Heathkit AA-161 Coversion Project - Plate Voltage concerns

[bdgregory]

I'm working on converting this Heathkit integrated amp and have a few questions I hope I can get help with. I posted the schematic for the power side of the amp. The problem I am trying to solve is an intermittent issue causing the fuse to blow. This issue was present before I began any mods to the amp. The difference is before I used it as a hi fi amp, and the problem occurred once in a great while. Now, it happens when I try to overdrive the amp.

My questions relate to the voltage spec's and measurements. B+ is spec'd at 390V. The plate voltage spec's for the amp is 370V, with 380V on the screens. The actual measured B+ is 415V; Plates is 408V, screens 410V.

I know 408V on the plates of 6BQ5 is well over the design limits even though it seems reasonably within tolerance of the spec for the amp.

Questions:
Could the high plate voltage be the root of my fuse issue?
Note that the spec has a 10V delta between plates and screens. My measurements are 2V (nearly the same). Could this indicate an issue with my OT?
Can I insert a dropping resistor before the B+ to bring it down to a more reasonable level?

Suggestions/Recommendations?


By the way - the amp sounds fantastic as is - better than I hoped for, except I can't run any overdrive for any amount of time.

Thanks in advance!

 

[celeste]

Slow-blo or standard fuse?

The actual voltage that is important to a tube is Vak, voltage between the anode and cathode, not plate voltage to ground. In a fixed bias amp, cathode and ground are going to be pretty much the same, but in a cathode biased amp, you need to subtract cathode voltage from plate voltage if measuring from ground. So the situation with plate voltage is not as dire as it first seems.

measure your cathode voltage, and the cathode resistor if it is carbon comp, then figure the current your output current, does that seem out or reason? Because it is UL, I won't tell you to install screen resistors, but if you were to insert a resistor just to measure voltage across it, then you could figure plate and screen power to get a better idea how the higher B+ is effecting the operation.

My experience is that intermittent fuse blowing is not about voltage per say, but about voltage spikes causing arcing. The first places that come to mind are the coupling caps between the PI and power tubes, or a winding to winding short in the OT.

The reason the kind of fuse is important is because it gives you an idea of the over current is chronic or acute. A slow-blo will survive short term acute over current situations where a standard fuse would blow.

 

[bdgregory]

thanks Celeste - I'm using slo blo fuses as spec'd. .5 amp. The Vak in my case would be ~ 391 - I need to measure the cathode resistor to double check it - but assuming it's 210 watts as spec'd I get ~38-40 ma idle current.

Since my coupling caps (PI to Pwr tube) are original, I'll swap them out to see if that helps.

 

[directdriver]

The Heathkit AA-13 is almost identical to the AA-161, except its power transformer has two primary windings for 240vac. But the AA-13 schematic's voltage points are much more accurate in actual reading than AA-161's schematic. Use the AA-13 as a reference.

 

[Skylar]

Heathkit Aa-161 Board

• Skylar
• Dec 23, 2016

Hi Everyone, I'm new here. I can see it's been a while since this discussion but I am working on a similar project. I have a heathkit AA-161 that I've been using for a guitar amp through the phono mag. I want to rebuild the amp to fit in a fender style chassis for use in a combo amp. I'm going to post my modified layout here. If anybody sees anything sketchy and would like to offer some advice, i'd sure appreciate it.