Bias problem in my BM/HRM. Advice please.

[Raoul Duke]

Hi all,
Went to play my BM yesterday and got some weak fizzy output in the clean channel with marginally higher output in the OD channel - similar to a dying tube sound I’ve heard before. Tried a new tube in V1 to no avail just to rule that out; so decided to put it on the stand and do some measuring. Here’s what I found:

- looked at the power tubes and the ink has turned from red to white. Tells me something’s cookin’…
- powered it up and there’s no visible sign of red plating, heaters all at 3.2vac per side, plate voltage consistent at ~490vdc
- measure the bias coming off the rectifier board: -73vdc
- measure bias at power tubes’ 1R cathode resistors : V5= 50mvdc, V4= 17mvdc
- measure at bias pot wiper: -73vdc, wiper does not sweep voltage
- all nodes on the filter board are within 5% of specified voltages, all pre-amp tubes within 5% of specified voltages

What I’m thinking at this point is that the bias pot has failed and is blasting the power tubes - which may or may not be fried. The pot itself is a 10k branded for the amp part supplier that I bought it from - of which I had a similar bias pot fail in my 102 about a year ago. They might have even been purchased in the same order (or close).

So I shut everything down, discharge everything and test the pot in circuit with leads on the CCW lug and wiper. Full CCW limit is .5k, full CW limit is 10.4k. This sounds ok to me I think .

Now I’m kind of stumped at where to go next. Any suggestions are greatly appreciated!

Thanks!

[ijedouglas]

Hi all,
Went to play my BM yesterday and got some weak fizzy output in the clean channel with marginally higher output in the OD channel - similar to a dying tube sound I’ve heard before. Tried a new tube in V1 to no avail just to rule that out; so decided to put it on the stand and do some measuring. Here’s what I found:

- looked at the power tubes and the ink has turned from red to white. Tells me something’s cookin’…
- powered it up and there’s no visible sign of red plating, heaters all at 3.2vac per side, plate voltage consistent at ~490vdc
- measure the bias coming off the rectifier board: -73vdc
- measure bias at power tubes’ 1R cathode resistors : V5= 50mvdc, V4= 17mvdc
- measure at bias pot wiper: -73vdc, wiper does not sweep voltage
- all nodes on the filter board are within 5% of specified voltages, all pre-amp tubes within 5% of specified voltages

What I’m thinking at this point is that the bias pot has failed and is blasting the power tubes - which may or may not be fried. The pot itself is a 10k branded for the amp part supplier that I bought it from - of which I had a similar bias pot fail in my 102 about a year ago. They might have even been purchased in the same order (or close).

So I shut everything down, discharge everything and test the pot in circuit with leads on the CCW lug and wiper. Full CCW limit is .5k, full CW limit is 10.4k. This sounds ok to me I think .

Now I’m kind of stumped at where to go next. Any suggestions are greatly appreciated!

Thanks!

What do you measure on pin 5?

[Raoul Duke]

Ian,
I get -71vdc on both tubes (I built a 50w).

[martin manning]

Does the bias pot resistance vary as expected when you measure from either end of the element to the wiper? Disconnect it to get a good reading.

[Raoul Duke]

Interesting result guys:

Completely out of circuit - the CCW to wiper functions as stated before, CW to wiper (where the resistor was attached) “OL”.

Bad pot then?

[martin manning]

Yes. The element must have broken near the CW end.

[Raoul Duke]

Martin,
I think that’s what happened to the last one that failed. I’ll have to go back and check. I’ll replace the pot and see where that lands me. I appreciate your help as always!

Thanks all! I can always count on the experts here for guidance and I truly appreciate that!

[Raoul Duke]

All right, new bias pot in and everything is stable and running/sounding as it should.

I put the EHs back in and they biased up fine - so I ran it for a half hour, then played it for about 45 minutes, all the while rechecking voltages and bias just to see if things stayed consistent - and they did. Very relieved that it was a pretty easy fix.

Really a little gun shy about those bias pots though. I looked back through my records and both failed pots were ordered within a month of each other and failed the exact same way. I wonder if it’s a bad batch or is it something I’m doing? Tried to be extra gentle torquing the new one in and made sure I didn’t go hard on it. The new one is a Fender pot - but comparing it to the two dead pots I’m certain it’s the same manufacturer. Strange…

Anyhoo, thanks again!

[martin manning]

I looked back through my records and both failed pots were ordered within a month of each other and failed the exact same way. I wonder if it’s a bad batch or is it something I’m doing? Tried to be extra gentle torquing the new one in and made sure I didn’t go hard on it. The new one is a Fender pot - but comparing it to the two dead pots I’m certain it’s the same manufacturer. Strange…

So it broke at the red X in the schematic below? That would take bias to 0V, and things would get hot. I suspect a manufacturing defect rather than any kind of mishandling on your part.

I just posted a wiring mod in the ODS DIY Eyelet Boards (https://ampgarage.com/forum/viewtopic.p ... 08#p365408) that will cause the bias to fail cold in the event of a bias pot wiper failure or an element failure such as you had.

[Raoul Duke]

Hey that’s great Martin, thanks!
I just downloaded it and will start upgrading boards after I put together a Mouser order for some resistors.

I appreciate the solutions you come up with and the time you take to design and illustrate them for us all here.

Thanks again!

[martin manning]

Really all you need to do is change two resistors and add one new one on the rectifier board, and make a simple change to the bias pot connections.

[pottedplant]

Yes. The element must have broken near the CW end.

I would like to add that the rivets holding these solder lugs to the wafer are so pathetically fragile and I've had so many issues with them in guitars I've worked on for people over the years because the previous "tech" decided to bend the leg back to make the ground connection to the pot casing. I don't mind doing that but they don't realize they're putting the force entirely on the rivet. I bend them with 2 pliers supporting the solder lug at the base above the rivet so that the rivet sees no force. They just dont make them like they used to

I imagine if Raoul were to take the pot out and send signal through it and listen while jiggling the lugs it would be intermittent. This same failure is the reason why you get max volume when turning all the way down on some guitars who have had aforementioned process done to them.

[Matt J]

I looked back through my records and both failed pots were ordered within a month of each other and failed the exact same way. I wonder if it’s a bad batch or is it something I’m doing? Tried to be extra gentle torquing the new one in and made sure I didn’t go hard on it. The new one is a Fender pot - but comparing it to the two dead pots I’m certain it’s the same manufacturer. Strange…

So it broke at the red X in the schematic below? That would take bias to 0V, and things would get hot. I suspect a manufacturing defect rather than any kind of mishandling on your part.

I just posted a wiring mod in the ODS DIY Eyelet Boards (https://ampgarage.com/forum/viewtopic.p ... 08#p365408) that will cause the bias to fail cold in the event of a bias pot wiper failure or an element failure such as you had.

Thank you for adding that bias mod to your Eyelet board designs, Martin! I was just about to work on the bias board for my 3rd Gen build and this will be great to include.
Matt J.

[martin manning]

You're welcome. Good timing!

[martin manning]

...will start upgrading boards after I put together a Mouser order for some resistors.

I was just about to work on the bias board for my 3rd Gen build and this will be great to include.

Post back with results on the bias voltage and trimmer position you end up with. I'm interested to see if the resistor values in the fail safe bias document are good.