Bassman AA864 Build hum / buzz issue

[Jamlid]

Hi folks, having issues with my AA864 build. Here's what's happened so far.

Upon first power up there was a loud hum, then smoke from the PT - This, it turns out, was most likely due to a short in the heater wiring (resistor leg across both solder terminals on the lamp holder).

After I replaced the PT the hum still persisted. I took it to a tech who pointed out that I'd grounded everything incorrectly. Once they sorted that out everything seemed to work fine for a time. No hum or anything concerning.

Up until this point I hadn't used a current limiter or variac. However, once it burned through one of the 470r 1W screen grid resistors I thought best to replace the 6L6GC power tubes and use a current limiter upon next start up. Worth mentioning that when this happened I had the amp at about 2/3rds volume and was driving it with a pedal. I saw the output tube arc and the mains fuse was done for, along with previously mentioned resistor.

So, after finding new output tubes and replacing the burned tube socket and resistor I went through all of the steps deemed necessary by Rob Robinette for starting up a new build and all seemed fine. I had no variac but the limiter behaved as you'd expect. No obvious catastrophic shorts. All voltages seem fine when the power switch is closed, but when I close the standby switch is when this new hum gradually fades in. The voltages in the HT circuit start where you'd expect but then gradually lower to the voltages listed below, this coincides with the hum. All of these voltages were measured with a 100W bulb current limiter. (I’m in the UK btw).

- Pins 3 and 4 on both power tubes - start off at 440 ish VDC then decrease over a 10/20 seconds - when at around 370VDC that’s when hum/buzz becomes audible. Then voltages settle ~340VDC.

Another thing I found when when closing the standby switch was that the heater AC voltage behaves strangely (I think). See below:

heater circuit no power tubes (w/ limiter):
pwr on standby off 6.4VAC
pwr on standby on 6.33VAC

heater circuit with power tubes (w/ limiter):
pwr on standby off 6.08VAC
pwr on standby on 5.29VAC

I'm using new tubes and a new screen grid resistor, same spec as schematic - 470r 1W. Are these voltages lowering because of the current limiter? It just seems strange that the hum and voltage drop seem related.

Might I have damaged other components when the tube arced? What should I be looking for/measuring now? Is it worth going back through all preamp tube pins and measuring voltages there?

Thanks!

[mresistor]

Yes the voltages will be lower on the current limiter -  We don't check operational voltages while on a LBL current limiter.

First, do you have 1 ohm 1W 1% resistors on cathodes of the power tubes ( pin 8 ) to ground?  If not install a couple.   Then are the power tubes matched?  If the power tubes currents gets unmatched enough that can cause hum. The one ohm resistors make it easy to check whole tube current. 


The current limiter is only used to check for initial shorting inside the amp for a fresh build or after repairs or modifications. Take the amp off the limiter and make sure first that the bias is set to max negative voltage on the 6L6s pin 5 then adjust the bias properly under operating conditions. Bias should probably be set to 55-65% of max idle current for a 6L6  which is around 16.5W to 19W or so.  It can go to 70% but check how it sounds at lower current levels. Check the bias and dissipation for both tubes.

[Esquirefreak]

One or several parts of the circuit is drawing way too much current.

I would NOT fire the amp up before thoroughly going over your build, checking for possible shorts. That is - bad wire/component insulation for example. Something making contact to chassis where it shouldn't, etc.

After that I'd start up without tubes and make sure there's a healthy amount of negative voltage on the 6L6 sockets (pin 5).

If you keep blowing fuses in a known working circuit, the circuit itself is not the issue.

And make sure you didn't hurt the OT. Check resistance across the windings.

/Max

[Jamlid]

Thanks for the replies.

I haven't got any of those resistors at the moment. I will install them when I get chance in order to measure tube current then dissipation. Tubes are indeed matched. I also have a hum pot installed.

