Bandmaster - 23 watts clean?

[lowell]

This amp came in with complaints of lower headroom. It's an AB763. I found some incorrect resistors in the PI, and a couple Rk were wrong in the preamp. Aside from that, there is a bit more B+ voltage but I think that's normal for the increase in mains voltage over the years. ?? Anywho I've biased the power tubes, checked every voltage on all tubes' pins, even measured dropping resistors, Ra, Rk, grid voltages etc... All checks out good. However, the PI cathode voltage is at 85v, NOT the specified 110v. All components are correct and connections good. Don't think that would cause such a noticeable problem though. The Vg/k on the PI is ~-2v. I put a 1k test signal in at 200mv and connected the speaker out to a 4ohm dummy load. The distortion on my scope is visible at 23watts RMS of output... or ~13.5ACV. Is this normal? Not familiar with headroom in this amp.

[GregS]

Is this normal?

No. I've worked on a couple of them. One I hooked up to the scope and a dummy load and measured over 40W rms clean output. They are rated for 40W so 23W is not in the ballpark. There is definitely something wrong. It may be running out of headroom at an earlier stage such as the phase inverter or a pre-amp stage rather than the power amp. If you have a dual trace scope you can hook one of the probes across the load at the output of the amp and the other at the output of an earlier stage. If you see the distortion appearing simultaneously on both channels with the same shape waveform you know the headroom is being exceeded at either that stage or an earlier one rather than the power amp. You can then trace back to find the stage it originates at. If the output of the amp is running out of headroom before the output of phase inverter then you know the headroom is being exceeded at the power amp.

Hope this helps.

[lowell]

Hi Greg thanks. So the distortion starts at the grid of the PI where the NFB is connected. The other grid looks good. I'm confused as to whether this is a PI issue or PA issue because of the feedback.

[Bruce / Mission Amps]

What? Somebody is goofed up. Get some facts straight first.
13.5vac rms across a 4 ohm load is well over 40 watts....

[lowell]

Ha haaaa! Thanks Bruce. Brain fart, don't know WHAT I put into my TI82.

[JoeM]

You were thinking 13.5V peak AC not RMS. Unless you really mean peak, it would then be around 23W.

[lowell]

Good Math Joe but I think I had the 8ohm load measurement mixed up w/ the 4ohm load.

[GregS]

To clarify, what peak to peak voltage did you read off the scope and with what load connected?

[lowell]

My scope has proven unreliable. I used an RMS meter.

[Tubebass]

Is the correct tube (12AT7?) installed at the PI?

[lowell]

Yep

[Tubebass]

I only ask because I got a Pro in to look the other day which had its tubes seemingly in random order!

[Bruce / Mission Amps]

Ha haaaa! Thanks Bruce. Brain fart, don't know WHAT I put into my TI82.

I just tuned one up last week and was getting about 12.7 vac across a 100 watt 4 ohm dummy load! ha ha ...