Jump to content

On the bench today.......


Recommended Posts

On 30/12/2020 at 14:36, dave said:

I am thinking of using the AD1845 device

Retrospective brain fart correction, actually AD1865 DAC device.

14 hours ago, toprepairman said:

Why not just use a resistor to drop the odd 1.3V ?

Yes I did try that, but the rectifier heater winding is quite thin, and felt warm to the touch with 2 amps going through it. I try not to stress old transformers unnecessarily.

Link to post
Share on other sites
  • Replies 80
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Popular Posts

Latest 'use up some components' power supply now up and running. This will probably go into service with the new phono stage when I finish it. It seems to be as low noise as I'd hoped. CLCLCR, then sh

Yes, I suppose it is another way of saying I am testing my self. Give it my best shot, and if it doesn't work out, I learn something fixing it. If it does work first time, it's a bit of an anticlimax.

On the bench today is my prototype "simple" 26 preamp on one chassis. If it sounds good I can build more. It's always a popular choice of preamp and i have a few 26 valves. I have plenty of parts to b

Posted Images

On 12/10/2020 at 16:57, dave said:

Xu7fylQ3t9kgRvhJn07ac3jvZP1a_ynGbfyMMPf9

Note directly heated monode to the right.

Directly heated monode - I love it.

Link to post
Share on other sites
On 05/12/2020 at 20:43, dave said:

I think the 6080 is doomed in the short term, I was quite optimistic that the 2k transformer would work with the 6080, I did not manage to find a spec so I guessed based on what other people had used. It does work, but it is marginal, I can get just about 3 watts, but I need a voltage swing of almost 300 on the grid to do it, which is not very practical with 260volt supply. From this I surmise that the output transformer should be around 1k.  I know I am doing a shoot first ask questions later approach, this is all for my own amusement and learning. It will have to go to one side till I can either find some suitable transformers, or different output valves.... Hmm EL36.... I wonder.. supposed to be quite good in triode mode...

The Western approach, shoot first.. Shame you couldn't get it working. I have a 6080 own build amp but it is OTL, puts out 6W into 8 ohms, sounds great. Complex psu though.

  • Like 1
Link to post
Share on other sites
5 hours ago, Firebottle said:

The Western approach, shoot first..

Yes, I suppose it is another way of saying I am testing my self. Give it my best shot, and if it doesn't work out, I learn something fixing it. If it does work first time, it's a bit of an anticlimax. But that doesn't happen very often.

  • Like 1
  • Haha 2
Link to post
Share on other sites

It lives, in stereo, no feedback though, thought I would try it without, sounds very transparent but a little rough, and messed with the frequency balance, as expected, and also some of my local mains noise break through. need to look a that. Although high gain, that will probably disappear into the noise with some feedback. It is drawing 60 watts, and the mains transformer gets a little hotter than I would like, so that combined with about 280v HT, I think solid state rectification is the way to go. Less heat and more power. Quite impressed by PCL86, all NOS, 2 Mullards a Siemens a Pinnacle. Very stable and consistent right out of the box. Big red Power light in homage to its ancestry.

nDmpKEtqn4BroYJvGQtrTM7xM5azXj9tDmneFBTlg

Link to post
Share on other sites
Posted (edited)

Sulk. Thinking warm fluffy almost finished thoughts this afternoon, but no, I am not sure what did what first, will have to investigate. I put in silicon diodes, and feedback. This has slightly thrown the operation of one channel, with asymmetry, and pronounced power supply bounce effects (a pet hate), magnified because the amp is class AB1, so the power supply sags a bit at full output. Therefore in the space left by the EZ81, will go two 0A2 150 volt regulator tubes to shunt the input / phase splitter power supply. and I need to sort out the feedback as the amp now rings joyfully.

