Silverface Fender Bassman 100 - Checking Voltage Question?

[BonesMachine]

Jon... 99% ok. are you talking about the main power transformer? ... ok so i isolated the output transformer and did some basic loose OT multimeter resistance/continuity tests including:

- secondary to secondary ohm: .4 ohm
- primary to secondary continuity: none - good
- all wires. shorts to case/ground: none - good
- resistance between the primary red wire and blue/brown wires - weird?

i think all looks normal except for the resistance between the primary red wire and blue/brown wires. i am getting a very low ~4ohm reading from red to blue and also red to brown. from what i have seen this should be higher ~100ohm range? is what i'm seeing not normal? thanks for baring with me on this. i bought the amp to learn but i was hoping to just do a basic check up of the amp while in working condition

[Helmholtz]

i think all looks normal except for the resistance between the primary red wire and blue/brown wires. i am getting a very low ~4ohm reading from red to blue and also red to brown. from what i have seen this should be higher ~100ohm range?

That looks like a shorted primary.

Normally I would expect something like 15R per side.

https://www.hammfg.com/files/parts/pdf/1750W.pdf

[Chuck H]

Normally I would expect something like 15R per side.

Could that be a typo? Aren't we use to seeing 100 to 200 ohms here?

If it's a cheap little meter it could be an insensitive meter reading? Low testing voltage bogged down by inductance?

Were the leads removed from the tube sockets? Still a chance there's something wrong with the sockets.

[Helmholtz]

Could that be a typo? Aren't we use to seeing 100 to 200 ohms here?

No, that high DCR might be acceptable for a 20W to 40W OT but would cause excessive losses with a 100W OT.
My 100W Plexi OT measures 17R per side.

To avoid problems with the inductance, short the secondary while measuring primary DCR.

[BonesMachine]

i am indeed using a cheap digital meter. the OT is pulled from the amp. i shorted the secondary and am still getting ~4 ohm from primary (red to brown) and (red to blue). i think i saw a dime sized red hot spot on the OT while it was clicking. i had a bright light on but still think i saw something. the hot spot also left a bit of a scar. so its not looking so good for the OT, eh? is this a definite dead OT result from this measurement?

[Chuck H]

i am indeed using a cheap digital meter. the OT is pulled from the amp. i shorted the secondary and am still getting ~4 ohm from primary (red to brown) and (red to blue). i think i saw a dime sized red hot spot on the OT while it was clicking. i had a bright light on but still think i saw something. the hot spot also left a bit of a scar. so its not looking so good for the OT, eh? is this a definite dead OT result from this measurement?

I think we need a "don't like" tab for episodes like this

I guess the good news here is that the issue seems to be sorted out quickly (we hope). Certainly continue to suspect the tubes because a failure there is the most likely cause for an OT failure. So it's likely an OT and new tubes. Maybe screen resistors (they often fail with the tubes). And hopefully that's the end of it.

[BonesMachine]

thanks for all the help! i'm getting inconsistent readings on my cheapo meter. maybe those low readings are no good? i just saw an in the ballpark reading, my meter is all over the place? i'm gonna buy a fluke or something and check them again...who knows right now? nothing is certain at the moment

[Chuck H]

Well good. I hope it's just a cheap meter issue. I got my refurbed Fluke 87 through e-bay for under a hundred bucks. The read looked good. There was still screen protector plastic on the display and it had the original rubber slip and "leatherette" zip bag. It's been a great meter.

FWIW there are A LOT of new meters available in that price range anymore. Most are just fine and sometimes more versatile. Though not likely as rugged. I'll endorse the used Fluke option from my own experience, but there are plenty of options. My fluke will probably still be around when some new offerings are in the land fill. But it doesn't have a built in thermometer or a decent small value cap test option. Those would actually be pretty cool to have. Priorities. As long as it's not Radio Shack or the Ace Hardware $19.99 special (with the needle meter) I think you can't go wrong with most new offerings that have a good read. My first meter was a B&K with a lot of functions that I bought for seventy bucks in 1989 from a brick and mortar store. It lasted a long time and performed very well until the dial switch went hinky. No replacement available. Point is, you don't need to get a top of the line DMM to get good performance from such a tool. Just don't buy a cheap one.

[BonesMachine]

thanks for the meter advice. buying used isnt really an option. i was looking into fluke but am on a budget and didnt want to spend bucks on their made in china stuff. also some of the model specs on their site arent accurate like features it doesnt even have. you have to download the manual to find out. not cool! $30 basic alligator clips? i guess? i went with a klein mm400 which is way better than what i have at least. hopefully i wont be using it that much : ) i could buy 2 to cross reference measurements instead of 1 lower end fluke

[BonesMachine]

assuming the OT could be bad or good and just about to go. would it be safe as far as causing further damage at this point to turn it back on with new tubes and making sure the screen resistors are good or replaced? and also using a dummy load and bulb limiter. dont want to fry new tubes. i guess i'm wondering if just a bad OT could damage anything assuming everything else is ok?

[The Dude]

Don't run the amp with a shorted OT. You can check the screen resistors with an ohm meter.

[BonesMachine]

can the screen resistors be tested in circuit?

[The Dude]

Yes. The screen grid isn't physically connected to anything in the tube, so basically one end of the resistor is already disconnected (as far as DCR is concerned).

[Jazz P Bass]

If a screen resistor is open (bad) then that tube will not conduct/

[BonesMachine]

So its been a while but i finally got around to fixing my amp. I had turned it on with no speaker hooked up... I replaced power tubes, new filter caps, new bias caps and turned it back on with a bulb limiter. I think things look good so far the bulb starts bright then dims. I played my guitar through it with no bulb limiter very briefly. I didnt replace the screen grid resistors because they measured within spec. The OT passed some basic loose tests done with a multimeter. I guess there could be other damage to the OT. So possibly just a bad tube as far as getting it working!? I did some voltage tests on the filter caps with the amp on and standby on. Again I'm a novice can i get some feedback on the results they're not all the same so just wondering if things are looking good. The voltage readings are from where it likes to idle steady. Here are the readings:

3 x 22uf 500v filter caps
- 456 v
- 438 v
- 372 v
2 x 100uf 350v filter caps
- 216 v
- 438 v