B&H 302 B+ problem

[Paul1453]

I bought a Bell & Howell Filmosound 302 a while back.
It came with an external PT and speaker.
They said it worked, so when I got it I tried it out and it did work as a guitar amp.
But it did not sound very good and was not very loud for a 12-15W amp.
After a few minutes, I noticed the Output tubes were very hot and beginning to red plate.
I shut her down, waited for her to cool off some, and removed all tubes but the 5Y3 rectifier.
Checked the output of the rectifier, supposed to be 350V, and I got 500+V!   
Tried a different 5Y3 still 500V.

I thought maybe if the CT isn't right I might get this symptom?
I opened the PT and 11 pin connector and checked out all the wiring.
Tested continuity of the PT as shown in the diagram, and all checked out good.
The CT on pin 11 is connected to Ground in the amp chassis, but there is no continuity to the core of the PT.

I have two ideas what might be causing this problem now.
1.  The PT is wired correctly, but it is not for this 6V6 amp but a similar 6L6 model instead.
2.  The CT is grounded to the chassis, but the PT is not bolted to it so the PTs core doesn't have any Ground.

I plan to test #2 when I get home tonight, by bolting the PT to the chassis somehow and rechecking B+.
Do you think that will lower the voltage?

If it is the wrong PT, do you think the existing components could be modified to work with 2 6L6s?
I'm thinking the OT won't be right for 6L6s at a minimum, and there are probably many more issues.
Besides that, the guts of this amp is tightly packed with components.
It makes just sticking a test lead in there to check a voltage quite a challenge, and considerably dangerous with 500+V running around in there. 

400 VAC on pins 4 & 6  of the rectifier socket with the PT grounded to the chassis.   
This doesn't seem to be the right PT for this amp.   

[silverfox]

The voltage from that PT sounds about right but I could be wrong. Seems to me it should be something like: 1.4 x 400 -75, which would indicate the wrong PT. Who removed it from the chassis? Another consideration is the line voltage may be higher now then when that amp was manufactured.

silverfox.

[Paul1453]

The voltage from that PT sounds about right but I could be wrong. Seems to me it should be something like: 1.4 x 400 -75, which would indicate the wrong PT. Who removed it from the chassis? Another consideration is the line voltage may be higher now then when that amp was manufactured.

silverfox.

No Idea. 

I take my cheap shots on Ebay and learn to live with the results.

I'm going to try feeding her the right voltages on it's PT socket from my bench supply and see if she works properly then. 

[sluckey]

...and removed all tubes but the 5Y3 rectifier. Checked the output of the rectifier, supposed to be 350V, and I got 500+V!

...400 VAC on pins 4 & 6  of the rectifier socket

You must have a load on the power supply for voltage to read correctly. No tubes = no load = no current = no voltage drop.  400VAC will produce 565VDC with no load, doesn't matter whether you use a 5Y3, 5U4, or silicon diodes. So, put your tubes back in and recheck the voltages.

I plan to test #2 when I get home tonight, by bolting the PT to the chassis somehow and rechecking B+.
Do you think that will lower the voltage?

Bolting the PT to the chassis will make no difference.

[Paul1453]

...and removed all tubes but the 5Y3 rectifier. Checked the output of the rectifier, supposed to be 350V, and I got 500+V!

...400 VAC on pins 4 & 6  of the rectifier socket

You must have a load on the power supply for voltage to read correctly. No tubes = no load = no current = no voltage drop.  400VAC will produce 565VDC with no load, doesn't matter whether you use a 5Y3, 5U4, or silicon diodes. So, put your tubes back in and recheck the voltages.

I plan to test #2 when I get home tonight, by bolting the PT to the chassis somehow and rechecking B+.
Do you think that will lower the voltage?

Bolting the PT to the chassis will make no difference.

Bolting PT to chassis makes no difference.

All tubes back in for voltage check.
I'm pretty sure I did this 1st, but didn't write that part and got much too high B+ with all the tubes in, like still 470+, and V4 in particular goes to red plating pretty quickly.

With my bench supply, if I feed my 6.6 VAC to pins 1 & 2, V's 1,2,3,4 won't light their heaters.
When I put one side of my 6.6 VAC to pins 1&2, and the other side to pin 5, all tubes light their heaters except V1 & V2.
V1 & V2 are fed a rectified and filtered DC heater voltage labeled A?
With all but these tubes lit up, when I feed pin 11 Gnd and pin 7 with around 350VDC from my bench supply, it pulls right around 100mA of current and V4 doesn't red plate.
I can't pass signal through V1 & V2 without getting their heaters to light.
Otherwise I think it would be playing again if they were.   

The schematic I found in the manual online is not complete, it's partly cut off.
I attached what I could of the sections in question with some points highlighted.

[Paul1453]

I was planning on trying to sell this back on Ebay, for a decent profit. 

Money is tight until next payday, and now this won't make it in time.   

I can't just sell it with known problems and have a clear conscience.   


Unknown PT and all tubes back in.  Sanity returns.   

All heaters light again, and sound output reappears.

Now I can get 379 VDC on the output of that 1st resistor.   

It's a weird adjustable one.  Wire wrapped with a clamp you have to unscrew to loosen, move the clamp, and tighten again.

It's clamped on with as much Rs as I can get from it, and now reads 379 VDC on the output.

V4 is no longer red plating, but the amp still sounds like shit with low output.

Removed exciter lamp tube and it's 6V6 driver tube.

All is well, but still sounds like shit.

Even with a premium 12AX7 and red tipped 12AU7 swapped in.  The red tipped 12AU7 did help some.

It only works with the input on the Phonograph port, and set to the magnetic recording function.

Also needs the speaker it came with plugged into the dual output port.  It has a smaller than 1/4" plug, so I can't just plug in 1 of my other speakers.

At least it is not red plating V4 any more.  Output tubes seem to be running much cooler now.

It has a Bias adjustment pot, which I didn't touch.  I'm not sure if that is for all 6V6s, or just the exciter lamp circuit.   

It seemed to me, when I read that part of the manual,  that it was only adjusted for the lamp circuit which is no longer needed.

12AY7 and 12AX7 don't like chopstick tapping, I put their metal shields back on now.