12AY7 vs 12AV7

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Dave1234

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So I' m thinking that 12AV7 valves are like a tenth of the price of 12AX7. I know they are lower gain, but can they be used in a 5f1 without any issues? Did try one in my 5e3 just to try it out, but not for any extended time. Any ideas anybody?
 

tubeswell

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A 12AV7 is in that 'hotter tube' territory, similar power-rating to a 12AU7 or a 12AT7 and with an amplification factor that is mid-way between those other two types, but with a lower plate resistance and a wider bandwidth than any of the 12A_7 family. It would be a good sub for a 12AT7 in BF amps to tame down the reverb driver side, or further tame down the LTP in BF amps and give them a tamer warmer sound. Or if you have a 12AU7, subbing in a 12AV7 would increase the gain, so that's thinking for example in something like a Maggie vibrato circuit.

Having that lower plate resistance and wider bandwidth, it would 'sing' more if the load was lowered and the bias-point was cooler, compared the operating conditions you get in a typical 12AX7 or 12AY7 stage, in order to take advantage of the wider possible range of operating points.

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Taking the load line in the datasheet I linked to above, a 20k load with an HT of 350V would would yield a plate voltage of about 175 @8.75mA with a bias voltage of about 3V, which requires a bias resistor (cathode bias) of 340R, so you'd use 390R, which would run it a little cooler. At this operating point the tube would be idling at 1.5W, so its still within its Pa(max) but because of the higher current, you'd need a minimum 3W 20k plate resistor if it was a simple inverting stage. 1/4W resistor would be fine for the cathode resistor.

In such a configuration the tube could take a 6Vp-p input signal cleanly and deliver about 180Vp-p cleanly, so it'd find it most optimal use further along the signal chain is you were looking for clipping, or in V1 if you want a nice clean sounding jazz amp or bass amp.
 
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Ricky D.

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Based on a little web surfing research, it looks to me like it should be OK. You've got a 41 gain factor for the 12AV7 vs. 45 for the 12AY7 and 100 for the 12AX7.

The only issues I think you'll have will be the difference in the sound, and that's 100% your department.

Here's what I found: http://www.300guitars.com/articles/preamp-tube-gain-factors-and-substitution-chart/

12AX7 Family Tube Substitution

The 12AX7 family of dual-triode preamp tubes consists of the 12AX7, 5751, 12AT7, 12AY7, 12AV7 & 12AU7. These are all pin compatible with one another, the only differences being the gain factor of each tube. A common substitution is to replace a 12AX7 with a 5751 or a 12AY7 to tame a preamp that tends to overdrive too easily, allowing you to get a better ‘clean’ sound out of your amp.

Tube Gain Acceptable Substitutes
12AX7 100 5751 12AT7 12AY7
5751 70 12AX7 12AT7 12AY7
12AT7 60 5751 12AY7
12AY7 45 5751 12AT7 12AV7
12AV7 41 12AY7 12AU7
12AU7 19 12AV7

It is important to note that the above is not carved in stone and any of these tubes can be substituted for any other. For example in the 4×10 5F6-A Bassman the stock first gain preamp tube is designated as a 12AY7 which has a gain factor of 45. Replacing it with a 12AX7 that has a gain factor of 100 dramatically changes the tone and feel of the amp. Experiment and see what works best for you.
 

Dave1234

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Thanks for the input, it is very much appreciated. I have already constructed the 5f1 circuit based loosely on the Weber layout, but using 2 x 16uf caps in place of the original 8uf in the filter stage and a 25uf in place of the original 16uf, also in the filter stage. I was going to use SS rectification, but now have space for a full wave 5y3 with an A.c. Input of 275. I am guessing my B+ will be lower than 350, but I am unsure of the math to be honest, I will just measure it when I get the HT side done.

I have a few 12ax7's kicking around, some TAD's out of my Laney which I re-valved a while back. I might go with those to save pulling resistors off the board.

Thanks again

Dave
 

kleuck

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Beware, 12AV7 weren't intended nor built for audio use at, and most of them are microphonics ime.
 

EdMax

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Beware, 12AV7 weren't intended nor built for audio use at, and most of them are microphonics ime.

+1

best for switching operations, there were used heavily in computers because the held up well during long periods of cut-off..
 

kleuck

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I like 12AY7, but if cost is the parameter, just go for 6N2P-EV (simple rewire required) 12AY7 are not cheaper than 12AX7.
Anyway, these preamp tubes have a long life-time, cost is not really an issue imo.
 

Silverface

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Dropped in a little late but think this should be noted:

The 12AT7 is NOT always a simple drop-in replacement for a 12AX7/7025 or 5751.

It has a much lower internal plate resistance, resulting in a significantly higher current draw - and if your schematic and readings for a 12AX7 in a particular circuit were in the 200 or so VDC range with the 12AT7 you'll likely see a drop to 120VDC or so. This will not only reduce gain but thin your sound (and possibly create an excessive load on that part of the circuit, and a "domino effect" of needed plate resistor & coupling cap changes to regain some tonal "body" & reduce strain on the that part of the amp.

You really need to be able to read & unferstand tube data sheets (& be sure tubes used have been tested under load - which the average Hickok & other tube testers don't due (they can weed out duds, but unless you build your own a high voltage tester for reality testing & matching a 2nd phase of in-amp testing...preferably in a serviced but NOT valuable amp is needed for plate voltage, under-load and accurate matching/bias testing.

Chances are that swapping will be Ok - but it's that one time out of 10 when its not that expensive things go "boom".

So please - do your homework first!
 

Dave1234

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Cheers for all your input guys. I will be sticking with the 12AY7 I have from EH. I will upgrade to a 5751 NOS when one comes up at a reasonable price!
 

laird

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Alternatively you could use a 6BQ7A (38 mU gain factor) http://frank.pocnet.net/sheets/049/6/6BQ7A.pdf
being 6v tubes the heater legs are wired to pins 4 & 5 respectively. Pin 9 is not wired.
They are around $5 each or less and sound good as well.

Those look interesting! With such high transconductance and low plate impedance they'd probably make great tremolo oscillators or cascode gain stages. Another similar tube in 7-pin form (with shared cathodes) is the 6J6, also easy to find for under $5.

-Laird
 

onenotetom

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The cheap side of me is always looking for ways to substitute something to save a couple of bucks. So I am thinking yah, just change or resolder a few wires and I can start saving money on tubes. Then I think about the 76 Twin Reverb that I recently purchased that has what I believe are the original tubes.

With the number of amps that I own, and the amount that I play, if I live long enough to warrant tube replacement on any of my amps I should be happy to pay the price.

Still love these discussions though!
 
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The cheap side of me is always looking for ways to substitute something to save a couple of bucks. So I am thinking yah, just change or resolder a few wires and I can start saving money on tubes. Then I think about the 76 Twin Reverb that I recently purchased that has what I believe are the original tubes.

With the number of amps that I own, and the amount that I play, if I live long enough to warrant tube replacement on any of my amps I should be happy to pay the price.

Still love these discussions though!

Cheap you say? http://tubes-store.com/advanced_search_result.php?keywords=6v6&imageField.x=-484&imageField.y=-206
They use to be even lower price a couple years ago. Those 6V6 equivalents can handle way over 250V as listed on data sheet.Some run the at 450v. Use the search button to type in tubes you are interested in.
 
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