Ampeg V4 bias supply issues

[donegone]

Every time I turn the unit on, the current jumps to about 300ma and drops slowly and roughly levels out at 120ma. Its the solid state rec. but I am having problems finding an exact schematic. The bias section seems to be a mixture of two schematics (attached). The configuration that I see for the bias supply is a connection off of the leg of the output TX and the rec. it goes to the .047uf 600v coupling cap, then a 56k to ground, the diode, 75k to ground, a 100uf 100v filter cap to ground, then the negative voltage goes to the 100k resistors to the grids. When I first measure the neg. DC voltage before I close the standby connecting the B+ filter caps to ground, the DC voltage reads roughly -40vdc and it does flux a little +/-. As soon as I flip the switch and apply the B+, the bias supply voltage begins to drop rapidly down to -30 and even -20vdc. So far, I have disconnected the .33uf coupling caps that connect the PI to the Output stage. I have checked the filter caps but my B+ looks good. I even pulled the 3 section caps that provide power to the output stage and tested to see if they were leaking and they test fine. I have even hooked up a new power supply thinking that was the issue. The problem still happens. The only things I can say are that when I first received the amp there were 6L6's installed instead of 7027's, one of the 10ohm 5w plate resistors was blown. I figured that is what blew the resistor. I replaced the resistor and tubes. I checked the primary on the output transformer and from leg to center tap on both sides, it reads roughly 60ohms. All grid, plate, and screen resistors check good on the output section. The bias supply filter cap, resistor, and diode all check good. I replaced the bias supply cap 100uf 100v just to see if it wasn't intermittent. It still produces the problem. I am at a loss. Help?

[Enzo]

This is unclear to me. Your current starts at 300ma then reduces to 120ma. WHAT current? Through all the tubes? B+ line? MAins?

Then you describe a falling bias voltage. Falling bias voltage should INCREASE tube current, so regardless of where that current was measured, your symptom seems in reverse of the readings.

WHat exactly is the problem we want to solve? Does the amp not work right? Or are you just concerned there is an initial current spike when first turned on? Or are you having low bias voltage? If the bias is low at the tubes, then what is it at the bias filter. If it is falling there too, then the power tubes are probably not responsible.

SInce the ground is open in standby, the bias readings in that state don't mean much. If your bias is low, and you replaced the filter cap, did you replace the film cap? Try that.

And those resistors for each tube? The burnt one was likely due to a failed tube, rather than a bias issue.

6L6s will work in the 7027 sockets.

[Enzo]

Here is another V4 drawing

http://www.schematicheaven.com/ampeg..._vt22_vt24.pdf

[donegone]

This is still an incorrect schematic. The current I am refering to is that which is being pulled through the tubes. The initial reading is 300mA through the tubes when the standby switch is thrown which immediate knocks my bias from -40vdc to -20, (i understand that there is an initial spike when the amp is first turned on) then after a minute or so the bias voltage is around -30vdc and the tubes draw about 120mA and my negative bias voltage still bounces around +/- 5vdc. I am taking my measurements for the bias supply right off of the filter cap leg of the bias supply. I didn't think the output tubes were at fault since I tried several Quads. The only thing I haven't tried was that film cap. I thought about replacing the film cap on the bias supply so now I will. Does that cap act as a coupling cap and if so where is the DC voltage we are blocking since its coming right off of the secondary of the TX? Also, I know that the 6L6's will work but I thought 7027's have a higher plate voltage rating?

[Enzo]

Caps block DC, but they also pass AC, and that is what this coupling cap does. It passes enough AC to allow a bias supply to form.

OK tube current. 120ma for four tubes, or 30ma each? SOunds reasonable. And it is normal for these amps to take a minute to settle down.

Yes, 7027s are cooler tubes than 6L6s, I like them a lot. What I meant by 6L6s will work is that I don;t think any problems you might be having are due to the tubes being 6L6s. 6L6s should work just fine if the amp is OK.

I am still unclear what we are trying to fix. Is the amp performance compromised in some way? AM I misreading the 30ma/tube 120ma total? Did you mean 120ma per tube or something? Certainly 30ma is no problem for the tube, even at 550v or more. Are you just concerned the amp doesn;t snap right to the voltages immediately upon switching on?


yeah, I think the last schematic there is farther from your amp than the two you posted. I just thought it might be interesting to see the range of configurations the V4 can be found in. It sounds to me like you have already drawn out the bias supply schematic for the amp at hand. You may never find a published schematic that is 100% like your amp. That is actually not uncommon at all. And especially if you ever work on old Gibson amps. SO if yours has something like the V3 and V4 combined, as long as you know what is what, and it makes the necessary voltage, you are covered.

[donegone]

Yes the 120mA is for each tube. I apologize for not including specifics. I have a bias tool that puts my multimeter in series with the cathode so I can read the current passing through each tube. What seems odd about all of this is that the schematic I posted showed -54vdc after the filter cap in the bias supply. I need to obtain this negative voltage so the tubes can be properly biased around 30mA. The only time I can obtain this value on the bias supply after having supplied the B+ is when the output tubes are removed from the amplifier. Whenever I supply the B+ while the tubes are installed, it causes the bias supply to become less negative. When measured with a multimeter from the leg of the filter cap in the bias supply, the voltage approximately reads -30vdc and fluctuates by +/- 15%. It will periodically rise and fall by +/- 30%. On the scope, it looks similar to a DC load line of a leaky filter cap. It seems very unstable and it causes the individual tube current to be all over of the place but all readings tend to be relatively high whenever the circuit stabilizes for a moment. I just cant figure out why I can't obtain my -54 volts on the bias supply. Tomorrow I am going to look into that coupling capacitor you suggested.

[g1]

There is supposed to be a schematic stuck to the reverb tank...
You should have around -50VDC at the bias filter cap. Some of them have extra filtering in the bias circuit.
The .047 drops the ac to the bias circuit. You don't want full ac to the bias diode. Sounds like yours is dropping too much.
Here's another drawing from the other V4 bias circuit thread:
http://music-electronics-forum.com/a...40-ampegv4.gif
NOTE! This drawing has the factory misprint. C16 R50 junction does not connect to bottom of R49 (ground).
NOS 7027's were spec'd at higher plate voltage. There is some controversy over whether they were just repinned good 6L6's. Some of the newer types were. Not sure if anyone is building real ones now. I have been running JAN 6L6WGB in my V4 with more than 540VDC on the plates with no issues.

[Enzo]

Well certainly 120ma per tube is way off.

SO I ask again, did you change out the film cap? That .047uf?

[g1]

Seems I am wrong about the .047 being there to drop ac voltage. Not sure of it's exact purpose. One thing I know for sure is when it goes bad you lose bias!
topic is also being discussed here:
Ampeg v4 bias supply issue

[g1]

Sorry about the flip flop, the .047 is indeed there to drop voltage. It's impedance is around 60k so it forms a voltage divider with the 56k resistor and drops roughly half the ac from the p.t. secondary.