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Homebrew amp

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Re: Homebrew amp

Post by Doz » Sat Jan 09, 2016 2:29 pm

Following this with interest. I have a box of 1625's (807 with different basing, and 12.6V heater). I have a couple of Hammond output TX left over from something-or-other, and I've recently got a couple of suitable mains transformers, so the idea is to create couple of Monoblocks.

I *think* the OPTs are 4.3K plate to plate, so I may just put the 8ohm speakers on the 4 ohm tap to raise that a little and see how I get on.

I may even get to make a start tomorrow. I thought to try 2 parrallel 1625's in push pull. I don't think I have enough bases to do 6...

 
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Re: Homebrew amp

Post by Dr Wobble » Sun Jan 10, 2016 7:15 am

Glad my efforts have inspired you to knock up a couple of mono blocks Andy, the first time you power it up, and get music out is worth the effort..I have some NOS 1625's somewhere, you can get theUX7 bases and top caps on ebay I toyed with the idea of making one stereo amp, but it will weigh a ton. I'm not bothered about having the valves on show - I've always thought that a bit daft - the power supply for this will need to be substantial so may have to the amp seperate to the PSU.

Progress of sorts, I've eliminated the hum by improving the layout and upping the capacitance to 440u for the res cap and 47u each for the front end and PS. But the real improvement came from poking the amp with a plastic pen; shifting the tag of anode two on the PS made the hum disappear.

I've changed the PS back to the 6SL7 as I need more gain. Am a bit fed up with changing components, am going through some solder, but as I'm inexperienced and not that knowledgeable, this is the way it has to be. The OP stage halves the signal from the PS. May need to put a driver stage in, but impedance calculations arn't my strongpoint, and another stage may complicate things when applying NFB . I'm going to try using a BJT CCS for the PS to get better balance as well, though again I'm a bit vague about the practice, fortunately valves are quite forgiving.

Another problem needs solving which is the signal riding on the back of a saw tooth wave of 2v P-P. Going to try using DC HTR supply.

It's good to hear this thread is of some interest,please chip in with your comments, Andy.

 
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Re: Homebrew amp

Post by GlowingAnode » Sun Jan 10, 2016 10:04 am

Andy, reference the 2v sawtooth. A little trick you can try. When you switch off the mains the heaters will stay warm for a couple of seconds and the smothers will maintain the HT. Check to see what happens whether the sawtooth disappears immediately or fades away.
Cheers Rob.

 
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Re: Homebrew amp

Post by Doz » Sun Jan 10, 2016 10:39 am

I've built quite a few amps over the years, including a 12V driven 6550 push pull amp which did about 25 watts , man that turned some heads! The inverter was a real fun thing to design and build! It suffered with input tubes (6n6p's) going microphonic. I eventually mounted them on a little srpung sub chassis. It warmed the boot up quickly!

 
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Re: Homebrew amp

Post by Dr Wobble » Sat Jan 16, 2016 8:04 am

Spent the last week trying to get the LTP phase splitter working on a CCS. Unfortunately I can't get it running right, which is down to not knowing what the feck I'm doing to be honest. I can't figure out how to bias the grids or the best way to arrange the grid leak R's. Chris- Mr Herald has given me some advise which I'll try. but I think I may go back to using resistors as this amp is taking a long time and I still have a long way to go.

More soon, Andy.

 
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Re: Homebrew amp

Post by GlowingAnode » Sat Jan 16, 2016 8:25 am

Good call Andy, a ccs is a nice to have, once the rest of it is working properly.
Keep going mate, you'll get there.
Rob.

 
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Re: Homebrew amp

Post by Dr Wobble » Sun Jan 17, 2016 7:46 am

Yesterday I replaced the CCS with R's, bish bash bosh jobs a good un. So I have an amp that works, I put quiet music in one end and it comes out louder out the other end. However for six 807's I'm getting about 28w, which considering the OPT can handle 120w isn't efficiant . I could get the same out of two with a different OPT. Re power output, I've known for a while that you don't need mega watts but was surprised yesterday when I cranked my Yamaha tranny amp up to quite loud, loud enough to make Molly run away and found the amp power was running at 2w !

While I had the amp running I fiddled with the sig gen. The amp goes from 10hz to 50khz with obviously less amplitude on higher frequencies. So I need to to put some filters in to roll off the highs. BTW Rob I got rid of the saw tooth, you were right it was the heaters, AC sneeking in through the cathodes of the LTP.

