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 Post subject: Champeg guitar amp build
PostPosted: Oct Wed 18, 2017 9:55 pm 
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I started on a guitar amp project a while back but never finished it due to life circumstances. I would like to follow through on that one someday but I'm still getting back on my feet so I need something simpler. They don't get much simpler than a Fender Champ 5F1!

I want to use as much stuff as I can out of the junk box. I don't have any 6V6GT power tubes but I do have 5V6GT. Likewise, I only have one or two 12AX7 tubes but a lot of 3AV6 (1/2 12AX7). As luck would have it, the 5V6GT and 3AV6 have the same heater current so I can string two 3AV6's and a 5V6GT in series and run it off a 12 volt transformer. The heaters add up to 11 volts so 12 volts is within the 10% tolerance but I'll still add a resistor in the string to drop a volt.

For the power transformer I'll use a small "isolation" transformer to step up the line voltage to 240 volts then use a full-wave bridge rectifier which should put it pretty close to what the Champs were running. Just for fun, I've got a little panel mount Ohmite VT-02 variac that I got in a box of junk at a garage sale. I want to try wiring it into one of the 120 volt outputs of the transformer with the other 120 volt output in series. That way the voltage can be adjusted from 120 to 250 volts. I've heard of guys plugging tube amps into a variac to lower the headroom but they were limited on how far they could crank it down since the heater supply was dropping with the B+. Here is a schematic of what I had in mind; http://www.gofastforless.com/junk/Champeg.gif

The Champ amp will be built in this handsome Ampeg cabinet, thus Champeg.

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 Post subject: Re: Champeg guitar amp build
PostPosted: Oct Thu 19, 2017 12:48 am 
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:) It looks like a good project to me

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 Post subject: Re: Champeg guitar amp build
PostPosted: Oct Mon 23, 2017 6:10 am 
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I like the way you modified the original Champ schematic.

Years ago, I built a few single ended amps using 12AX7's but with 50L6 power tubes. I collected transformers from old stereos and tape decks, and hooking 2 together by their secondaries gave me various heater voltages and an isolated B+.
If you use the same values of resistors and caps in the preamp stages as shown in the Fender schematics, you can get a good sound from a variety of power tubes. The speaker you choose may take some experimenting. I ran across a few that just didn't work well with low power.

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 Post subject: Re: Champeg guitar amp build
PostPosted: Oct Mon 23, 2017 7:18 pm 
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westcoastjohn wrote:
The speaker you choose may take some experimenting. I ran across a few that just didn't work well with low power.


Do you have any suggestions? This is the speaker I bought https://www.parts-express.com/eminence-6-1-2-guitar-speaker-20w-4-ohm--299-402 I don't have high hopes for a $12 speaker but figured it would be good enough to get the amp up and running. The cab will actually accept an 8" speaker.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Oct Mon 23, 2017 8:35 pm 
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I started on some of the mechanical work this weekend. This was a little transistor bass amp so the speaker box was enclosed.

Image

A hammer and chisel made short work of removing the back boards. I haven't decided how I'm going to finish off the opening.

Image

The chassis will be reused, it was quite large for a simple little amp. Unfortunately, it wasn't large enough for the variac to fit inside. After considering several options, I decided to enclose the variac and mount it to the bottom of the chassis. The shaft will be extended through the chassis to a knob on the top panel. Two soup cans were used for the enclosure.

Image

The chassis had three 1/2" holes on the bottom for input/ouput jacks. I opened up the outer two to 5/8" to accommodate the 7-pin sockets and drilled a 1" hole for the 8-pin socket. Also drilled a 3/8" hole to mount the variac.

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With sockets riveted into place, I bolted up the variac and slapped in some tubes just to see how it will all look when put together.

Image

I'm going to reuse the AC Input socket (with built-in fuse holder) and power switch. The variac knob will go where the headphone jack was. I'll need to make a face-plate to cover up all the unused holes

Image

That's as far as I could go with the layout until the transformers arrive. Somehow, I misplaced a whole box of salvaged transformers so I had to order some new ones. I'm sure the old box will show up right after the new ones get here.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Oct Tue 24, 2017 8:34 pm 
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It will be loud with a 5V6 and that speaker. Too loud for a house in the suburbs, unless you muffle it with pillows in a closet. :lol:

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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 01, 2017 6:54 pm 
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The new transformers arrived. I penciled out a basic layout but when I went to start assembly, I could not find my bin of terminal strips anywhere. Grrrrrrr! It's hard to build something out of the junk bin when you keep losing the junk.

