1st Champ build newbie help request

[Retiarius]

Hello everybody,

I'm building my first amp, a 5F1 Tweed Champ clone from a Mojotone kit. To be honest a friend, better qualified in the safe handling of these things, is actually doing most of it with my 'help' with the hope that I will be able to complete the next build alone.

Today we finally switched it on and, after the valves had warmed up it emitted a very high pitched squeal, persistent in pitch and volume. The squeal reacted to the volume pot.

We took out the negative feedback loop by removing the resistor and the squeal stopped. However, although the amp worked - in as much as I could plug a guitar in and play, there was a lot of hum. Also, to my ears, the tone seemed coloured by valve distortion (i.e. quite pleasant, but not what you'd expect at such low levels) even though the volume on the amp was only 1 and the volume on my guitar was turned down low. Further, I had to turn the tone pot right down on the guitar to achieve a tone that was not very bright indeed.

The speaker was not fitted into a cab, but loose.

I would be very grateful for any ideas.

Thanks in advance,

Retiarius

[soma_hero]

You probably have the Output Transformer plate leads reversed. That will give you a big scream! Switch them around and you should be fine.

[M Fowler]

What soma_hero noted would be my first guess.

Mark

[Retiarius]

Thanks soma_hero and Mark for your replies - much appreciated.

May I beg your indulgence and ask a further (possibly dumb?) question: why did the squeal stop when we took out the negative feedback?

Regards,

Retiarius

[paulster]

With the output transformer primary leads round the wrong way and therefore the signal was out of phase, your negative feedback was actually positive feedback.

In that regard it's behaving just like getting feedback from a mic.

[Retiarius]

paulster,

Thanks for your input - makes complete sense - thought it was a slightly dumb thing to ask!

Thanks again.

Regards,

Retiarius

[Structo]

Don't be afraid to ask questions when starting out.
That's how you can learn about things you don't quite understand.
But, be sure to research tube theory to get a better understanding what is going on inside the amp.

[Retiarius]

Tom - thanks for the input.

Regarding the OT being wired out of phase - there was an additional document included in the OT's box saying to wire it opposite to the way the schematic showed - rather annoying if it turns out to be incorrect!

I have to say that I don't think the Mojotone documentation is as sound as it could be - there are a couple of discrepancies between the schematic and the layout docs which have resulted in the build being more laboured than, perhaps, it should have been.

Anyway, my mate's having a look at it tomorrow, so hopefully, all will be well. I'll let you know if we fix it.

Thanks again to all for your help.

Regards,

Retiarius

[labb]

If you would care to read a nice little write up on building a Champ go to Angela Instruments, select the How-to-section down at the bottom then select the Angela single ended 6V6 guitar amp. It is a write up on building a Princeton 5F2 which is the same as a Champ 5F1 except that the Princeton has a tone control.

[Retiarius]

Labb - thanks for the link, nice one.

Well, we wired up the OT the other way and, hey presto, she wails! Thanks very much for your help chaps.

Just one thing. I know a Tweed Champ is going to hum, but how much is too much? I know it's a difficult thing to describe, but there is quite a lot of hum when the volume's turned past 12 o'clock, and it gets louder from there. You can't hear the hum when playing - it's totally swamped by the noise (of the guitar) - so it doesn't matter in that respect, but for recording it would be a problem. Any ideas?

Apart from the hum this amp is fabulous.

Regards,

Retiarius

[paulster]

Can you identify whether it's 60Hz or 120Hz hum? Hint, go and place your guitar pickup close to a mains transformer to get an idea of what 60Hz hum sounds like if you don't have any other way of identifying it.

If it's 60Hz hum then it's noise from the heaters getting amplified. In this case check your heater wiring to ensure that it is twisted sufficiently and is layed out away from the other wiring going to the tubes. When wires cross the heater wiring they should ideally do so at 90degrees to minimise coupling.

If it's 120Hz hum then it's noise on your high voltage supply. This can be one of two major things typically. One is your grounding scheme. Make sure that your grounds are all sound, especially if you've soldered directly to the chassis or to the pot backs. The other likely candidate is insufficient filtering in the power supply. This is fairly likely in a Champ build as it's single-ended and since large capacitors were difficult/expensive to obtain back in the 50s and this amp was supposed to be as cheap as could be. If that's the case then you'll have to live with it or accept that you could increase the filtering but it would change the feel/tone of the amp.

[sliberty]

Hum can be caused by a wide variety of layout problems. Maybe a few photos of the amp would be helpful for us to see if we notice anything wrong.

[M Fowler]

How about the house outlet your plugged into, any other noise source such as lights, heaters or other appliances?

I have never seen mojo build instructions but they must discuss grounding scheme or provided the grounding spots in their chassis?

Mark

[sliberty]

How are your heaters wired? Do they follow the Fender layout for this model? i.e. Did you ground pin 9 on V1 and pins 1 and 2 on V2?

I have heard some say that the "single sided" heater arrangement of the traditional 5F1 Champ layout was inferior to the more common "twisted pair" heater arranegement used in later Fenders and most other later amps. If you followed the Fender layout, "upgrading" the heater wiring might help.

[paulster]

I'd forgotten that the early ones had one side of the AC heaters soldered directly to the chassis.

Good, close-up photos would definitely help.