Maximum clean volume SE octal 6L6 amp?

[Jerry garrcia]

I’m planning a build of a small sized amp that will be placed in a tight cabinet.
- 12” speaker (15 ohms),
- SS rectifier,
- 125ESE OT and will have to use the 5K primary for a 16 ohm speaker,
- PT from a reel to reel tape recorder that puts out around 265 VAC (so probably around 360-370 VDC).
- Single 6L6 power tube
- 6SJ7 preamp tube with @printer2 bias switch between grid leak/cathode bias.
- NFB and boost switch (when in cathode biased mode).
- Princeton tone stack
- an extra 47uF filter stage before B+ node.

Initially I wanted to go PP but it will not fit very well in the closed cabinet.

The goal is as usual to get maximum cleans out of the 6SJ7 but now also maximum volume out there fore the 6L6 (have some EL34’s and a KT 120 (but a bit to big).
Read somewhere that a single 6SJ7 will have problems to fully drive a 6L6 to maximum output (probably 350V on the plate).
1) Is that correct?
2) If so should I add an extra gain stage with a tube that can drive the 6L6?
2) If so which tube should I use to do so but still keep the maximum cleans? I have some 6SQ7’s, 6SK7’s (could triode wire it) and 6SN/SL7’s (don’t care to much to use them since I hate to only use half of a tube).

Any inputs appreciated.

 

[James Knox]

I like the 6SL7 dual triode on my octal preamps.

 

[2L man]

To get everything out of 6L6GC to 5K SE OT the B+ voltage should be higher, at least 420VDC. Its loadline came to right. B+ 370VDC loadline came to left.

For SE loadlines I set Headroom so that there come about same 2nd and 3rd harmonics because it is one meaningful variable. 5K OT is not linear so there comes more 2nd harmonics and then also 3rd comes higher. For 2,5k OT there comes less distortion. But when bias current comes higher the SE OT comes heavier and more expensive so for guitar amp it is more economical to use higher voltage and lower current OT when distortion is not too negative feature.

 

[Jerry garrcia]

To get everything out of 6L6GC to 5K SE OT the B+ voltage should be higher, at least 420VDC. Its loadline came to right. B+ 370VDC loadline came to left.

For SE loadlines I set Headroom so that there come about same 2nd and 3rd harmonics because it is one meaningful variable. 5K OT is not linear so there comes more 2nd harmonics and then also 3rd comes higher. For 2,5k OT there comes less distortion. But when bias current comes higher the SE OT comes heavier and more expensive so for guitar amp it is more economical to use higher voltage and lower current OT when distortion is not too negative feature.

Once again thanks! Been reading up on the SE 6L6’s and a lot of different opinions. And what the optimal primary impedance of the OT should be depending on plate Voltages. On the data sheets no more info above 350V so I thought that I’ll get a little higher than the 350V to handle a primary impedance of approximately 5K (it will be that for a 16ohm load but I’ll be having a 15ohm).

Thanks for an objective “answer”. I have a couple of old radio PT that will be able to deliver the 450V plate voltage and the sufficient current on the HT (>80mA) and 6.3V windings (2A). Unfortunately they are old and ugly but checks out fine. Might have to find a PT cover for it so nobody gets chocked if they are fiddling around in the amp.

Will a single 6SJ7 be able to fully drive an 6L6 to the maximum amount of volume with the maximum amount of headroom? Is there a way to find out with the knowledge of these larger brains in this forum?
One reason that I ask is that people has been saying that one of the reasons that Fender changed the preamp tube in the 5C1 amp to a 12ax7 was that the 6SJ7 didn’t deliver the sufficient amount of gain compared to the double triode. I might, as usual, mix the concept of gain with volume out.
I’m looking for maximum volume cleans out of the 6SJ7 tone and a 6L6.
If another tube is needed for that I will be able to squeeze it in.

Another question, will need to attack the subject of load lines. Any tips for some good tutorials?

 

[Jerry garrcia]

I like the 6SL7 dual triode on my octal preamps.

Me too in some amps but in this case the 6SJ7 will be the preamp tube. Loves the sound of it and have a bunch that needs to be used.
Will save the 6SL7 I have for a two channel PP amp. Thanks for the input

 

[echuta13]

I haven't built an 6SJ7 preamp yet, but at some point I really have to (I have a bunch of the 5693 reds on the shelf somewhere)! I have built a pentode (EF86) single stage SE amp before and wasn't ultimately satisfied with it. It just didn't have enough "kick" to really drive the output tube (6V6GT). I suspect if you used an EL84 it may work out, but you're probably going to run into the same issue (even more so with the 6L6). With all of that being said, the amp did sound great, just not enough oomph! I wouldn't hesitate to use a 6SJ7 as a single stage pre followed by a paraphase or long-tail PI/PP amp (use a 6SL7 for mo mojo), but in SE you may be a bit dissappointed.