I did play around with the new N-68X transformer. The original Champ layout diagram shows 340 volts at the first filter cap. Running the transformer "backwards" won't get it there. However, since this amp only pulls about 43mA B+, I can feed only one of the primaries with 120 volts then use the other primary as a secondary. That has less voltage drop than running it backwards plus there is a voltage step on the other [actual] secondary. With the variac wired to the step-up secondary and configured to give an additional step-up it has 150 VAC + 120 VAC from the other "primary" in series for a total of 270 VAC. Those are open circuit numbers, it will drop some with load but not much given the fairly light load. I can experiment with different dropping resistors [after the rectifier] to alter the voltage and sag.

I ordered some neat Russian surplus terminal strips so it will probably be a couple more weeks until assembly begins. Oh well, that'll give me a chance to finish recapping the neighbor's arcade game [seen lurking in the background].


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Thu 23, 2017 1:08 pm 
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The terminal strips showed up this week. I started putting the amp together last night and this evening I finished wiring the chassis. The only change made from the original concept schematic was to put the Variac on the primary of the power transformer so the B+ voltage would go down to zero.

Image

Image

The initial test run went well, it actually works perfectly right out of the gate. With the Variac turned up all the way, the B+ is 320 volts. I'm glad I wired the Variac to go all the way down instead of just half way. The amp still sounds pretty good with the B+ all the way down to 35 volts!

The Variac is absolutely brilliant! Want SWEET distortion at bedroom volume? No Problem. Crank up the volume on the guitar until it distorts just right, then turn down the Variac to set the volume. It gets quieter with little to no change in distortion.

This is the coolest, best sounding, practice amp I have ever heard.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Sat 25, 2017 5:57 am 
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When I was testing the amp, it was plugged into a 2X12 speaker cabinet. The first time I fired it up after installing it in the little Ampeg it howled loudly. Put my finger in the first pre-amp tube and it is dead quiet, remove finger and it howls. The 3AV6 in that position is the most microphonic tube I have ever heard. When I "plucked" the nipple on the top of the tube it made a sound through the amp like I was plucking a string on the guitar. All I had to do to remedy the howling was to swap the two 3AV6 tubes.

Either this cheap 6.5" speaker is really good or I am blessed with a tin ear. I honestly can't tell any difference between it and the two 12" alnico speakers in the larger cab.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Mon 27, 2017 5:56 am 
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I added a second 470 Ohm resistor parallel to the 470 Ohm resistor on the output of the rectifier. That brought the B+ up to 340 volts.

With a resistive dummy load [and scope] on the output and the input fed with a function generator, it only put out 2.75 Vrms across a 3.8 Ohm load before starting to distort. If I'm doing the math correctly, that makes it a 2 Watt amp. Not that there is anything wrong with that [the kids keep telling me to turn it down], but these little Champs are typically called a 5 Watt amp. My voltages are now spot-on with the Fender 5F1 schematic and the only change I made to the actual amplifier circuit was a 1K screen resistor. Is this just a case of creative marketing or is there something off in my amp?

Cranking it up further, you can see how it distorts (input signal superimposed over clipped output).

Image

Sure looks a lot harder edged than the "creamy smooth" distortion picture that the tube amp fans extol. Sounds fine to me.

It was interesting to play with the Variac setting and input amplitude and see what effects they had. Altering the B+ only affected what point it would start to distort (headroom). If the input wasn't high enough to clip, then increasing the B+ had zero affect on the function of the amp.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Mon 27, 2017 1:52 pm 
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Hi mrriggs,

You might check bias on the 6V6 and that the output transformer is properly matched to the tube.

I think either of these could lead to early clipping.

Tim


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Mon 27, 2017 6:02 pm 
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Tkilboy wrote:
Hi mrriggs,

You might check bias on the 6V6 and that the output transformer is properly matched to the tube.

I think either of these could lead to early clipping.

Tim


The bias on the output tube is -17 volts, schematic shows -18 volts. It does saturate before cutoff so it would likely benefit from a bit lower bias voltage.

The output transformer is a new reproduction that claims to be a direct copy of a genuine 5F1 Champ output transformer. I suppose that doesn't necessarily guarantee it is actually matched to the output tube.

