7802 twin triode

[Dr. Toobz]

There has been mention of the seemingly rare 7802 pass tube as a possibility for the output position in the Crack, an interesting one since the transconductance of the tube is much higher than a 6AS7 or even 5998 at 20,000 mho! This should lead to a lower output impedance, and possibly, a better damping factor on some "slow" sounding headphones (even my HD 650's seem to like being driven from a lower impedance source than a higher one, but perhaps that's a matter of taste).

There's a few of these tubes "out in the wild" but my concern would be the possibility of oscillation, as I remember PJ bringing up in some buried thread. With a high transconductance tube like this, would simply adding a grid stopper to the circuit prevent the possibly of the tube oscillating, and if so, how might I calculate the appropriate value?

[Paul Birkeland]

With a high transconductance tube like this, would simply adding a grid stopper to the circuit prevent the possibly of the tube oscillating, and if so, how might I calculate the appropriate value?

It's not a bad idea, anything from 200 Ohms to 1K should be acceptable. 

If you try it, be sure to let us know how you like it!

[Dr. Toobz]

I lucked out and snagged two NOS Tung-Sol 7802's on eBay for about $15/each - try that with a 5998! In any case, first impressions are very positive - huge soundstage, low distortion, and a very crisp sound, similar to the change I saw when going from a 6AS7 to the 2399 (e.g., 5988), but more pronounced. I can verify that the output impedance appears to be much lower than usual, as a pair of AKG's plugged into the Crack actually have bass, and a pair of Grados (32 ohm) play loudly and don't seem to have much different of a sound signature/balance when going between the Crack and an iPhone. (Normally, this is certainly not the case - they sound very strange when driven from 120 ohms). Coupling caps are 100uF JJ electrolytics, which I seem to like a lot, at least more so than the Dayton film caps I had in there previously. Sacrilege, I know.... 

Since these are supposedly NOS tubes, the transconductance has to be at or above 20,000 mho, which should mean about a 50 ohm output impedance in the cathode follower arrangement, correct? The 5998 has double the transconductance of a 6AS7 (e.g., 14,000 vs 7,000 mho), and the 7802 is almost three times the 6AS7. Even my 300 ohm Senns sound a bit better than usual, which makes me wonder if lowering the source impedance is either a) lowering distortion, which is already very low, or b) changing the damping factor and/or response curve of the drivers?

I am looking forward to seeing if this really makes a difference once my 600 ohm DT880's arrive, which I am going to give a try after not liking the 250 ohm "pro" variant with my pre-Bottlehead gear.

[Paul Joppa]

Just a detail - transconductance is a function of plate current and (to a lesser degree) voltage. At Crack currents (around 30mA) all of these tubes have more like half the quoted transconductance - the spec sheets always give gm at or near maximum current.

Nevertheless, they still rate in the same order.

[Dr. Toobz]

Ah, that makes sense. This must be how the "high current Crack" experiment (which I am now assuming is vaporware?) lowered its output impedance. If I were to somehow run the 7802 tube at 60mA (presumably by ditching the Speedball on the power tube, cap coupling the two stages and altering the cathode resistor value), wouldn't this make the amp truly viable for 32 ohm phones? I've yet to hear a transformer I've liked as much as a tube in OTL arrangement 

[Paul Joppa]

Ah, that makes sense. This must be how the "high current Crack" experiment (which I am now assuming is vaporware?) lowered its output impedance. If I were to somehow run the 7802 tube at 60mA (presumably by ditching the Speedball on the power tube, cap coupling the two stages and altering the cathode resistor value), wouldn't this make the amp truly viable for 32 ohm phones? I've yet to hear a transformer I've liked as much as a tube in OTL arrangement 

So far, none of us really knows how low an output impedance is necessary to properly damp most 32 ohm phones, so I can't really answer your question.

The higher-current Crack is still an active project; most likely as Crack II or something. The power transformer limits the available current, and so does the heat generated by more current through the cathode resistor. As I'm sure you have noticed, Crack already gets pretty hot. :^)

There is no need to give up the direct coupling; all of these tubes can carry quite a lot of current with only 50-70 volts plate to cathode. But more current means more heat from the power supply resistors as well as the cathode resistor; and lower voltage unless the power supply resistors are lower resistance. And both higher current and smaller resistors means more hum, so then you need another PSU filter stage or larger caps ....  as you can see it's not just a smaller cathode resistor or bigger Speedball!

[ice9mike]

I just purchased a Tung Sol 7802WA. I found that it worked in my Crack (with Speedball) without any modifications. Is there any risk in running this setup?

Sonically, it sounds clearer and louder into my DT880 Pros (250 ohm). Bass has more impact. I have run various 6080 (GE, RCA, Raytheon) and 6AS7G (RCA, GE), but the 7802 is by far my favorite power tube.

