6SJ7 Pentode Operating Conditions

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rp
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6SJ7 Pentode Operating Conditions

Post by rp »

6sj7.jpg
I'm going to try and set up the input pentode on my 5C1 clone to cathode bias to make it more pedal friendly. This is an operating conditions table from the actual data sheet. Seems they did the hard work of curve reading for you? Do I just pick an operating point - in this case Plate Voltage and Ra, which I don't feel like changing, and read across and use those values for the rest?

I looked around at schematics of amps with pentodes and they are all over the place but the earliest GA-40 is set up much like the chart at 180V with a 250K resitor. The plan is to copy that. Is that all there is to it?

I read through Merlins background on small signal pentodes and he recommends sticking with a voltage gain around 100 but the 5C1 has only one gain stage so maybe go for more? The points I'm choosing would give a VG of 161.

http://www.valvewizard.co.uk/pentode.html

FYI right now just clipping in resistors I have w/ 2.2k Rk: 1.45v bias, 178V on the plate, 35V G2 or w/ 1k Kr: .83 bias 166V, 25V G2 which is a tad louder, I haven't put in bypass caps on the cathode or G2 yet. The node feeding the tube is 285V.

Any ideas where to head first?
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Stevem
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Re: 6SJ7 Pentode Operating Conditions

Post by Stevem »

The nice thing about playing with a Pentode stage is that you can get some good compression taking place before it clips and when they do clip they have a bit more even order harmonics to them for a tad sweeter sound if you do not drive them really hard into clipping!
My thoughts are kind of , if your only going to use a mire 100 gain out of what a Pentode can offer, then why use it?
Also do not confuse signal level gain with sustane, that cones from compression.
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rp
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Re: 6SJ7 Pentode Operating Conditions

Post by rp »

Stevem wrote: Thu Mar 09, 2017 12:59 pmMy thoughts are kind of, if your only going to use a mire 100 gain out of what a Pentode can offer, then why use it?
I thought that too and why I asked, it is rock 'n' roll after all.

I was hoping for specific guidance to values to use not as I'm too lazy to try things out but it's PTP and a PITA, and testing makes me trash old cc's. But now that you've called me out on being a gain wimp, if I use the values across the 180V/470K Ra row I can leave the 2200 Rk and the 2.2M Rg2 I've got and just change Ra add the bypass caps and blast off. Starting at max is one way to find out. Stay tuned.
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Phil_S
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Re: 6SJ7 Pentode Operating Conditions

Post by Phil_S »

A while back, I built a GA20 deviant. I haven't used it in quite a while, but remember being very pleased with the result. I recall that I thought it was very Jimi-ish when running P90's through it. While this doesn't directly address your question directly, here is what I built. I had to fiddle with the B+ ladder a bit to get a higher plate voltage on the 6SJ7. I felt it was too muddy with the GA20 schematic values in the B+ supply. This was a PTP build. There is probably a thread about it here at TAG with pictures. You might search for it.
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Re: 6SJ7 Pentode Operating Conditions

Post by tubeswell »

rp wrote: Thu Mar 09, 2017 12:39 pm ... 2.2k Rk: 1.45v bias, 178V on the plate, 35V G2 ... HT=285V
1.45/2200 = 0.065mA tube current - way below what might be expected (The RCA datasheet says to expect around 3.8mA with HT = 250V , Vg1 = -3V, Vg2 = 100V, Va = 250V)

What resistance are the plate and screen resistors?
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rp
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Re: 6SJ7 Pentode Operating Conditions

Post by rp »

That row that gives a VG of 192, with Ra of 470K is not good, too much dirty bassy drive and btw gave me a Vp 95v, I paralleled another 470K across it for 220K and it was much better sounding, Vp was now 185v. I will next try another 2200 across the Rk for 1100, then parallel the 2.2meg g2, which would make it like the GA40 and see where that goes.

What I think I'm asking is how to set V1 up so I have the same amount of drive into the 6V6 as I had with the grid leak set up originally. I liked it a lot with grid leak, had a clang about it with the Edcor, something I associate with some headroom, but it got a really nice bite when you dug in, pretty cool for a low power amp. Sounds more like a 5F1 now - maybe still too much drive?