There is a healthy amount of negative voltage on both tubes' pin 5 without tubes present in circuit. ~ -62V. This is with the bias set to maximum negative voltage. (No limiter)

Also before measuring this I checked over my build and as far as I can tell there are not accidental errors/shorts. I did reflow some of the joints that looked as though there was a chance of them being cold, then tested with a MM after I did so. All seemed fine to me.

I then fired up the amp WITH the limiter and no tubes to make sure I hadn't shorted anything during reflowing. I then installed all tubes and fired up the amp WITH the limiter again, no problems indicated by said limiter. I then removed the limiter and fired up the amp, still with bias set to max negative voltage. I was going to set up proper bias voltage (schematic says -44V, but I hear that new production tubes can't handle being biased as hot, so was planning on setting it at more like -50V first of all). However, I started hearing the same hum again. I turned off the amp pretty quickly as I didn't want to do the same as before and have the amp die altogether/go through another set of pwr tubes. (Those are expensive at the moment!)

Lastly I checked the resistance across the OT windings:

Primary winding end to end: ~100R
Blue wire to red centre tap wire: ~53R
Brown wire to red centre tap wire: ~50R

Secondary winding ground wire to 8ohm wire: 2.7R

Also measured across the primary and secondary wires and found no shorts there. Do these measurements look okay? (There are two more taps for 4 and 16ohms but they are heat shrunk and out of the way, should I test those too?)

Next I plan on installing those 1R 1W 1% resistors on the cathodes of each pwr tube so I can test dissipation.

If this seems fine then should I work my way backwards through the circuit and test voltages?

Thanks

[Jamlid]

Have managed to do some more work on this today, adding the 1W 1R 1% cathode resistors to ground.

As always after working inside the amp, I firstly powered it up using the limiter. No problems. I then removed it so all voltages below are without it.

I set the bias colder than the schematic: -50VDC when the power switch was closed, which dropped to -49.6VDC once the standby switch was also closed.

However, one 6L6GC plate measures at 477VDC and the other at 468VDC. This is much higher than the 420 stated on the schematic.

Cathode resistor voltage drop was around 0.051VDC for both tubes, which seems low to me.

I'm aware that I need to subtract cathode voltage from plate voltage and use the result to calculate dissipation, but the former was so low I just entered 477 into Rob Robinette's bias calculator. It ended up telling me that the tubes are running at 23.4W or 78% dissipation.

It occurred to me that I might not be using the best MM and probes. (It's a cat II 600V rated MM, and the probes I'm using are nothing special). Could this be causing inaccurate readings?

If the reading are accurate, what does this mean?

[sluckey]

Adjust the bias to lower the cathode resistor voltage drop. Recalculate dissipation and repeat until you are happy.

[Jamlid]

Thanks Sluckey. Will do. But do you think the plate voltages are alarmingly high?

[sluckey]

Definitely high but I'm not alarmed. What readings do you have on the PT secondaries? Can you post a datasheet for your PT?

[Jamlid]

Mains voltage reads at: 244.8VAC
PT bias circuit reads at: 53.4VAC/53VAC - standby open/standby closed
PT heater circuit reads at:  3.380 + 3.257 (respective to grnd)/3.368 + 3.329 - standby open/standby closed
PT HT circuit reads at: 347.8VAC + 347.8VAC (as above)/344.9VAC + 344.4VAC - standby open/standby closed

The PT is a Hammond 291EEX:
https://www.hammfg.com/files/parts/pdf/291EEX.pdf

[sluckey]

The schematic shows 305VAC for secondary. Your PT puts out 345VAC. That explains the higher dc voltages. It's a case of wrong PT. However, your amp should work just fine with the higher voltages if the filter caps are rated high enough.

[Jamlid]

Well, that's one hell of an oversight. If there's one thing I'm learning from this hobby is that there are always more things to check and check again.

Will check filter caps, hopefully they're alright!

[Jamlid]

So the filter caps I have installed currently are rated at 350V (the two in series connected to ground), and the other three are rated at 500V. Is this a bit too close for comfort?