Harp amp mod for dummies

[sgfable]

Just got into playing amplified and picked up a VOX AC4TV to try out as a harp amp. It has an EL84 with a 12AX7 in the pre-amp. Would swapping out one or both of these tubes result in better volume/less feedback? Any easy DIY mods for the novice for this thing?

[sbluestubes]

Swap out the 12AX7 with a 12AU7

[Jaybird]

If you want to try tube swaps, I would suggest trying a 12ay7 or a 5751 for the preamp tube. A 12au7 will make your sound weak and muddy as it has only a gain factor of 19. An 12ax7 being 100, 5751 70, An 12AT7 60 (with poor tone qualities), 12AY7 45, and the AU7 19.
This will help some feedback problems caused by high gain. Sometimes tone searches can be solved with different speakers.

[Jaybird]

"The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 type has a gain factor of 100, while a 5751 (which is often used in place of a 12AX7) has a gain factor of 70. This means that if you plug a 5751 into a socket that expects a 12AX7, the pre-amp will have about 30% less gain. Not only will this make the amp quieter, but it can also alter the sound by making the power section work harder when you turn it up. Many guitar amp users (particularly harmonica players) like to reduce the gain to get a different sound or calm the amp down to prevent feedback."
Guitar amp tweak - change the gain factor of your pre-amp

[Jaybird]

An added note to my first suggestion of tube replacements, I just installed an `12AT7 in my amp to see what it sounded lie, and I think it sounds just fine. Better than the 12AY7 that was in. A little more clean headroom with no loss of tone to my ear. I use my amp for harp, so with a guitar, you may hear it different. The 12AU7 just took all life out of the amp.

[Howlin' Mad Mac]

12AT7's can sound just great & easier to find than 5751's... problem is a lot of them are microphonic so as long as you've got a good one should work just fine

[pops]

I like the 12AY7 for harp, if you need more umph give it more volume and turn the tone controls up. Usually get more volume before feedback and a warmer tone. But then that's just me

[RitzRed]

I have a little Kent 3 tube amp that is a little quiet for me it has one 35W5 and one 50C5 And one 12AU7 tube in it do you think it will get more gain and or more volume, if I swap out the 12AU7 and replace it with a 12AX7 and as I am new to all this, does it have the potential of harming anything in the amp by swapping out the tubes

[JHow]

I have a little Kent 3 tube amp that is a little quiet for me it has one 35W5 and one 50C5 And one 12AU7 tube in it do you think it will get more gain and or more volume, if I swap out the 12AU7 and replace it with a 12AX7 and as I am new to all this, does it have the potential of harming anything in the amp by swapping out the tubes

Ritz, man, doesn't your Kent have a 12AV6? Take a look at the data sheets, its totally a different animal than the 12AX7, starting with the number of pins, but also and mainly in the fact that inside the tube is completely different as well. Your 12av6 is one triode and a couple of diodes in the same package. The 12ax7 is two triodes in one package.

You can download data sheets here.

electron Tube Data sheets - 1

One of the things you could consider, because I don't think one single book will do it all, is if you live near a university or college with an engineering school library, or physics library, or similar, you may be able to still find old textbooks dealing with vacuum tube circuits. I say this because reading the materials is usually free (even if you aren't a student), but also because old textbooks will explain tubes alongside of basic electronics concepts (like ohms law) at the same time, so you kill two stones with one bird and learn electronics and the tubes that you are interested in (just ignore it if you run across a section on "waves in ether" as I have seen in a 1946 vintage radio book - ha ha). Also look for any old RCA tube manual which usually has intro sections on how tubes work.

You might explore some of the intro chapters in some of the books here to see what I mean:

Technical books online

[RitzRed]

yea after looking at it again it does have a 12AU6 different all together I have been working on a different amp that has all the same components as a Fender Princeton I have not gone all the way through the schematic matching it to the amp I have but so far everything is spot on the same resistors tubes and caps all wired the same in the same spots I am hoping to make this amp into a harp amp