An all-octal pentode/parallel triode 'Deluxe' build?

[capohk]

Inspired by @Jerry garrcia and @printer2 in the epic thread here, as well as what is left in the "widows and orphans" pile of components from my haul of local junk, I think this will be the last project I can make without getting more donor stuff. Here is what I have:

c.1935 Wards Airline chassis with more holes than a Swiss cheese

Hammond 290BBEX Deluxe PT

Choke from 1930s - nice component ~10H 160mA

either: Japanese-made Tamura HiFi OT 5K pri, 2/4/8/16 sec (massive lump of iron, not sure how that would sound or work, might need to use one impedance up for a reflected load of 10K)
or: cheap and nasty Blues Jr OT that was upgraded by a bandmate (tiny little thing with 8ohm sec only)

Lovely set of tubes including NOS French Mazda 5y3, a pair of English Haltron 6v6G coke bottles and RCA/GE 6SL7WGT tubes and a Special Red 5693 metal case 6SJ7.

I will make a turret board for this one as it was a success in my last build, easier to do the thinking in DIYLC than trying to design while soldering PtP.

Borrowing heavily from the afore-linked thread, here are my initial thoughts on a schematic:

Not really done any thinking about power rail voltages or dropping resistors yet, the 290BBEX puts out 355-0-355 HT to the rectifier which the power calculator seems to think will result in a B+ of ~440VDC, which seems a bit high for a Deluxe.

Some pics for context below

 

[capohk]

 

[NickK_chugchug]

Nice - what's the reason for paralleling the 6SL7? (lower noise?)

 

[capohk]

Nice - what's the reason for paralleling the 6SL7? (lower noise?)

Good question - because I don't like to see a triode wasted? I am also currently planning a ef86/18w 2 channel amp with EL84s that was drawn out by Steve Luckey which has a similar topology and it was the way that @Jerry garrcia had designed his. The alternative is to cascade them, so it would end up a bit like a single channel 6g3, but with paraphrase and cathode bias (so not really). I'd need to drop some gain though. I'm hoping the pentode channel will sound awesome, so maybe the other dual triode could be used in some other way? It'll be a head, reverb might be a stretch.

 

[NickK_chugchug]

Parallelling tubes provides a higher signal to noise ratio (mathematically) - noise introduced by the 6SL7s would be different hence not add up, but at the same time, the signal (both from the input and from the B+) should be identical and so the signal output adds - so you get twice the signal but only the same noise as a single tube. Also .. if you have the triode laying around why not

Interested to hear what this will sound like (I like taking designs and then creating something similar from bits).

 

[Jerry garrcia]

I’m glad to se that I inspired someone. In retrospect I’d built the amp with a 6SL7, 6SQ7 or a 6SF5 instead of the 12ax7. Just for the mojo and my dislike of the noval sockets. What I would do on the pentode channel is to add the grid leak/cathode bias switch (curtesy of @printer2 ). It gives the amp more versatility and an easy add. Use it now on all my 6SJ7 builds. But with 0.047 and 33-47K as grid stoppers

I’ve also gone from 0.047 to 0.022 coupling caps on most of the builds. Except on the inputs and from the screen to ground.
Another thing I’ve done is to bias the 6SJ7 a bit different.
This example is just cathode biased but I think it sounds a bit better with this setup for the Rp, Rg2 and Rk.

If you don’t want to use the parallel connection on the 6SL7 you could use it to drive a LND-150 reverb (Tube Town has a nice one and DON’T forget the shielded cables…)

 

[2L man]

A HiFi OT in guitar amp should make sound more detailed. Also spread the frequency range to highs and lows.

To me it looks and sounds that typical 5E3 loud sound has OT core saturation distortion which come when Cathodyne distortion first make power tube signals different and this imbalance cause OT saturation starting from low notes. This then also increase HV sag. So that should lessen at least slightly? When there comes more highs the sound can become irritating but that should be easy to lessen installing filters to circuit.

You could get third input using 6LC7 triodes series and geting Low gain making input between them and High gain before them. Then 6SJ7 input should be about Normal gain.

I think the Choke comes mostly a dead weight when there is 5k after it Possibly Choke then also collect more the EM strays which modern world has?

Installing a Capacitor to 0VDC between Choke and 5k Resistor make the Choke function as LC filter with the Capacitor!

If the Schematic is your drawing you should edit the return 0VDC currents to match the feed currents A, B, C and D. Now there is no logig. A and B should be together! Other stage return currents can be separate. Also try to find other symbol for Mains Safety Earth? Better symbol is _____ which have ///// under. In schematic it separate primary SE (Chassis) from secondary 0VDC symbols.

