Tweed Champ (5F1) in Europe?

[tunghaichuan]

Don't worry about the slight increases in resistance. Those old carbon comp resistors are typically +/- 20% so they can be 20% under or over the value and not cause any problems.

I checked the data sheet for the 6V6 and the maximum resistance for the grid load resistor (the 220K) is 500K, so at +/- 20% you're well below the max.

The main problem with carbon comps is that they drift in value over time compared to metal film, metal oxide, or carbon film resistors. Sometimes when they drift just right the amp sounds very good. Other times, it doesn't doesn't work out so well. That's partially why some vintage amps sound great and others don't.

They can be noisy in certain positions, such as the plate load (100K). But some think that there is some magic/mojo if you put carbon comp resistors in those positions. I have not tried it myself, but I think it would be hard to tell the difference in a double blind test.

I've decided to stick to carbon comp resistors just because Victoria uses them and I love the way the 518 sounds. They're probably overkill to use all over the place, but it makes my life simpler, and if the noise they add won't be more than what the 518 puts out, I'll be happy.

I've sourced some new carbon comps (still waiting for a couple of 100k ones that turned out to be harder to get) and started wiring the board and soldering them in place. This time no surprises with the resistivity rating after soldering.

But - the two 68k grid resistors came at 71-72k and turned 73-74k after soldering. And the 220k power tube grid load resistor came at 250k and went to about 254k after soldering.

I don't think that either drift is spectacular or particularly important for the well-being of the finished amp, but as CCs will do, they will most likely continue to slowly drift upwards. The two "68k"s won't make a noticeable difference, but I'm worried about the "220k" - what if it goes to 300k? I'm assuming it won't affect the tone but will affect the power tube "loudness", and the power tube itself.

Hmm, maybe that's actually a good place for a metal or carbon film. If I understand it's purpose correctly, no signal actually goes through that resistor, except maybe to "go away" to ground.

 

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Don't worry about the slight increases in resistance. Those old carbon comp resistors are typically +/- 20% so they can be 20% under or over the value and not cause any problems.

I checked the data sheet for the 6V6 and the maximum resistance for the grid load resistor (the 220K) is 500K, so at +/- 20% you're well below the max.

The main problem with carbon comps is that they drift in value over time compared to metal film, metal oxide, or carbon film resistors. Sometimes when they drift just right the amp sounds very good. Other times, it doesn't doesn't work out so well. That's partially why some vintage amps sound great and others don't.

They can be noisy in certain positions, such as the plate load (100K). But some think that there is some magic/mojo if you put carbon comp resistors in those positions. I have not tried it myself, but I think it would be hard to tell the difference in a double blind test.

Once again, a quick and very clear answer! If you're ever in Romania, let me know because I owe you some glasses of your favourite beverage :beer:

I'm sorry for my novice questions, I'm learning as fast as I can and it's quite a lot to absorb. I've looked everywhere BUT at the 6V6 data sheet. :thwap Sometimes I get a bit tired with the research and the everyday stuff, that I tend to overlook obvious things...

I've read R.G. Keen's article about carbon comp resistors and he seems to make a case for the mojo factor. If that's one of the main differences between how the Swart Space Tone 6V6LE (which I've only heard online) and a Victoria 518 (which I've played) sound, there's definitely a warmer vibe to the Victoria. Not that there's anything wrong with either approach, just a matter of taste.

Any opinion on whether I should try to check the voltages inside with my cheapo multimeter/probes?

If anyone else stumbles upon this thread and needs more info about the 5F1, here's a very nice explanation of the preamp stage in a 5F1: http://www.classictubeamps.com/preamp.html

Thanks again, tunghaichuan!

 

[tunghaichuan]

Once again, a quick and very clear answer! If you're ever in Romania, let me know because I owe you some glasses of your favourite beverage :beer:

I'm sorry for my novice questions, I'm learning as fast as I can and it's quite a lot to absorb. I've looked everywhere BUT at the 6V6 data sheet. :thwap Sometimes I get a bit tired with the research and the everyday stuff, that I tend to overlook obvious things...

