Help with Tubes for an AC-30

[jsm610]

Apologies in advance for my complete ignorance, but I need to replace the tubes in my AC-30/4 and have no idea what I'm doing. I've searched for "tube layout" "tube config" - everything I can think of, but can't find any info on what tubes go where on the chassis?

Can anyone help me? What tubes should go where (I labeled the tubes 1-10 on this chassis photo)? Mucho thanks to anyone who can help!

[johnallg]

Tubes 7 , 8, 9, and 10 (V3, V4, V5, V6) are the EL84/6BQ5 tube finals. Tube 6 is the 5AR4/GZ34 (V10) rectifier. There are four 12AX7/ECC83 tubes (V1, V2, V7, V9), and one 12AU7/ECC82 (V8) tube. Those are your tubes 1-5, but I'm not sure which one is the 12AU7. The tubes should have the numbers on them, I would think. Here is the schematic from the Vox site: http://www.voxamps.co.uk/downloads/circ ... 301960.jpg

[jsm610]

Thanks John. But I'm confused: I thought the AC-30/4 had an EF86 (or more than one) - the research I've done indicates thats the main difference between the AC-30/6 and AC-30/4?

[sharkboy]

Aside from the power tubes (EL84 tubes) and GZ34 rectifier, I am having trouble correlating the exact tube locations of the preamp tubes. I agree that an AC30/4 should have one EF86.

From this schematic http://www.bnv-gz.de/~ooehmann/schemati ... AC30-4.gif , you can see that there is an EF86 as V1 (their numbers do not correlate you yours), two 12AX7s in the other preamp, another 12AU7 in the reverb circuit and a 12AX7 in the phase inverter/splitter section. That should account for the number of tubes, but I'll keep looking around for more info on the specific locations of preamp components.

[jsm610]

Ok - thanks to all of your feedback, plus some input from the seller of the chassis and some input from Glen Lambert, I think this is what we have:
AC-30/4 Tube Layout (number per the above graphic, 1-5 are the pre-amp tubes from the end closest to the input jacks, 6 is the rectifier, and 7-10 are the power tubes)
1 - 12ax7
2- 12ax7
3 - 12au7
4 - EF86
5 - 12ax7
6 - GZ34 (5AR4)
7 - EL84
8 - EL84
9 - EL84
10 - EL84

These 4-input AC-30's are supposedly really rich and brilliant sounding - can't wait to hear it fired up!

[johnallg]

Thanks John. But I'm confused: I thought the AC-30/4 had an EF86 (or more than one) - the research I've done indicates thats the main difference between the AC-30/6 and AC-30/4?

That was what was confusing me about the /4 - I had seen the schematics with the EF86, but the ones I saw didn't have the other 12AX7 tubes. Then I saw the schematic on the VOX website and thought that was it. I had the other 5 tubes right!

[Nickcha]

Ok - thanks to all of your feedback, plus some input from the seller of the chassis and some input from Glen Lambert, I think this is what we have:
AC-30/4 Tube Layout (number per the above graphic, 1-5 are the pre-amp tubes from the end closest to the input jacks, 6 is the rectifier, and 7-10 are the power tubes)
1 - 12ax7
2- 12ax7
3 - 12au7
4 - EF86
5 - 12ax7
6 - GZ34 (5AR4)
7 - EL84
8 - EL84
9 - EL84
10 - EL84

These 4-input AC-30's are supposedly really rich and brilliant sounding - can't wait to hear it fired up!

Correct. These can always be confusing because the visual orientation of the layout does not line up with the V1 - V10 schematic. Below is a pic of my '60 AC30/4 (looking at the chassis from the front). From this orientation the 5 pre-amp tubes have covers on them and are perpendicular to the 4 EL84 power tubes. From L to R they read like this:
1. Phase Inverter 12AX7
2. Normal ch. EF86
3. Vib/trem modulator 12AU7
4. Vib/trem oscillator 12AX7
5. Vib trem input 12AX7

Obviously the larger upright tube to the left of the PI is the GZ34 rectifier.

On the AC30/6s that followed the difference was that the #2 position became a 12AX7 and being a double triode, it provided the preamp gain stage for both the Brilliant and the Normal chs. Other than that, the configuration remained the same until '64 when the TB circuit began to be integrated into the chassis rather than added on the back.

So please tell, how do you like your AC30/4. I've had many AC30s between '60 and '64 and the /4 is in a category of its own. I like the /6 for its better versatility, and the /4 is unparalleled for its thick chewy gain. However, that's basically the one thing the /4 does. But it does it like no other.

[paologregorio]

Thanks for posting all the pics of the vintage AC30s. It made me realize that I was missing the power tube spring retainers, and one of my tubes is making a microphonic buzz-most annoying. I ordered a set of fancy/deluxe EL84 spring retainers, along with a 12 AU7, as I had a 12 AX7 in that spot, which I didn't realize I had mistakenly placed in tube socket 3 until a few days back while I was going through the amp trying to source the microphonic tube, and referring back to my schematic from Aspen Pittman's The Tube Amp Book.

Good luck with the re-tube!

[Nickcha]

Stephen Grosvenor has compiled data and written an excellent handbook that I'd recommend for every vintage JMI VOX AC30/15/10/4/2 owner. It's called "A Service Engineer's Guide to the Vox AC30 Valve Amplifier", and he has a website called TheVoxAC30Guide.com, where you can find out about the manual. It's a treasure trove of good data and specs for anyone getting under the hood and messing around with one of these beauties. He covers the entire AC series beginning with the 30/4 in 1959.

[jsm610]

So please tell, how do you like your AC30/4. I've had many AC30s between '60 and '64 and the /4 is in a category of its own. I like the /6 for its better versatility, and the /4 is unparalleled for its thick chewy gain. However, that's basically the one thing the /4 does. But it does it like no other.

Sadly, been too busy to put it all together. Its sitting in the corner of my office wanting....

See you're down the street - greetings from St. Pete!

[Nickcha]

John, why don't you come to TONEfest this Fri & Sat, in Clearwater.