V-4 vs V-4B preamp difference: Capacitor question

StuartV · #1 01-31-2012, 01:29 PM

Can anybody tell me which capacitor value an actual early 70s V-4B has for C7? 0.1uf or 0.01uf?

Schematic for V-4:
http://www.drtube.com/schematics/ampeg/vt22-v4-74.gif

Ampeg drawing of V-4B schematic:
http://www.ampegv4.com/images/schematics/V4B.jpg

Joe Piazza drawing of V-4B preamp schematic:
http://www.drtube.com/schematics/ampeg/v4bpre-jp.gif

All 3 agree that the preamps are different on the C3 and C6 capacitors.

But, the Ampeg V-4B shows capacitor C7 at 0.1uf and the JP drawing shows C7 at 0.01uf (which is the same as the V-4 value for C7).

So, I'm curious whether JP's drawing is wrong, or whether Ampeg drew it up one way and then ended up building them another (i.e. like the V-4)?

Thanks for any help.

nashvillebill · #2 01-31-2012, 01:42 PM

I'd say the Ampeg schematic is more likely to be right, based on experiences I've seen posted on the old Ampage forums.

Not meaning to dis JP's work, by any means, we all appreciate his efforts, it's just easy to occasssionally make a mistake.

Now notice that the other coupling caps upstream are 0.01 regardless of model. So even if the first two gain stages of the V4B pass slightly lower frequencies, they will be blocked later.

And the two amps share the same OT, which is really going to be the limitation on the bass response of the amps.

StuartV · #3 01-31-2012, 01:47 PM

Quote:

Originally Posted by nashvillebill

Now notice that the other coupling caps upstream are 0.01 regardless of model. So even if the first two gain stages of the V4B pass slightly lower frequencies, they will be blocked later.

Which caps are you talking about? I thought the other coupling caps were C3 and C6 and those are different between the V-4 and V-4B (according to Ampeg and JP). V-4 is 0.01 and the V-4B is 0.1.

nashvillebill · #4 01-31-2012, 01:58 PM

Other couplingcaps include C201, C202, C8, and C10 on the V4B. The signal has to go through all of these as it passes from one gain stage to the next.

(sorry I hit enter by mistake too soon)

sonicblue62 · #5 01-31-2012, 01:58 PM

While the electronics jargon above makes little sense to me, I have been interested in one of these amps and have read that they are essentially the same amp - is that really the case? Is the V4B a better bet for bass (since it was apperantly made as a "bass amp") or will the V4 get the same warm fat tube sound that we all know and love? While I'm here, what about the VT-22 (which is also supposedly the same amp)? Sorry for my ignorance and the slight derail, I just need to figure this out before I spend the cash.

nashvillebill · #6 01-31-2012, 02:03 PM

Quote:

Originally Posted by sonicblue62

While the electronics jargon above makes little sense to me, I have been interested in one of these amps and have read that they are essentially the same amp - is that really the case? Is the V4B a better bet for bass (since it was apperantly made as a "bass amp") or will the V4 get the same warm fat tube sound that we all know and love? While I'm here, what about the VT-22 (which is also supposedly the same amp)? Sorry for my ignorance and the slight derail, I just need to figure this out before I spend the cash.

If you look at the readily available schematics, its very easy to see that the V4 and the VT-22 are the exact same amp, only the VT-22 is a combo unit while the V4 is a head only.

The schematics also show that the V4 and V4B are identical save for a couple of minor differences in the preamp and the lack of a reverb on the V4B.

StuartV · #7 01-31-2012, 02:06 PM

Quote:

Originally Posted by sonicblue62

I have been interested in one of these amps and have read that they are essentially the same amp - is that really the case?

The exact differences are detailed here:

stonemarmot.com/rants » How To Make The Ampeg V4 Better For Bass Guitar

StuartV · #8 01-31-2012, 02:18 PM

Quote:

Originally Posted by nashvillebill

Other couplingcaps include C201, C202, C8, and C10 on the V4B. The signal has to go through all of these as it passes from one gain stage to the next.

(sorry I hit enter by mistake too soon)

I really appreciate you taking the time to educate me. I see the caps you're talking about on the schematics. And, I think I see how C201, C8, and C10 would possibly filter the whole signal.