So I need to have a think here, whether to use EL34's to get more power OP or not use the UL taps and use higher HT on the anodes or make it into a guitar amp.

Andy.

 
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Re: Homebrew amp

Post by Hartley118 » Mon Jan 18, 2016 4:34 pm

As you point out, it's disappointing to be only getting 28W. from 6 x 807s. Something must be clipping prematurely.

What is it that happens as you reach 28W? Is it the top or the bottom of the waveform that clips? Or is it happening in a stage before the output valves?

Martin

 
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Re: Homebrew amp

Post by Dr Wobble » Tue Jan 19, 2016 1:30 pm

Martin,the 807's are fixed bias at 30v about 40mA, there is enough drive from the LTP. Simply put when the OP from the LTP reaches 60v P-P the top of the wave gets cut off. Because I'm running as UL HT can't be more than 300v , so with a OPT pri at 2k ish the load line is steep and so the OP voltage swing is limited, so not much gain.

I'm going to try disconecting the screens from the UL taps and up the anode and may try it in AB2, the worlds my mollusc as they say.

Andy.

 
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Re: Homebrew amp

Post by Herald1360 » Tue Jan 19, 2016 6:36 pm

If the 807s are biassed at -30V, it sounds like the drive flat topping is simply the low impedance of the forward biassed 807 grids clipping the positive peaks of the phase splitter drive. To go into AB2 you'll need to provide a low impedance drive for the 807 grids. A pair of cathode followers is one possibility. Have a search specifically for AB2 807 drive circuits and see if anything useful appears. Or a word with the Scandinavian guy over on UKVRRR who has done some clever stuff driving very awkward valves with lots of grid current.

See also this thread of yours......

http://www.vintage-radio.net/forum/show ... p?t=106886

 
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Re: Homebrew amp

Post by GlowingAnode » Tue Jan 19, 2016 9:31 pm

A cathode follower will give the low kmpedance drive as s required. However you have to watch bias drift and what happens during valve warm-up if directly coupled.
Consider using a coupling transformer instead, you will be amazed by the increase in output.
A good trick is to use a small pp op tx backwards driving the grids, fed from a small audio amp.
I have a driver tx I was given by VB, happy to pass it on for you to play with.

Rob.

 
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Re: Homebrew amp

Post by Dr Wobble » Thu Jan 21, 2016 2:49 pm

Chris, why are the 807 grids forward biased? Can you expand on what you mean a bit more please? I understand about the OP stage being low impedance, would using a higher current valve help or is this definately a job for a cathode follower?

Thanks Rob for the offer of a driver tfmr, I might take you up on that for experimentation.

I've run out of steam with this amp at the moment so taking a day off and doing a bit of research. As it stands it has enough power ( had to knock up a test "speaker" using an old KEF X-over and odd speaker cabs- see pic ) for me and sounds quite good, it's just not running to it's potential in terms of power and valve count. I know how to improve the front end by improving the frequency response of input stage and direct couple to the LTP and use a tranny CCS, just want to get the OP stage sorted.

Have looked into AB2 but am worried it may be a bit beyond me at present. However I did find a useful paper by F Langford Smith with an interesting circuit - see attached PDF. I've also dropped Tricomp a PM. Driving the amp also involves AFAIK regulating the screen supply which is another kettle of maths fish to wrestle with. I'm a bit mathed and theoried out, need to go and dig the garden or wrestle a bear, something physical that doesn't involve equations... going a bit math'd : )

Bless you all for sticking with me, Andy.
Attachments
amp 10 UT 001.jpg
AB2 F Langford Smith_page1_image1.tif

 
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Re: Homebrew amp

Post by Dr Wobble » Thu Feb 18, 2016 12:02 pm

Back working on this amp after a spell away. I've now got the OP power up to 90watts, which is a ridiculous amount of power, I'll have to build some speakers just to test the bugger with music content, but as I've come this far I'll see if I can squeeze a bit more out.

This involves running the 807's at 400v on screens and anodes which means I'm going to have to wind my own tfmr as the big isolation tfmr and variac and PSU I'm running it on presently only goes up to 380v. Finding enough capacitance is problematic too, I've got six 220u caps in parallel at present running very close to their max voltage.
I phoned the chap at Hi Fi collective who bought out Audio cap to see if he had any F&T 2200u 500v caps ( 2200u for 1A, amp runs at nearly that at full whack) which of course he didn't.

