Single Ended 6V6/EL34/KT66/6550/KT88 Amp

[mooreamps]

TVW..

1. I see he pulled the bypass caps.
2. on the cathode follower ; bias diode in the wrong place.
3. inside the P/I, bias diode in the wrong place..

-g

[kg]

i always use a slow blow primary winding fuse.

i also always use a fast blow HV fuse. the smallest i can get away with without blowing. sometimes i go through a few figuring out where that point is.

sometimes i'll fuse the heater circuit but not always.

jm2c
ken

[R.G.]

i always use a slow blow primary winding fuse.

Yeah, that's a practical necessity. Anything other than slow blow there has nuisance trips because of the big input surge in the power transformer when it turns on.

i also always use a fast blow HV fuse. the smallest i can get away with without blowing. sometimes i go through a few figuring out where that point is.

Yep again. If you put this in the DC side, it is great for the times when a tube goes nuts or shorts. But it won't protect the transformer winding from a failed rectifier or shorted filter cap. Fusing the HV winding will do this. But if it's a CT winding, you have to use two fuses, one in each side. For some faults, it's OK to put one fuse in the CT return, but that won't protect your transformer against a rectifier short from side to side.

sometimes i'll fuse the heater circuit but not always.

It's a choice. I'd be one of the last people saying that it always has to be done. If nothing else, we have a major fraction of a century of guitar amps which don't have these. But it can save a power transformer in some cases. It's a choice to make.

I always think that having the right data to make choices is the best way to go.

[jbefumo]

My first bottom-up design . . .

Hi All,

Have built a few amps over the years, always simply following a stock schematic, and not really knowing what I was doing. This time I actually studied up, read some books, and took a shot at designing my own.

So far, it's pretty promising, and I'm optimistic that it will ultimately sound pretty decent. Having a problem that has me a bit baffled, illustrated in the attached schematic.

Basically, when I engage the cathode bypass capacitor in the second preamp stage, a get a high-pitched squeal - not overwhelmingly loud, but loud enough. Happens regardless of the value of capacitor I use.

This only happens when I'm running through the power stage. If I remove the power tube and tap the signal as shown, and run it to an external amp, everything sounds fine, so I'm guessing maybe some kind of feedback issue from the power amp?

Any tips would be much appreciated. Really just shooting in the dark at this point.

Thanks.

Joe

[loudthud]

The schematic shows a common ground between C5 and C6. Is the amp wired that way? Those grounds should be separated.

[tubeswell]

Basically, when I engage the cathode bypass capacitor in the second preamp stage, a get a high-pitched squeal - not overwhelmingly loud, but loud enough. Happens regardless of the value of capacitor I use.

This only happens when I'm running through the power stage. If I remove the power tube and tap the signal as shown, and run it to an external amp, everything sounds fine, so I'm guessing maybe some kind of feedback issue from the power amp?

So you've tried it with the NFB disconnected already?

[mooreamps]

Hi All,

Have built a few amps over the years, always simply following a stock schematic, and not really knowing what I was doing. This time I actually studied up, read some books, and took a shot at designing my own.

So far, it's pretty promising, and I'm optimistic that it will ultimately sound pretty decent. Having a problem that has me a bit baffled, illustrated in the attached schematic.

Basically, when I engage the cathode bypass capacitor in the second preamp stage, a get a high-pitched squeal - not overwhelmingly loud, but loud enough. Happens regardless of the value of capacitor I use.

This only happens when I'm running through the power stage. If I remove the power tube and tap the signal as shown, and run it to an external amp, everything sounds fine, so I'm guessing maybe some kind of feedback issue from the power amp?

Any tips would be much appreciated. Really just shooting in the dark at this point.

Thanks.

Joe[ATTACH=CONFIG]20894[/ATTACH][ATTACH=CONFIG]20894[/ATTACH]

The last time I had a problem like this, a ground wire was shorted to the chassis... Now I know this may sound a little strange, but I was using single-point grounding, and this created a high impedance ground loop...

For this problem, check your wiring on your power amp...

-g

[jbefumo]

for some reason, I'm not receiving notification of replies, so I apologize for the delay in responding.

Can't see the PDF I posted, so not sure about C5/C6 -- searching my drives now to try and find it.

Thanks for the tip, though.

In any case, I ultimately got it working ;^)

Joe