Scratch build valve power amps

[Rroff]

Wish I knew more about electronics in that respect - I was messing about with I think 4.7uf electrolytic->100uH inductor (I think)->0.1uf ceramic in a kind of pi filter type arrangement between the DC-DC POL (to create negative rail) and regulators but not sure it actually accomplishes much LOL.

I've always had some residual hum on all five of my amplifiers which has made me suspicious of there being some slight DC offset.

Not so applicable for what you are doing but for opamp based stuff I've been a fan of the DCP series POL devices such as this https://www.ti.com/lit/ds/sbvs012i/sbvs012i.pdf it seems in my application ground loop type conditions for some reason are the primary cause of any upsets/hum/noise.

 

[NickK]

N

Doubt it matters in this case as they are only 16V capacitors. My valve amps don't have bleeders either. When switched off, the caps discharge completely. I built a Carlson's Lab style capacitor discharge tool to be sure when dealing with capacitors anyway.

I can report that it has made the transformers quieter on all of the amps which is nice.

Cool -- I just had visions of 240V (due to the earth wire) caps charged. With the caps charged do you not find you have HT over the valves without the heaters/grid operational?

I've sort of pretty much decided on a simple single end design initially for mine, but I'm still exploring.

 

[Kei]

N


Cool -- I just had visions of 240V (due to the earth wire) caps charged. With the caps charged do you not find you have HT over the valves without the heaters/grid operational?

I've sort of pretty much decided on a simple single end design initially for mine, but I'm still exploring.

This is the schematic for the DC trap. https://sjostromaudio.com/hifi_pics/hifi_100pr/dct03r0schema_p1color.png

The caps in the valve amps are so small (47uF is the biggest, the rest are just 10uF) that they barely last a full second on shut down, so the HT collapses very quickly.

 

[NickK]

This is the schematic for the DC trap. https://sjostromaudio.com/hifi_pics/hifi_100pr/dct03r0schema_p1color.png

The caps in the valve amps are so small (47uF is the biggest, the rest are just 10uF) that they barely last a full second on shut down, so the HT collapses very quickly.

Still not sure I like the idea of 16V caps being used for 240V AC, even if the AC ripple itself is ±1V for example (thus below 16V) things like inrush etc. Obviously there's some design concept I'm missing.

I can see what he's doing - using the diodes to essentially provide a bypass for the full 240V with the drop down at any point being 3-4V below so essentially you get a cut under the ripple but then using a set of capacitors to provide a further smoothing and thus the theory is that the caps only see a small difference between the two sides...
Any transient RF noise in the incoming AC could pop the caps however I assume that an EMI filter is on the transformer mains input.

I've seen a design that uses -132V and -137V and 5V digital chips floating at that - it works, the ESD 2000V for 5-10uS could cover a fast start transition but the system slowly ramps the voltages thus keeping the difference at maximum 5V. In the design a 50MHz high speed gavonic would simplify and make the design safer...

If the amp has a failure - ie short to earth, that voltage drop is then 240V until the fuse blows.

I'll butt out

 

[NickK]

QQ did you opt for a regulator in your DC HT power supply?

I’m considering if I should put something after the smoothing caps but before the HT power distributions (it will have filter caps for each tube or stage)

 

[Kei]

QQ did you opt for a regulator in your DC HT power supply?

I’m considering if I should put something after the smoothing caps but before the HT power distributions (it will have filter caps for each tube or stage)

There is no regulation in my amp. B+ sags by about 20V once there is a decent level output. Maybe a future upgrade along with dc heaters for the first stages.

 

[NickK]

There is no regulation in my amp. B+ sags by about 20V once there is a decent level output. Maybe a future upgrade along with dc heaters for the first stages.

Ok, thank you. Reading around it seems that regulation is all or nothing. If you put regulation in one area, as that no longer follows mains float so causes more problems.

 

[Kei]

Added another pair of JJ KT77's and JJ GZ34 to complete the collection. I also picked up a pair of 6BR7 as they are no longer made so having a decent stash is going to save having to rewire the socket back to an EF86 for a few years. (CV4006/6059 is just the industrial version of 6BR7)


The collection has gotten large enough that I'm now considering a plastic storage box for them.

 

[Kei]

Turns out, one of the 6BR7 was faulty. It produced virtually no gain making one amp very quiet vs the other. After some actual measurements today, I can see why.

Static Voltages with 0V at G1:

Faulty 6BR7
Anode: 236V
Cathode/G3: 0.95V
G2: 236V

Other supplied 6BR7
Anode: 105V
Cathode/G3: 1.98V
G2: 96V

Known good 6BR7 from my own collection
Anode: 88V
Cathode/G3: 2.1V
G2: 86V

Oscillograms with 150mV 1KHz sine input: CH1 is speaker out, CH2 is anode out of the 6BR7. Explains why one channel was extremely quiet as the output voltage was almost a quarter of what it should have been. In power terms, 2.86V into 8 ohms from a good valve is a shade over 1W. 0.76V into 8 ohms is a measly 0.07W


Suffice to say, it's on it's way back to the seller.

 

[Kei]

After someone mentioned trying to build a Mullard 5-20 using 807 output valves on DIYaudio, I got it into my head to look at whether it was possible using my spice simulation as the ideal output transformer for 6L6 based valves is 6.6K. (which is what the 5-20 happens to use) Turns out, based on my simulation, 6L6GC and it's 30W+ derivatives will work just fine in my amp. So on a whim, I bought some of the high dissipation (35W) 7851A's to try out.

With all of my output valves side by side, they look pretty similar to the EH 6CA7 on the far left, but the primary difference is that the cathode is quite a bit smaller. It does seem odd that it has a 35W dissipation limit whilst the rest are all 25W.


Fitted and running. Cathode biasing seems to work nicely. Plates sit at 428V and screens just below at 427V via the 43% ultra linear taps. Small bonus these have is the reduced heat output from the lower heater current (0.9A vs 1.5A) After a couple of hours listening, it sounds every bit as good as the other EL34's and their derivatives. I figure the ~20dB of negative feedback is minimizing any audible differences there may be between the various valves I've tried. A big added aesthetic bonus is that these have the blue glow on steroids compared to the rest.

 

[NickK]

Quick question - which solder did you go for?

I’ll be using ptfe 1000V copper - I can’t remember if that was silver coated copper but if it was I’m going to have to go with a couple of % silver mix - Mundorf do an on free mix with 3.8%.

 

[Kei]

Quick question - which solder did you go for?

I’ll be using ptfe 1000V copper - I can’t remember if that was silver coated copper but if it was I’m going to have to go with a couple of % silver mix - Mundorf do an on free mix with 3.8%.

Sorry for the slow reply, I didn't get a notification of your message. I used standard multicore solder 60/40. I have always stuck with lead solder as the joints seem to last exceptionally well. I've not had any trouble with the amps so far and I doubt I ever will. It would have been lead solder back in the day so I don't see an issue.

 

[NickK]

Sorry for the slow reply, I didn't get a notification of your message. I used standard multicore solder 60/40. I have always stuck with lead solder as the joints seem to last exceptionally well. I've not had any trouble with the amps so far and I doubt I ever will. It would have been lead solder back in the day so I don't see an issue.

I’ve ummed and erred over this. In the end I found some old unleaded silver rosin that seems to work well on the 25W iron. So I may continue down that route.
In the past I didn’t like it with a 80W gas iron as it seemed not to work well.