6V6s - cathode bias versus fixed bias question...

[HTH]

ok, I've got an amp I built which is essentially the normal channel on an AC30 mated with a pair of cathode biased 6V6s.

It's a tiny bit too quiet for band rehearsals, I need maybe another two notches on the volume (if you get me).

Would switching to fixed bias give me a noticeable volume increase?

Also, I'm using the AC30 LTP PI arrangement - would switching to 'Fender' values give me more punch? (i.e. 470ohm and 10k versus 1k2 and 47k)

Thanks... Ian

[stokes]

I've got a 5E3 which has the same out put tubes.Some of the mods I did to get more percieved power was to stiffen up the power supply with 50uf filters on the plate and screen supply,really improved headroom and bass response.I have a 5V4 instead of the 5Y3 rectifier.I also did the Paul C mod to the PI,but that wont help in your case.These all gave me more headroom and better fidelity.A big improvement was using an EVM 12L speaker.For the caps I would recomend Sprague.Dont use those Xicon or any of the Asian manufactured crap.I have done tests with many amps over the years and I wont use anything but Sprague or F&T brand,they really outshine the Jap caps.Fixed bias would help some too.

[HTH]

thanks for that Stokes,

I've already tweaked the amp a bit in the meantime... went and changed to 'Fender' LTP values rather than 'Vox' and lost some grind. Think I'll end up somewhere in the middle.

I've also noticed on schematics where they're using a LTP PI and 6V6s, the usual 1M resistors are halfed to 470k. I'm gonna do this too and loose a bit bass (got plenty).

The EF86 channel is basically unused so I'm gonna wire up a CF stage and run a tonestack - should make it ballsier.

Oh, and the first gain stage is run with the triodes in parallel - it's a subtle but noticeable difference. It's not as focused, but works well to fatten up the tone for rock stuff.

Lastly, I used a crossline master volume - putting 10k resistors before it really made a positive difference is making it sound good at low volumes.

[Jammin'John]

Fixed

Fixed bias will make it a little tighter & louder but you will lose some compression & sustain & harmonics if that's what you want.
A speaker swap may get you there.
Of course the "other" players could turn down a little.
It's NOT a gig.


JJ

[aprilia1k]

With 6V6's, switching to fixed bias should give you noticeably tighter bass, and power - you'll lose the compression and "singing" quality that cathode bias provides - the tube actually increases it's output as the signal from a held note decays, as well as compressing the front of the note as the tube's bias lags behind the attack a bit (nice attributes of cathode bias).

According to Gerald Weber though - if you play with a loud drummer, you really need to have the punch/attack, power and tight bass of fixed bias. Sounds right to me (have both fixed and cathode bias amps). Using a cathode bias circuit just doesn't "stand out" in terms of attack (and unless it's 50w or more, it's hard to compete with a "pounder" on drums).

aprilia1k

[black_labb]

the el84's of the vox have more gain that the 6v6's, so putting a bit more gain into the preamp might be a good idea, but it looks like you did that. if you think you could turn it up louder without it getting muddy then maybe tweak some things to get more gain from the preamp. but if its too muddy then fixed bias would probably be your best bet imo.

[HTH]

In the end, I pretty much ended up with this except for a master volume before the PI (sounds great)...



I used an OT I had ordered a few years back from Ingo at Shinrock - it was a custom order wound a bit beefier than he normally would for a 2x 6V6 OT.