AC30 with 6V6s, help with tweaking...

[HTH]

ok, I've built an AC30 preamp stage into a pair of cathode biased 6V6s.

the first gain-stage differs slightly from the usual AC30 one since I'm running both triodes in parallel (like Matchless do). got a 100k plate load and unbypassed 1k5 cathode - plenty of gain going on, actually theres too much (the signal is severly clipped at anything over 3/4 on the gain.

the PI is a typical AC30 setup but with lower grid resistors... 470k, 470k, 1k2, 47k (no NFB). tried NFB but the tone became overly fizzy and harsh.

any tips on tuning this in? - I think the 1st stage is where I'm getting alot of gain. would a lower plate load (68k ???) or higher cathode resistor (2k2, 4k7 ???) be adviseable?

cheers

[stokes]

If your aim is to reduce gain in that stage then,yes a lower value plate resistor will achieve this.To achieve the gain chracteristics of a single triode with a 100k plate load resistor and a 1.5k cathode resistor with the two triodes paralelled like you have you should have a 50k plate resistor and an 820ohm cathode resistor.I would try the lower value plate resistor first.Since your cathodes are sharing a resistor the 1.5k should be high enough.

[HTH]

hey Stokes,

thanks for the advise - I'm using a shared 100k plate load and unbypassed 1k5 cathode so it's close to a typical AC30 stage with the 220k plate and 1k5 cathode.

I unbypassed the cathode since there was way too much clipping going on. I think a 68k shared plate load might be just right, will give it a go.

I've also used a 220k/330k voltage divider after the tonestack going into the PI to tame the signal a bit (helped some).

The coupling caps are already quite small all through the amp (0.022uF is the biggest). I went through a bunch of leaky PIO 500pF caps and that was causing me grief for a while (heh).

[HTH]

Just an update in case anyone is interested...

I've removed the crossline MV and put a typical pre-PI MV using a 500k pot. The amp has opened up ALOT, the bass is clearer, the amp is punchy and the tonestack doesn't sound quite so phasey.

It seems louder too, cranked it up full and it was still nice and tight.

gonna try bringing the gain UP a little now (lol).

[stokes]

Try bypassing the cathode on the stage we discussed earlier to bring the gain up some,before getting too crazy with gain increase mods,might be all you need.

[HTH]

Try bypassing the cathode on the stage we discussed earlier to bring the gain up some,before getting too crazy with gain increase mods,might be all you need.

hey Stokes, I'm trying a 25uF cap on the first gainstage cathode - that should be plenty.

if that's not enough then I'll try bypassing the voltage divider I have going into the PI.

[HTH]

ok, I've got a 150k plate resistor on the first stage (parallelled triodes) and a 1k5 cathode resistor bypassed with a 25uF cap.

I've probably got all the gain I can get out of this setup without it getting fizzy.

Maybe a cascaded 1st stage like a 2203 would work better (???)

[stokes]

If you mean doing away with the paralleled dual triode and running one into the other,that will definately increase the gain.

[AdmiralB]

Part of your problem, relative to a 'real' AC30, is the 6V6s. Pentodes in general and EL84/EL34 specifically have a LOT more gM than most beam tubes (actually, more than any beam tube I can think of right now). So it takes a lot more signal to drive them (the beam tubes) to full power, which means your preamp is likely to clip a lot more for a given output.

I had a Lexicon 284 amp for a long time. I converted one side of it to run a 6V6 and I had to dime that channel to compete with the stock EL84 side at about '3'.

[Bruce / Mission Amps]

Get your plate voltages of the first preamp triodes around 170-190vdc and leave the preamp plate load resistors at around 100K.
Try changing the PI cathode resistor to 820-910 ohms and replace the 47K ohm tail resistor with a 22K to 27k ohm.
The Mission Aurora 18 amp is similar to what you are doing and it has no trouble what so ever swinging a clean PI signal far enough to clip 6V6s.
Even with the MV set lower so the preamp is starting to fold over a bit, it still has enough left swing left to over drive the 6V6s.

By the way, considering an average guitar amp with EL84s, proper bias and plate voltages... it only takes about 24vac-34vac of PI (peak to peak) swing to seriously over drive the EL84s.
The same amp with 6V6s, biased correctly, proper bias and plate voltages...
will take as much as 55vac-75vac of PI drive (peak to peak) swing to do the same thing.
Totally different animals.
Bruce

[HTH]

hi Bruce,

I'll try the 820ohm and 22k in the PI but can you explain what effect this had over the 1k2 and 47k I'm using right now? (also using 470k over 1M resistors in the PI for less bass)

I've been checking out a Tophat schem where they're using the exact same preamp as I've used into a pair of 6V6s using a bog standard AC30 type PI.

Just wondering, not arguing or saying you're wrong or anything - all hail Bruce

Many thanks in advance