Puzzled by Amp failure.Maxim (Australia)

[maxjt]

Hi
Been reading this off and on for a while but now cant remember my original name and password;so have reregistered.
I have a Maxim head,built in Melbourne in the early seventies.2 x KT88 cathode bias.When i got this the original GEC KT88s were smashed so were replaced with JJ's.Also i replaced all the ps caps which were well gone.A few out of spec resistors here and there and the amp was so quiet i couldnt tell if it was on or not but a bit of volume and it sounded superb;loud full and very touch sensitive.
A few months ago i switched on and was playing for a while then the guitar signal disappeared to be replaced with a loud hum.Since then i have checked all the caps and associated circuitry but cannot locate anything obvious.On flipping standby i get clean signal which diminishes to be replaced by a loud hum.Voltages are correct with all tubes out but on replacement B+ at the KT88's plates decreases to about half of the expected value.If i had hair i'd be tearing it out at this stage.
Can anyone think of more possibilities and i'm really hoping the output valves arent roasted.

[martin manning]

Did you try a new rectifier tube?

[maxjt]

Yeah,i've swapped that one out for a new one along wth the 6v6 voltage regulator ,preamp valves and drivers.Exactly the same result.

[martin manning]

On flipping standby i get clean signal which diminishes to be replaced by a loud hum. Voltages are correct with all tubes out but on replacement B+ at the KT88's plates decreases to about half of the expected.

Do I understand correctly that when you switch from standby to play you get signal coming through for a short time and then the hum?

Also, do all the voltages look good with all tubes except the KT88's in?

[ampdoc1]

I'd suggest you get a can of freeze spray and hit all the resistors and caps, one at a time. I'd bet you will find a component that is changing value as it heats up.

If you aren't near a store with freeze spray, just get a can of computer cleaner. Turn it upside down so only the freon escapes, and hit each component with a small blast.

a'doc1

[Structo]

Can someone tell me what the 6V6 does in that circuit?

Is it a cathode follower type voltage regulator?

[xtian]

The photos you posted; are those BEFORE replacing the power supply caps? Because those look old. Also, ALL the other electrolytics should be replaced.

[martin manning]

Are the two X'ed circle things off the standby switch incandescent lamps, and are they working?

[teemuk]

Can someone tell me what the 6V6 does in that circuit?

Is it a cathode follower type voltage regulator?

Sort of... since it practically regulates with sheer capacitance and ultimately voltage potential shifts in the input will also reflect to output, unlike in regulators that use some sort of feedback to held up steady voltage.

Anyway.... The pentode is conencted as triode. Grid voltage is derived from the 150K/220K voltage divider. Filter cap bypasses AC ensuring DC voltage with very little ripple (8uF value is ample enough due to high R constant in the RC circuit). Cathode tracks grid's voltage and effectively current gain of the tube also multiplies capacitance at the grid circuit.

I think they commonly refer to such setup as "capacitance multiplier" but I have heard other titles for the circuit as well, including "fixed bias triode mode regulator".

[maxjt]

Hi
The 6v6 is a regulator for the screen supply and its been replaced.
On switch on you get signal for prehaps 10 seconds which diminishes and is replaced by a loud speaker damaging hum so yeah it is heat related.
The photos are indeed before filter cap replacement;i've also replaced all the electrolytics in the amp together with the coupling caps and any out of spec resistors.
All voltages are ok without KT88's in although the line voltage here is currently 283v apparently because of the localised addition of everyone having solar panels feeding into the local grid.(according to the power company).
The two lamps (on and standby) are working ok.
I'll need to check again but from memory the unloaded plate voltage on the outputs is 720v.Screengrid 380v.With output valves in this drops to 460 and screens to well under 300v.
Time for a coffee

[martin manning]

283V?! It's normally ~240, right? Wow!

I see the 6V6GT's heater is on a separate winding, likely due to the fact that it's cathode is sitting up around screen voltage and there is a need to keep the Vh-k within limits. But, the KT88 heaters are also on that winding, and their cathodes are going to be at a few tens of volts. What this means is that the Vh-k on the KT88's is probably extreme. Design max is 200V, absolute 250V per the MOV data sheet. Since screen voltage is approximately 2/3 of HT, Vh-k must be right up there. I know the cathode resistor is bypassed, common to both output tubes, and there is common-mode cancellation, but I'm thinking perhaps a new pair of KT88's is needed.

[maxjt]

Thanks for your reply.

I suspected hk breakdown but i think i was clinging to the hope that it was something less expensive.
Its a fun amp to use but i'm none to sure if its really worthwhile to replace the output valves and wait for the next failure.
I find the overvoltage in this area to be a problem as it seems that i'm running all my amps way over spec.I doubt that an old ac30 which is marginal anyhow really appreciates a line voltage which leads to filament voltages of 7.6v
I wonder is this becoming more common?

[martin manning]

If I were in your situation I think I would make up a box with a bucking transformer to get a source of voltage that is at a more reasonable level. I'd be very reluctant to run any amp at the overvoltages you are seeing. In the US most places have between 120 and 130VAC (120 is nominal), and vintage tube gear that was originally designed for 110-117VAC.

[Reeltarded]

I would take a slingshot and a trunk full of rocks on a solar panel hunt. If you do it at night black paint might work until it was scraped off, but rocks are forever and frontal attacks in daylight send a different message.

[Richie]

You have a 390ohm resistor for 2 x KT88 with 700+ with tubes out.

Have you measured the resistor/s value with the bypass cap, then without it or lift the positive end,and measure again?

I would look at that too,but like Martin said, you may have to try some other KT88. And have some other voltage issues from the wall to work out.

If, the cathode resistor values are off,or the bypass cap is shorting, it could also cook a new set of tubes. The data also says they need to use their own cathode resistor.
http://www.hebertech.com/datasheet/genalex/kt88.html