 

[rdjones]

. . . . The goal is as usual to get maximum cleans out of the 6SJ7 but now also maximum volume out there fore the 6L6 (have some EL34’s and a KT 120 (but a bit to big).

Just be sure to wire pin 1 to accomodate the EL34 so you can freely swap between 6L6, etc and EL34s.

Read somewhere that a single 6SJ7 will have problems to fully drive a 6L6 to maximum output (probably 350V on the plate).
1) Is that correct?
2) If so should I add an extra gain stage with a tube that can drive the 6L6?
2) If so which tube should I use to do so but still keep the maximum cleans? I have some 6SQ7’s, 6SK7’s (could triode wire it) and 6SN/SL7’s (don’t care to much to use them since I hate to only use half of a tube).

Any inputs appreciated.

6J5 is an octal triode that is often seen as a driver stage. 6J5 is equal to ½ of 6SN7. 6C5 can be used as a substitute for 6J5.

 

[Jerry garrcia]

Just be sure to wire pin 1 to accomodate the EL34 so you can freely swap between 6L6, etc and EL34s.

6J5 is an octal triode that is often seen as a driver stage. 6J5 is equal to ½ of 6SN7. 6C5 can be used as a substitute for 6J5.

Once again I’m stunned by the willingness to share knowledge in this forum. Thanks. Will wire so I can use both power tubes.
I think I have a couple of 6j5’s. If need I’ll use one of them. But is there a way to be kind of certain without having to breadboarding the whole thing? Theoretical calculations or experience?

 

[2L man]

Another question, will need to attack the subject of load lines. Any tips for some good tutorials?

Giuseppe Amato keep Universal Loadline Calculator and to support him buying his book does it. Visit ULLC and there is its cover. I bought it electric and for only $5 it is excellent. ULLC is very good because it is fast to use.

Only what might confuse is that some pentodes have few positive drid lines as well when most pentodes the highest is zero but just moving cursor over lines show the voltage.

 

[NTC]

2) If so which tube should I use to do so but still keep the maximum cleans? I have some 6SQ7’s, 6SK7’s (could triode wire it) and 6SN/SL7’s (don’t care to much to use them since I hate to only use half of a tube).

Use a 6SQ7 as the second gain stage. It has a gain of 100 - you should have PLENTY of gain with that and the pentode. You may need a resistor divider to get rid of some of the gain.

 

[Jerry garrcia]

Giuseppe Amato keep Universal Loadline Calculator and to support him buying his book does it. Visit ULLC and there is its cover. I bought it electric and for only $5 it is excellent. ULLC is very good because it is fast to use.

Only what might confuse is that some pentodes have few positive drid lines as well when most pentodes the highest is zero but just moving cursor over lines show the voltage.

Just did

 

[Jerry garrcia]

Use a 6SQ7 as the second gain stage. It has a gain of 100 - you should have PLENTY of gain with that and the pentode. You may need a resistor divider to get rid of some of the gain.

Or maybe a triode wired 6SK7? So many options and so many decisions. Ok. Will start with the schematic.
Good thing that I quit my job this Thursday and will have some time to finish all the fun projects. Got some old cigar boxes and also some nice, cheap Telecaster rip off necks. Might be a productive month

 

[2L man]

You could try OT cathode feedback which is often used on HiFi amps. Basically OT secondary is connected series between power tube cathode resistor and Common. It increase lows and decrease distortion. Gain is low only about 3 or 4db but compensate the loss what Hammond 125DSE and 125ESE cause to 80Hz because of their low inductance ~10H.

 

[Jerry garrcia]

You could try OT cathode feedback which is often used on HiFi amps. Basically OT secondary is connected series between power tube cathode resistor and Common. It increase lows and decrease distortion. Gain is low only about 3 or 4db but compensate the loss what Hammond 125DSE and 125ESE cause to 80Hz because of their low inductance ~10H.

Thank you again. I was looking through my stach of old tubes. Got a few EF86 (not tested yet. Will get my tube tester, DuoKit, this week). If I run that in triode connection couldn’t that be a good driver stage for the 6L6. Non octal but what the h**l. Stupid or good?

