70s Ampeg V4B Blows Fuses

[ca7922303]

Replaced c15, c16, c23, r28. At 60v on variac, amp out of standby, c16(150.8v & 212mv), c15(-0.00mv +, -267mv -) c23(-0.00mv +), (-267mv -), r28(39mv & 16.63v and increases the constant hum when touched) power and preamp tubes are slightly lit, preamp tubes are cool, power tubes(2) are hot.also, no preamp control except for mid-range. Control knob and rocker arm have full affect on hum tone. All other control knobs and rocker switches do not.

[ca7922303]

C16 after replacement reads 150vac & 1.4 vac @ 60v on variac.

[g1]

How about DC volts at D6 anode or negative end of C15.
What is the make and model of your multi-meter?

[ca7922303]

Craftsman autoranging #82139, - end of c15(-0.588vdc)

[ca7922303]

Tried to go north of 60v on variac with 2 power tubes installed, shut it down when tubes started to red plate. C15,16,23 and r28 didn't solve it.

[Leo_Gnardo]

Tried to go north of 60v on variac with 2 power tubes installed, shut it down when tubes started to red plate. C15,16,23 and r28 didn't solve it.

From what I can see in your list 'o statistics, there's still no bias voltage in spite of having replaced critical components in the bias supply. Have to admit, I am stumped. With no bias voltage no wonder your output tubes are going red-plate.

[ca7922303]

Also, I forgot to mention that there is no hum balance pot and wiring on this one, so once I get the other problems worked out, I'd appreciate any info I could get on replacing that. Thanks.

[ca7922303]

I guess I'll start making a list of every components vdc reading at 60v on variac and post it to see if anything stands out?; No tubes installed and standby on.

[Leo_Gnardo]

I guess I'll start making a list of every components vdc reading at 60v on variac and post it to see if anything stands out?; No tubes installed and standby on.

You could do that, but I expect it's an exercise in futility UNTIL you see some significant voltage in the bias supply. With 60V AC line voltage, I'd expect about -30V bias. There is none, and until there is, you're chasing your tail. I hope some other MEFster can come up with a proposed solution, otherwise I'd have to say it's time to put the amp in the hands of an experienced tech and be prepared to pay the bill. To your credit, you have done what seems necessary. It's just not paying off.

[g1]

Remove the power tubes and do not install them til the bias circuit is fixed. Redplating the tubes damages them. Until you have the correct DC voltage at pin5 of the power tubes they are going to redplate.
You replaced C15 and C16? In a bias circuit, their positive ends should go to ground, is that how you installed them? Many people make the mistake of installing them backwards.

[Dave H]

C16 after replacement reads 150vac & 1.4 vac @ 60v on variac.

Have you replaced D6? Assuming C16 is OK, 160Vac on one side of C16 but only 1.4Vac on the other side points to a shorted D6. If D6 is shorted C16 forms an AC potential divider with C15

[ca7922303]

D6 has not been replaced and reads in circuit 0.75 k resistance with leads reversed and 248 mvdc & 1.7 mvdc with variac @ 60v. I can pull D6 and measure out of circuit?

[ca7922303]

Here's a photo of c15,16, & 23. c16 does not have +/- on board or component, do I put it back with numbers like I believe original was.

[ca7922303]

So*

[vintagekiki]

Have to admit, I am stumped.

How can you not understand. The boy is tireless and has crazy fun with us.
He fires something that means nothing, and laughs from the sidelines.

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