Peavey Classic 400 schematic ?

[glebert]

I wonder if the Peavey techs could provide what the signal levels should be at various points in the signal path. It would be nice if that was in the service manual.

[Jazz P Bass]

According to the owners manual, the amp ts rated at 400 watts, 4 or 8 ohm.
420 watts at clipping.

The overall power consumption is 1000 watts from the mains.
That is where I would look next.

[J M Fahey]

In a very simplified way, "power tubes are inherently current limited" .
Itīs very hard to pull many electrons from metal surface and to make them travel in VACUUM!!!!
Which is good as an excellent built in short protection and helps having a "Rock" guitar sound and clipping but is a PITA when you try to pull more power than usual from them.
And current capability only goes *down* every hour that tube is ON, even faster if abused.

I trust Peavey, and they have shown to be conservative in general, so I bet that amp can put out 400W RMS "barely clipping" or not much less (say 350W+?) when leaving Factory, which means fresh tubes and settings.

After a few years?
Who knows, but power out spec can only go down, never up.

[Enzo]

I wonder if the Peavey techs could provide what the signal levels should be at various points in the signal path. It would be nice if that was in the service manual.

It is just another tube amp, the voltages within will all be similar to any other amp.

[vintagekiki]

Does this amp really do 400W? ... ...

In the beginning to understand.
Each factory stands behind the power specified for its amplifier, provided that all tubes are 100% new, from the same manufacturer and that the bias is set to the specified value for the types of power tubes that are installed.

When replacing power tubes, the change complete set (in this case 8 x 6550) because the amplifier does not know the "strong and good" power tubes, the amplifier knows only new tubes.
What we call "strong and good" power tubes does not have to mean that all 8 x 6550 are matched as an octet and that for bias (for example -56.5V) they have approximately the same current (+/- 5%) because the power decreases with the square current.

In other words if output tubes instead of 100% have a 90% emission, amplifier gives 80% power output. Power tubes generally change when their emission drops to 70% because the amplifier drops power to 50%.
The bias is properly adjusted to the quiescent current (n x 25-35 mA) because it is high probability that all power tubes at high power will have approximately current.
It is desirable not to mix power tubes from different manufacturers, because during exploitation emission do not drop at the same time

Useful link: http://music-electronics-forum.com/t8291/

[nsubulysses]

Sorry no scope output pics yet

These two 100 ohm resistors to ground in filament supply are open. They look normal but if you see the underside it's burnt.

Would this make filament supply low and tubes will have weak emission ?

[Jazz P Bass]

The 100 ohm resistors are like a 'hum balance'.

They will tend to fail if a tube fails B+ to heater.

If they are open, then the heater supply should go up.

[g1]

I'm a bit surprised your B+ is a tad low, rather than higher which is what we usually see with modern line voltage.
Have you looked at AC ripple on the main caps at idle and at full tilt?

[nsubulysses]

I forgot to mention I measured power consumption as JPB suggested and it was 240W at idle, 760W when amp is driven to clipping.

This was before I found open 100R resistors.

I am checking this thing out now so I'll check for ripple when I put it back together too. However, if there was excessive ripple how would this make the power output less than expected? just asking because I wouldn't think to look for that when an amp doesn't put out full power. thanks

[g1]

Excess ripple will basically reduce the B+ available to the tubes.

[nsubulysses]

Here's what the amp does at clipping. About 34V into 4 ohms. it says it in RMS in bottom left side


And the 400W output as Enzo requested. So yeah it can put 40V into 4 ohms for 400W but hmmm


B+ and screen supplies have about half a volt AC at idle and about 2VAC at clipping.

B+ and screen supplies drop about 100V each from idle to clipping, 680V B+ drops to 598V and 392V screen supply goes to 298V

Power tubes are doing about 100-110mA at clipping. Seems like amps usually do 120-150mA at clipping typically. maybe this is less so since there are 8 power tubes and it's only doing 290W at clipping.

[J M Fahey]

Ok, what you have is *reality* .
300W RMS as is, period; *might* do (or approach) 400 RMS with all fresh power tubes, if good family.
Itīs hard for tubes to put out clean high power, easy task for SS.

[nsubulysses]

Maybe I am at war with reality

I have an SVT-II here so I might as well give it a look for comparison

here's how it does 34V into 4 ohms

B+ is 655V and drops to 565V at clipping. Screen supply drops less compared to Peavey Classic 400, 385V at idle and 327V.

SVT power consumption is 220W at idle and 650W doing 34V into 4 ohms.

SVT power tubes (I only measured one) are doing 132mA when amp is clipping

Maybe this Peavey classic 400 is different because it has a saggier screen supply and 2 additional power tubes so it does similar power to SVT but is supposed to be easier on the tubes.

Seems disapppointing but possibly real??

[The Dude]

Did you ever check for ripple as g1 suggested?

[nsubulysses]

yes in post 26.

I checked with DMM on AC on power supply nodes