Jump to content

SE 300B Custom Amp Build


Curious_George

Recommended Posts

1 hour ago, captainbeefheart said:

 

Sounds like low frequency instability where rolling the bass off with a smaller cathode bypass capacitor lowers the gain enough of the low frequencies to keep it flat.


After this amp build, I’m going to experiment with another amp and ensure I have “low frequency boost” and increase / decrease power supply capacitance and see how this affects things. 

  • Like 1
Link to comment
Share on other sites

5 hours ago, Curious_George said:


After this amp build, I’m going to experiment with another amp and ensure I have “low frequency boost” and increase / decrease power supply capacitance and see how this affects things. 

 

I remember an old console amp I salvaged a couple years back that was a single ended EL84 output and it had a strange bump in the bass region even after the tone control and RIAA eq were removed. All that was left was 6EU7 preamp stage (no cathode bypass cap) and the EL84 output which was bypassed with a 30uF capacitor placing the -3db around 50Hz. The coupling stage -3db was around 30Hz. It had global feedback, I don't remember how much but there was a filter network at the feedback junction on the 6EU7 cathode which looked to boost the low end so the bump on the output made sense to me at the time but I wish I analyzed it a little more. I gave it to a friend that wanted to try tubes but had no money.

  • Like 1
Link to comment
Share on other sites

20 minutes ago, captainbeefheart said:

Which tube rectifier are you going with?

 

I mostly use directly heated types and use the 5v winding center tap to feed B+ to the first filter.

 

GZ34/5AR4 are overrated


i usually use a JJ GZ34/5AR4 or USA NOS 5V4GA. The JJ seem to be decent quality for the price. They were $15 recently, now they are $20. 
 

I also like the 5Z4, but it is metal, so not too good looking, if that is a concern. 
 

 

Link to comment
Share on other sites

7 hours ago, Curious_George said:

Plus, I like the slow warm up of the indirectly heated rectifiers.

 

This does allow one to use lower voltage rated power supply caps, basically rated to working voltages or slightly above. This simplifies the power supply because most common filter value electrolytics are under 500v. Tons of 350v even 450v choices, slims down quite a bit around 500v. With a 400v B+ and using an indirectly heated rectifier you can get away with one 450v cap, using SS rectifiers or directly heated tubes you end needing at least a 600v supply.

 

BUT if the amp is ever turned on without power tubes you will get these voltages even with a 5AR4. I had universal power supply boards made up that can be configured a number of ways and it is setup for series caps with balance resistors. I use two 350v caps in series for most amps giving a 700v rating for worst case scenario no load condition. 

 

I can see why the 5AR4/GZ43 is more popular for Class AB amps as they have lower impedance vs the more common indirectly heated rectifier tubes like 5U4 or 5R4Y but I build Class A amps and so load stays consistent enough where you will not get any rectifier induced voltage drop compression with a higher impedance rectifier.

 

I scored a ton of NOS NIB 5931 rectifier tubes a long time ago and have been using them pretty much exclusively because they just won't die, they are very rugged and robust, they seem to handle more capacitance without shortening their life. Two of my amps I have 100uF as the reservoir capacitor and it's been going on almost 20 years with the same 5931 in them.

Link to comment
Share on other sites

4 hours ago, captainbeefheart said:

 

This does allow one to use lower voltage rated power supply caps, basically rated to working voltages or slightly above. This simplifies the power supply because most common filter value electrolytics are under 500v. Tons of 350v even 450v choices, slims down quite a bit around 500v. With a 400v B+ and using an indirectly heated rectifier you can get away with one 450v cap, using SS rectifiers or directly heated tubes you end needing at least a 600v supply.


Exactly. At 500V your choices are limited and price goes up fast. 
 

 

Link to comment
Share on other sites

19 hours ago, Curious_George said:

In order to keep the layout clean

 

Tisk tisk tisk....  We've heard/had the  conversation about black verses red verses yellow zip ties.....and it looks like you chose black.

 

Otherwise, to my (very ignorant) eye, looks great!!

 

 

Link to comment
Share on other sites

23 minutes ago, Coytee said:

 

Tisk tisk tisk....  We've heard/had the  conversation about black verses red verses yellow zip ties.....and it looks like you chose black.

 

Otherwise, to my (very ignorant) eye, looks great!!

 

 


I’m a firm believer in the Coriolis Effect making a huge difference in sound. Obviously, the Northern Hemisphere sounds better than the Southern Hemisphere.

  • Like 1
  • Haha 1
Link to comment
Share on other sites

7 hours ago, Curious_George said:


Exactly. At 500V your choices are limited and price goes up fast.

 

Which is why a while back when radial electrolytics became the king of the hill where all the best caps are radial and axial took a back seat I made a universal board that takes advantage of increasing improvement in radial capacitors where they can be wired in series for high voltage ratings. I am surprised to see so many builders still using axial electrolytic capacitors, the choices are slim, ratings not so great, and the price is more than double!!

 

Another wonderful rectifier is NOS Russian 5C3S which is similar to the 5U4G, the filament of the 5C3S looks amazing when powered up, much different vs the 5U4G.

Link to comment
Share on other sites

On 12/4/2021 at 8:41 PM, Curious_George said:

Plus, I like the slow warm up of the indirectly heated rectifiers. I have 5R4, 5U4 and 5Y3GT too and use them as well. 

 

My 2A3 amps used GZ37s... they've got a lot of hours on them.

Link to comment
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.
Note: Your post will require moderator approval before it will be visible.

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.

×
×
  • Create New...