Wiring up 5E3 / AC15 combination. Help wanted.

[platonicbomb]

I have some old chassis from Filmosound projectors. They are 15W, 2 6V6 for power, typically 5y3 or 12ax7 for pre-amp. I've decided to gut one and create a new amp that is basically a 5E3 in the power section with a Vox AC15 front end (only the EF86 channel & no tremolo).

I don't have a great deal of experience here, but since I have a chassis with the power & input transformers & tubes mounted I believe I can handle the modification with some help. Is anyone interested in working through this with me?

THe first thing I want to make sure I understand is how to join those two circuits successfully, since there are schematics for everything on either side.

[loudthud]

Not sure exactly which AC15 schematic you are using. I'm looking at this:

http://www.el34world.com/charts/Sche...AC15_MK_II.pdf

You won't need all those .005 caps and 1meg resistors. They're part of the tremolo circuit. You are going to have quite a bit of gain so a good layout is a must. Decide where the input jack and pots will go. Hopefully the input jack will be far away from the output and power supply. Locate the EF86 socket close to the input jack. The basic topology has to be:

Input jack - EF86 - Tone/Volume controls - 12AX7 (5E3 driver) - Concertina PI - 6V6s

Locating the EF86 socket close to the input jack will allow short wires to be used. I suggest using terminal strips as opposed to an eyelet board. Solder components between the tube socket pins and terminal strips when possible. Keep all wires short. Do you have any pictures of the chassis?

[platonicbomb]

pics of chassis

these are pics i grabbed off the web, so not the actual amp i'm dealing with but the same model:

http://www.scottmcknight.com/FOSX1.JPG
http://www.scottmcknight.com/FOSX2.JPG

The inside is a rat's nest of course, one reason I'd like to gut it. But there won't be a great deal of room regardless. The chassis is about 10" long, 5" wide and MAYBE 2" deep.

[loudthud]

What are the two tubes on the end farthest from the PT? I would strip those out and install the EF86 socket there close to the microphone jack.

I can see that one of the tubes right next to the PT is a 5Y3, is the other one a 6V6? I can see one metal 6V6 towards the back, where's the other one?

[platonicbomb]

tube config, original schematic

You're correct, 2 6v6's and a 6sL7 all in a row. Then a 6v6 as an oscillator along with a 5y3 closest to the power tranny. Actually that 6v6 may be one of the power tubes and one of the others the oscillator, which doesn't really matter once the wiring's gone.

Then next to the output transformer on the far end, situated above the original 1/4" input jack, are a photo tube and a 6J7. The schematic for the original amp is here:
http://www.film-tech.com/warehouse/w...443&category=2

The schematic for this particular amp is on page 173. Good resource for learning about the iron in there.

By the way I think these little amps sound great on their own, but there's so much junk in the chassis relating to the projector input that I suspect I'll have a hard time getting a clear guitar signal unless I can figure out what to remove. If you want to tackle THAT with me, let me know. I have several of these.

I appreciate your help here.

[Chris / CMW amps]

Why not build "just a 5E3(style) amp"?

A VOX AC15 EF86 pre-amp will overdrive the 5E3 driver/splitter way too much imho...

but:
- try adding some resistors to attenuate (typo?) the signal a bit
- OR: have a look at the old Gibson GA20 schematic

Hope this helps.

[platonicbomb]

Why not build "just a 5E3(style) amp"?

Chris,
No doubt I have a lot to learn here. The idea of combining the preamp of an AC15 with a 5E3 was suggested by Antonio at Texeira. It seems like a good idea to me, though some mods may be necessary to marry the two happily. I appreciate any insight.

A VOX AC15 EF86 pre-amp will overdrive the 5E3 driver/splitter way too much imho...

but:
- try adding some resistors to attenuate (typo?) the signal a bit
- OR: have a look at the old Gibson GA20 schematic

I may get a couple of Weber's 'dial-it-in' devices to see what resistors are going to work the best.
Moses

[Bruce / Mission Amps]

From the looks of the schematic, you should be able to build a killer 5E3 type amp with this... however, wire the preamp tube sockets to use two 6SL7 preamp tubes instead.

[platonicbomb]

5E3 vs 5E3+AC15

I can sense a small consensus building toward a 5E3... any other opinions on the likelihood of success with an AC15 front end?

I have about 5 of this particular chassis with tubes and iron all mounted, so I'm not limited to doing just one thing. What I will need is a clear diagram/schematic to pull it off, not having actually designed an amp before.

Here's a link to the 5E3 schematic I've been looking at: http://www.dockeryamps.com/web/Docs/...-schematic.pdf. What changes do I need to make to it?

Thanks!

[loudthud]

I would stay with octal preamp tubes just to make things easier. You could certainly use a lower gain tube (6SN7 mu=20) as the 5E3 driver and still use a pentode in the first gain stage. If that doesn't work, a 6SQ7 is a great single triode with mu=100. All kinds of possibilities.

[platonicbomb]

$400 about right for this?

So it turns out I have found an amp tech who will do this whole project for $400 including materials. He has used these filmosound chassis to make custom amps several times. I note that a Weber kit minus the tubes and transformers will cost me $350, so I'm leaning toward what looks like a no-brainer... have a pro build this, learn what I can from the process and replicate it for myself once I have the real thing to compare my work to. Seem like a reasonable approach?

[hasserl]

I've built a 5E3 style amp with a 6AU6 pentode front end, it sounds fantastic! It's my main amp used for blues and classic rock, and has even covered metal with a dirt pedal pushing the front end (not by me, by a guy in a different band that played a set at one of our gigs using our gear). The pentode is a great way to go IMO. The gain is not too much for the amp, and depending on the guitar used you can dial in just the right amount of gain using the Volume control. I highly recommend it.

[platonicbomb]

from the possible builder:

The tech I'm discussing this project with says:

"The AC-15/5E3 would be an excellent choice. Pentodes on the front-end will definitely give you an amp that doesn't sound like anything you have right now. Also, one pentode gain stage will give you as much gain as two triode stages. The 53 Deluxe is also a good choice with its arcane circuit topology.

"Do any of these units have 9-pin miniature sockets instead of octals? That would certainly expand the possibilities. Of course 9-pin sockets could always be retro-fittted or adapters made for the octal sockets. Instead of using the EF-86 I'd probably opt for one of the many triode-pentode tubes available,e.g. 6U8, 6GH8, etc. With these tubes I can use the triode section wired as a follower to drive a more elaborate tone control stack. Followers also give an amp a nice right hand response feel."

Any thoughts?

[black_labb]

sounds good, but deffinately getting out of 5e3 territory, but some experementing is always good.

[hasserl]

The AC15 uses a LTPI, the 5E3 a split load PI, whichever one you go with will have a large impact on the sound. I went with the split load in my amp, I wanted that sound, I just wanted to have the pentode front end with it. Turned out great IMO.