Chinese 5F1 Champ Clone with weird volume pot cap wiring

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Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby Seiche » 07 Sep 2019, 11:30

I have two handwired chinese 5f1 champ clones, one a "Golden Ton" Smart-10 and one a "Storm" Smart-10.
The look identical but they sound very different. I would like to understand why.

One is quite bright and the volume control works as expected. It also takes a while to warm up and pass sound. Let's call it "A".
The other one passes sound instantly, is darker tonally and the volume control works almost like a linear pot, even though it is an audio pot. Let's call it "B".

Today I took them apart to see what the difference is (look at cap and resistor values an so on). All the values are the same, except for the treble(?) bleed caps around the volume control:
- Amp A has a 220pf (221) treble bleed cap similar to a guitar's volume control between pins 2 and 3 of the volume pot.
- Amp B has two 220pf (221) caps. One is going from pin 3 to ground and one is going from pin 2, which is connected to the grid on the second half of the input tube, to ground. I don't understand this configuration at all.

The funny thing is, I like the darker amp better for effects and overdriving, as it is less harsh, but it can be quite dark if I don't use bright settings on pedals and my bridge pickup. And the volume control is difficult to set as it is quite loud quite early on the dial.

Amp A
Amp_A-1.jpg

Amp B 1
Amp_B-1.jpg

Amp B 2
Amp_B-2.jpg
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby phatt » 07 Sep 2019, 13:06

If I'm reading it right then Amp B has both 220p caps go to ground.
If so both caps on Amp B are cutting excess treble, in different places. (Grid to ground and plate to ground).

While Amp A is (as you noted) a treble bleed/bypass on the volume pot.

Regards plate to ground cap;
I'm sure you have seen circuits where they add a small cap across the 100k plate resistor to wipe off excess treble. As this is for AC signal it matters little if the 220p cap goes from plate to HT or to ground. As far as the AC signal is concerned the B+ is just another common point so the result is the same,,,you can do it either way. :thumbsup

IME, if you want lotsa distortion then you have to cut higher freq otherwise it just gets very harsh.
Which is the reason you like the darker amp for overdriven sound.
Most Amps In the shops now have WAY TOOOOO much bandwidth and sound crap.
Same goes for a lot of those all in one digi pedal devices.
HTH,, Phil.

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Seiche (07 Sep 2019, 13:26)
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby Seiche » 07 Sep 2019, 13:26

The funny thing is that the tweed champ has neither cap.
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby phatt » 07 Sep 2019, 13:30

Bear in mind the OT's will likely be different than an original Champ and speakers do alter the final outcome,, sometimes in a big way.

Phil.
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby Seiche » 08 Sep 2019, 17:57

I removed all the caps and now amp A sounds great and just how I imagined.

However, amp B when dimed (volume on 8-10) has a farting noise that comes and goes and depends on the notes played (mostly bass notes on the low E-string).

I have tried different speakers and tubes, but am not sure my tubes are good or not, as some that I thought would be good made it worse than others.

Is there another explanation for this? I resoldered the caps to Amp B and it makes no difference, so likely they are not the reason.
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby phatt » 09 Sep 2019, 04:52

I assume Amp A is the *Golden ton Smart10*? Being the schematic you posted?
In which case yes removing the treble bleed across the volume would even out the tone.

If you removed the caps on the other amp,, with no schematic. (Assuming that is Amp B ??)
then it would be brighter but farts on low notes at high volume.
Then you simply reduce the value of the plate coupling caps until you find a balance. So if they are 22nF then go down to 10nF on first triode.

Also there is no series resistor on the grid of the 6L6 and that can cause nasty glitching at high volume. (Try 5~10k)

Be aware that power supply voltage nodes make a HUGE difference to Valve circuits and how they respond.

The circuit you posted shows the first supply resistor (screen) is 10k Try 4k7 for a cleaner head room.
The point at which the power tube breaks up is governed by the screen voltage.
HTH, Phil.

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Seiche (12 Sep 2019, 08:00)
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby Seiche » 09 Sep 2019, 20:34

Interesting.

Amp A and amp B are identical circuitwise now that I've remove the blue caps on both. They are both the circuit that I've posted (I've checked all the values).

The wiring is a bit different. As if they were build in different batches. But they are not very noisy.

