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Maximum Screen Voltage for Triode Strapped 1625 or 837
Maximum Screen Voltage for Triode Strapped 1625 or 837
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Old 28th December 2015, 10:58 PM   #1
rcollege is offline rcollege  United States
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Default Maximum Screen Voltage for Triode Strapped 1625 or 837

Hi all,

I have some 1625's, 837's and 802's I was going to use for SE. All take the same socket.

These are all transmitting tubes...just wondering if anyone has any experience with the screen voltage limits for these when triode strapped.

I have seen it said (written) that the limit can generally be ignored as long as the plate Voltage never go below the Screen (G2) voltage. (I do not think this is true for all- if any tubes)

I plan on using higher voltage with lower current...I have some 7.8k and 13.5k output xfmers.

Any input is very appreciated
Thanks

Last edited by rcollege; 28th December 2015 at 11:01 PM.
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Old 29th December 2015, 04:14 AM   #2
Miles Prower is offline Miles Prower  United States
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The 1625 is internally the same as the 6L6, and you can get pseudotriode data from the spec sheet: 6L6: Frank's.

As for using the other types, not seeing any specs on pseudotriode operation.
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Old 29th December 2015, 05:40 AM   #3
smoking-amp is offline smoking-amp  United States
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For the 837, from the RCA datasheet, page 8. When the screen and plate are at 200 V, and grid1 is at 0V, then the plate draws 100 mA and the screen 22 mA. The curves look pretty flat at higher plate V, so you can probably assume the screen will draw 22/100 x plate current for higher V and other currents from the plate curves. Then figure the 5 Watt max for the screen dissipation at idle from screen current x screen V.

802 looks about the same.

http://frank.pocnet.net/sheets/049/8/837.pdf
http://frank.pocnet.net/sheets/049/8/802.pdf


For a cheap and super linear triode, there is the pentode in the 38HE7, available for $1 at ESRC. (they were just $ 0.35 in quantity from Vacuumtubes.net, I got a box of them) The pentode can be operated alone by putting 21V across pins 10 and 12, and should be good for 15 W Pdiss in that configuration. Triode curves below. 50 mA/div Vert., 50 V/div Horiz., 8 V steps on g1. Mu 4.2, Gm 8800 at 60 mA, Rp in triode is about 477 Ohm at 60 mA, 230 mA max. DC. Don't overdo the plate V on these, use the high current to advantage. Screen rated 3.5 Watt. Screen intercepts 1/20 of plate current.

Put 2 in parallel to upgrade a 300B (300B: gm 5500 at 60 mA, Rp 700 Ohm, 100 mA max DC).
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Last edited by smoking-amp; 29th December 2015 at 05:56 AM.
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Old 29th December 2015, 06:01 PM   #4
smoking-amp is offline smoking-amp  United States
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I checked another point on the 837 screen current curves (page 8). With 200V on screen and plate, but +60V on g1, the screen current is 90 mA and the plate current is 480 mA. So the screen intercept ratio actually decreases slightly with the higher current to 0.19 from (the earlier) 0.22 of plate current.

So, screen dissipation can be safely calc'd as plate V x 0.22 plate current (for triode mode).
Which makes the screen dissipation 0.22 x plate dissipation. Max screen diss of 5 Watts can then be assured if plate diss. stays below 5/0.22 = 22.7 Watts. Since plate dissipation is limited already to 12 Watts, all is well.

One could put a limiting screen stopper resistor in series just to be absolutely safe. 220 Ohms would drop 20V at the 90 mA screen current, so would drop the screen V by 20% when near maximum conduction on a loadline. (probably can't get near that conduction level in triode mode anyway)

802 looks similar.

38HE7, at just 0.05 plate current intercept (has aligned grids), and 3.5 Watt max for the screen grid, would lead to 3.5/0.05 = 70 Watts max plate diss. without melting g2 (in triode mode), which is well above the 15 Watt plate limit. Should be fine, as long as the screen voltage is not high enough to arc over to grid1. Works fine on the curve tracer here up to 1000V for a short term test, but I would not operate it that way. Maybe keep peak plate V to 450V to be safe. (its got plenty of current capability)

Last edited by smoking-amp; 29th December 2015 at 06:23 PM.
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Old 30th December 2015, 01:56 AM   #5
rcollege is offline rcollege  United States
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Thanks Miles and Smoking-amp.

Do the 837's look like good candidates for an SE triode mode amp? The pentode lines look linear to me for both 837's and 802's.

Have either of you or anyone had good luck with 807's or 1625's in SE? I read they weren't good in this role...even for background music... my curiosity will probably have me build it anyway...but other opinions are good.
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Old 30th December 2015, 02:16 AM   #6
smoking-amp is offline smoking-amp  United States
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I've only used the 1625, and that was in pentode mode for an Amateur Radio xmtter a long time ago. My 1st question would be whether you can plot a useful power output load line for the 837 or 802, without having to resort to positive grid 1. I wouldn't normally consider zero bias tubes, especially for low Watts out. Then an Xmit type HV power supply and a plate cap would definitely kill the deal.

Last edited by smoking-amp; 30th December 2015 at 02:18 AM.
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Old 6th January 2016, 11:07 PM   #7
DualTriode is online now DualTriode  United States
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rcollege,

You sent me a pm asking me to join in.

I have played with these tubs minus the 802. I also tried 307a’s and 6BQ6’s.

The large coke bottle shape is nice. The top cap adds to the look as well.

The plates are ample for the rated power. Keep in mind hard work may shorten life.

The fun I had with these tubes was as a headphone amplifier. I ran them at low plate voltage and higher current in an effort to minimize plate resistance and output impedance.

All these SET headphone amplifiers were nice however none of them were the Holy Grail. The music is in your head between your ears. There are no delayed reflections, no interaction with the space.

DT
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Old 7th January 2016, 02:37 AM   #8
rcollege is offline rcollege  United States
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Thank you for your response DualTriode.

It seems that after reading the responses, I may have to put these tubes away for a bit.
I have low funds, and little room...and small OPT's...along with small 90dB speakers.

I need power, but I don't want to use tons of current, as it is easier/ smaller to produce high voltage than high current. It allows for smaller iron.

I may just find tubes that can handle higher voltages, JJ KT88 go up to 800V...with using only 40mA I can get 4.5W with a 7.8K OPT...good distortion figures as well.

I could easily build a PP, but I love the sound that I get from SE.
I have a Dynaco ST35... I see no reason for another PP amp.

I'll let you all know.
Thanks
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Old 19th January 2016, 08:56 AM   #9
jazbo8 is offline jazbo8
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Maximum Screen Voltage for Triode Strapped 1625 or 837
The following information is roughly translated from a Japanese website, it basically says that we should forget about fearing screen grid meltdown when the multi-grid tubes are triode connected, run them not at the maximum screen grid voltage but at the maximum anode voltage, in fact, even that can be exceeded - as long as the Pda is observed, there is no problem at all. The tradeoff are higher required grid bias voltage/drive and somewhat higher RL.
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Old 19th January 2016, 10:08 AM   #10
TURING is offline TURING  China
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I am also planning to produce 837,1624,307A, 12E1 single-ended tube amp. In addition to 1624 using the pentode connection, other are used triode connected.But we have been unable to determine the 837 (rk44) anode consumption and can not determine the operating point. You have information 837 can determine the maximum anode power it? I can only conservative tentative in 16w (anode + G2 + G3).

802, significantly less than the 837 anode power.

1625 same as 807.

Last edited by TURING; 19th January 2016 at 10:10 AM.
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