Question for tube amp gurus

[father of fires]

A buddy of mine has a vt-22 (2x12 guitar cab version of the v-4) that has been coverted into a head. It is sitting in his house unused. He said I could have it but, it doesn't work and it has no tubes. I haven't seen it so, I don't know what kind of shape it is in. I have built and serviced a couple of tube amps. I am by no means an expert but, I can find bad parts and replace them. Outside of that, I'll need to get a tech. The theory of tube amp design is still way over my head.

Anyways...

One thing I would like to do is try running it as a preamp with a power amp. I have played v-4s and I love their sound. I just want to experiment with a little more headroom. I find thier "grindy-ness" to harsh for some applications and I think it is most likely a byproduct of power tube distortion. At low volumes they always sound sweet.

My first thought was to tube it, get some sort of dummy load and run the preamp out into my power amp.

Then I started to wonder if I could run it without having to buy power tubes. I could still plug in a dummy load or an attenuator if I had to but, It would be cool to be able to try my preamp experiment before shelling out for power tubes.

I have no practical applications for this, I kinda just want to hear what it sounds like.

To sum up. Is it safe to run a tube amp without power tubes just to use it as a preamp?

[BassmanPaul]

Yes BUT where do you plan to take your signal from? I assume you are planning to drive a SS stereo power amp? SS equipment typically runs at much a much lower impedance than tubes. You'll need to interface the V4 preamp such that the output is sufficient to drive the power amp and the impedance changed suitably. It's not such a simple task. You also have to remember that some of the mojo of a tube amp comes from the the output stage which you will not be using.

You say that you have built tube electronics before so why not just build a suitable pre-amp and use that. It makes more sense to me than dragging around a full tubeless amp just to use the pre-amp!

Paul

[father of fires]

Originally Posted by BassmanPaul

Yes BUT where do you plan to take your signal from? I assume you are planning to drive a SS stereo power amp? SS equipment typically runs at much a much lower impedance than tubes. You'll need to interface the V4 preamp such that the output is sufficient to drive the power amp and the impedance changed suitably. It's not such a simple task. You also have to remember that some of the mojo of a tube amp comes from the the output stage which you will not be using.

You say that you have built tube electronics before so why not just build a suitable pre-amp and use that. It makes more sense to me than dragging around a full tubeless amp just to use the pre-amp!

Paul

I'm not dragging it around this would be for home use. The v4 that I played had a preamp out jack on the back. I assume the VT-22 does as well (which I could be wrong). But, wouldn't that jack have the appropriate impedance? I plan to fix up the amp and restore it. I was just wondering if it was safe to do as an experiment.

Would I need a dummy load to keep the OT from blowing or would the lack of power tubes keep this from happening?

Thanks for your help. I was hoping you'd see this thread. You seemed quite knowledgeable in other tube amp threads.

[Cates]

How high are you cranking the V4(V22) to get the grit? If you ran your amp into a PA or into a sound board for recordings you could just have the amp @ a low volume and then raise the level of the bass so you could get more volume but keep the tone. It does seem a little excessive to haul around such a good amp and only use it as a preamp lol

[father of fires]

Originally Posted by Cates

How high are you cranking the V4(V22) to get the grit? If you ran your amp into a PA or into a sound board for recordings you could just have the amp @ a low volume and then raise the level of the bass so you could get more volume but keep the tone. It does seem a little excessive to haul around such a good amp and only use it as a preamp lol

Once again, this is just an experiment I want to try to satisfy a little curiosity. I like to experiment with tones. I go out of my way sometimes to try different rigs. Even ones I'll never get to use just so I can say, "hmmm that sounds cool!"


P.S. I like to do things the hard way.

[Cates]

Well the line out (ext amp) on the V4 just sends out the sound without sending out any POWER like the "speaker" jack does, so I doubt it would really HURT another amp that you would be connecting it to. I think it would be worth a try.

[father of fires]

Originally Posted by Cates

Well the line out (ext amp) on the V4 just sends out the sound without sending out any POWER like the "speaker" jack does, so I doubt it would really HURT another amp that you would be connecting it to. I think it would be worth a try.

Thanks but, that is not my question. I know the line out will work. I am just trying to figure out if it is safe to run the VT22 as a preamp without the power tubes. If so, what is the best way to do it?

Thanks,

Jeff

[BassmanPaul]

I don't have the schematic handy but IIRC from the other Ampeg amps I have worked on the pre-amp out is taken from just before the phase splitter. That is a high impedance output. The best way to proceed is to purchase a DI transformer with a high primary impedance.

What you could try is a microphone matching transformer. These are for running low impedance mics into a PA with only high impedance inputs. They have an XLR on one end and a bulky phone plug with a built in transformer on the other. You'll need an XLR gender bender to connect to the amp.

Leaving the output tubes out negates the need for a dummy load as the OPT is not connected to anything. The B+ voltage is going to be higher than normal as the power supply is running without a significant load. What that may do to the old filter caps is questionable.

Paul

[father of fires]

Originally Posted by BassmanPaul

I don't have the schematic handy but IIRC from the other Ampeg amps I have worked on the pre-amp out is taken from just before the phase splitter. That is a high impedance output. The best way to proceed is to purchase a DI transformer with a high primary impedance.

What you could try is a microphone matching transformer. These are for running low impedance mics into a PA with only high impedance inputs. They have an XLR on one end and a bulky phone plug with a built in transformer on the other. You'll need an XLR gender bender to connect to the amp.

Leaving the output tubes out negates the need for a dummy load as the OPT is not connected to anything. The B+ voltage is going to be higher than normal as the power supply is running without a significant load. What that may do to the old filter caps is questionable.

Paul

Thanks. This is the answer I was looking for. Instead of messing with transformers, what about using the the effects loop or line in on one of my bass amps? Aren't they set up for high impedance?

[beans-on-toast]

In the VT22, the output tube just prior to the line out is a cathode follower with a low impedance output. What you have is a high quality effects send.

As Paul said, removing the power tubes will protect your output transformer.

Use a shielded instrument cable (not a speaker cable) to run from the line out to the input of an effects loop on another amp or a power amp.

[suraj]

I think high or low impedences will just make a difference in the volume. If this is just an experiment, you can probably just go ahead without buying a di.. Hopefully won't damage anything

[father of fires]

Thanks guys. When I get it set up I'll post a review.