Little SE amp, 6SL7 and 6V6

[John]

Just following up from long ago about a little CSC amp I bought off eBay. PRR and others had pointed a few things out to me that should have been obvious to me, but weren't.  Anyway, FINALLY got it built on the breadboard, and a few musings for any other newbies that might be reading this.

The output was fluctuating pretty badly. I double checked everything, checked some more and then did what I should have done right off- traced each wire and connection according to schem. Found a small thing wrong, changed it, voila. Like Doug says, if it was wired right it would be working.

The 6SL7 sounds really nice. Definitely has a sweet spot for me according to what plate R I used. Started out with 86k, too gainy/mushy. Dropped to 56k, little anemic. 68k is juuuusst right. I switched between NOS Sylvania and/or GE and Russian no-name tubes for both the 6SL7 and the 6V6 and could not tell a significant difference. Then again, I've spent many years on open tractors with no hearing protection, so don't go by me.

Was using my Triplett analog to measure voltages. Couldn't figure out why when I cranked up the volume the plate voltage on the power tube would go way, like to 600 volts! Finally, hooked up a different meter (duhhhh) and everything held rock steady (and double checked with yet another one). My theory is that the Triplett is reading AC voltages as well (?) Still don't know enough to make a good guess, but as has been said many times, if the voltages aren't making sense, try a different meter.

From here on out I am going to make a written note of every tip I read on here, since I sure can't remember them all! I don't post much, but read a lot, and I'd like to once again thank all the members for all the questions and answers on here.

[PRR]

> Doug says, if it was wired right it would be working.

A very wise man. I try to follow that teaching.

> My theory is that the Triplett is reading AC voltages as well

Very likely.

For some reason, analog meters *tend* to read DC+AC, VTVMs (and most DMMs) *tend* to try to ignore the AC riding on DC.

> plate voltage on the power tube would go way, like to 600 volts!

If the idle B+ was 300V, the tube were perfect, signal was MAX, and meter was peak-catching, then 600V would be the right answer.

Tube isn't perfect and meter may read closer to Average, but B+ in gitar 6V6 amps is usually 350V-400V, so 600V is still in a right ballpark. (400V idle, 300V peak signal, 210V average signal, is 600V near-enuff.)

> hooked up a different meter

OR.... you "know" the OT winding can't support any large DC voltage (sudden changes yes, not steady level). So you poke the "cold" (non-Plate) end of the OT and watch that voltage. You "know" the plate "must" be at that voltage, minus a few V for resistance, plus/minus signal-swing. i.e. if the cold side of the OT primary is 400V, the plate is at 380V plus signal swing.

In a single-ended amp, ideally the B+ (and the cathode current) should not change from idle to MAX. If it changes a lot, the load is mis-matched. In real life, tube curvature suggests a 5%-10% rise. That's for dummy-load. With speaker the load is never really matched, though mostly the effect is less current-change. So if you do see a large change of B+ (or Vk) with MAX signal, re-ponder your load impedance. (But if it sounds good, don't fret.)

[John]

For some reason, analog meters *tend* to read DC+AC, VTVMs (and most DMMs) *tend* to try to ignore the AC riding on DC.

Ahhh. You just saved me hours of trying to find the "bad spot" in the meter.