But, C202 looks like it only affects part of the signal chain. Am I reading that wrong? I am definitely ignorant on reading schematics basic electronics in general, pretty much. Please don't take me as being argumentative. I'm just trying to understand these things (which I clearly don't now).

nashvillebill · #9 01-31-2012, 02:25 PM

V201 is a three-stage tube called a compactron. The signal goes through C201 into the grid of the first gain stage of V201. Then the signal comes off the plate at pin 2 and into the coupling cap C202. The signal goes into the grid of the second stage, pin 7,and is pulled off the plate pin 5. Then the signal goes into the grid of the third stage, pin 9, and this last stage is a called a cathode follower because the signal is pulled off the cathode pin 4 before going on to the next gain stages of the 12DW7.

StuartV · #10 01-31-2012, 02:31 PM

Got it. I had no idea how to read the schematic of a tube before. Thanks again!

nashvillebill · #11 01-31-2012, 02:39 PM

A quick Google turned up this explanation of a basic triode gain stage SIMPLE VACUUM TUBE DESIGN BASICS - TRIODE VOLTAGE AMPLIFIER

StuartV · #12 01-31-2012, 04:21 PM

Thanks for the link, Bill. Unfortunately, it is mostly a bit over my head.

And, really, my question remains, can anybody confirm which capacitor value was actually used in a V-4B for the C7 capacitor? 0.1uf or 0.01uf?

Did Joe Piazza make a mistake? Or did he actually create his diagram by looking at an amp and found that it had a different cap than the Ampeg schematic calls out?

beans-on-toast · #13 01-31-2012, 04:31 PM

According to a V4B Ampeg schematic that I have dated 5-71, the value of C7 is 0.1uF 400V.

StuartV · #14 01-31-2012, 04:35 PM

Is that the same one I have linked to above (also dated 5/71)?

The other schematic listed above says 0.01.

The question is, which one is correct (i.e. by looking at an actual V-4B preamp, not a schematic)?

SwansonEricJohn · #15 01-31-2012, 04:38 PM

I've had this done to mine:
stonemarmot.com/rants/?p=38

And noticed a slight improvement. It wasn't expensive for a tech to do it, either. Choose one or the other inputs so that the other stays stock (guitar). the V-4 is a pretty cool (and LOUD) guitar amp, too!

StuartV · #16 01-31-2012, 04:43 PM

Good input (so to speak

).

That's the same article I linked to in several recent V-4 threads. I'm taking the amp in to get serviced and the Reverb fixed anyway (just 'cause it needs to be serviced and I like stuff to be 100%, even if I'm not using it). So I was thinking that, if it's cheap and it makes one channel the same as the V-4B, why NOT do it while it's in the shop anyway?

Thanks!

beans-on-toast · #17 01-31-2012, 04:53 PM

Mine is the same one that you have referenced from AmpegV4.com. I also see Ampeg schematics for the V4/V22 that indicate that this value is 0.01uF as you pointed out.

I've found that both Ampeg and Piazza have some errors in their schematics. Sorry, I don't have a V4B in front of me to check.

If it helps, if you look at a vintage SVT schematic, I can confirm that they use a 0.1uF in that position feeding into the tone stage.

StuartV · #18 01-31-2012, 05:24 PM

Thanks, beans. I didn't even think to look at an SVT schematic. But, thinking about it, I'm not sure how much that sways me. The SVT midrange crossover points are not the same as the V-4B. The V-4 and V-4B both are switchable from 300Hz to 1000 or 3000.

The V-4 and V-4B being SO nearly identical, it makes me very willing to believe they would both use the same cap in the C7 spot.

OTOH, with the V-4B being different in C3 and C6, I can also easily believe the Ampeg schematic.

Really, it wouldn't surprise me at all to learn that some V-4Bs have 0.1uf for C7 and others have 0.01uf for C7.

Especially if it's the case, as NashvilleBill said, that the later caps, which are 0.01uf make the point moot anyway, as far as the sound in the final output.

beans-on-toast · #19 01-31-2012, 06:23 PM

You might find it interesting to check this out.

The calculator shows you what varying the coupling capacitor will do in general terms. The default values are about right. Output impedance is around the plate resistor parallel to the plate resistance, the default 38K is fine. Choose 1M for the volume resistor. The plots are interesting to look at and compare.