ampeg sb12 very high voltages

[freeformfx]

I have a 70's 6L6 epuipped SB12 I've been struggling with for awhile. I've already replaced output transformer on it which tested bad, now I can't get anything near normal voltages from the secondaries of the PT or from the GZ34. The secondaries measure 415v per side and the heater voltages are high as well.
The B+ is measuring a whopping 619v with standby off, 580 with standby on. I've actually tested this with the output tubes IN to see if they brought down the B+ but then that same voltage stays on the tubes so I can't run it very long and variacing isn't very useful with a tube rectifier.
Is this PT possibly just bad?
also i have the 1k 10watt first dropping resistor removed right now to completely isolate the power supply

[g1]

You can still use the variac, you said the heater voltage was high, the rectifier shouldn't drop out till it's heater is substantially less than 5V.
Could there be some big ripple on the cap that is confusing your meter?

[freeformfx]

I am using the variac right now set at about 120v rather than the 125v in my shop.
I will test for ripple
I did put in a 2.5k 10w resistor ( all I had around) and it brought the b+ down to 519v, still too high. I will recheck heater voltages with everything under load later, maybe they're closer now. i've only not installed the 12ax7's because the leads from the OT get caught under the board and block off the socket from setting propely. PITA

[freeformfx]

anyone have any ideas on whether the PT may be suspect on this one??
I'm hesitant to run it as it is. And what about the 2.5k 10w in place of the 1k?
that should only help things right

[Enzo]

Suspect of what? Transformers are simply coils of wire around an iron core. The voltages coming out are determined by the voltage going in, and the turns ratio. The transformer is not likely to have grown extra winding turns. The transformer has internal resistance, and that is effectively in series with your B+ load, so that is part of why the voltage sags with tubes. But that won't change either, unless the wire itself changes.

Voltages are often higher than original these days due to higher mains voltage. But your 125v instead of 120v is less than 5% high, so then your secondaries will be a similar percentage high. So if it was meant to be 500v, then 5% higher will be 525v.

If you remove that first dropping resistor, then none of the preamp tubes are powered, nor are the power tube screens. So under those conditions, there is nothing to pull down the B+. One way to turn a power tube off is to remove its screen voltage. That is in fact what Peavey does for its standby circuit. SO with that resistor removed, your power tubes will not conduct any current to speak of, plus the screen drain will be gone.

You will pretty much have to put preamp tubes in it to get anywhere with the downstream B+. That first resistor sets the screen voltage as well as the basis for the tubes further down the string. It surely won't hurt the amp to have 2.5k instead of 1k, but you ought to measure the voltage drop across it - with all the tubes in - so we know how much current is being drawn.


I am not sure why increasing that resistor would reduce the B+, unless the comparison is voltage without the resistor compared to voltage with the resistor.

[TigerAmps]

I agree with Enzo. The voltage very well could be higher than what you expect.
New filters with higher values could cause a slight increase in voltage.
Old (original caps) could cause issues as well. What is the state of the power supply caps?
If you measure 415 on the secondaries, you should see 540V (1.3 * 415) on the plated before you throw the standby.
After you throw the standby switch to on, it should drop to around 525V which is what Enzo eluded to above.
If you see over 500V, the bias definite will need an adjustment!
Check the output jack wiring as well. Make sure the external speaker jack is making a good connection.
With everything connected (including the speaker load), what is the idle current of the tubes and the plate voltage?
Using today's tubes, most of the B-25's I fix definitely require a bias resistor change (R25).
If you changed the OT, I would definitely check the bias. What OT did you put in? Mercury replacement or a clone/substitute?

[gearhead63]

I agree with Enzo an TigerAmps. I have a Crate BV60H, runs 6L6GC power tubes, and even with all tubes in circuit it runs quite a bit higher B+
voltages than what the schematic shows (I can't tell you exactly what the voltage is, I don't have my paperwork here). And the dropping resistors right after the PT are new and in spec. as well as the filter caps.
My mains voltage runs about 118 most of the time.
But it didn't affect the bias adjustment. And works just fine.

[J M Fahey]

to completely isolate the power supply

Well, we don't actually want it isolated, but loaded, fully loaded.
Meaning all filaments, all plates and screens sucking current.

To boot, tube rectifiers on their own have, say, up to 60V drop across them ... but only if full current is passing through them, not otherwise.