 

[Jerry garrcia]

You could try OT cathode feedback which is often used on HiFi amps. Basically OT secondary is connected series between power tube cathode resistor and Common. It increase lows and decrease distortion. Gain is low only about 3 or 4db but compensate the loss what Hammond 125DSE and 125ESE cause to 80Hz because of their low inductance ~10H.

I’m finally free from the hassle of work for a couple of weeks at least. Still a lot to do at home.
Got the Giuseppe Amato book and he describes the OT feedback as well.
So my almost mature idea for a design is as following.
1) Four filter caps and B+ runs from the second.
2) B+1 ~420V
3) 6sj7 preamp
4) 6sn7 CF that also drives a more complex tone stack like the 5F6-A
5) 6L6 with OT cathode feedback
Sounds ok?

Edit. Starting to look at the chassis that will fit. I might not have room for a three pot tone stack. Don’t have any 6J5’s at home so either a 6SK7 (to little gain?), 6SQ7 (to much gain?) or EF86 (non octal). Hard for me to use a 6SN7 since only half of the tube will be used.
Or back again to a two channel amp that utilises the second half of the 6SN7 as a gain stage

 

[radiocaster]

Enough for what?

Recording? Plenty.

Playing over drums/gigging? YMMV

 

[Jerry garrcia]

Enough for what?

Recording? Plenty.

Playing over drums/gigging? YMMV

For gigging but not heavy metal drums. Jazz drums in small clubs. 12” speaker with closed cabinet.
Not used to a SE 6L6 amp. So have no references to compare with. My SE 6SJ7 + one triode 6SC7 and a single 6V6 Can get pretty loud but then it starts to distort at the appropriate volume for my gigging needs. So want louder cleans.
Also planning on wire the power tube socket so it can handle an EL34. Got some KT66 and EL37’s but not enough so I have spare ones. Vast number of good old 6L6’s and EL34’s.

 

[radiocaster]

For gigging but not heavy metal drums. Jazz drums in small clubs. 12” speaker with closed cabinet.
Not used to a SE 6L6 amp. So have no references to compare with. My SE 6SJ7 + one triode 6SC7 and a single 6V6 Can get pretty loud but then it starts to distort at the appropriate volume for my gigging needs. So want louder cleans.
Also planning on wire the power tube socket so it can handle an EL34. Got some KT66 and EL37’s but not enough so I have spare ones. Vast number of good old 6L6’s and EL34’s.

A regular 6L6 puts out more than an EL34. A 5881 less. Also will depend on biasing, preamp, etc.

Jazz with brushes could be o.k.

 

[Jerry garrcia]

A regular 6L6 puts out more than an EL34. A 5881 less. Also will depend on biasing, preamp, etc.

Jazz with brushes could be o.k.

Thanks! Maybe a KT120? Got two for free yesterday

 

[2L man]

Briefly checking it looks Vitaly does not show any Local Cathode Feedback unless he has updated the book?

I thought it would be easy to find more info so I have not bookmarked any pages but it wasn't. Here it is discussed in guitar amps and on post 6 is first "correct" circuit and better drawn on post 28.

Adding NFB to power tube cathode?

Hoffman Amplifiers Tube Amp Forum, Adding NFB to power tube cathode?

el34world.com

CFB using OT secondary in cathode current circuit is not very effective but better than nothing.

Perhaps you should buy Ultralinear OT because it work!

If you don't like amp coming heavy then Mennovanderveen toroid SE OT are lightest and they have UL option. Menno used to stock some 12W SE in Holland but two other OT I have bought came direct from Romanian factory so postage was bit higher and tax increased the price some more. He reply emails fast so you can ask if any are on EU sell?

I have used VDV-2512-SEE (only 955g) in few circuits and test UL as well. UL did not improve "my guitar sound preference" but if low distortion is needed it work very well. I did test EL34 for about 12W, KT66 about 13W and KT88 about 15W and all made very loud sweet sounding amp. So I bought VDV-1324-SEE (only 1750g) and going to drive it using KT150 but have not build it. It should be able to produce even 30W when impedance is set to 2,5k, bias is 160mA and B1 450VDC. It lists only 99 euros but it came from Romania which increased its price but I don't remember how much?

Toroidy.pl OTs are heavy but for guitar frequency range should be able to produce double the rated power. Not to exceed max bias using higher voltage and possibly doubling the impedance ot using parallel tubes is needed and then their weight comes reasonable. Anyone who want a monster SE guitar amp the TTG-6C33CSE 600mA bias is enought for even six parallel KT88

Best buy conventional SE OT should be Tubetown 2k5-5/4-8-16 for only 61 euros if it is good for 160mA bias. That OT is very high on my build order after I finish two builds.