And Steve's right about the compression, it distorts more balls out like a 5F1 but has a Voxy compression. I like a quad of EL84s but like less as you go down, not a fan of SE El84. Maybe a 5F1 with a well set up pentode might make for a very nice little SE.

Thanks Phil, did you keep it grid leak or try cathode bias too?
Last edited by rp on Thu Mar 09, 2017 3:34 pm, edited 1 time in total.
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rp
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Re: 6SJ7 Pentode Operating Conditions

Post by rp »

What resistance are the plate and screen resistors?
I understand the current is important to setting up a pentode right, but that's beyond me.

Right now v1 is:
Kr 2220k 25mf bypass 1.4V, 220K Plate 185v, 2.2M G2 with .05mf (to the cathode as Merlin says) at 35v. Next is to try a Kr of 1100, then a G2 of 1meg. The supply node is 285v.
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Re: 6SJ7 Pentode Operating Conditions

Post by rp »

Man, the voltages sure move around with the changes, I've been paralleling resistors and it's all 2X. I'm way out of my league here, I can just go by my ears. I'm not on that table anymore as the Vp is now 100v and I've no idea if I'm upping or dropping gain or have decent current draw for the pentode to work well, but set up with GA-40 values it sounds the brightest, crispest, punchiest yet, has the right snap and tight distortion now. I'm running it through a G12H30 anni for the better spectrum and it seems nice with the below values. Anyone wanna confirm the math that the pentode is in a happy place?

1100 Rk / 220K Ra / 1.1M Rg2 - 1.3v / 100v / 40v. 22mf and .05 bypass caps. 286v supply voltage.

This is kind of fun, wish I understood it better. No wonder peeps stick to triodes.
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Re: 6SJ7 Pentode Operating Conditions

Post by tubeswell »

You need to make a tube characteristics chart and plot the Vg curves on it (for the Vg2 voltage you are aiming at), in order to estimate where the Vg0 knee is and put the load line a bit below that. I take it you haven't got Merlin's 1st or 2nd pre-amp book?

When setting the Vg2, the important thing is to keep the Ia:Ig2 ratio about the same as the data sheet's model/typical characteristics (in this case the data sheet gives a couple of examples; 3:0.8 where Va = 250V, 2.9:0.9 where Va = 100V, so that's somewhere between 1:0.267 and 1:0.3). This will ensure that the screen dissipation is kept in check.
Last edited by tubeswell on Thu Mar 09, 2017 6:02 pm, edited 1 time in total.
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rp
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Re: 6SJ7 Pentode Operating Conditions

Post by rp »

You need to make a tube characteristics chart and plot the Vg1 lines on it (for the Vg2 voltage you are aiming at),
That's asking a lot of me, especially as I'm in the dumbing epoch of my life :lol: I think if the Vp goes down the current goes up, so that's good. Can I just calculate the current across Ra or measure it, I'm not sure what happens when you add in a G2 here, or if it must be included. Googling I read that 2-3mA is recommended for the 6SJ7 plate current. Is is reasonable to play around 'till I get that and a sound I like?
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Re: 6SJ7 Pentode Operating Conditions

Post by tubeswell »

rp wrote: Thu Mar 09, 2017 5:35 pm ... I think if the Vp goes down the current goes up, so that's good. ...
Yes, but where Vp drops below the knee of the Vg0 curve for the screen voltage you are operating at, Ip suddenly drops, and Ig2 suddenly rises by the same amount. This causes the screen to burn up,
rp wrote: Thu Mar 09, 2017 5:35 pm ... Can I just calculate the current across Ra or measure it, I'm not sure what happens when you add in a G2 here, or if it must be included. ....

