Any way to use a tube head as a preamp only?

justinhedrick · February 16, 2011, 04:10:41 PM

is there a way to disable the power amp section of an amp to use just the pre-amp? i want to use 2 amp heads, 1 power amp, and 1 speaker cab. there probably isn't a way to do this easily, just thought i'd ask.

zachoff · February 16, 2011, 04:33:55 PM

If it's slave-able. Basically, you can use the slave out to the external amp in of the amp you wish to use as the power section.

justinhedrick · February 16, 2011, 04:44:48 PM

Quote from: zachoff on February 16, 2011, 04:33:55 PM
If it's slave-able. Basically, you can use the slave out to the external amp in of the amp you wish to use as the power section.

right, but in that scenario you have to have a speaker cab plugged into the amp you are wishing to use as the premp. i'm wanting to have a chain that goes:

amp 1 -> power amp -> cab
amp 2 -> same power amp -> same cab

Hemisaurus · February 16, 2011, 05:08:34 PM

If it has an FX loop yes, connect the effects send to your power amp, and remove the power tubes from your tube amp else the power tubes or the output transformer will blow.

If it doesn't have an FX loop, you either want to build one in, ie. solder a jack just past the tone stack, probably with a trim on it, (and follow the above) or buy a hotplate or some other kind of attenuator with a line out or build one, few power resistors hooked to the speaker jack, etc. etc. probably an online guide somewhere.

OK re-reading the question, you want to run two preamps into one power section?

Same as before, but on the amp you are wanting to use as the power amp AND preamp you'll need to take the signal out the effects send, mix it with the signal coming from the other amp with the disabled power section, and then feed it back into the effects returm. A simple passive mixer would do the job, you can build one yourself or a few companies make 'em.

Again if you don't have effects loops, your on a bit of a stickier wicket.

Alternatively, and this may be easier for you. If your cab has multiple speakers, say it's a 4x12, have it rewired for stereo, plug one amp into the left half, and one amp into the right half. I have a buddy that does this, he runs two heads into a stereo 4x12, and uses a ABY box to switch between amps.

Whatever you do, make sure you are not running a tube amp, with a power section active and no cab, pull the power tubes out of the amp that's not connected to a cab

Same goes for zachoff, never slave a tube amp without both amps having a cab connected, or the power tubes pulled. The slave out is designed to let you run two amps (with cabs) plugging your guitar into the first amp, and the slave out from the first amp to the second amp.

justinhedrick · February 16, 2011, 05:20:50 PM

right on. now, i've tried plugging the effects send of my VTM 60 into my power amp and couldn't get any sound to come out of it. maybe i was doing something wrong?

Lumpy · February 16, 2011, 06:34:18 PM

Quote from: justinhedrick on February 16, 2011, 04:44:48 PM
i'm wanting to have a chain that goes:
amp 1 -> power amp -> cab
amp 2 -> same power amp -> same cab

You cant run two amps into the same cabinet, unless its bi-ampable (some bass cabinets) or a stereo cab with two separate inputs. Typically, if you see two input jacks on the back of your cab, one is an input, and the other is usually an output for daisy-chaining another cab.

If you need more volume, get more speakers... get another cab. 3 amps/1 cab is not really the best formula.

justinhedrick · February 16, 2011, 06:52:04 PM

Quote from: Lumpy on February 16, 2011, 06:34:18 PM
Quote from: justinhedrick on February 16, 2011, 04:44:48 PM
i'm wanting to have a chain that goes:
amp 1 -> power amp -> cab
amp 2 -> same power amp -> same cab

You cant run two amps into the same cabinet, unless its bi-ampable (some bass cabinets) or a stereo cab with two separate inputs. Typically, if you see two input jacks on the back of your cab, one is an input, and the other is usually an output for daisy-chaining another cab.

If you need more volume, get more speakers... get another cab. 3 amps/1 cab is not really the best formula.

if i can use the amps as just the pre-amps it would be 2 preamps, 1 mono power amp with 2 inputs, and 1 cab.

