****JGR's 6SL7 preamp proto is done!***

JGR · #1 10-21-2012, 06:27 PM

Some good folks encouraged me to look into the 6SL7 tubes a while back, and I'm glad they did. I just finished the first prototype - it uses the new 2U format which I'm going to stick with going forward for whatever I'm building - it allows me more flexibility with the power supply, as well as the addition of a Jensen balanced out (in addition to the Jensen DI) - the balanced out was another thing some folks recommended that I'm glad I tried. The Jensen balancing tranny is pretty $$, but I think it does add a bit of sonic mojo. I also added a mute switch (there ya go Frank!

) since I had a bit more real estate on the front panel - it also allowed me to use my cool stove knobs which don't fit on the 1U.

Some initial thoughts and impressions....

I can see why folks like these tubes - kind of what I was hoping them to be. They are big and bold, but have a nice softness to the initial attack, and a very musical character to them. The tone is big and weighty, and they seem to impart the "all tube" vibe, even when running SS power.

This circuit follows the current iteration of my original design which has a tweaked gain structure compared to the original five I've built thus far, but I did make a fair amount of changes to the gain structure and EQ based on what I thought would work for the 6SL7 front end, as well as some tweaks to the power section to keep heater-to-cathode voltages in spec. With the lower gain, I voiced it a bit wider since I can get away with it - the EQ is the same as my other builds with familiar shelving bass and treble, and dual inductor driven low/high mids (200Hz, 1.5 kHz). The 6SL7's make it a different beast, but it's not a total departure, and I think folks that generally prefer the 6SL7 would still dig the other design too. Gain range is great - plenty of clean range, and a beautiful overdrive with things cranked. The OD is very useable, and has a nice soft nature compared to the 12ax7's. The gain of the 6SL7's seem about ideal - 12ax7's can be a bit too much, the 12au7's sometimes not quite enough, and the 12at7's are a bit lacking tonally IMO. Plenty of output through the unbalanced and balanced outs, I'm diggin the beefed up power supply that the 2U allows me.

The bad - my NOS Tung Sol is noisy - got some background waterfall, static, and occasional wierd microphonics. I hope TC Tubes takes it back, because it was supposed to test premium. The RCA is great so far, and it should be for the $$$. Other bad is I don't have any spares on hand, so I'm going to order some new Tung Sol reissues - hopefully they will be good. I don't like screwing around with NOS anyway. The only negative I can see to this whole deal is having to sort through tubes which I don't have the patience for. Hopefully I'll have some here mid week so I can give things a proper workout.

Haven't tidied things up yet, but here's some proof:

I'll get some clips up as soon as I get the new tubes in.

K73 · #2 10-21-2012, 06:30 PM

Sounds awesome. I hope the sound it makes turns out to be equally so.

B-string · #3 10-21-2012, 09:05 PM

Seems like only a couple years ago 6SL7 and 6SN7 NOS were cheap????

JGR · #4 10-21-2012, 09:17 PM

Some of them are still reasonable. I payed a fair chunk (for me anyway) for a NOS RCA that tested top notch as well as Tung Sol which was a good bit less but also tested well. I have some new production Tung Sol's on the way which hopefully won't be crummy.

B-string · #5 10-21-2012, 09:33 PM

I have always believed some magic came from them in the B15's. I bought one to replace a bad valve in an old Magnavox console radio a couple years back. At that time a NOS RCA cost me less than $10 shipping included. Sometimes the internet ain't such a good thing?

Congrats on a nice build!

Passinwind · #6 10-21-2012, 11:20 PM

Nice Jon! I'm still diggin' mine a lot. As you say, the 6SL7 sits right in the sweet spot for bass apps in many ways. Hope you find some good examples, as I eventually did. Still haven't tried any new stock ones yet.

SactoBass · #7 10-21-2012, 11:27 PM

Quote:

Originally Posted by JGR

Some good folks encouraged me to look into the 6SL7 tubes a while back, and I'm glad they did. I just finished the first prototype - it uses the new 2U format which I'm going to stick with going forward for whatever I'm building - it allows me more flexibility with the power supply, as well as the addition of a Jensen balanced out (in addition to the Jensen DI) - the balanced out was another thing some folks recommended that I'm glad I tried. The Jensen balancing tranny is pretty $$, but I think it does add a bit of sonic mojo. I also added a mute switch (there ya go Frank!

) since I had a bit more real estate on the front panel - it also allowed me to use my cool stove knobs which don't fit on the 1U.

Some initial thoughts and impressions....

I can see why folks like these tubes - kind of what I was hoping them to be. They are big and bold, but have a nice softness to the initial attack, and a very musical character to them. The tone is big and weighty, and they seem to impart the "all tube" vibe, even when running SS power.

This circuit follows the current iteration of my original design which has a tweaked gain structure compared to the original five I've built thus far, but I did make a fair amount of changes to the gain structure and EQ based on what I thought would work for the 6SL7 front end, as well as some tweaks to the power section to keep heater-to-cathode voltages in spec. With the lower gain, I voiced it a bit wider since I can get away with it - the EQ is the same as my other builds with familiar shelving bass and treble, and dual inductor driven low/high mids (200Hz, 1.5 kHz). The 6SL7's make it a different beast, but it's not a total departure, and I think folks that generally prefer the 6SL7 would still dig the other design too. Gain range is great - plenty of clean range, and a beautiful overdrive with things cranked. The OD is very useable, and has a nice soft nature compared to the 12ax7's. The gain of the 6SL7's seem about ideal - 12ax7's can be a bit too much, the 12au7's sometimes not quite enough, and the 12at7's are a bit lacking tonally IMO. Plenty of output through the unbalanced and balanced outs, I'm diggin the beefed up power supply that the 2U allows me.

