Jump to content

Little Sweetie Forum amplifier project


henry4841
 Share

Recommended Posts

The basics from the Mullard 3 watt amp is what I was trying to get people to mod the Sweetie amp to;

 

Run the first stage in starvation mode.

 

DC couple the two stages by increasing the 6Y6 bias resistor.

 

Use the 6Y6 cathode voltage to power the screen of the 6SJ7

 

All can be easily done if one wanted to try different schemes of the Sweetie's setup. I can work out the exact values and voltages for the changeover.

 

As for output tubes, why not something more powerful like a 6BG6GA which is cheap and plentiful but also rated for 20 watts. Mind you that rating is for horizontal deflection duty and for audio applications is more a 30 watt tube like the 6L6GC. I use them a lot and they are fantastic tubes, plus the plate top cap looks neato.

  • Like 1
Link to comment
Share on other sites

If one is happy with a few watts there is no better tube in triode mode than the Russian 4P1L.

 

It is more linear than actual real triodes like the 45 or 2A3.

 

In open loop (with no feedback) it will outperform EL84, 6V6, 6Y6 etc..  when strapped as triodes in both performance (low distortion) and sound quality.

 

I have built so many amplifiers I cannot even keep track of them. I have designed many amplifiers and heard many many more. From this experience I am here to say the 4P1L is the best triode strapped tube out there, it just sounds heavenly. It's easy to drive so driver performance plays a role in the performance also. Magical tube if there ever was one. Parallel a pair if one wants more than 3 watts.

 

I haven't experimented with push pull designs with the 4P1L because it sounds so lovely in single ended mode but it's on the eventual to do list.

 

I sent Nick some to try, hoping he likes them as much as I do.

Link to comment
Share on other sites

6 minutes ago, captainbeefheart said:

The basics from the Mullard 3 watt amp is what I was trying to get people to mod the Sweetie amp to;

 

Run the first stage in starvation mode.

 

DC couple the two stages by increasing the 6Y6 bias resistor.

 

Use the 6Y6 cathode voltage to power the screen of the 6SJ7

 

All can be easily done if one wanted to try different schemes of the Sweetie's setup. I can work out the exact values and voltages for the changeover.

 

As for output tubes, why not something more powerful like a 6BG6GA which is cheap and plentiful but also rated for 20 watts. Mind you that rating is for horizontal deflection duty and for audio applications is more a 30 watt tube like the 6L6GC. I use them a lot and they are fantastic tubes, plus the plate top cap looks neato.

I think those suggestions would make an excellent DIY amp. It would make the Zen sound like a function generator... 

  • Like 1
Link to comment
Share on other sites

11 minutes ago, captainbeefheart said:

The basics from the Mullard 3 watt amp is what I was trying to get people to mod the Sweetie amp to;

 

Run the first stage in starvation mode.

 

DC couple the two stages by increasing the 6Y6 bias resistor.

 

Use the 6Y6 cathode voltage to power the screen of the 6SJ7

 

All can be easily done if one wanted to try different schemes of the Sweetie's setup. I can work out the exact values and voltages for the changeover.

 

As for output tubes, why not something more powerful like a 6BG6GA which is cheap and plentiful but also rated for 20 watts. Mind you that rating is for horizontal deflection duty and for audio applications is more a 30 watt tube like the 6L6GC. I use them a lot and they are fantastic tubes, plus the plate top cap looks neato.

If anyone wants the PDF of the Mullard "Circuits for Audio Amplifiers" here it is. It is a trove of DIY building knowledge. This book was geared toward Mullard getting people attracted to their tubes and encouraged DIY.

 

https://archive.org/details/19400164MullardCircuitsForAudioAmplifiers

  • Like 2
  • Thanks 1
Link to comment
Share on other sites

I was thinking more about the Mullard 3-3 amp and the sweetie, the output tube would have to be pentode strapped to reduce bias voltage down low enough to DC couple the stages. 30v bias is just too much to put at the screen of the 6SJ7 for starved operation. Ideally, we would want a bias voltage of 10-15v. With the increased bias resistor set the cathode for 30v, and the plate of the 6SJ7 can be put at 20-15v for proper bias of 10-15v.

 

With too much voltage on the 6SJ7 screen it pulls too much current for starved operation and it won't bias up properly for grid leak bias.

 

With the 6Y6 triode strapped and a bias of 30v, it would need to be about doubled to 60v with increased bias resistor. The plate would then be run at 30v giving us the -30v bias from grid to cathode. I just think 60v would be too much on the screen for the 6SJ7 and would increase current too much for starved operation. 

 

I'm not 100% positive, it's just preliminary thinking. I would need to breadboard and play around with it to see how well it works. But ideally it would be better to have an output tube with a much lower bias voltage, something in the area of 10-15v.

Link to comment
Share on other sites

Actually the triode strapped 4P1L could ideally have a low enough bias voltage to directly couple them like the Mullard 3-3 circuit.

 

That actually might make one amazing SET amplifier right there. Instead of the Pentode output stage and 20db of feedback we can run the 4P1L as a triode and still have enough gain for some negative feedback. Since the 4P1L is so linear as a triode we won't need much feedback but it will help with damping, bandwidth, distortion and noise.

