PI input voltage?

[daz]

How much VAC should a marshall 50 watt style amp show with the master maxed at the PI grid? also while i'm at it, what amount of AC should be seen at the speaker jack when cranked?

[GainFreak]

How much VAC should a marshall 50 watt style amp show with the master maxed at the PI grid? also while i'm at it, what amount of AC should be seen at the speaker jack when cranked?

20-30V depending on Treble and Middle settings.

[defaced]

also while i'm at it, what amount of AC should be seen at the speaker jack when cranked?

Ohm's law. 8 ohms (resistive load), 50 watts, gives 20 volts AC.

[daz]

What would happen if that were 65-70vac with tones all at 12:00?

[Robert M. Martinelli]

RMS or P-P ? (There's a HUGE difference between these two values).
Cheers
Bob

[Chuck H]

Since it's a 50 watt amp I'll have to assume there isn't 70VAC at the speaker (600+ watts @ 8 ohms)

I would think that 70VAC at the PI input could throw the bias on the PI and cause blocking distortion.

Chuck

[Robert M. Martinelli]

Since it's a 50 watt amp I'll have to assume there isn't 70VAC at the speaker (500+ watts @ 8 ohms)

I would think that 70VAC at the PI input could throw the bias on the PI and cause blocking distortion.

Chuck

Hi Chuck!
How r u doin'?

That's the reason I asked him if the readings were RMS or P-P, I strongly suspect those 65-70 VAC he was talking about were P-P values.

Cheers

Bob

[daz]

Sorry. As Chuck well knows i'm not tech, so i'm not sure about the difference there and all i was doing is measuring AC voltage at that point with a meter and tone controls at noon or thereabouts. But heres the thing....i just checked my amp which is the same design pretty much, and they have the same AC voltages there and everywhere i checked them except one, and it very telling ....the speaker on his has 8VAC while mine is double that. Thing is, i tried a different OT and his still did has the problem im trying to troubleshoot, tho it was a smaller 25 watt OT.That problem being the amp starts distorting more as i turn it past 10:00 instead of getting louder, and i thing the speaker voltage is telling me something. Someone elsewhere said it sounds like one tube not working, and thats exactly what i thought once he mentioned it. but i've check everything aside for the real fluke kind if things like a funky socket or such. (tho they are beltons and very good and don't looked arc'd)

All that said, i meant to say the EL34 grids, not then PI ! So is that too much there? Also, is there anything else you can think of that might cause this loss of volume possibly due to one tube not working besides the things i checked which are tubes, OT, cathode resistors and caps, screen resistors, impedence switch and speaker jack? i checked a lot of other things, but now with 1/2 the VAC at the speaker i'm thinking it's something happening AFTER the EL34 grids.

[MWJB]

Open PI plate resistor? As you say the amp "lost volume" (e.g. it had it at one time?) sonds like a component failure?

What VAC do you get on the good amp at the EL34 grids?

You could have significantly more VAC at the speaker when cranked than 50W RMS might suggest.

Let's make a reasonable assumption that the amp makes 35W RMS (relatively clean). Into an 8 ohm load you'd expect to see 16-17VAC at the speaker. So set up the dummy load apply 1000HZ until you see 16VAC or so at the speaker (all controls fully clockwise) and see what you get working back towards the front of the amp. The typical 100mVAC signal (all pots fully CW) could well be enough to be pushing things way above the amps RMS rating. Compare the 2 amps

[MWJB]

Unplug the power tubes, disconnect the amp from the wall, put your 1/2VAC on the speaker secondaries (measure that it is actually 0.5VAC with your meter), what do you get, end to end, on the OT primaries? Any discrepancy when measuring either end to CT?

[daz]

I found the problem. It's the worse news. yup...OT. Clipped in the OT from my amp which is the same OT, and it works fine. $80 down the tubes. I only wish i knew why it went bad to begin with

[MWJB]

Maybe the owner tried a different speaker/cab & caused a mismatch.

I'd apply 100mV at the input, wind all the controls fully CW and see what develops at the speaker...you get 32W with everything set to noon (could be as little as 10% of pot value in some cases), perhaps this guy likes to play with everything dimed? It's worth considering you may need to upgrade the OT.

[daz]

I asked him if he could have mismatched or played for any length of time with the amp on and speaker unplugged. But he said no. he's a friend so i don't think he'd BS me. And i know he can't play dimed because he gets complaints from the neighbors even at fairly low volumes, plus he doesn't play out.

[pdf64]

I've had a intermittant bad tube red plate, and overheat the OT primary so as to create a short across some windings.
So it might yet be a bad tube, or bad connection in the grid circuit of the el34, creating temporary bias loss.
Or was it an OT you did development tests with? If you ran the el34s for awhile at high standing currents ie close to, or at, class A, then the OT primary might have overheated and created a weak point. Which was walking wounded, then subsequently failed. OT manufacturers will assume class AB unless otherwise specified, the primary winding will have a max continuous current rating that they might not publish. Peter.

[daz]

Or was it an OT you did development tests with?

yes, as a matter of fact it was that amp i developed the current design with. The design i built my second one with and it's a great amp. So you could be right because i basically learned a lot with that amp and i'm sure i abused the heck out of that OT, tho i don't recall it being low volume when i sold it to him. i suppose the winding could have been on the verge of shorting. One side IS low resistance wise....45 ohms and 35 on the other. My amp's OT is 5 ohms apart instead of 10 like his.

i took the OT out and inspected it with the bells off and theres nothing onvious outside, so there is obviously a shorted winding.