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Hi Guys
Is it possible to replace 12ax7s with 12at7s. From reading the web I can see there will be a reduction in the gain. Will that be the only difference?. I would be using them in a Yaqin Mc-100b. I understand that the front end of this amp can be overloaded by the the stock 12ax7s.
Thanks for your help
Regards
Follow Ups:
I agree that 5751 is your first port of call. More likely to fit the circuit. If you are just trying plug'n play remember the E80CC. Expensive and basically just a noval ECC40, but it's a better sound than the 12AT7. It's a bit taller, though, so factor that in.
andy
I replaced the tube in my phono preamp from a 12ax7 to a 12at7 and it works great. If you want it even lower you can try a 12au7. It all has to do with the output of the cartridge I use. The higher the output the lower the gain of the tube. You can get some at7's or au7's cheap,it will not hurt to give them a try.
A 12AX7 is usually set up with about 1ma of plate current. The 12AT7 should use 3-10ma. Thus with the stock 12AX7 plate & cathode resistors the 12AT7 plate voltage will drop perhaps as low as 60 volts or in a less linear part of the plate curves.
Does direct drop-in work? Yes. However, a better drop-in 12AX7 type tube with lower gain is a 5751 (70 vs 100).
Where did you get that the 12AX7 will overload in this amp? You won't clip the AX7 with any source, at any reasonable level.
The 12AT7 may work, it won't sound right, and will probably overload sooner than the 12AX7 simply because it won't be running properly.
Hi
Thanks for the reply. It would appear that the MC-100b has a gain problem in the front end. This is from various sources on the web. One solution is to use 5751s this I have done with what I feel some success. I just happen to have a set of 12at7s and was wondering what the effect would be. It's not clear to me if these are a direct replacement for the 12ax7s. I am a tube novice and dont want to cause any damage to the tubes or the amp.
Regards
Your amp has a gain switch. Set to the 600mv setting it reduces the gain of the 12AX7 stage.
Hi Blackdog
With the 12ax7 (which were the chineses stock tubes) I did feel that it was providing some distortion. With the change to the 5751s I felt that there was a more refined sound with everything more under control and I have been using it like this for nearly a year now.
I dont use the input switch on the front only the RCA sockets on the rear as I dont have a separate pre-amp and have more than one source.
In passing I wonder if you happen to know if the TJ full music 6SN7s will actually fit. The tube sockets are quite close and from pictures I've seen the TJs seem to be a bit fatter. I was looking at some Black Treaures but these appear to be far to big.
Regards
"With the change to the 5751s I felt that there was a more refined sound with everything more under control"
That could be because the 5751's are older American made tubes, and the 12AX7's were newer imports? Older American made stuff always sounds better IMHO.
You won't damage anything, but it probably won't sound quite right. I've an MC100 on the bench and didn't find any issues with the 12AX7. How does it sound to you with the 12AX7?
If excess gain is the issue, try the 12AT7. If this the input stage that you are experimenting with? Heck even try a 12AY7. One you've found a tube that gives you the gain you can live with, you may need to tweak the parameters around the tube to get the most desirable response. Sometime, unit don't even need this tweaking... and seem to be fine with the altered tube.
Anyhow, good luck!
We've all been through this before - a change to a 12AT7 doesn't change the gain much.
While indeed the 12AT7 has 40% or so lower mu than the 12AX7, it also has much lower plate resistance; at 250 volts the 12AX7 has a plate resistance of about 62.5K while the 12AT7 is about 10.9K at the same voltage. This means that much more of the voltage drop will occur across the plate load resistor instead of the plate. And that means more gain from a common cathode stage. With a fully bypassed cathode resistor and a 100K plate load resistor the gain of a 12AX7 is about 61.5 and the gain of a 12AT7 is about 54. So there is not very much difference, only about 13% less with a 12AT7 - all else being equal.
Add in the gain leveling effect of NFB - either local or loop - and it's going to be difficult to find much gain difference in most cases.
The largest issue you face in a swap is that the operating point of the 12AT7 and the 12AX7 are significantly different. Generally I find the 12AT7 likes to run about 3X the plate current of a 12AX7.
So swapping the two really has little effect on gain, but some effect on tone. Some like the tone after a swap, some don't.
To get any significant gain drop you need to use a 12AY7/6072 which has 1/3 the plate resistance of a 12AX7, but it also has only about 40-44% of the mu. Using the same operating conditions as above, putting a 12AY7 in the circuit would yield a gain of about 34.4, roughly half of a 12AX7.
Jim
Some say to drop the 12at7s in the mc240s and the mc60s in place of the 12ax7 in the final stage..Its a cathode follower but its plate is loaded with the OT primary.I don't recall hearing that much difference.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
Well, as a cathode follower it will always be less than unity gain, so I doubt the mu difference is going to be audible.
The 12AT7 has a much higher Gm though, which may well be audible. I've never understood why Mac ran the relatively beefy tubes (12BH7, 12AU7, etc. ahead of the drivers and then used such a wimpy low transconductance tube as the driver. Especially as the signal voltage drives the power tube grids close to zero volts the drivers are called on to supply a bit of current - and a 12AT7 can do that much better than a 12AX7. The lower output impedance of a 12AT7 follower would be useful in that position I'd think.
But the amp is a classic, so?????
The 12AT7 has a much higher Gm though, which may well be audible. I've never understood why Mac ran the relatively beefy tubes (12BH7, 12AU7, etc. ahead of the drivers and then used such a wimpy low transconductance tube as the driver"
thank You Jim,
I thought was just a little dense and missing something when I went over Mac schemos a while back.
Yes, the amp is a classic? the iron is classic, and the unity coupled circuit, I will give them that.
The Mind has No Firewall~ U.S. Army War College.
Jim
Thats actually what they did with mc275,they used the 12at7 in the final stage.
I've never understood why Mac ran the relatively beefy tubes (12BH7, 12AU7, etc. ahead of the drivers and then used such a wimpy low transconductance tube as the driver.
Its kind of strange but the 12uau7 of course is the phase inverter and the 12bh7 is where the voltage gain takes place.Mac does most all their voltage ampification in those primary stages and once its fed to the output stage,the kt88s produce a 2 to 1 gain but its also amplifying current with the unity coupled circuit.You can actually pull the output tubes out of a mac and drive most speakers to listenable level.You won't have any bass due to the impedance mismatch but you can drive them.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
What about that nasty 12AU7?
I change those to a 6cg7.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
Jim, yeah I usually have costumers try various 12A(_)7 types, to see if anything, it a head in the correct direction. Or a move away from the desired endpoint. After say, someone settles on a 12AT7, I will look at the scheatic and try to adjust the parameters (idle bias current), etc to get the tube more into the linear region--- if desired by the owner.
A lot of ppl want to keep this stage (usually input) as amendable to the various tube types. To keep swapping tubes with various songs, CD's, guitars, styles of music wanted.
For those new on the market DIY 5E3 and Champ 5F1 kits... the manufacturers really over drive the input 12AX7. Way over the original tweed settings. This leads to a not-so-subtle onset of OD esp with guitars that have hot pickups. Or gain in the stomp boxes. The 12AY7 is a great sub (almost an automatic sub) for these DIY. To get them to sound like what a tweed should sound like.
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