No problem, glad to help out. Definitely Google the 6V6GT data sheets. FWIW, the 6V6GTA is the same tube but is rated as a 14W tube for maximum dissipation. The 6V6GT is only rated at 12W. The "A" designator means slow warmup for use in tube equipment that runs the filaments of the tubes in series. (Filaments are wired in parallel in the 5F1.) The 'GT is identical to the 'GTA except for the "A." The 14W vs. 12W discrepancy is due to the way the tube industry rated the tube. You'll need this number if you want to check the bias of your 6V6 tube. Be aware that the JJ/Slovak 6V6S is not really a 6V6 tube, it can dissipate up to about 18 watts or so so it is more like a metal 6L6 or 6L6G in terms of power dissipation.

The bottom line is that some 6V6 tubes can be biased up to 14W with no problems, while other will over dissipate ("red plate") and will have to be biased down to 12W.

I've read R.G. Keen's article about carbon comp resistors and he seems to make a case for the mojo factor. If that's one of the main differences between how the Swart Space Tone 6V6LE (which I've only heard online) and a Victoria 518 (which I've played) sound, there's definitely a warmer vibe to the Victoria. Not that there's anything wrong with either approach, just a matter of taste.

RG Keen has forgotten more about electronics and tube circuits than I'll ever know, so I will defer to his judgment on this

Any opinion on whether I should try to check the voltages inside with my cheapo multimeter/probes?

That meter should be fine. When measuring the B+ or any other high voltage DC use the 600V setting set for DC. If you want to measure the raw AC coming out of the PT, use the 600V setting set to AC and measure from one end of the high voltage lead to the center tap. Don't measure across the entire HV winding or you might burn out the meter.

To measure cathode voltages, set the scale to the nearest appropriate voltage level on the DC setting. There is about 1v on the 12AX7 cathodes and about 20V or so on the 6V6's cathode.

You might want to get a set of retractable hooks for the meter. One for the black and one for the red lead. You can hook up the black lead to ground and then measure the voltages with one hand. Put the other hand in your pocket or behind your back as a safety precaution.

If anyone else stumbles upon this thread and needs more info about the 5F1, here's a very nice explanation of the preamp stage in a 5F1: http://www.classictubeamps.com/preamp.html

Thanks again, tunghaichuan!

Thanks for the link, I had not seen that site before.

 

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RG Keen has forgotten more about electronics and tube circuits than I'll ever know, so I will defer to his judgment on this

Oh I'm sorry, I thought the link to the article had already been posted somewhere in this thread. Here it is:

http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

You might want to get a set of retractable hooks for the meter. One for the black and one for the red lead. You can hook up the black lead to ground and then measure the voltages with one hand. Put the other hand in your pocket or behind your back as a safety precaution.

Yep, that's the plan. Thanks!

 

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Another noteworthy piece of information I've come across is that Victoria and the Weber 5F1 kit use a 25uF capacitor in parallel with the 1K5 resistor coming from pin 3 of the 12AX7.

Here's the Weber 5F1 layout:

http://taweber.powweb.com/store/5f1_layout.jpg

And a short film from Victoria humorously illustrating a 5F1 build:

http://www.youtube.com/watch?v=1xj6d9MLCpo

Supposedly this is a bypass cap, added in order not to screw up the 12AX7 bias as far as DC goes, but boosts the mid and low frequencies (or rather cuts some of the high ones?) in the AC current.

In any case, I don't have a quality electrolytic capacitor to add there, and I'm fed up as it is with waiting for parts so I'll just try the 5F1 I have as soon as I can, and if there'll be any problem with the tone I'll make that adjustment then.

 

[tunghaichuan]

Putting a cathode bypass cap across that 1.5k resistor is a common mod. I believe that some 5F1 Champs came out of the factory with it, stock, although it was not on the schematic.

