Hitting low strings hard when amp cranked...

[rhaigh]

I have a Northland Baffin II tube amp(same as Teisco Checkmate 18 without the reverb). When I play my strat through the amp and turn the up on 8-10, it sounds great. Breaks up real nice. When I use my tele with hotter humbuckers, and crank up the amp again to 8-10, it sounds great unless I hit low notes on the low E string. When I do this, the note gets this weird out of phase type sound for a quick moment(less than a sec) and then the sound returns to normal as I let it ring out (almost sounds like a laser in a sci-fi movie). Could this be a power tube biasing issue? The plate voltage is 330V. 11.75v across the 130ohm cathode resistor gives 90mA or approx. 45mA from each tube. Or should I be looking to the preamp tube biasing? Attached is the schematic (mine is the same with no reverb). Thanks.

[Jazz P Bass]

I would try to lower the first stage cathode bypass capacitor.

30uf will pass too much bass frequencies.

[Zozobra]

It could be the cathodyne PI. They can do some odd things when overdriven sometimes.

The Valve Wizard

[rhaigh]

Yeah, It's definitely the bass frequencies. The notes get all squishy and drizzly if you know what I mean (i'll try to post a link to a sound sample later today). I'll play with that 30uF bypass cap. Any other recommendations to tighten up the low end? The amp is late 60's/early 70's and is in absolute mint condition but it does have the original caps. I already replaced the tremolo caps to get it working. Which caps would you replace first to clean up these low notes? Thanks.

[eschertron]

It could be the cathodyne PI. They can do some odd things when overdriven sometimes.

The Valve Wizard

I agree with this. If the problem doesn't appear until the MV is up to near max (or some level) then you need to look in the power amp half of the circuit. And the 'voltage doubling' effect Merlin mentions with respect to overdriving the PI sounds like it might be the same as the 'spaceship' sound you get. Merlin's fix is a resistor or three. Check it out first as a cheap, easy fix.

[Jazz P Bass]

I would not replace any caps until the Vac ripple was measured with a meter.

By simply placing a suitable high voltage cap in parallel, if the ripple drops, that cap is suspect.

[pdf64]

Have you tried it with a different speaker (of the correct impedance)?
Any/all ecaps may be life expired if it's been used a lot, or deformed if not.
It would be a good idea to identify failed parts and get the amp working as intended, before attempting mods.

[rhaigh]

I tried adding cathodyne grid stoppers(100k, 470k, and 1M) but no change. The drizzly sound is still there on the low notes. When I get a change i might replace the caps in the power amp.