though the voltage may show good at low load. For this reason, when battery biasing in the way you biased the grid (i.e. through the transformer secondary), bypassing the batteries is essential. Seeing how the secondary of the interstage transformer has a 90kΩ secondary, to ensure a good low end rolloff (say 20Hz), the batteries should be bypassed with a capacitor of at least C=1/(2π*f*Z) or 0.09µf. I would us a good quality 0.1µf orange drop.

Why don't you give this a try and see if it has any effect?

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 22 Jun 2013, 19:38 
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Suncalc wrote:
mwhouston wrote:
Looking at some past notes for 5V input I was getting 13V out. I didn't expect this amp to have huge gain so these figures look about right. But negative gain? never.
Any suggestions where to look first? {from the "Show us pictures of your system" thread}

Mark;

A thought occurred to me. Are you using battery biasing? The bias has to be pure DC and the AC impedance to ground must be close to zero. One of the effects of battery aging is increased ESR even though the voltage may show good at low load. For this reason, when battery biasing in the way you biased the grid (i.e. through the transformer secondary), bypassing the batteries is essential. Seeing how the secondary of the interstage transformer has a 90kΩ secondary, to ensure a good low end rolloff (say 20Hz), the batteries should be bypassed with a capacitor of at least C=1/(2π*f*Z) or 0.09µf. I would us a good quality 0.1µf orange drop.

Why don't you give this a try and see if it has any effect?

Over the last few days I have been thinking the same thing. How does the AC get through the batteries to ground? If the batteries are bypassed there is a route for the AC. I was going to try it but a thought I may need a bigger cap say 220uf. Good to know a small quality poly will do the job. I will try it over the next few hours and post a reply. If not I may ask you to design a front end for the amp say based around a 12AX or AU7. Really I like the amps unusual design so if I can get it to work with this config. the better.

I checked the original cct. There is no cap across the battery.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 23 Jun 2013, 00:04 
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I knew I wasn't going crazy. This amp has tested with gain in the past on two other occasions I can think of. Today I connected the test gear and pesto, 1.5V in and 3.75V out. So carefully I carry it to my listening area, plug it up to test gear and no gain. I bring it back to the bech and no gain

I tried the cap bypass for the bias volage but had no effect both when it had gain and when it had none. Infact on the left channel it reduced gain further. I tried a bigger cap 470uf, and this kills any signal getting trough. The original cct. Didn't have a bypass cap.

I replaced all the batteries, no effect, I ran the amp and taped and poked around the cct. Thinking there is a short or bad connection. No effect. I am about to try a different rec tube but at all times the is 175V present.

Anyone got any bright ideas what the hell I should look for? I'm about to put the amp through the window with the window closed.

I tried a new rec tube, no better. For 1.5V in I get about 340mv out. This appears to stay constant though if i stick in 20v i get 5.5v out. i tried the amp with no tubes. Nothing out. I thought maybe the plate and grid pins were not contacting the tube pins but they are.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 23 Jun 2013, 03:28 
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I have no explanation but after spending all day trying to find why this amp mostly ill-performs and occasionally works as it should on putting it back in its box, it worked perfectly. I have no explanation why it was faulty or now why it works and sounds excellent. Tomorrow I will re- test and have a big listening session.

As long as the volume is moderate this little amp sounds fantastic. Looking forward to a big day of music. Thanks to those who made suggestions to find the fault.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 21 Oct 2013, 21:39 
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I was using Intermezzo as a headphone and. There was too much noticeable hum when the music was quiet but I proved the amp can work well as a head phone amp. I decided to chase the hum once more and found that the AC HT lead was laying across the grid wire of the left channel. By lifting the wire away and making that area neater I dropped the hum to 1.4mV RMS in both channels. I doubt if you could hear this in speakers, even efficient ones, but in phones it is still audible.

At least now I can build a very unique HP amp with old tubes (and old rec tube) all that is required is to make a really quiet PS. The PS in Intermezzo is half wave rectified. With full wave rectification and additional chokes and caps I should be able to get the PS dead quiets. The choke in Intermezzo are only 5H. Not hard to improve on that.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 22 Oct 2013, 07:30 
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Nice work Mark.
Please report back with your alterations to reduce hum. I think my 6k6 amp is going back on the bench to get rid of some pesky hum, too.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 22 Oct 2013, 12:28 
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While I had the amp on the bench I ran a test CD to see bass response. The -3db point was 40hz with 33ohm phones connected. I'm pleased withe that. Taking a look at the sine wave at 1000hz showed a perfectly formed sine wave. Putting the amp back on the overly efficient Fostex boxes the only way I could hear anything at all was by putting my ear right into the cones.

I may try adding one more 100uf cap to the PS. The last cap is 100uf but I may be able to add another one. Noise level of about 1.4mV is pretty low though.

On another note, does any know the difference between UX-171A and just a 171A? I am bidding for both on eBay.

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 23 Oct 2013, 17:24 
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Added the additional 100uf cap but this had no effect on noise level. It remains at 1.5mV. No good for phones but great for speakers. Even on the Fostex you hear nothing until your ear is right in the cone. Next PS full wave and a lot quieter and remote!

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 24 Oct 2013, 08:04 
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Just had a little laugh to myself. I have Intermezzo powering (yes all 0.5W) the Fostex FE206 double chamber bass boxes each of my 46" LED TV. I just finished watched Oceans 12 at more than enough volume, crystal clear no distortion and enjoyed every second of the movie. Then I realised the whole movie sound was being powered by 1920s tubes with an amp that only has two trannis in the whole signal cct. Lately my tube and chip 80W amps resides.

So a modern day digital TV showing a recent movie the sound of which is powered by a 0.5W amp with 1920s tubes. Ha-bloody-ha! It was great!!

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 Post subject: Re: 1930 171-A SE amp
PostPosted: 30 Apr 2014, 23:46 
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Attachment:
IMG732.jpg
Am image with Intermezzo and and the new preamp (Parley - tube with jFet buffer). Also the Fostex FE206s in the simpler ported box.


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