Edited by dave
Link to post
Share on other sites
Posted (edited)

Warm fluffy thoughts of almost finished back this morning. Put some more hours in to it last night, so now has less ringing and 10 dBs of feedback, and sounding fairly civilised. Bit worried about inrush, and I think I will try and search out some method of soft start, if it is easy. My idiosyncratic front panel adjustments include overall bias for all tubes, and two pots to balance the pairs. For these I will eventually find some way of recessing the pots into the front panel for screwdriver adjustment, and volume. Had an interesting time tuning it up on the distortion analyser. As a load line virgin, it is interesting to see that it does do what I thought it should, and it is definitely Class AB, and works best like that, (very mysterious). At this point the amp now draws about 50 watts (down from 60 with valve rectification) and about 6 watts anode dissipation in each PCL86, one of which is a bit low in retrospect, and about 10 watts output.

Below the circuit I based the amplifier on. In this almost finished project vacuum, my mind wanders, along with the DAC and Valve crossover, to the still at the thought stage, Class AB2 triode 20 ish watts, using PL36s. I now have a list of definite circuit requirements. stabilised power supply for front end, and a differential phase splitter, with cathode follower buffers. I visualise mono blocks in a similar form to Quad iis. All this may change but I will have to check out my iron pile.

rld_signal_circuit.jpg?phpMyAdmin=CayBDF

https://syclotron.com/

Edited by dave
  • Like 1
Link to post
Share on other sites

Sad to see a KW500 leave the premises, but in for a loud hum on the RHC and a low level hum through the speakers that sounded freakishly identical to TX hum. Another firm favourite in this workshop. Adieu my heavy PSU'd friend ;-) Refreshed and ready to enter battle once again !

 

2021-01-06 14.01.50.jpg

Edited by Dave Brown
  • Like 1
Link to post
Share on other sites
  • 3 weeks later...

Building a EL33 tube preamp - very basic simple stuff, but high quality parts. Just finished the PSU. 12X4 rectifier, all polypropylene caps, 2.2uF first cap > 10H/100mA choke > 47uF > dropper resistor > 47uF. Bleeder 100K 12W. I get 270v off load. I may possibly use a tube voltage regulator, will see. 

I'll load the EL33 with a plate choke. I'm using PL33s in fact - 19v at 300mA heaters. I have around 30 of them. I really like the sound of this tube. It's pretty much an EL3n with an octal base, and the EL3n had lots of fans. One of the best indirectly heated tubes I've ever tried - warm clear sound something like a 26 DHT. 

Update:   Finished it and used it to drive my SE output stage. It's really good - better than I expected. I usually use DHTs but this is very close. Quite holographic, punchy bass, nice treble, excellent on vocals. This is a decently priced preamp in parts - nothing boutique, though everything is individually picked for optimum performance, like teflon coupling caps. I think this would perform as well as some very costly preamps. 

Edited by pmcuk
  • Like 1
Link to post
Share on other sites

In this amp the HT is quite saggy, and the class AB does not help, with it saging by up to 60 volts during transients. In the end I decided to make a shunt regulator for the input stage. This is a hybrid design, a small NPN transistor lots of zener diodes and a valve, an EF80 in this instance as I have lots of those. This holds the input / phase splitter power supply constant. At no load it pulls 4.5mA, about 1.4 watts, which is quite acceptable.

The weirdness I tracked down to a dodgy Siemens valve. Without a suitable valve tester I don't know what is wrong, it just seems much slower to heat up and different from the others, and prone to oscillation. The current meter by some fluke has a little green line in almost the right place for setting the bias current and when the meter goes in to the red, clipping is most likely.  The red glow on the ceiling is from the Shunt regulator reference, it goes out when it runs out of headroom. If I can find a suitable led, I will put it on the front as a headroom indicator.

hgOOiu_OAvkUUPmcB8Px6VwDQ9CLdD02pVwzpykP

I need to do some in depth thinking about the power supply, to workout how much I can increase the main reservoir capacitor without hurting the transformer, that way I should be able to get  some more dynamic power for transients. I think I am ready to download PSUD2. I am going to try an inrush limiter somewhere as the turn on is pretty savage.

On 06/01/2021 at 10:12, toprepairman said:

You could try a small capacitor across R4 to cure the ringing, likely to be between 50 and 1000 pF.

This was the most sensible way to fix the ringing in the end. 