You can use values from .68uF to 22uF. The practical range is from 2uF-4uF for humbucking pickup guitars and 4uF-10uF for single coil equipped guitars.

Another noteworthy piece of information I've come across is that Victoria and the Weber 5F1 kit use a 25uF capacitor in parallel with the 1K5 resistor coming from pin 3 of the 12AX7.

Here's the Weber 5F1 layout:

http://taweber.powweb.com/store/5f1_layout.jpg

And a short film from Victoria humorously illustrating a 5F1 build:

http://www.youtube.com/watch?v=1xj6d9MLCpo

Supposedly this is a bypass cap, added in order not to screw up the 12AX7 bias as far as DC goes, but boosts the mid and low frequencies (or rather cuts some of the high ones?) in the AC current.

In any case, I don't have a quality electrolytic capacitor to add there, and I'm fed up as it is with waiting for parts so I'll just try the 5F1 I have as soon as I can, and if there'll be any problem with the tone I'll make that adjustment then.

 

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It works! It sounds good "out of the box" , but it's getting better fast - the filter caps are probably settling in, maybe some undecided carbon comp resistors still figuring out what value they want to be, and definitely the speaker getting more and more in a vibrating mood.

Thanks for all the help, tunghaichuan!

 

[tunghaichuan]

Glad to hear everything worked out for you. It may take a while for the speaker to break in, but it will only get better sounding. Great job, I like the looks of that little amp. :AOK

It works! It sounds good "out of the box" , but it's getting better fast - the filter caps are probably settling in, maybe some undecided carbon comp resistors still figuring out what value they want to be, and definitely the speaker getting more and more in a vibrating mood.

Thanks for all the help, tunghaichuan!

 

[red]

Glad to hear everything worked out for you. It may take a while for the speaker to break in, but it will only get better sounding. Great job, I like the looks of that little amp. :AOK

Thanks! It looks even better in real life but there was not enough light and the flash kept going off. I actually took a bunch of pictures but that turned out to be the best of them, hehe.

Future plans for the amp include changing the red pilot light with a blue one, and add that bypass cap across the 1K5 resistor (I do like those low frequencies a lot, and I understand that it will also reduce some of the noise I inevitably get as a single-coil guitars fan).

Other than what I get because of the single coils, the amp is pretty silent. There is a tiny hum when the volume knob is at 12 (maximum), but it's definitely not louder than what I've heard from the 518, so I'd say the carbon comp resistors aren't any inconvenience here.

Now I have to try and find the best books to have on tube amps and start learning about it in more detail. And maybe start thinking about the cheapest way to get quality parts for a 5E3 .

 

[tunghaichuan]

Other than what I get because of the single coils, the amp is pretty silent. There is a tiny hum when the volume knob is at 12 (maximum), but it's definitely not louder than what I've heard from the 518, so I'd say the carbon comp resistors aren't any inconvenience here.

With carbon comps, it is usually hiss, not hum so it sounds like you lucked out.

Now I have to try and find the best books to have on tube amps and start learning about it in more detail. And maybe start thinking about the cheapest way to get quality parts for a 5E3 .

Here is a site to get you started on building amps:

http://www.pmillett.com/tecnical_books_online.htm

There is enough info there to keep anyone interested learning for a long time.

 

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Here is a site to get you started on building amps:

http://www.pmillett.com/tecnical_books_online.htm

There is enough info there to keep anyone interested learning for a long time.

Thanks!

 

[red]

With carbon comps, it is usually hiss, not hum so it sounds like you lucked out.

I don't know, maybe I did. With no input and the amp turned all the way up on the Champ clone, there's a very small amount of hiss. That's exactly what I hear out of my Peavey Classic 30 with no input on the clean channel turned up about halfway - and there are no carbon composition resistors in the PC30.

Actually the loudest thing I hear in the situation described is a tube. Probably the power tube. Cooking away.