Get 3 sets of voltage drop reading across the plate resistor and across the screen resistor as well as across the cathode resistor, for each bias resistor or screen resistor or plate resistor you try - in order to calculate the plate and screen current in each case. Aim for an Ia:Ig2 ratio of around 1:0.3 where the plate voltage is sitting nearer to 100V, and 1:027 where the plate voltage is sitting nearer to 250V.
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Re: 6SJ7 Pentode Operating Conditions

Post by Phil_S »

rp wrote: Thu Mar 09, 2017 3:28 pm Thanks Phil, did you keep it grid leak or try cathode bias too?
I did not try cathode bias. This was a rare instance when it crossed my mind to leave well enough alone and then I did just that. I was actually hoping to increase plate voltage to around 180-ish, but I couldn't seem to do it. The record says I finished this build November 2009. That was when I knew a whole lot less. I could have easily reduced the last dropping resistor in the string to 5K and maybe as low as 1K and it probably would have been more than enough decoupling. Have you tried using grid leak? I'm not clear from the thread content. If you haven't, is there some reason why?
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Re: 6SJ7 Pentode Operating Conditions

Post by martin manning »

rp, a couple of operating points to try:

Vbb = 290, Ra = 47k
Vg1 = -3.0, Va = 150, Ia = 3mA
Ig2 = 0.82 mA, Ik = 3.82 mA
Rg2 = 232k, Rk = 785

Vbb = 290, Ra = 100k
Vg1 = -4.0, Va = 140, Ia = 1.5 mA
Ig2 = 0.4 mA, Ik = 1.9 mA
Rg2 = 500k, Rk = 2k1
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rp
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Re: 6SJ7 Pentode Operating Conditions

Post by rp »

Thanks Martin, I will try that out over the weekend. Too bad I didn't see it sooner, I buttoned it up.

Phil, I lent my 5F1 clone to a friend long term and I want to play some electric again, at home, not bug the neighbours, and unboxed my pedals. And I’ve been meaning to convert this to cathode bias for years just to hear the difference.

I played around with the three resistors until I started to hate this hobby. Rg2 makes as big difference as the plate resistor in listening trials. I thought this would be an easy 20 minute job to swap from grid leak to cathode bias and be like setting up a pentode output stage - which unless you have an extreme set of tubes or a wrong PT is rather easy. Setting up a pentode preamp tube is like playing Whack-a-Mole, it’s not really about reading across the table I posted as the voltages move around - a lot. I didn’t have the patience to make small changes just 2X changes paralleling resistors. Maybe more subtlety was needed which is what Martin provided.

So, without the knowledge to read the charts and understand the proper currents I settled on voltages that most matched the original grid-leak set up and listened if it sounded ok. Sounds great with the below set up, I hope I have enough plate current for it to work well, and that I’m not melting the screen (didn’t know that was a possibility in a preamp tube). Sounds more like a 5F1 now, more aggressive, less unique, lost it’s 5C1 charm. It does have a pentode chime. I like it better than an AC-4, but I’m using a G12-30-anni and never tried an AC-4 except through the old stock speaker. This is the values I settled on, the V’s and I’s, the later measured with a meter. Not sure if the plate and screen current is across the resistor or to ground. If anyone sees that I’m over dissipating the screen on my pretty, red 5693 let me know quick! Data sheet says .9W for g2.

Rk 1100 / .8V / .7mA to ground

Ra 220K / 162V / 1.1mA to ground, .63mA across resistor

Rg2 2.2M / 24.2V / .14mA to ground, 4.76mA across resistor
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Re: 6SJ7 Pentode Operating Conditions

Post by didit »

Hello --

I just this past week first powered-on a prototype build GA-50 inspired design. Single 6SJ7 into 6SN7 paraphase and 6L6 PP. Component values cribbed from a "reliably marked-up" schematic found several years ago. Will post worked schematic, pictures and more separately -- when it's closer to finished. It works. Seems to fulfill goal of delivering objective of big warm "Jim Hall" sounds. Need to live with it, get a few of more serious player friends to test fully, etc... Not necessarily the sonics you we're looking for but perhaps a touch-point regardless.

The V1 is modeled on "microphone channel" so cathode bias: Rk = 1K w. 22uF bypass; Ra = 120K; Rg2 470K w. 0.047uF. Rk is lower than Gibson's reference schematic but is correct from the working amp that was "blueprinted". Rough static voltages recorded so far: Ebb = 288; Va = 195; Vg2 = 32; Vg1 = 0.2. Future experimentation as time allows. Will be putting trimmers in for Ra & Rg2, and perhaps Rk. Blencowe 2nd Ed. was referenced in research. EF86 is his worked example. Lots of good info. Impossible to summarize in a few sentences. Paraphrasing and extrapolating one recommendation that I quickly took to heart---use ears, as it's really beyond simple maths.

Best .. Ian
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