1976 Ampeg VT40 Guitar Combo....help please!

[axismatt]

Hi,
My VT-40 combo suddenly died on me a month or so ago, with the output dropping to a 2W fuzzy mush where it had previously been deafening!
Its definitely the power amp section, as the preamp send works fine when I send it to another power amp. I suspect the output transformer, but don't really know how to test it and what readings to expect. Plus I've had a couple of hefty 'jolts' from those large Caps, even after it had been off for an hour :evil: :evil: so I'm getting close to giving up, as I can't find anyone around here who'll take it on.
Is anyone familiar with these, or can anyone offer any advice on repairing it or finding a replacement output transformer please?
thanks,
matt

 

[CJ]

Is this a tube amp?

 

[VacuumVoodoo]

Yep, it's a tube amp. The schematic of the power section is here http://users.aol.com/portaflex/schems/vt40powa.gif

Assuming the tubes are healthy and since the HV supply seems ok - jolts from the filter caps - i would suspect a fault in the bias supply.

 

[kiira]

[quote author="axismatt"]Hi,
My VT-40 combo suddenly died on me a month or so ago, with the output dropping to a 2W fuzzy mush where it had previously been deafening!
Its definitely the power amp section, as the preamp send works fine when I send it to another power amp. I suspect the output transformer, but don't really know how to test it and what readings to expect. Plus I've had a couple of hefty 'jolts' from those large Caps, even after it had been off for an hour :evil: :evil: so I'm getting close to giving up, as I can't find anyone around here who'll take it on.
Is anyone familiar with these, or can anyone offer any advice on repairing it or finding a replacement output transformer please?
thanks,
matt[/quote]

I have the V4 head version of this amp. OMG it's loud. Are you still using the 7027As in it? I had some fried resisters in mine from the proximity of the bias circuit boards to um, I forget now.., hot things, but I had to trace through the circuit and check the voltages. kinda scary. Be carefull!

Uncle Ned at triodeelectronics.com might be able to help with an output Xformer if that's what ya really need because he is an Ampeg fan. I canverte mine to fixed adjustable bias.

 

[CJ]

Thanks for the schematic!
Ohm out the output transformer. (blu-red brn)
No need to remove tubes.
Usually it's the primary that gets popped due to smaller wire and more turns which means more heat and inductive kick.

 

[NewYorkDave]

This might be a common problem with V4s. I know I've seen it at least a couple of times. Matter of fact, there's a V4 with a blown output xfmr that's been sitting in my junk pile for years. I don't know why I hold on to it...

 

[soundguy]

man, the midrange boost on the V4 is totally insane. Get that feeding back on itself and if you have P90's tony iommi would ask YOU how you got your guitar tone... v4's: worth the trouble.

dave

 

[CJ]

I bet those output tubes want 4.7k input swampers instead of 47k.
Probably a typo.

 

[axismatt]

hi, thanks for the replies. The circuit diagram for this model is quite a bit different to the one linked to above - I've scanned it and would love to paste it up here but don't know how (I don't have a url for it) - could someone who's done this on here please enlighten me? :idea: [/img]

 

[axismatt]

ok, I've attempted to measure the output transformer windings...

primary -

brown to blue - 550ohms when I first put the meter on, but falling fairly rapidly, settling at around 290 ohms

brown to red(centre tap?) - settled at about 150ohms

blue to red - settled at about 160ohms

secondary -
I don't know if it implies a fault but I couldn't really measure them on my crappy DVM as the minimum range on it is 2000ohms - all 3 windings (2, 4 and 8ohm) didn't even register 1 ohm


so, what do you guys think? What readings should I be expecting?
thanks in advance
matt

 

[NewYorkDave]

If your meter is working correctly, you could have an open secondary. This is pretty rare...

 

[VacuumVoodoo]

A DVM with 2000 ohm as the lowest range would show 0 when measuring secondary windings of tube OT, these are noprmally less than 5 ohms.

If your DVM shows 0 and not open circuit than your transformer is OK.

To make sure, take measurments between primary and secondary windings, this should register as open circuit.

Are you able to measure bias voltage of output tubes ? Take a measurment between the wire labeled "Output Bias" and ground as shown in the schematic i referred to. Should be around -65V and should not change more than a few % with tubes in and out of the sockets.

 

[NewYorkDave]

I think he probably meant that the lowest range on his meter is 0-2000 ohms. I have never seen an ohmmeter or DMM that wouldn't measure below 2000 ohms.

 

[axismatt]

sorry, let me try and be clearer...yes the dvm range is 0-2000ohms and it was indicating o ohms, ie short circuit on the secondary...but i have no way of knowing whether its a 'real' short circuit or not due to the vagueness of my dvm!
thanks, i'll get back to you about the bias voltage
matt

 

[NewYorkDave]

Ah, then your secondary is probably OK. Like VaccumVoodoo says, the secondary winding is of very low DC resistance and may read as a short on less-precise ohmmeters.

 

[axismatt]

well, i guess thats good news then as the transformer's an expensive and hard to find item - does that primary resistance value sound ok too?

so what does that leave??? What could cause the bias supply to change, as it was working fine until recently...i'm a bit stumped on this one!!

 

[Guest]

Bias supply going out could be caused by a resistor frying, a cap going out, a diode going...

 

[adamasd]

If it is the output transformer you can get a replacement from,
http://www.mercurymagnetics.com/
They are abit pricey but, they are going to be closer to the original then anything else you can find to fit in there.

[quote author="NewYorkDave"]Matter of fact, there's a V4 with a blown output xfmr that's been sitting in my junk pile for years. I don't know why I hold on to it...[/quote]

If you want to get it out of your junk pile I will take it.


adam

 

[axismatt]

Hi, hopefully this is a link to the actual circuit diagram that came with the amp...

http://img.photobucket.com/albums/v436/axismatt/ampeg1976vt40.jpg

Alex, I measured the resistance between primary and secondary windings and it rose on connecting the dvm to around 84k - is this ok iyo?

Regarding bias voltage, a tech friend of mine had a look at this amp and reported it as being -60v, which tends to agree with the value on the other circuit diagram you found at http://users.aol.com/portaflex/schems/vt40powa.gif
HOWEVER if you look at my circuit diagram it says it should be -28.5V?!!?!
...which one is right? very confusing.....

 

[axismatt]

Hi, hope I'm not being a pain but does anyone know what the bias voltage should be here, as -28 compared to -60v seems a big difference on the 2 fairly similar circuit diagrams.

many thanks
matt