Hemisaurus · February 16, 2011, 08:25:57 PM

Quote from: justinhedrick on February 16, 2011, 05:20:50 PM
right on. now, i've tried plugging the effects send of my VTM 60 into my power amp and couldn't get any sound to come out of it. maybe i was doing something wrong?

Maybe? Does it normally work? Have you tried putting an effect in it? Or even a cable between the send and return jacks.

Perhaps it's a low level output, try hooking the send to a boost, overdrive or distortion, and hooking that to your power amp. If that works, you got a low level effects send, as oppose to a line level. You could also check your manual and find out. Perhaps Peavey put it earlier in the circuit, I'd normally expect it post tonestack, but maybe that is not the case here.

Hemisaurus · February 16, 2011, 08:38:20 PM

Dude you got to read your manual.

Quote
Note Effects Loop is compatible with low-level instrument signals!!!

but also

Quote
Booster Output - A preamp signal which can be routed to another power amp

and there was I re-inventing the wheel

justinhedrick · February 17, 2011, 09:19:14 AM

Quote from: Hemisaurus on February 16, 2011, 08:38:20 PM
Dude you got to read your manual.

Quote
Note Effects Loop is compatible with low-level instrument signals!!!

but also

Quote
Booster Output - A preamp signal which can be routed to another power amp

and there was I re-inventing the wheel

thanks for that. i was under the impression that the booster output was somewhere in the power section. i guess that might not be the case!

Hemisaurus · February 17, 2011, 09:30:48 AM

According to the manual, both effects send and booster output are -10dBV into 10Kohms, they list that as instrument level but it's also the level of most home stereo equipment, so I'd hope there would be enough signal for a power amp.

What is amp#2?

justinhedrick · February 17, 2011, 09:48:04 AM

amp #2 is a sovtek mig50h head. it has a "line output" as well but i need to get it repaired because a tech had to rob the jack to use when i snapped of an input jack years ago (the are shitty plastic).

Hemisaurus · February 17, 2011, 10:31:56 AM

If you replace it, don't upgrade it to a metal jack, unless you are sure that it's already grounded to chassis.

I've seen a few cases where people replace shitty plastic jacks with metal ones, and create shorts that do horrible things to their amps

Don't know if you know this, but in Urbana you have access to electronic parts at the ECE stores on campus, as long as you, or someone you know, has a valid campus email UIUC/UIC/UIS any will do.

Google 'em.

justinhedrick · February 17, 2011, 10:49:53 AM

yep, i'm familiar w/ ECE. haven't been there though.

good call on the metal/plastic jack comment. i never would have thought of that.

now, here is 1 more question for your Hemi sized brain:

the sovtek has 3 pre amp tubes, the peavey has 4 (but i guess one of them has something to do with the power section?). would there be phase issues between these two if they are being fed into the same power amp?

Hemisaurus · February 17, 2011, 11:09:16 AM

I can give you a definite maybe

There's no easy way to determine phase shift in a amp, you could put it on an oscilloscope (I have a spare, you got anything to trade

) or just suck it and see.

If it is a problem, you might be able to just flip the polarity of one of the cables but this could be an issue, if your amps have a common ground path, for example through the power amp, you'd be shorting out the line out on the flipped cable.

I'd just try connecting them normally first, and worry about the rest after. How are you connecting two amp outs to one power amp input btw

justinhedrick · February 17, 2011, 11:12:47 AM

Quote from: Hemisaurus on February 17, 2011, 11:09:16 AM
I can give you a definite maybe

There's no easy way to determine phase shift in a amp, you could put it on an oscilloscope (I have a spare, you got anything to trade ) or just suck it and see.

If it is a problem, you might be able to just flip the polarity of one of the cables but this could be an issue, if your amps have a common ground path, for example through the power amp, you'd be shorting out the line out on the flipped cable.

