Subbing 6SL7's in 12AX7 circuits

[gingertube]

Schematics of my amp referred to above are posted here:
http://www.diyaudio.com/forums/instrume ... ing-2.html
Cheers,
Ian

[Phil_S]

I'm not sure which amp(s) I built doing exactly what you describe, using 6SL7's in place of 12AX7's. It was most definitely not a high gain type of amp, and probably something Fender-ish. Having never heard the original amp, there was no basis for comparison. However, with that understanding, I was very happy with the result.

I believe one build was a single channel Gibson GA-20 and the other was a Princeton 5F2 (the one with the choke.)

I bought some old stock 6SL7's on eBait for a reasonable price -- considerably less than what you'd pay for old stock 12AX7's. I did not encounter the microphonics that I sometimes see people here mention and discuss. Maybe I just got lucky? I believe I obtained examples of Sylvania, RCA, and Tung Sol. The Sylvania's are the nicest looking, with a shiny dome. All the brands were good.

I just treated them like 12AX7's and built per the schematic.

[johnnyreece]

Yeah, they'll work. I mean, one could optimize the values a little better, but it's close enough for rock 'n roll.

[Mark]

I have toyed with replacing the 12AX7 in my Champ clone with an octal tube as I've heard they sound better. I might have to try it over The Xmas holidays, I'll have to check but I suspect a purchase is in order.

I seem to recall 6SC7's were popular and hard to get hold of.

[Merlinb]

Something else to consider is that it is usually harder on the heater elements to run them on DC if they are not specifically designed to do so.

Not sure where you've picked this idea up from, but it's not true. DC is not worse for the heater than AC.

[rp]

I have toyed with replacing the 12AX7 in my Champ clone with an octal tube as I've heard they sound better. I might have to try it over The Xmas holidays, I'll have to check but I suspect a purchase is in order. I seem to recall 6SC7's were popular and hard to get hold of.

Not better at all, just different. Mostly they just look cool, big and chunky parts is the best part of tube amps to me so bigger chunkier pre valves is added fun.

They hum more, I don't think they have spiral filaments which seems to have come in with the minis, but one won't be a problem. they are more microphonic as their inner structure is bigger. Much of the mojo is from the primitive circuits (grid leak bias, cathodyne) they went into rather than the tube itself. Try them for the fun not to expect a revelation.

The champ tube is the 6SJ7 - a pentode and part of the mojo of the 5C1. The 6SC7 has a shared cathode and won't sub into a 5F1.

[Cliff Schecht]

Something else to consider is that it is usually harder on the heater elements to run them on DC if they are not specifically designed to do so.

Not sure where you've picked this idea up from, but it's not true. DC is not worse for the heater than AC.

I guess for most any tube we deal with in guitar amps, yes this isn't true. It's really an issue with directly heated cathodes in higher power tubes. I learned about it from some older ham radio friends. This guy provides a clear explanation (http://www.antiqueradios.com/forums/vie ... p?t=123840):

It is the higher filament directly heated tubes that may give you a problem. Such tubes were the 203A and 211. Operating them on DC meant that one end of the filament was always more negative than the other end, and most emission would take place from that end. And the filament would wear thin at that point.

Early shipboard CW transmitters that used these tubes had a polarity reversal switch on the front panel and it was up to the operator to insure that every day, the switch was thrown to the opposite position so it balanced out the filament wear over time.

However using indirectly heated cathodes, it does not make a bit of difference. And with tubes like the 26, where the filament voltage is only 1.5 volts, this "more emission from one end of the filament" is just not going to happen. At the plate current you are going to be seeing, the filament is not called on to do that much emission as in transmitting tube service.
Curt

[Merlinb]

I guess for most any tube we deal with in guitar amps, yes this isn't true. It's really an issue with directly heated cathodes in higher power tubes.

Ah yes, DHTs (directly heated triodes). That makes more sense.

[Cliff Schecht]

wasn't there also about DC being non-optimal for tubes with controlled heater starts too? For some reason, I think this came up in a discussion of less-common variants of otherwise once-common octals, maybe for a PI?

The idea is that when the heaters are cold, they have a high inrush current and can sometimes develop the so called hot spots momentarily that weaken the heater more quickly over time. I'll leave it up to you to find some reference material on how true this is .

[angelodp]

I realize that this is a very old thread. I am contemplating a Tweedle Dee with 6SC7 in V1 and 6SL7 for PI. Based on what I read here it seems doable. Love to hear if any have done an Octal front end for a Tweedle Dee.

A

[Stevem]

If the amp your building has the room for a few more 12ax7s you can run there 2 sections in parallel and get all the full good effects of the Octal s with none of the down sides, in fact a 12BH7 is a nice sounding big plate 9 pin tube!

I love the sound of SL7s and SN7s, but not in a combo amp of 25 watts rms or more that gets cranked since they are far more microphonic than the 9 pin family of tubes with that level of SPL hitting them.