New build squealing

[TarheelTechinTraining]

Never thought about the meter being off. Another question how quickly should the filter caps discharge?

[The Dude]

That's going to depend on the amount of capacitance, the voltage across them, and how much if any bleeding is going on at turn off. Best to check with a meter before sticking your fingers in there.

[TarheelTechinTraining]

Absolutely. It just seems they drop pretty quick. Like under 10 minutes. I'm using 600v caps instead of the 500v like the schematic

[The Dude]

You probably get this, so apologies if I'm "dumbing it down", but the voltage rating of the cap matters less than what the voltage is across the cap when fully charged. In other words, a cap with 300V across it is going to discharge similarly whether it's rated at 500V or 1,000V.

[TarheelTechinTraining]

No worries all "good" advice accepted. The filter caps are getting about 540v. Thought the 500v wouldn't /couldn't take it

[TarheelTechinTraining]

Oh by the way. All the folks on this forum are wonderful. I won't have this fire breathing beast of an amp. Its all thanks to you guys.

[TarheelTechinTraining]

Actually I did. What did swapping the primaries do anyway? Seems to me I didn't have enough NFB with the 120k resistor. Can someone explain?

[TarheelTechinTraining]

Another noob question. The heater voltage is sort of high, like 7.2 vac. Should I worry? The specs for the Svetlana's is 6.9 max

[Chuck H]

Actually I did. What did swapping the primaries do anyway? Seems to me I didn't have enough NFB with the 120k resistor. Can someone explain?

Swapping the leads reversed the phase of the amps output. Which is where the signal voltage is derived for the NFB loop. If the loop is in opposite phase with the signal present where it's applied (the PI tail) then you get a negative feedback loop. If the loop is in like phase (as it was before swapping the leads) you have a positive feedback loop. A positive feedback loop will usually oscillate like the low frequency voltage modulation of a tremolo circuit (an oscillator circuit) or the high frequency squeal you heard from your amp.

[Chuck H]

You surely want to get that down. A recent discussion here covered this ever more common problem and revealed that tube life can suffer greatly. There is a small life advantage to running the filaments on the low side, but too low can lead to poor audio performance. Most of my amps were running high between 6.9something and 7.1something. I used equal value resistors on either side of the filament supply to bring it down to 6.2something. Here's a good link written by one of our members that covers it.

The Valve Wizard

[Jazz P Bass]

10% either way is fine.

The difference between 6.93 & 7.2 is, what, 0.039%.
I would not sweat it.

I have seen many 'modern' amps (& old ones) come in at 7.2 Vac.

[Chuck H]

10% either way is fine.

The difference between 6.93 & 7.2 is, what, 0.039%.
I would not sweat it.

I have seen many 'modern' amps (& old ones) come in at 7.2 Vac.

My recollections are from this thread where R.G. gets jiggy and breaks it down to settle a disagreement with SGM. Read posts #47 through #56. If this is even half right I think observing correct filament voltages deserves some attention.


http://music-electronics-forum.com/t35157-2/