Heater Pin-outs; similar tubes

[Tim Hall]

Hi all,

I have a Gilmore Jr kit amp from Guytronix/Gerhart, and I want to test out some different output tubes. The stock tube is 6N1P, and this can be substituted with a 12AU7, 12AT7 or ECC99 if the pinout for the heaters is changed.

As far as I can tell all of these tubes have an identical pinout, so I can't quite understand why the 6N1P seems to be the exception here. The 6N1P has heater wires going to #4 and #5, but not #9. I assume, but haven't verified, the 12A?7 would have wires going to #4/5 and #9. Can somebody explain this to me? I'm new to tube tinkering.

Also, the 6CG7 (noval replacement for 6SN7) seems like it would also work as a substitute. Does anyone know which heater pinout configuration this tube would require (i.e. same as 12AU7 or same as 6N1P).

Thanks in advance for any help here,

Tim

[Tim Hall]

Ok, on further inspection it seems I misinterpreted the pinout diagrams. I may have answered this question for myself (?). The 6N1P & 6CG7 have a shield seperating the two triodes, connected to pin #9, whereas the 12A?7 has the filament connected to #9. Is this correct?

Also (and don't laugh too hard at this one) do you always read pin-out diagrams as though you were looking at the bottom of a tube (or soldered end of a socket)?

[steve]

Tim,

Hi all,
I have a Gilmore Jr kit amp from Guytronix/Gerhart, and I want to test out some different output tubes. The stock tube is 6N1P, and this can be substituted with a 12AU7, 12AT7 or ECC99 if the pinout for the heaters is changed.

As far as I can tell all of these tubes have an identical pinout, so I can't quite understand why the 6N1P seems to be the exception here. The 6N1P has heater wires going to #4 and #5, but not #9. I assume, but haven't verified, the 12A?7 would have wires going to #4/5 and #9. Can somebody explain this to me? I'm new to tube tinkering.

The heaters on the some 12AX7/12AU7 and similar are wired in parallel to work on 6.3v. The tube can be wired to run on 12.6v if you use pins 4 & 5 only. Pin 9 is a center tap for the filament.

To wire a 12AU7 into that circuit you'd connect pins 4 and 5 together and connect this to one side of the heater winding. Then you connect the other side to pin 9. Since pin 9 is an internal shield on the 6N1P, you may have to unhook any other connections, it could be grounded but I'm not sure as I don't have the schematic in front of me.

Also, the 6CG7 (noval replacement for 6SN7) seems like it would also work as a substitute. Does anyone know which heater pinout configuration this tube would require (i.e. same as 12AU7 or same as 6N1P).

Thanks in advance for any help here,

Here is a great resource for looking up tube data:

http://www.duncanamps.com/tdslpe/index.html

Download the software and run it any time you want to look up pin outs for tubes. There is also a feature that displays tube numbers with common pin outs, whether or not the ratings are the same.

The 6CG7 has the same pinout as the 6N1P according to the Duncan Amps software.

To answer your second post, you read the pin diagrams as though looking at the bottom of the tube. The pins are read clockwise starting from the gap, or key.

HTH,

steve

[Tim Hall]

Thanks Steve. That confirms my understanding now. I feel a little more confident with my tinkering.

The shield from the 6N1P is not connected to ground, and it occured to me the shield should (or could) be grounded once I figured out what I was looking at. Would anyone care to postulate what effect grounding the shield would have?

[hasserl]

Nevermind, Steve already answered, & better than I did at that.

[Tim Hall]

Well, after making sure I knew how to read a pin-out diagram I got to tinkering. I built an adapter to use 12A?7-type tubes in the output, and I built an adapter to test a 6SL7 in the 9-pin preamp. I was a little surprised with the resulting combinations...maybe some of you could chime in and offer some thoughts on what I heard, and what one might expect to hear? I know this gets very subjective.

In the preamp I tested: 12AX7 (stock), 12AT7 (Groove Tubes), 5751 (Sovtek), 6SL7 (Sovtek)
In the power section: 6N1P (stock), 12AT7

First I tested the 12AT7 in both the the output an preamp. I didn’t have very high expectations for this tube since it doesn’t seem to be one of those tubes “audiophiles” do a lot of chatting about. However, this one really surprised me. In the power section this tube was noticeably more articulate than the 6N1P...in fact after swapping tubes out several times, I got the distinct impression the 6N1P was sorta “blurry” sounding compared to the 12AT7. The 12AT7 also offered slightly more output and headroom. In the preamp it was ok, but nothing special.

The other tube that surprised me was the 6SL7. This does seem to be one of those tubes that audiophiles rave about, and Alessandro uses them in many of his preamps (which is where I got the idea). Despite having the same mu-factor as the 5751, this tube was very weak sounding by comparison. The tone was also rather flat sounding. Is it possible that I could be blown away by the sound of a choice NOS 6SL7? Or does this Sovtek version give me a good idea of what the 6SL7 sounds like in general?

Of all the preamp tubes, I liked the 5751 the best. Even with less gain than the 12AX7, the 5751 seemed to offer the most dynamic response to pick attack, and the “fullest” sound.

I would appreciate it if any more experienced tinkerers would share their thoughts on this, since this is my first time doing this sort of thing.