This is the original schematic, not the redrawn one, and carries suggested normal operation voltages:
http://www.vintage-blue.com/blog/sch...ic_6l6_ver.pdf

[g1]

And note that the schematic Juan posted shows 117V line voltage. With your variac giving you 117V, and the supply fully loaded with tubes etc., see if the voltages are still more than 20% over what is shown on the schematic. That standard +/- 20% of the voltages means with 117VAC in, you can have up to 570VDC at the plates in run mode.

[freeformfx]

I realize the secondaries may not show spec'd voltage unless everything is loading it but the heater voltages are high even then
the filter caps are an increased from 30uf, 40uf 500v for the first (though it's a weber cap stated tested up to 600v) two more of these 40uf's for the next two and I temporarily stuck a 22uf 350v cap in for the last filter I should test it it could be bad for all I know.
I've ordered proper replacements for those two out of spec.

Here's the update: Enzo the drop across that 2.5k10w resistor is 16.5volts
with all the tubes in I have the B+ down to 491, OT plates at 485 screens at 471v. The bias voltage at pin 5 is -45.5v
The amp IS working but now I'm in a new pickle with it as I have a really low volume and a serious hum that can't be adjusted out by the hum balance pot.
I do notice that at the last dropping resistor the reading is very low at 275v, all other voltages look pretty good to me. So I wonder more about that 22uf 350v old Illinois cap I have there.
thanks for input

[freeformfx]

Oh and unfortunately for some unknown reason to me I can't check bias (current draw) because somehow my bias probe socket WILL NOT fit in the socket of the amp! I have even forced it and closely examined the situation but just will not insert. Even though the pins look identical to the pins of the tube that's in it , SO I can't tell the bias of the 6l6's. All I know is the bias voltage at pin 5 is -45.5v and the plate 485v. frustrating

[nickb]

anyone have any ideas on whether the PT may be suspect on this one??
I'm hesitant to run it as it is. And what about the 2.5k 10w in place of the 1k?
that should only help things right

In no way wishing to detract from all the other sage advice here, but in direct answer to your question you can 'ring' the transformer to detect shorted turns, in this case on the primary. I would expect the transformer to run hot and have a high magnetizing (unloaded) current too, if that were the case.

http://www.electro-tech-online.com/a...test-gif.4290/

[freeformfx]

Man, just as soon as I have convinced myself my PT has got a clean bill of health. or at least is still hanging on and not causing any of the problems I'm having.
thanks
sorry if this ends up a double post but my new ones aren't showing
ok a little more info, and i am crossing my fingers that the problem isn't in the ampeg tone module, but turning the treble pot through it's paces has a wacky effect on the buzz gating distortion turning it all the way down nearly sends the amp into runaway. The bass pot effects the sound this way a bit too.
I found that although i do have -49v on pin 5 of V3 BUT have -59.8 on V4, what could be causing this?
also if i pull one end of the 100k 2w resistor going to the diode in the bias supply I get a much stronger fuller full volume signal, though a bit distorted but much better. This resistor is carbon comp and has drifted to 125k , could it be the problem.
I've been working on this amp on and off for over 4 years so i am pretty out of ideas any help is sooo appreciated!

[Jazz P Bass]

Oh and unfortunately for some unknown reason to me I can't check bias (current draw) because somehow my bias probe socket WILL NOT fit in the socket of the amp! I have even forced it and closely examined the situation but just will not insert. Even though the pins look identical to the pins of the tube that's in it , SO I can't tell the bias of the 6l6's. All I know is the bias voltage at pin 5 is -45.5v and the plate 485v. frustrating

Are you set up to measure the mains current that the amp is drawing?

Two 6L6's biased warm will pull maybe 80-90 watts from the wall.

Not that I am in any way advising that is the way to SET the bias, simply a health check.

[freeformfx]

no not really. I just used the method resistance across red and blue OT leads and then voltage across them. Then divided voltage by resistance to get the current draw. I got about 39ma. at 485 plates that's a much higher dissipation figure than I usually go with but Is ok I guess?
as the volume control was doing nothing I found a broken trace between outer leg of volume pot and pin 7 of V1, funny how I was getting volume at all. Well after repairing that I am left with a very low volume fizzy distortion that when you let the note die out it cleans up a little.

Followed along with my "listening amp" and probe and was good signal all the way through all the coupling caps when I suddenly f'd up the input of my little probing amp when i touched pin 5 of the output tube. I've never had anything hurt an amp I was probing through before.
Any ideas what this is? bad volume pot?