I'd just try connecting them normally first, and worry about the rest after. How are you connecting two amp outs to one power amp input btw

parallel ins on a single channel power amp. not even sure if THAT would work. it's a solid state power amp, if that matters.

Hemisaurus · February 17, 2011, 11:22:34 AM

Doesn't matter that its SS, but it does depend on what they mean by parallel. What is the make / model?

90% of the time it won't matter if you connect to line outs together, it's the same as connecting two input jacks together, you are tieing the ground planes of two amps together.

Do you own a voltmeter? You could check between the grounds of the two line out jacks. Make sure you plug them both into the same outlet or power strip, and hopefully they both have a ground pin on the power cord, so that the polarity is always the same way round?

justinhedrick · February 17, 2011, 11:31:22 AM

Quote from: Hemisaurus on February 17, 2011, 11:22:34 AM
Doesn't matter that its SS, but it does depend on what they mean by parallel. What is the make / model?

90% of the time it won't matter if you connect to line outs together, it's the same as connecting two input jacks together, you are tieing the ground planes of two amps together.

Do you own a voltmeter? You could check between the grounds of the two line out jacks. Make sure you plug them both into the same outlet or power strip, and hopefully they both have a ground pin on the power cord, so that the polarity is always the same way round?

my buddy has one. the power amp is a peavey m3000. you can find the manuals online here: http://www.peavey.com/assets/literature/manuals/80370325.pdf

Hemisaurus · February 17, 2011, 03:30:26 PM

That manual certainly gives the impression that all the inputs are, are a pair of jacks wired together. So if you are atall worried about the line outs of your amps, do check the ground planes between them. If they are both in the same outlet, with a ground pin on the power lead, they will likely be on the same plane.

Just FYI, you know that a Mig50 and a VTM60 together will probably be as loud, possibly louder, than a 300W SS amp?

I'd seriously consider the stereo cab route or in a full stack, one amp per cab, if it's feasible for you to do? If you already have two 1/4" sockets on the back of your cab, you just need to drill a hole for a toggle switch as in the below diagram.

Taken from the Celestion website

justinhedrick · February 17, 2011, 03:50:19 PM

i already have two 2x15 cabs, so using both heads isn't an issue. i just wanted to be able to run the rigs together to possibly a/b the clean and dirty sounds into 1 cab.

Hemisaurus · February 17, 2011, 05:05:11 PM

Hmm, I guess that's one way to do it, but your going to miss all the harmonics you get from power tube distortion.

You could rig a speaker switch box, 2 ins, 2 outs, 1 out goes to your cab, the other to a dummy load the same resistance as your cab. That way you could switch one amp out to the dummy load, thus ensuring the safety of the power tubes, and the other amp would switch to the actual cab. You could maybe get away with DPDT and common ground it, but I'd probably go 4PDT so you can switch and keep each amps output totally separate. I see Small Bear Elec has some 4PDT stomp switches.

It's probably easiest though, just using an ABY box in front of the amps, and giving each amp it's own cab, but that's just me

Good Luck.

justinhedrick · February 17, 2011, 05:44:02 PM

hemi,
thanks for all yr help man. i probably will just do the a/b/y pedal in front of the amps.

by the way, i like your idea about the dummy load and switch. and what you are suggesting seems fairly cheap, but webber sells something that does just that, but its like $200. any thoughts on this?

Hemisaurus · February 17, 2011, 05:53:59 PM

Weber's is much fancier than my idea

It has selectable impedances for both the amp and the dummy load, whereas mine would be fixed impedance, dependent on what value load you bought, and it looks like he uses a relay and a remote footswitch, where I would have the signal running through the actual footswitch (or toggle if you prefer).

It's kind of like the difference between a turbocharger and a supercharger, the first is subtle and well thought out and the other is more of a blunt instrument.

If you know in advance the impedance of the cab you are using, and you are going to stick with 60W amps (assuming the switch has a minimum capability of around 4A) you can use the $30 solution

If you're like me, your favourite sound will probably be in Y with both amps blaring.