[Paul Birkeland]

What DC voltage is present on the O pads of the larger Speedball PCB?

[ice9mike]

I did the full voltage measurement using my Fluke 87III with an RCA 6080 and the Tung Sol 7802:

Terminal     6080     7802

SB-O1        105.4    83.4
SB-O2        104.9    63

1                77.5      76
2                168.3    176.4
3                0           0
4                168.8    176.6
5                79.7      78.6
6                0           0
7                102.6    66
8                0           0
9                104       83.3
10              0           0
11              0           0
12              0           0
13              170.2    176.7
14              0           0
15              189.9    195
21              208.7    213.4

A1              78.8      78.1
A2              0           0
A3              1.55      1.54
A4              0           0
A5              0           0
A6              76         75.4
A7              0           0
A8              1.55      1.50
A9              0           0

B1              75.6      75.4
B2              175.5    176.2
B3              104.2    82.2
B4              77.8      75.9
B5              175.4    176.2
B6              105.4    84
B7              0           0
B8              0           0

I would have thought that O1 and O2 would match better in the 7802 but they don't. O2 also oscillated between 56V and 65V. Does this indicate a problem?

Thanks.

[Paul Birkeland]

Looks good, the cathode voltages are a bit lower, so the dissipation across the Speedball heatsinks will drop a little.

[ice9mike]

Thanks PB for checking it out and replying late on a Saturday. This is a good community that I have learned a lot from by just reading old posts. I hope to add some wisdoms of my own. I just posted my solution to fastening the top plate to the frame. I think it works great, but there are some trade offs. The great things about this amp is that it is not limited to one design.

ice9mike

[Snarii]

An update on this ancient thread that might be of interest for anybody interested by a 7802.

I've just acquired a very nice 7802WB from a fellow forumer. He didn't have any problem with it, but on my stock crack+speedball it had an huge hum centered on both channels. I had to try something...
According to Paul advice a few posts ahead, I make a quick try with a 220ohm (1/4W that's only what I had on hand, no idea if it's suitable) on one grid of the 7802. Of the two, the pin 4 grid was the most accessible...
And voila ! the hum is gone, only very very faint distant hum. I of course intend to suit the symetrical resistor on pin 1, but that single grid stopper resistor was enough to do the trick, as if the oscillation was internaly induced in the 7802.
I still have to check the sonic impact of the modification on my other output tubes...

I didn't have my best input tube on for the first run, but as far as I can say, sonically the 7802WB is a very nice tube, very dynamic !

[hpjun]

I have the 7802 Tung-Sol tube and it sounds amazing.

Now I would like to get the output impedance down even further. I heard for cathode followers, lowering the cathode resistor will drop the output impedance and also the gain as well. But the crackatwoa is CCS, is there a resistor on the CCS that I can adjust to drop the output impedance and gain as well?

So far, none of us really knows how low an output impedance is necessary to properly damp most 32 ohm phones, so I can't really answer your question.

The higher-current Crack is still an active project; most likely as Crack II or something. The power transformer limits the available current, and so does the heat generated by more current through the cathode resistor. As I'm sure you have noticed, Crack already gets pretty hot. :^)

There is no need to give up the direct coupling; all of these tubes can carry quite a lot of current with only 50-70 volts plate to cathode. But more current means more heat from the power supply resistors as well as the cathode resistor; and lower voltage unless the power supply resistors are lower resistance. And both higher current and smaller resistors means more hum, so then you need another PSU filter stage or larger caps ....  as you can see it's not just a smaller cathode resistor or bigger Speedball!

Now that the Crackatwoa exists, I want to know if I can modify it for high current? Is higher current more linear?

[Paul Birkeland]

So, looking at the datasheet, running the 7802 in place of the 6080 will drop the output impedance to about 100 Ohms.  This is insignificant.  (The tube may still sound different/better, I won't argue that)

Lowering the cathode resistor value on a cathode follower changes the operating point, this is sort of like advancing the timing on an engine.  When you go too far, you get pinging and the engine explodes.

If you want to lower the output impedance, you can add loop feedback.

[hpjun]

would running 2 cathode followers in parallel work without messing up the amp on the crackatwoa?

Was thinking of something like this: maybe using 2 6080 or 2 7802 or 2 6BL7.
http://m.ebay.com/itm/1pc-Gold-Dual-6SN7-6BL7-TO-6AS7-tube-adapter-/201511879930


Also I am still interested in experimenting on the cathode follower current and resistor values. Goal is still to lower the output impedance more. If the cathode follower was operated at a higher current it would also have lower impedance right? Also If I could I would trade away gain for lower impedance I would do it. If I had to run 2 cathode followers I would do it. Just want to get the impedance below 50 ohms without global feedback or transformer. If you have any ideas let me know. I just wouldn't want to make it sound bad.