Ampeg V4 Bias Problem?

[plexi50]

I have an older 1977 Ampeg V4 head with reverb. The bias resistor is 75K. I am getting -60mv. The schematic says -52 mv.  It does sound like the bias is very cold with that slightly distorted sound you would get from an amp that isnt bias correctly. I have changed the power supply & bias cap's as well.  I have not had this issue before with any other Ampeg V4 or VT22 amps.

Does this sound like a set of power tubes? Coupling caps? The power tubes all test strong but do have high miles on them. ???

[PRR]

> The schematic says -52 mv.

Schematic says -52 Volts, not "mv".

Can you tap some 1 Ohm resistors, or a Bias-right, and measure the actual current in the tubes?

[plexi50]

Yes i will do that later this morning PRR and post the results.

[plexi50]

Hoffman bias socket and the current value in pic. 525 plate volts. Should be 540 according to the schematic. Frustrated.

[sluckey]

Fluke says 6.5. But what? Volts? mVolts? Amps? Ohms? Dollars?

If that is 6.5mV or 6.5mA then that tube is biased very cold or it's just weak. Either would cause plate voltage to be high. What do the other tubes look like?

I would plug in a fresh set of tubes first. If still cold, decrease the bias voltage to about -50V and check again. You may even want to install an adjustable bias circuit.

[plexi50]

The voltage is 6.5 ma. Ice cold is the word. I decreased the bias resistor from 75k to 60 k and i now have -52.5 volts on all 7027 tubes @ 22.0 ma.
But now the plate voltage is 508 VDC. Shouldnt the schematic be reflecting 540-530 under load?

I will check the rest of the 7027 ma readings.

[plexi50]

Took out the 60k bias r and put the 75k stock value back in place.
 
#1 10.5 ma
#2 6.3 ma
#3 11.0 ma
#4 9.0 ma

Dont have a new set of 7027's laying around at the moment. I have this feeling something else is wrong.

New power tubes have been ordered.

[DummyLoad]

thinking that it probably needs new PS filters and possibly a new set of outputs. 


--pete

[TubeGeek]

Sorry to hijack your thread but I have a VT-22 on my bench right now that is acting strange and I thought since these amps are pretty much the same thing I'd just ask the question here. Maybe we can help each other out.


When I power the amp up I get no filaments lighting up in any of the tubes. 


First thing comes to mind…ok check the 6.3VAC winding from the PT.


1.7mVAC across pins 2 and 7 of a power tube socket.


Hmmm.


Removed the filament winding from the power tube sockets, secure them and test voltage. 


I get 6.7 VAC.


I resolder the filament wires to pins 2 and 7 of the power tube socket.  Turn the amp on and measure across the pins, getting 1.7mV.


I expect the PT needs to be replaced.  But why?  It works when it is not connected to the tube sockets.


I double, triple checked the filament wiring on all tubes, looks good to me. I don't see anything shorting out.  I resoldered all filament connections for good measure.  The 100 ohm hum balance pot has also been replaced.


Why would the PT measure correct 6.3VAC out of the amp and then nothing when it is installed?


 

[plexi50]

Removed the filament winding from the power tube sockets, secure them and test voltage. 
I get 6.7 VAC.

I resolder the filament wires to pins 2 and 7 of the power tube socket.  Turn the amp on and measure across the pins, getting 1.7mV.



I remove the hum pot period and go with (2) 1 watt 100 ohms from pin2 and pin7 to ground.  I dont  trust hum pots. If any one ever turns the hum pot or plays with it the filament voltage be will unbalanced and who knows what the filament voltages would be on each side of the tubes. Unless you knew how to use a DVM or what you were looking for. Ive had a lot of amps come in over the years with a bad or miss adjusted hum pot.
Ampeg,Twin Reverb,Peavey,etc

[sluckey]

Why would the PT measure correct 6.3VAC out of the amp and then nothing when it is installed?

Maybe there is a dead short across the filament circuit that drags the voltage down to nothing. But if that's the case the filament leads would likely be melting the insulation and/or the PT would be hot, or a fuse may even pop.

While the filament leads are connected as usual, pull all tubes and check the voltage. If dead, look for a short in the filament circuit. If good, put the tubes back in, one at a time, while monitoring the filament voltage. I would start with the little tubes.

Maybe the winding is shot but you still have 6.3v under no load but dies as soon as a load is applied. I would disconnect the filament leads again and put a high wattage 2Ω resistor across the filament leads. This will provide a load of about 3 amps. If the voltage still dies under this load test then the PT is bad. If the voltage stays good, then look for a short in the amp's filament circuit.

Doing these tests should tell you if you have a problem in the filament circuit wiring, a shorted tube, or a bad PT.

[sluckey]

I remove the hum pot period and go with (2) 1 watt 100 ohms from pin2 and pin7 to ground.  I dont  trust hum pots. If any one ever turns the hum pot or plays with it the filament voltage be will unbalanced and who knows what the filament voltages would be on each side of the tubes.

