• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

the grand return. 3d21A triode PP

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
The 6N6P's haven't arrived in the mail yet, so I put the 6BZ7 and the 33K LTP plate resistors back, just to be able to hear something.

The good news is that hum is at acceptable levels. One amp buzzes a tiny bit but I don't hear it from more than a couple feet away. I'll double check the wiring and maybe try switching some tubes between channels.

The bad news is that the amps sound pretty overdriven. I'm thinking the 6N6P substitution should clear that up but if not, I'll have to do some more checking.

I just wanted to thank everyone for the patient answers and helpful advice over this 1 1/2 year thread :)
 
so i've got the 6N6P's in there now with 15K anode load resistors with 6mA feeding the LTP. the distortion is down quite a bit but not gone. bass response seems most flat overall when turned down around 12dB. any louder and it gets quite distorted but not louder.

so i'm scratching my head wondering where this distortion could be coming from. if anyone could point me to what might more be more likely i'd really appreciate it!

- the 6J6 triodes are very unmatched
- the 6N6P LTP still isn't running well
- the 6K6:4/8/16 transformer is not wired how i thought, or a different impedance

maybe it's a combination of all three? an easy thing to try would be just going off the ground and 8 ohm taps of the OPT, in case the secondaries aren't symmetrical enough to do the 0-4-16 balanced scheme i have going.
 
so I switched around the OPT secondaries, grounding the COM putting just the 8 ohm on the red binding post, but the sound remains exactly the same.

gain is quite low, with the bass and mids getting crunchy and overdriven once actual listening volume is reached.

from the voltages i'm measuring, i should have quite a few watts of headroom before reaching clipping based on my driver stage. anode voltage on the 6n6P's is 250V, with them pulling 8mA total. with a supply voltage of 385v. voltage on the 6J6 anode is 185V, bias is sitting around 3.5V on the cathode with a 400 ohm cathode resistor. and the 6P20S' are sitting around 75V of bias. so even though that 6J6 is riding a little high, there should still be quite a bit of swing before the driver stage is clipping, no?

my next best theory is that it's the 6J6's. i had calculated that they should be drawing a lot more than 0.88mA of current at that bias point, more like 3mA, especially since it's two triodes paralleled. they're old, used tubes I found lying around so i'm hoping they're just burnt out and the cause of all my worries. i have new matched (whatever that means) 6N15P's coming in the mail to complete the all-russian signal tube vibe i have going, which i hope will pull more current and actually get down to the 130V anode voltage that the datasheets said i'd be getting.

but other than that, i'm pretty much at a loss. this signal circuit with the direct coupled grounded cathode worked quite well in my last 6L6GC implementation, except I didn't use CCS' in the cathodes of those output tubes, just manually adjustable fixed bias. to think I thought i'd be saving hassle this time around :p

anyone have any ideas?? i'd be eternally grateful..
 
Those experiments were done 8 years ago. I don't remember the details, and whatever notes I took back then are still stuffed into boxes packed when we had 3 weeks to get out of Florida. There are still things I can't find 3 years later.

From post #332 in that thread:

I was over 100 watts with 450 volts in triode. Distortion at 75 watts was about 1.8%.

The power supply voltage into the OPT was 450 volts. It's safe to assume that 75 watts isn't coming from those tubes in class A, so it had to be AB1 or AB2, the driver was capable of AB2, I don't think I even scoped the grids to see if they were driven positive, but it is a possibility.

I got a dozen 7403's

PM me with your source if there are any more available. I don't have enough of them to make an amp, but I do have all the other parts to put together a 75 to 100 watt triode amp.....just no big enough triodes that I have at least 8 of.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.