SLO ef80 to 40 watt with loop

[Fictitiousfreedom]

Hello amazingly knowledgeable amp friends!

Around 6 months ago i built the Slo mikro ef80 amp from Robs site. I almost immediately regretted choosing that version, even before i finished it. I could have built the 40watt for almost the same price but i finished it anyway. I tried just about everything and never could get it to sound good. Recently i gathered the parts to upgrade it to the 40watt from robs site. I just finished drilling out the old chassis, luckily i used one that was oversized for the mikro. Before i commit fully to this i need to figure something out. I really would prefer to add the fx loop back into the circuit, which i am confident that with a little more research and time i can work out myself.

My question is should i add a passive loop and use it as i plan to use all of my other amps with a dumbleator or should i add the extra tube and go that route? I have all the parts for a dumblator, the buffered version i found on this site. The main problem i have with this approach is that i had attempted to add a passive loop when it was the ef80 version and could not get it to work. I probably just lacked the knowledge i needed but im not sure. An Fx loop is basically mandatory for me and i know it is one of the advantages of the original slo100. If i need to add another tube socket and a few components than its no problem. Another thing i really want on this amp is a slave in/out, i normally use the fx loop for this but id prefer to have seperate jacks if that is a better option. Im curious as to what the best way to do this is and also which approach yields a better slave in/out.

Thanks for your time guys!

[Ten Over]

Do you want a transparent loop or a loop that plays a part in the sound of the amp like the SLO 100?

[Fictitiousfreedom]

Do you want a transparent loop or a loop that plays a part in the sound of the amp like the SLO 100?

Thats a great question. I dont care too much as long as it sounds good. I did think about getting the proper iron and doing it as a slo50 clone but decided to save some time and money by doing robs, especially since it required minimal changes to the layout. As long as it serves its purpose and doesn't cause any signal degradation than ill be happy. I dont care about it being like the original. When i say signal degradation i mean large losses of signal level, increased noise, and impedance mismatches ect..

Thanks for your time man.

[Lynxtrap]

If you use a passive loop you may need to place it after the master volume. If you put it where the active loop is in a SLO100, the signal will be way too hot.

[Ten Over]

Is this anything like what you are talking about?

[Fictitiousfreedom]

Is this anything like what you are talking about?

Yes! That is exactly what im talking about. That was not expected and very much so appreciated, thank you! That should give me a nice head start.

I finished the conversion, not counting the planning, drawing up everything, and ordering parts i had it ready and biased in about 3 days. It is insane how much better it sounds. Its got otherworldly bottom end and the overdrive is so thick i think that i lost a shoe.
I wanted to complete it as close to the design as i could at first for reasons everyone is familiar with but now i need an fx loop bad, its essential for me. Ill start possibly with a passive loop just to see how it works but i absolutely have to learn how to add active loops to my builds. Im making a dumbleator but i still need to understand what it entails.

Then we have the slave out/in, it seems you wouldn't need a buffer or anything specifically active because its already the correct level leaving the amp, am i right in this assumption? I have no idea and its a little tough to find a straight answer, just like most things. I want to build a high power stand alone power amp that is specifically for running rack mounted preamps. That is why i want to know how to add the slave jacks. Seems like it would be much cheaper, more versatile, and honestly easier to do separate preamps and i have seen a few people doing it, but not many. I assume there is a reason that i am missing due to my limited experience but That is a can of worms for another time.

If you use a passive loop you may need to place it after the master volume. If you put it where the active loop is in a SLO100, the signal will be way too hot.

I think that may be what my problem was, im going to try it again right now so ill let you know if it works. Thanks man!

For anyone reading this in the future that is doing this same conversion i have some insight. I personally would convert it into one of robs other mikro amps and if you want a Slo start from scratch. It would be only a little more costly to do a scratch build and something i learned through this process is that its much easier to start from scratch than it is to convert an existing build. It will be much cleaner and more polished that way. My chassis looks like swiss cheese, i had to create sheet metal covers for the really big holes like the cap can hole, and my filter cap board had to be raised up so high that the caps are almost even with the bottom of the chassis. That being said it is so much better now, i hated it before. So it is worth it just some good for thought.

[Ten Over]

[quote=Fictitiousfreedom post_id=455044 time=1691019971 user_id=26418
Then we have the slave out/in, it seems you wouldn't need a buffer or anything specifically active because its already the correct level leaving the amp, am i right in this assumption? I have no idea and its a little tough to find a straight answer, just like most things.
[/quote]

No. The slave out signal level needs to be attenuated to line level and the slave out output impedance needs to be low.

Power amps are designed for line level voltages. Here are some examples of line level voltages:

Consumer Audio Level = -10dBv = 0.3162Vrms = 0.447Vp
Professional Audio Level = +4dBu = 1.2277Vrms = 1.736Vp
0dBV = 1.0Vrms = 1.414Vp
0dbu = 0.7746Vrms = 1.095Vp

The voltage at the treble wiper can be on the order of 90Vp (note that I moved the tone stack back up to the cathode on the cathode follower where it should be).

The output impedance of the slave out forms a low pass RC filter with the capacitance of the patch cable. If the cable is long enough and the output impedance is high enough, then noticeable high frequency loss will occur. Can't do anything about the cable capacitance, but the output impedance can be made low by using a buffer.

Ask me a straight question and I will give you a straight answer.

[Lynxtrap]

I'm not sure why you need to have separate jacks for slave in/out? If you want effects on the sound going to an external power amp, the signal from the effects return needs to go via the slave out jack.

[Fictitiousfreedom]

[quote=Fictitiousfreedom post_id=455044 time=1691019971 user_id=26418
Then we have the slave out/in, it seems you wouldn't need a buffer or anything specifically active because its already the correct level leaving the amp, am i right in this assumption? I have no idea and its a little tough to find a straight answer, just like most things.