 

[mountainhick]

Just a thought, I recently did a similar amp, two channel except 9 pin tubes, 5879 and split hi low single/parallel 12AY7 input stage, similar topography straight into PI. The split is useful for single coil vs humbuckers. I used two jacks, but you could do a switch as well.

 

[mountainhick]

I’m glad to se that I inspired someone. In retrospect I’d built the amp with a 6SL7, 6SQ7 or a 6SF5 instead of the 12ax7. Just for the mojo and my dislike of the noval sockets. What I would do on the pentode channel is to add the grid leak/cathode bias switch (curtesy of @printer2 ). It gives the amp more versatility and an easy add. Use it now on all my 6SJ7 builds. But with 0.047 and 33-47K as grid stoppers
View attachment 1122336
I’ve also gone from 0.047 to 0.022 coupling caps on most of the builds. Except on the inputs and from the screen to ground.
Another thing I’ve done is to bias the 6SJ7 a bit different.
This example is just cathode biased but I think it sounds a bit better with this setup for the Rp, Rg2 and Rk.
View attachment 1122337

If you don’t want to use the parallel connection on the 6SL7 you could use it to drive a LND-150 reverb (Tube Town has a nice one and DON’T forget the shielded cables…)

Jerry, what subjectively soundwise is desirable from that cathode switch? How do you actually use it?

 

[printer2]

Jerry, what subjectively soundwise is desirable from that cathode switch? How do you actually use it?

I will let Jerry comment on the sound, the switch goes between cathode bias and grid leak bias. grid leak bias was only used on early guitar amps and some cheap line powered amp (no power transformer) usually with a 12AU6 in order to get the most gain out of the tube. They did use a high plate resistor also to get the most signal level into the output tube.

 

[mountainhick]

I will let Jerry comment on the sound, the switch goes between cathode bias and grid leak bias. grid leak bias was only used on early guitar amps and some cheap line powered amp (no power transformer) usually with a 12AU6 in order to get the most gain out of the tube. They did use a high plate resistor also to get the most signal level into the output tube.

From a couple youtube vids of early fenders and gibsons with grid bias, the more archaic version seems grungier. Is that your perception as well?

 

[printer2]

From a couple youtube vids of early fenders and gibsons with grid bias, the more archaic version seems grungier. Is that your perception as well?

Yes it can be.

 

[Jerry garrcia]

Jerry, what subjectively soundwise is desirable from that cathode switch? How do you actually use it?

When in grid leak you definitely get more gritty gain with more attack. Also a bit more background noice (not much but noticeable). The gritty gain is kind of pleasing when aiming for lap steel type of sound or when really digging in for a grungier sound. Earlier distortion. But I also really like it when playing old school jazz because you can really get some nice tones out of it if you can control your picking and technique. To get those sweet tones (not the gritty cigar box type tones) it demands more of you as a player. Just look at Pasquale Grasso. Also my son really likes it since it makes him focus on controlling his technique.
The major drawback and the reason why I install the switch is that grid leak doesn’t like a boost pedal at all.

When in cathode biased mode it behaves more or less like a “normal” modern amp from the 50-60’s. Although you still get the pleasant 6SJ7 tone but more tamed and easier to control.
So in conclusion, if you have room in the chassis and a spare 0.02-0.047uF cap I would definitely install it (SPDT ON-ON) in all pentode based preamps.

 

[NTC]

The schematic in the first post shows two triodes in the pi but gas 6SJ7 next to them. That should be 6SL7, yes?

 

[capohk]

The schematic in the first post shows two triodes in the pi but gas 6SJ7 next to them. That should be 6SL7, yes?

Yes. Thank you

 

[Jerry garrcia]

For one of the triodes in the preamp 6SL7 (if you don’t want to go parallel) I think Sluckey posted a 6SL7 driven reverb or was it a tremolo. I’ll dig in in my unorganized library to se if I can find the schematic.

 

[Jerry garrcia]

For one of the triodes in the preamp 6SL7 (if you don’t want to go parallel) I think Sluckey posted a 6SL7 driven reverb or was it a tremolo. I’ll dig in in my unorganized library to se if I can find the schematic.

Was a tremolo. Or not a but many different

Which is the better tremolo circuit in SE amp ?

Hoffman Amplifiers Tube Amp Forum, Which is the better tremolo circuit in SE amp ?

el34world.com

 

[Jerry garrcia]

This was the one I was looking for.

 

[Jerry garrcia]

And here is Tubenits 6SL7 driven reverb.

 

[capohk]

Thank you for all of this. I'll need to make some decisions about physical layout, cabinet size etc. I did also think about using a 6SF5 instead of the dual triode, super cheap and an octal to boot.