diy guitar tube amp help.

Ok, somewhere, someone is going to read this any wonder why.....

About 3 years ago, I found a Hewlett Packard Audio Oscillator, Model 200B, a little rough.... but in working condition. I played around with it for a while and then put it on a shelf and forgot about it until tonight.

Lately, I've been reading about DIY tube amps and started thinking if it is possible to use some of the parts in the Oscillator to make a guitar tube amp?

I looked at the tubes in it...... 5V4G Rectifier Tube and Two 6K6GT tubes. I still don't see anything info wise on the Transformers, but then again...this was built in 1939 so I'm sure that dust from over the years is covering that up quite nicely.

I've done some searching on the internet but found sites that when you do the pricing for parts....well, I want to spend little as possible.

Is it possible to use some parts on this to make a guitar tube amp? Or should I just say forget it and try to sell it to someone that will actually use it and enjoy 30's Technology?

Any help for this learning DIY'er would be great!!!!!

Here is a pic of the cover off and looking down inside of it......
 

Attachments

  • top view.jpg
    top view.jpg
    88.6 KB · Views: 305
If it goes, then you can use it.

I wouldn't turn it on for too long thouugh, the old components might go pop. Don't re-use any old capacitors or resistors, they might be a fire risk.

I would guess that if the oscillator has speaker outputs, then the transformer on the right is an output transformer, probably single ended. Otherwise you will need to buy one.

What you could do is completly strip that cool looking chassis down and build something like this Fender Princeton on it. Sub one 6K6 for a 6V6 and a 5V4 for a 5Y3, and all you need is a 12AX7.

http://www.kbapps.com/audio/schematics/tubeamps/fender/princeton5f2a.html

If you can show the connectors and the underside of the unit, I can tell you more.

Also please read the safety thread :)
 

EC8010

diyAudio Moderator Emeritus
2003-01-18 7:57 am
Near London. UK
Of course, some people would say that the HP200B is an important part of audio test equipment history and that you shouldn't destroy it...

I doubt if any of it is suitable for conversion. Many old oscillators did have a loudspeaker amplifier in them, but it would only be a Watt, or so.
 
a 6K6 is just a slightly smaller 6V6. it looks like you might even have a P-P 6K6 output stage! see how they are wired to the output transformer. Also see if the thing can drive a speaker. If the output transformer can't drive a speaker there is no point is using this chassis. It looks like there are at least two small pentodes in there. use these for preamp and Phase inverter if needed.

First check to see if the HP200 is worth anything. If it is, resotore it and either hold on to it, use it (I find my function generator very useful for amp debugging), or sell it to buy more old tube junk!
 
if it's 600 ohm output, you should just clean it and replace the capacitors and use it. Especially if you don't already have a function generator. quite usefull things. I'd say the order you should get test equipment should be....

working brain, then a good multimeter, then a scope, and then a function generator. Once you get a scope and 'generator, you don't want to go back to the days where you only had a lowly multimeter....
 
I wasn't calling the HP a function generator. I was just saying that some sort of generator is useful on the bench. I use a HP3312A function generator, but I mostly use it to generate audio frequency sine waves when I'm messing around with audio circuits (and I don't play with square wave response on guitar amps).

I was just aying that if that HP oscilator worked and had adjustable output level and could be moved accross most of the audio band it would serve just fine for guiitar amp debugging purposes and be a very useful tool.
 
I looked around this morning to see if I could find out the value of this but no luck. The closest value I could find was on ebay and that was under $30.

Besides, I'm looking at the value for myself....I tinker with simple op amps and that's about it. I still have so much to learn it's crazy. I still get caught up in schematics and sometimes look for pictures of the boards to go by.

I also sat down and cleaned out the insides a bit. It had some foam packing material in it!!!!! I also cleaned off the tubes and even the 3w light bulb. Then I took some pictures and tested it out.

I hooked up a spare speaker to the front connectors and fired it up. I works just fine. The dials are good, no pops or cracks when I turn them. So this baby works just fine.

Here is a pick of the bottom with the cover off.
 

Attachments

  • underside.jpg
    underside.jpg
    87.3 KB · Views: 134
I have dissasembled several HP oscillators that were slightly newer than the one you describe. I have used the power transformers and filter chokes for guitar amps. The power transformer in these units is good for a 15 to 30 watt guitar amp. The 6k6 tubes could also be used in a guitar amp. Find a schematic that uses 6V6's.
The output transformer in this unit has a 600 ohm secondary, so it is probably useless for driving a speaker. It might be possible to take the output transformer apart and rewind the secondary, but due to its age, this is not likely. A new output transformer will be needed. Find a transformer made for two 6V6 tubes in push pull. About 6000 ohms centertapped. I used Schumaker transformers that I got cheap on Ebay.
As stated earlier I wouldn't trust the capacitors and resistors because of their age, but these are usually pretty cheap.
If budget is tight, and you are careful you can re-use the chassis and tube sockets. Since the sockets are all 8 pin octal, and most guitar amp schematics use 12AX7's for preamp tubes, you will have to find an octal preamp tube. I have used a 6SL7 for this purpose.
 
Oddly, I have several of these here.

I looked at them and said to myself:

- great air variable cap, suitable for medium power RF, yummy!
- (iirc) nice reduction drive too... or was it a worm drive to the cap?
- single ended broadband output xfrmr that is gonna be good out to 20kHz or better... (ok so they used feedback and it was only sinewaves)
- power transformer (ho hum)
- real tubes! :cool:

Might make a decent amp, but as noted, it's a 600 ohm output in all likelyhood. :bawling:

BUT, maybe not - gotta measure.

Anyhow I kept a few of them here...

_-_-bear :Pawprint:

PS. think I'd prefer a PP output guitar amp... not sure though.