Tubes don't care what the filament voltages measure with respect to ground. They only care that there is 6.3VAC (or whatever is required) ACROSS the filament. A good humdinger pot is no more untrustworthy than a pair of 100Ω resistors. There is no danger with anyone 'misadjusting' the pot. No more dangerous than turning a tone pot from one end to the other.

The humdinger does not affect the filament voltage fed to the tube. The filament voltage will always be the same across the tube filament regardless of the position of the pot. The pot simply changes to ground reference for the filament circuit, which allows you to adjust for minimum hum, either by ear, or putting an AC voltmeter across the speaker leads.

If I had the money, I'd put a humdinger in every tube thing I build. But a CT (real or artificial) is usually good enough for guitar amps.

[John]

If I had the money, I'd put a humdinger in every tube thing I build. But a CT (real or artificial) is usually good enough for guitar amps.

I have incorporated them in my last several, and plan to keep doing it. I keep (trying to) refining the humdinger board so that I can put a turret board on top of the PT and have everything right there. I put 100 ohm resistors in series with the pot for the "fuse" factor in case something shorts out. It  increases the cost and time for that portion though, for sure.


I doubt I'd bother if I was using the standard 12ax7 tubes, but feel it's definitely worth the extra trouble with the octals and their "unbalanced heaters". (in quotes because I don't know that for a fact, but have read it on the 'net) 

[plexi50]

The humdinger does not affect the filament voltage fed to the tube. The filament voltage will always be the same across the tube filament regardless of the position of the pot. The pot simply changes to ground reference for the filament circuit,

Ah ok. Im used to grounding the DVM lead and then checking each tubes filament voltage to be roughly the same. I learned something. Across the heater leads is all that is important? I feel like 

[TubeGeek]

Why would the PT measure correct 6.3VAC out of the amp and then nothing when it is installed?

Maybe there is a dead short across the filament circuit that drags the voltage down to nothing. But if that's the case the filament leads would likely be melting the insulation and/or the PT would be hot, or a fuse may even pop.

While the filament leads are connected as usual, pull all tubes and check the voltage. If dead, look for a short in the filament circuit. If good, put the tubes back in, one at a time, while monitoring the filament voltage. I would start with the little tubes.

Maybe the winding is shot but you still have 6.3v under no load but dies as soon as a load is applied. I would disconnect the filament leads again and put a high wattage 2Ω resistor across the filament leads. This will provide a load of about 3 amps. If the voltage still dies under this load test then the PT is bad. If the voltage stays good, then look for a short in the amp's filament circuit.

Doing these tests should tell you if you have a problem in the filament circuit wiring, a shorted tube, or a bad PT.

This is exactly the help I was looking for. Thanks.


I removed all the tubes, powered on with DVM across the filaments, measured 0.744 VAC.


Installed the tubes one by one, did not change.


Next I connected a 2Ω 50W resistor across the filament winding, powered up and measured 20mVAC on the DVM.


Lastly just to be sure I measured across the filament winding with no load…I measure 6.76VAC.


It looks like this PT is defective and will need to be replaced.

[HotBluePlates]

Next I connected a 2Ω 50W resistor across the filament winding, powered up and measured 20mVAC on the DVM.

This was with the filament wires not connected to the sockets? If so, then the PT is the culprit.

If the filament wires were connected to the sockets during this test, it doesn't tell you anything useful (other then there is a probable short across the filament pins somewhere in the filament wiring).

[TubeGeek]

Next I connected a 2Ω 50W resistor across the filament winding, powered up and measured 20mVAC on the DVM.

This was with the filament wires not connected to the sockets? If so, then the PT is the culprit.

If the filament wires were connected to the sockets during this test, it doesn't tell you anything useful (other then there is a probable short across the filament pins somewhere in the filament wiring).

Yes.  The filament wires were disconnected from the socket.  I had the 2Ω 50W resistor in a clamp and then soldered the filament leads to each end of the resistor and tested.

[sluckey]

You've got the problem diagnosed properly but I wanted to comment on this statement just so you know what was happening...

I removed all the tubes, powered on with DVM across the filaments, measured 0.744 VAC.

You basically got no voltage even with all the tubes pulled. But pulling all the tubes did not completely remove all the load. I had forgotten about the 100Ω humdinger pot that was still connected to the filament circuit. Your PT could not even power that 100Ω load even though that only amounts to 63mA to a 6.3v winding! That's a real dead PT!

It's possible (maybe even likely) that one of the filament wires is broken or came unsoldered from the actual end of the 6.3v winding, resulting in a high resistance connection. That PT is probably gonna cost big bucks. Use your ohm meter to check the resistance of the filament wires. A good winding should be near zero ohms, but yours is probably very high resistance. If you have some needle piercing probes then pierce the insulation of the wires near where they exit the PT. If you now get a good reading, then just splice on some new wires.

I'm betting it will still be a very high resistance though. My suspicions are a bad solder joint inside the PT. If the bells are removable I would remove them and peel back some tape or paper to examine the actual solder connection. I've repaired a couple low voltage PTs this way. The wire will be big enough to work with your bench tools and if you're lucky, the 6v winding may even be the top wound coil. Got nothing to lose except a little time.