So What next. Been rummaging through my transformer pile to find some combination to make an amplifier without having to buy to many parts. This time I am going to gratuitously prototype, rather than build a box and hope my guess is good. I found a small ish pair of mains transformers so I think time for a pair of SE GU50 triode wired mono blocks. GU50 has anode dissipation of 40 watts so should be good for a bit of power....

Edited by dave
Link to post
Share on other sites
23 hours ago, dave said:

So What next. Been rummaging through my transformer pile to find some combination to make an amplifier without having to buy to many parts. This time I am going to gratuitously prototype, rather than build a box and hope my guess is good. I found a small ish pair of mains transformers so I think time for a pair of SE GU50 triode wired mono blocks. GU50 has anode dissipation of 40 watts so should be good for a bit of power....

The GU50 has been on my radar for a while but I don't have any. There's an interesting design on Bartola Valves. If you want a driver valve I'd certainly recommend the EL33 from my most recent builds - it's quite surpassed my expectations. 

Link to post
Share on other sites
32 minutes ago, pmcuk said:

The GU50 has been on my radar for a while but I don't have any. There's an interesting design on Bartola Valves. If you want a driver valve I'd certainly recommend the EL33 from my most recent builds - it's quite surpassed my expectations. 

GU50s are plentiful and cheap, and they have strange sockets that look like tractor exhaust pipes. I must say I was thinking of using something in the ECF8x(x) family. I don't have any EL33s or even PL33s. Some of the other types that could be interesting would be PCL84 for example. The pentode is a video amplifier, so it should be fairly linear and fast, and a but more power than an ECF8x, but not as much as an output tube.

Do you have any thoughts on 6V6 as a preamp, there is a long thread on DIYAudio about them, 

Link to post
Share on other sites

I've recently done a lot of testing of medium mu tubes in triode, including the 6V6. I was looking for a driver tube with decent current capability, like 15mA. The 6V6 was OK but not my favourite. I liked the EL33/PL33, followed by the KT61, followed by EL84, EL41 and E80L. I also have some EL12n which I bought cheap from a German dealer, BTB. They're an E.German tube, plenty of new stock, and very interesting as output tubes, Ri of around 1K in triode so will work into 3K primary. 

Edited by pmcuk
  • Like 1
Link to post
Share on other sites
  • 2 weeks later...

Rats nested GU50 amp. Bit of a slow start as eventually realised the tube I was using was very low output. Wouldn't do anything without the grid glowing red, and far to docile. Found in a box a couple of new in box unopened, brought over from Siberia by my (now ex) brother in law, thanks Dimitry, from a stash in his dad's shed. That one has fire in its belly. Got about 7 watts SE out of in in triode mode, driven by a PCL84. Kind of acceptable.

T4Yoii4RAD6ZIJkszmbKWz6huH2mdFJVRxV7EwGl

But got me reflecting on what I want to achieve, looking at the transformers can't help feeling the need to make it a Pye mozart homage. That with the output transformer not really that good, and rolling off above 15kHz, and distortion not really going below 2% and about 4% at full output, although it could be noise related, my analyser is missing the 400Hz high pass filter. I think I was hoping for a bit better performance from my "triode" amplifier. So I am going to now redo it with an EL34, as I think that is what the output transformer was originally meant for. The pye mozart has an interesting circuit, which includes the transformer secondary connected to the cathode of the output valve.

mozartpower.jpg

It may well look like a cross between this:

pye-black-box-amp-02.jpg

 because it is compact and has 2 transformers, and this,

220f5d7d-0535-4d78-b748-3a3f95aaab4c.jpg

in Pye blue, 

178801906_pyeamp).thumb.jpg.51e8349777903a7eaefd2f73eea75477.jpg

For my "Triode " no feedback amplifier I think I am going to have to splash on some better iron. I do believe that low measured distortion is a worthy goal, and past experience has shown that I cannot really cope with prematurely rolled off highs. I think I need to focus myself now on making worthy amps that are as good as I can make them, rather than some thing that works with tube I think looks nice, which is quite tempting. Damn you measurements, I was fairly happy with this one till then, it looked pretty clean on the scope.

Edited by dave
  • Like 1
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...