Link to comment
Share on other sites

Good thinking you guys, This is what this thread is all about. But power numbers with our speakers is just not necessary. I measured the power I was using last night at what I call a reasonable level, my GF would say too loud, and on peaks I was measuring with my AC meter at the speaker terminals at .200Vs. For those not electronically inclined 2.84V at 1 meter is 1 watt is the way speaker efficiency is measured.  Doing the math using Ohms law I came up with practically nothing in watts being used. I do have big horns. Now 1 watt is not going to shake walls but I could hear my system at what I was measuring in another room with the door shut. It is all the power our great, great Grandpa had in his mono console radio. Using a sub with it makes this Sweetie needing nothing more in the way of power in my book. 

  • Like 1
Link to comment
Share on other sites

12 minutes ago, Curious_George said:

Such as the EL84. Any brand of EL84 wold make a great amp in my opinion unless you just got a hold of a real dawg.

 

Nope, I was wrong and you are correct. I just looked at some curves for triode operation of the 6BQ5 and it shouldn't change bias/drive voltage all that much.

 

Some tubes have very low drive voltage in pentode mode and then require much more for triode mode but that's not the case here.

 

We also have 6AQ5 options as well.

Link to comment
Share on other sites

6 minutes ago, henry4841 said:

Good thinking you guys, This is what this thread is all about. But power numbers with our speakers is just not necessary. I measured the power I was using last night at what I call a reasonable level, my GF would say too loud, and on peaks I was measuring with my AC meter at the speaker terminals at .200Vs. For those not electronically inclined 2.84V at 1 meter is 1 watt is the way speaker efficiency is measured.  Doing the math using Ohms law I came up with practically nothing in watts being used. I do have big horns. Now 1 watt is not going to shake walls but I could hear my system at what I was measuring in another room with the door shut. It is all the power our great, great Grandpa had in his mono console radio. Using a sub with it makes this Sweetie needing nothing more in the way of power in my book. 

I agree to a point Henry, but when I listen to my ported Heresy's, I need a bit more power and one pair does not have a sub in my particular listening area and more power is needed to get some bottom end that I like. 

  • Like 2
Link to comment
Share on other sites

4 minutes ago, Curious_George said:

I agree to a point Henry, but when I listen to my ported Heresy's, I need a bit more power and one pair does not have a sub in my particular listening area and more power is needed to get some bottom end that I like. 

Any 5 watt and under tube amp is going to benefit adding a sub. That is if you are a deep, strong bass lover. Me, not so much. It is lovely vocals and midrange that I am more concerned with and this is where the Sweetie shines. Nothing I have built is any better there than this amp. Many are almost as good and possible equal but at much more expense. 

 

I am enjoying the tube amp discussions we are having but if you get pass high school level electronics you are not talking to me. 🙂

 

This is ya'lls thread and not mine so any discussion is fine with me here. Have all the fun on this thread as you possible can. This is the way threads are supposed to be. 

Link to comment
Share on other sites

I agree, just about anything drives my La Scala's but my Heresy's, not the case.

 

5 watts is close to being acceptable if you don't want to rock out but to work well with all heritage series speakers I'd shoot for around 10 watts output so everyone can get in on the fun and not just the horn guys.

 

I did a sim of the EL84 triode strapped and 6SJ7, pretty much dropped right into the sweetie with the exception of changing the bias resistor to 270 ohms and increasing the plate supply to get 250v across the EL84.

 

2 watts output at 4.5% THD

 

But it only takes 170mV peak to drive to full output so we have plenty of gain to add feedback which will only make the amp better anyway.

Link to comment
Share on other sites

4 hours ago, henry4841 said:

It is being hard for me to sell this Sweetie after I have finished it. Like one of your kids leaving for college.

I think I can understand. I have a couple of motorcycles for sale that I put together. It is HARD to let go. I hope it helps to know your efforts are appreciated.

  • Like 1
Link to comment
Share on other sites

7 minutes ago, captainbeefheart said:

I agree, just about anything drives my La Scala's but my Heresy's, not the case.

 

5 watts is close to being acceptable if you don't want to rock out but to work well with all heritage series speakers I'd shoot for around 10 watts output so everyone can get in on the fun and not just the horn guys.

 

I did a sim of the EL84 triode strapped and 6SJ7, pretty much dropped right into the sweetie with the exception of changing the bias resistor to 270 ohms and increasing the plate supply to get 250v across the EL84.

 

2 watts output at 4.5% THD

 

But it only takes 170mV peak to drive to full output so we have plenty of gain to add feedback which will only make the amp better anyway.

Just for fun yesterday while testing the amp on some 85db shop speakers, plenty loud for me in small room, I decided to just open the amp wide open and see how it sound. I know it had to be clipping something awful but I could listen to it. Possible just me and my crappy hearing or that tube amps clip so pleasantly. 

Link to comment
Share on other sites

6 minutes ago, henry4841 said:

I am loving the interest this thread is having. Keep it going guys. Looking at the date I started this build up top it took me 20 days to build this amp. That is fast for me these days. A good talented young man could do it in 2 days at most. 

After all the holes are drilled in the chassis, that is when the fun starts.

  • Like 1
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.

 Share

×
×
  • Create New...