replace one or all speakers?

[flavourful]

So I got this cool old ampeg vt-40 amp off ebay, but it needs some work. I'm in the process of ordering fresh tubes, but I found that it has two blown speakers as well. At the moment the distortion channel sounds like my amp is dieing, and the clean isn't too pretty either. It's set up with 4 10" speakers like the fender deville. so, the question:

Do I replace two speakers or replace all four?

Will the thing sound funny with mismatched speakers, or won't it matter so much? Also the speakers I'm looking at are the Electro-Harmonix '10cs' ones by eminence. They were recommended by an ampeg enthusiast site and are really cheap. Has anyone here tried em'? Are they any good?

(btw I can't match whats in there. it's two different types already and they both have no labeling. I also can't find what kind of speakers it was originally made with, if you know a good site for this type of vague information tell me!)

[Rob Mercure]

The VT-40 came with CTS 10" speakers - there should be a date/manufacturers code on such speaker that starts with the numbers "137."

Rob

[flavourful]

thanks for the speaker info, but it looks like they're replacements. I think I'm just going to replace em' all. But now I have a new problem. I was looking at the schematics:

Pre: http://www.drtube.com/schematics/ampeg/vt40pre-jp.gif
Power: http://www.drtube.com/schematics/ampeg/vt40pwr-jp.gif

Now I'm not that great with schematics and such, but it seems like there should be 9 tubes total, right? It goes up to V6, but there's also a V201, a V202, and a V203, making 9 tubes. The problem is when I looked at my amp there's 8. What happened to that last one?

There's also the problem of some weird tubes. the power tubes were unmarked aside from company and the numbers (76-13 274), there's a strange '6an8a' one in here as well as a '7247'. obviously this is not what I expected. are these close replacements for some of the stock tubes? They're numbers I don't recognize.

This mismatched number of tubes thing is completely throwing me, and I can't even identify which one is missing with the weird ones that are in there. Any input? I could sure use it...

[Rob Mercure]

My Ampeg service manual lists the "4-71" circuit which is obviously different from yours - perhaps a thorough web search will provide other circuits. Have you checked out the "Unofficial Ampeg" site?

Anyhoo, the circuit I've got has 9 tubes - 3-12AX7, 1-12AU7, 1-12DW7 (7247), 1- 6CG7, 1-6K11, and 2-7027s.

Rob

[Keystring]

Originally Posted by flavourful

thanks for the speaker info, but it looks like they're replacements. I think I'm just going to replace em' all. But now I have a new problem. I was looking at the schematics:

Pre: http://www.drtube.com/schematics/ampeg/vt40pre-jp.gif
Power: http://www.drtube.com/schematics/ampeg/vt40pwr-jp.gif

Now I'm not that great with schematics and such, but it seems like there should be 9 tubes total, right? It goes up to V6, but there's also a V201, a V202, and a V203, making 9 tubes. The problem is when I looked at my amp there's 8. What happened to that last one?

There's also the problem of some weird tubes. the power tubes were unmarked aside from company and the numbers (76-13 274), there's a strange '6an8a' one in here as well as a '7247'. obviously this is not what I expected. are these close replacements for some of the stock tubes? They're numbers I don't recognize.

This mismatched number of tubes thing is completely throwing me, and I can't even identify which one is missing with the weird ones that are in there. Any input? I could sure use it...

The 7247 appears to be an industrial replacement for the 12AU7. They both have the same 'amplification factor' of 17.

Not sure why there is a 6an8 in there. Unless it was used to replace the 6cg7 which is used for the reverb circuit.

If that's the case, then it's possible that the tube socket was rewired because pin 9 on the 6an8 is tied to the first grid and cathode. And the 6cg7
has pin 8 tied to the cathode only.

You need to find the correct schematic for your amp. Although I did do a search and the one you found seems to be the only one out there.

Your amp may have been 'modded'.

You also say that your amp has 8 tubes, instead of 9. Is there an open 'hole' in the chassis where another tube socket might have been?

[Rob Mercure]

Keystring,

The 12DW7/7247 is a dissimiliar twin triode - 1/2 of a 12AU7 and 1/2 of a 12AX7 - not an "industrial replacement for the 12AU7" which would be the 5814, 5963, 6067, 6189, etc., etc., - there are numerous industrial and other variants of the 'AU7. And the 6AN8 is a triode/pentode not a dual more or less "power" triode. While it is possible that there was some mods - after all the 6FQ/CG7 was Ampeg's all time "favorite" reverb driver - I suspect that this is simply a model variant.

What would probably help is if flavourful were to date some of the amplifier's components to give us a general idea of the manufacture year.

Rob

[flavourful]

hehehe... now I feel like an idiot. on the back in tiny print it lists what tubes go where in the amp, and most of mine are correct. The 7247 is in place of what should be a 12dw7, which is kinda weird. anyways the thing is dated between 76'-79' and the model # is 1103, though I couldn't find anything about that on the web, but I don't really need the schematics anymore anyways. Thanks for the input, it really did help. well I'm off to work, and when I get back I'm ordering tubes. Thanks!

[NoFi]

Hi, there are several variations of the Ampeg V series circuits.
If your amp has a distortion knob, then the schemo for your amp is this one (maybe you'll need it someday) :
http://www.ampegv4.com/images/VT22_Schem2.jpg

It's a common shcematic for VT40/VT22/V2/V4 models with the distortion knob.
The distortion thing will always sound dodgy, it's the design.

And for the record my VT40 (export model) came with Rola speakers.

[Rob Mercure]

Hey NoFi,

Thanks for the schemo - I've added it to my electronic collection. But I just wanted to note that my 1977 Phillips/Magnavox V-4 master volume model uses the most commonly available V-4 schematic - 6-70, part # 4010244 - with R22 or R21(can't remember right now) in the cathode of "section 2" of the 12DW7 (the first one that the signal encounters - for some reason the triode using pins 1-3 is referred to as unit "2" in 9A basing) replaced with a potentiometer for the master volume. The schemo you provided is missing the lower right hand corner
"info" box but it is from 2-6-76 and is part # 591761-1

Rob