top mount ampeg VT40

methodofcontrol · 10-04-2006, 01:24 AM

i've got an old top-mount VT40 that's got minor issues, the reverb seems to get stronger the longer the amp is on...is that a tube issue? i've got a similar vintage V4 that i can swap tubes with.
also, w/ the 300hz mid cut the amp sizzles and crackles w/ the low notes. but when i bring the 300hz back up to flat, the problem goes away.
lastly (for the moment) the power light does not work. it's wired directly to the power transformer and power switch. i've tried jiggling the wires to see about a bad connection, but i get nothing. is it likely a bad bulb? (i've heard that neon bulbs never really go bad, so i thought it more likely a connection issue.)
i'm in need of a "helmet a" logo for this amp, a standby switch, black "period correct" ampeg knobs, period correct fuse holder, and possibly a power light.
hopefully someone knows where i can strip an old junker of these parts!
thanks

Enzo · 10-04-2006, 02:03 AM

The power and standby lights are indicator assemblies, not simple bulbs. But all that means is there is a resistor in series with the neon bulb inside. So the whole thing can be slapped across the mains. They are available on the parts market. I mean like Mouser, not antique AMpeg parts.

Of course neon bulbs can fail, there is not a component made that can't fail. Neon bulbs naturally darken with age - this can diminish the effect of a trem for example. I used to buy the neon bulbs for strobe tuners by the hundred. But beyond that, wires can break off, the bulb can crack losing its neon, the resistor can open, the connection within can fail.

I'd make sure the mid switch was super clean. Aside from that, there is only so much the switch circuit involves.

Growing reverb? COUld be a tube, could be a cap that wakes up, could be a resistor value shifting as it warms. Determine if the drive is getting hotter or if the recovery stage is. Scope the drive signal and watch its level. Apply a stewy signal to the return jack and see if the output to the amp is stable or not.

CEDist has an AMpeg logo, see if it suits - stock S-M920

methodofcontrol · 10-04-2006, 02:51 AM

thanks for the tips enzo.
the logo available from CEdist is a current production model. not the same as the vintage ones.
as for neon bulbs, yeah...i know ANYTHING can fail, i just thought it was more likely a problem other than the bulb itself. if i pull the bulb and hook it up to a 9v battery...should it light if it's good? also...i know the standby lights in v-series ampegs typically have resistors in series w/ them. i've replaced a bunch of them. but according the schematic and my visual inspection, the power light goes straight to the tranny.
thanks for your help, i'll swap the pre tubes from my V4 and see which problems go away. i'll post back for more info/help.

Enzo · 10-04-2006, 07:30 AM

You didn't quite follow. The 120VAC neon pilot light is an assembly with a resistor INSIDE it in series with the bulb. The whole thing runs on 120VAC. It is wired to the mains, just as is the tranny. It isn't wired to the tranny in the sense that the tranny matters to it. Remove the tranny and the powr light should still come on. 9V will never light a neon bulb. SInce the standard indicator lamp assemby expect to run on 120VAC, when they standby light is wired to the B+, that is hundreds of volts higher, so an ADDITIONAL series resistor is added external to the lamp assembly.

I forget, but it seemed to me that the basic NE2 neon bulb needed about a 220k resistor in series for operation across the 120VAC. That is more or less what is in the indicator lamp assembly - little bulb and high value resistor. That resistor value might be wrong, I forget stuff easy.

Look in Mouser under Indicator, they have it broken down so there are neons on three pahes. IN catalog 627 page 29, 33,78. CHicago Mini on page 29 has a ton of them in various shapes and colors.

Plain old neon lamps on page 32 shows typical working voltages for them.

methodofcontrol · 10-05-2006, 05:35 AM

thanks for the tip, enzo!
i can't figure out how to view pictures on mouser...i would need to see what the part looked like to be sure of what i'm ordering. i thought viewing the item was an option on mouser....but i'm an idiot, and i can't figure it out. any help?

Enzo · 10-05-2006, 08:49 AM

If you don't have the paper catalog, request one. They send them for free. It is so much easier to browse in the book than it is online. Ther aer pictures on the page, though they are small.

But once you get to the listing, for example stock 607-2110A1

If you were doing a search, there is a shart of all the matches that comes up. Note the part number at left is underlined and blue. It is a link, click it, and you get to the page for the part. Usually there is a photo there along with how many they have etc. In this case no photo, but on that page you can click on data sheet, and get the manufacturers data sheet, which does have pictures. Also, the search results always has a column where you can click DATA SHEET for ones where it is available.

That is a long list of options on page 29. You can chose color of light, and the shape of the exposed part, etc.

Mark Black · 10-06-2006, 07:53 PM

FWIW every Ampeg of that era I have worked on recently had an open or way-out-of-spec resistor in the power indicator circuit. It's a 180K 1/2W and should be on the far end of the PCB closest to the indicator lamp.

I have also found numerous cases of cracked solders and/or broken pins at the custom inline "chip" in the tone control circuit on the front PCB, which might account for your crackling noise. Try retouching all the solders on that thing and see if one (or more) of the pins fall out, revealing they were cracked and barely touching.

In many cases some of the soldering on the main PCB seems to be marginal, and I have done several 100% retouch depending on how widespread the problem seemed. Even found one factory-looking joint where the solder never wicked up the component lead - just a wire in the middle of a solder doughnut which leaves me wondering how it got through these many years.

Haven't experienced the reverb problem you're having, but totally agree with Enzo's suggestions.

Good luck!

Enzo · 10-07-2006, 08:17 AM

That resistor is on the standby light, not the main power light.

Mark Black · 10-08-2006, 05:35 PM

Quote:

That resistor is on the standby light, not the main power light.

Oops - you're right (my bad).