[HotBluePlates]

Agreed on all counts.

[TubeGeek]

Awesome info.  I am learning here again.


I measure 2kΩ resistance across the filament leads.


I will remove the PT in the next day or two and have a look.  It'd be nice if I could repair this instead of ordering a replacement.  A new PT for this amp is $189.

[HotBluePlates]

I measure 2kΩ resistance across the filament leads. ...  It'd be nice if I could repair this instead of ordering a replacement.  A new PT for this amp is $189.

The high resistance obviously is no-good. But a lot of times you'll find the break or high-resistance point close to where the wire leads attach. That's what I found after cutting open the paper wrap on one transformer: there was a break in the fine wire coil only a turn or 2 in from where it gets soldered to the heavier wire lead (this was a high-voltage winding).

The upside is you have nothing to lose: if you really, really butcher the PT, you still had to buy a new one.

[TubeGeek]

Well I pulled the PT out today, opened it up and had a look at if it is repairable.  It doesn't look like I can save this one.  Luckily there are replacements out there from Heyboer.

[PRR]

> removed all the tubes, ...measured 0.744 VAC.

With 100 Ohm pot across?

Winding acts-like about 8 times 100r, or 800r.

> measure 2k? resistance across the filament leads

Should be well under 1 Ohm. 2K is close-enough to 800r measured by light-load drop. Chalk the difference to bad connection being inconsistent. (If the hum-pot is 250 Ohms, the two readings agree exact.)

Pull the iron, peel the bell-covers off, follow the heater leads into the paper and splices. Be gentle, because if you can't fix it, the ugly-hack is to get a 6V 5A transformer and find a place to bolt it. You don't want to mangle the 115V and 600V windings. But an added heat tranny is so ugly that I would not shed extra tears if I did bung-up the PT guts and was forced to buy a neat/clean replacement PT.
___________________________________

> Across the heater leads is all that is important

Take a 6V battery and a 6V lamp up in a tree. The to-ground voltage each side is zero, yet the lamp lights fine.

> grounding the DVM lead and then checking each tubes filament voltage to be roughly the same.

When one meter-probe is nailed to chassis, looking for 3.1V AC each side is quick and confirms it aint dead (or way-low or high). If the tubes also light, forget the heater system. But if the heaters are NOT working right, "no filaments lighting up", then this check is incomplete. In a new-wired amp (or an amp with heater fuse) you could have the same-phase 3.1V on both sides (zero V across). On old-old amps it is normal to have 6.3V one side and zero on the other. Many amps add a DC voltage to the heater system to reduce cathode leakage, and some meters do not resolve smaller AC in presence of large DC. "Across" tells you for-sure that the heater "should" light. (Remember to switch to AC, that gets me every time.)

[TubeGeek]

I thought about adding just a filament transformer too but in the end the customer and I agree that a new PT is in order.  The owner is a big Ampeg fan.  Apparently he bought another V4b off eBay with a dead PT so I may be in for another job or two on the next one.

[plexi50]

The amp is done and sounds great. Bad power tubes. Found a 12AX7 in V4 causing distorted tone where a 12AU7 should have been. Used ARS cap cans as well. The schematic that was on the underside chassis panel is different as well from any other Ampeg V4 i could find online. -62 volts bias grid coupling is what  this Ampeg V4 called for.

[TubeGeek]

I used CE cap cans and wish I would have used ARS now because the cap cans are hot.  Don't ask me how I found out they were hot.

[plexi50]

Don't ask me how I found out they were hot:

I have done that to. Flirting with disaster. Good thing the plastic covering was in place. Or was it?

[TubeGeek]

Nope. The cardboard covers were off.  That is when I discovered they were hot. It zapped the sh*t out of me.

[plexi50]

Wowza! Thats why when i saw 687VDC on the plates of the Sound City 200 i used an oven mitten just in case. I can take 525VDC.
 Im not trying 687VDC.

[TubeGeek]

Wowza! Thats why when i saw 687VDC on the plates of the Sound City 200 i used an oven mitten just in case. I can take 525VDC.
 Im not trying 687VDC.

687...jeez that's scary


I always say thank god I am fat, the extra cushion absorbs these shocks

[plexi50]

Yeah it is scary. Nothing to take lightly. Pay no attention to us younger people. High voltage is a seriously dangerous thing.
Take all precautions when fiddling around in side an amp. Never get to relaxed and rest your arm or palm on the chassis/

[TubeGeek]

Yeah it is scary. Nothing to take lightly. Pay no attention to us younger people. High voltage is a seriously dangerous thing.
Take all precautions when fiddling around in side an amp. Never get to relaxed and rest your arm or palm on the chassis/

That's for sure.  I am 36 and do not want to expire anytime soon.  I joke about it now but it is something to be taken seriously.

[plexi50]

725VDC without a load. 687VDC with a tube load. Im 57 and i think i want to hang around a little longer.