B-52 AT100 Low output

[drjones1106]

Hey all, I've got a B-52 AT100 that I got for a song, that has never put out the volume I would expect from a 100 watt amp, much less a 15w. My orange terror pushes a 412 louder than this thing's clean channel.
when I got it it was non functioning due to no tubes (some idiot had hot glued random tubes on top of the sockets) and a fried bias trim pot. Those got replaced and it fired right up, but quietly. It spent a year just chilling, then I tried some random spare power tubes I had, and V7 shorted frying a few resistors. boy do I feel stupid.
Long story short, everything has been replaced and is back to the state it was before I made stoned troubleshooting decisions. Tried everything with new tubes, and no change.
So output is low, clean channel measuring 33W clean and 57.75W master and channel volume full. I didn't measure the dirty channel output as I'm not sure how to tell since it'll already be clipped on the Oscope, but it is super quiet, like bedroom level at 9 o'clock on the master.
B+ is 557, 530ish when pushed. all the pre-amp tubes are getting proper voltages, I forgot to write them all down. I'll double check when I have time.
I suspected that OT but everything checks out, no shorts across the windings and both sides have expected resistances.
I've done a very thorough inspection, all of the solder joints look fine, chopsticking hasn't relieved anything. no fried resistors or swelling caps, etc. Since voltages are within reason I'm reluctant to start replacing caps as the circuit board is a pain in the ass.
so I'm at a loss. I would like to get this thing up and running, or if not verify that I can reuse the transformers for a frankenamp. but I've done all I can think, though I there's so much I don't know. any help is appreciated.

[WimWalther]

"LOAD IMPANDANCE"

"STANDBAY SW"

So it's Chinese, I take it? =)

[WimWalther]

First thought.. nearly 30V of sag seems like an awful lot for a 5AR4-based supply. You're sure that rectifier tube is totally healthy? Do you have a solid-state plug-in you can try, to see how the amp behaves?

Easy enough to make if you don't have one on-hand.

[Ronquest]

First thought.. nearly 30V of sag seems like an awful lot for a 5AR4-based supply. You're sure that rectifier tube is totally healthy? Do you have a solid-state plug-in you can try, to see how the amp behaves?

This amp has the switchable "tri-mode rectifier" tube to ss options on the back.  5AR4 has about 30V of drop compared to diodes.  Not sure if it correlates to what he's doing/getting.  Voltages seem high at 557V as the B+ caps are rated 500V and the 5ar4 isn't data sheet rated for that either.

Are your power tubes pulling current, all biased correctly.

This is a quote from another board on voltages, seems more with what you should be getting.   "With The B+ lines disconnected from the circuit, measuring right out of the power supply, I get the following readings: B+1 514v, B+2 512V, B+3 507v, B+4 497v." 

[drjones1106]

hey all here's a short update.
First, a big typo in OP. B+ is 457 not 557. my bad. I re-measured voltages and took better notes, so B+ is at 450 idle and 430 under test.
voltages all look normal, nothing is wildly out of wack, though I can post the full list if anyone cares to double check me.
the 6L6s are all pulling current and though not balanced very well are all fine 13.8Wplate dissipation for one side 18.7W for the other.
I have more info I'll post tomorrow but my wife says its time to go to bed.

[tubeswell]

“13.8W plate dissipation for one side 18.7W for the other.“


Suggest you try swapping the pairing of the 6L6s around to get it more even and bias the tubes up to 21W (assuming they’re 30W tubes).

[Dave]

If you discover that the OT is bad, let me know. I have at least one.


Dave

[drjones1106]

tubeswell you're right I should setup my power tubes to functioning closer to 30W if I'm concerned with output volume.
Dave, thanks, I'll let you know but as of now I don't think its the OT. No shorts across any windings, no opens, voltages at the power tubes are all good.
so last night I checked all the preamp voltages, and everything was looking fine. nothings unexpected. 
I re-measured my output wattage (0.5v @ 1khz sine, VAC measured across a dummy resistor 9.6ohms) and found that I was getting 22.38VAC with the output sine clean for a 52.17W output on the clean channel. when I crank the channel and master volumes to full I'm getting 33.62VAC for 117.74W! so maybe I'm just wasting everyone's time here. So I moved on to the dirty channel, and setting all the various gain and volume pots to get the clipped signal as "clean" as reasonable, I'm measuring 6.96VAC for 5.04W. With the master around noon I'm getting a very distorted signal at 18.64VAC for 36.19W. Dimed I'm back to 32.64VAC for 110.97W. So maybe the issue isn't with the function of the amp, but my expectations of this cheap Chinese triple rec clone. Maybe the master volume sweep just needs to sit in the last quarter to get the volume I'm expecting out of a 100W amp. It still doesn't sit right with me tho. My bassman 70 will rattle my neighbors fillings with the master above 3. I would just expect more from an amp like this.
I'll re bias the output to be more to spec and maybe double check the caps in the dirty channel to see if I'm losing signal somewhere in there. I'll update hopefully in the next day or so.

[PRR]

(in fixed bias) The idle dissipation has almost nothing to do with Maximum output power.

The numbers on a gain pot ("Master Volume") do not control output Power. (You can always play louder.)

[drjones1106]

(in fixed bias) The idle dissipation has almost nothing to do with Maximum output power.

The numbers on a gain pot ("Master Volume") do not control output Power. (You can always play louder.)

Well thanks for the dressing down PRR, I'll try and keep my ignorance in check.
I realize that both these are not going to dictate output power, but the amp still feels weak to me. I'm trying to understand the relationship between an amps advertised wattage, what I should reasonably expect for measured watts, and precived volume in the room.
If you have some suggested reading I'll try and come back with more intelligent questions.

[sluckey]

Well thanks for the dressing down PRR,

That's not a dressing down. I'm in charge of that!