As it said on The Merry Pranksters bus -----> Further, A.

 
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Re: Homebrew amp

Post by Norman Raeburn » Thu Feb 18, 2016 12:50 pm

Hi Andy, As a work around for the voltages on the smoothers, you can put two in series with balancing resistors across them to equalise the voltage there by reducing the voltage across them by half. This is common practice in a lot of amplifiers I work on. Norman

 
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Re: Homebrew amp

Post by Refugee » Thu Feb 18, 2016 5:43 pm

My cinema amps are 25W and they have two double 32uf 450V cans with one can servicing rail decoupling in the front end. The HT is 390V to 415V in normal service. I have re-stuffed some of the cans with those slim 33uf 450V ones without issue.
2200uf sounds a bit huge to me.

 
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Re: Homebrew amp

Post by Dr Wobble » Fri Feb 19, 2016 7:40 am

Thanks Norman, I was aware you can connect caps with bleeder/equalising R's in series, which is what I may have to do, but as you know prices go up for caps especially when you get above 200v. So to get 2200u/500v, I'd need two 250v 4700u caps, which are also tricky ish to source. 2200u per amp is a "rule of thumb" I've come across in a few articles on amps and power supplies. Open any 80's amp running on 60v XA rails and you find big caps around 10000u or more.

One other thing I read that stuck in my mind went something like " the power supply in an amplifier is what you end up listening to" Andy.

 
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Re: Homebrew amp

Post by crackle » Fri Feb 19, 2016 9:09 am

Andy
Have you noticed a rise in the OP power since fitting the larger smoothing caps?
Or is the higher power as a result of other changes?

 
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Re: Homebrew amp

Post by raditechman » Fri Feb 19, 2016 12:22 pm

refugee said
My cinema amps are 25W and......
.

What amplifiers are these, do you own or run a cinema?. Or have you salvaged an old amplifier from one.

I have a couple of 807's in a home brew 80m linear amp which I no longer use, I have been thinking using the chassis and valves to build a small audio amplifier at some stage. Not until the weather warms up though!.

John

 
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Re: Homebrew amp

Post by Refugee » Fri Feb 19, 2016 2:44 pm

raditechman wrote:refugee said
My cinema amps are 25W and......
.

What amplifiers are these, do you own or run a cinema?. Or have you salvaged an old amplifier from one.

I have a couple of 807's in a home brew 80m linear amp which I no longer use, I have been thinking using the chassis and valves to build a small audio amplifier at some stage. Not until the weather warms up though!.

John

They were recovered from a skip in the early 1980s when valve amps were 10 a peny.
I did a thread a long time ago.
viewtopic.php?t=1702&p=14832#p14822

 
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Re: Homebrew amp

Post by Doz » Sat Aug 27, 2016 7:21 pm

Doz wrote:Following this with interest. I have a box of 1625's (807 with different basing, and 12.6V heater). I have a couple of Hammond output TX left over from something-or-other, and I've recently got a couple of suitable mains transformers, so the idea is to create couple of Monoblocks.

I *think* the OPTs are 4.3K plate to plate, so I may just put the 8ohm speakers on the 4 ohm tap to raise that a little and see how I get on.

I may even get to make a start tomorrow. I thought to try 2 parrallel 1625's in push pull. I don't think I have enough bases to do 6...


Tomorrow never comes... and then, it did...

A "roughly" ultra-linear williamson design. 2 1625's in push pull. 6SN7 phase-splitting and driving. Output stage has about 450V on load. Yet to complete the input stage. Hoping to get some audio out of it tonight.

chassis.jpg
lit up.jpg
underside.jpg

 
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Re: Homebrew amp

Post by Doz » Sat Aug 27, 2016 11:58 pm

15 watts into 8 ohms... quite pleased with that.

Some instability to track down.

 
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Re: Homebrew amp

Post by Dr Wobble » Tue Sep 06, 2016 2:39 pm

Nice one Andy. Its about time I dug mine out and got it going properly, I have a driver stage I want to try out that uses a cascade configuration. Who dig the metal work for yours?

You say you have 450v HT, I presume on the anodes of the 1625's, what are you running the screen grids at?

Andy.

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