DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently December 4th, 2020, 7:25 pm

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 54 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6  Next
Author Message
PostPosted: May 11th, 2020, 12:48 pm 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 491
Roscoe,

I guess that is an option, but it is a single 55V tap off the main HV winding and have been using a 1/2 wave rectifier. I ran across a better solution, though, looking at a Keroes article on a Williamson amp using 6550 output tube, he used a 6.3V 1A/117VAC transformer backwards off the heater winding. Actually, that is a pretty good idea, since the heaters would tend to stabilize the voltage. I have a pair of 5 or 10VA 7V toroids that are really small and can be added powered right off the KT88 heater. That should give me around 105 VAC or so, and halfwave rectified to average DC of around 95V. Just adjust the filter network to drop a bit of voltage (I have a 150H/8mA choke that should do very well in the middle of a PI filter), and I should be it the right place.

David


Top
 Profile  
 
PostPosted: May 11th, 2020, 1:13 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 277
I didn't realize you were using KT88s. I would definitely check the feedback level when you get it up and running again. Their gain is quite a bit lower than the EL34's. You might make up for that with the ECC99, but it's worth checking. It's easy to do this for this amp, assuming Hafler maintained the standard 20dB feedback. Remove the feedback loop from the output transformer and ground it, then apply a signal to the input until you get 5 volts output at the 16 ohm tap with a 16 ohm load resistor. Then reconnect the feedback loop and apply the same input voltage. You should get .5 volts at the output. If less, then there's too much feedback and you'll need to raise the feedback resistor a bit. If you get more, the feedback is reduced and you can go with that or drop the feedback resistor accordingly.


Top
 Profile  
 
PostPosted: May 11th, 2020, 1:20 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 277
David, have you seen this article?

https://dalmura.com.au/static/High%20po ... 201955.pdf

Check his bias voltage arrangement, might be helpful. Also, I see he uses the same feedback arrangement as the EL34 version, so that should be okay. With the change to the ECC99 it still might be worth checking the feedback level to make sure it hasn't increased.


Top
 Profile  
 
PostPosted: May 11th, 2020, 2:08 pm 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 491
Grover,

I do not understand how the divider network actually works to achieve the bias voltage needed. I would appear to me to have too high of a voltage output. The 0.05uF cap input into the 47K resistor shunt looks to me like a highpass filter network, and analyzing the values results in a pole of 67Hz. That would attenuate the waveform (perhaps by around 50%). So instead of a 400VAC, it would be around 200VAC into the half wave rectifier. A selenium rectifier stack with a PIV needed for handing 400VAC, 25VPIV per diode, with a bit of safety (say total 600V PIV) would be 24 diodes, 1V drop each. So down to 176VAC peak after the rectifier before any filtering.

I feel a little more secure with adding a transformer.

David


Top
 Profile  
 
PostPosted: May 11th, 2020, 2:18 pm 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 491
Grover,

One thing I wanted to mention. The ECC99 is fairly equivalent in gain to the 6SN7, it has a mu of 22 vs 20 for the 6SN7. That is pretty close and should not be a problem. The transconductance is considerably higher, and the current sink in the tail might help achieve the full gain out of the differential amp, so there may be an increase overall. Perhaps that is what you meant. I am picking the ECC99 for drive capability into the KT88 grid. I debated going with 5687, but that would limit tube swapping should I choose to due to the basing.

David


Top
 Profile  
 
PostPosted: May 11th, 2020, 3:05 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 277
David McGown wrote:
Grover,

I do not understand how the divider network actually works to achieve the bias voltage needed. I would appear to me to have too high of a voltage output. The 0.05uF cap input into the 47K resistor shunt looks to me like a highpass filter network, and analyzing the values results in a pole of 67Hz. That would attenuate the waveform (perhaps by around 50%). So instead of a 400VAC, it would be around 200VAC into the half wave rectifier. A selenium rectifier stack with a PIV needed for handing 400VAC, 25VPIV per diode, with a bit of safety (say total 600V PIV) would be 24 diodes, 1V drop each. So down to 176VAC peak after the rectifier before any filtering.

I feel a little more secure with adding a transformer.

David


I agree that the reversed transformer is probably the best solution. Just thought I'd throw it out there. ;-)


Top
 Profile  
 
PostPosted: May 11th, 2020, 3:19 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 277
David McGown wrote:
Grover,

One thing I wanted to mention. The ECC99 is fairly equivalent in gain to the 6SN7, it has a mu of 22 vs 20 for the 6SN7. That is pretty close and should not be a problem. The transconductance is considerably higher, and the current sink in the tail might help achieve the full gain out of the differential amp, so there may be an increase overall. Perhaps that is what you meant. I am picking the ECC99 for drive capability into the KT88 grid. I debated going with 5687, but that would limit tube swapping should I choose to due to the basing.

David


That wasn't what I meant, I was just thinking about the slight increase in gain. But now that you mention it, if the current sink allows for full gain of the ECC99, that really is a big jump--from 12.7 for the 6SN7 drivers to 22. That's going to affect your feedback arrangement significantly, no maybe about it, I think. ;-)

I understand that you want to improve the driver capability but I don't think it's necessary and I'm concerned that you may not hear what it is the original front end imparts--an ease and naturalness to the sound that makes it so lovely. Since you already have one channel built, consider trying the original circuit with a 6CG7 in place of the ECC99 just to see what it sounds like.

ETA: Also, if the overall gain is increased that much, your input sensitivity is going to be so high that I'm not sure it would be practical.

Forgive me, I don't know that much about current sources and current sinks so I'm only guessing here.


Last edited by Grover Gardner on May 11th, 2020, 3:46 pm, edited 1 time in total.

Top
 Profile  
 
PostPosted: May 11th, 2020, 3:41 pm 
Offline

Joined: February 28th, 2013, 1:19 pm
Posts: 491
Grover,

It is easy enough to rewire should I need to. All I need to change is to replace the 20K plate resistors with the original 47K and the current sink with a 470ohm resistor and use 12FQ7s. Either that, or just take out the current sink and replace it with a resistor, and keep the ECC99 in circuit. Should have less impact on gain.

Things to try out.

David


Top
 Profile  
 
PostPosted: May 11th, 2020, 3:51 pm 
Offline

Joined: June 4th, 2013, 2:39 pm
Posts: 277
Okay. A dded a note to my previous comment about input sensitivity, something to consider as well.

Speaking of the 470 ohm cathode resistor, one thing you can do is raise that to 680 ohms. This is a common "blessed" tweak to the Williamson that increases the voltage swing of the driver stage and reduces distortion.


Top
 Profile  
 
PostPosted: May 11th, 2020, 3:53 pm 
Offline

Joined: April 22nd, 2013, 12:58 pm
Posts: 213
Isn't that one of the two resistors setting the feedback ratio?

Grover Gardner wrote:
Okay. A dded a note to my previous comment about input sensitivity, something to consider as well.

Speaking of the 470 ohm cathode resistor, one thing you can do is raise that to 680 ohms. This is a common "blessed" tweak to the Williamson that increases the voltage swing of the driver stage and reduces distortion.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 54 posts ]  Go to page Previous  1, 2, 3, 4, 5, 6  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 7 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group