Why is the 65 Amps London's EF86 channel so chimey....

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dehughes
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Post by dehughes »

Merlinb wrote:
dehughes wrote:...and this took the voltage from about 120v down to 77v or so on the grid, and RAISED the voltage on the plate from 125v to about 172v. Interesting....anyone know why that happened?
Lower screen voltage compresses the gid curves, = lower quiescent anode current, but bias voltage doesn't drop proportionately because it also handles screen current via the cathode, hence the increased anode voltage.
I'm going to have to read that answer a few more times....but thank you! :)
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Post by JimiB »

great thread!
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Post by rjgtr »

This thread got me curious so I converted the normal channel to cascode and I really like it! It is a fatter sound than the parallel with good complexity.
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dehughes
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Post by dehughes »

Yeah, this thread got me 'a tinkering....

So, I've done a few things:

1) Wired up my EF86 stage just like an AC30/4.

2) Wired up my EF86 stage just like a DC30.

3) Wired up my EF86 stage with a 220k plate resistor and a 1M screen resistor with another 1M screen resistor to ground, in parallel with the .1uf cap (attempting to replicate the London EF86 voltages).

For reference, here is that handy-dandy chart so wonderfully stored on this site:

Image

My question now pertains more to what is going on with a pentode when the screen drops in voltage but the plate stays the same.... I noticed that going between the AC30/4 and DC30 options the plate voltage stayed at about 126ish, but the screen voltage went from 118v to about 72v, respectively. Now, when putting a 1M across the .1uf cap to ground (and with a 220k plate and 1M screen) the plate jumped up to 172v and the screen was down to about 75v. We've cleared up here why that happens, so my question is more towards what is happening when you have a 125v plate and a 118v or 72v screen...is the former more of a "cold biased" effect and the latter more a "warmer biased" effect? I seem to remember that being mentioned somewhere...

I'm curious mostly because I'd like to have an explanation for what I'm hearing. With the AC30/4 values and voltages I seem to have a somewhat darker but fatter vibe that is not terribly harsh in the upper frequencies, but articulate enough, and very deep and robust in the low end. With the DC30 values I'm hearing more articulation and a bit more harshness in the upper mids, and not as deep and grand a low end. I'm leaning more towards the AC30/4 values and voltages...but I'm going to give the DC30 setup another day or so...
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Post by PeterS »

Okay, prompted by this thread, I started tinkering too. I put the AC30/4 front end on my Lite IIb with 6v6s.

Haven't had chance to play it much, but what I've heard so far sounds great.

Measured voltages---- B+ (for this stage) is 260, screen is at 90 volts, plate is at 70 volts!! Seems odd that the plate voltage is so low. I'd swap the tube and see if it changes, but I only have ONE EF86 (NOS Amperex).

Does it matter that the plate is below the screen? Should I be increasing the screen resistor, which I assume will lower the screen and therefore reduce the current and so also raise the plate?

This pentode stuff is just what we didn't need--- more parameters! :lol:
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Post by PeterS »

Hmm, so I scrounged up another ef86 and some e80f tubes to try.
I get the same voltages (fairly closely) with all of them.

This I find surprising, because I was expecting the screen voltage to be very dependent on tube characteristics--- the fact that it is connected through a 1Meg resistor means that a small difference in the current to the screen will change the voltage by a lot.

SO I changed the screen resistor to 1M2, now the screen is below the plate... still sounds good... actually sounds better to use the "lo" input, on the "hi" input there's no clean (with an SG)....
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Post by geertjacobs »

I used the AC15/30 EF86 values in the reference chart in a Vox AC15 Lite and it was by far the best sounding amp I built (and even played).

I have some parts lying around for a SS recto 2xEL84 amp and I'm really (really!) intrigued by the "cascoded twin-triode operating in Pentode mode".
It would be a real achievement to get that EF86 magic sparkle and balanced harmonics with an "ordinary" 12ax7.
The Aiken posts on cascode are very promising, so if anyone has any pointers on the "cascode in pentode mode" I'm definitely interested...
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PeterS
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Post by PeterS »

I used the AC15/30 EF86 values in the reference chart in a Vox AC15 Lite
Did you measure voltages? I'm curious whether your screen came out higher than the plate with those values...
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Post by geertjacobs »

I didn't take any voltages at the time (converted the amp a few years ago for a friend, so I don't have it anymore)
The amp was an electrically exact copy of the AC15 1960 schematic minus the tremolo channel (I believe I grounded that side of the PI). Only the preamp component values were so badly readable that I used the ones on the EF86 reference chart.
What I found somewhat amazing in the AC15 schematic were the small filter caps (8uF). I wonder how much they contribute to the sound.
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Post by soundmasterg »

Sorry it took me so long to get back to this. The voltages on my brother's Silvertone with the added EF86 had the screen higher than the plate by about 30 volts. Plate was around 75v and screen was around 105v. This was with the stock AC30/4 values. After I increased the screen resistor then I got where it is now, with plate at about 106v and screen at 92v. I've read that the screen should be about 30% to 40% lower than the plate for proper operation, but in my case it sounded great where it was so I didn't bring the screen down any lower. If I was going to though, I would probably split the screen value between two resistors, or better yet, a resistor and a pot so I could control the tone even more.

Greg
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Post by PeterS »

Plate was around 75v and screen was around 105v.
Similar to my voltages with those resistor values. Makes me think possibly a real VOX would also have those voltages.

Is there any reason why it would be bad to have the screen above the plate? It means that any secondary electrons that get knocked off the plate get accelerated towards the screen, but at these low voltages I don't think that matters.
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Re: Why is the 65 Amps London's EF86 channel so chimey....

Post by VTAMPSUSA »

Sorry this is an OLD, OLD thread. But has anyone considered NOT fully bypassing the EF86? 8O


Like, maybe, without the boost mode, they only have a very small bypass cap that only boosts the very high frequencies that get attenuated by a large anode resistor? And when they boost, they fully bypass. I'd guess they are also using a bright cap directly after.

I'd done this with triodes using 22n-10n and you can get an incredible amount of chime independent of volume position.
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