Your suggestions would change them from the champ circuit, so I'm reluctant to try them. My research suggests the power tube might be running too hot and I'm thinking about measuring the bias and changing the cathode resistor for a slightly higher value.

What do you think?

Also there is no series resistor on the grid of the 6L6 and that can cause nasty glitching at high volume. (Try 5~10k)

What kind of wattage do I have to have there? 1w like the power supply resistors?
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby phatt » 10 Sep 2019, 06:04

Grid stopper resistor is only signal so 1/4 watt is all you need.
I use 1 watt as they have nice thick pigtails and stay where you put them. :wink:

I'm not sure what schematic you refer but there are at least a half dozen differences that I can see,
so although similar they may well sound/respond quite different. [smilie=a_whyme.gif]

Differences;
SS rectifier
All supply Filter caps are larger values
Input resistor Higher value.
V1a Cathode resistor lower value.
V1a cathode, bypass cap added.
Power Valve cathode Cap value larger.

Add to that, speaker likely different
OT is likely different.
End result the sound will likely be different. :|

If it helps the voltages on the power rails should around~~
360V HT
320V Screen
and 250V for V1
Phil.

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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby Seiche » 10 Sep 2019, 11:19

Input resistor Higher value.
V1a Cathode resistor lower value.
V1a cathode, bypass cap added.
Power Valve cathode Cap value larger.

I am talking about the 5F1 schematic as a comparison.
The input resistors are 68k in both cases
The cathode resistors are the same, the cathode caps are also the same (even though 5f1 schem doesnt show it, but the layout does).
Power valve cathode cap values are also the same (the smart-10 schem shows a different value, but both amps A and B have 22uf-50v).
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby Seiche » 10 Sep 2019, 20:42

Hey Phil,

I appreciate your help a lot! I started reading up on tube circuits and how tubes work etc. and what each part does and your comments have helped understand a bit better.
I have a few more questions though. :blackeye
phatt wrote:Also there is no series resistor on the grid of the 6L6 and that can cause nasty glitching at high volume. (Try 5~10k)

Just to make sure I understand you correctly, is this how what you're proposing to avoid the nasty glitching at high volume?
mod_grid_stop_resistor.png

phatt wrote:The circuit you posted shows the first supply resistor (screen) is 10k Try 4k7 for a cleaner head room.
The point at which the power tube breaks up is governed by the screen voltage.

Actually I want this amp to be dirty and I'm thinking about removing the NFB. So you're saying I should increase the 10k to make this amp less clean?

What would I need to change to use a 6V6GT?
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Re: Chinese 5F1 Champ Clone with weird volume pot cap wiring

Postby phatt » 13 Sep 2019, 04:15

To save me writing and confusing you it's better to read it from the experts. :thumbsup
https://robrobinette.com/Reading_Tube_A ... matics.htm

That site covers most of what you need to know about these amp circuits plus ways to mod.

Yes you can remove NFB for more dirt.
Re 6V6.
Here is a dead easy way to get your head around the differences in power valves.
It's called *Transconductance* TC for short, usually shown in data sheets but not always.

(((transconductance is the ratio of the change in output current to the change in input voltage across a circuit)))
If you are into high performance car engines it is how fast it revs,, or gets to full Horsepower. :mrgreen:

TC's of common Power Valves;
Tetrodes
6V6 = 3,800
6L6 = 8,000

Pentodes
EL84 = 11,000
EL34 = 11,000

Hence TC of 6V6 is low and needs a few tricks to make them OD.
Often just dropping the screen grid voltage is all that is needed.
Look up all the early 6V6 Deluxe Schematics and note the screen drop resistor value over time.
The early units like 5B3 had ~10k screen drop then down to 5k in later versions.

The early units had less clean headroom an less power while later units had a bit more clean head room and hence a little more volume.

So if you build a simple 6V6 amp circuit and change nothing much except swap to EL84 valve then obviously it will distort earlier because of the TC diff.

In general,
A Tetrode requires about 70Volt swing on the input grid to get to full power while a Pentode only requires about 30Volt swing.
Hence you often hear the term a British sound verses a American sound.
Easy to see why, as most Brit amps used pentodes while USA used Tetrodes.
HTH,,, Phil.

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