will this kill my power amp/OT?

[nicodemus]

I have a crate v32 palomino that is too loud for late apartment use, I also have a jan6ak6 0.5W power amp I built a while back. My question is, could I connect the crate's power amp up to a small 2W 8 ohm speaker (think alarm clock size) with no signal going to the output section, & run the preamp out to my 0.5W power amp, without harming my crates power section? In my mind 8ohms is 8 ohms, but I never have claimed to know everything. Thank you, Jason

[Rob Mercure]

Jason,

I'm not familiar with the particular Crate and too lazy to look it up right now (6:30 in the morning and breakfast is cooking) but if it's a tube amp just pull the output tubes and then drive your smaller amp. Much simpler and no chance you might accidentally turn up the Crate - wake your neighbors/wife/orangutan, et al - and blow the small speaker.

Rob

[Andy]

I think it would be better to use a 30W resistor rather than a speaker that you know won't take the power if something fails. it would fail catastrophically and probably fry your amp. especially if somehow the volume on the amp got cranked by mistake.

[nicodemus]

Thanks guys. I tried it with the power tubes out, and taking my signal post P.I. sounds great, I did have a 30 Vdc jump on the 12ax7 plates, wouldn't think it should hurt anything. Any reason I couldn't install a suitable switch to lift my power tube plate supply voltage? I'm also going to try and hunt down a 30W resistor, as it is a P.I.T.A. to get the tubes in and out of this thing & even with the tubes pulled, it would nice to have a fail safe for the time it's late @ night & I do something stupid. Thank's guys, Jason

[Rob Mercure]

Well Jason you probably don't need a 30W resistor unless you plan on turning up the amplifier volume to max with no speaker. In general you could install a resistor that's 10X - 20 X the speaker impedance across the output jack terminals (if this amp had a speaker jack, if not, as close to the OT speaker terminals as possible) of about 5-10W. The idea is to have some load on the OT at all times to prevent 'flyback" spikes that will punch through the isulation on the windings of your OT and short it out - this is usually done to protect against an open speaker, open speaker cord, or a speaker plug that is accidentally pulled. But since this resistor is always in circuit you want the value to be at least ten times the speaker load so that it doesn't draw significant power away from the speaker. Now being 10 X the speaker impedance, for example, the resistor should only dissipate 1/10 the speaker power so you can use a much smaller wattage resistor. I've got a bunch of 180 ohm/10 W mil spec resistors I got surplus a few years ago that I use exclusively for this purpose. The idea is not to create a "power soak" but to protect the OT and the higher impedance reflected into the OT secondary will just limit AC current flow through the output tubes.

Rob

[Enzo]

I wouldn't interrupt the output plate voltage. I would prefer to open the cathode legs if possible - consider the copper art - or turn off the screens. And I seconf teh idea of a smaller resistor. You don't need a full dummy load, just something to suppress spikes.

[nicodemus]

Thanks guys, I'll definitely install the protection resistors on my 3 amps. Just to confirm the set up, Preamp (Palomino or other amp) running to my JAN6AK6 power amp, no input to the power amp (palomino or other), & speaker unplugged with a 180 Ohm 10W resistor across the OT output terminal. If I wanted to use an amp for silent recording with my DI box (no load on OT), I should then use the 8 Ohm 30W resistor. I was thinking of getting a Weber speaker motor attenuator, but I already have an analog DI speaker emulator (surprisingly close to a miced cab), the only problem is you have to hook it up to a speaker for the load & this pretty much defeats the purpose for my needs. Thank you guys very much for all the help, Jason

[BobW]

Nico, Why not just simply pull the output tubes? That would be less intrusive into your Palomino and wouldn't require any resistive or speaker loading. Since you wouldn't have any signal into the output transformer, it isn't providing any transformer action and doesn't need any reflected loading, ie pwr resistor or speaker required.

[Enzo]

That would work too, though it would cause the B+ to rise quite a bit. WHich may or may not affect the tone.

[BobW]

Enzo, I agree the B+ would increase slightly but it's basically the same as if the amp were placed on standby minus any drop due to preamp current draw. The schematic shows a loaded B+ is 320V, Caps C16 and C19 are rated at 450V. I haven't seen anything more than a 50V increase in a standby vs. operating mode in a Fender. Should be OK to remove the tubes, and agree don't know how it will affect the tone thanks for pointing that out

[Enzo]

Yep, I am not worried about the parts, but adding 50v to the B+ might well affect the tone or the dynamics of the preamp. Or not.

[nicodemus]

Thanks for all the help guy's, been away from the computer for a bit. I've had a chance to experiment with pulling the tubes & it looks like a 32Vdc B+ jump (with a good tone), I'll probably just go with this for now, it's a pain in the butt to get the tubes in and out, but it makes more sense & less work for now, I'm lacking the time I wish I had to tinker. I plan to brighten up the drive circuit later to match the clean & @ that time, I'll look into lifting the cathodes or experiment with a P.P.I master. I'm also curious, looking @ the 15W version it seems I could convert mine easily & run into a 16 Ohm speaker instead of an 8 (or add that 8 Ohm 30W resistor), of course I would have to adjust bias for the 2 missing tubes, but I have the 15W schematic for help. I know pulling 2 tubes does not drop the perceived power much, but playing the 15 & 30W's in the store there seems to be a significant difference. It seems I can get a better tone with the power amp getting more signal to it, but it may be in my head. Thanks again, Jason