70s Ampeg V4B Blows Fuses

[ca7922303]

Back at post #270 I advised you to disconnect C106, which you said you did and it did not make a difference. When you cut the leg in post #271, was it some other cap than C106? Did you reconnect it?

On my board C106 is where schematic shows C112 and I did reconnect it right after I tested it before . Now both C106 and C112 have one(1) leg lifted and are reading 3.3k @R14/15 with either position of ultra low switch.

[g1]

So not just errors on the schematic but on the layout drawings as well. Sure makes troubleshooting a lot more difficult.
Once you get new caps in there (for C106 & C112) as well as the 12DW7, it will be worth it to do one more run through of heater, plate, and cathode voltages for the tubes.

[ca7922303]

So not just errors on the schematic but on the layout drawings as well. Sure makes troubleshooting a lot more difficult.
Once you get new caps in there (for C106 & C112) as well as the 12DW7, it will be worth it to do one more run through of heater, plate, and cathode voltages for the tubes.

I'll post those voltages after I get hum balance pot installed, also do you think either one of these film capacitors would work in C106/112 while I wait for them to be deluvered?

[vintagekiki]

I'll post those voltages after I get hum balance pot installed, also do you think either one of these film capacitors would work in C106/112 while I wait for them to be deluvered?

Unfortunately no.
C106 / 112 is 6.8uF / 20V electrolytic capacitors

[ca7922303]

Unfortunately no.
C106 / 112 is 6.8uF / 20V electrolytic capacitors

How about a 30 uf / 20v, I found a couple laying around?

[ca7922303]

So not just errors on the schematic but on the layout drawings as well. Sure makes troubleshooting a lot more difficult.
Once you get new caps in there (for C106 & C112) as well as the 12DW7, it will be worth it to do one more run through of heater, plate, and cathode voltages for the tubes.

Any information on setting bias on this amp? Thanks.

[vintagekiki]

How about a 30 uf / 20v, I found a couple laying around?

It can. Just check them first to see if they are correct, and pay attention to the polarity

[g1]

Any information on setting bias on this amp? Thanks.

There is no bias adjustment on this amp. They usually do fine without it.
If you want to, you can check the idle current of the tubes by a few different methods.
One is to cut the ground connection from pin8 on each power tube and install 1 ohm cathode resistors. Then you measure the DC voltage at idle across those resistors.

There is another way that does not require any circuit modifications. You measure the resistance of the output transformer primary sections and record them. Then you measure the idle DC voltage across those sections and calculate the idle current, using the resistance numbers that you recorded..
Checking the idle current of the tubes will only tell you how well matched they are.

[ca7922303]

There is no bias adjustment on this amp. They usually do fine without it.
If you want to, you can check the idle current of the tubes by a few different methods.
One is to cut the ground connection from pin8 on each power tube and install 1 ohm cathode resistors. Then you measure the DC voltage at idle across those resistors.

There is another way that does not require any circuit modifications. You measure the resistance of the output transformer primary sections and record them. Then you measure the idle DC voltage across those sections and calculate the idle current, using the resistance numbers that you recorded..
Checking the idle current of the tubes will only tell you how well matched they are.

I installed the hum balance pot and that made all the difference on the hum, it's now at normal levels I helieve for this amp( I had a V4 stack in 1875). It sounds great. I really appreciate all the help I got on this. I'm extremely grateful. You guys are top notch and I appreciate the patience you had to have to deal with my lack of experience/knowledge. I have a bias adapter from eurotubes if that would work just to see what it's doing. Also, can you tell me again what measurements youdy like for me to post now that it's working. Can you be specific as to whether vdc/vac and what pin/tube to ground or not. I've caused so many posts on this one it will take me all evening to go back and find. Thanks again.

[vintagekiki]

Any information on setting bias on this amp?

Setting bias is done if you have previously repaired the V4b preamp, and if the power amp is correct.

Red 1 on schematic
In the cathode tube V5 (pin 8) make a test circuit for testing and matched tubes 6L6GT.

Red 2 on schematic
Replace bias resistor R49 (75k) with resistor 68k ¹/₂ W
Connect 68k resistor in series with trim pot 50k, so you can regulate bias voltage.

