Preamp Tube Substitutions

Fresh_Start

Fresh_Start

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Glassman was nice enough to send me a PM and straighten me out on a couple of misconceptions I had about electronics, specifically in post #10.

Most folks tend to think that if a tube is called 12A?X, then it can be substituted for any other 12A?X tube. That is not necessarily a good assumption.

12AX7 mu = 100
5751 mu = 70
12AY7 mu = 44

These three tubes have very similar specifications are should be compatible in a 12AX7 spot other than the differences in the amplification factor ("mu").

Depending on your amp's circuit, it is possible that a 12AT7 and 12AU7 tube could cause problems.

If an amp has a mixture of tubes and solidstate devices, even if the ICs, FETs, or whatever they are only serve switching functions, then excessive current might damage the solid state circuitry. If you or your amp tech are sufficiently knowledgeable about your amp's circuit, then swap away. However, sticking with the first three tubes listed above may be safer if you are unsure.

Here's a piece by Bill Machrone:
http://billmaudio.com/wp/?page_id=131

Please note that several of the comments on Bill's page from guitarists report good results using a 12AT7, 12AU7 or even 12DW7 (half AX7/half AU7) tube.

Tube data sheet search for anybody who really wants to dig into this subject:
http://www.mif.pg.gda.pl/homepages/frank/vs.html

That site also is useful because it shows alternative tubes which are direct substitutes.

Cheers,

Chip

Please note - I thought seriously about just deleting this thread. Glassman advised me to correct what I could and emphasize that the conclusion(s) aren't etched in stone, so I've left it.
 
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Re: Preamp Tube Substitutions

Chip

I have used a an ancient RCA 12at7 in V1 of my Blues Jr for about a year. Will this harm my amp? The tone is very big with the 12at7 and the amp really shines when pushed by an OD......Would a 5751 be a good alternative?

Thanks,

Joe
 
Re: Preamp Tube Substitutions

this got me all worked up so i did some tube swaps! Trouble is, i have a shortage of AT7s...so all i did was try a few different brands of ax7 in my 1967 fender twin.
Funnily enough, the JJ which is often my least favourite came out as the best sounding in that amp! Maybe cos its fatter rather than bright, and all that prodigious clean volume sounds more balanced and warm with the smooth tops of the jj tube. I was expecting the tung sols to sound the best, but i think maybe they are better suited to dark amps, or where you want to use some mild overdrive. They are my favourite in my princeton reverb II because. The top end gives some cut. EH are nice and stong and competent for all round use - they work nice in my 900 master volume marshall. I have a bunch of old haltrons and philips etc, but seeing how few people are likely to use them im wont comment.
What i did find today is that some tube brands work in some amps better than others. There is no best brand for everything eh?
 
Re: Preamp Tube Substitutions

Gibson, try EH preamp tubes, they're kinda dark as well, you may like them. Also shuguang, or groove tube gt12ax7c's. same thing, but they're a little brighter than the EH
 
Re: Preamp Tube Substitutions

yeah i have a heap of eh preamp tubes, and soms shugang, and a few others (rubys, jolidas etc including phillips and haltron).
For me the EH, tung sols and jjs are all really good for different applications. Ive also used GTs but they are really sovteks. wa, wb and wc are not as good as the EH i dont reckon.
The only current 12ax7 tube i havee not tried is the winged C, but between EH, jj and tung sol, i reckon anything i need is covered for various amps.
 
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Re: Preamp Tube Substitutions

I've found that subbing a 12AX7 for a PI in my DRRI (normally AT7) gave earlier breakup with a little less clarity. I changed back pretty quick so that I could really get used to the normal config. but sometime I'll switch back again and really see what I like more.
 
Re: Preamp Tube Substitutions

Not sure if I'd agree with the statement about the extra current being harmful. A the idle current between a 12AX7, 12AT7 and 12AY7 is not obscenely huge. Let us take the case of something approximating the input stage on a Fender or Marshall or other amplifier. Plate resistor of 100kΩ, cathode resistor of 1.5kΩ, 1MegΩ grid leak, 68kΩ grid stopper. 300V from the power supply to the plate. Drawing a load line, a 12AX7 will idle around 1mA, a 12AT7 will idle around 1.5mA, and a 12AY7 will idle at 1.6mA. Worse case instantaneous current through the tube (with the grid at 0), is 2mA for 12AX7, 2.7mA for a 12AT7, and 2.2mA for a 12AY7. If you could get the grid positive enough to pull the plate voltage down to 0 (which you can't) the maximum current through the tube would be 3mA for all three tubes, as that's the maximum current you're gonna get through a 100kΩ plate resistor with a supply voltage of 300V, period (Ohm's law).

The variation here is on the order of tenths of a milliamp, which in of itself is already a 1/1000 of an amp. I doubt a JFET or relay (especially a relay) tied to the plate or cathode is going to be damaged by few extra 1/10000s of an Ampere. Also, the extra current through the tube isn't going to be seen anywhere further along in the signal chain. With a Fender style plate-driven tonestack, that extra current is going to be sucked away by the tonestack's insertion loss (that thing which makes it useful as a tonestack). Also, in every amplifier circuit I've ever looked at, the individual stages are separated by capacitors, which permit the signal to pass through (AC) but block DC (those extra tenths of a milliamp we're worried about). So the next stage doesn't even see the extra current.

Finally, the plate dissipation ratings aren't even going to come into play. All those are is how much power (volts and amps combined) you can put through the plates before you're at risk of overheating and damaging them. At the voltages and currents found in 12A*7 tubes in a preamp application, you're not even close to overheating the plates.
 
Re: Preamp Tube Substitutions

bungalowbill said:
Chip

I have used a an ancient RCA 12at7 in V1 of my Blues Jr for about a year. Will this harm my amp? The tone is very big with the 12at7 and the amp really shines when pushed by an OD......Would a 5751 be a good alternative?

Thanks,

Joe
Click to expand...

Joe -

If nothing bad has happened in a year of regular use, my guess is that you're fine. If I were starting from scratch, then yes I'd try a 5751 instead. Actually, I have tried a 5751 in the V1 spot of a Blues Jr. and it could be a great tone depending on what you're looking for. In a Blues Jr., the Fat switch introduces a cathode bypass cap for the second gain stage (part of V1) via a FET. Otherwise, there aren't any solid state components tied into V1.

Koreth - you're probably correct about the current differences not being big enough to do "bad things" to solid state components under normal operating conditions. Although going from 1ma to 1.7ma is a 70% increase. However, what happens at initial turn on? A spike in the input signal or the AC supply voltage? Why take the chance?

[deleted incorrect statements]

Bottom line for me is that if it's a all-tube amp, it shouldn't hurt anything. If there are solid state switching elements, etc. however, I'm sticking with the substitutes I listed earlier unless you or your amp tech really understand the circuit.

Hope that helps,

Chip
 
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