No. The slave out signal level needs to be attenuated to line level and the slave out output impedance needs to be low.

Power amps are designed for line level voltages. Here are some examples of line level voltages:

Consumer Audio Level = -10dBv = 0.3162Vrms = 0.447Vp
Professional Audio Level = +4dBu = 1.2277Vrms = 1.736Vp
0dBV = 1.0Vrms = 1.414Vp
0dbu = 0.7746Vrms = 1.095Vp

The voltage at the treble wiper can be on the order of 90Vp (note that I moved the tone stack back up to the cathode on the cathode follower where it should be).

The output impedance of the slave out forms a low pass RC filter with the capacitance of the patch cable. If the cable is long enough and the output impedance is high enough, then noticeable high frequency loss will occur. Can't do anything about the cable capacitance, but the output impedance can be made low by using a buffer.

Ask me a straight question and I will give you a straight answer.
[/quote]


I still have a huge amount to learn, and always will i hope. Thanks for your time man, i am very much aware of how valuable a straight answer is, especially in the world of today. Its still safe to use a passive loop as a slave out/in right, Just not ideal as far as level and impedance is concerned?

I'm not sure why you need to have separate jacks for slave in/out? If you want effects on the sound going to an external power amp, the signal from the effects return needs to go via the slave out jack.

Yeah, i understand now. It wasn't so much the effects i was wanting the slave jacks for. What i was thinking was that the ideal requirements for an effects loop probably were different than that for the slave jacks if thats not the case than there is probably no need for them but im still not 100% sure if that is the case or not.

[Fictitiousfreedom]

Also forgot to add, the passive loop does work if placed after the MV. I ran some time based effects and It sounds pretty good. I also ran the preamp out into my Boogie MkV25 and it also sounded pretty good although I have a real problem with knowing something could be better, which is one of the many obsessive qualities that led me to amp building lol
Im building a D’lator right now and it should be done within the next few days. Ill see how it sounds and even then i am going to try and add the active loop back. Ill keep this thread updated with my progress for anyone in the future.

[Ten Over]

Its still safe to use a passive loop as a slave out/in right, Just not ideal as far as level and impedance is concerned?

Yes.

[Ten Over]

What i was thinking was that the ideal requirements for an effects loop probably were different than that for the slave jacks if thats not the case than there is probably no need for them but im still not 100% sure if that is the case or not.

My effects loop above can function as either an effects loop or slave out/slave in. My slave out above would not be appropriate for stomp boxes that don't have an input level control. It is intended for line level devices that do have input level controls. Other than that, the two loops have the same ideal requirements.

[Ten Over]

Also forgot to add, the passive loop does work if placed after the MV. I ran some time based effects and It sounds pretty good. I also ran the preamp out into my Boogie MkV25 and it also sounded pretty good although I have a real problem with knowing something could be better, which is one of the many obsessive qualities that led me to amp building lol
Im building a D’lator right now and it should be done within the next few days. Ill see how it sounds and even then i am going to try and add the active loop back. Ill keep this thread updated with my progress for anyone in the future.

Here is an effects look with no bypass switch for your consideration:

[Fictitiousfreedom]

Also forgot to add, the passive loop does work if placed after the MV. I ran some time based effects and It sounds pretty good. I also ran the preamp out into my Boogie MkV25 and it also sounded pretty good although I have a real problem with knowing something could be better, which is one of the many obsessive qualities that led me to amp building lol
Im building a D’lator right now and it should be done within the next few days. Ill see how it sounds and even then i am going to try and add the active loop back. Ill keep this thread updated with my progress for anyone in the future.

Here is an effects look with no bypass switch for your consideration:

Thank you! I finished my dumbleator and it sounds great with my Dumble style amp but horrible with the Slo for some reason, it makes it extremely dark and muffled and then the bright switches make it nasty and overly bright. I will absolutely try your circuit in the next week or so. Ill keep you updated on how it goes. The 40 watt slo i built has half of an 12ax7 that is unused. I wish i could find something to use it for, it just seems like a waste not to.

Again, thank you for your generosity it is very much appreciated!

[Fictitiousfreedom]

I posted a long reply last night but it seems to have vanished.

Anyway, i got a little adventurous and went for your schematic. I built it exactly as your drawing other than the change from a 1M master volume to the 100K, i didnt have an appropriate 100k pot on hand, so i left the 1M in just to see if it worked. It did totally work!

Let me say first, the conversion from a cathode biased 2w “mikro amp” with a toroidal power xformer, a cap can for filtering, and two noval output tubes to a fixed bias 40w amp with a laydown power x former, large 80uF filter caps, huge output iron, huge choke, and two octal output tube sockets was traumatic to say the least. Adding another tube and small board was not the best idea but its already a pretty gnarly amp so i said why not.

I was honestly shocked it worked on the first go. That being said, there are some things that need tweaking for sure. The send pot kind of works like a MV to an extent, it wont bring the entire amps level any higher than the actual MV is set but if you turn it down it does cut almost all of the volume. There is a very nasty thin sounding distortion that is still audible. The strange thing is that as you turn it up it sounds as though two signals that are out of phase are mixing together. There is always a strange comb filtering type overtone that becomes less obvious as i turn the send pot up but its still there. Also, my fx that i put in the loop sound weird, their level is way lower than it should be and it almost sounds as though there is a small delay or latency and they honestly sound strange. There is also a lot more noise but i expected that and can address it after i get it working.

Is there some type of tests i can run to diagnose something like this, i have a signal generator, scope, multimeter, dummy load. And most of the common tools? I can take pictures tonight and clean up my layout, and even make some sound clips if itll help, if so i can post them soon.

Thanks Again man, i feel very fortunate to have access to people like yourself!