With trim pot (50k) set bias voltage (on schematic point -64V) to max.
Insert one tube 6L6GT into the tube socket V5, and leave for 5 min to tube warm up.
Turn on StandBy, and with 50k trim pot set the bias to value 25mV (200mV DVM range)

[Helmholtz]

I had a V4 stack in 1875.

Wow! You must have a time machine.

[vintagekiki]

( I had a V4 stack in 1875)
I have a bias adapter from eurotubes if that would work just to see what it's doing. ¹⁾

In year 1875, no make guitar amp or tubes as we have now.
¹⁾
Since you have a bias adapter (for octal tubes) skip Red 1 on schematic.
Bias adapter mount in tube socket V5
Setting bias is continued, according to the procedure described under Red 2 on schematic

[ca7922303]

In year 1875, no make guitar amp or tubes as we have now.
¹⁾
Since you have a bias adapter (for octal tubes) skip Red 1 on schematic.
Bias adapter mount in tube socket V5
Setting bias is continued, according to the procedure described under Red 2 on schematic

Meant 1975(Magnavox), head was $350.00, 4x12 cabs with cts spkrs we're $250.00 each.

[g1]

Suggest you use the bias probe to test the tubes idle current first, then decide if you need to install bias adjustment. Have you used it before or do you have instructions for it's use? (edit: see bottom of this post for eurotubes bias probe instructions)

can you tell me again what measurements youdy like for me to post now that it's working. Can you be specific as to whether vdc/vac and what pin/tube to ground or not. I've caused so many posts on this one it will take me all evening to go back and find. Thanks again.

I'll make up a list later.
Glad to hear you got it up and running.

*Instructions for Eurotubes Biasprobe:
"Our probes measure directly in DC Milliamps so you will need to set your multimeter to the DCA mode (DC Amps, NOT VOLTS!) and if the meter is not auto sensing then select the 200mA scale. If you have purchased one of our “Cheap Multimeters” then you will find this setting right between 3 to 4 O’clock. Insert the black plug from the probe into the “Common” input and the red plug into the positive input which is usually marked mA. DO NOT CHANGE THE FUNCTION SWITCH ON YOUR MULTIMETER WHILE IT IS HOOKED UP TO THE AMP! This will blow the fuse in the meter and you will need to replace it! Make sure the meter is set properly and the probe is plugged into the meter before you plug the probe into your amp. Remove one of your power tubes and plug the probe in. Place the tube into the top of the probe. If you had to remove the amp from the chassis then make sure that you have plugged the speaker back in. You must have a speaker load on the amp when you are adjusting the bias! Turn on the meter and then turn on the amp leaving it in the standby mode for about a minute, then switch to the play mode. Your meter will read out the plate current in milliamps."

[ca7922303]

Suggest you use the bias probe to test the tubes idle current first, then decide if you need to install bias adjustment. Have you used it before or do you have instructions for it's use? (edit: see bottom of this post for eurotubes bias probe instructions)


I'll make up a list later.
Glad to hear you got it up and running.

*Instructions for Eurotubes Biasprobe:
"Our probes measure directly in DC Milliamps so you will need to set your multimeter to the DCA mode (DC Amps, NOT VOLTS!) and if the meter is not auto sensing then select the 200mA scale. If you have purchased one of our “Cheap Multimeters” then you will find this setting right between 3 to 4 O’clock. Insert the black plug from the probe into the “Common” input and the red plug into the positive input which is usually marked mA. DO NOT CHANGE THE FUNCTION SWITCH ON YOUR MULTIMETER WHILE IT IS HOOKED UP TO THE AMP! This will blow the fuse in the meter and you will need to replace it! Make sure the meter is set properly and the probe is plugged into the meter before you plug the probe into your amp. Remove one of your power tubes and plug the probe in. Place the tube into the top of the probe. If you had to remove the amp from the chassis then make sure that you have plugged the speaker back in. You must have a speaker load on the amp when you are adjusting the bias! Turn on the meter and then turn on the amp leaving it in the standby mode for about a minute, then switch to the play mode. Your meter will read out the plate current in milliamps."

I appreciate that. I've used it before on some newer tube amps that had bias adjustment pots already installed.