The bad - my NOS Tung Sol is noisy - got some background waterfall, static, and occasional wierd microphonics. I hope TC Tubes takes it back, because it was supposed to test premium. The RCA is great so far, and it should be for the $$$. Other bad is I don't have any spares on hand, so I'm going to order some new Tung Sol reissues - hopefully they will be good. I don't like screwing around with NOS anyway. The only negative I can see to this whole deal is having to sort through tubes which I don't have the patience for. Hopefully I'll have some here mid week so I can give things a proper workout.

Haven't tidied things up yet, but here's some proof:

I'll get some clips up as soon as I get the new tubes in.

Holy freakin smokes, Jon! I didn't even know you were developing such a preamp! Very nice! How do you think this compares to Jule's Monique, tonewise?

christw · #8 10-21-2012, 11:57 PM

Too cool! I think we need more 6SL7 preamps in the world. I also feel like they may contribute a tad to the B-15 mojo.

Foz · #9 10-22-2012, 02:21 AM

nice...

JGR · #10 10-22-2012, 07:19 AM

Thanks guys. I definitely hear the B15 vibe in these tubes as others have mentioned.

Sacto, I haven't had the pleasure of hearing/playing a Monique in person - I would imagine there are some similarities due to the 6SL7's, but enough differences in overall design - gain staging/voicing, EQ, power supply (rectification, voltage levels, filtering), and driver circuitry to give them their own unique personalities in terms of tone and feel/response.

BurningSkies · #11 10-22-2012, 07:29 AM

Looks great JGR. I'm glad that they 6SL7's ended up delivering what everyone promised.

I think more guys will be on board with this tube compliment once they hear what they add to the overall tone and playing dynamic. I haven't missed the 12AX7 since moving over.

JimmyM · #12 10-22-2012, 09:40 AM

Certainly won't diss the venerable 12ax7 when SVT's are full of them

but 6SL7's do rule. Looking forward to hearing your clips, Gustav!

christw · #13 10-22-2012, 10:54 AM

Last year I bought a bunch of 12AX7 to 6SL7 converters to try in my amps because I dig the 6SL7 mojo and I knew it'd be years until I actually designed myself a preamp. Unfortunately, they don't fit any of my amps. I have recessed tube sockets and limited space all around!

MAMMOTHvolume · #14 10-22-2012, 04:23 PM

Quote:

Originally Posted by JGR

The bad - my NOS Tung Sol is noisy - got some background waterfall, static, and occasional wierd microphonics. I hope TC Tubes takes it back, because it was supposed to test premium. The RCA is great so far, and it should be for the $$$. Other bad is I don't have any spares on hand, so I'm going to order some new Tung Sol reissues - hopefully they will be good. I don't like screwing around with NOS anyway. The only negative I can see to this whole deal is having to sort through tubes which I don't have the patience for. Hopefully I'll have some here mid week so I can give things a proper workout.

Have you had it on the scope to make sure it is the valve making the noise? That kinda noise is normally generated by resistors (make sure you got metal film not chinese ones that look like metal film)

JGR · #15 10-22-2012, 04:29 PM

It's not a component issue; I use high quality caps and metal film resistors throughout. Definitely a tube; the RCA is dead quiet using the pre-EQ out which only uses the first tube. Tung Sol is noisy in that position. New tubes should be here tomorrow.

JGR · #16 10-27-2012, 02:06 PM

Alrighty, tubes finally showed up - a pair of reissue Tung Sols. Popped 'em in and all is well, everything is quiet as a church mouse, even at full gain. No hiss, microphonics, pops - nada.

Compared to my 12ax7 preamp, this one is more weighty and vintage sounding - it's a bit darker and looser, but it's not muddy or sloppy - still plenty of definition, just a different vibe with the 6SL7's. There is certainly enough to differentiate them, but at the same time, I could use either one and be happy if that makes sense.

Here's a video - had to re-shoot because the camera was getting overloaded and I was getting a lot of distortion and rattles from the other equipment in the room - this one is a little better...

http://youtu.be/i9eMqyUYErU

JimmyM · #17 10-28-2012, 12:39 AM

If we're going by Youtube vids, i like this pre better. I agree with your descriptions and I like this one better.

5StringPocket · #18 10-28-2012, 09:35 AM

No hiss or noise at high gain, useful eq points, good presence and balance to the sound with rich harmonic content. Very nice indeed!

JGR · #19 10-28-2012, 10:25 AM

I'm getting ready to do some direct A/B comparisons between the two preamps, so I switched back to the unbalanced pre out on the new build to make it apples to apples since I don't have a balanced out on my other one - I hadn't played through the unbalanced out since I got the new tubes yesterday. The Jensen balancing transformer definitely contributes to the sonic footprint in a very pleasing way.

The unbalanced out is extremely quiet as long as you use a normal patch cable from preamp to amp, which is what you would do it a rack. When I ran a ten foot cable from the pre to the power amp across all the other cables, etc., it picked up noise whereas the balanced remains quiet. But that is to be expected. Going to go back and play for a while and wait for the rain....

Bassmec · #20 10-28-2012, 10:26 AM

I am so pleased that people haven't decided that pre amp tubes are just pre amp tubes they all do sound quite different to me.
That's one of the best dual triodes RCA ever made.
I also have a lot of love for the old RCA grid cap, steel envelope 1620/6J7 pentode pre amp tubes, they used in studio equipment of the 40's and 50's.