10-04-2006, 01:24 AM	#1
methodofcontrol Senior Member Join Date: Aug 2006 Posts: 158	top mount ampeg VT40 i've got an old top-mount VT40 that's got minor issues, the reverb seems to get stronger the longer the amp is on...is that a tube issue? i've got a similar vintage V4 that i can swap tubes with. also, w/ the 300hz mid cut the amp sizzles and crackles w/ the low notes. but when i bring the 300hz back up to flat, the problem goes away. lastly (for the moment) the power light does not work. it's wired directly to the power transformer and power switch. i've tried jiggling the wires to see about a bad connection, but i get nothing. is it likely a bad bulb? (i've heard that neon bulbs never really go bad, so i thought it more likely a connection issue.) i'm in need of a "helmet a" logo for this amp, a standby switch, black "period correct" ampeg knobs, period correct fuse holder, and possibly a power light. hopefully someone knows where i can strip an old junker of these parts! thanks

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10-04-2006, 02:03 AM	#2
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,277	The power and standby lights are indicator assemblies, not simple bulbs. But all that means is there is a resistor in series with the neon bulb inside. So the whole thing can be slapped across the mains. They are available on the parts market. I mean like Mouser, not antique AMpeg parts. Of course neon bulbs can fail, there is not a component made that can't fail. Neon bulbs naturally darken with age - this can diminish the effect of a trem for example. I used to buy the neon bulbs for strobe tuners by the hundred. But beyond that, wires can break off, the bulb can crack losing its neon, the resistor can open, the connection within can fail. I'd make sure the mid switch was super clean. Aside from that, there is only so much the switch circuit involves. Growing reverb? COUld be a tube, could be a cap that wakes up, could be a resistor value shifting as it warms. Determine if the drive is getting hotter or if the recovery stage is. Scope the drive signal and watch its level. Apply a stewy signal to the return jack and see if the output to the amp is stable or not. CEDist has an AMpeg logo, see if it suits - stock S-M920

10-04-2006, 02:51 AM	#3
methodofcontrol Senior Member Join Date: Aug 2006 Posts: 158	thanks for the tips enzo. the logo available from CEdist is a current production model. not the same as the vintage ones. as for neon bulbs, yeah...i know ANYTHING can fail, i just thought it was more likely a problem other than the bulb itself. if i pull the bulb and hook it up to a 9v battery...should it light if it's good? also...i know the standby lights in v-series ampegs typically have resistors in series w/ them. i've replaced a bunch of them. but according the schematic and my visual inspection, the power light goes straight to the tranny. thanks for your help, i'll swap the pre tubes from my V4 and see which problems go away. i'll post back for more info/help.

10-04-2006, 07:30 AM	#4
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,277	You didn't quite follow. The 120VAC neon pilot light is an assembly with a resistor INSIDE it in series with the bulb. The whole thing runs on 120VAC. It is wired to the mains, just as is the tranny. It isn't wired to the tranny in the sense that the tranny matters to it. Remove the tranny and the powr light should still come on. 9V will never light a neon bulb. SInce the standard indicator lamp assemby expect to run on 120VAC, when they standby light is wired to the B+, that is hundreds of volts higher, so an ADDITIONAL series resistor is added external to the lamp assembly. I forget, but it seemed to me that the basic NE2 neon bulb needed about a 220k resistor in series for operation across the 120VAC. That is more or less what is in the indicator lamp assembly - little bulb and high value resistor. That resistor value might be wrong, I forget stuff easy. Look in Mouser under Indicator, they have it broken down so there are neons on three pahes. IN catalog 627 page 29, 33,78. CHicago Mini on page 29 has a ton of them in various shapes and colors. Plain old neon lamps on page 32 shows typical working voltages for them.

10-05-2006, 05:35 AM	#5
methodofcontrol Senior Member Join Date: Aug 2006 Posts: 158	thanks for the tip, enzo! i can't figure out how to view pictures on mouser...i would need to see what the part looked like to be sure of what i'm ordering. i thought viewing the item was an option on mouser....but i'm an idiot, and i can't figure it out. any help?

10-05-2006, 08:49 AM	#6
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,277	If you don't have the paper catalog, request one. They send them for free. It is so much easier to browse in the book than it is online. Ther aer pictures on the page, though they are small. But once you get to the listing, for example stock 607-2110A1 If you were doing a search, there is a shart of all the matches that comes up. Note the part number at left is underlined and blue. It is a link, click it, and you get to the page for the part. Usually there is a photo there along with how many they have etc. In this case no photo, but on that page you can click on data sheet, and get the manufacturers data sheet, which does have pictures. Also, the search results always has a column where you can click DATA SHEET for ones where it is available. That is a long list of options on page 29. You can chose color of light, and the shape of the exposed part, etc.

10-06-2006, 07:53 PM	#7
Mark Black Senior Member Join Date: May 2006 Location: Near Seattle, WA, USA Posts: 328	FWIW every Ampeg of that era I have worked on recently had an open or way-out-of-spec resistor in the power indicator circuit. It's a 180K 1/2W and should be on the far end of the PCB closest to the indicator lamp. I have also found numerous cases of cracked solders and/or broken pins at the custom inline "chip" in the tone control circuit on the front PCB, which might account for your crackling noise. Try retouching all the solders on that thing and see if one (or more) of the pins fall out, revealing they were cracked and barely touching. In many cases some of the soldering on the main PCB seems to be marginal, and I have done several 100% retouch depending on how widespread the problem seemed. Even found one factory-looking joint where the solder never wicked up the component lead - just a wire in the middle of a solder doughnut which leaves me wondering how it got through these many years. Haven't experienced the reverb problem you're having, but totally agree with Enzo's suggestions. Good luck!

10-07-2006, 08:17 AM	#8
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,277	That resistor is on the standby light, not the main power light.

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