The Champ was an inexpensive amp that was basically just copied from the back of a tube manual. I imagine they were built with whatever parts they could dig up. I'm not inclined to believe that every amp Fender made was 100% correct.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Tue 28, 2017 11:00 pm 
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Tkilboy wrote:
Hi mrriggs,

You might check bias on the 6V6 and that the output transformer is properly matched to the tube.

I think either of these could lead to early clipping.

Tim


Thanks, Tim. I think you nailed it with the mismatched transformer.

Somehow I neglected to notice the primary impedance of this Classictone (40-18110) output transformer is 17k Ohms! A "Champ" transformer is usually around 7k Ohms. They give no explanation why the impedance is so high other than saying they copied a transformer they found in a 5F1 amp.

When I draw a load line with this impedance, it definitely clips too soon and the peak voltage is WAY off the chart. Seems odd that they would sell a transformer that is so poorly matched.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 12:51 am 
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Just thinking out loud here...

When looking at the 17K load line, I was a little puzzled by why it would be cutting off when it does (top of the wave form in the pic above). The clipping on the bottom of the waveform is from tube saturation.

The cutoff at the top happens at almost the same point as the saturation even though the voltage on the grid should be high enough to not cutoff. I'm guessing that this is because the output transformer simply doesn't have enough energy stored in it to swing any higher? That would explain why it kind of peters off instead of being cutoff sharply. Would a cooler bias on the output tube "square up" the clipping on the top of the wave form since it would be cutoff by the grid rather than being starved by the output transformer?


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 1:21 am 
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mrriggs wrote:
Tkilboy wrote:
Hi mrriggs,

You might check bias on the 6V6 and that the output transformer is properly matched to the tube.

I think either of these could lead to early clipping.

Tim


Thanks, Tim. I think you nailed it with the mismatched transformer.

Somehow I neglected to notice the primary impedance of this Classictone (40-18110) output transformer is 17k Ohms! A "Champ" transformer is usually around 7k Ohms. They give no explanation why the impedance is so high other than saying they copied a transformer they found in a 5F1 amp.

When I draw a load line with this impedance, it definitely clips too soon and the peak voltage is WAY off the chart. Seems odd that they would sell a transformer that is so poorly matched.
Sounds like they measured it with an 8 Ohm load and then built it for 4 Ohm.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 2:14 am 
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Flipperhome wrote:
Sounds like they measured it with an 8 Ohm load and then built it for 4 Ohm.


Wouldn't just changing speakers cure the problem?


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 2:41 am 
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Mike Toon wrote:
Flipperhome wrote:
Sounds like they measured it with an 8 Ohm load and then built it for 4 Ohm.


Wouldn't just changing speakers cure the problem?
Yes, if you go to 2 ohm.


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 6:05 pm 
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I measured the output transformer last night and came up with 16.9k primary impedance (with 4 ohm secondary), so it is a 17k transformer.

With a 1.6 Ohm load on the secondary it should put a 7k load on the output tube. I tried that and the waveform didn't look much different, maybe a little more rounded on the corners but still flat on the bottom and sloping on top. Power output did go up, to a whopping 2.5 Watts! The highest I managed to get it was 2.75 Watts with a 2.8 Ohm load.

A search of the vast interweb has turned up no solid numbers from an actual Champ amp, only the 5 Watt figure based off of the load line calculation. I'm just curious why my amp is the way it is and if it is typical for this design. I'm not in any hurry to change it. It's pretty cool, just the way it is.

Here is an updated schematic. http://www.gofastforless.com/junk/Champeg2.gif


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 6:21 pm 
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I'm a bit behind so dunno your actual B+ but with 250v plate & screen, a 6V6 into a 5k load produces 4.5W...

My guess is you'll never get the "rated" output without a OPT change...

http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6V6GT


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 Post subject: Re: Champeg guitar amp build
PostPosted: Nov Wed 29, 2017 9:20 pm 
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35Z5 wrote:
I'm a bit behind so dunno your actual B+ but with 250v plate & screen, a 6V6 into a 5k load produces 4.5W...

My guess is you'll never get the "rated" output without a OPT change...

http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6V6GT
This amp has a 5V6 on a variac, and maybe a lowered B+ is affecting the power output.

How about a voltmeter on the B+?
Also it will be interesting if you find a different output transformer to compare waveforms.
Congrats on your success so far.

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